MUTATION

There is ample evidence that mutations are causally related to cancer, a prominent age-related disease. Since the 1950s the accumulation of spontaneous mutations in the DNA of organs and tissues has been hypothesized to underlie aging itself (e.g., Failla, 1958). What are mutations, and why are they there? First, it is necessary to distinguish DNA mutations from DNA damage. DNA damage consists of chemical alterations in DNA structure, leading to a structure that can no longer serve as a substrate for faithful replication or transcription. DNA damage cannot be copied to end up in daughter cells. DNA mutations are heritable changes in a DNA sequence of an organism, which can be part of a gene, a gene regulatory region, or some noncoding part of the genome. Mutations are usually introduced as a consequence of misreplication or misrepair, for example, due to the presence of DNA damage. Hence, DNA damage can lead to mutations when it is not correctly repaired. Mutations can vary from point mutations, involving single or very few base pairs to large deletions, insertions, duplications, and inversions. In organisms with multiple chromosomes, DNA from one chromosome can be joined to another and the actual chromosome number can be affected.

Mutations are inevitable. Indeed, they fuel the survival of cells and organisms in times of stress. They are the substrate for evolution, which gave rise to different life forms. In both prokaryotic and eukaryotic cells mutation rate can be greatly increased, causing many cells to die but also giving rise to cells with the necessary attributes to survive and expand. Cancer cells, for example, can undergo mutations in genes that control the mutation rate (for example, genes involved in DNA repair). Such "mutator phenotypes" allow them to accelerate the acquisition of novel attributes (e.g., drug resistance, tissue invasivity) through gene mutation. While in such cases mutations are detrimental for the host, they are beneficial for the cell. In most cases, mutations will have adverse effects on both the host and the cell. In the somatic cells of multicellular organisms, however, mutations usually have adverse effects.

Gross chromosomal alterations

The earliest and still the most popular way to look at mutations is by cytogenetic means. This necessarily precludes the detection of mutational events smaller than a few million base pairs and also limits the target tissue to cells in metaphase, usually white blood cells. A noteworthy exception to this latter limitation is the original work of the late Howard Curtis (1963), who with coworkers examined mouse liver parenchymal cell metaphase plates after partial hepatectomy and found considerably higher numbers of cells with abnormal chromosomes in old, compared with young, animals (i.e., from about 10 percent of the cells in mice four to five months old to 75 percent in mice older than twelve months). Later, large structural changes in DNA were observed to increase with donor age in white blood cells of human individuals, from about 2–4 percent of the cells having chromosomal aberrations in young individuals to about six times higher in the elderly. The recent use of more advanced methods, such as chromosome painting, have confirmed the increase in cytogenetic damage with age in both human and mouse. In both human and mouse lymphocytes the increase in chromosome aberrations appeared to be exponential.

Mutations detected in selectable marker genes

With the development of tests based on selectable endogenous marker genes, it became possible to assess mutation frequencies at these loci in T-cells from human and animal donors (for a review see Vijg, 2000). Using the hypoxanthine phosphoribosyl transferase (HPRT) locus test, investigators have shown that mutation frequencies at this locus increase with donor age. For example, results obtained with this assay suggest that mutation frequencies in humans increase with age from about 2 × 10^-6 in young individuals to about 1 x 10^-5 in middle aged and old individuals. In mice mutation frequencies have been reported from about 5 x 10^-6 in young animals to about 3 x 10^-5 in middle aged mice (Dempsey et al., 1993). However, in both mice and humans these values could be underestimates, due to the loss of HPRT mutants in vivo or in vitro. Indeed, results from Grist et al. (1992), who assayed the HLA locus (using immunoselection for mutationally lost HLA antigen) in human lymphocytes, indicate mutant frequencies two to three times higher. Values higher than HPRT were also found with other assays involving selectable target genes. The discrepancy has been explained in terms of the inability of the HPRT test to detect mitotic recombination events (HPRT is X-linked) and a relatively strong in vivo selection against mutants.

In mice subjected to caloric restriction, the only intervention demonstrated to increase life span (Masoro, 1993), HPRT mutation frequencies were found to increase with age at a significantly slower rate than in the animals that fed at will. Dempsey et al. (1993) studied both types of animals for twelve months. They saw an age-associated accumulation of mutations in the mice that fed at will, but not in the mice that experienced calorie restriction. Their conclusion was that the great majority of endogenous mutations are related to diet. However, it is not clear if this effect of diet is the result of intake of multiple exogenous mutagens or is related to the metabolism of food.

The results of Dempsey et al. suggest that the level of accumulated somatic mutations reflects biological rather than chronological age. This conclusion was further strengthened by Odagiri et al. (1998), who demonstrated accelerated accumulation of mutations in peripheral blood lymphocytes of so-called senescence-accelerated mice (SAM). Although the SAM model is not generally accepted as a mouse model of accelerated aging, these findings nevertheless demonstrate a link between somatic mutation rate and physiological decline. Interestingly, the HPRT test has been used on tubular epithelial cells of kidney tissue from human donors two to ninety-four years old. The mutation frequencies found were much higher than the values for blood lymphocytes and also increased with age (Martin et al., 1996). The high mutation frequency in the kidney cells could reflect a relatively slow turnover as compared to T-cells.

Mutations in transgenic mouse reporter genes

With the development of transgenic mouse models harboring chromosomally integrated reporter genes, it became possible to directly test the hypothesis that somatic mutations in a neutral (with no function) gene accumulate with age in various organs and tissues (Gossen and Vijg, 1993). Using one of these models, harboring the lacI gene as a target, Lee et al. (1994) were the first to demonstrate an age-related increase in mutation frequency in spleen from about 3 × 10^-5 in mice of a few weeks old to 1-2 × 10^-4 in 24-month-old animals. Subsequent results from other laboratories indicated age-related increases in mutation frequency in some, but not all, organs. Dollé et al. (1997), for example, demonstrated that mutation frequencies at a lacZ transgene increase with age in the liver, while such an increase was virtually absent in the brain. The increased susceptibility to spontaneous mutagenesis of liver versus brain corresponds to observed higher frequency of focal pathological lesions in the mouse liver as compared with the brain (Bronson, 1990). More recently, the pattern of organ specificity in age-related mutation accumulation was expanded with the observation of age-related increases in mutation frequencies in spleen, heart, and small intestine, but not in testes (Dollé et al., 2000).

Essentially the same results obtained by Dollé et al. (1997) were found by Ono et al. (2000). These investigators used the original mouse model of Gossen et al. (1989), harboring the same transgene. They also observed an age-related increase of mutation frequency in the liver, heart, and spleen, but virtually no increase in the brain and testes. Dollé et al. (1997; 2000) also observed striking organ specificity with respect to the mutational spectra of the old animals. While in the small intestine and the brain virtually only point mutations accumulated (the small increase in the brain was almost totally due to point mutations), in the liver and especially in the heart large deletion mutations were a prominent part of the spectrum (Dollé et al., 1997).

In interpreting these data it should be noted that the observed increases were modest (varying from less than twofold, to more than fourfold) and appeared to level off at middle age (Lee et al., 1994; Dollé et al., 1997). The relatively small age-related increase of mutant frequencies can be interpreted as evidence against a major role for somatic mutations in aging (Warner and Johnson, 1997). However, although transgenic reporter genes do not suffer from a selection bias (as is the case with most selectable endogenous targets), it still provides an underestimate of the real mutation load and its adverse effects. Homologous (mitotic) recombination, for example, leading to deletion of entire reporter gene copies is a frequent mutational event and goes undetected in the transgenic assays. Most of the transgenic models also do not account for mutational hot spots, and such important functional end points as cell death are missed. Indeed, to put the results on mutant frequencies of different organs and tissues at various age levels into context, it will be necessary also to assess cell proliferation and cell death. Most important, it will be necessary to determine at some point the critical level of cellular mutation loads in terms of physiological consequences. The question is whether the mutation loads observed in a tissue at old age have physiological consequences. A glimpse of an answer can possibly be obtained from another type of model system.

Models for genome instability

Evidence that mutation accumulation does play a role in the functional decline and increased incidence of disease associated with aging can be derived from the work with mouse models having genetically engineered defects in genome stability systems. For example, cells in highly proliferating organs of telomerase-null mice (knockout or defective mice) showed erosion of telomeres, resulting in high levels of genetic instability (fusion and loss of chromosomes), accompanied by increased programmed cell death and a compromised capacity for cell renewal in spleen and bone marrow. In a subsequent study of third-generation telomerase-null mice, shortened life span was found to be accompanied by reduced capacity to respond to stresses, such as wound healing, and by an increased incidence of spontaneous malignancies (Rudolph et al., 1999). The results of these studies underscore that in proliferative organs (highly proliferating cells in organs of telomerase defective mice) the early initiation of genetic instability due to telomere erosion can greatly accelerate age-related loss of cell viability and increased tumor formation.

Other examples of genome stability mutants that point to accelerated aging are mouse models with inactivated genes involved in double-strand break repair. These animals prematurely exhibit symptoms of age-related degeneration in liver, skin, and bone (Vogel et al., 1999). Hence, it appears that genetic defects promoting genomic rearrangements are associated with symptoms of accelerated aging (for a review see Vijg, 2000). This would be in keeping with the results of studies involving patients with Werner syndrome. This genetic disease, caused by a heritable mutation in a single gene (the WRN gene), is characterized by the accelerated occurrence of certain aspects of the senescent phenotype, including cancer. The WRN gene contains both a helicase and an exonuclease function and is thought to play a role in suppressing genomic instability. Indeed, cultured somatic cells from patients with Werner syndrome display an increased rate of somatic mutations and a variety of cytogenetic abnormalities, such as deletions and translocations (Fukuchi et al., 1989). This high level of genomic instability could be the cause of the severe limitation of in vitro life span demonstrated in these cells. Other so-called progeroid syndromes, such ataxia telangiectasia and Bloom syndrome show increased genomic instability (for a review, see Turker and Martin, 1999).

Summary and future prospects

In summary, there is now conclusive evidence that mutations accumulate with age in most organs and tissues of the mouse and in white blood cells of mice and humans. A considerable fraction of this loss of stability of the nuclear genome consists of genome rearrangements. Genome rearrangements in the form of illegitimate recombinations are likely to be due to misrepair and misannealing of double-strand breaks or other DNA lesions opposite one another on the two DNA strands. In view of the elevated occurrence of this type of mutation in white blood cells of patients with segmental progeroid syndromes, it is tempting to speculate that if mutations contribute to the adverse effects associated with aging, genome rearrangements play a major role. Indeed, it is unlikely that randomly induced point mutations will have a major effect on cell functioning. Cellular systems are robust, and insensitive to many mutations. However, sizable genome rearrangements, even a relatively small number, could seriously affect normal regulation, through gene dosage or position effects. A dosage effect is another standard term in biology and means that an additional copy of the gene (or additional copies) will increase the amount of proteins that are produced. (And the other way around, i.e., when one of your two copies of gene X is deleted you may produce less of protein X). In actively proliferating cell compartments one of the predicted effects would be hyperplasia, neoplasia, and tissue atrophy. Hyperplasia is like neoplasia, but a forestage. In many cases (perhaps most) it stays with that and a tumor never results. Atrophy simply means a reduction in mass because of cell loss. In postmitotic cells it could affect a variety of functional pathways leading to a mosaic of cells at different stages and finally to cell death.

Future research in the area of mutation accumulation as a possible cause of aging is likely to focus on two topics. First, more and more emphasis is now given to mouse models genetically manipulated to have defects in genome stability systems. As outlined above, many of these mice also show signs of accelerated aging. By using so-called knock-in models in which natural genes are replaced by genes with subtle alterations rendering them less effective, it should be possible to generate models with overall less effective genome preservation systems without the total absence of one important gene function. If aging is caused by mutation accumulation, it is likely that such mice will mimic the aging phenotype more fully than single-gene knockouts.

Second, a more recent approach, made possible by the completion of the Human Genome Project, is to analyze all genome instability genes for polymorphic variation in different populations of elderly individuals. Gene variants, alone or in combination with others, can then be studied for association with natural differences in life span, functional decline, and age-related disease among elderly persons. Studies of individuals over one hundred years old have provided evidence that genes may play an increasingly prominent role in the ability to achieve older and older age beyond average life expectancy (Perls et al., 1998). It is possible that a combination of optimal genome stability genotypes contributes to the longevity in centenarians.

Jan Vijg

See also Accelerated Aging: Animal Models; Accelerated Aging: Human Progeroid Syndromes; Cellular Aging: Cell Death; Genetics; Longevity: Reproduction; Longevity: Selection; Nutrition: Caloric Restriction; Stress; Theories of Biological Aging.

BIBLIOGRAPHY

Bronson, R. T. "Rate of Occurrence of Lesions in 20 Inbred and Hybrid Genotypes of Rats and Mice Sacrificed at 6 Month Intervals During the First Years of Life." In Genetics of Aging, Vol. 2. Edited by D. E. Harrison. Caldwell, N.J.: Telford Press, 1990. Pages 280–358.

Curtis, H. J. "Biological Mechanisms Underlying the Aging Process." Science 141 (1963): 686–694.

Dempsey, J. L.; Pfeiffer, M.; and Morley, A. A. "Effect of Dietary Restriction on in Vivo Somatic Mutation in Mice." Mutation Research 291 (1993): 141–145.

DollÉ, M. E. T.; Giese, H.; Hopkins, C. L.; Martus, H. J.; Hausdorff, J. M.; and Vijg, J. "Rapid Accumulation of Genome Rearrangements in Liver but not in Brain of Old Mice." Nature Genetics 17 (1997): 431–434.

DollÉ, M. E. T.; Snyder, W. K.; Gossen, J. A.; Lohman, P. H. M.; and Vijg, J. "Distinct Spectra of Somatic Mutations Accumulated with Age in Mouse Heart and Small Intestine." Proceedings of the National Academy of Sciences of the United States of America 97 (2000): 8403–8408.

Failla, G. "The Aging Process and Carcinogenesis." Annals of the New York Academy of Science 71 (1958): 1124–1135.

Fukuchi, K.; Martin, G. M.; and Monnat, R. J., Jr. "Mutator Phenotype of Werner Syndrome is Characterized by Extensive Deletions." Proceedings of the National Academy of Sciences of the United States of America 86 (1989): 5893–5897.

Gossen, J. A.; de Leeuw, W. J. F.; Tan, C. H. T.; Lohman, P. H. M.; Berends, F.; Knook, D. L.; Zwarthoff, E. C., and Vijg, J. "Efficient Rescue of Integrated Shuttle Vectors from Transgenic Mice: A Model for Studying Gene Mutations in Vivo." Proceedings of the National Academy of Sciences of the United States of America 86 (1989): 7971–7975.

Gossen, J. A., and Vijg, J. "Transgenic Mice as Model Systems for Studying Gene Mutations In Vivo." Trends in Genetics 9 (1993): 27–31.

Grist, S. A.; McCarron, M.; Kutlaca, A.; Turner, D. R.; and Morley, A. A. "In Vivo Human Somatic Mutation: Frequency and Spectrum with Age." Mutation Research 266 (1992): 189–196.

Lee, A. T.; DeSimone, C.; Cerami, A.; and Bucala, R. "Comparative Analysis of DNA Mutations in lacI Transgenic Mice with Age." Federation of American Societies for Experimental Biology Journal 8 (1994): 545–550.

Loeb, L. A. "Endogenous Carcinogenesis: Molecular Oncology into the Twenty-First Century—Presidential Address." Cancer Research 49 (1989): 5489–5496.

Martin, G. M.; Ogburn, C. E.; Colgin, L. M.; Gown, A. M.; Edland, S. D., and Monnat, R. J., Jr. "Somatic Mutations are Frequent and Increase with Age in Human Kidney Epithelial Cells." Human Molecular Genetics 5 (1996): 215–221.

Masoro, E. J. "Dietary Restriction and Aging." Journal of the American Geriatrics Society 41 (1993): 994–999.

Odagiri, Y.; Uchida, H.; Hosokawa, M.; Takemoto, K.; Morley, A. A.; and Takeda, T. "Accelerated Accumulation of Somatic Mutations in the Senescence-Accelerated Mouse." Nature Genetics 19, no. 2 (1998): 116–117.

Ono, T.; Ikehata, H.; Nakamura, S.; Saito, Y.; Hosoi, Y.; Takai, Y.; Yamada, S.; Onodera, J.; and Yamamoto, K. "Age-Associated Increase of Spontaneous Mutant Frequency and Molecular Nature of Mutation in Newborn and Old lacZ-Transgenic Mouse." Mutation Research 447 (2000): 165–177.

Perls, T. T.; Bubrick, E.; Wager, C. G.; Vijg, J.; and Kruglyak, L. "Siblings of Centenarians Live Longer." Lancet 351 (1998): 1560.

Rudolph, K. L.; Chang, S.; Lee, H. W.; Blasco, M.; Gottlieb, G. J.; Greider, C.; and DePinho, R. A. "Longevity, Stress Response, and Cancer in Aging Telomerase-Deficient Mice." Cell 96 (1999): 701–712.

Turker, M. S., and Martin, G. M. "Genetics of Human Disease, Longevity and Aging." In Principles of Geriatric Medicine and Gerontology, 4th ed. Edited by W. R. Hazzard, J. P. Blass, W. H. Ettinger, Jr., J. B. Halter, and J. G. Ouslander. New York: McGraw-Hill, 1999.

Vijg J. "Somatic Mutations and Aging: A Re-Evaluation.". Mutation Research 447 (2000): 117–135.

Vogel, H.,; Lim, D. S.; Karsenty, G.; Finegold, M.; and Hasty, P. "Deletion of Ku86 Causes Early Onset of Senescence in Mice." Proceedings of the National Academy of Sciences of the United States of America 96 (1999): 10770–10775.

Warner, H. R., and Johnson, T. E. "Parsing Age, Mutations and Time." Nature Genetics 17 (1997): 368–370.

MUTATION

CONCEPT

A word familiar to all fans of science fiction, mutation refers to any sudden change in DNA—deoxyribonucleic acid, the genetic blueprint for an organism—that creates a change in an organism's appearance, behavior, or health. Unlike in the sci-fi movies, however, scientists typically use the word mutant as an adjective rather than as a noun, as, for example, in the phrase "a mutant strain." Mutation is a phenomenon significant to many aspects of life on Earth and is one of the principal means by which evolutionary change takes place. It is also the cause of numerous conditions, ranging from albinism to cystic fibrosis to dwarfism. Mutation indicates a response to an outside factor, and the nature of that factor can vary greatly, from environmental influences to drugsto high-energy radiation.

HOW IT WORKS

DNA, Chromosomes, and Mutations

Deoxyribonucleic acid, or DNA, is a molecule in the cells of all life-forms that contains genetic codes for inheritance. DNA, discussed elsewhere in this book, is as complex in structure as it is critically important in shaping the characteristics of the organism to which it belongs, and therefore it is not surprising that a subtle alteration in DNA can produce significant results. Alterations to DNA are called mutations, and they can result in the formation of new characteristics that are heritable, or capable of being inherited.

Every cell in the body of every living organism contains DNA in threadlike structures called chromosomes. Stretches of DNA that hold coded instructions for the manufacture of specific proteins are known as genes, of which the human race has approximately 40,000 varieties. If the DNA of a particular gene is altered, that gene may become defective, and the protein for which it codes also may be missing or defective. Just one missing or abnormal protein can have an enormous effect on the entire body: albinism, for instance, is the result of one missing protein.

Mutations also can be errors in all or part of a chromosome. Humans normally have 23 pairs of chromosomes, and an extra chromosome can have a tremendous negative impact. For example, there should be two of chromosome 21, as with all other chromosomes, but if there are three, the result is Down syndrome. People with Down syndrome have a unique physical appearance and are developmentally disabled. Nor is an extra chromosome the only chromosomal abnormality that causes problems: if chromosomes 9 and 22 exchange materials, a phenomenon known as translocation, the result can be a certain type of leukemia. Down syndrome also results from translocation.

Germinal mutations are those that occur in the egg or sperm cells and therefore can be passed on to the organism's offspring. Somatic mutations are those that happen in cells other than the sex cells, and they cannot be transmitted to the next generation. This is an important distinction to keep in mind in terms of both the causes and the effects of mutation. If only the somatic cells of the organism are affected, the mutation will not appear in the next generation; on the other hand, if a germinal mutation is involved, what was once an abnormality may become so common in certain populations that it emerges as the norm.

The Role of Mutation in Evolution

Most of the forms of mutation we discuss in this essay appear suddenly (i.e., in a single generation) and affect just a few generations. Yet even such seemingly "normal" characteristics as our ten fingers and ten toes or our two eyes or our relatively hairless skin (compared with that of apes) are ultimately the product of mutations that took shape over the many hundreds of millions of years during which animal life has been evolving. Evolution, in fact, is driven by mutation, along with natural selection (see Evolution).

Over the eons, advantageous mutations, examples of which we look at later, have allowed life to develop and diversify from primitive cells into the multitude of species—including Homo sapiens —that exist on Earth today. If DNA replicated perfectly every time, without errors, the only life-forms existing now would be those that existed about three billion years ago: single-cell organisms. Mutations, therefore, are critical to the development of diverse life-forms, a phenomenon known as speciation (see Speciation). Mutations that allow an organism to survive and reproduce better than other members of its species are always beneficial, though a mutation that may be beneficial in some circumstances can be harmful in others. Mutations become especially important when an organism's environment is changing—something that has happened often over the course of evolutionary history. And though we cannot watch evolution taking place, we can see how mutations are used among domesticated plants and animals, as discussed later.

REAL-LIFE APPLICATIONS

Ethnicity and Mutation

Every single human trait—blue eyes, red hair, cystic fibrosis, a second toe longer than the big toe, and so on—is the result of some genetic mutation somewhere back down the line. Traits that are shared by all people must have arisen long ago, while other traits occur only in certain populations of people. Traits may be as innocuous as eye color or hair texture or as grave as a shared tendency toward a particular disease. Cystic fibrosis, for instance, is most common in people of northern European descent, while sickle cell anemia (see Amino Acids) occurs frequently in those of African and Mediterranean ancestry. A fatal disorder known as Tay-Sachs is found primarily in Jewish people whose ancestors came from Eastern Europe. In many cases, the particular mutation, while harmful in one regard, proved to be a useful one for that population. We know, for example, that while two copies of the mutant sickle cell anemia gene cause illness, one copy confers resistance to malaria—a very useful trait to people living in the tropics, where malaria is common.

THE PIMA "FAT-STORAGE MUTATION."

Researchers have noted a high incidence of obesity among the Pima, a Native American tribe whose ancestral homeland is along the Gila and Salt rivers in Arizona. The Pima tend to eat a diet that is no more fatty than that of the average American—which, of course, means that it is plenty fatty, complete with chips, bologna, ice cream, and all the other high-calorie, low-nutrient foods that most Americans consume. But whereas the average American is over-weight, the average Pima is more dramatically so. This suggests that long ago, when the ancestors of the Pima had to face repeated periods of famine in the dry lands of the American Southwest, survival favored the individual or individuals who had a mutation for fat storage. It so happens that today, there is more than enough food at the local supermarket, but by now the Pima as a group has the fat-storage gene. Therefore, many members of the tribe have to undergo strict dietary and exercise regimens so as not to become grossly overweight and susceptible to heart disease and other ailments.

Favorable Mutations

As with other mutations relating to ethnic groups, scientists have hypothesized that some advantage must be conferred upon people with single copies of the cystic fibrosis gene or the Tay-Sachs disease gene. Though many mutations are harmful, others prove to be beneficial to a species by helping it adapt to a particular environmental influence. Useful mutations, in fact, are the driving force behind evolution.

The processes of evolution are usually much too slow for people to discern, but it is possible to observe the effects of selective breeding when applied to domesticated animals and plants. The artificial selection of pigeons by breeders, in fact, provided the English naturalist Charles Darwin (1809-1882) with a model for his theory of natural selection, discussed in Evolution. Likewise, animal and plant breeders use mutations to produce new or improved strains of crops and livestock. Careful breeding in this manner has spawned the many different breeds of dogs, cats, and horses—each with their characteristic coloring, size, temperament, and so on—that we know today. It also has resulted in crops that are resistant to drought or insects or which have a high yield per acre. Likewise, goldfish, yellow roses, and Concord grapes are all descendants of ancestors with specific mutations.

Diseases and Mutation

The majority of mutations, however, are less than favorable, and this is illustrated by the relationship between mutation and certain hereditary diseases. An example is Huntington disease, a condition that strikes people in their forties or fifties and slowly disables their nervous systems. It produces shaking and a range of other symptoms, including depression, irritability, and apathy, and is usually fatal. The gene associated with Huntington's is dominant.

The horrible degenerative brain condition known as Creutzfeldt-Jakob disease, discussed in Diseases, is usually caused by another mutation. (Though it can be caused by infection, most cases of the disease are the result of heredity.) As with some of the other conditions we have mentioned, this one seems to affect particular groups more than others. Whereas the worldwide incidence of this rare condition is about one in one million, among Libyan Jews the rate is higher. The disease is a type of spongiform encephalopathy, so named because it produces characteristic spongelike patterns on the surface of the brain. Spongiform encephalopathies are caused by the appearance of a prion, a deviant form of protein whose production typically is caused by a mutation.

Most hereditary diseases are, by definition, linked with a mutation. Such is the case with hemophilia, for instance (see Noninfectious Diseases), and with cystic fibrosis, a lethal disorder that clogs the lungs with mucus and typically kills the patient before the age of 30 years. Cystic fibrosis, like Huntington, occurs when a person inherits two copies of a mutated gene. In 1989 researchers found the source of cystic fibrosis on chromosome 7, where an infinitesimal change in the DNA sequence leads to the production of an aberrant protein.

Congenital Disorders

In the past, all manner of superstitions arose to explain why a child was born, for instance, with a cleft palate, a situation in which the two sides of the roof of the mouth fail to meet, causing a speech disorder that may be mild or severe. Once known as a harelip, the cleft palate was said to have formed as a result of the mother's being frightened by a hare while she was carrying the child. In fact, it is just one example of a congenital disorder, an abnormality of structure or function or a disease that is present at birth. Congenital disorders, which also are called birth defects, may be the result of several different factors, mutation being one of the most significant. Among the many examples of congenital disorder are the hereditary diseases we have already mentioned, as well as dwarfism, Down syndrome, albinism, and numerous other conditions.

DWARVES AND MIDGETS.

The term dwarf has many associations from fairy tales—an example of the combined fascination and revulsion with which people with congenital disorders have long been treated—but it also is used to describe persons of abnormally short stature. A dwarf is distinguished from a midget in a number of ways, all of which indicate that the features of a midget are less removed from the norm. Midgets, while small, have bodies with proportions in the ordinary range. Likewise, the intelligence and sexual development of an adult midget are similar to those of other adults, and a midget or midget couple typically produces children of ordinary size. Pygmies, a group of people in southern Africa, appear to be midgets through a germinal mutation, but in many populations the mutation is somatic, occurring only occasionally in families whose other members are of ordinary size.

Dwarfs, by contrast, have several different disorders. One variety of dwarfism, known in the past as cretinism, is characterized by a small, abnormally proportioned body and an impaired mind. On the other hand, several forms of hereditary dwarfism carry with them no ill effect on the mental capacity. For example, people with the type of dwarfism known as achondroplasia have short limbs and unusually large heads, but the life span and intelligence of someone with this condition are quite normal. In the case of diastrophic dwarfism, the brain is fine, but the skeleton is deformed, and the risk of death from respiratory failure is high in infancy. Persons with diastrophic dwarfism who survive early childhood, however, are likely to enjoy a normal life span.

DOWN SYNDROME.

Like people with many other congenital disorders, those with Down syndrome used to be called by a name that now is considered crude and insensitive: mongoloid. The term, when used with a capital M, refers to people of east Asian descent and is analogous to other broad racial groupings: Caucasoid, Negroid, and Australoid. In the case of people with Down syndrome, mongoloid referred to the unusual facial features that mark someone with that condition.

A person with Down syndrome (caused by an extra chromosome in the 21st chromosomal pair) is likely to have a wide, flat face and eyes that are slanted, sometimes with what is known as an inner epicanthal fold —all facial characteristics common among people who are racially Mongoloid. Numerous other facial features identify a person with Down syndrome as someone who suffers from a specific congenital disorder, including a short neck, ears that are set low, a small nose, large tongue and lips, and a chin that slopes. People with Down syndrome are apt to have poor muscle tone and possess abnormal ridge patterns on their palms and fingers and the soles of their feet. Heart and kidney problems are common with Down syndrome as well, but one feature is most common of all: mental retardation. The condition occurs in about one of 1,000 live births among women under age 40 but about one in 40 live births to older women. Overall, the incidence is about one in 800 live births. As noted earlier, the cause of Down syndrome is translocation, but the reason translocation occurs is not known.

ALBINISM.

Compared with dwarfism or Down syndrome, albinism is not nearly as severe in terms of its effect on a person's functioning. A condition that results from an inherited defect in melanin metabolism (melanin is responsible for the coloring of skin), albinism is marked by an absence of pigment from the hair, skin, and eyes. The hair of an albino tends to be whitish blond, the skin an extremely pale white, and the eyes pinkish. Albinism occurs among other animals: hence the white rats, rabbits, and mice almost everyone has seen. Domestic white chickens, geese, and horses are partial albinos that retain pigment in their eyes, legs, and feet. As was once true of people with other congenital disorders, human albinos once inspired fear and awe. Sometimes they were killed at birth, and in the mid-nineteenth century, albinos were exhibited in carnival sideshows. In these cruel spectacles, sometimes whole families were put on display, touted as a unique race of "night people" who lived underground and came out only when the light was dim enough not to hurt their eyes.

On the other hand, some ethnic groups experience enough albino births that another one causes no excitement. For example, among the San Blas Indians of Panama, one in approximately 130 births is an albino, compared with one in 17,000 for humans as a whole. Albinism comes about when melanocytes (melanin-producing cells) fail to produce melanin. In tyrosinase-negative albinism, the most common form, the enzyme tyrosinase (a catalyst in the conversion of tyrosine to melanin) is missing from the melanocytes. When the enzyme is missing, nomelanin is produced. In tyrosinase-positivealbinism, a defect in the body's tyrosine transportsystem impairs melanin production. One inevery 34,000 persons in the United States has tyrosinase-negative albinism. It is equally common among blacks and whites, while more blacksthan whites are affected by tyrosinase-positivealbinism. Native Americans have a particularlyhigh incidence of both forms of albinism.

Mutagens and Other Causes

As might be expected, cells that divide many, many times in a lifetime are more at risk of errors and mutations than cells that divide less frequently. In a human female, egg cells are fully formed at birth, and they never divide thereafter. By contrast, sperm cells are being produced constantly, and the older a man is, the more frequently his sperm-producing cells have divided. By age 20 they will have divided 200 times and by age 45 about 770 times. This has led scientists to hypothesize that when a baby is born with a congenital disorder caused by an error in cell division, the father is the parent more likely to have contributed the gene with the mutation.

This is just one example of why mutation occurs. Many mutations are caused by mutagens—chemical or physical factors that increase the rate of mutation. Some mutagens occur naturally, and some are synthetic. Cosmic rays from space, for instance, are natural, but they are mutagenic. Some naturally occurring viruses are considered mutagenic, since they can insert themselves into host DNA. Hydrogen and atomic bombs are man-made, and they emit harmful radiation, which is a mutagen. Recreational drugs, tobacco, and alcohol also can be mutagens in the bodies of pregnant women. The first mutagens to be identified were carcinogens, or cancer-causing substances. Carcinogens in chimney soot were linked with the chimney sweep's cancer of late eighteenth-century England, discussed in Noninfectious Diseases. In fact, cancer itself is a kind of mutation, involving uncontrolled cell growth. Other environmental factors that are known to bring about mutations include exposure to pesticides, asbestos, and some food additives, many of which have been banned.

WHERE TO LEARN MORE

"Are Mutations Harmful?" Talk. Origins (Web site). <http://www.talkorigins.org/faqs/mutations.html>.

Human Gene Mutation Database, Institute of Medical Genetics, University of Wales College of Medicine (Web site). <http://archive.uwcm.ac.uk/uwcm/mg/hgmd0.html>.

Kimball, Jim. Mutations. Kimball's Biology Pages (Web site). <http://www.ultranet.com/~jkimball/Biology-Pages/M/Mutations.html>.

"Mutations." Brooklyn College, City University of New York (Web site). <http://www.brooklyn.cuny.edu/bc/ahp/BioInfo/SD.Mut.HP.html>.

Patterson, Colin. Evolution. Ithaca: Comstock Publishing Associates, 1999.

Reilly, Philip. Abraham Lincoln's DNA and Other Adventures in Genetics. Cold Spring Harbor, NY: Cold Spring Harbor Laboratory Press, 2000.

Twyman, Richard M. Advanced Molecular Biology: A Concise Reference. Oxford, UK: Bios Scientific Publishers, 1998.

Weinberg, Robert A. One Renegade Cell: How Cancer Begins. New York: Basic Books, 1998.

KEY TERMS

AMINO ACIDS:

Organic compounds made of carbon, hydrogen, oxygen, nitrogen, and (in some cases) sulfur bonded in characteristic formations. Strings of amino acids make up proteins.

CHROMOSOME:

DNA-containing bodies, located in the cells of most living things, that hold most of the organism's genes.

CONGENITAL DISORDER:

An abnormality of structure or function or a disease that is present at birth. Congenital disorders also are called birth defects.

DNA:

Deoxyribonucleic acid, a molecule in all cells, and many viruses, containing genetic codes for inheritance.

GENE:

A unit of information about a particular heritable trait. Usually stored on chromosomes, genes contain specifications for the structure of a particular polypeptide or protein.

GERMINAL MUTATION:

A mutation that occurs in the egg or sperm cells, which therefore can be passed on to the organism's offspring.

HERITABLE:

Capable of being inherited.

MUTAGEN:

A chemical or physical factor that increases the rate of mutation.

MUTATION:

Alteration in the physical structure of an organism's DNA, resulting in a genetic change that can be inherited.

NATURAL SELECTION:

The process whereby some organisms thrive and others perish, depending on their degree of adaptation to a particular environment.

ORGANIC:

At one time chemists used the term organic only in reference to living things. Now the word is applied to compounds containing carbon and hydrogen.

POLYPEPTIDE:

A group of between 10 and 50 amino acids.

PROTEINS:

Large molecules built from long chains of 50 or more amino acids. Proteins serve the functions of promoting normal growth, repairing damaged tissue, contributing to the body's immune system, and making enzymes.

RNA:

Ribonucleic acid, the molecule translated from DNA in the cell nucleus, the control center of the cell, that directs protein synthesis in the cytoplasm, or the space between cells.

SOMATIC MUTATION:

A mutation that occurs in cells other than the reproductive, or sex, cells. These mutations, as contrasted with germinal mutations, cannot be transmitted to the next generation.

SPECIATION:

The divergence of evolutionary lineages and creation of new species.

TRANSLOCATION:

A mutation in which chromosomes exchange parts.

Mutation

Mutations are physical changes in genes and chromosomes . They may be confined to a single cell or may be transmitted from one cell to another within a multicellular organism (somatic cell mutation), or may be transmitted from one generation to another through mutation in the gametes (germ-line mutation). Mutations may be caused by natural events within the environment, by action or inaction of deoxyribonucleic acid (DNA) repair enzymes , and by human production of chemicals or high-energy radiation (mutagens). Mutation rates vary from organism to organism, from gene to gene, from time to time, and from place to place. They can have a significant effect not only on the individual, but on the evolution of species.

Causes of Mutations

Since genes are composed of DNA, nearly anything that can change the structural composition, sequence, physical integrity, or length of a DNA molecule can cause mutations. Breakages may be caused by physical damage such as being severed by ice crystals in a frozen cell or violent agitation from high temperature. Exposure to high-energy radiation (bombardment by alpha, beta, or gamma particles) or ultraviolet light can have a similar effect. A variety of chemicals act as mutagens. Some chemicals, such as bromouracil, are structurally similar to DNA bases, and are inserted in place of normal bases. Ethidium bromide has a structure that allows it to wedge within the DNA double helix (intercalation), and is used as a stain for DNA. Many other chemicals, such as peroxides and mustard gas, chemically modify DNA.

Mutagens, which affect DNA, are distinct from teratogens , which influence the embryological development of an individual without necessarily affecting DNA structure. For example, thalidomide, a tranquilizer, causes nongenetic birth defects such as shortened limbs. Sensitive tests for identifying mutagens, like the Ames test, frequently also identify teratogens.

Spontaneous mutations can appear in DNA for many reasons, including faulty proofreading during replication. The fidelity of replication is greatly influenced by the cutting activities of DNA polymerases, which usually cut out incorrectly added nucleotides . Study of bacteria with high mutation rates (mutator strains) has shown they often have DNA polymerases with limited 3′ to 5′ (three-prime to five-prime) exonuclease activity. An exonuclease removes nucleotides at the end of the DNA chain. Low exonuclease activity means they are less able to remove incorrect nucleotides once added. On the other hand, antimutator strains often have DNA polymerases with very efficient 3′ to 5′ exonuclease activity. Due to these and other enzymes, a large number of different rates of mutation occur in different systems. Normally, the rate of change is about one in ten billion nucleotides per cell division, but the variance is wide and can be as high as one in ten thousand per generation. Human cells have approximately nine billion nucleotides, and so on average, about one mutation should occur in each round of DNA replication.

Types of Mutations: Structure and Information

Mutations can be classified in terms of the structural changes they cause, and in terms of the changes in the genetic information they produce. Point mutations are those affecting a single nucleotide. Point mutations may be deletions or insertions of nucleotides, or changes from one nucleotide to another (substitutions).

To understand the types of changes, it is useful to remember that the DNA nucleotides are adenine, thymine, cytosine, and guanine (abbreviated A, T, C, G). Canonically, A pairs with T, C pairs with G. Because of their chemical structures, A and G are referred to as purines, while C and T are pyrimidines. Substitutions, then, may be from purine to purine or from pyrimidine to pyrimidine (transitions), or purine to pyrimidine or vice versa (transversions).

The DNA within a gene codes for the amino acid sequence in a protein , and so DNA mutations can lead to protein changes. The code is read in triplets, sequences of three nucleotides. From this, it is readily seen that any insertion or deletion will change the triplet groups, and so may have major effects on the amino acids coded for. This is called a frame-shift mutation. Frame-shift mutations almost always result in nonfunctional proteins.

Transitions and transversions often have less drastic effects. In some cases, there is no effect at all. This occurs when the change is from one "synonym" to another in the genetic code ; that is, when the new triplet codes for the same amino acid as the old one. A "nonsense" mutation is much more serious, since this converts a triplet coding for an amino acid (sense) into one with no corresponding amino acid (nonsense). This causes protein synthesis to stop (such triplets are called stop codons ). A missense mutation is also potentially serious, since this changes one amino acid to another. When the new amino acid is chemically similar to the old one, there may be little effect on the protein structure and function. When they differ in size, polarity, or charge the effect may be profound.

SUGIMURA, TAKASHI (1926–)

Japanese biologist who demonstrated that chemicals, X rays, and other agents that cause cancer often do so by causing mutations in the deoxyribonucleic acid (DNA) of cells. Sugimura, along with American Bruce Ames, won the prestigious Japan Prize in 1997.

Such is the case with the sickling variant of the hemoglobin gene. In the 1940s, Nobel laureate Linus Pauling suggested, and, in the 1950s, Verne Ingram demonstrated, that the first well-described "molecular disease" namely sickle cell disease, was due to a mutation that affected just one position in the amino acid sequence of the hemoglobin (Hb) molecule that carries iron in human blood. The underlying mutation was later shown to be a transversion from thymine to adenine. This converts an amino acid near one end of the beta chain of human hemoglobin from a glutamic acid side to a valine. This change, from a negatively charged hydrophilic side chain to a hydrophobic side chain, converts HbA to HbS. This alters the way hemoglobin molecules aggregate at low oxygen concentrations; HbS molecules cause the red blood cells that contain them to bend into a sickle shape. When these misshapen cells obstruct blood flow, an affected individual experiences great pain.

Mutation in Evolution

Mutation is one of the four forces of evolution; the others are selection, migration, and genetic drift. For a century after the publication of The Origin of Species by English naturalist Charles Darwin in 1859, mutation was often discussed as a source of new variation, but it was seldom considered to be highly important except in rare instances. However, in the 1960s, mutation became a major focus of evolutionary research.

The central question regarding mutation in evolution is to what extent mutations are harmful, harmless, or useful. In two experimental papers in 1966, Richard Lewontin and John Hubby demonstrated that many more individual fruit flies are heterozygous (meaning they have two different alleles at a genetic locus ) and their populations had many more polymorphisms (the number of genes with more than one allele present) than could be accounted for by classical population genetic theory. R. K. Selander and others then extended this work for a broad phylogenetic spectrum of organisms. This gave strong support to the ideas of two population geneticists from Japan, Motoo Kimura and Tomoka Ohta, who hypothesized that most mutations were selectively neutral instead of being deleterious, as the standard view was at the time. In their view, mutations increase genetic diversity by giving rise to harmless differences in a gene that can be maintained in a population over long periods. These changes are reflected in the number of alleles (gene forms) within the population.

Neutralists (such as Kimura and Ohta) argued that most alleles at a genetic locus were either neutral or likely to have nonsignificant deleterious consequences. If alleles are principally neutral, then changes in alleles frequencies will be driven fundamentally by random forces (principally genetic drift). On the other hand, selectionists thought that alleles are predominantly harmful (with a view that only rare alleles have beneficial contributions), and, hence, natural selection would act to change allele frequencies in a predictable fashion, eliminating most new ones.

Kimura and Ohta's recognition of the neutral value of most mutations allowed the estimation of divergence times between related species by analyzing accumulated gene changes; the so-called molecular clock. Parts of proteins that were indispensable to function would be very well preserved and hence have few preserved mutational changes in their related gene sequences. Dispensable portions would have many more mutations. Changes in noncoding DNA regions, such as introns and "junk DNA," can accumulate even more mutations without effect.

In the last two decades of the twentieth century, two other major advances were made in the understanding of mutation. First, site-specific mutagenesis allowed molecular biologists to mutate genes almost letter by letter. With this approach, they can look at the impact of changing single amino acids on the structure and function of proteins.

Second, a debate on the role of mutation rate and the direction of mutations has been rekindled. In the 1940s, Salvador Luria and Max Delbrück showed definitively that mutations did not arise that specifically addressed some biochemical inability of the organism, such as an ability to metabolize a new food source or to resist pathogenic infection. Instead, random mutations are produced, and those populations with beneficial adaptations survived better than other populations.

However, in the 1980s, John Cairns and others challenged the orthodoxy of this view with a variety of new experiments, which they thought indicated that mutations with adaptive value preferentially arose in some bacterial populations.

The response from the majority scientific community was rapid. In 1999, Croatian scientist Miroslav Radman, working in Paris, provided the most widely accepted resolution to this conflict. Namely, he and others believe that some selective agents (in many experiments stress was induced by starvation) led to an increase in the overall rate of mutation rather than to an increased production of adaptive mutations. This increases the rate of all types of mutations, including adaptive ones.

see also Blood; Chromosome, Eukaryotic; Chromosome Aberrations; DNA; Gene; Genetic Code; Genetic Diseases; Nucleotides; Replication

John R. Jungck

Bibliography

Alberts, Bruce, et al. Molecular Biology of the Cell, 4th ed. New York: Garland Publishing, 2000.

Atherly, Alan G., Jack R. Girton, and John F. McDonald. The Science of Genetics. Philadelphia, PA: Saunders College Publishing, 1998.

Cooper, D. N. Human Gene Mutation. Bios Scientific Publishers Ltd., 1997.

Kimura, Motoo. Population Genetics, Molecular Evolution, and the Neutral Allele Theory: Selected Papers. Chicago: University of Chicago Press, 1994.

Lewontin, Richard C. The Genetic Basis of Evolutionary Change. New York: Columbia University Press, 1974.

Ohta, T. "The Nearly Neutral Theory of Molecular Evolution." Annual Review of Ecology and Systematics 23 (1992): 263–286.

Radman, Miroslav. "Mutation: Enzymes of Evolutionary Change." Nature 401 (1999): 866–868.

Woodruff, R. C., and John N. Thompson, eds. Contemporary Issues in Genetics and Evolution, Vol. 7: Mutation and Evolution. Dordrecht, The Netherlands: Kluwer Academic Publishing, 1998.

Mutation

A mutation is any heritable change in the genome of an organism. For a population, heritable mutations provide the source of genetic variation, without which evolution could not occur: If all individuals of a species were genetically identical, every subsequent generation would be identical regardless of which members of the species reproduced successfully. For an individual organism, mutations are rarely beneficial, and many cause genetic diseases, including cancer. For researchers, mutations (either spontaneous or introduced) provide important clues about gene location and function.

Phenotypic Effects and Evolution

Mutations in the germ-line cells are heritable and provide the raw material upon which natural selection operates to produce evolution. Mutations in somatic cells, which are cells that are not germ line, are not heritable but may lead to disease in the organism possessing them.

Most mutations do not cause disease and are said to be "silent" mutations. This is for at least two reasons. First, most DNA does not code for genes, so changes in the sequence do not affect the types or amounts of protein made and there is no change in the phenotype of the organism. Second, most sexually reproducing organisms are diploid, meaning they possess two copies of every gene. Many types of mutation simply disable one copy, leaving the other intact and functional. Therefore these mutations display a recessive inheritance pattern, with no effect on phenotype unless an individual inherits two copies of the mutation. Diploid species can accumulate a large pool of such recessive mutations, which are mostly disadvantageous and thus contribute to the burden of genetic disease.

Some mutations lead to detrimental alterations of the normal pheno-type and are, therefore, selected against. Very occasionally, the mutant phenotype is superior and provides a selective advantage, which leads to an increase in the frequency of this mutant allele and, thus, to evolution of the population. Alternatively, a disadvantageous mutation in one environment may become advantageous in another, again leading to increased frequency of this allele.

Molecular Basis of Mutations

DNA is composed of a double helix, each side of which is a long string of four types of nucleotides. Each nucleotide possesses identical sugar-phosphate groups that contribute to the DNA backbone but differs in the structure of the base suspended between the two backbones. The bases are adenine, thymine, cytosine, and guanine (A, T, C, G). Because of their structure, A pairs only with T across the double helix, and C only with G.

Within genes, the sequence of DNA encodes a sequence of amino acids used to build a protein. The DNA is read in triplets of bases, with each triplet coding for an amino acid. With the recognition that the genetic information lies in the sequence of bases in the DNA, it became possible to understand the chemical nature of gene mutations and how these could be as stable as the original allele of the gene.

Consideration of the genetic code linking DNA and amino acids reveals how mutations can either alter a protein, have no effect, or prevent it from being produced entirely. Mutations fall into four broad categories (point mutations, structural chromosomal aberrations, numerical chromosomal aberrations, and transposon-induced mutations), each of which may be subdivided further.

Point Mutations

"Point mutations" are small changes in the sequence of DNAbases within a gene. These are what are most commonly meant by the word "mutation." Point mutations include substitutions, insertions, and deletions of one or more bases.

If one base is replaced by another, the mutation is called a base substitution. Because the DNA is double-stranded, a change on one strand is always accompanied by a change on the other (this change may occur spontaneously during DNA replication, or it can be created by errors during DNA repair. Consequently, it is often difficult to know which base of the pair was mutated and which was simply the result of repairing the mismatch at the mutation.

For example, the most common mutation in mammalian cells is the substitution of a G-C pair with an A-T pair. This could arise if G is replaced by A and subsequently the A is replicated to give T on the other strand. Alternatively, the C could be replaced by a T and the T could then be replicated to give an A on the complementary strand, the final result being the same. It is believed that the G-C to A-T conversion most commonly begins with a C-to-T mutation. This is because most of these mutations occur at DNA sequences in which C is methylated (i.e., chemically modified by the addition of a-CH₃ group). The methylated form of C can be converted to a base that resembles T (and thus pairs with A) by removal of an-NH₂ group (deamination)—a relatively common event.

Base substitution mutations are classified as transitions or transversions. Transitions are mutations in which one pyrimidine (C or T) is substituted by the other and one purine (G or A) is substituted on the complementary strand. The G-C to A-T conversion is a transition mutation, since C becomes T.

Transversions are mutations in which a purine is replaced by a pyrimidine or vice versa. Sickle cell anemia is caused by a transversion: T is substituted for A in the gene for a hemoglobin subunit. This mutation has arisen numerous times in human evolution. It causes a single amino acid change, from glutamic acid to valine, in the β subunit of hemoglobin. Sickle cell anemia was the first genetic condition for which the change in the protein was demonstrated in 1954 by Linus Pauling (a Nobel laureate from the California Institute of Technology) and subsequently shown to be a single amino acid difference by Vernon Ingram (a Nobel laureate from the Massachusetts Institute of Technology).

Base substitutions are sometimes silent mutations—mutations that do not change the amino acid sequence in the protein encoded by the gene. Silent mutations are possible because the original and mutated sequences can code for the same amino acid, given the redundancy of the genetic code. In the divergence between sea urchins and humans, for example, one of the histone proteins has only two amino acid substitutions, although the gene has many base pair substitutions. Histones are proteins around which DNA is wrapped in chromosomes. The very close similarity in sequence between such distantly related organisms is an indication of how critical the structure is for the function: Most mutations that change it are very disadvantageous.

One type of substitution mutation that almost always inactivates the gene is mutation to a stop codon . A stop codon ends the assembly of the protein, and a truncated protein is usually not active biochemically. Many recessive genetic diseases occur when a mutation converts a coding triplet to a stop codon.

Other mutations involve the insertion or deletion of one or more base pairs in the DNA. When they occur in genes, such mutations typically inactivate the encoded protein, because they change the "reading frame" of the gene. The DNA sequence is translated in groups of three nucleotides. Insertion or deletion of a nucleotide changes the sets of triplets, and thus every subsequent amino acid is altered, changing the protein completely, as shown in Figure 2. Stop codons also frequently arise from insertions or deletions.

Naturally occurring trinucleotide repeat sequences (e.g., CAGCAG CAGCAG) are hot spots for certain important human mutations that involve the insertion of more copies of the repeated sequence. For example at the locus for Huntington's disease, a sequence of 10-29 copies of CAG is normal and stable, but if there are 30-38, there is a high rate of mutation to increased numbers of copies, and if there are 39 or more copies, middle-age dementia called Huntington's disease results.

Functional Consequences and Inheritance Patterns.

Mutations can be classified by their functional consequences. Mutations that inactivate the resulting protein, or prevent it from being made at all, are called loss-of-function mutations. These are usually recessive, since the organism still retains one functional copy on the other chromosome. Loss-of-function mutations may be dominant if the organism cannot compensate for the loss by using the other gene copy. Gain-of-function mutations are those in which the protein takes on a new function, or loses the ability to be regulated by other proteins. These mutations are typically dominant, since the new function may be deleterious even in the presence of a normal protein, encoded by the other gene copy.

Chromosomal Aberrations and Transposons

"Structural chromosomal aberrations," the second category of mutations, arise when DNA in chromosomes is broken. The broken ends may remain unrepaired or may be joined with those of another break, to form new combinations of genes, such as translocations. A translocation between chromosomes 8 and 21 in humans causes acute myeloid leukemia by increasing the activity of c-myc, a gene involved in cell replication.

Translocations often cause human infertility, because they interfere with the normal distribution of chromosomes during meiosis. Chromosomes pair up before separating, as eggs or sperm are formed, and the correct pairing depends on matching sequences between them. Structural aberrations also include inversions and duplications of pieces of chromosomes.

Most chromosomal aberrations lead to the formation of chromosomal fragments without centromeres . Centromeres are crucial for proper chromosomal division, during both mitosis and meiosis. Therefore a chromosomal fragment is likely to be lost from one of the daughter cells formed after cell division.

Structural aberrations are nonetheless common in evolutionary history. As a result, although the chromosomes of mouse and man are quite different in appearance, most genes have the same neighbors in the two species, representing the ancestral mammalian arrangement, even if they have been moved to another chromosome as shown in Figure 3.

"Numerical chromosomal aberrations," the third category of mutations, are changes in the number of chromosomes. In some cases, the whole genome has been duplicated (called polyploidy ) and the mutant has, for example, four of each chromosome (and is thus tetraploid) rather than the usual two (diploid, as in humans). These are much more common in the evolution of plants than animals. In other cases, only one or a few of the chromosomes are involved, which is referred to as aneuploidy. Down syndrome, in which a person has an extra chromosome 21, is an example of such a mutation. Aneuploidy may also involve the loss of a chromosome. The absence of one of the sex chromosomes, X or Y, is a mutation in humans that results in Turner's syndrome, in which there is only one X.

"Transposon-induced mutations" are the fourth category of mutations. Transposable genetic elements (transposons) are pieces of DNA that can copy themselves and insert into a new location in the genome. They were first discovered by Barbara McClintock, a U.S. geneticist and Nobel laureate in 1950. When transposons jump into a new position, the insertion may disrupt a gene and thus mutate it, usually inactivating it. Sometimes the transposon jumps again, and the activity of the gene it leaves is restored. Often, however, the transposon stays in the original position, permanently disrupting the gene. Some forms of hemophilia are due to transposon insertion. Transposon mutations have been extremely common in human evolution, and such mutations are still occurring.

Mutations in Research and Medicine

Early geneticists treasured mutations in the organisms they studied, since no characteristic can be studied genetically unless heritable variants exist. If, for example, everyone had brown eyes, nothing could be learned about the inheritance of eye color, as all generations would have the same color of eyes. For this reason, geneticists collected and propagated all the mutants they could find, and methods were developed to deliberately induce mutations, a process called mutagenesis. Such techniques include exposing their experimental organisms to X rays and chemicals.

Transposons can also be deliberately used to introduce mutations in model organisms. In the plant Arabidopsis thaliana and in the fruit flyDrosophila melanogaster, transposon mutagenesis is often used to induce mutations, as the mutation can be very rapidly cloned and mapped with the transposon's DNA sequence as starting point.

Comparing existing mutations can help determine the evolutionary relatedness of two organisms. During evolution, there has been a relatively constant rate of accumulation of mutations in genes for a number of proteins, so the number of changes can be used to estimate the time since two species had a common ancestor. This is called the molecular clock and is illustrated in Figure 1. Each gene has evolved at a characteristic rate—the result of mutation rates, selection, and chance changes in the gene pool.

Advances in genetics have only intensified the search for mutations, especially in complex traits such as behavior and cancer, as the key to finding the genes involved and then unraveling the underlying mechanisms. This involves mapping the mutations, cloning the genes, and studying the mutants to discover what biochemical processes are changed in the mutants.

Mutations are believed to underlie most, if not all cancers. Cancer-causing mutations found so far include genes involved in communication between cells (signal transduction) and in the control of cell division. Many of these genes have been categorized into two broad classes: oncogenes and tumor suppressor genes. The mutation that has been found most often, in a tumor suppressor gene called p53, usually arises as a somatic mutation but can also be inherited as Li Fraumeni syndrome.

Xeroderma pigmentosum is an autosomal recessive condition in which the ability to repair DNA damage induced by UV light is defective. Many mutations are produced, and the affected people have large numbers of skin cancers.

see also Carcinogens; Chromosomal Aberrations; DNA Repair; Gene; Hemoglobinopathies; Mutagenesis; Mutation Rate; Polyploidy; Transposable Genetic Elements.

John Heddle

Bibliography

Drake, John W. "Spontaneous Mutation." Annual Review of Genetics 25 (1991): 125-46.

Hartwell, Leland H., et al. Genetics: From Genes to Genomes. Boston: McGraw-Hill,2000.

Lewis, Ricki. Human Genetics: Concepts and Applications, 4th ed. Boston: McGraw-Hill,2001.

Pauling, Linus, et al. "Sickle Cell Anemia, a Molecular Disease." Science 110 (1949):543-548.

Internet Resource

International Agency for Research on Cancer. <http://www.iarc.fr/>.

Mutation

A mutation is the alteration in the composition in deoxyribonucleic acid (DNA) . Mutations that are inherited can change the character of a species . Living organisms rely upon change as a means of adapting to new environments or conditions. Change is a crucial survival mechanism. Evolution hinges on the appearance and inheritance of mutations. Mutations are the source of genetic variation in humans and other life forms, and are a feature of all life, from microorganisms to humans.

Mutation involves a change in one or more of the constituents of deoxyribonucleic acid (DNA). Because DNA provides the blueprint of an organism's operation and appearance, a genetic change in DNA will be evident as a change in an organism's appearance, behavior , or health. Mutated offspring can be quite different in appearance from their parents. For example, a mutation in the gene that determines the production of skin pigment can produce the fair skin, white hair, and eye difficulties that are characteristics of albinism . Dwarfs are an example of a mutation that affects growth hormones .

Mutations can be harmful or can be beneficial. If harmful, the mutation will be selected out over time . The beneficial mutations will be retained. The change in a species over time is the underpinning of evolution.

Mutational errors in DNA

For much of our recorded history, the sudden appearance of an altered person, plant , animal , or other living creature was mysterious and feared, and was typically attributed to some divine intervention. Beginning with the observations of Gregor Mendel on the effects of breeding peas of differing appearance, the genetic basis of mutation became recognized and accepted. A greater understanding of the mechanics of mutation came from the experiments conducted by Thomas H. Morgan in 1910 with fruit fly mutations, and George W. Beadle and Edward L. Tatum in the 1940s on bread mold mutations. The publication of the structure and composition of DNA by Francis Crick and James Watson in 1953 paved the way for the understanding of the molecular basis of mutation.

In eukaryotic organisms such as humans every cell contains DNA arranged on threadlike structures. The structures are called chromosomes. Within the chromosomes lie the regions of DNA called genes that contain the information for proteins . Human beings carry about 30,000 genes on their chromosomes. If the DNA of a particular gene is altered, that gene will become changed. If the change is minor, the mutation may not even be apparent or may be irrelevant to the three dimensional structure and the function of the protein that is produced. But, even an alteration at a single site in a gene can produce drastic alteration of the protein. The protein may not function correctly or may not function at all.

Just one missing or abnormal protein can have an enormous effect on the entire body. For example, the multiple effects that are associated with an albino are the result of one missing protein.

DNA is made up of subunits known as nucleotide bases. There are four kinds of bases: adenine (A), cytosine (C), guanine (G), and thymine (T). The arrangement and different combinations of the bases determines the genetic information, analogous to the arrangement of letters to form words. Mistakes in the genetic code occur when any of these nucleotide bases are absent or duplicated or misplaced. Mutation can be thought of as a kind of molecular typographical error.

As an example, a stretch of DNA could have the following arrangement of bases: ATCTTTGGT. A mutation could occur that produces repeats of some of the bases, as in the italicized region that follows: ATCATCATCTTTGGT. The added bases disrupt the arrangement of the bases, and so disrupt the information contained in the base sequence. An example is Huntington's disease . The presence of two copies of a mutated gene (one from each parent) causes the progressive degeneration of the nervous system and leads to death of the afflicted person in their 40s or 50s. The Huntington's disease mutation is caused by a repeating sequence of three bases in the gene.

Other mutations include the presence of an incorrect base at a certain point, and a missing base (a deletion mutation).

An example of a disease that results from a deletion mutation is cystic fibrosis . The presence of mutated genes in which three thymine bases are absent produces cells in the lungs that are defective in the transport of molecules such as sodium . A result is the accumulation of mucus in the lungs. Bacteria readily colonize the mucus and become resistant to treatments intended to kill them. As well, the host's immune response to the invading bacteria causes progressive damage to lung tissue . The infections and impaired lung function can cause a premature death.

Mutational errors can extend to include more than just one base of a chromosome . Humans normally have 23 pairs of chromosomes. But mutations can produce a fetus that has an extra copy or copies of a chromosome. The unique physical appearance and retarded mental faculties associated with Down syndrome arise when three copies of chromosome 21 are present. Another type of chromosomal mutation occurs when portions of two adjacent chromosomes swap places with each other. Such a translocation mutation between chromosomes 9 and 22 lead to a certain type of leukemia .

Mutations that occur in the egg or sperm cells of a eukaryote are called germinal mutations. These mutations can be inherited by subsequent generations. In contrast, somatic mutations, which occur in cells other than sex cells, cannot be inherited.

Causes of mutation

Mutations also arise naturally during the manufacture of DNA. Thus, the opportunity for error exists every time a cell replicates. Even so, DNA is correctly made almost always. Predictably, cells that divide numerous times are more at risk for errors than cells that divide less frequently. For example, the egg cells are present in a female at birth and never undergo division, while sperm cells in males are constantly being produced. A theory proposes that the cause of birth defects including a type of dwarfism, Marfan syndrome , and myositis ossificans are typically the result of a defective gene contributed by a mutated sperm.

Developing embryos and fetuses are especially at risk for mutation. Their cells divide very rapidly and become increasingly specialized for specific tasks. Pregnant women must be careful to avoid x rays , almost all medication, and even the extreme temperatures of hot tubs and saunas.

Mutations can occur after birth as well. For example, evidence is mounting that environmental influences can trigger genetic changes. A compound that is responsible is termed a mutagen . The known association between some types of cancer and smoking, for example, may result from a mutation in lung cells that overrides the natural controls to cell growth and division. The rampant growth that can occur produces a cancer.

Mutation and evolution

Mutation conjures up images of a sudden and dramatic alteration in appearance, behavior, or some other characteristic of living organisms. Indeed, this aspect was the basis of the term mutation when it was coined in 1901 by Dutch botanist Hugo De Vries. But, the term mutation also refers to the inheritance of an altered gene, even through multiple generations.

Every human genetic trait is subject to mutation. Indeed, some mutations—like hairless skin—must have occurred long ago because they are shared by all humans. Other traits occur only in certain populations of people. Cystic fibrosis, for instance, is most common in people of northern European descent. Sickle cell anemia , a serious blood disease, occurs frequently in people of African and Mediterranean ancestry. Tay-Sachs disease , a fatal disorder, is found primarily in Jews with eastern European ancestors. This suggests that the first person in whom such a mutation occurred came from that particular ethnic group. It may be that not enough time has lapsed, evolutionarily, to allow the mutation to spread to the wider population. Or, it can also be true that a mutation confers a selective advantage on a certain group of people. An example of the latter point is sickle cell anemia , which, in native Africans, can be protective against malaria .

Over millions of years, advantageous mutations have allowed life to develop and diversify from primitive cells into the multitude of species on Earth today, including Homo sapiens. Indeed, the appearance of a mutation, the "testing" of that mutation, and the subsequent inheritance or noninheritance of the mutation is the driving force of evolution. If DNA always replicated perfectly and with no change, every life form from bacteria to humans would have remained unchanged for the entire time of their existence on Earth. Since the planet has and continues to experience change, the inability to adapt to the changes would doom a species to extinction .

Mutations have been exploited by man for commercial purposes. Animal and plant breeders use mutations to produce new or improved species of crops and livestock . Careful breeding in this manner has spawned all the different species of dogs and horses we know today. It has resulted in crops that are resistant to drought or insecta and whose yield is improved. Controlled mutation and breeding has produced goldfish, yellow roses, and Concord grapes .

The processes that generate mutations are collectively termed mutagenesis . In the laboratory, mutagenesis can be accomplished in a controlled and precise manner. This genetic technique is called insertional mutagenesis. It is used to selectively disable genes, in order to find out what functional significance the gene product has to the cell. The ability of microbiologists to introduce controlled and precise mutations in the genetic material of bacteria and viruses can increase the understanding of the operation of the organisms, and of the mechanisms they use to cause disease. Other uses of the technique include the study of gene expression and the study of how a protein's three-dimensional structure influences its function.

See also Carcinogen; Genetic disorders; Genetics.

Resources

books

Lee, Thomas F. The Human Genome Project. New York: Plenum Press, 1991.

Pollack, Robert. Signs of Life: The Language and Meanings ofDNA. Boston: Houghton Mifflin Company, 1994.

Wilcox, Frank H. DNA: The Thread of Life. Minneapolis: Lerner Publications Company, 1988.

periodicals

Angier, Natalie. "Genetic Mutations Tied to Father in Most Cases." New York Times (May 17, 1994): B9.

Angier, Natalie. "Researchers Trace the Primary Cause of Cystic Fibrosis to the Stone Age." New York Times (June 1, 1994): A13.

Blakeslee, Sandra. "Genes Tell Story of Why Some Get Cancer While Others Don't." New York Times (May 17, 1994): B6.

Nickerson, M.L., Weirich, G., Zbar, B., L.S. Schmidt. "Signature-based Analysis of MET Proto-oncogene Mutations Using DHPLC. Human Mutation 16 (2000): 68-76.

Brian Hoyle

KEY TERMS

Genome

—The complete set of genes an organism carries.

Germinal mutation

—A mutation in the germ cells (sperm or egg cells). This mutation can be passed on to succeeding generations.

Nucleotide base

—One of the four chemical subunits of DNA: adenine, cytosine, guanine, and thymine.

Somatic mutation

—A mutation in the body cells. This mutation cannot be passed on.

Translocation

—A genetic term referring to a situation during cell division in which a piece of one chromosome breaks off and sticks to another chromosome.