Bardet-Biedl syndrome

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Bardet-Biedl syndrome

Definition

Bardet-Biedl syndrome (BBS) is a condition that primarily affects vision, kidney function, limb development, growth, and intelligence.

Description

BBS expresses itself differently from person to person, even among members of the same family. However, certain features frequently appear.

Genetic profile

BBS is a genetically heterogeneous condition; this means that it has more than one known genetic cause. One of these causes is a mutation in the MKKS gene , located on chromosome 20. When working properly, this gene appears to produce a chaperonin, a factor needed to process proteins. Without the chaperonin, the proteins cannot work properly.

Using linkage analysis, researchers have connected some BBS cases to other chromosomes . Linkage analysis is a method of finding mutations based on their proximity to previously identified genetic landmarks. As of February 2001, the specific genes responsible for these BBS cases remain unknown. However, several potential locations of BBS genes have been recognized. These sites are named for the number of the chromosome on which they are found, the arm of the chromosome ("q" for long arm, "p" for short arm), region of the arm, and band within the region. For example, "11q13" means chromosome number 11, long arm, region 1, band 3. In studies of families with BBS, researchers have found that a significant number of cases link either to 11q13, 15q22, or 16q21. In other families, researchers have linked BBS to either 2q31, 3p12, or 20p12. This last site is the location of the MKKS gene.

Regardless of the site involved, BBS displays an autosomal recessive inheritance pattern. This means that the condition occurs only when an individual inherits two defective copies of a BBS gene. If one copy is normal, the individual does not have BBS. This individual is called a carrier of BBS and can pass the gene on to the next generation.

Research indicates that people who inherit one abnormal BBS gene and one normal gene may be at risk for some of the health problems seen in BBS. Compared to the general population, these BBS gene carriers are more likely to develop high blood pressure, diabetes mellitus , and kidney disease, including kidney cancer.

Demographics

BBS affects people around the world. However, it is most common in the Middle East, especially in the Arab and inbred Bedouin populations of Kuwait. In these groups, it may affect as many as one in 13,500 individuals. The incidence is almost as high in Newfoundland, where as many as one in 16,000 individuals has BBS. Outside of these areas, researchers estimate that BBS affects only one in 160,000 people.

The specific genetic cause of BBS differs by family and geographic location. For example, in the Middle East, BBS appears to link to 16q21 or 3p12. However, in patients of European descent, BBS appears to link to 11q13 or 15q22.

Signs and symptoms

If the newborn with BBS has finger or toe abnormalities, these are apparent at birth. However, these defects have a variety of congenital causes, meaning they originated during development of the fetus and were not inherited. For this reason, medical care providers may not immediately suspect BBS. It becomes a consideration as the child develops and additional abnormalities emerge. In boys, genital abnormalities become evident soon after birth. In almost all patients, obesity and retinal degeneration begin in early childhood. Learning disabilities, if present, are identified in school-aged children, if not earlier. Failure to menstruate leads to diagnosis of some adolescent girls. Infertility brings some young adults to medical attention. Kidney disease is progressive and may not become obvious until adulthood.

Due to progressive degeneration of the retina, vision damage occurs in all patients. Specific vision defects include poor night vision during childhood, severe myopia (nearsightedness), glaucoma , and cataracts. A few patients suffer from retinitis pigmentosa , a condition in which the field of vision progressively narrows. Most individuals affected with BBS are blind by age 30.

Many infants with BBS are born with a kidney defect affecting kidney structure, function, or both. The specific abnormality varies from patient to patient and may be aggravated by lifelong obesity, another common problem for BBS patients. The complications of obesity, such as high blood pressure (hypertension) and insulin-resistant diabetes mellitus, contribute to kidney disease.

BBS patients may have extra fingers or toes (polydactyly), short fingers (brachydactyly ), or broad, short feet. Some patients have a combination of all three of these features. Alternately, polydactyly may be limited to one limb, hands only, or feet only. Syndactyly, the fusion of two or more fingers or toes, may also occur. In some BBS families, all affected members display at least some of these limb abnormalities.

Many individuals with BBS have genital abnormalities. Most boys with BBS have a very small penis and some also have undescended testes. Men with BBS are usually unable to have children. In women with BBS, the genitalia, ovaries, fallopian tubes, and uterus may or may not be underdeveloped. The vagina may not be completely formed. Though some women with BBS do not menstruate, others menstruate irregularly, and some women are able to have children. In both sexes, there may be birth defects in the urinary or gastrointestinal tract.

Some research indicates that people with BBS have characteristic facial features, including a prominent forehead, deep-set eyes, flat nasal bridge, and thin upper lip. Teeth are small and crowded, and a high, arched palate is common.

Occasionally, individuals with BBS have liver disease or heart abnormalities.

In addition to the physical effects of the condition, intelligence is sometimes affected. While some BBS patients show normal intelligence, others have mild to moderate learning disabilities. These patients are often developmentally delayed—they are slower than most children to walk, speak, or reach other developmental milestones. Difficulty with language and comprehension may continue into adulthood. In a few people with BBS, more severe mental retardation occurs. In some patients, vision handicap and developmental delay appear to be related.

Some parents report that their children with BBS have behavioral problems that continue into adulthood. These include lack of inhibition and social skills, emotional outbursts, and obsessive-compulsive behavior. Most people with BBS prefer fixed routines and are easily upset by a change in plans.

Diagnosis

Diagnosis of BBS is a challenge for medical professionals. Not only do the symptoms of BBS vary greatly from patient to patient, but some of these symptoms occur in other conditions, many of which are more common than BBS.

Though available on a research basis, genetic testing for BBS is not yet offered through clinical laboratories. Instead, it is the association of many BBS symptoms in one patient that generally leads to a clinical diagnosis. Therefore, patients must have a thorough genetic evaluation. This provides a chance to rule out other disorders with similar symptoms. Because symptoms emerge throughout childhood, patients diagnosed as infants require regular exams to confirm proper diagnosis. Some disorders historically confused with BBS include Lawrence-Moon syndrome, Kearns-Sayre syndrome, and McKusick-Kaufman syndrome . This last syndrome is also caused by mutation in the MKKS gene; in fact, the gene took its name from McKusick-Kaufman syndrome. While people with this syndrome show some of the same symptoms as BBS patients, the specific MKKS mutation differs between the conditions. This explains how one gene can be responsible for two distinct yet similar disorders.

Six major criteria form the basis of BBS diagnosis. These are retinal degeneration, polydactyly, obesity, learning disabilities, kidney abnormalities, and genital defects (in males). To confirm diagnosis, the patient should receive three particular diagnostic tests. An eye exam called an electroretinogram is used to test the electric currents of the retina. An ultrasound is used to examine the kidneys, as is an intravenous pyelogram (IVP). An IVP is an x-ray assessment of kidney function.

Treatment and management

Unless they have severe birth defects involving the heart, kidneys, or liver, patients with BBS can have a normal life span. However, obesity and kidney disease are major threats. If unchecked, obesity can lead to high blood pressure, diabetes mellitus, and heart disease. Untreated kidney disease can lead to renal failure , a frequent cause of early death in patients with BBS. Some patients require dialysis and kidney transplant. Therefore, it is very important to monitor and manage patients with BBS, and to promptly treat any complications. Affected individuals should eat a well-balanced, low-calorie diet and exercise regularly.

Because BBS carriers also appear prone to kidney disease, parents and siblings of patients with BBS should take extra precautions. These include baseline screening for kidney defects or cancer, as well as preventive health care on a regular basis.

In order to conserve vision to the extent possible, retinal degeneration should be carefully monitored. Therapy, education, and counseling help prepare the patient for progressive loss of vision. The Foundation Fighting Blindness, a support and referral group, offers help to BBS patients and their families.

Though not life-threatening, learning disabilities and reproductive dysfunction need attention in order to maximize the quality of life for patients with BBS. Affected people benefit greatly from special or vocational education, speech therapy, social skills training, and community support services. Some adult patients may never be able to live independently and may remain with their families. In these cases, families should plan future living arrangements in case the patients outlive their caregivers.

Genital abnormalities may require hormonal treatment or surgical attention. Sometimes removal of undescended testes is necessary to prevent cancer. Patients with genital and reproductive dysfunction may need counseling to help them deal with the personal, familial, social, and cultural impact of the condition. Genetic counseling is available to help fertile BBS patients address their reproductive choices.

Prognosis

The outlook for people with BBS depends largely on the extent of the birth abnormalities, prompt diagnosis, and follow-up care. At this time there is no treatment for the extensive retinal damage caused by BBS. However, good health care beginning in childhood can help many people with BBS avoid other serious effects of this disorder. Researchers are actively exploring genetic causes, treatment, and management of BBS.

Resources

BOOKS

"Bardet-Biedl Syndrome." In Smith's Recognizable Patterns ofHuman Malformation. 5th ed. Philadelphia: W. B. Saunders, 1997, pp. 590-591.

PERIODICALS

Beales, P. L., et al. "New Criteria for Improved Diagnosis of Bardet-Biedl Syndrome: Results of a Population Survey." Journal of Medical Genetics 36 (1999): 437-446.

Foltin, Lynn. "Researchers Identify Inherited Obesity, Retinal Dystrophy Gene." Texas Medical Center News 22 (2000): 17.

Hrynchak, P. K. "Bardet-Biedl Syndrome." Optometry andVision Science 77 (May 2000): 236-243.

ORGANIZATIONS

Foundation Fighting Blindness. Executive Plaza 1, Suite 800, 11350 McCormick Rd., Hunt Valley, MD 21031. (888) 394-3937. <http://www.blindness.org>.

Genetic Alliance. 4301 Connecticut Ave. NW, #404, Washington, DC 20008. (800) 336-GENE (Helpline) or (202) 966-5557. Fax: (888) 394-3937. info@geneticalliance. <http://www.geneticalliance.org>.