The Bardet-Biedl syndrome is a a syndrome characterized mainly by obesity, pigmentary retinopathy, polydactyly, mental retardation, hypogonadism, and renal failure in fatal cases.

The syndrome is named after Georges Bardet and Arthur Biedl.

Two forms have been identified:

Bardet-Biedl syndrome 1 (BBS1) has no linkage to chromosome 16
Bardet-Biedl syndrome 2 (BBS2) is mapped to markers on chromosome 16.

Laurence-Moon-Biedl syndrome and Laurence-Moon-Biedl-Bardet syndrome are no longer considered as valid terms in that patients of Laurence and Moon had paraplegia but no polydactyly and obesity which are the key elements of the Bardet-Biedl the syndrome. Laurence-Moon syndrome is a separate entity.

Major features

Eyes: Pigmentary retinopathy.
Hand and foot: Polydactyly.
Cardiovascular system: Hypertrophy of interventricular septum and left ventricle and dilated cardiomyopathy.
Gastrointestinal system: Fibrosis.
Urogenital system: Hypogonadism, renal failure, urogenital sinuses, ectopic urethra, uterus duplex, septate vagina, and hypoplasia of the uterus, ovaries, and fallopian tubes.
Growth and development: Mental and growth retardation.
Behavior and performance: Poor visual acuity and blindness.
Heredity: The syndrome is familial and is transmitted as an autosomal recessive trait. chromosome 3 locus appears to be linked to polydactyly of all four limbs, whereas chromosome 15 is associated with early-onset morbid obesity and is mostly confined to the hands, and chromosome 16 represents the "leanest" form.
Additional features: Obesity.

Cause

The detail biochemical mechanism that leads to BBS is still unclear. Recently, eight genes (BBS1 to BBS8) that are responsible for the disease when mutated have been cloned, and most of the gene products encoded by these BBS genes are located in the basal body and cilia of the cell. It has been postulated that these BBS gene products might involve in the cell signaling pathway in the cilia, and these signaling systems play an essential role in the normal development so that a malfunction in these systems causes the diverse pathological effects of the Syndrome.

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GENOMICS: Bardet-Biedle Gene Identified
From Applied Genetics News, 9/1/00

Researchers have identified a gene mutation that causes Bardet- Biedl syndrome (BBS), a rare genetic disorder that can lead to mental retardation. The discovery by Anne M. Slavotinek and Leslie G. Biesecker at the National Human Genome Research Institutes, and Val C. Sheffield and colleagues at the University of Iowa (Iowa City, IA) is reported in the September 2000 issue of Nature Genetics.

"BBS is a relatively rare disorder, but it has an interesting constellation of features, including obesity, mental retardation, polydactyly [additional digits], genital abnormalities and retinitis pigmentosa, which can lead to blindness," says Sheffield, a Howard Hughes Medical Institute researcher.

BBS can arise from defects in at least six different genetic loci. "We had found an inbred population of Bedouins in Israel with BBS and mapped the gene to a locus on human chromosome sixteen," says Sheffield. "We then identified a second Bedouin population with the disease, and found that their affected gene didn't map to [the] same location. This gave us the first clue that there was genetic heterogeneity in people with BBS."

Sheffield and his colleagues first zeroed in on the gene MKKS because Biesecker and his colleagues had established that it caused McKusick-Kaufman syndrome (MKS) in a large Amish population. "The one phenotypic feature shared by people with BBS and MKS is extra digits," says Sheffield. There was also evidence that there may be a BBS locus on chromosome 20, where the MKKS gene resides. "So, we screened the MKKS gene for mutation in BBS families whose disease showed no evidence of linking to any of the other known loci."

The screening revealed that 4 of the 34 people with BBS had mutations in the MKKS gene. Importantly, the mutations in MKKS were of the type that completely knocked out the gene's function, leading the scientists to hypothesize that MKS might be due to a crippled form of the MKKS gene, whereas BBS is caused by a complete knockout of the gene. While the protein product of the MKKS gene is still unknown, the gene has a similar sequence to other genes that produce chaperonins, proteins that aid in the proper folding of newly synthesized proteins.

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