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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