This is a proposal to identify the molecular defect in Bardet-Biedl syndrome, an autosomal recessive disorder characterized by retinal degeneration, obesity, renal problems and mental retardation. The applicant and coworkers have identified three large unrelated inbred kindreds with Bardet-Biedl syndrome in genetically-isolated Bedouin tribes. The disorder maps to chromosome 16 in one kindred, chromosome 3 in another and chromosome 15 in the third. Specific aim one is to genetically fine-map the Bardet-Biedl syndrome candidate regions on chromosomes 16, 3 and 15. The mapping will be done by analyzing short terminal repeat polymorphisms (STRPs) in these families. The applicant, in conjunction with the Cooperative Human Linkage Center (CHLC), has developed 2000 markers, which can be screened in multiples by PCR. The locus has been mapped to within 20 cM for the chromosome 16q-linked disorder and 10 cM for the chromosomes 3- and 15-linked disorders. The applicant estimates that with the available resources, linkage can be narrowed to 1-2 cM for chromosome 16 and <5 cM for chromosomes 3 and 15. Specific aim two is to employ positional cloning strategies to isolate the Bardet-Biedl syndrome gene on chromosome 16q. Closely-linked markers will be used to isolate YACs, which will be assembled into a YAC contig map. The YACs will be used to isolate cosmid clones from the region, and also to identify individual genes by exon trapping. Genes isolated in aim two will be sequenced. Those encoding "developmentally important" proteins such as growth factors, transcriptional factors, and homeotic genes will be priority. They will be studied by looking for mutations in Bardet-Biedl syndrome DNA samples by DGGE and/or SSCP analysis. This will be followed by sequence analysis. Specific aim three is to screen known candidate genes mapping to chromosomes 16, 3 and 15 that have been identified in other laboratories. Genes will be selected on the basis of several criteria: (1) homology to other genes that cause retinal degeneration; (2) developmental regulation; (3) genes expressed in retina and other affected tissues; and (4) genes expected to have pleiotropic effects. Candidate genes will be evaluated by genetic mapping and mutational screening in Bardet-Biedl syndrome patients.