Phenotypic differences among patients with Bardet-Biedl syndrome linked to three different chromosome loci

Bardet-Biedl syndrome (BBS) is an autosomal-recessive disorder of mental retardation, obesity, retinal dystrophy, polydactyly, and hypogenitalism. Renal and cardiac abnormalities are also frequent in this disorder. Previous clinical suggestions of heterogeneity of BBS were confirmed recently by the identification of four different chromosome loci linked to the disease. In this study we compared clinical manifestations of the syndrome in patients from 3 unrelated, extended Arab-Bedouin kindreds which were used for the linkage mapping of the BBS loci to chromosomes 3, 15, and 16. The observed differences included the limb distribution of the postaxial polydactyly and the extent and age-association of obesity. It appears that the chromosome 3 locus is associated with polydactyly of all four limbs, while polydactyly of the chromosome 15 type is mostly confined to the hands. On the other hand, the chromosome 15 type is associated with early-onset morbid obesity, while the chromosome 16 type appears to present the "leanest" form of BBS. Future cloning of the various BB genes will contribute to the understanding of the molecular basis of limb development and to the identification of human obesity-related genes.