Bardet-Biedl syndrome (BBS) is a ciliopathy characterized by retinal degeneration, obesity, polydactyly, renal abnormalities, and cognitive impairment for which 15 causative genes have been identified. Here we present the results of a mutational analysis of our multiethnic cohort of 83 families (105 cases); 75.9% of them have their mutations identified including 26 novel changes. Comprehensive phenotyping of these patients demonstrate that the spectrum of clinical features is greater than expected and overlapped with the features of other ciliopathies; specifically Alström and McKusick-Kauffman syndromes.

While overlap between the MKKS and BBS phenotypes has previously been reported for cases with BBS6 mutations, we also observed MKKS phenotypes involving BBS10 and BBS12 and Alström-like phenotypes associated with mutations in BBS1, BBS2, BBS6, BBS7, BBS9, BBS10 and BBS12 for the first time.

Bardet Biedl syndrome is a genetically heterogeneous ciliopathy with fourteen genes currently identified. To date, mutations in BBS7 and TTC8 (BBS8) were reported in 4.2% and 2.8% of BBS families respectively. We sequenced the coding regions of BBS7 and TTC8 in 35 BBS families of diverse ancestral backgrounds. In addition, the role of putative modifier genes on phenotype severity; NXNL1 and MGC1203 c.430C>T, was assessed. Genotype-phenotype correlation was explored in patients with identified mutations. Four novel pathogenic BBS7 changes were identified in 2/35 families (5.7%). In one family with two affected individuals with BBS7 mutations, a more severe phenotype was observed in association with a third mutation in BBS4. The overall retinal phenotype appeared more severe than that seen in patients with BBS1 mutations. This study confirms the small role of BBS7 and TTC8 in the overall mutational load of BBS patients. The variability of the ocular phenotype observed, could not be explained by the putative modifier genes; NXNL1 and MGC1203 c.430C>T.