Exome sequencing identifies a novel and a recurrent BBS1 mutation in Pakistani families with Bardet-Biedl syndrome

Purpose: To determine the genetic cause of Bardet-Biedl syndrome (BBS) in two consanguineous Pakistani families. Methods: Clinical characterization of the affected individuals in both families was performed with ophthalmic examination, electroretinography, electrocardiography, and liver and renal profiling. Seventeen genes are known to be associated with BBS, so exome sequencing was preferred over candidate gene sequencing. One affected individual from both families was selected for exome sequencing. Segregation of the identified variants was confirmed with Sanger sequencing. Results: Retinitis pigmentosa, obesity, and learning difficulties were present in the affected individuals in both families. In family A, a sixth finger (polydactyly) of the proband's sister was removed by a surgical operation leaving a scar on the little finger. Polydactyly was also present in both affected individuals from family B. All diagnostic symptoms were characteristic of BBS in both families. In both affected individuals from family A, exome sequencing identified a novel homozygous mutation (c.47+1G>T) in BBS1 that inactivates the splice donor site at the end of exon 1. In family B, a previously reported mutation, c.442G>A; p.(Asp148Asn), was detected. Conclusions: Exome sequencing is an efficient and cost-effective technique for identifying mutations in genetically heterogeneous diseases. In addition, intrafamilial phenotypic variability in family A argues for the modifying effect of other still unknown modifier alleles.