Candidate exome capture identifies mutation of SDCCAG8 as the cause of a retinal-renal ciliopathy

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Abstract
Friedhelm Hildebrandt and colleagues combine homozygosity mapping with candidate exome capture and high-throughput sequencing to identify SDCCAG8 mutations as the cause of a retinal-renal ciliopathy. They further show that SDCCAG8 localizes to centrioles and that its depletion causes renal cysts and cell polarity defects. Nephronophthisis-related ciliopathies (NPHP-RC) are recessive disorders that feature dysplasia or degeneration occurring preferentially in the kidney, retina and cerebellum. Here we combined homozygosity mapping with candidate gene analysis by performing 'ciliopathy candidate exome capture' followed by massively parallel sequencing. We identified 12 different truncating mutations of SDCCAG8 (serologically defined colon cancer antigen 8, also known as CCCAP) in 10 families affected by NPHP-RC. We show that SDCCAG8 is localized at both centrioles and interacts directly with OFD1 (oral-facial-digital syndrome 1), which is associated with NPHP-RC. Depletion of sdccag8 causes kidney cysts and a body axis defect in zebrafish and induces cell polarity defects in three-dimensional renal cell cultures. This work identifies loss of SDCCAG8 function as a cause of a retinal-renal ciliopathy and validates exome capture analysis for broadly heterogeneous single-gene disorders.