Loss of CEP162 function at the primary cilium delays ciliogenesis and causes retinal ciliopathy in humans

Abstract

ABSTRACTCiliopathies often comprise retinal degeneration since the photoreceptor’s outer segment is an adapted primary cilium. CEP162 is a distal end centriolar protein required for proper transition zone assembly during ciliogenesis and whose loss causes ciliopathy in zebrafish. CEP162 has so far not been implicated in human disease. Here, we identified a homozygous CEP162 frameshift variant, c.1935dupA (p.(E646R*5)), in retinitis pigmentosa patients from two unrelated Moroccan families, likely representing a founder allele. We found that even though mRNA levels were reduced, the truncated CEP162-E646R*5 protein was expressed and localized to the mitotic spindle during mitosis, but not at the basal body of the cilium. In CEP162 knockdown cells, expression of the truncated CEP162-E646R*5 protein is unable to restore ciliation indicating its loss of function at the cilium. In patient fibroblasts, cilia overcome the absence of CEP162 from the primary cilium by delaying ciliogenesis through the persistence of CP110 at the mother centriole. The patient fibroblasts are ultimately able to extend some abnormally long cilia that are missing key transition zone components. Defective transition zone formation likely disproportionately affects the long-living ciliary outer segment of photoreceptors resulting in retinal dystrophy. CEP162 is expressed in human retina, and we show that wild-type CEP162, but not truncated CEP162-E646R*5, specifically localizes to the distal end of centrioles of mouse photoreceptor cilia. Together, our genetic, cell-based, and in vivo modeling establish that CEP162 deficiency causes retinal ciliopathy in humans.SIGNIFICANCE STATEMENTCiliopathies often comprise retinal degeneration since the photoreceptor’s outer segment is an adapted primary cilium. CEP162 is a basal body protein required for proper transition zone assembly during ciliogenesis and has so far not been implicated in human disease. Using genetic, cell-based, and in vivo studies, we show that a biallelic mutation of CEP162 causes late-onset human retinal degeneration. This mutation specifically hinders CEP162 function at the cilia, leading to impaired transition zone assembly and delayed formation of dysmorphic cilia. Studies in mouse photoreceptors support that absence of CEP162 could lead to defective outer segments. The discovery of novel ciliopathy genes, such as CEP162, advances our insight into cell-specific functions of the primary cilium.