Expansion of the role and functional characterization of a missense variant in CEP78 associated with cone-rod dystrophy and hearing loss.

Abstract

Purpose : Inactivating CEP78 mutations have been found in typical cone-rod dystrophy with hearing loss (CRDHL)(MIM 617236), distinct from Usher syndrome (USH). We previously identified the first biallelic missense variant in CEP78 in a Belgian family with CRDHL and male infertility. Here we aimed to expand its role in inherited retinal disease (IRD) and to functionally characterize the effect of this missense variant.

Methods : Whole exome sequencing (NextSeq 500, HiSeq 3000, Illumina) data were revisited in our IRD cohort. Single nucleotide polymorphisms and microsatellite markers were used for haplotype reconstruction. In silico protein modeling was combined with Western Blot analysis on primary fibroblasts obtained from healthy controls, heterozygous and homozygous individuals.

Results : The c.449T>C, p.(Leu150Ser)(NM_001098802.2) change has been identified in two unrelated families. Haplotype reconstruction revealed a common haplotype of ~2.8 Mb. Homology protein modeling and FoldX calculations suggested a detrimental effect of the variant on protein stability, which was subsequently confirmed using Western Blot on fibroblast protein lysates. Lower to absent CEP78 protein was detectable in heterozygous and homozygous individuals in comparison with controls, respectively. In addition, protein modeling suggested that similar variants changing the first leucine to serine in other proteins containing leucine-rich repeat (LRR) domains have a putative strong effect on the overall protein stability. As the male affected individual in the additional family did not display infertility, other potential causes for male infertility in the original family were investigated. Interestingly, a complex allele c.[4127del; 946G>T] was found in the SPAG17 gene (NM_206996.2), in which a biallelic variant has previously been linked to male infertility. Although no second SPAG17 mutation was found here, its role in the male infertility cannot be excluded.

Conclusions : This study revealed a founder and loss-of-function effect of the CEP78 p.(Leu150Ser) variant. Recently a functional relationship has been shown between CEP78 and CP110, an important actor in centriole length and ciliogenesis. We hypothesize that the lower amount of CEP78 dysregulates pathways controlled by CP110 in the retina. Finally, CEP78 loss of function may underlie additional cases with atypical or misdiagnosed USH.

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.