A Nonsense Mutation in CEP250, a Mammalian-Specific Homolog of Rootletin, Causes Usher Syndrome

Abstract

Purpose: To use the combination of homozygosity mapping and whole exome sequencing to identify the disease-causing gene in members of the same family presented with different phenotypes.

Methods: A consanguineous family (MOL0028) of Iranian Jewish origin including six siblings affected with SNHL and retinal degeneration without vestibular dysfunction. A combination of homozygosity mapping and whole exome sequencing (WES) in MOL0028 were applied on DNA samples of memebers with different clinical phenotypes combined with electrophysiological tests and laboratory molecular assays.

Results: We identified in family MOL0028 nonsense mutations in two genes encoding ciliary proteins: c.3289C>T (p.Q1097*) in C2orf71 (reported to cause nonsyndromic retinal degeneration) and c.3463C>T (p.R1155*) in the centrosome-associated protein CEP250 (encoding the C-Nap1 protein). The CEP250 gene has not been associated thus far with any inherited human disease and the c.3463C>T mutation was absent in 160 ethnicity-matched control chromosomes and in about 13,000 chromosomes reported at the exome variant server. Patients who were double homozygotes for both mutations had SNHL accompanied by early-onset and severe retinitis pigmentosa (RP), while patients who were homozygous for the CEP250 mutation (and heterozygous for the C2orf71 mutation) had SNHL with a relatively mild form of retinal degeneration. No ciliary structural abnormalities in the respiratory system were evident by electron microscopy analysis in patients with either genotype combination. RT-PCR analysis in blood samples of patients homozygous for the CEP250 mutation revealed expression of a mutant transcript at levels similar to the expression of the wildtype transcript in normal control individuals. The mutant transcript results in the production of an abnormal truncated protein lacking the NEK2-phosphorylation region. By evolutionary analysis we show that C-Nap1 and rootletin are paralogues that arose by genomic duplication about 500 million years ago.

Conclusions: Our data indicate that a homozygous nonsense mutation in CEP250 causes a novel type of Usher syndrome, characterized by early-onset hearing loss and a relatively mild retinal degeneration. The more severe retinal involvement in the double CEP250 and C2orf71 homozygotes may indicate an additive effect caused by nonsense mutations in genes encoding two ciliary proteins.

Keywords: 696 retinal degenerations: hereditary • 539 genetics • 696 retinal degenerations: hereditary