CEP78 is mutated in a distinct type of Usher syndrome

Abstract

Background: Usher syndrome is a genetically heterogeneous disorder featured by combined visual impairment and hearing loss. Despite a dozen of genes involved in Usher syndrome having been identified, the genetic basis remains unknown in 20-30% of patients. In this study, we aimed to identify the novel disease-causing gene of a distinct subtype of Usher syndrome.

Methods: Ophthalmic examinations and hearing tests were performed on patients with Usher syndrome in two consanguineous families. Target capture sequencing was initially performed to screen causative mutations in known retinal disease-causing loci. Whole exome sequencing (WES) and whole genome sequencing (WGS) were applied for identifying novel disease-causing genes. RT-PCR and Sanger sequencing were performed to evaluate the splicing-altering effect of identified CEP78 variants.

Results: Patients from the two independent families show a mild Usher syndrome phenotype featured by juvenile or adult-onset cone-rod dystrophy and sensorineural hearing loss. WES and WGS identified two homozygous rare variants that affect mRNA splicing of a ciliary gene CEP78. RT-PCR confirmed that the two variants indeed lead to abnormal splicing, resulting in premature stop of protein translation due to frameshift.

Conclusions: Our results provide evidence that CEP78 is a novel disease-causing gene for Usher syndrome, demonstrating an additional link between ciliopathy and Usher protein network in photoreceptor cells and inner ear hair cells.

Keywords: CEP78; Usher syndrome; cone-rod dystrophy; next-generation sequencing.

Figure 1

Clinical manifestations of the patients with atypical Usher syndrome. Fundus images of Family 1: II-2, OD (A) and OS (B), Family 2: V-1, OD (C) and OS (D). Optical coherence tomography images of Family 1: II-2, OD (E) and OS (F), Family 2: V-1, OD (G) and OS (H). Fundus autofluorescence results of Family 1: II-2, OD (I) and OS (J). Fundus fluorescein angiography of Family 2: V-1, OD (K) and OS (L). The results of scotopic electroretinograms (ERGs) 0.01 (M), scotopic ERG 0.3 (N), oscillatory potential ERG (O), photopic ERG 3.0 (P) and flicker ERG 30 Hz (Q) of normal control (left panel), Family 1: II-2 (middle panel) and Family 1: II-1 (right panel). Audiogram images of Family 1: II-2 (R) and Family 2: V-1 (S). In the audiogram images, cross or circle labels indicate air-conduction hearing, and right angle labels indicate bone-conduction hearing.

Figure 2

Genetic findings in the patients with Usher syndrome. (A) CEP78 mutations were identified in two unrelated consanguineous families. The variants cosegregate with the disease phenotype. Arrows indicate probands. (B) RT-PCR and Sanger sequencing confirmed the splicing-altering effect of the variant c.1254+5G>A in Family 1. In two patients’ mRNA, the 46 bp exon 10 is completely skipped compared with normal control. (C) RT-PCR and Sanger sequencing confirmed the splicing-altering effect of the variant c.1629–2A>G in Family 2. In the probands’ mRNA, a 10 bp-long region in CEP78 exon 14 was deleted, while in the unaffected heterozygous carrier, wild-type/mutant hybrid sequence was observed.