Introduction: Usher syndrome (USH) is an autosomal recessive disorder that predominantly affects hearing, vision, and, in some cases, vestibular function. USH, according to the onset age, severity, and progression of symptoms, is categorized into four main types. In addition, there are a significant number of reports that patients' manifestations deviate from canonical phenotypic criteria of main types of USH, which are named atypical USH. CDH23 is the second most common USH gene in which its defects result in USH1D, non-syndromic autosomal recessive deafness-12 (DFNB12), and in a few cases, atypical USH1D. While some studies have suggested that missense and truncating damaging variants in the CDH23 gene cause DFNB12 and USH1D, respectively, no genotype-phenotype correlation for atypical USH1D has been established.
Methods: Using whole-exome sequencing, we studied an Iranian family with two affected siblings who manifested congenital bilateral hearing loss, late-onset nyctalopia, retinitis pigmentosa, and normal vestibular function, indicating that their clinical symptoms are consistent with USH2.
Results: Whole-exome data analysis revealed a novel bi-allelic nonsense variant (c.6562G>T; p.Glu2188Ter) in the CDH23 gene, which was confirmed by Sanger sequencing. Surprisingly, CDH23 is a member of the USH1 genes; therefore, our patients suffered from atypical USH1D. Also, by conducting a literature review, we provided a clinical and mutational profile of all reported patients with atypical manifestations or those who refuted the claimed genotype-phenotype correlation.
Conclusion: By reporting a novel damaging variant, we expand the mutational spectrum of the CDH23 gene that leads to atypical USH1D. Also, reviewing the literature shows that, contrary to previous claims, different genotypes occur in the CDH23 gene allelic disorders, and there is no clear-cut genotype-phenotype correlation.
Keywords: Atypical Usher syndrome; CDH23 gene; Genotype-phenotype correlation; Usher syndrome.
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