| Description |
Usher Syndrome (USH) is the leading cause of deaf-blindness worldwide. Patients with USH have hearing loss, balance problems, and retinal degeneration. To date, eleven genes have been associated with USH. Interestingly, mutations in several USH genes lead to discrete diseases as well. Despite extensive studies on USH in the past, over one-hundred years since USH was originally identified, the molecular function of USH genes remains understudied. This incomplete understanding greatly limits therapeutic development for USH. To understand the molecular mechanisms underlying variable disease manifestations, I studied an USH2 gene, DFNB31, which causes USH2D when mutated in its N-terminal region, and autosomal recessive nonsyndromic deafness type 31 (DFNB31) when mutated towards its C-terminal region. DFNB31 encodes a protein called whirlin that was previously shown to have multiple mRNA variants in human and mouse tissues. I hypothesized that whirlin isoforms have unique functions and disruption of different whirlin isoforms is the cause of various disease manifestations. To test this hypothesis, I utilized region-specific whirlin antibodies and Dfnb31 mouse models that mimic human DFNB31 mutations and disease outcomes. I found that alternative splicing and alternative use of promoters produce several Dfnb31 mRNA variants that are translated to full-length (FL), N-terminal and C-terminal whirlin isoforms, which localize at different subcellular positions in the inner ear hair cells and retinal photoreceptors. Studies in Dfnb31 mutants show that FL-whirlin isoform is required at the hair cell stereociliary bases and retinal photoreceptor periciliary membrane complex to form a stable USH2 protein complex, whereas C-whirlin isoform is required at the stereocilia tips for stereociliary elongation. I found that mutations in N-terminal region of Dfnb31 lead to loss of FL- and N-whirlin isoforms and cause USH2D-like symptoms. On the other hand, mutations in the C-terminal region of Dfnb31 lead to loss of FL- and C-whirlin isoforms and cause DFNB31-like symptoms. Considering the presence of multiple splicing isoforms for several other USH genes and the variable phenotypes caused by mutations in these genes, differential disruption of the splicing isoforms is likely the mechanism underlying different disease manifestations upon mutation in these USH genes. In addition, I found vestibular deficits in Dfnb31 mutants, which was surprising because USH2 patients were thought to have normal vestibular function. My findings present a rationale for vestibular analysis of all USH2 patients at the clinics to comprehend the pathogenesis and mechanism of USH. In summary, my findings will help improve differential diagnosis between USH and its related diseases and is expected to contribute to the development of USH therapies. |
