Mutation of beta-tubulin 4B gene (TUBB4B) causes autosomal dominant retinitis pigmentosa with sensorineural hearing loss in a multigenerational family

Purpose: Members of a multigenerational Canadian family presented to an inherited retinal degeneration (IRD) clinic with retinitis pigmentosa (RP) and sensorineural hearing loss, reminiscent of an Usher syndrome phenotype. Biallelic disease-causing variants in the known Usher syndrome genes were not identified. Therefore, we enrolled further family members in this study and examined whether other IRD gene variants could explain the phenotype in the family. Methods: Family members underwent a comprehensive ophthalmic examination, including best-corrected visual acuity, direct and indirect ophthalmoscopy, fundus photography, visual field testing, spectral-domain optical coherence tomography, audiological examination, and genetic testing. Some patients also had autofluorescence imaging. Loss-of-function testing was initiated by antisense morpholino knockdown of tubb4b in zebrafish. Results: Multimodal clinical testing in affected patients revealed an autosomal dominant late-onset presentation of RP associated with progressive, bilateral sensorineural hearing loss that occurred in the second to third decades of life with no vestibular involvement. Panel-based genetic testing revealed a heterozygous c.1168C>T, p.Arg390Trp variant in the beta-tubulin 4B gene (TUBB4B) only in affected family members. Based on in silico analysis, segregation analysis through the family, and literature evaluation, this variant is likely to be the disease-causing variant inherited in an autosomal dominant manner. We searched our local database of ~1,000 patients with IRD, and no other TUBB4B variants were identified, confirming this is a rare disease variant. Knockdown of tubb4b in zebrafish revealed cone and rod photoreceptor abnormalities in the retina and hydrocephalus in the developing brain, resulting in early larval lethality. Conclusions: For the first time, we describe a multigenerational family with a TUBB4B gene variant p.(Arg390Trp) segregating with deaf-blindness, establishing autosomal dominant inheritance. This further confirms that the Arg390 codon is a mutation hotspot. We also expand the range of phenotypes seen with the p.(Arg390Trp) TUBB4B gene variant to include typical RP as well as a milder, pericentral RP. Furthermore, our studies suggest there is conservation of TUBB4B ciliary function between zebrafish and humans, making zebrafish a better model system for studying vision loss than the mouse model.