A Novel Variant in TUBB4B Causes Progressive Cone-Rod Dystrophy and Early Onset Sensorineural Hearing Loss.

Background: Sensorineural hearing loss (SNHL) is a frequent manifestation of syndromic inherited retinal diseases (IRDs), exemplified by the very rare form of autosomal-dominant Leber congenital amaurosis with early onset deafness (LCAEOD; OMIM #617879). LCAEOD was first described in 2017 in four families segregating heterozygous missense mutations in TUBB4B, a gene encoding a β-tubulin isotype. To date, only eight more families with similar TUBB4B-associated sensorineural disease (SND) have been reported. Most cases harbored missense variants affecting the same amino acid (Arg391) and only three families segregated variants involving different residues (Tyr310, Arg390).

Methods: We performed whole-exome sequencing and a full ophthalmological and audiological examination of the affected members in an Italian family segregating syndromic IRD with early onset deafness.

Results: We identified a novel, ultra-rare, disease-causing variant in TUBB4B (NM_006088.6:c.1049A>C) that replaces a highly conserved lysine with threonine at amino acid position 350. The functional impact of the Lys350Thr substitution was supported by protein structure modeling studies. The variant segregates in the family members presenting retinal disease with early onset SNHL. Detailed ophthalmological assessment of the affected subjects diagnosed a progressive cone-rod dystrophy.

Conclusion: These findings expand the limited number of disease-causing TUBB4B variants, corroborating their association with SND forms, and suggest Lys350 is an important residue for β-tubulin function. Interestingly, our results demonstrate that TUBB4B mutations can cause cone-dominated retinal phenotypes.