Cooperation between SOX9 and SOX10 is required for hearing via regulation of genes involved in inner ear fluid homeostasis

Abstract

Altered fluid homeostasis in the inner ear is a major cause of congenital hearing loss. The embryonic endolymphatic sac mediates fluid resorption for which anion exchangers such as SLC26A4, and its transcriptional activator FOXI1, are required. Besides FOXI1, not much is known about transcriptional regulators of this fluid balance. In campomelic dysplasia (CD), caused by heterozygous mutations in SOX9, in addition to skeletal malformation, some patients are deaf, with an unknown etiology. In a mouse model of CD caused by the SOX9Y440X mutation, which results in a truncated protein lacking the transactivation domain, we show severe endolymphatic dysfunction leading to deafness and vestibular problems. Adult heterozygous Sox9Y440X mice that express the mutation in the developing inner ear display sensorineural deafness combined with endolymphatic hydrops and lack of endocochlear potential. The mutant inner ear is enlarged from E15 with fewer Slc26a4-expressing cells in the endolymphatic epithelium. The cochlea overexpresses Aqp3, which encodes a water channel. We find by biochemical and cell-based transactivation assays and genetic interaction tests that SOX9 and SOX10 work co-operatively to repress Aqp3 transcription: SOX9Y440X blocks this repression by dominant interference. Our study reveals the key roles of SOXE transcription factors in the development of endolymphatic sac, and highlight a molecular mechanism whereby aquaporin expression must be repressed to sustain fluid homeostasis in the inner ear.