Identification of pathogenic mechanisms of COCH mutations, abolished cochlin secretion, and intracellular aggregate formation: genotype-phenotype correlations in DFNA9 deafness and vestibular disorder

Mutations in COCH (coagulation factor C homology) cause autosomal-dominant nonsyndromic hearing loss with variable degrees of clinical onset and vestibular malfunction. We selected eight uncharacterized mutations and performed immunocytochemical and Western blot analyses to track cochlin through the secretory pathway. We then performed a comprehensive analysis of clinical information from DFNA9 patients with all 21 known COCH mutations in conjunction with cellular and molecular findings to identify genotype-phenotype correlations. Our studies revealed that five mutants were not secreted into the media: two von Willebrand factor A (vWFA) domain mutants, which were not transported from the endoplasmic reticulum to Golgi complex and formed high-molecular-weight aggregates in cell lysates, and three LCCL domain mutants, which were detected as intracellular dimeric cochlins. Mutant cochlins that were not secreted and accumulated in cells result in earlier age of onset of hearing defects. In addition, individuals with LCCL domain mutations show accompanying vestibular dysfunction, whereas those with vWFA domain mutations exhibit predominantly hearing loss. This is the first report showing failure of mutant cochlin transport through the secretory pathway, abolishment of cochlin secretion, and formation and retention of dimers and large multimeric intracellular aggregates, and high correlation with earlier onset and progression of hearing loss in individuals with these DFNA9-causing mutations.