COCH5B2 is a target antigen of anti-inner ear antibodies in autoimmune inner ear diseases

Objective: This study was designed to identify the 58-kDa inner ear protein against which the sera of some patients with idiopathic, progressive senso rineural hearing loss or Meniere's disease strongly react. Background: We and other groups have previously demonstrated that a 58-kDa protein extracted from guinea pig or bovine inner ear tissue is a target of antibodies in serum samples from some patients with autoimmune inner ear d iseases. Methods: After separation of inner ear proteins by 10% sodium dodecyl sulfa te polyacrylamide gel electrophoresis, the bands corresponding to 58 kDa we re localized and excised from the gel. The concentrated protein was then di gested with trypsin, and the peptide fragments were separated by high-press ure liquid chromatography. Three fractions were subjected to amino acid seq uencing by the classic Edman degradation. Results: The sequence of a stretch of 14 amino acids of the first fragment was identical to that of amino acids 526 to 539 of the COCH5B2 protein. The sequences of 11 and 10 amino acids of the second and third fragments, resp ectively, also were identical to residues 417 to 427 and 396 to 405 of the COCH5B2 protein. These data, together with two-dimensional gel electrophore sis followed by Western blot experiments, confirmed that the 58-kDa inner e ar protein is the COCH5B2 protein. Discussion: These findings indicate that the 58-kDa target protein of antib odies in serum samples of patients with autoimmune inner ear diseases is th e COCH5B2 protein, a molecule that is highly and specifically expressed in the cochlea and vestibule.