Protection against cisplatin-induced ototoxicity by adeno-associated virus-mediated delivery of the X-linked inhibitor of apoptosis protein is not dependent on caspase inhibition

Hypothesis: Gene therapy with an adeno-associated viral (AAV) vector encoding the X-linked inhibitor of apoptosis protein (XIAP) in an animal model of cisplatin-induced ototoxicity can elucidate apoptotic pathways in the inner ear. Background: Cisplatin is limited clinically by ototoxicity associated with apoptosis in the inner ear. The relevant intracellular apoptotic pathways, however, are unknown. XIAP is an antiapoptotic protein that both inhibits caspases and reciprocally regulates the proapoptotic Smac/Omi proteins. AAV-mediated delivery of various XIAP mutants could distinguish between these antiapoptotic pathways in the ear and further the development of specific reagents for gene therapy-mediated prevention of cisplatin-induced ototoxicity. Methods: We administered unilaterally through the round-window AAV-harboring genes encoding wild-type dXIAP, yellow fluorescent protein, or either of two dXIAP point mutants-one deficient in caspase inhibition (dXlAP-d) and the other additionally deficient in the binding of Smac/Omi (dXIAP-t). All rats received a 3-day systemic course of cis-platin. Functional hearing loss was measured by shifts in auditory brainstem response (ABR) thresholds after cisplatin treatment, and hair-cell loss was assessed by whole-mount phalloidin staining of cochlear turns. Results: Uninjected ears universally displayed high-frequency-specific hair-cell loss and ABR threshold shifts upon cisplatin treatment. Although yellow fluorescent protein had no effect, ears injected with dXIAP exhibited 68% less ABR threshold shift at 32 kHz and 50% less basal-turn outer-hair-cell loss compared with contralateral untreated ears. This protection was maintained in ears injected with dXIAP-d but was abolished in those expressing dXIAP-t, which is incapable of blocking Smac/Omi. Conclusion: Hair-cell apoptosis induced by cisplatin involves the Smac/Omi pathway. Thus, gene therapy with either wildtype dXIAP or Smac/Omi-selective dXIAP-d may be effective to protect against cisplatin-mediated ototoxicity.