Age-related hearing loss involves mitochondrial DNA instability and copy number depletion in the cochlea: Insights from in vivo and in vitro models.

Abstract

Age-related hearing loss (ARHL) is a highly prevalent sensory neurodegenerative disorder that involves various molecular mechanisms. The present study investigated if age-related oxidative stress (OS) induces alterations in mitochondrial DNA (mtDNA) copy number and heteroplasmy, using complementary in vivo and in vitro models. Aged (30-month) CBA/CaJ mice have elevated auditory brainstem response (ABR) thresholds amplitudes vs. young (3-month) CBA/CaJ mice, a key physiological characteristic of ARHL The in vitro experiments employed the strial SV-K1 cochlear cell line treated with hydrogen peroxide (HO). Both the aged cochleae and HO-treated cells exhibited a significant (∼50 %) reduction in mtDNA copy number compared to their respective controls with enhanced levels of malondialdehyde (MDA). Therefore, in both the in vivo and in vitro models, OS drove mtDNA depletion. High-depth sequencing employing nuclear mtDNA pseudogene (NUMT)-avoidant methodologies revealed non-random distribution of heteroplasmic variants. Mutation hotspots were identified within the mitochondrial genome, particularly in regions encoding cytochrome c oxidase subunit 1 (COX1) and cytochrome b (CYTB) in both models. While HOtreatment induced a more widespread expansion of low-frequency variants, aging mice primarily showed shifts in the allele frequency distribution of existing variants rather than an accumulation of novel mutations. These findings demonstrate that OS is also a key factor of mtDNA regional mutational burden in the aging cochlea. The parallel mtDNA alterations observed in aged tissues and OS cells underscore mitochondrial genomic instability as a central mechanism in ARHL, highlighting potential targets for interventions aimed at preserving mitochondrial integrity and therefore auditory function.