Marginal Cell Damage in the Inner Ear of mtDNA4834-Deficient Rats is Associated with Methylation-Induced Downregulation of SOD2

Abstract

Objective: Presbycusis, the most prevalent sensory impairment in the elderly, is associated with the loss of mitochondrial (mt)DNA4977 (corresponding to the loss of mtDNA4834 in rats). Superoxide dismutase 2 (SOD2) is a key factor involved in aging. Therefore, this study aimed to investigate the relationship between SOD2 and aging-associated loss of mtDNA4834 in rats. Methods: A rat inner ear aging model was established by mimicking the aging-associated mtDNA4834 deletion mutation with D-galactose. In addition, SOD2 was either overexpressed or inhibited to investigate its effects on marginal cell (MC) damage in mtDNA4834-deficient rats. The expression and methylation status of SOD2 as well as the expression of SOD2 transcriptional regulatory elements in DNA4834 deletion mutant marginal cells (MCs) and normal MCs were compared. The effect of SOD2 expression on MC phenotype was simulated by transfection. Results: SOD2 was downregulated in mitochondrial DNA (mtDNA)-deficient MCs, and this process was associated with the methylation of the SOD2-encoding gene. In addition, the overexpression of SOD2 in mtDNA4834-deleted MCs resulted in increased cell viability and reduced apoptosis, as demonstrated by the upregulation of anti-apoptotic proteins and downregulation of pro-apoptotic proteins. Moreover, SOD2 overexpression suppressed mtDNA4834 deletion mutation and increased the copy number of mtDNA4834, whereas SOD2 silencing resulted in the opposite effect. Conclusion: These findings indicate that aging-related mtDNA4834 deletion is associated with SOD2 deficiency, suggesting that SOD2 is a potential target for the treatment of mtDNA4977 deletion-related presbycusis.