Mitochondrial dysfunction and DNA methylation in type 2 diabetes and cognitive impairment

Background. Mexican American populations are disproportionately affected by type 2 diabetes (T2D) and Alzheimer’s disease (AD). Although commonly characterized by the accumulation of amyloid plaques and tau tangles, the role of mitochondrial dysfunction (changes in mitochondrial dynamics, apoptosis, and/or oxidative stress signaling) in AD pathophysiology has become further elucidated over recent years. Methylation of key nuclear-encoded genes may be involved in regulation of critical mitochondrial processes. Evidence suggests that differential methylation and expression of mitochondrial related genes (e.g. POLG) may correlate with mitochondrial DNA copy number (mtDNACN). Hypothesis. We tested the following hypotheses: (1) methylation of mitochondrial-related genes is negatively correlated with their respective gene expression, (2) methylation of mitochondrial biogenesis genes (e.g., TFAM, POLG) negatively correlates with mtDNACN per cell, and (3) methylation of genes related to oxidative stress response, mitophagy, endosomal/exosomal trafficking and apoptosis correlates with cell-free mtDNA (mtDNACF) levels. Methods. DNA from 14 female Mexican American subjects enrolled in HABLE, the Healthy Aging Brains of Latino Elders cohort, was used for this study. Subjects were grouped based on T2D diagnosis, and were matched across groups based on age and cognitive status. For mitochondrial-related gene expression, cDNA was synthesized from blood buffy coat RNA and tested using the RT2 Profiler™ Human Mitochondria Array. For methylation analysis, nDNA from the blood buffy coat extract was bisulfite-converted and methylation levels were determined using the MethylationEPIC™ beadchip. Data was analyzed using Genome Studio. Buffy coat mtDNACN and plasma mtDNACF were quantified using TaqMan®-based qPCR. Methylation levels of CpG regions around candidate genes were then correlated with (1) gene expression, (2) mtDNACN, and (3) mtDNACF. Results. Preliminary results indicate that hypomethylation of some mitochondrial-related genes corresponds with increased expression (e.g., COX10); methylation of two sites associated with a known CpG island for POLG are negatively correlated with mtDNA copy number per cell. Conclusions. Significant correlations between mitochondrial phenotypes and candidate gene epigenetic loci may point to novel regulatory mechanisms of mitochondrial function. Future studies will include exploratory analysis at the genome-wide level using a larger cohort.