Why diabetes matters in dementia studies: Failure to account for diabetes status masks regional mitochondrial DNA copy number changes in human hippocampus, amygdala, and cerebellum in Alzheimer's disease.

INTRODUCTION

Diabetes is a major risk factor for Alzheimer's disease (AD), and both diseases involve mitochondrial dysfunction. Changes in mitochondrial DNA (mtDNA) can influence energy metabolism, cellular signaling, and epigenetic programming, thereby affecting cellular health. We hypothesized that AD is associated with reduced mtDNA copy number (mtDNA-CN) in vulnerable brain regions, and that diabetes modifies these changes.

METHODS

MtDNA-CN was quantified by absolute quantitative polymerase chain reaction (qPCR) in post-mortem hippocampus, amygdala, and cerebellum from cognitive controls (CCs), AD, and diabetes groups (CC+D, AD+D; N = 66-77).

RESULTS

MtDNA-CN differed by region, being highest in amygdala, intermediate in hippocampus, and lowest in cerebellum (503, 421, and 160 copies/cell; p < 0.0001). Pooled analyses showed modestly lower mtDNA-CN in AD. However, inclusion of diabetes appeared to mask differences. Donors with diabetes (CC+D and AD+D combined) had higher mtDNA-CN than non-diabetic (CC and AD combined) in hippocampus (+37.5%; p = 0.0012), amygdala (+30.6%; p = 0.0057), and cerebellum (+48.5%; p = 0.0002). This diabetes-associated increase was also present within AD, where AD+D had higher mtDNA-CN than AD in all regions. Removal of diabetic donors showed that AD was associated with a 19%-30% lower mtDNA-CN, reaching significance in the amygdala and cerebellum after correction for multiple testing.

DISCUSSION: We show that AD is associated with multiregional mtDNA-CN loss most clearly in donors without diabetes, whereas diabetes is associated with higher mtDNA-CN and can mask AD-related mitochondrial changes. The functional significance of higher mtDNA-CN in the diabetic brain remains unclear, but evidence that diabetes acts as an effect modifier has important implications for dementia studies.