Defective mitochondrial DNA homeostasis in the substantia nigra in Parkinson disease

Abstract

Increased somatic mitochondrial DNA (mtDNA) mutagenesis causes premature aging in mice, and mtDNA damage accumulates in the human brain with aging and neurodegenerative disorders such as Parkinson disease (PD). Here, we study the complete spectrum of mtDNA changes, including deletions, copy-number variation and point mutations, in single neurons from the dopaminergic substantia nigra and other brain areas of individuals with Parkinson disease and neurologically healthy controls. We show that in dopaminergic substantia nigra neurons of healthy individuals, mtDNA copy number increases with age, maintaining the pool of wild-type mtDNA population in spite of accumulating deletions. This upregulation fails to occur in individuals with Parkinson disease, however, resulting in depletion of the wild-type mtDNA population. By contrast, neuronal mtDNA point mutational load is not increased in Parkinson disease. Our findings suggest that dysregulation of mtDNA homeostasis is a key process in the pathogenesis of neuronal loss in Parkinson disease.

Neuronal complex I deficiency occurs throughout the Parkinson’s disease brain, but is not associated with neurodegeneration or mitochondrial DNA damage

“…Moreover, ultra-deep-sequencing studies in single neurons and brain homogenates have shown no difference in the point mutational burden of mtDNAencoded complex I genes between PD individuals and controls, suggesting that mtDNA point mutations do not contribute to the observed complex I deficiency [8,13]. Here, we show that complex I deficiency occurs in neurons from at least eight different brain areas in PD, whereas levels of mtDNA damage only exceed those of controls in the SNc.…”

“…a-e Magnification ×400, scale bar 50 µm. h Magnification ×200, scale bar 100 µm harbored significant mtDNA changes in PD in the form of higher deletion levels and wild-type mtDNA depletion [13]. The remaining neuronal populations showed no significant mtDNA differences between PD and controls.…”

“…6. Data from the SNc, frontal cortex and cerebellum from the same individuals have been reported previously [13]. 4 Semiquantitative analyses of complex I deficiency in PD and control neurons.…”

“…The data from the SNc, cerebellum and frontal cortex have been described in detail in our previously published work using consistent subject ID's [13]. To allow for a more complete correlation with our complex I data, we expanded the experiment to include pyramidal neurons from the CA2 and CA3 area of the hippocampus (47 neurons from nine individuals with PD and 34 neurons from four controls) and principal neurons of the putamen (72 neurons from nine individuals with PD and 65 neurons from eight controls).…”

“…To allow for a more complete correlation with our complex I data, we expanded the experiment to include pyramidal neurons from the CA2 and CA3 area of the hippocampus (47 neurons from nine individuals with PD and 34 neurons from four controls) and principal neurons of the putamen (72 neurons from nine individuals with PD and 65 neurons from eight controls). Total mtDNA copy number and the fraction of major arc deletion were determined using a duplex real-time PCR assay as previously described [13] in 7-12 single neurons per brain region from each PD individual or control.…”