Abstract
Introduction: Parkinson's disease (PD) is a neurodegenerative movement disorder, characterized by progressive loss of dopaminergic neurons and accumulation of proteinaceous Lewy body inclusions. Mitochondrial dysfunction has emerged as a prominent cellular mechanism involved in neurodegeneration, including PD. The contribution of mitochondrial DNA (mtDNA) variants to PD pathogenesis has long been debated but is still not clearly answered. Several nuclear genes driving inherited forms of PD, such as PRKN, are linked to mtDNA quality control mechanisms. Accordingly, somatic mtDNA variants have been suggested to contribute to the impairment of mitochondrial function in PD. (reviewed in 1)
Material and Methods: mtDNA copy number, D-Loop activity and major arc deletion were analyzed in whole blood, lymphoblasts, fibroblasts, induced pluripotent stem cells (iPSC) and iPSC-derived neuronal progenitor cells and neurons of PD patients, healthy heterozygous PRKN variant carriers, and matched controls by digital droplet PCR and real-time PCR. mtDNA sequencing was performed by Next Generation Sequencing methodology at high coverage.
Results: mtDNA copy numbers tended to be increased in iPSC and derived neurons, as compared to respective cells of origin. D-loop transcriptional/replicative activity, as measured by 7S DNA quantities, appeared to be decreased in iPSC-derived neurons. No significant increases of large mtDNA deletions were detected in iPSC-derived neurons. An accumulation of heteroplasmic variants was observed in cell models compared to blood DNA in mutation carriers and controls. Interestingly, we noted an altered correlation of mutation frequency in iPSC and derived neurons between controls and PRKN variant carriers.
Conclusion: mtDNA copy numbers and function correlate with cell type. mtDNA variants are mainly observed in clonally derived cells and propagate to varying amounts of heteroplasmy levels. Deep mtDNA genotyping provides evidence for an altered mitochondrial quality control mechanism in PD-related mutation carriers. Therefore, advanced cellular models are indicated for the study of mitochondrial genotypes and its contribution to the phenotype in neurodegenerative diseases, as exemplified by PD.