a Circulating cell free mitochondrial DNA copy number variation in neurodegenerative disease

Abstract

Circulating cell free nuclear DNA (ccf-nDNA) is a widely observed phenomenon that has proved useful in non-invasive prenatal screening and as a potential biomarker of cancer. Circulating cell free mitochondrial DNA (ccf-mtDNA) has subsequently been identified, which has prompted a new wave of research to assess if this also has predictive ability in disease predisposition or progression. Previous studies of ccfmtDNA in Alzheimer’s disease (AD) and pilot work in our lab investigating ccf-mtDNA and Parkinson’s disease (PD), suggest that ccf-mtDNA may indeed serve as a biological marker of neurodegenerative disease (NDD). Given the rising prevalence of NDD in our aged population and the limited accuracy of current clinical tests of NDDs considerable effort has been placed into the identification and validation of novel fluid based biomarkers of NDD. With this in mind, I aimed to assess the utility of ccf-mtDNA as a biomarker for NDD as well as increase our understanding of the mechanism of ccf-mtDNA release. To achieve this I have used three, large, CSF cohorts: a diverse cohort of NDDs, a cohort of progressive multiple sclerosis (PMS) and a large-scale longitudinal study of PD. Ccf-mtDNA levels were assessed using qPCR and ccf-mtDNA integrity was assessed using both qPCR and next-generation-sequencing. Ccf-mtDNA levels and integrity were compared between cases and controls, correlating results to clinical and neuropathological measurements of disease as well as both nuclear and mitochondrially encoded protein levels. Similar to previous work, we found significant differences in the abundance but not integrity of ccf-mtDNA between patients and controls, particularly in PMS patients. Large-scale replication of ccf-mtDNA in PD revealed that, although ccf-mtDNA levels were indeed lower in cases compared to controls, this was significantly modulated by a number of confounding factors such as treatment, treatment duration and comorbidity. Whilst we conclude that ccf-mtDNA levels are unlikely a viable biomarker for NDD, this work has highlighted important considerations relating to the heterogeneity of ccf-mtDNA abundance and its utility as a biological marker in disease. Additionally, this work has revealed a novel interaction between therapeutic interaction and the release of ccf-mtDNA, which may expand our understanding of disease mechanism and serve as an independent measure of therapeutic efficacy in future trials.