Distinct age-related pattern of mitochondrial somatic mutations across multiple sclerosis phenotypes

Background: Mitochondrial dysfunction has been proposed as a contributor to neurodegeneration in multiple sclerosis (MS). While the accumulation of somatic mitochondrial DNA (mtDNA) mutations with age is well documented in other neurodegenerative conditions, its role in MS progression remains largely unexplored. The aim of this study was to investigate the association between age and somatic mtDNA mutation burden in MS patients and evaluate whether any difference exists according to disease course. Methods: A total of 404 MS patients were recruited. Whole mtDNA was sequenced from blood-derived DNA using long-range PCR and the Illumina® Nextera XT kit. Somatic mutations were defined based on heteroplasmy levels between 1-5%. Linear regression models were used to assess the association between age and mutation rate. Results: We observed a significant age-dependent increase in low-frequency non-synonymous mtDNA mutations in MS. Analyses stratified by disease course revealed that this effect was substantially driven by PPMS patients (n = 238, P = 2.71 × 10-3), while no association was seen in RRMS (n = 155, P = 0.35), suggesting course-specific mitochondrial trajectories. Furthermore, fast-progressing patients showed a positive linear relationship between age and mtDNA mutation rate (P = 0.017), while slow-progressing ones showed an opposite trend (test for interaction P = 0.013). Conclusions: These findings support the existence of a differential age-related accumulation of somatic mtDNA mutations detected in blood across MS courses.