Multiple sclerosis (MS) is a multifaceted neuroinflammatory and neurodegenerative disease of the central nervous system characterized by demyelination, axonal injury, and progressive disability. Although inflammatory demyelination predominates during the early relapsing-remitting phase, insidious neurodegeneration drives long-term functional impairment. Emerging evidence highlights energy failure and oxidative stress as primary catalysts of axonal loss. Within this pathophysiological cascade, the accumulation of low-frequency somatic mtDNA mutations represents a critical mechanism underlying mitochondrial dysfunction. A landmark study published in the Journal of Neurology evaluates how these age-dependent mitochondrial genomic alterations diverge across distinct clinical phenotypes of multiple sclerosis, offering critical insights into disease pathogenesis.

Biological Role of Somatic mtDNA Mutations in Neurodegeneration

Mitochondria generate the vast majority of cellular adenosine triphosphate through oxidative phosphorylation. Consequently, neurons and glial cells remain exceptionally vulnerable to disruptions in mitochondrial respiratory chain integrity. Mitochondrial DNA lacks protective histones and possesses limited repair mechanisms compared to nuclear chromatin. Therefore, reactive oxygen species produced during chronic neuroinflammation readily induce structural mitochondrial genome alterations over time.

As individuals age, somatic mtDNA mutations naturally accumulate across post-mitotic and highly proliferative tissues. However, excessive mutational burden disrupts electron transport chain subunits, exacerbating oxidative stress and precipitating bioenergetic crisis. In multiple sclerosis, persistent microglial activation, peripheral immune infiltration, and chronic demyelination subject axons to intense metabolic stress. When cumulative mitochondrial damage surpasses critical cellular thresholds, focal respiratory failure occurs. This energetic deficit impairs calcium homeostasis and accelerates axonal transection. Understanding the rate at which these mutations accumulate provides essential context for explaining why neurodegenerative features intensify with advancing patient age.

Methodological Framework and Heteroplasmy Detection

To characterize mitochondrial genomic instability across clinical phenotypes, investigators recruited a cohort of 404 individuals diagnosed with multiple sclerosis. The study evaluated peripheral blood samples to capture accessible genomic biomarkers reflecting systemic mitochondrial changes. Researchers utilized long-range polymerase chain reaction amplification followed by high-throughput next-generation sequencing using specialized sequencing chemistry to sequence whole mitochondrial genomes.

A rigorous bioinformatic approach was implemented to identify true low-frequency somatic events while filtering technical artifacts and nuclear mitochondrial pseudogenes. Somatic variants were defined by heteroplasmy levels ranging strictly between 1% and 5%. By setting this narrow window, the investigators successfully isolated newly acquired somatic shifts from inherited germline polymorphisms. Subsequently, multivariable linear regression models evaluated the association between biological age and mutation burden. The team adjusted for potential confounders, including disease duration and biological sex, ensuring reliable statistical stratification across relapsing and progressive patient cohorts.

Divergent Mutation Trajectories: PPMS Versus RRMS

The study demonstrated a statistically significant age-dependent rise in low-frequency non-synonymous mitochondrial variants across the overall multiple sclerosis cohort. However, subsequent stratified analyses revealed profound differences between distinct clinical phenotypes. The observed age-associated increase was substantially driven by individuals with primary progressive multiple sclerosis (PPMS, n = 238, P = 2.71 × 10⁻³). In stark contrast, individuals with relapsing-remitting multiple sclerosis (RRMS, n = 155, P = 0.35) exhibited no significant correlation between chronological age and somatic mutational burden.

These divergent trajectories indicate that primary progressive disease is biologically distinct in its handling of mitochondrial genomic stress. In relapsing disease, transient inflammatory episodes may temporarily alter cellular metabolism without immediately accelerating systemic mitochondrial mutational accumulation. Conversely, primary progressive disease features persistent smoldering neuroinflammation and accelerated biological aging. This chronic environment promotes progressive mitochondrial DNA damage. Consequently, these findings validate the concept that different MS phenotypes follow distinct biological pathways rather than representing a simple chronological continuum.

Clinical Progression Rates and Mitochondrial Genome Instability

Beyond categorizing patients by classical disease phenotype, the investigators examined whether the rate of physical disability accrual correlated with mitochondrial mutation dynamics. The study cohort was stratified into fast-progressing and slow-progressing patient groups based on verified disability outcome measures over longitudinal follow-up. This granular evaluation uncovered a significant interaction between patient age, progression velocity, and mutational load (test for interaction P = 0.013).

Fast-progressing patients demonstrated a robust positive linear relationship between advancing age and the accumulation of low-frequency somatic variants (P = 0.017). Conversely, slow-progressing patients showed an inverse or stable trend. This divergence highlights that mitochondrial genome instability directly mirrors the aggressiveness of the clinical course. In rapidly deteriorating individuals, impaired DNA repair or excessive free radical generation may create a vicious cycle of accelerating mitochondrial dysfunction and tissue loss. Identifying these dynamic differences underscores the value of mitochondrial metrics in stratifying prospective disease trajectory.

Translational Insights and Biomarker Potential in Clinical Practice

The discovery of distinct age-related somatic mitochondrial variations in blood-derived DNA introduces valuable translational opportunities for clinicians and researchers. Traditionally, assessing central nervous system pathology requires neuroimaging or invasive cerebrospinal fluid sampling. However, measuring low-frequency heteroplasmy in peripheral blood offers a minimally invasive window into systemic metabolic resilience and genomic vulnerability.

Furthermore, these insights may guide future therapeutic strategies for progressive multiple sclerosis. Current disease-modifying therapies predominantly target adaptive immune pathways, demonstrating limited efficacy against smoldering neurodegeneration and disability progression in PPMS. Interventions targeting mitochondrial bioenergetics, mitochondrial biogenesis, or targeted antioxidant defenses could potentially mitigate somatic DNA damage. As precision neurology advances, incorporating mitochondrial genomic profiling may help clinicians identify patients at high risk for rapid disability accrual, paving the way for personalized neuroprotective regimens.

Frequently Asked Questions

What causes somatic mtDNA mutations to accumulate in multiple sclerosis?

Somatic mitochondrial DNA mutations accumulate due to persistent oxidative stress, chronic neuroinflammation, and limited mitochondrial repair mechanisms. Over time, reactive oxygen species damage mitochondrial DNA, generating low-frequency heteroplasmic mutations that disrupt cellular energy metabolism and accelerate neuroaxonal degeneration across susceptible neurological tissues.

How do mitochondrial mutation patterns differ between PPMS and RRMS?

Patients with primary progressive multiple sclerosis show a significant age-dependent accumulation of non-synonymous mitochondrial mutations in blood. In contrast, patients with relapsing-remitting multiple sclerosis show no significant association between age and mutational load, indicating distinct biological trajectories and metabolic vulnerabilities between phenotypes.

Can peripheral blood testing accurately reflect mitochondrial changes in MS?

Peripheral blood testing detects low-frequency heteroplasmic mutations between 1% and 5% using advanced deep sequencing. Although central nervous system tissue directly experiences demyelination, peripheral blood cells mirror systemic metabolic stress and genomic instability, providing an accessible, minimally invasive biomarker for assessing clinical progression.

Disclaimer: This content is for informational and educational purposes only. It does not constitute medical advice, diagnosis, or treatment recommendations. Refer to the latest local and national guidelines for clinical practice.

References

1. Esposito F et al. Distinct age-related pattern of mitochondrial somatic mutations across multiple sclerosis phenotypes. J Neurol. 2026 Jun 06. doi: 10.1007/s00415-026-13926-7. PMID: 42251197.
2. Mahad DH, Trapp BD, Lassmann H. Pathological mechanisms in progressive multiple sclerosis. Lancet Neurol. 2015;14(2):183-193.
3. Campbell GR, Worrall JT, Mahad DJ. The central role of mitochondria in axonal degeneration in multiple sclerosis. Mult Scler. 2014;20(14):1806-1813.
4. Mao P, Reddy PH. Is multiple sclerosis a mitochondrial disease? Biochim Biophys Acta. 2010;1802(1):66-79.