Altered levels of circulating mitochondrial DNA in elderly people with sarcopenia: Association with mitochondrial impairment.

Background: Age-related chronic inflammatory process is often referred to as "inflammaging", which had been described in several human disorders, including sarcopenia. Recently, mitochondrial DNA (MtDNA) has moved into the spotlight as a "damage-associated molecular pattern" (DAMP) agent that can potentially elicit inflammation. Yet, the roles of this mitochondrial DAMP have never been investigated in sarcopenia.

Design: Cross-sectional study.

Participants: From January 2021 to June 2021, elderly outpatients ≥65 years and able to finish a comprehensive geriatric assessment were recruited in our study.

Methods: Participants were divided into sarcopenia group and non-sarcopenia group according to the DXA scans and grip strength. Genomic DNA was extracted from plasma and peripheral blood mononuclear cells (PBMCs), and changes in MtDNA copies were quantified using qPCR. Plasma levels of inflammatory cytokines were measured using ELISA kits. Loss of mitochondrial membrane potential (Δψm) in PBMCs was analyzed using the fluorescent probe JC-1.

Results: Participants with sarcopenia were significantly older, more likely to be physically inactive, and had higher levels of circulating cell-free MtDNA (ccf-MtDNA) (all p < 0.05). After adjusting for potential confounders, ccf-MtDNA was independently associated with increased odds of sarcopenia (adjusted odds ratio (AOR), 1.576; p = 0.009). Furthermore, ROC curve analysis showed that ccf-MtDNA had an area under the curve (AUC) of 0.726 (95% CI: 0.607-0.844; p < 0.05) for distinguishing elderly subjects from sarcopenia. Compared with non-sarcopenia subjects, plasma interleukin (IL)-6 and IL-8 were significantly higher in sarcopenia subjects (both p < 0.05). By performing a correlation test, it was found that the level of IL-6 was positively correlated with ccf-MtDNA (r = 0.301; p < 0.05). Then, PBMCs were used as surrogates for mitochondria-rich cells, and the results showed that the relative amplification of MtDNA in PBMCs was significantly reduced (p < 0.05), whereas the depolarization of Δψm was significantly increased in sarcopenia subjects (p < 0.05).

Conclusions: Taken together, our data suggested that circulating MtDNA might be a novel and important source of inflammatory stimuli potentially relevant for sarcopenia in elderly people, and this would provide an attractive therapeutic target to improve this disease.