Clinicopathological and circulating cell‐free DNA profile in myositis associated with anti‐mitochondrial antibody

OBJECTIVE: Anti‐mitochondrial antibodies (AMAs) are associated with idiopathic inflammatory myopathies (IIMs). We aimed to summarize the clinicopathological characteristics, assess circulating cell‐free mitochondrial DNA (ccf‐mtDNA), and circulating cell‐free nuclear DNA (ccf‐nDNA) in AMA‐associated IIMs. METHODS: Medical records of 37 IIMs patients with AMAs were reviewed. Circulating cell‐free mtDNA and ccf‐nDNA levels in sera from IIMs patients with AMAs (n = 21), disease controls (n = 66) and healthy controls (HCs) (n = 23) were measured and compared. Twenty‐eight immune‐mediated necrotizing myopathy (IMNM) patients, 23 dermatomyositis (DM) patients, and 15 anti‐synthetase syndrome (ASS) patients were enrolled as disease controls. Correlations between variables were analyzed. RESULTS: Limb weakness was observed in 75.7% and neck weakness in 56.8% of patients. Cardiac involvement occurred in 51.4% of patients. Muscle pathology revealed 81.1% of IMNM, 5.4% polymyositis, and 13.5% nonspecific myositis. Microinfarction was observed in 8.1% of patients. Serum ccf‐mtDNA levels in AMA‐associated IIMs were significantly higher than those in HCs (p < 0.001), but no significant differences between AMA‐associated IIMs and IMNM, DM, or ASS. Serum ccf‐nDNA levels in AMA‐associated IIMs were significantly higher than those in HCs (p = 0.02), and significantly lower than those in DM (p = 0.02). Serum ccf‐nDNA levels correlated negatively with MMT8 total scores (rs = −0.458, p = 0.037) and positively with mRS scores (rs = 0.486, p = 0.025). Serum ccf‐nDNA levels were significantly higher in the non‐remission group (p < 0.01). INTERPRETATION: AMA‐associated IIMs exhibit distinct clinicopathological features. Serum ccf‐nDNA may serve as a potential marker for disease severity and prognosis in AMA‐associated IIMs.