Reduced Expression of m6A Demethylases FTO and ALKBH5 in Monocytes from the Site of Inflammation in Patients with Juvenile Idiopathic Arthritis.

N6-methyladenosine (m6A) has recently emerged as a post-transcriptional modulator governing cell-specific gene expression in innate immune cells, particularly in monocytes. Disruptions in m6A homeostasis, manifested as the altered expression of m6A-related proteins and m6A levels, have been implicated in autoimmune disorders. Perturbations in m6A dynamics within total Peripheral blood mononuclear cells (PBMCs) have shown strong correlations with disease severity in rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). It remains unclear in which specific cell type(s) m6A homeostasis is disturbed, and also whether other rheumatic diseases such as juvenile idiopathic arthritis (JIA) show similar features. Here, we assess the involvement of m6A and m6A-regulatory proteins in JIA monocytes. Notably, the diminished expression of m6A-eraser proteins FTO and ALKBH5 was observed in JIA monocytes extracted from the inflamed joint. This resulted in increased m6A-methylated transcripts in monocytes from these patients. Correspondingly, we observed that culturing monocytes in the presence of synovial fluid from JIA inflamed joints reduced the expression of both FTO and ALKBH5. The knock-out of FTO in human monocytes of healthy controls increased monocyte activation, indicating the relevance of FTO and m6A in the context of JIA. These findings underscore the potential of ALKBH5 and FTO expression as a biomarker in JIA and identify the m6A machinery as a potential therapeutic target for the treatment of JIA and possibly other autoimmune diseases in the future.