Background and Objectives A large body of evidence supports the role of microRNAs (miRNAs) in inflammatory diseases. Concerning rheumatoid arthritis (RA), several miRNAs exhibiting deregulated expression pattern were identified in various fluids (serum or synovial) or different cell types (macrophages, synoviocytes). The potential interest of these molecules as biomarkers is increasingly considered, but their precise mode of action still remains largely hypothetical because the identification of their physiological mRNA targets is challenging. To evaluate the overall importance of miRNAS in the pathogenesis of RA, we investigated the expression of genes involved in miRNA biogenesis in FLS isolated from healthy donors and RA patients. We also used a mouse mutant line carrying a hypomorphic mutation in the Dicer gene to monitor the impact of reduced miRNA maturation on experimental arthritis triggered by K/BxN serum transfer.

Materials and Methods Human FLS were isolated from synovial tissues from RA and OA patients and healthy controls after informed consent. Mice FLS were isolated from Dicer d/d and littermates controls. IL-6 release was measured in culture supernatants and following RNA extraction using TRIzol and reverse transcription, real-time quantitative RT-qPCR was performed. Handling of mice and procedures were in accordance with the French Law for the Protection of Laboratory Animals and reviewed by the Regional Ethical Committee for Animal Experimentation (CREMEAS) of the Strasbourg University (authorisation number A-67-345). Serum collected from 9-week old arthritic K/BxN mice was used to trigger arthritis in adult Dicer ^d/d and + / + mice by two successive i.p. injection (150 μL). The articular arthritis index was visually determined on a 0–4 scale (0 = no swelling or erythema, 4 = excessive edema with joint rigidity).

Results Dicer is the only gene involved in the miRNA biogenesis pathway showing significantly reduced expression in FLS isolated from RA patients compared to controls or OA. Similarly, mutant mice (Dicer ^d/d) with reduced Dicer expression exhibit more severe arthritis symptoms upon K/BxN serum transfer.

Conclusions miRNAs are important players to keep pro inflammatory molecules under tight control. Mutations globally affecting mature miRNAs production represent a previously unidentified risk factor for RA.