Digital PCR-Based Assessment of Piwi-RNA PIR35982 as a Supporting Biomarker in Rheumatoid Arthritis

Rheumatoid arthritis (RA) is a chronic autoimmune inflammatory disorder with an etiology that remains incompletely understood. Its development is influenced by both environmental and molecular factors, with growing attention to epigenetic mechanisms. Among these, PIWI-interacting RNAs (PIRs) have emerged as molecules of interest. PIRs are small, single-stranded noncoding RNAs, and play a crucial role in maintaining genome stability in germline cells; however, they were also found in somatic cells. The aim of this study was to identify PIRs whose expression may be associated with the severity of RA. A total of 57 individuals were enrolled in the study, including 37 patients with RA and 20 healthy controls (HCs). Quantitative real-time polymerase chain reaction (qPCR) and digital polymerase chain reaction (dPCR) were used to evaluate the plasma concentrations and peripheral blood mononuclear cells (PBMCs) expression levels of selected PIRs. In PBMCs, PIR27124 showed elevated expression in RA compared to HCs (p=0.04). Additionally, both PIR27731 and PIR35982 showed increased expression in rheumatoid factor (RF)-negative RA patients relative to RF-positive patients (p-values 0.045 for both). In plasma, PIR35982 showed a decreased concentration in RA compared to HCs (p=0.001). Furthermore, both qPCR and dPCR confirmed that RF-negative RA patients had decreased plasma levels of PIR35982 compared to HCs (p=0.01 and p=0.02, respectively). PIR35982 can be considered a potential molecular marker associated with RA and may be useful for identifying RF-negative patients. Moreover, dPCR represents a promising tool for the discovery and evaluation of novel biomarkers, such as PIRs.