OP0297 ABERRANT ADENOSINE TO INOSINE RNA EDITING IN ACTIVE RHEUMATOID ARTHRITIS
Authors: Nikolaos Vlachogiannis, Aikaterini Gatsiou, Kimon Stamatelopoulos, Maria Tektonidou, Petros Sfikakis, Konstantinos Stellos
Journal: Annals of the Rheumatic Diseases (2019)

<h3>Background</h3> Adenosine to inosine (A-to-I) RNA editing is a widespread post-transcriptional RNA modification mainly located in repetitive <i>Alu</i> elements and mediated by the enzyme adenosine deaminase acting on RNA-1 (ADAR1). A-to-I RNA editing controls various aspects of RNA metabolism, which may affect tissue-specific gene expression. Although deregulation of RNA editing has been previously reported in various human diseases including cardiovascular disease and cancer (Stellos et al., <i>Nat Med</i>, 2017; Liu et al., <i>Nat Med</i>, 2019 and Ishizuka et al., <i>Nature</i>, 2019), its role in autoimmune diseases and especially in rheumatoid arthritis (RA) remains unknown. <h3>Objectives</h3> To study whether A-to-I RNA editing is involved in the pathogenesis of RA and to determine the impact of anti-rheumatic treatment on RNA editing. <h3>Methods</h3> We first analysed the expression of ADAR1 in 185 RA synovial tissues versus 76 healthy/osteoarthritic synovia derived from 4 independent RNA-sequencing and microarray datasets. We validated the findings in peripheral blood mononuclear cells (PBMCs) derived from 19 patients with active RA vs 14 controls and performed an additional ADAR1-isoform analysis (ADAR1p110/ADAR1p150) by RT-qPCR. Further, we studied in single nucleotide level the A-to-I RNA editing levels of the pro-inflammatory gene cathepsin S (CTSS) 3'-untranslated region (3'UTR), a matrix degradation enzyme which is a well-established target of ADAR1, by<i> Alu</i> Sanger sequencing and RNA editing analysis. Last, we examined the effect of anti-rheumatic treatment on RNA editing. <h3>Results</h3> Expression of the RNA editor ADAR1 was significantly increased in RA synovium compared to healthy or osteoarthritic synovium. Similarly, a significant increase of ADAR1, mainly due to an increase of the interferon-inducible ADAR1p150 isoform, was observed in PBMCs from active RA. Next, we studied the RNA editing levels in PBMCs from active RA patients before and after 12-week treatment versus controls. RNA editing of CTSS 3'UTR <i>AluSx<sup>+</sup></i> was increased in active RA (6-47% increase in editing rate of 8 individual adenosines, all P&lt;0.05). Increased CTSS mRNA expression in RA PBMCs was associated with both ADAR1p150 expression (r=0.623, P=0.004) and RNA editing rate of 12 individual adenosines (r=range 0.45-0.72, P&lt;0.05 for all) located within the CTSS 3' UTR <i>AluSx<sup>+</sup></i>. The correlation between CTSS and ADAR1 was also observed in synovial tissue. Notably, ADAR1p150 expression and RNA editing rate reached control levels after 12-week treatment with methotrexate ± corticosteroids and/or biologics in patients with good clinical response (EULAR responders) but remained unchanged in the EULAR moderate/non-responders. <h3>Conclusion</h3> ADAR1-mediated RNA editing is increased in active RA inducing the expression of pro-inflammatory genes thus representing a novel drug response biomarker and a potential therapeutic target in EULAR moderate/non-responders. <h3>References</h3> [1] Stellos, et al., Nat Med 2016;22:1140–50. doi:10.1038/nm.4172 [2] Liu, et al., Nat Med 2019;25:95–102. doi:10.1038/s41591-018-0302-5 [3] Ishizuka, et al., Nature 2019;565:43–8. doi:10.1038/s41586-018-0768-9 <h3>Disclosure of Interests</h3> None declared