Mosaic loss of chromosome Y characterizes late-onset rheumatoid arthritis and contrasting associations of polygenic risk score based on age at onset.

Objectives: mosaic chromosomal alterations (mCAs) increase with age and are associated with age-related diseases. The association between mCAs and rheumatoid arthritis (RA), particularly late-onset rheumatoid arthritis (LORA), has not been explored. Methods: mCAs were detected in peripheral blood samples from two independent Japanese datasets (Set 1:2,107 RA cases and 86,998 controls; Set 2:2,359 RA cases and 86,998 controls). The associations between mCAs and RA were evaluated in each dataset using logistic regression models and meta-analysis. In each dataset, we evaluated the effect sizes of mosaic loss of Y (mLOY) and polygenic risk score (PRS) of RA in males and subsequently performed a meta-analysis. The interaction between mLOY and PRS was assessed. These models were applied separately to RA, LORA, and young-onset RA (YORA). Results: mLOY increased significantly in LORA (OR=1.43, P=0.0070). We observed a negative association between mLOY and YORA (OR=0.66, P=0.0034). On the other hand, we found consistently negative associations of autosomal mCAs or mosaic loss of X (mLOX) with RA, LORA, and YORA. The effect sizes of PRS were lower for LORA than for YORA. mLOY with a high cell fraction strengthened the association between PRS and LORA (P=0.0036), whereas the association with YORA was independent of mLOY. Conclusion: LORA was characterised by the presence of a high burden of mLOY. The observed interaction between mLOY and PRS in LORA, but not in YORA, supports different gene-environment interactions between the subsets. These data suggest that distinct pathophysiological mechanisms underlie the development of LORA and YORA.