Association of FCGR3A and FCGR3B Copy Number Variations With Systemic Lupus Erythematosus and Rheumatoid Arthritis in Taiwanese Patients

IgG Fcγ receptors (FcγRs) mediate a variety of immune functions that are critical in immune responses. In humans, 5 classic low-affinity FcγRs (FcγRIIA, FcγRIIB, FcγRIIC, FcγRIIIA, and FcγRIIIB) are coded by 5 genes (FCGR2A, FCGR2B, FCGR2C, FCGR3A, and FCGR3B, respectively) in the FCGR cluster on chromosome 1. Activating FcγRs (FcγRIIA, FcγRIIC, FcγRIIIA, and FcγRIIIB) mediate immune cell activation promoting inflammation, while the classic inhibitory FcγRIIB dampens immune responses and restricts inflammation (1,2). Activating FcγRs mediate functions including immune complex clearance, phagocytosis, antigen presentation, antibody-dependent cellular cytotoxicity, and cytokine production (3). In contrast, the inhibitory FcγRIIB abrogates immune cell activation. FcγRIIB also plays a role in the maintenance of peripheral B cell tolerance and the prevention of autoimmunity (2). The variations in FcγR expression significantly affect IgG immune complex–mediated signal thresholds (3,4). Notably, proinflammatory and antiinflammatory cytokines could modulate the expression levels of activating and inhibitory FcγRs (5) that affect the threshold immune cell response to IgG immune complexes (6). FcγR-knockout mouse models indicate that both activating and inhibitory FcγRs influence the development of autoimmune diseases (7–9). The contributions of FcγR genes to autoimmune diseases have attracted substantial attention, and functional FcγR polymorphisms have been reported to play important roles in the pathogenesis of autoimmune diseases (4,10,11). Gene copy number variation (CNV) is a rich source of genetic heterogeneity (12,13). The FCGR cluster on chromosome 1q23 shows a complex pattern of CNVs. Among 5 FcγR genes in the cluster, 3 genes (FCGR3A, FCGR2C, and FCGR3B) have CNVs, while FCGR2A and FCGR2B do not have CNVs (14,15). CNVs play important roles in human disease pathogenesis (16). FCGR3B copy number deficiency is associated with autoimmune disease, including systemic lupus erythematosus (SLE) (17–19), Sjogren's syndrome (20), and systemic sclerosis (21). Although FCGR3B CNVs were reported to associate with rheumatoid arthritis (RA) (22–24), no association between RA and FCGR3B CNVs was observed in other studies (25,26). Moreover, no association between FCGR3A CNVs and RA was observed (24,27). In the current study, we investigated whether FCGR3A and FCGR3B CNVs are associated with susceptibility to SLE and RA in Taiwanese individuals. The results provide new insights into the role of FcγRIII family members in the pathogenesis of SLE and RA in an Asian population.