Many of the FcγR-encoding genes show variation in SNPs, which may determine the IgG binding characteristics of the various FcγRs. The impact of genetic variation is not known for all receptors, but some functional FcγR polymorphisms have been characterized (Fig. 4, reviewed in [38]). check details The best-known SNP variant is R131H in the FcγRIIa, whereby an arginine at position 131 changes to histidine, which facilitates binding to IgG2 and enables phagocytosis of IgG2-coated particles. Homozygous carriers of arginine at this position may experience

increased risk of infection, whereas those homozygous for histidine may be at higher risk for autoimmune disorders. A SNP (I232T) in the transmembrane area of the inhibitory FcγRIIb may impact the receptor’s inhibitory activity. FcγRIIIa may express either a valine or a phenylalanine at position 158 (V158F). The V158 allotype has a higher affinity for IgG1 and IgG3 subclasses compared to 158F. In another example, the human neutrophil antigen (NA) is present on FcγRIIIb and expresses two allotypes (NA1 and NA2) which impact receptor binding. NA1 shows higher binding and phagocytosis of IgG1- and IgG3-coated particles and higher affinity for IgG3 in comparison to the NA2 allotype. In addition to SNPs, copy number variation (CNV) is now also being recognized as an important factor of variation. Gene dosage effects may click here occur as a functional consequence of CNV. Recently, an association between

a low copy number of FCGR3B and glomerulonephritis in systemic Tacrolimus (FK506) lupus erythematosus (SLE) has been reported [33,34]. The low gene copy number correlates with reduced FcγRIIIb expression and is likely to contribute to the impaired clearance of immune complexes, a feature of SLE [33]. Recent studies identifying CNV in the human genome suggest that large areas at chromosome 1q23–24 exhibit a high degree of variation in gene copy number [39]. Indeed, FCGR3A, FCGR2C and FCGR3B show CNV

at variable degrees of co-segregation, while FCGR2A and FCGR2B do not show CNV [36,37,40,41]. CNV may thus be an indicator for interindividual differences, including differential responsiveness to infection or predisposition to autoimmune disease as a result of unbalanced immunity [34]. The Multiplex Ligation-dependent Probe Amplification (MLPA) method was used to study FCGRs in a cohort of patients with idiopathic thrombocytopenic purpura (ITP) versus a control group of healthy volunteers [35]. Both control and ITP groups showed no variation in FCGR2A and FCGR2B. MLPA showed that FCGR2C, FCGR3A and FCGR3B CNV are present in the normal population. CNV was not associated with susceptibility to ITP in this cohort. A stop codon in exon 3 of FCGR2C suggests that it is a pseudogene (Table 4). A SNP at this site changes the region to an open reading frame (ORF). In healthy volunteers, STOP allele frequency was found to be 91·2% of all alleles and ORF frequency was 8·8%.