FCGR3B Gene Variants In Brazilians: New Alleles Should Be Considered?

FCGR3B gene encoding the receptor FcγRIIIb has three polymorphic forms (FCGR3B*01, FCGR3B*02 and FCGR3B*03) which represent the human neutrophil antigens HNA-1a, -1b and -1c, respectively. It has been postulated that variants of these alleles might represent new FCGR3B alleles, however there are few studies assessing the frequency of such variants in different populations. In this study we performed an analysis of the frequency of the FCGR3B alleles and its variant forms examining the polymorphic region of the FCGR3B gene in a large Brazilian population sample including healthy blood donors and patients with systemic lupus erythematosus (SLE).

A segment containing the polymorphic nucleotide positions 141, 147, 227, 266 and 277 in exon 3 was sequenced from genomic DNA of 615 individuals including 310 blood donors and 305 patients with SLE. Additionally, nucleotide databases were screened for variant FCGR3B sequences.

The allele frequencies for FCGR3B*01, FCGR3B*02 and FCGR3B*03 were 0.37, 0.59 and 0.04, respectively. In two patients with SLE (0.3%) there was no gene amplification characterizing the FCGR3B-null phenotype. Considering the 5 FCGR3B polymorphic sites, 4 variant forms were observed. One variant was linked to the FCGR3B*01 allele with SNPs occurring simultaneously in two positions: A227G and G277A (n=2; 0.3%). The other three variants, linked to the FCGR3B*02 allele were: A277G (n=8; 1.3%); C141G (n=3; 0.5%); and T147C (n=1; 0.2%). All mutations observed were missenses. The results are summarized in the Table. In 103/613 (16.8%) sequenced individuals we found 19 SNPs on 14 polymorphic sites distinct from those which characterize the FCGR3B alleles. Among the most frequent, T230A was observed in 36 cases (5.9%), T230G in 20 cases (3.3%), G249A in 6 cases (1%) and C337A in 6 cases (1%), all shown as heterozygous. The comparative analysis between the two groups of individuals revealed an association between the presence of the allele FCGR3B*01 and the HNA-1a/a and HNA-1a/b genotypes with increased susceptibility to SLE (p <0.001; p = 0.028; and p = 0.012, respectively), however there was no significant correlation between these genotypes and disease severity.

The observed frequencies of the FCGR3B*01, FCGR3B*02, and FCGR3B*03 alleles were similar to those reported by genotyping studies which included Europeans or North Americans individuals. The frequency of the null phenotype (0.3%) was similar to that reported by Europeans (0.2-0.8%). The variant FCGR3B*02A277G showed a frequency (1.3%) sufficiently high to be considered a new allele as proposed by the last Granulocyte Working Party/ISBT 2013. Interestingly, this variant was significantly more prevalent in patients with SLE than in blood donors (p=0.03). All the variants described in this study have been observed in different populations however we emphasize the presence of SNPs (T230A, T230G, G249A and C337A) in polymorphic sites different from those that characterize the FCGR3B gene alleles. We conclude that the FCGR3B gene is highly polymorphic and that the occurrence of new alleles must be considered, however the functional consequences of such changes has not yet been elucidated.

No relevant conflicts of interest to declare.