An arginine residue (R409) in the CH3 domain name of IgG4, unique to the IgG4 subclass, is necessary for Fab-arm exchange(4). the putative immunoregulatory properties of this isotype(3). Fab-arm exchange is usually facilitated by residues in the hinge region as well as in the CH3 domain name of IgG4 that are unique to this subclass (4). An arginine residue (R409) in the CH3 domain name of IgG4, unique to the IgG4 subclass, is necessary for Fab-arm exchange(4). Analysis of the crystal structure of the CH3 domain name of IgG4 suggests that this arginine residue (R409) facilitates Fab-arm exchange by preventing the proper formation of an inter-chain hydrogen bond network that is generated in other IgG isotypes which all have a lysine in position 409 (5). A single nucleotide polymorphism (SNP) in the CH3 exon of IgG4 results in a non-synonymous switch in codon 409 (AGG AAG) and alters the R409 residue, which is critical for Fab-arm exchange. This SNP was previously recognized as an isoallotypic variant of IgG4 in which a lysine (K409) Griseofulvin is present in place of arginine (R409) in the CH3 domain name of IgG4(6). The K409 variant of IgG4 resembles IgG1, Griseofulvin IgG2 and IgG3, which also encode a lysine at this position (Physique 1), and do not undergo Fab-arm exchange(5). Serum concentrations of IgG4 correlate with certain IgG allotypes, some of which are genetically linked to the K409 variant of IgG4 (7). It has been speculated that this K409 variant of IgG4 might be enriched in IgG4-RD subjects and that this polymorphic variant could contribute to the Griseofulvin pathogenesis of IgG4-related disease (IgG4-RD), a fibroinflammatory disorder of possible autoimmune etiology, which is usually characterized by elevations in circulating IgG4 levels as well as an growth of IgG4+ plasma cells in the affected tissues (8, 9). Although considerable genetic studies in IgG4RD have not yet been reported, it remains likely that IgG4-RD is usually caused by environmental triggers in a genetically susceptible background. Open in a separate window Physique 1 Single nucleotide polymorphisms in the CH3 exon of IgG4SNP’s (strong) in the CH3 exon of IgG4 are depicted in an alignment of the CH3 exons of IgG1, IgG2, IgG3, IgG4 and IgGP. The amino acid translation of the IgG4 CH3 exon is usually shown below the alignment. The positions of the primers utilized for specific amplification of the CH3 exon of IgG4 are indicated by arrows. The minor allele of rs77498506 is usually identical to the K409 MMP8 variant of IgG4 and results in a non-synonymous substitution from arginine to lysine (AGG AAG). The codon encoding arginine (R409) is usually underlined. We have evaluated the occurrence of the K409 variant of IgG4 in a cohort of 25 subjects with IgG4-RD who offered to the rheumatology medical center at the Massachusetts General Hospital. All patients signed written informed consent for the investigations explained. All experienced biopsy-proven IgG4-RD affecting one or more of the following organs: pancreas, lacrimal gland, submandibular gland, parotid gland, biliary tree, retroperitoneum, kidney (tubulointerstitial nephritis), lymph node, lung, mediastinum, aorta, common carotid artery, palate, pharynx, larynx, lymph node, and skin. Nineteen patients self-identified as White, 3 as Asian, and 2 as Black. One patient declined to provide information about race. Due to the high degree of nucleotide sequence conservation among the constant regions encoding IgG subclasses and the IgGP pseudogene, the K409 variant of IgG4 is not included in most high-throughput genotyping panels (Physique 1). We therefore designed primers for the specific amplification of the CH3 exon of IgG4 (5-CAACAAAGGCCTCCCGTCCT-3 and 5-GGGGCTTGCCGGCCCTG-3). PCR was performed for 35 cycles at a Tm of 67C using the KAPA2G HotStart ReadyMix from KapaBiosystems using 50-100 ng of genomic DNA as a template. The PCR products were sequenced using the Sanger method. The producing sequences included several bases that were specific to IgG4, which were used to confirm that this amplified sequences were indeed IgG4. The amplified region includes 5 single nucleotide polymorphisms (SNPs): rs56133431, rs8010914, rs77498506, rs17841088, and rs201617483 (Physique 1). The K409 variant of IgG4 corresponds to the minor allele (T) of the rs77498506 SNP. The rest of the SNPs encode synonymous substitutions. The SNP frequencies in the CH3 exon of IgG4 observed in the subjects with IgG4-RD are summarized in Table 1. The SNP frequencies in control populations from dbSNP are also outlined for comparison. We did not identify any subject with the K409 variant of IgG4 in our study cohort. These data suggest that this variant is not a major contributor to disease susceptibility in IgG4-RD. Statistical analyses have limitations when the frequency of an allele is usually 0 in a population, and it is affordable to presume that the frequency of the K409 polymorphism is in the same.
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