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To determine whether specific isoforms of IRF5 are transcribed in patients with systemic lupus erythematosus (SLE) who have risk genotypes in the exon 1B donor splice site at single-nucleotide polymorphism (SNP) no. rs2004640.
Peripheral blood mononuclear cells were obtained from SLE patients and healthy controls from Argentina, Spain, and Germany and from trio families from Spain and Denmark. A reporter assay was used to investigate the role of SNP no. rs2004640. IRF5 expression in relation to the genotypes of functional SNPs was analyzed using quantitative polymerase chain reaction. Sequencing and genotyping of the IRF5 gene was performed.
Sequencing of complementary DNA from individuals with different genotypes showed 4 basic isoforms transcribed from all 5′-untranslated regions (5′-UTRs), suggesting no preferential isoform transcription based on rs2004640 genotypes. Analysis of translation efficiency showed that exon 1A was the most efficient in initiating protein synthesis. We identified a novel polymorphic insertion/deletion that defines the pattern of expression of isoforms of IRF5. The insertion consists of 4 repeats in exon 6 affecting the protein interaction domain. The insertion segregates in the risk haplotype with the high expression allele of a poly(A) site SNP no. rs10954213 and the exon 1B donor splice allele of the 5′-UTR SNP no. rs2004640. The poly(A) polymorphism correlated with levels of IRF5 in cells stimulated with interferon-α. The SNP most strongly associated with SLE was SNP no. rs2070197 (P = 5.2 × 10−11), which is a proxy of the risk haplotype, but does not appear to be functional.
None of the functional variants investigated in this study is strongly associated with SLE, with the exception of the exon 1B donor splice site, and its functional importance appears to be small. Our results suggest that there may be other functional polymorphisms, yet to be identified, in IRF5. We did not observe evidence of epistatic interaction between the functional SNPs.