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ARL15 is a novel susceptibility gene identified in a recent GWAS in a north Indian rheumatoid arthritis (RA) cohort. However, the role of ARL15 or ARF family genes in RA aetiology remains unknown. Therefore, we aimed to i) establish the expression of ARL15 in rheumatoid arthritis synovial fibroblasts (RASF) and ii) its functional characterisation by assessing its effects on major inflammatory cytokines and interacting partners using a knockdown approach.RASF were cultured from synovial tissue obtained from RA patients (n=5) and osteoarthritis (OA) patients (n=3) serving as controls. Expression of ARL15, ARF1 and ARF6 in RASF was checked by semi-quantitative PCR and western blots; and altered expression of ARL15, if any, by induction of RASF with TNF using real-time PCR. The effect of ARL15 on the expression of adiponectin, adiponectin receptor I, IL6 and GAPDH and on cell mobility by invasion and migration assays were assessed by siRNA mediated gene knockdown.Expression of ARL15, ARF1 and ARF6 was confirmed in RASF and OASF samples but ARL15 expression remained unaltered on TNF induction. Notably, ARL15 knockdown resulted in downregulation of IL6 and GAPDH, upregulation of adiponectin and adiponectin receptor I genes; and significant reduction in migration and invasion of RASF. Genemania showed significant interactions of ARL15 with genes responsible for insulin resistance and phospholipase D.This first report on ARL15 expression in RASF and its likely role in inflammation and metabolic syndromes through a TNF independent pathway, encourages hypothesis-free studies to identify additional pathways underlying RA disease biology.

Genome-wide association studies (GWAS) and their subsequent meta-analyses have changed the landscape of genetics in rheumatoid arthritis (RA) by uncovering several novel genes. Such studies are heavily weighted by samples from Caucasian populations, but they explain only a small proportion of total heritability. Our previous studies in genetically distinct North Indian RA cohorts have demonstrated apparent allelic/genetic heterogeneity between North Indian and Western populations, warranting GWAS in non-European populations. We undertook this study to detect additional disease-associated loci that may be collectively important in the presence or absence of genes with a major effect.High-quality genotypes for600,000 single-nucleotide polymorphisms (SNPs) in 706 RA patients and 761 controls from North India were generated in the discovery stage. Twelve SNPs showing suggestive association (P5 × 10(-5)) were then tested in an independent cohort of 927 RA patients and 1,148 controls. Additional disease-associated loci were determined using support vector machine (SVM) analyses. Fine-mapping of novel loci was performed by using imputation.In addition to the expected association of the HLA locus with RA, we identified association with a novel intronic SNP of ARL15 (rs255758) on chromosome 5 (Pcombined = 6.57 × 10(-6); odds ratio 1.42). Genotype-phenotype correlation by assaying adiponectin levels demonstrated the functional significance of this novel gene in disease pathogenesis. SVM analysis confirmed this association along with that of a few more replication stage genes.In this first GWAS of RA among North Indians, ARL15 emerged as a novel genetic risk factor in addition to the classic HLA locus, which suggests that population-specific genetic loci as well as those shared between Asian and European populations contribute to RA etiology. Furthermore, our study reveals the potential of machine learning methods in unraveling gene-gene interactions using GWAS data.