Rheumatoid arthritis (RA) is an inflammatory autoimmune disease, characterized by chronic synovial hyperplasia associated with infiltration of immune cells and soluble mediators that amplify and perpetuate inflammation. The small and large peripheral joints are the primary target, and joint degeneration eventually occurs (Jang et al., 2022). The global prevalence of RA has been estimated at 460 per 100,000 population during the period 1980–2019, and women are three times more likely to develop the disease than men, particularly during the age of 30–50 years (Almutairi et al., 2021). In 2019, a lower prevalence of RA has been reported in Iraq (101.2 per 100,000 population), but women are also at a higher risk than men (Mousavi et al., 2022).

Although the pathogenesis is not well defined, dysregulated immune function is a recognized pathophysiologic feature of RA in which immune cells, such as CD4 + T cells, B cells, and macrophages, are functionally abnormal (Wu et al., 2021). Moreover, a complex network of different cytokines is implicated in the pathogenesis of RA and has a role in triggering synovial cell proliferation and mediating cartilage and bone damage (Kondo et al., 2021). Cytokines are glycoprotein molecules that participate in cellular signaling to coordinate immune functions and play a critical role in regulating inflammatory reactions through pro-inflammatory and anti-inflammatory effects (Liu et al., 2021). Animal model studies and human observational studies have indicated a significant role for a number of pro-inflammatory and anti-inflammatory cytokines in the pathogenesis of RA such as interleukin (IL)− 1, IL-4, IL-10, IL-12, IL-17, interferons (IFNs), transforming growth factor (TGF)-β, tumor necrosis factor (TNF)-α (Chen et al., 2019, Kondo et al., 2021). Moreover, there is compelling evidence that these cytokines are associated with RA activity and severity and may be considered as biomarkers in monitoring disease outcome as well as potential therapeutic targets (Skrzypkowska et al., 2022). In addition, genome-wide association studies (GWASs) have revealed that cytokine gene polymorphisms (i.e. single nucleotide polymorphisms; SNPs) are associated with susceptibility to RA in different ethnic groups; some are associated with an increased risk of disease and others with a decreased risk of disease (Qian et al., 2022, Yucel, 2020).

Among the cytokines that have recently attracted attention in RA is IL-40. Four studies have reported up-regulated levels of IL-40 in RA patients and proposed an etiological role for this cytokine in the pathophysiology of the disease (Al Ghuraibawi et al., 2022, Navrátilová et al., 2023, Navrátilová et al., 2022, Navrátilová et al., 2021). In addition, elevated serum levels of IL-40 have recently been demonstrated in another rheumatic disease, ankylosing spondylitis (Jaber and Ad’hiah, 2023) and a low-grade inflammatory disease, type 2 diabetes mellitus (T2DM) (Nussrat and Ad’hiah, 2023). IL-40 is a novel cytokine discovered in October 2017. In IL-40-deficient mice, immunoglobulin A (IgA) levels were significantly decreased and associated with a decreased number of IgA+ B cells. Therefore, its role in controlling the immune response and inflammatory reactions, particularly in relation to B cells, has been suggested (Catalan-Dibene et al., 2017). IL-40 is a low molecular weight protein (27 kDa) expressed by activated B cells, bone marrow cells, and fetal hepatocytes. It is encoded by the chromosome 17 open reading frame 99 (C17orf99), a gene mapped to human chromosome 17 (17q25.3). IL-40 is considered an orphan cytokine because it shows no structural homology to known cytokine families (Catalan-Dibene et al., 2018). The C17orf99 gene is rich in SNPs and harbors approximately 65 SNPs with a minor allele frequency of ≥ 10% (http://www.ensembl.org). To date, none of these SNPs has been investigated for association with risk of any human disease. Some cytokine SNPs have also been proposed to control serum cytokine levels (Qian et al., 2022); therefore, it may be reasonable to hypothesize that altered serum levels of IL-40 in RA patients may be affected by C17orf99 SNPs, which may also influence RA risk. Candidate SNPs in this regard are those located in intergenic regions, where not only do SNPs affect gene expression, but approximately half of disease-associated SNPs are found within these regions (Li et al., 2016).

In this preliminary study, two intergenic SNPs of the C17orf99 gene (rs2004339 A/G and rs2310998 G/A) were analyzed for the first time in a cohort of Iraqi women with RA with the aim of determining their role in susceptibility to disease. In addition, serum IL-40 levels were also analyzed and evaluated for their association with SNP genotypes and clinical indicators of RA.