Over the past decade, biomarkers such as various genes and their SNPs have been analyzed as indicators of susceptibility to MS development (Axisa and Hafler, 2016). Thus, genetic polymorphism studies are required to be performed in different populations with various ethnic roots to assess their significant roles in the probable risk of MS, narrowing the time window until diagnosis and treatment and providing vital insight into the disease etiology, causing the development of early diagnostics and rational therapies to slow the development of the disease and improve the clinical consequences (Axisa and Hafler, 2016). The influential involvement of cytokines throughout the disease course is exhibited by the induction of the blood-brain barrier (BBB) disruption, the migration of innate and adaptive immune cells, and the stimulation of proinflammatory cytokines that are responsible for both the neuroinflammatory cascade in the CNS and the deregulation of anti-inflammatory cytokines (Göbel et al., 2018, Tempest et al., 2022).
The recent study analyzed whether the IL-18-137G/C (rs187238) and the IL-21-1472G/T (rs2055979) SNPs possibly impact the risk of developing MS. In addition, this study examined their associations with the progression and clinical outcomes of the disease, including the severity scores. This research was the first to investigate the aforementioned polymorphisms in Egyptian MS patients.
When comparing the genotype and allele frequencies in MS patients and controls to detect the relationship between IL-18-137G/C (rs187238) SNP and the risk of MS, the GC genotype and C allele were significantly related to the elevated risk of MS. Moreover, when comparing the RR-MS and progressive-MS patients, the GC and CC genotypes and C allele were significantly associated with the higher risk of developing progressive-MS. According to the current results, this SNP could be a significant molecular factor in the disease's susceptibility and progression. The recent findings followed those reported by Karakas Celik et al. (2014), who revealed that Turkish MS patients had a higher frequency of the IL-18 CC genotype and C allele than healthy individuals—in contrast to the results of the present research, Orhan et al. (2016) and JahanbaniArdakani et al. (2019) observed no significant differences in the recurrences of the genotypes and alleles of the IL‐18-137G/C (rs187238) SNP between the MS patients and healthy controls. The discordance between the results of the present study and those of prior studies conducted in different populations might be attributed to various factors comprising the different pathogenesis and molecular mechanisms included in the disease occurrence. Moreover, racial and ethnic background, age differences, and environmental factors involved in the disease pathogenesis may be reasons for different results in different studies. Furthermore, the differences in genotyping tools used for SNP detection and the dissimilarities in sample sizes were involved in additional studies.
The recent results could be elucidated by the fact that the SNPs of cytokine genes, which are linked to the promoter region, do not influence the sequence of protein amino acids but can affect cytokine production, which might cause a modification of the immune response related to the cytokine (Cheng et al., 2014). IL-18 could be involved in MS via its functional polymorphisms. The IL-18-137G/C (rs187238) SNP is located on the IL-18 promoter area, and the transition from G to C at this position disrupts the human histone H4 gene-specific transcription factor-1 nuclear factor binding site (H4TF-1), which leads to the changing of the transcriptional activity of the promoter area and results in a low IL-18 level (Giedraitis et al., 2001). By Gutcher et al. (2006) the role IL-18 and its receptor was examined in the experimental allergic encephalomyelitis (EAE), an ideal model for investigating mechanisms of the disease with possible relevance for MS, and it was noted that IL-18-deficient mice were susceptible to developing EAE. This remark could support the current study findings that the IL18-137C allele was linked to low IL-18 production, which was linked to MS; thus, the IL18-137C allele might carry a risk factor for MS.
IL-18 is a significant proinflammatory cytokine with numerous biological functions responsible for autoimmune mechanisms (Xiao et al., 2018, Niu et al., 2019). IL-18 might contribute to drive T helper 1 (Th-1) and natural killer (NK) cell immune responses by producing interferon-γ (IFN-γ) in the presence of IL-12, and IFN-γ has a noticeable and stage-specific role in the MS pathogenesis(Arellano et al., 2015). Without IL-12, IL-18 modifies to drive Th2 differentiation and produces Th2-type cytokines. Therefore, IL-18 appears to be a balancing factor between Th1 and Th2 and plays a vital role in the modulation of immune status (Lu et al., 2015). In addition, IL-1β is processed by the inflammasome, which is supposed to be involved in the inflammatory response in MS (Keane et al., 2018, Govindarajan et al., 2020).
In the recent study, considering the IL-21-1472G/T (rs2055979) SNP, no significant difference was observed when comparing total MS patients and healthy controls. However, when comparing the diseased forms, the TT genotype and T allele were associated with a reduced risk of progressive MS. Some kinds of literature have examined an association between genetic variations in the IL-21 gene and MS. Fedetz et al. (2009) and Linden et al. (2011) noticed no proof of the association of IL-21 SNPs with MS in the Spanish and Swedish populations, respectively. In addition, Gharibi et al. (2015) revealed that the genotypic and allelic distributions of the IL-21 rs2055979 SNP did not differ between Turkish MS patients and healthy controls; however, they indicated the possible association between this SNP and the disease progression, proposing that the G allele induced or the T allele protected versus the disease progression. These prior findings agree with the findings of this research. Contrary to the results of this study, Ali et al. (2022) observed that the wild-type GG genotype of this SNP could be a predisposing factor for MS. This conflicting data may be due to definite IL-21 genetic polymorphisms amongst specific geographic regions, ethnicities, and populations. The IL-21-1472G/T (rs2055979) SNP is located in a non-translated region of the IL-21 gene (intron 2), and the action of the intronic SNP is not yet fully elucidated. This mutation does not denote an actual risk genetic mutation; on the other hand, it could be a probable substitute indicator for another efficient mutation. Additionally, this SNP may work synergistically with various genetic modifications in the IL-21 gene to magnify the production of IL-21 (Ding et al., 2012, Wang et al., 2014). However, it has been suggested that it could be associated with the controlling of gene expression via regulating the mRNA splicing as well as the translation (Sauna et al., 2011).
Moreover, IL-21 modifies both T cell and B cell responses, which are essential in the immune responses associated with the pathogenesis of several diseases, including MS. (Spolski et al., 2012). Concerning MS, IL-21 has meaningful action in stimulating the differentiation of naive CD4 + T cells into Th17 cells, which play an essential role in the MS pathogenesis, where these cells present at a high rate in active lesions of MS along with activated B cells and monocytes and have the ability to stimulate an inflammatory cascade followed by the axonal destruction, the CNS demyelination, and the development of plaques, destroying neuronal signaling (Dendrou et al., 2015, Choi et al., 2017). It can directly influence the B cell proliferation, responsiveness, the differentiation into plasma cells, and the production of immunoglobulin; therefore, the IL-21 effect on B cells may lead to the development of autoimmune diseases (Xing et al., 2018). Furthermore, synergy with IL-15 or IL-18 can elevate the cytotoxicity of natural killer cells and CD8 + T cells (Gharibi et al., 2016). Additionally, in synergy with TGF-β, IL-21 can stimulate the expression of IL-17 and RORγt in naive T cells and support the Th17 cells differentiation (Messaoudi et al., 2011, Maddur et al., 2012). Thus, it is reasonable to suppose that IL-21 is a possible candidate gene for autoimmune diseases such as MS.
The possible significant role of these SNPs in the phenotype expression of the disease, especially in MS severity, has not been clarified yet. Many baseline features, including the age of onset, MSSS, and EDSS, need to be considered. Thus, the second goal of the current study was to analyze the impact of these SNPs on the clinical outcomes of the disease. For the IL-18-137G/C (rs187238) SNP, the research results revealed that MS cases carrying the GC genotype and C allele exhibited a more significant increase in the mean values of the disease duration, EDSS, and as well as MSSS than those cases carrying both the GG and CC genotypes and the G allele. MS cases carrying the CC genotype and C allele exhibited a significant increase in the mean values of the age of onset compared to those cases carrying GG and GC genotypes and the G allele. This pointed out that the heterozygous GC genotype and C allele are noticeably linked to the disability progression and the disease severity.
Furthermore, multiple regression analysis exhibited that the IL-18-137G/C (rs187238) SNP was a predictor of disease disability according to EDSS values, which strengthened the aforementioned outcomes. Whereas, in the case of IL-21-1472G/T (rs2055979), considering the severity of the disease, no significant association was observed between this SNP and EDSS and MSSS, and multiple regression analysis supported these results. Contrary to the recent results, Gharibi et al. (2015) revealed no significant association between the age of onset of MS and this SNP; nevertheless, in agreement with the present results, they did not observe any significant association between other clinical parameters like EDSS and the duration of disease and the IL-21-1472G/T (rs2055979) SNP in the Iranian population. However, Gharibi et al. (2019) observed a positive correlation between EDSS scores and IL-21 mRNA levels.
In conclusion, this is the first literature in Egypt examining the IL-18-137G/C (rs187238) and the IL-21-1472G/T (rs2055979) SNPs and MS susceptibility and possible interactions of these SNPs with clinical profiles of MS patients. This study suggests that the IL-18 -137GC genotype and C allele might be a risk factor for MS susceptibility, disability progression, and as well as disease severity. On the contrary, the TT genotype T allele of the IL-21 1472G/T SNP could be linked to decreased risk of progressive MS in the Egyptian population. Although the importance of these SNPs in MS was analyzed, this study has some limitations. First, the associations might be initiated by synergistic interactions with other SNPs in the same genes or other candidate genes. Second, these findings must be established in other populations with a larger sample size. Finally, the IL18 and IL-21 genes' possible active mechanisms must be clarified more.