Lack of Association Between MIF Gene -173G>C Polymorphism with Multiple Sclerosis

Discussion

Although the etiopathology of MS remains unknown, it is considered to trigger an inflammatory cascade resulting in axonal and neuronal degeneration in CNS induced by myelin-reactive CD4⁺ T-lymphocytes crossing the blood-brain barrier after being activated in peripheral tissues (2, 9). The initial phase of the disease includes adaptive immunity and CD4⁺ T-helper (Th)1, Th-17 and CD8⁺ T-cells, all of which are modulated by T-regulator, CD4⁺ Th2, B cells, macrophages, microglia and natural killer cells. On the other hand, disease progression includes the innate immunity (22). MIF is a pleiotropic pro-inflammatory cytokine involved in the regulation of both innate and adaptive immunity (23). It is mainly secreted by activated T-lymphocytes and monocyte/macrophages, up-regulates the pro-inflammatory actions of these cells, and stimulates them to produce a wide range of pro-inflammatory molecules that play a vital role in the MS pathogenesis, including interleukin (IL)-1beta, IL-6, IL-8, interferon-gamma, tumor necrosis factor-alfa, prostaglandin E2, nitric oxide, cytosolic phospholipase A2 and cyclooxygenase-2 (23-26). MIF inhibits the random migration of macrophages, concentrating macrophages at the inflammatory site and it is thought to play an important role in cell-mediated immunity. In addition, MIF enhances Toll-like receptor-4 expression on the macrophage surface, increases phagocytosis and intracellular killing. It up-regulates the expression of intracellular adhesion molecule-1 and vascular cell adhesion molecule-1, and matrix metalloproteinases, leading to enhanced blood-brain barrier permeability (10, 23, 24, 27). Thus, MIF induces the accumulation of neuroantigen-reactive T-cells and other inflammatory cells in the CNS, facilitating the development of EAE and MS. MIF reduces the population of regulatory T-cells, which suppresses the immune functions of other cells, allowing inflammation in the context of MS in both the brain and spinal cord (4, 10). Furthermore, it inhibits p53-dependent apoptosis of lymphocytes and macrophages. A failure of autoreactive T-cells to undergo apoptosis due to higher levels of MIF expression may lead to inappropriate persistence of these cells and cause harmful immunoreactivity in MS (8, 10). MIF also stimulates the antibody production in B-cells, which have been demonstrated to exhibit clonal enlargement (22). As a neuro-endocrine mediator, MIF is secreted by anterior pituitary cells and counter-regulates the immunosuppressive and anti-inflammatory actions of glucocorticoids. Hence, genetic predisposition to higher MIF production may increase the magnitude, duration and possibly the chronicity of immune and inflammatory responses (5, 27, 28).

MIF is necessary for the progression of EAE but is not required for initial activation of auto-reactive lymphocytes. It has been demonstrated that the detection of infiltrating monocytes in the CNS of EAE mice is correlated with substantial clinical disability (10, 29). MIF synthesis is known to be significantly increased in active MS plaques constituted mainly by T lymphocytes, macrophages and microglia (9, 10). To further implicate a role for MIF in MS, Denkinger et al. (4) have demonstrated that anti-MIF treatment in mice with acute EAE decreases accumulation of encephalitogenic T-cells and other inflammatory cells into the CNS and expands a population of regulatory T lymphocytes, reducing the severity of clinical signs and accelerating recovery from the disease. It has also been suggested that inhibition of MIF reduces relapses of disease in a relapsing/remitting model of EAE (10). MIF levels have been found to increase in the CSF and sera/blood of patients with active/relapsed multiple sclerosis and have therefore been associated with the disease activity (7, 8, 30, 31). Measurement of MIF levels in healthy subjects and in patients with auto-immune inflammatory diseases, such as rheumatoid arthritis and juvenile idiopathic arthritis showed that serum and tissue MIF levels are significantly higher in MIF gene -173*C allele carriers than in MIF -173GG homozygous subjects. Increasing data from literature have linked MIF gene -173G>C polymorphism with susceptibility to or severity and outcome of inflammatory and/or autoimmune diseases, in which increased systemic or local MIF concentrations have been associated with severe clinical manifestations, high severity scores, and often poor outcome (6, 12, 28). These findings suggest that MIF may be a common risk gene for a number of autoimmune disorders. Therefore, we assumed that the MIF gene -173G>C polymorphism may be also associated with MS. To validate the hypotheses, we examined the association of MIF - 173G>C polymorphism with MS and found that allele and genotype frequencies of this polymorphism do not show any statistically significant difference between MS patients and controls. There was no significant association observed when the patients were compared with the controls according to GG genotype versus GC+CC genotypes and GG+GC versus CC genotypes (Table III). Also, there as no statistically significant association between MIF -173G>C polymorphism and baseline clinical and demographic characteristics of MS patients (Table II). Our findings conflict with the previous data reported by Akcali et al. (32). In that study including 120 RRMS patients and 120 control subjects from Turkish population, MIF-173 CC and GG genotypes were found to be statistically higher and lower in MS patients, respectively (p<0.001 and p=0.004, respectively). Moreover, patients with the MIF-173 CC genotype had a significantly lower age of disease onset compared to those with the MIF-173 CG and MIF -173GG genotypes. In the study of Akcali et al. on MIF-173 gene polymorphism frequencies, GG genotype was 37% vs. 55%, GC genotype was 43% vs. 41%, and CC genotype was 20% vs. 4.2% in MS patients vs. control subjects, respectively. In the present study, the frequency of GG genotype was 75.2% vs. 70.5%, GC genotype was 23.5% vs. 25.2%, and CC genotype was 1.3% vs. 4.3%. There is a remarkable difference between these two groups in terms of genotype and allele frequencies of MIF gene -173G>C polymorphism. One possible explanation for the conflicting results of the previous report could be the population heterogeneity. Because the study of Akcali et al. and the present study included individuals from the Southeastern Anatolia vs. Central Black Sea regions, conflicting results might be attributed to the differences between these two populations or unknown genetic variations.

The etiology of MS is complex and includes the interaction of multiple environmental and genetic factors. Both genetic and environmental factors may affect disease susceptibility and phenotypic characteristics, but we cannot not yet understand their exact roles. Among the environmental factors most commonly emphasized factors are the Epstein-Barr virus presence, vitamin D deficiency, ultraviolet exposure and smoking. MS is a polygenic inherited disease. Because the odds ratio for the many genes out of HLA region –the major risk factor for MS- is very low, the size of data set needed for finding an association increases, decreasing the importance of a single gene in genetic predisposition to MS. All these facts can explain the negative association found between MS and MIF -173G>C polymorphism in the present study (22). On the other hand, although MIF -173G>C polymorphism is considered as a risk factor for many inflammatory and autoimmune diseases, studies on some inflammatory and autoimmune diseases such as giant cell arthritis and cutaneous vasculitis have failed to find an association (33, 34). This suggests that the physiopathological processes may vary between autoimmune diseases and that we may have not understood/identified the effects/roles of the MIF and MIF gene polymorphisms in inflammatory diseases completely. The present study suffers several limitations; the sample size was small, which may result in false-positive and false-negative results in genetic association studies by decreasing the statistical power of the study. Moreover, another limitation was the fact that serum and/or CSF MIF levels were not measured in the patient and control groups.

In conclusion, the results of the present study suggest that MIF gene -173G>C promoter polymorphism is not associated with MS susceptibility or clinical presentation in the present Turkish population, but our study has a preliminary character and should be extended on a larger population.