**4.** *HLA* **genes and MS**

activated [4]. The immune regulatory defects and increased migration of autoreactive lymphocytes within the brain, that are the typical traits in MS, lead to the process of inflammation, myelin sheath breakdown, demyelination, remyelination, neuronal and axonal degeneration, and subsequent deterioration of neurological functions [5]. Neurodegenera‐ tion, neuronal and axonal damage that correlate with the progression of the disease can be a process partly independent from inflammation and demyelination or even can be the cause of demyelination occurring from the disease onset. Axonal damage in MS is a result of many

It is still unclear as to why MS is so devastating and rapidly progressive in one patient and less so in another. Because the etiopathogenesis of MS is very complex, disease develop‐ ment as well as the characteristics of disease progression is probably the consequence of multifactorial interaction. Our work is dedicated to genetic and biochemical markers that were chosen according to their possible role in the modulation of the immune response in MS patients and thus could be associated with MS risk and disability progression. In our work, we discuss the immune response-related genetic factors associated with MS that can be generally classified into *HLA* genes and *non-HLA* genes. Since vitamin D can have an important role in the pathogenesis of MS, great part of our work is dedicated to its metabo‐ lism, functions, mechanisms of action in MS and genetic factors that can modify these effects. In this work, we also present the results of our own analysis of genetic and biochemical markers that we found to be associated with MS risk or progression in the group consist‐ ing of MS patients with clinically diagnosed MS and healthy individuals from the region of Central Slovakia. To evaluate the disease progression rate, we used the widely accepted multiple sclerosis severity score (MSSS, score range 0.01–9.99) [8] that considers the neuro‐ logical impairment of the functional systems (expanded disability status scale score) [9] together with disease duration. For the purpose of the association analysis of these mark‐ ers with the rate of disease disability progression, we stratified MS patients by MSSS scores to three groups—slowly progressing MS (MSSS < 3), mid-rate progressing MS (MSSS 3–6)

**2. Immune response-related genetic factors in the risk and progression of**

MS is a typical gender-dependent disease; a higher risk of MS is observed in women than in men in all populations and races. A study conducted in Canada found female to male ratio in individuals affected by MS to be 3:1 [11]. The risk of MS development in siblings of an affected individual is estimated to be 5%, in children 2%, in monozygotic twins 25% [5]. However, it has been shown that genetic predisposition is not strong enough to induce disease develop‐ ment, and appropriate environmental triggers are necessary to start the disease process [1, 12]. In general, the MS-associated genes can be classified into genes of the HLA-complex

pathological processes [6, 7].

4 Trending Topics in Multiple Sclerosis

and rapidly progressing MS (MSSS > 6) [10].

**multiple sclerosis**

and*non-HLA* genes [3].

Antigen expression that is inducible by cytokines is different on the various immune cells. Major histocompatibility complex II (MHCII) antigens are transmembrane proteins localized on the immune cells, thus having an important role in the process of exogenous antigen presentation to Tcells. MHCII molecules are coded by the gene of human leucocyte antigens D (*HLA-D* gene) that is localized on chromosome 6 and has three regions—HLA-DP, HLA-DQ and HLA-DR [17]. The susceptibility of the population to autoimmune diseases depends on the individual ability to express HLA-DQ and HLA-DR antigens. This expression can be induced by virus infection, most likely by EBV, influenza or paramyxovirus [18]. *MHCII* gene expression is regulated by vitamin D through its binding to the *vitamin D–responsive ele‐ ments*(*VDREs*) that are localized in the promoter region of *HLA-DRB1* gene. This fact can explain the interaction between vitamin D, that is an important factor modifying MS devel‐ opment and disease course, and genetic predisposition to MS represented mainly by a highly conservative allele HLA-DRB1\*1501. HLA-DRB1\*1501 allele is in general considered to be the most important susceptibility allele of MS [19–24]. This allelewas found to be present in over 50% of MS cases [25, 26]. The increased frequencies of the DRB1\*15 allele in MS patients have been described in Northern Europeans [23, 27], South and North Americans [20, 28], Medi‐ terraneans [29, 30] and African Americans [21]. In Spanish cohorts, the DRB1\*03 was the second most frequent allele associated with MS, but only after eliminating HLA-DRB1\*15 [29]. The DRB1\*03 allele has also been found to be significantly associated with the increased risk of MS in Scandinavians [27], Sardinians [31], and Australians [32]. Fernández et al. [33] found that the DRB1\*13 allele is protective against MS development in Spaniards. A protective effect of the alleles DRB1\*01, DRB1\*07, DRB1\*12 and DRB1\*14 was confirmed in the recent metaanalysis in Caucasians [24]. The allele DRB1\*07 was found to be protective against MS also in Scandinavians [34]. The DRB1\*13.03 allele was found to be the primary risk allele in MS patients of European descent [23]. The protective effect against MS has also been shown for the HLA-DRB1\*11 allele [24, 29].

The DQB1\*06:02 allele was found to be linked to the increased risk of MS with a proved tight linkage disequilibrium between DRB1\*15 and DQB1\*06 in Caucasians [35]. As the risk factor of MS, DQB1\*06:02 allele has also been identified in a cohort of Afro-Brazilians [36] and Spaniards [33]. Kaushansky et al. [37] suggested that the role of the DRB1\*15:01 and DQB1\*06:02 alleles in MS depends on the heterogeneous interaction of target antigen, geno‐ type, and phenotype. On the contrary, Isobe et al. [38] found none of the HLA-DQB1 alleles to be associated with MS in African Americans. According to the combinations of HLA-alleles, the association of HLA-DRB1\*15/\*15 genotype with MS was identified by several studies [32, 34, 39]. In multi-case MS families, Barcellos et al. [39] identified a high risk DRB1\*15/\*08 genotype and protective DRB1\*15/\*14 genotype. The study of Sawcer et al. [23] indicates that in all populations of North-European ancestry, a predisposition to MS is linked with the DRB1\*15:01-DQB1\*06:02 haplotype. Furthermore, Link et al. [34] in a Scandinavian cohort showed that risk haplotypes for MS are almost all DRB1\*15 bearing haplotypes, while protective effect against MS development are HLA class I A\*02 allele-bearing haplotypes. In Sardinian MS patients, Cocco et al. [40] confirmed a positive association of the haplotype HLA DRB1\*03:01-DQB1\*02:01 with MS.

In the study from our laboratory, we analysed the association of the *HLA-DRB1/DQB1* genes, alleles and their combinations with susceptibility to MS in the population from central Slovakia. We found that the increased risk of MS is in individuals carrying alleles HLA-DRB1\*15, DRB1\*03 and DQB1\*06, genotypes HLA-DRB1\*15/\*15 DQB1\*06/\*06 and haplotype DRB1\*15-DQB1\*06. In addition, we also found that HLA-DRB1/DQB1 class II alleles DRB1\*07, DRB1\*13, DQB1\*03, genotypes DRB1\*13/\*11, DQB1\*05/\*03 and haplotypes DRB1\*13-DQB1\*06 and DRB1\*11-DQB1\*03 are associated with the protection against MS development. We cannot exclude that the proposed protective effects of the DRB1\*11-DQB1\*03 and DRB1\*13-DQB1\*06 haplotypes in our cohort could be, at least partially, due to the linkagedisequilibrium with alleles in the HLA class I region which is primarily associated with MS [41].

### **5.** *Non-HLA* **genes and MS**

Gene products of *non-HLA* genes can contribute to the genetic risk of MS by modulation of different processes. These genes are involved in the regulation of functions of T- and Bcells, dendritic cells, NKcells, cytokine signalization, metabolism of interferons, vitamin D metab‐ olism, neuronal regeneration and many others [3]. It has been found that these genes can contribute not only to the increased inherited risk of MS development but also to the risk of other autoimmune diseases [42, 43]. The examples of the SNPs involved in the etiopathogenesis of MS are summarised in **Table 1**.


**Table 1.** Gene polymorphisms involved in the etiopathogenesis of MS [3, 44–48].

analysis in Caucasians [24]. The allele DRB1\*07 was found to be protective against MS also in Scandinavians [34]. The DRB1\*13.03 allele was found to be the primary risk allele in MS patients of European descent [23]. The protective effect against MS has also been shown for

The DQB1\*06:02 allele was found to be linked to the increased risk of MS with a proved tight linkage disequilibrium between DRB1\*15 and DQB1\*06 in Caucasians [35]. As the risk factor of MS, DQB1\*06:02 allele has also been identified in a cohort of Afro-Brazilians [36] and Spaniards [33]. Kaushansky et al. [37] suggested that the role of the DRB1\*15:01 and DQB1\*06:02 alleles in MS depends on the heterogeneous interaction of target antigen, geno‐ type, and phenotype. On the contrary, Isobe et al. [38] found none of the HLA-DQB1 alleles to be associated with MS in African Americans. According to the combinations of HLA-alleles, the association of HLA-DRB1\*15/\*15 genotype with MS was identified by several studies [32, 34, 39]. In multi-case MS families, Barcellos et al. [39] identified a high risk DRB1\*15/\*08 genotype and protective DRB1\*15/\*14 genotype. The study of Sawcer et al. [23] indicates that in all populations of North-European ancestry, a predisposition to MS is linked with the DRB1\*15:01-DQB1\*06:02 haplotype. Furthermore, Link et al. [34] in a Scandinavian cohort showed that risk haplotypes for MS are almost all DRB1\*15 bearing haplotypes, while protective effect against MS development are HLA class I A\*02 allele-bearing haplotypes. In Sardinian MS patients, Cocco et al. [40] confirmed a positive association of the haplotype HLA

In the study from our laboratory, we analysed the association of the *HLA-DRB1/DQB1* genes, alleles and their combinations with susceptibility to MS in the population from central Slovakia. We found that the increased risk of MS is in individuals carrying alleles HLA-DRB1\*15, DRB1\*03 and DQB1\*06, genotypes HLA-DRB1\*15/\*15 DQB1\*06/\*06 and haplotype DRB1\*15-DQB1\*06. In addition, we also found that HLA-DRB1/DQB1 class II alleles DRB1\*07, DRB1\*13, DQB1\*03, genotypes DRB1\*13/\*11, DQB1\*05/\*03 and haplotypes DRB1\*13-DQB1\*06 and DRB1\*11-DQB1\*03 are associated with the protection against MS development. We cannot exclude that the proposed protective effects of the DRB1\*11-DQB1\*03 and DRB1\*13-DQB1\*06 haplotypes in our cohort could be, at least partially, due to the linkagedisequilibrium with

Gene products of *non-HLA* genes can contribute to the genetic risk of MS by modulation of different processes. These genes are involved in the regulation of functions of T- and Bcells, dendritic cells, NKcells, cytokine signalization, metabolism of interferons, vitamin D metab‐ olism, neuronal regeneration and many others [3]. It has been found that these genes can contribute not only to the increased inherited risk of MS development but also to the risk of other autoimmune diseases [42, 43]. The examples of the SNPs involved in the etiopathogenesis

alleles in the HLA class I region which is primarily associated with MS [41].

the HLA-DRB1\*11 allele [24, 29].

6 Trending Topics in Multiple Sclerosis

DRB1\*03:01-DQB1\*02:01 with MS.

**5.** *Non-HLA* **genes and MS**

of MS are summarised in **Table 1**.
