The role of genetic factors in multiple sclerosis susceptibility

A complete genomic screen for multiple sclerosis underscores a role for the major histocompatability complex. The Multiple Sclerosis Genetics Group

Multiple sclerosis (MS), an inflammatory autoimmune demyelinating disorder of the central nervous system, is the most common cause of acquired neurological dysfunction arising in the second to fourth decades of life. A genetic component to MS is indicated by an increased relative risk of 20-40 to siblings compared to the general population (lambda s), and an increased concordance rate in monozygotic compared to dizygotic twins. Association and/or linkage studies to candidate genes have produced many reports of significant genetic effects including those for the major histocompatability complex (MHC; particularly the HLA-DR2 allele), immunoglobulin heavy chain (IgH), T-cell receptor (TCR) and myelin basic protein (MBP) loci. With the exception of the MHC, however, these results have been difficult to replicate and/or apply beyond isolated populations. We have therefore conducted a two-stage, multi-analytical genomic screen to identify genomic regions potentially harbouring MS susceptibility genes. We genotyped 443 markers and 19 such regions were identified. These included the MHC region on 6p, the only region with a consistently reported genetic effect. However, no single locus generated overwhelming evidence of linkage. Our results suggest that a multifactorial aetiology, including both environmental and multiple genetic factors of moderate effect, is more likely than an aetiology consisting of simple mendelian disease gene(s).

Clinical concordance in sibling pairs with multiple sclerosis

As part of a linkage study, we obtained clinical, demographic, and genetic information on 210 families with siblings concordant for multiple sclerosis (MS). Twenty-eight pairs were excluded and information was incomplete in a further 16 pairs; intrafamilial comparisons of the clinical course are reported on the remaining 166 families (155 pairs and 11 trios) in whom complete data sets were available. The demographic characteristics were comparable to those of recently performed prevalence studies in the United Kingdom, supporting the application of results in these families for genetic linkage studies in MS. We observed no significant correlation for age at onset after correction for selection bias but found a minor correlation for year at onset, which we speculate is due to earlier recognition of symptoms in second affected siblings. There was no pair-wise concordance for presenting symptoms or disability at time of assessment. However, there was a strong correlation for disease course and to a lesser degree for gender. In addition, the familial recurrence rate was 33%, almost twice that previously recorded in a local prevalence study. These results suggest that the etiology of MS involves random exposure to an, as yet unidentified, environmental trigger and the clinical features of familial disease are modified by inherited factors. That the risk of developing MS is not spread uniformly among families has important implications for the counseling of individuals with familial disease.

Genetics of demyelinating diseases

Multiple sclerosis (MS), the prototypic demyelinating disease in humans, is the most common cause of acquired neurological dysfunction arising between early to mid adulthood. MS is an inflammatory disorder and is believed to result from an autoimmune response, directed against myelin proteins and perhaps other antigens, resulting in demyelination and dense astrogliosis. A genetic component in MS is indicated by an increased relative risk to siblings compared to the general population (lambda s) of 20-40, and an increased concordance rate in monozygotic compared to dizygotic twins. Association and/or linkage studies to candidate genes have yielded a considerable number of reports showing significant genetic effects for the major histocompatibility complex (MHC), immunoglobulin heavy chain, T cell antigen receptor, and myelin basic protein loci. With the exception of the MHC, however, these results have been difficult to replicate or apply beyond isolated populations. Recently, a multi-analytical genomic screen effort was completed to identify genomic regions potentially harboring MS susceptibility genes. Nineteen such regions were identified. The data confirm the reported genetic effect of the MHC region. However, no single locus generated overwhelming evidence of linkage. These results suggest a multifactorial etiology, including both environmental and multiple genetic factors of moderate effect.

Deficient expression in multiple sclerosis of the inhibitory transcription factor Sp3 in mononuclear blood cells

To evaluate differential gene expression in multiple sclerosis (MS) patients and control subjects, we used differential display to screen for messenger RNAs that are differentially expressed in peripheral blood mononuclear cells from monozygotic twins who are discordant for MS. We identified a 232-bp complementary DNA fragment, present only in material from the normal twin, that exhibited 100% identity with the inhibitory transcription factor Sp3. Oligonucleotide primers corresponding to Sp3 messenger RNA sequences amplified complementary DNA of appropriate size from 83% of control subjects but from only 21% of MS patients (p < 0.001). These results suggest that Sp3 gene transcription is suppressed in peripheral blood mononuclear cells from most MS patients and that other genes whose expression is normally suppressed by Sp3 in immune cells may consequently be overexpressed.

Epidemiology of multiple sclerosis

The inhomogeneous distribution of MS may represent an important clue about the cause of the disease. The prevalence of MS appears to be increasing in many regions of the world, although it is likely that in a majority of regions surveyed, this increase is attributable to differences in ascertainment over the relatively short period that MS has been surveyed worldwide, which is almost exclusively the last 50 years. MS occurs most frequently in regions populated by northern Europeans, and in these areas, it is much more common in the northern European population than in the indigenous populations. MS occurs in relatives of MS patients at a rate 10- to 50-fold greater than in the general population. On average, the absolute risk to a first-degree relative of an MS patient is 2% to 5%, depending on the exact degree of relatedness. Although the mode of transmission is not certain, a multigenic pattern best fits the observed pattern of transmission. A majority of monozygotic twins are discordant for MS, indicating that a major component of MS susceptibility is environmentally determined, although recent observations in adopted relatives suggest that this risk is not transmissible. Rare epidemics of MS involving small numbers of individuals in geographically isolated regions have been reported. A number of migrant studies suggest that the risk of acquiring MS may be altered by migration, although some also support genetic factors. Analytic approaches to address the role of various risk factors include ecologic studies, case-control studies, and cohort studies. Ecologic studies are best suited for addressing risk factors that influence overall population risk, whereas case-control studies are best suited for addressing risk factors that pertain to individuals within a population. If a risk factor is distributed homogeneously in a given population, a case-control study is insensitive to its effect. Improved methodology to diminish the biases inherent in case-control studies and identification of other important risk factors from basic scientific studies undoubtedly will be important for analytic epidemiologic studies of the future. Furthermore, analysis of highly informative populations, such as discordant identical twins and adoptive siblings of MS patients, likely will improve the specificity of case-control studies by minimizing the vast number of potential differences between cases and controls.

No genetic linkage between multiple sclerosis and the interferon alpha/beta locus

Multiple sclerosis (MS) is probably caused by an interaction of genetic and environmental factors. The genetic component is reflected by a ten-fold higher concordance rate in monozygotic (27%) compared to dizygotic (3%) twin pairs. Treatment with interferon was recently reported to have a favorable effect in patients with relapsing-remitting MS. In the present familial study, we have investigated the possibility of a genetic association between the highly polymorphic Interferon alpha beta locus and the development of MS. Based on our data, we conclude that there is no linkage between the Interferon alpha beta locus and MS.

Tumor necrosis factor alpha-308 alleles in multiple sclerosis and optic neuritis

Tumor necrosis factor-alpha (TNF-alpha), a proinflammatory cytokine, is believed to play an important role in multiple sclerosis (MS) pathogenesis. A bi-allelic polymorphism in the TNF-alpha promoter region (TNF alpha-308), has been reported to influence levels of TNF-alpha production. In the present study, we investigated the TNF alpha-308 polymorphism in 93 patients with MS, 17 patients with optic neuritis (ON) and 95 healthy individuals using an allele-specific PCR technique. Allelic genotype was compared with TNF-alpha mRNA expression levels and HLA class II phenotypes. No significant difference regarding the TNF alpha-308 polymorphism was observed between MS patients and controls. Specifically, the less common allele, TNF2, which is associated with higher expression levels of TNF-alpha, was somewhat less frequent among MS patients. In fact, analysis of 19 patients homozygous for the MS associated HLA-DR-DQ haplotype HLA-Dw2 showed that this haplotype does not carry the TNF2 allele. In addition, in 47 patients, the TNF-alpha alleles did not correlate with expression levels measured as numbers of TNF-alpha expressing cells. Thus, we found no evidence for an important role of TNF alpha-308 polymorphism for genetic susceptibility to MS.

Alterations in the activity of phospholipases A2 in postmortem white matter from patients with multiple sclerosis

Activities toward arachidonyl-labelled phospholipase A2 substrates were assayed in fractions of white matter and cerebral cortex from control subjects and in fractions of demyelinated plaque, normal-appearing white matter and cerebral cortex from subjects who died with multiple sclerosis. Membranous activity at pH 8.6 in the presence of Ca2+, characteristic of 14 kDa "secretory" phospholipase A2, in either multiple sclerosis white matter or cortex did not differ from controls, whereas membranous activity at pH 4.5 in the absence of added Ca2+, characteristic of lysosomal enzymes was increased over controls in both plaque and normal-appearing white matter but not cerebral cortex. Activity in the cytosol fraction, at pH 8.6 in the presence of Ca2+ and glycerol characteristic of the "cytosolic" 85 kDa enzyme was decreased by greater than 50% in both white matter and cortex samples from multiple sclerosis subjects. Immuno-precipitation and -blotting confirmed that the deficient activity was largely attributable to the 85 kDa enzyme although the enzyme protein was not similarly reduced.

Genetic control of multiple sclerosis: increased production of lymphotoxin and tumor necrosis factor-alpha by HLA-DR2+ T cells

Lymphotoxin (LT) and tumor necrosis factor-alpha (TNF-alpha) play an important role in the pathogenesis of multiple sclerosis (MS). MS is associated with the HLA-DR2, Dw2, DQ6 HLA class II haplotype. Because both LT and TNF-alpha are encoded in the HLA region, the HLA association of MS may be related to the production of these cytokines. To test this hypothesis, we investigated the production of LT, TNF-alpha, and interferon-gamma (IFN-gamma) by CD4+ T-cell lines (TCLs) specific for myelin basic protein (MBP) or tetanus toxoid (TT) isolated from MS patients and normal controls. After stimulation with specific antigen but not mitogen, TCLs from HLA-DR2+ donors produced significantly more LT and TNF-alpha than TCLs from DR2- donors. In contrast, HLA-DR2+ and DR2- TCLs did not differ in the production of IFN-gamma, a cytokine also produced by T cells but not encoded in the HLA region. Increased secretion of LT and TNF-alpha was unrelated to the specificity (MBP vs TT), MHC restriction (HLA-DR2 vs other DR molecules), or source (MS vs normal) of the TCLs. There was no significant association of the cytokine production with individual LT or TNF-alpha alleles, indicating that the increased production of these cytokines may be linked to other polymorphic genes in this region. The results suggest that the association of MS with HLA-DR2 implies a genetically determined propensity of T cells to produce increased amounts of LT and TNF-alpha.

Critical influences of the cytokine orchestration on the outcome of myelin antigen-specific T-cell autoimmunity in experimental autoimmune encephalomyelitis and multiple sclerosis

In EAE/MS, effector molecules are produced as a result of the interaction between T lymphocytes and antigen-presenting cells and the spectrum of cytokines produced is likely to decisively influence the disease outcome. These events may be more important, or at least more easily accessible to therapeutic intervention, than particular autoantigen specificities. Data from EAE suggest that cytokines connected to the Th1 phenotype of lymphocytes, especially IFN-gamma but also TNF-beta, TNF-alpha and IL-12, may promote inflammation while cytokines connected to the Th2 subset, IL-4, IL-10 and TGF-beta, may potentially have a role in disease limitation. It will be important to accurately study cytokines during immunotherapeutic interventions and in relation to immunogenetic variables in order to aim at immunotherapeutically intervening in the Th1, Th2 balance as well as counteracting disease-promoting cytokines such as IFN-gamma and TNF-alpha or promoting the action of downregulatory cytokines such as IL-10 and TGF-beta.