Genetic Factors in Multiple Sclerosis

Multiple sclerosis genetics

A broad scientific consensus has emerged linking multiple sclerosis (MS) risk to multiple independent and interacting DNA variants that are relatively frequent in the population and act in concert with environmental exposures. The multifactorial, polygenic model of heritability provided the rationale and impetus to pursue genome-wide association studies (GWAS), which have been highly successful in uncovering genetic variants influencing susceptibility. Over 200 loci have been firmly associated with MS susceptibility. The main association signal genome-wide maps to the major histocompatibility complex ( MHC) gene cluster in chromosome 6p21. This association has been observed across all populations studied. However, a significant proportion of MS heritability remains unexplained. Decoding the genetics of MS represents a long-standing and important research goal in this disease, as the demonstration of even modest functional genomic effects on risk or the course of MS is likely to reveal fundamental disease mechanisms and possibly yield new therapeutic opportunities.

The Genetics of Multiple Sclerosis: From 0 to 200 in 50 Years

Multiple sclerosis (MS) is a common autoimmune disease that targets myelin in the central nervous system (CNS). Multiple genome-wide association studies (GWAS) over the past 10 years have uncovered more than 200 loci that independently contribute to disease pathogenesis. As with many other complex diseases, risk of developing MS is driven by multiple common variants whose biological effects are not immediately clear. Here, we present a historical perspective on the progress made in MS genetics and discuss current work geared towards creating a more complete model that accurately represents the genetic landscape of MS susceptibility. Such a model necessarily includes a better understanding of the individual contributions of each common variant to the cellular phenotypes, and interactions with other genes and with the environment. Future genetic studies in MS will likely focus on the role of rare variants and endophenotypes.

Theiler's murine encephalomyelitis virus infection of SJL/J and C57BL/6J mice: Models for multiple sclerosis and epilepsy

Mouse models are great tools to study the mechanisms of disease development. Theiler's murine encephalomyelitis virus is used in two distinct viral infection mouse models to study the human diseases multiple sclerosis (MS) and epilepsy. Intracerebral (i.c.) infection of the SJL/J mouse strain results in persistent viral infection of the central nervous system and a MS-like disease, while i.c. infection of the C57BL/6J mouse strain results in acute seizures and epilepsy. Our understanding of how the immune system contributes to the development of two disparate diseases caused by the same virus is presented.

Is size an essential criterion to define tumefactive plaque? MR features and clinical correlation in multiple sclerosis

Tumefactive multiple sclerosis is an inflammatory demyelinating disease of the central nervous system. It has recently been described as a rare subtype of multiple sclerosis (MS) characterised by the appearance of solitary or multiple space-occupying lesions associated with imaging characteristics mimicking neoplasm. Atypical features include plaque size >2 cm with mass effect, oedema, and/or ring enhancement on magnetic resonance (MR) images.This study is a retrospective review designed to evaluate the prevalence of tumefactive plaques in a selected population of 440 MS patients referred to our MS centre in Southern Italy between 2005 and 2014. We analysed the radiographic features of lesions ranging in size from 0.5 to 2 cm to establish whether smaller plaques with MR characteristics similar to tumefactive plaques present different symptoms, disease evolution and prognosis. We also aimed to ascertain if MR features suggestive of biological aggressiveness could be useful prognostic criteria for a correct diagnosis of the disease and subsequent treatment. Our data suggest that lesions 0.5-2 cm and >2 cm have similar MR features and clinical evolution.

Clinical characteristics and long term prognosis in early onset multiple sclerosis

The clinical features, disease course and long term prognosis of early onset multiple sclerosis (EOMS) are variable. A preponderance of female sex, a high incidence of visual and brainstem symptoms at onset and a relapsing-remitting disease course with slower progression rate all have been considered relatively frequent in this subgroup of MS patients. Unfortunately, follow-up in previous studies has usually been limited to less than 10 years, precluding general statements on the long term evolution and prognosis in EOMS. In this retrospective study of EOMS with a substantially longer follow-up of 20.8 years, clinical characteristics, disease course and long term prognosis are described in 49 EOMS patients. In a representative subgroup of 28 patients disability was scored using Kurtzke's Functional System (FS) and Expanded Disability Status Scale (EDSS). The mean period of follow- up for these patients was 28.7 years. The present study confirms that several specific clinical characteristics can be identified in EOMS patients, such as a mainly relapsing-remitting disease onset and frequent presentation with brainstem- cerebellar dysfunction (28.6 %), pyramidal symptoms (18.4 %) and optic neuritis (14.3%). However, after a long period of follow-up the overall disease course and prognosis do not seem to differ from that in adult onset MS. By consequence a young age at onset in MS cannot be considered a favourable prognostic factor.

Theiler's virus infection: a model for multiple sclerosis

Both genetic background and environmental factors, very probably viruses, appear to play a role in the etiology of multiple sclerosis (MS). Lessons from viral experimental models suggest that many different viruses may trigger inflammatory demyelinating diseases resembling MS. Theiler's virus, a picornavirus, induces in susceptible strains of mice early acute disease resembling encephalomyelitis followed by late chronic demyelinating disease, which is one of the best, if not the best, animal model for MS. During early acute disease the virus replicates in gray matter of the central nervous system but is eliminated to very low titers 2 weeks postinfection. Late chronic demyelinating disease becomes clinically apparent approximately 2 weeks later and is characterized by extensive demyelinating lesions and mononuclear cell infiltrates, progressive spinal cord atrophy, and axonal loss. Myelin damage is immunologically mediated, but it is not clear whether it is due to molecular mimicry or epitope spreading. Cytokines, nitric oxide/reactive nitrogen species, and costimulatory molecules are involved in the pathogenesis of both diseases. Close similarities between Theiler's virus-induced demyelinating disease in mice and MS in humans, include the following: major histocompatibility complex-dependent susceptibility; substantial similarities in neuropathology, including axonal damage and remyelination; and paucity of T-cell apoptosis in demyelinating disease. Both diseases are immunologically mediated. These common features emphasize the close similarities of Theiler's virus-induced demyelinating disease in mice and MS in humans.

The genetics of multiple sclerosis. A review

Multiple sclerosis (MS) is characterized by chronic inflammation and demyelination in the central nervous system (CNS). Although the etiology of MS is unknown, both genetic and environmental contributions to the pathogenesis are inferred from epidemiologic studies. Geographic distributions and epidemics of MS and data from migration studies provide evidence for some, thus far unidentified, environmental effects. The co-occurrence of MS with high and low frequencies in ethnic groups often sharing an environment, the increased recurrence rate in families, and the high concordance rate among identical twins point to inheritable determinants of susceptibility. Based on the autoimmune hypothesis of demyelination, genetic studies sought associations between MS and polymorphic alleles of candidate genes which regulate either the immune response or myelin production. The most consistent finding in case-control studies was the association with the major histocompatibility complex (MHC) (also called human leukocyte antigen--HLA) class II, DR15, DQ6, Dw2 haplotype. Studies on other gene products encoded within or close to the MHC complex on chromosome 6p21.3 (e.g., HLA DP, complement components, transporter proteins, tumor necrosis factor, and myelin-oligodendrocyte glycoprotein) resulted in conflicting observations in different patient populations. The potential contribution of polymorphic alleles within the genes of the T-cell receptor alpha beta chains, immunoglobulins, cytokines, and oligodendrocyte growth factors or their receptors to MS susceptibility either remains equivocal or is rejected. Studies on families with multiple affected members have revealed that MS is a complex trait, that the contribution of individual genes to susceptibility is probably small, and that differences are possible between familial and sporadic forms. The development of molecular and computer technologies have facilitated the performance of comprehensive genomic scans in multiplex families, which have confirmed the possible linkage of multiple loci to susceptibility, each with a minor contribution. Several provisional sites were reported, but only 6p21 (MHC complex), 5p14, and 17q22 were positive in more than one study. The British update demonstrated segregation among regions of interest depending on DR15 sharing, and excluded a gene of major effect from 95%, and one with a moderate effect from 65% of the genome. The extended study by the US collaboration group revealed that the MHC linkage was limited to families segregating HLA DR2 alleles, which suggested that linkage to the MHC is related to the HLA DR2 association, and that sporadic and familial MS share at least one common susceptibility marker. Further identification of MS susceptibility loci may involve additional family sets, more polymorphic markers, and the exploration of telomeric chromosomal regions. Data from these studies may further elucidate pathogenic mechanisms of MS.

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.

Cranial MRI in idiopathic retinal vasculitis

Ten patients with clinically isolated idiopathic retinal vasculitis who had a positive family history for multiple sclerosis (MS) or positive typing for HLA B7 underwent magnetic resonance imaging (MRI) of brain and optic nerves in order to establish the frequency of clinically silent lesions. Brain MRI was normal in seven and abnormal in three: one had a single small white matter lesion, two had extensive white matter abnormalities resembling those seen in MS. In two patients a lesion was shown in the optic nerve. These findings suggest that a minority of patients with idiopathic retinal vasculitis have disseminated central nervous system lesions characteristic of MS, the frequency of such changes being less than in patients with isolated optic neuritis.

Immunological aspects of experimental allergic encephalomyelitis and multiple sclerosis

Multiple sclerosis (MS) is the most frequent, demyelinating disease of the central nervous system (CNS) in Northern Europeans and North Americans. Despite intensive research its etiology is still unknown, but a T cell-mediated autoimmune pathogenesis is likely to be responsible for the demyelination. This hypothesis is based both on findings in MS patients and studies of an experimental animal model for demyelinating diseases, experimental allergic encephalomyelitis (EAE). Experiments in EAE have not only demonstrated which myelin antigens are able to induce the demyelinating process but also have determined the characteristics of encephalitogenic T cells, that is, their fine specificity, major histocompatibility complex (MHC) restriction, lymphokine secretion, activation requirements, and T cell receptor (TCR) usage. Based on these findings, highly specific and efficient immune interventions have been designed in EAE and have raised hopes that similar approaches could modulate the disease process in MS. Although the examination of the myelin-specific T cell response in MS patients has shown parallels to EAE, this remains an area of intensive research because a number of questions remain. This review summarizes the important lessons from EAE, examines recent findings in MS, and discusses current concepts about how the disease process develops and which steps might be taken to modulate it.

The role of genetic factors in multiple sclerosis susceptibility

There has been increasing evidence that genetic factors have a role in determining susceptibility to MS. Re-examination of results from prevalence and migration surveys reveals that there remains considerable ambiguity in interpretation. Some patterns previously thought to decisively support environmental determination may still be explained, at least in part, on a genetic basis. It seems inescapable that MS is probably due to an interaction of genetic and environmental factors. It remains undetermined whether or not genes exist which are truly necessary for the development of the disease. Existing data are consistent with the notion that the study of MS susceptibility will parallel the findings in experimental models of spontaneous autoimmunity and that at very least, two genes and almost certainly several genes will be found to influence susceptibility and interact in as yet unknown ways. One of these loci appears to be the Class II MHC, although its role may be minor at the germ line level. Roles for the T-cell receptor alpha and beta loci appear to be minor and may even be non-existent in contributing to heritable susceptibility. We predict that additional loci will be identified which influence both susceptibility and outcome and will be more important. Furthermore, it is clear that the understanding of the contribution of individual susceptibility loci will continue to be difficult because of the constraints of human pedigree data. It is likely that further resolution of the questions posed above related to genetic susceptibility in MS will require multicenter collaboration.

Myelin basic protein-specific T-cell responses in identical twins discordant or concordant for multiple sclerosis

Although multiple sclerosis (MS) is thought to be an autoimmune disease, the target antigen of the immune response is unknown. Both myelin basic protein (MBP) and proteolipid protein (PLP) have been considered candidate autoantigens. Because the immune response to either foreign or self antigens is influenced by the genetic background of the host, the importance of these candidate antigens has been difficult to establish in humans because of genetic diversity. To eliminate genetic differences in MS patients and healthy controls, we have studied the MBP-specific T-cell response in 6 sets of identical twins, 3 of which were concordant and 3 discordant for MS. A total of 638 short-term T-cell lines were established and characterized for MBP-specific proliferative and cytotoxic activity, fine specificity, and human leukocyte antigen (HLA) restriction. Similar frequencies of MBP-specific T cells were observed in affected and unaffected individuals. A slightly higher percentage of cytotoxic T-cell lines was found in affected individuals. For most of the cell lines, the restriction elements were the HLA class II antigens that have been reported previously to be associated with MS; no important differences with respect to HLA restriction were found between the patients and healthy individuals. The peptide epitopes of MBP that were recognized most frequently by the T-cell lines were those previously shown to be immunodominant. Differences in specificity were seen in some discordant twins indicating that, despite genetic identity, the MBP-specific T-cell repertoire may be shaped differently. These findings indicate that differences in frequency, peptide specificity, or HLA restriction are not sufficient to implicate MBP-specific T cells in the pathogenesis of MS. However, the T-cell response to MBP may still represent one necessary component with disease occurring when this response is combined with other host characteristics such as regulation of cytokine-, adhesion molecule-, or HLA-antigen expression in the nervous system or immunoregulatory mechanisms.

Oligonucleotide dot-blot analysis of HLA-DQ beta alleles associated with multiple sclerosis

Oligonucleotide probes were used to investigate the role of DQ beta molecules in susceptibility to multiple sclerosis. Although shared amino acid and nucleotide sequences in DQ beta 1 have been suggested to be critical in disease development, we find that the distribution of sequences corresponding to residues 71-77 is not greater in patients versus controls.

Parent-child concordance in multiple sclerosis

It is now recognized that susceptibility to multiple sclerosis (MS) is determined in part by genetic factors. The gene loci influencing MS susceptibility are largely unidentified. In an attempt to better understand the mode of transmission, parent-child concordance for MS was studied in two large, population-based MS clinic populations. Among 75 parent-child pairs concordant for MS, we found 40 mother-daughter pairs, 13 mother-son pairs, 21 father-daughter pairs, and 1 father-son pair. Controlling for the known female preponderance in MS, the data show a paucity of father-son pairs. These data have implications for understanding the mechanisms of inheritance for MS susceptibility as well as for risk counseling in families of patients.

The inheritance of MS susceptibility

The object of this study was to determine, if possible, the mode of inheritance of the susceptibility to multiple sclerosis (MS). It was known that no single-gene model could fit the observations, so oligogenic models (models involving a small number of genes) were sought. Oligogenic hypotheses were tested against the available population data for MS until a reasonable level of agreement was found. The best-fitting simple hypothesis was this: MS occurs only in people who are homozygous for a recessive gene and carry a dominant X gene, and then only with reduced penetrance. The dangerous allele m- at the autosomal locus appears to be fairly uniformly distributed across England, Ireland and Canada, occurring in 10-30% of the gene pool. There are large variations in the frequency of the allele s- at the X-locus, ranging from 10% up to 72% of the gene pool. The penetrance varies significantly with geographical location, but nowhere approaches unity, so that the environmental factors are of great importance. The hypothesis explains the broad features of the population pattern of the occurrence of MS and it gives an outstanding fit to the best available data on MS in relatives. The result may assist attempts to map the genetic data on MS, opens the way for a reassessment of the attempts to identify the environmental factors, and it makes possible the completion of nonempirical risk tables for various countries. Similar techniques may be applied to other disorders with a genetic component in their aetiology.

Patients with multiple sclerosis carry DQB1 genes which encode shared polymorphic amino acid sequences

Of 61 Norwegian multiple sclerosis patients tested, 59, i.e., 97%, were positive for at least one of the HLA specificities DR2, DR4, or DRw6. Typing with sequence-specific oligonucleotide probes revealed that the same 59 patients carried DR2-, DR4-, or DRw6-associated HLA-DQB1 genes which encode shared polymorphic amino acid sequences in the membrane-distal part of their HLA-DQ beta chains. This shared DQ beta polymorphism may explain previously reported DR associations and could thus be the primary HLA association in MS.

Multiple sclerosis: updated risks for relatives

Two important characteristics of multiple sclerosis (MS) are familial clustering and a variable age of onset. There is increasing evidence for a genetically influenced susceptibility in MS. Because of this, patients and their relatives are increasingly asking about the risk for relatives of developing MS. In the MS Clinic in Vancouver, genetic histories are taken routinely for all patients and are updated annually. Patients do not attend the clinic specifically to participate in genetic studies, which could result in over-representation of familial cases. Data were available for 815 MS index cases and 11,345 of their relatives. Age-specific MS risks were calculated for first-, second-, and third-degree relatives of probands and are presented in an easy-reference format. In general, first-degree relatives of probands have a risk that is 30-50 times greater than the 0.1% risk for the general population.

Familial multiple sclerosis

Siblings of patients with multiple sclerosis have an increased risk of developing the disease. In this report we describe 3 families with multiple affected members, representing the largest published aggregation of cases in first degree relatives. In the 2 families in which HLA tissue-typing was performed the affected individuals shared part of the haplotype HLA-DR2 (+ HLA-DQW1), BfS (+ C2C), C4A3, C4B1. The implications of these findings for the aetiology of multiple sclerosis are discussed.

Heterotopic brain transplants in the study of experimental allergic encephalomyelitis

A heterotopic transplant paradigm was developed for its potential usefulness in dissecting genetically determined immune and central nervous system (CNS) components in the induction of experimental allergic encephalomyelitis (EAE). EAE is a cell-mediated, organ-specific, autoimmune disease producing inflammatory demyelination in the CNS. Susceptibility to EAE is determined by multiple genes and reflects both immune competence and target tissue responses. Syngeneic fetal CNS was heterotopically transplanted into the anterior chamber of the eye or beneath the capsule of the kidney of adult SJL or (SJL X BALB/c)F1 mice. Transplants usually survived better in the eye than the kidney. Six to eight weeks after transplantation, some mice were immunized for EAE. Immunized mice developed clinical and pathological signs of EAE in 12 to 15 days. The placement of CNS tissue into the eye or kidney prior to immunization did not suppress induction of EAE. Transplants in either location, in immunized mice, manifested perivascular inflammation and demyelination similar to that seen in the host CNS. However, transplants in mice not immunized for EAE, but maintained an equal time period after transplantation, did not demonstrate these features. The ability to produce the specific pathologic lesions of EAE in CNS tissue transplanted outside the CNS allows the design of studies of the tissue localization of genetic restrictions to development of EAE.

A population-based study of multiple sclerosis in twins

Results from studies of twin concordance in multiple sclerosis have not conclusively differentiated between environmental and genetic factors that determine susceptibility to the disease. Published studies that have been based on case finding by public appeal have been characterized by difficulties in ascertainment. The data reported here are from a large population-based study of multiple sclerosis in twins, in which ascertainment has been relatively unbiased and the cooperation of patients nearly complete. A total of 5463 patients attending 10 multiple sclerosis clinics across Canada were surveyed. Twenty-seven monozygotic and 43 dizygotic twin pairs were identified, and the diagnosis of multiple sclerosis was verified by examination and laboratory investigation. Seven of 27 monozygotic pairs (25.9 percent) and 1 of 43 dizygotic pairs (2.3 percent) were concordant for multiple sclerosis. The concordance rate for 4582 nontwin siblings of patients at two multiple sclerosis clinics was 1.9 percent, closely paralleling the concordance rate in dizygotic twins. To the extent that the difference in concordance rates between monozygotic and dizygotic twins indicates genetic susceptibility, the results of this study show a major genetic component in susceptibility to multiple sclerosis.

Prevalence of multiple sclerosis in British Columbia

A province wide prevalence study on multiple sclerosis (MS) was conducted in British Columbia (B.C.). The prevalence date was July 1, 1982. The major portion of this study was a review of all the files of neurologists practicing in B.C. as this was judged to be the most accurate source for identifying MS patients. 239,412 neurologists' files were hand searched by one researcher using modified Schumacher criteria for classification. Other sources used during the study for identifying MS patients were the MS Clinic, general practitioners, ophthalmologists, urologists, specialized facilities such as long term care facilities and rehabilitation centres, and patient self-referrals. A total of 4,620 non-duplicated cases were identified and classified. 4,112 of these (89%) were classified according to information contained in neurologists' records. The prevalence estimate for definite/probable MS in B.C. was 93.3/100,000 population. This increased to 130.5/100,000 population if possible MS and optic neuritis were also included. These rates are among the highest reported in Canada or elsewhere. The cooperation of B.C. neurologists made this study unique in its scope and accuracy of diagnosis.

Myelin basic protein and demyelinating diseases

Myelin basic protein (MBP) is a major protein component of myelin sheath. Primarily because of its ability to induce experimental allergic encephalomyelitis (EAE) in animals, this protein has been considered to play an important role in the pathogenesis of multiple sclerosis (MS), which is one of the most common demyelinating diseases. Its precise measurement in the cerebrospinal fluid (CSF) has been uncommonly difficult, mainly due to immunoheterogeneity of MBP or MBP-like material. More recently, highly sensitive radioimmunoassay techniques utilizing well-characterized antisera have been developed that facilitated its use in the management of MS. The clinical course of MS is highly variable, and the disease is characterized by periods of remission and relapses. Many studies have demonstrated the release of MBP during relapses and elevated levels of MBP in the CSF can be detected if lumbar puncture is performed within 7 days of the onset of neurologic symptoms suggestive of MS. However, the presence of MBP is not an absolute indicator of MS, as elevated MBP levels are also frequently observed in other demyelinating diseases.

Reproductive counselling for multiple sclerosis patients

The diagnosis of multiple sclerosis (MS) is frequently made during the reproductive years, and there is increasing evidence that it has a genetic causal component. An increasing number of couples are concerned about recurrence and they request reproductive counselling at genetic clinics or from their physicians. Here we outline factors to be considered in the reproductive counselling of such individuals. These include empiric recurrence risks, the effect of pregnancy and delivery on maternal MS, overall pregnancy outcome, longterm prognosis for the affected parent, and the possibility of sexual dysfunction in couples trying to conceive a child.