Susceptibility to multiple sclerosis and the immunoglobulin heavy chain variable region

Immunoglobulin germline gene variation and its impact on human disease

Immunoglobulins (Ig) play an important role in the immune system both when expressed as antigen receptors on the cell surface of B cells and as antibodies secreted into extracellular fluids. The advent of high-throughput sequencing methods has enabled the investigation of human Ig repertoires at unprecedented depth. This has led to the discovery of many previously unreported germline Ig alleles. Moreover, it is becoming clear that convergent and stereotypic antibody responses are common where different individuals recognise defined antigenic epitopes with the use of the same Ig V genes. Thus, germline V gene variation is increasingly being linked to the differential capacity of generating an effective immune response, which might lead to varying disease susceptibility. Here, we review recent evidence of how germline variation in Ig genes impacts the Ig repertoire and its subsequent effects on the adaptive immune response in vaccination, infection, and autoimmunity.

Differential reactivity of germ line allelic variants of a broadly neutralizing HIV-1 antibody to a gp41 fusion intermediate conformation

Genetic factors, as well as antigenic stimuli, can influence antibody repertoire formation. Moreover, the affinity of antigen for unmutated naïve B cell receptors determines the threshold for activation of germinal center antibody responses. The gp41 2F5 broadly neutralizing antibody (bNAb) uses the V(H)2-5 gene, which has 10 distinct alleles that use either a heavy-chain complementarity-determining region 2 (HCDR2) aspartic acid (D(H54)) or an HCDR2 asparagine (N(H54)) residue. The 2F5 HCDR2 D(H54) residue has been shown to form a salt bridge with gp41 (665)K; the V(H)2-5 germ line allele variant containing N(H54) cannot do so and thus should bind less avidly to gp41. Thus, the induction of 2F5 bNAb is dependent on both genetic and structural factors that could affect antigen affinity of unmutated naïve B cell receptors. Here, we studied allelic variants of the V(H)2-5 inferred germ line forms of the HIV-1 gp41 bNAb 2F5 for their antigen binding affinities to gp41 linear peptide and conformational protein antigens. Both V(H)2-5 2F5 inferred germ line variants bound to gp41 peptides and protein, including the fusion intermediate protein mimic, although more weakly than the mature 2F5 antibody. As predicted, the affinity of the N(H54) variant for fusion-intermediate conformation was an order of magnitude lower than that of the D(H54) V(H)2-5 germ line antibody, demonstrating that allelic variants of 2F5 germ line antibodies differentially bind to gp41. Thus, these data demonstrate a genetically determined trait that may affect host responses to HIV-1 envelope epitopes recognized by broadly neutralizing antibodies and has implications for unmutated ancestor-based immunogen design.

Th17 cell, the new player of neuroinflammatory process in multiple sclerosis

Multiple sclerosis (MS) is an autoimmune disease characterized by recurrent episodes of demyelination and axonal lesion mediated by CD4(+) T cells with a proinflammatory Th1 and Th17 phenotype, macrophages, and soluble inflammatory mediators. Identification of Th17 cells led to breaking the dichotomy of Th1/Th2 axis in immunopathogenesis of autoimmune diseases such as MS, and its experimental model, experimental autoimmune encephalomyelitis (EAE). Th17 cells are characterized by expression of retinoic acid-related orphan receptor (ROR)γt and signal transducer and activator of transcription 3 (STAT3) factors. Th17-produced cytokine profile including interleukin (IL)-17, IL-6, IL-21, IL-22, IL-23 and tumour necrosis factor (TNF)-α, which have proinflammatory functions, suggests it as an important factor in immunopathogenesis of MS, because the main feature of MS pathophysiology is the neuroinflammatory reaction. The blood brain barrier (BBB) disruption is an early and central event in MS pathogenesis. Autoreactive Th17 cells can migrate through the BBB by the production of cytokines such as IL-17 and IL-22, which disrupt tight junction proteins in the central nervous system (CNS) endothelial cells. Consistent with this observation and regarding the wide range production of proinflammatory cytokines and chemokines by Th17 cells, it is expected that Th17 cell to be as a potent pathogenic factor in disease immunopathophysiology. Th17-mediated inflammation is characterized by neutrophil recruitment into the CNS and neurons killing. However, the majority of our knowledge about the role of Th17 in MS pathogenesis is resulted in investigation into EAE animal models. In this review, we intend to focus on the newest information regarding the precise role of Th17 cells in immunopathogenesis of MS, and its animal model, EAE.

Influence of histocompatibility genes on disease susceptibility and treatment response in patients with relapsing-remitting multiple sclerosis treated with interferon β-1a

Multiple sclerosis (MS) is the most common, non-traumatic cause of neurological disability in young adults. The aim of this study was to investigate the influence of HLA class II alleles DRB1* and DQB1* on susceptibility to relapsing-remitting (RR) MS and response to interferon (IFN) β-1a treatment. A prospective observational study was conducted. Seventeen patients with clinically definite RRMS, attending a tertiary referral center for multiple sclerosis in Israel and receiving treatment with subcutaneous IFN β-1a, 22 mcg three times weekly were recruited between December 1998 and February 2000 and observed for 12 months. HLA genotyping was performed and clinical characteristics (relapse rate and disability progression) assessed at baseline and after 12 months. HLA data for a healthy control group were also used for comparison. HLA and the success of treatment with IFN β-1a in this group of RRMS patients were assessed. The frequency of DRB1*03 was six times higher in patients treated with IFN β-1a than in the healthy control group (n=100): 29% (5/17) versus 5% (5/100), respectively. Additionally, DQB1*03 and DQB1*02 were present in 82% (14/17) and 41% (7/17) of RRMS patients, but in only 33% (33/100) and 18% (18/100) of control patients, respectively. A better response to IFN β-1a treatment was seen in patients carrying these alleles than in patients without these alleles. Our results indicated that DRB1*03, DQB1*03 and DQB1*02 alleles may contribute to MS susceptibility and IFN β-1a responsiveness, and warrant further verification in a larger population.

Genetic diversity of the human immunoglobulin heavy chain VH region

The human immunoglobulin heavy chain VH region is one of the most complex regions in the human genome. The high level of diversity of this region has been shown by a number of studies. However, because of the limitations of the conventional experimental methods, it has been difficult to learn the extent of the diversity and the underlying mechanisms. This review describes a number of new genetic approaches developed in the authors' laboratory. By using these approaches, significant progress has been made in assigning different VH sequences to their respective loci, in learning the diversity of gene segment number and composition among the VH haplotypes, and in learning VH gene segment organization in individual haplotypes. Information obtained toward this direction could help in understanding the mechanisms underlying VH region diversity and the biological impact of the VH region diversity.

IgG allotypes and subclasses in Norwegian patients with multiple sclerosis

Multiple sclerosis (MS) is a multifactorial disease in which genetic and environmental factors apparently have a major influence on the susceptibility and course of the disease. In the present study we have investigated the genetic basis and subclass levels of IgG in MS. Hundred and thirty-six Norwegian patients with MS and 92 controls were genotyped for IgG allotypes of the GM and KM systems. IgG and IgG subclasses were quantified in sera from 115 MS patients and 20 controls. Neither GM nor KM allo-, haplo- or genotypes were significantly correlated with susceptibility, severity or course of the disease. The G1M (3) (3), G2M (23) (23) and G3M (5) (5) allotypes were significantly correlated with high serum levels of IgG3, whereas high IgG2 levels were correlated with G1M (3) (3) and G2M (23) (23) in both patients and controls. Serum levels of IgG subclasses were not significantly correlated with course or severity of the disease. The results indicate no major role for IgG allotypes or IgG subclass levels in the pathogenesis of MS.

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.

Genome-Wide Linkage Analysis of Chronic Relapsing Experimental Autoimmune Encephalomyelitis in the Rat Identifies a Major Susceptibility Locus on Chromosome 9

The immunization of inbred Dark Agouti (DA) rats with an emulsion containing homogenized spinal cord and CFA induces chronic relapsing experimental autoimmune encephalomyelitis (EAE), a disease with many similarities to multiple sclerosis. We report here the first genome-wide search for quantitative trait loci regulating EAE in the rat using this model. We identified one quantitative trait locus on chromosome 9, Eae4, in a [DA(RT1av1) × BN(RT1n)]F2 intercross showing linkage to disease susceptibility and expression of mRNA for the proinflammatory cytokine IFN-γ in the spinal cord. Eae4 had a larger influence on disease incidence among rats that were homozygous for the RT1av1 MHC haplotype (RT1av1 rats) compared with RT1n/av1 rats, suggesting an interaction between Eae4 and the MHC. Homozygosity for the DA allele at markers in Eae4 and in the MHC was sufficient for EAE. Thus, Eae4 is a major genetic factor determining susceptibility to EAE in this cross of DA rats. In addition, there was support for linkage to phenotypes of EAE on chromosomes 1, 2, 5, 7, 8, 12, and 15. The chromosome 12 region has been shown previously to predispose DA rats to arthritis, and the chromosome 2 region is syntenic to Eae3 in mice. We conclude that Eae4 and probably the other identified genome regions harbor genes regulating susceptibility to neuroinflammatory disease. The identification and functional characterization of these genes may disclose critical events in the pathogenesis of multiple sclerosis; understanding these events could be essential for the development of new therapies against the disease.

Comparison of clinical and demographic features between affected pairs of Italian multiple sclerosis multiplex families; relation to tumour necrosis factor genomic polymorphisms

We conducted a comparative analysis of clinical and demographic findings between pairs of relatives (36 sibling and 9 parent/child), concordant for Multiple Sclerosis (MS), from 40 MS Italian Multiplex families. A genetic TNF (alpha and beta) loci typing in 51 affected and 69 healthy relatives belonging to 25 of these families was also performed. The sib pairs resulted significantly concordant for age at onset (r=0.414, P<0.013), Progression Index (r=0.34, P<0.05) and sensory symptoms at onset (k=0.37), and significantly not concordant for sex (k=-0.37), whereas no concordance was found for year at onset and disease course. The only significant result in the small group of parent/child pairs was that parents developed MS at an age of 18.74 years significantly (P=0.020) greater than their children. Genomic analysis identified 13 variants of TNF-a alleles, 7 of TNF-b, 6 of TNF-d and 3 of TNF-e. No differences in the frequencies of the various TNF alleles were observed between affected and healthy relatives. The two-point lod-score analysis of the TNF locus showed not significant or negative results for the TNFalpha loci and slightly positive results (Zmax=0.4 at theta=0.2 cM) for the TNFbeta-b locus in the lowest penetrance dominant model. The Sib pair analysis, using combined TNFalpha and TNFbeta haplotypes, demonstrated a TNF allele sharing between affected sib-pairs which did not exceed the expected 50%. These results suggest that genetic factors may partially influence the disease onset and the progression rate in sibling pairs. A recall bias and/or an 'anticipation phenomenon' could explain the development of MS at an older age in parents than in their children. In this small-sized cohort of MS Italian families no significant associations were confirmed between TNF polymorphism and MS.

Changes in Bruch's membrane and related structures with age

Age-related macular disease is a major and growing public health burden in developed Caucasian societies, accounting for about 50% of blind registration. Evidence exists that this is an emerging problem in Eastern Asia, although the phenotype appears to differ from that seen in Western society. It is likely that several genes are involved, and that the genes or allelic variants conferring are common. Environment plays a major role in its pathogenesis, and it is believed that genetic susceptibility becomes apparent only if there are sufficient environmental pressures. There is no therapy currently available that will have an impact on the prevalence of blindness from age-related macular disease. It has been shown that visual loss occurs as a reaction to ageing changes in Bruch's membrane, which is interposed between the choriocapillaris and the retinal pigment epithelium. The age changes in all three structures have been partly characterised, and as a consequence, multiple putative pathogenic mechanisms have been proposed. Cross-sectional studies of populations with different genetic background and life styles would serve to prove the importance of inheritance and environment. Molecular genetic analysis of blood from affected sibling pairs from these sources may indicate the relevant genes, the prevalence of which may differ in different communities. Enquiries as to life styles may determine important environmental influences. Examination of donor eyes from these communities may reveal distinctive features that may reflect the variation in genetic predisposition and environmental pressures. It is hoped that the findings from such studies will lead to novel and potentially successful management strategies.

Molecular analysis of HLA class I (HLA-A and -B) and HLA class II (HLA-DRB1) genes in Japanese patients with multiple sclerosis (Western type and Asian type)

The types of HLA-A, -B and -DRB1 genes were studied in 146 Japanese patients with multiple sclerosis (MS) using polymerase chain reaction-sequence-specific oligonucleotide probe analysis. Fifty-seven patients who displayed selective clinical involvement of the optic nerve and spinal cord were classified as having Asian type MS. The other 89 patients had disseminated central nervous system involvement and were classified as having Western type MS. The frequency of HLA-B*5101 was increased in both types of MS patients compared with controls. The frequency of HLA-DRB1*1501 was increased in Western type MS and the frequency of HLA-DRB1*0802 was increased in Asian type MS compared to controls. After correction of P values, the association of Western type MS patients with HLA-DRB1*1501 was statistically significant (Pc=0.0003) whereas other HLA alleles showed no significant association. These results suggest that HLA class I (HLA-A and -B) alleles may not contribute to a strong susceptibility to MS in Japanese compared to HLA class II (HLA-DRB1) alleles.

HLA associations with multiple sclerosis in the Canary Islands

The study of small island populations has proved informative with respect to the epidemiology and genetics of many complex traits including multiple sclerosis. The class II major histocompatibility antigen DR15 is associated with multiple sclerosis in all groups except Sardinians, where the primary association is with DR4. We compared HLA-DR and -DQ allele frequencies in a representative sample of patients with multiple sclerosis from the Canary Islands with appropriate controls. There was a significant association with DR15 (patients 21/53: 40%: controls 11/55; 20%: chi2=4.09; pc=0.04; relative risk [RR]=1.98). DRB1*1501-DRB5*0101 was present in 17/53 (32%) patients in whom sub-types could be identified compared with 6/55 (11%) controls (chi2=7.21; pc=< 0.01; RR=2.94). All DR15 positive controls carried the DQA1*0102, DQB1*0602 haplotype whereas this was only present in 26/30 patients, suggesting that the primary association is with HLA-DR and not -DQ. We also found a significant increase in HLA-DR4 (16/53 [30%] in patients compared with 7/55 [13%] in controls; pc=0.05). This study contributes a new point on the immunogenetic map of multiple sclerosis in Europe, confirming the primary DR15 association with multiple sclerosis in a previously unstudied population but again highlighting the importance of DR4 in Mediterranean peoples.

Immune system genes in multiple sclerosis: genetic association and linkage analyses on TCR beta, IGH, IFN-gamma and IL-1ra/IL-1 beta loci

The role of genetic factors in the etiology of multiple sclerosis (MS) has been clearly demonstrated but the loci determining susceptibility to this disease remain largely unidentified. A contribution from several immune system genes has been suggested based on animal models and association/linkage analyses on MS patients and families. With the exception of the findings from the HLA complex, studies on candidate immune system genes have provided controversial results. Here we have performed genetic association and linkage analyses on four chromosomal regions containing immune system genes. A possible role for each of these loci in MS has been previously suggested. In data-sets derived from the Finnish population we found no evidence for contribution of the T-cell receptor beta chain (TCR beta chromosome 7q35), immunoglobulin heavy chain (IGH chromosome 14q32), interferon-gamma (IFN-gamma chromosome 12q14-q15) or interleukin-1 receptor antagonist/interleukin-1 beta (IL-1ra/IL-1 beta chromosome 2q14-q21) loci in the genetic susceptibility to MS.

No linkage or association of a VNTR marker in the junction region of the immunoglobulin heavy chain genes in multiple sclerosis

Multiple sclerosis (MS) is a demyelinating inflammatory disease of the central nervous system. Autoantibodies are though to participate in the pathogenesis. Previous reports on the role of immunoglobulin (Ig) variable gene segments in MS are contradictory. Here, by using a highly polymorphic variable number tandem repeat (VNTR) marker located in the centre of the IgH chain locus, we demonstrate a lack of linkage and association with MS in 34 multiplex families and 113 sporadic MS patients in Sweden. Stratification for the presence or absence of the MS-associated HLA-Dw2 haplotype did not influence the negative outcome. We conclude that the IgH chain genes are unlikely to play a role in genetic susceptibility to MS in the Swedish population.

The genetic analysis of multiple sclerosis

Although monogenic diseases often show extreme clinical phenotypes, the major burden of genetic ill health lies in the more prevalent polygenic disorders, such as diabetes, hypertension and multiple sclerosis. These conditions affect many thousands of individuals and their management consumes vast amounts of health care resources: in the UK some 80,000 people have multiple sclerosis; the estimated financial cost to society of introducing treatments, such as beta interferon, could be as high as 250 million pounds per year. Knowledge on the genetics of these common diseases is poor, but has potentially received a considerable boost with the arrival of whole genome screening. The genome screen in insulin-dependent diabetes mellitus (IDDM) reported in 1994 was the first in a human polygenic disease. Since this publication, whole genome screening has been performed in a variety of human polygenic diseases, including schizophrenia, bipolar affective disorder, non-insulin-dependent diabetes mellitus (NIDDM), inflammatory bowel disease, asthma and multiple sclerosis.

Polygenic control of experimental allergic encephalomyelitis in Biozzi ABH and BALB/c mice

Experimental allergic encephalomyelitis (EAE) is a chronic inflammatory disease of the central nervous system (CNS), with many similarities to multiple sclerosis (MS). Susceptibility to EAE is under genetic control of both the major histocompatibility complex (MHC) and unknown non-MHC gene products. This study uses a selective cross between EAE-susceptible ABH and low responder BALB/c mice, where disease is dominant and affects female mice significantly more than males. In a genome screen using microsatellite markers, linkage analysis suggests that genes encoded on chromosomes 4, 8, 10, 11, 12 and 17 contribute to the development of EAE (p < 0.05), although none of these putative EAE loci fulfilled the criteria for significant linkage. Interestingly, genotype frequency showed significant deviation from the expected random distribution of alleles on chromosomes 4, 8 and 17, (p < 0.001), with 32% of mice developing disease, exhibiting all 3 alleles (p < 0.001). This may indicate complex interactions amongst gene products in the EAE phenotype. This and other recent studies in different mouse strains underlies that EAE is a complex polygenic trait and may provide clues to the genetic mechanisms involved in autoimmune diseases such as multiple sclerosis.

A genome screen in multiple sclerosis reveals susceptibility loci on chromosome 6p21 and 17q22

The population prevalence of multiple sclerosis is 0.1%; however, the risk of the disease in the siblings of affected individuals is very much higher at 3-5%. The importance of genetic factors in accounting for this increased risk is confirmed by the results of twin and adoption studies. Despite the evidence for a strong genetic effect, a weak major histocompatibility complex (MHC) association is the only consistently observed feature in the genetics of multiple sclerosis. Other candidates have been proposed, including genes encoding the immunoglobulin heavy chain, T cell receptor beta chain and APOC2, but none has yet been confirmed. Evidence for linkage and association to the myelin basic protein gene has been reported in a genetically isolated Finnish population, but it has not been possible to reproduce these results in other populations. We used a two-stage approach to search the human genome for the genes causing susceptibility to multiple sclerosis. Two principal regions of linkage are identified, chromosomes 17q22 and 6p21 (MHC). Our results are compatible with genetic models involving epistatic interaction between these and several additional genes.

A full genome search in multiple sclerosis

The aetiology of multiple sclerosis (MS) is uncertain. There is strong circumstantial evidence to indicate it is an autoimmune complex trait. Risks for first degree relatives are increased some 20 fold over the general population. Twin studies have shown monozygotic concordance rates of 25-30% compared to 4% for dizygotic twins and siblings. Studies of adoptees and half sibs show that familial risk is determined by genes, but environmental factors strongly influence observed geographic differences. Studies of candidate genes have been largely unrewarding. We report a genome search using 257 microsatellite markers with average spacing of 15.2 cM in 100 sibling pairs (Table 1, data set 1 - DS1). A locus of lambda>3 was excluded from 88% of the genome. Five loci with maximum lod scores (MLS) of >1 were identified on chromosomes 2, 3, 5, 11 and X. Two additional data sets containing 44 (Table 1, DS2) and 78 sib pairs (Table 1, DS3) respectively, were used to further evaluate the HLA region on 6p21 and a locus on chromosome 5 with an MLS of 4.24. Markers within 6p21 gave MLS of 0.65 (non-significant, NS). However, D6S461, just outside the HLA region, showed significant evidence for linkage disequilibrium by the transmission disequilibrium test (TDT), in all three data sets (for DS1 chi2 = 10.8, adjusted P < 0.01)(DS2 and DS3 chi2 = 10.9, P < 0.0005), suggesting a modest susceptibility locus in this region. On chromosome 5p results from all three data sets (222 sib pairs) yielded a multipoint MLS of 1.6. The results support genetic epidemiological evidence that several genes interact epistatically to determine heritable 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).

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.

Multiple sclerosis and the HLA-D region: linkage and association studies

Inheritance patterns of multiple sclerosis (MS) in multiplex families suggest a complex aetiology involving environmental and genetically determined components. The association between the HLA class II DR15, DQ6, Dw2 haplotype and MS has been well documented in patients with ancestral origins in Northern Europe. Conversely, linkage analysis of this region in multiplex families, derived from a population base, has generated negative results. Thus, given the Dw2 specificity association, evidence implicating this locus in disease susceptibility appears contradictory. We have collected and determined the HLA-DR and -DQ haplotypes of 115 sibling pairs with multiple sclerosis, and confirm a significant association with the Dw2-associated haplotype, both in index cases and their affected siblings compared with controls. However, using a sibling pair linkage analysis that restricts haplotype sharing probabilities to defined genetic models, we have not observed linkage of this region to susceptibility in MS. We discuss the basis for association and linkage and conclude that the DR15, DQ6, Dw2 haplotype does represent a susceptibility locus but its contribution to the pathogenesis is small; although it may interact epistatically with other susceptibility genes, this haplotype is not necessary for disease expression.