Search

Multiple sclerosis (MS), a common central nervous system inflammatory disease, has a major heritable component. Susceptibility is associated with the MHC class II region, especially HLA-DRB5*0101-HLA-DRB1*1501-HLA-DQA1*0102-HLA-DQB1*0602 haplotypes (hereafter DR2), which dominate genetic contribution to MS risk. Marked linkage disequilibrium (LD) among these loci makes identification of a specific locus difficult. The once-leading candidate, HLA-DRB1*15, localizes to risk, neutral, and protective haplotypes. HLA-DRB1*15 and HLA-DQB1*0602, nearly always Iocated together on a small ancestral chromosome segment, are strongly MS-associated. One intervening allele on this haplotype, viz. HLA-DQA1*0102, shows no primary MS association. Two Canadian cohorts (n = 830 and n = 438 trios) genotyped for HLA-DRB1, HLA-DQA1 and HLA-DQB1 alleles were tested for association using TDT. To evaluate epistasis involving HLA-DRB1*15, transmissions from HLA-DRB1*15-negative parents were stratified by the presence/absence of HLA-DRB1*15 in affected offspring. All 3 alleles contribute to MS susceptibility through novel epistatic interactions. HLA-DQA 1*0102 increased disease risk when combined with HLA-DRB1*1501 in trans, thereby unambiguously implicating HLA-DQ in MS susceptibility. Three-locus haplotypes demonstrated that HLA-DRB1*1501 and HLA-DQB1*0602 each influence risk. Transmissions of rare morcellated DR2 haplotypes showed no interaction with HLA-DQA1*0102. Incomplete haplotypes bearing only HLA-DRB1*1501 or HLA-DQB1*0602 d id not predispose to MS. Balanced reciprocal transmission distortion can mask epistatic allelic association. These findings implicate epistasis among HLA class II alleles in human immune responses generally, provide partial explanation for intense linkage disequilibrium in the MHC, have relevance to animal models, and demonstrate key roles for DR2-specific interactions in MS susceptibility. MHC disease associations may be more generally haplotypic or diplotypic. genetics | MHC | linkage disequilibrium

The major locus for multiple sclerosis (MS) susceptibility is located within the class II region of the Major Histocompatibility Complex (MHC). HLA-DRB1 alleles, constituting the strongest MS susceptibility factors, have been widely exploited in research including construction of transgenic animal models of MS. Many studies have concluded that HLA-DRB1*15 allele itself determines MS-associated susceptibility. If this were true, haplotypes bearing this allele would confer equal risk. If HLA-DRB1*15 bearing haplotypes differed for risk, roles for other loci in this region would be implied and further study of the fine structure of this locus would be compelling. We have tested the hypothesis comparing haplotypes stratified by HLA class I tagging. We show here that HLA-DRB1*15-bearing-haplotypes in 1970 individuals from 494 MS families are indeed heterogeneous. Some HLA-DRB1*15 haplotypes determine susceptibility while others do not. Three groups of class I tagged HLA-DRB1*15 haplotypes were not over-transmitted: (i) HLA-DRB1*15-HLA-B*08 (TR = 25, NT = 23, Odds Ratio = 1.09), (ii) -HLA-B*27 (TR = 18, NT = 17, Odds Ratio = 1.06), and (iii) rare HLA-DRB1*15 haplotypes (frequency MHC | class II | transmission

The major locus for multiple sclerosis (MS) susceptibility is located within the class II region of the Major Histocompatibility Complex (MHC). HLA-DRB1 alleles, constituting the strongest MS susceptibility factors, have been widely exploited in research including construction of transgenic animal models of MS. Many studies have concluded that HLA-DRB1*15 allele itself determines MS-associated susceptibility. If this were true, haplotypes bearing this allele would confer equal risk. If HLA-DRB1*15 bearing haplotypes differed for risk, roles for other loci in this region would be implied and further study of the fine structure of this locus would be compelling. We have tested the hypothesis comparing haplotypes stratified by HLA class I tagging. We show here that HLA-DRB1*15-bearing-haplotypes in 1970 individuals from 494 MS families are indeed heterogeneous. Some HLA-DRB1*15 haplotypes determine susceptibility while others do not. Three groups of class I tagged HLA-DRB1*15 haplotypes were not over-transmitted: (i) HLA-DRB1*15-HLA-B*08 (TR = 25, NT = 23, Odds Ratio = 1.09), (ii) -HLA-B*27 (TR = 18, NT = 17, Odds Ratio = 1.06), and (iii) rare HLA-DRB1*15 haplotypes (frequency < 0.02). Rare haplotypes were significantly different from common haplotypes, and transmissions were remarkably similar to those for class-I-matched non-HLA-DRB1*15 haplotypes. These results unambiguously indicate that HLA-DRB1*15 is part of a susceptibility haplotype but cannot be the susceptibility allele itself, requiring either epistatic interactions, epigenetic modifications on some haplotypes, or nearby structural variation. These findings strongly imply that differences among HLA-DRB1*15 haplotypes will furnish the basis for MHC-associated susceptibility in MS and raise the possibility that the MHC haplotype is the fundamental unit of genetic control of immune response. [ABSTRACT FROM AUTHOR]