MHC transmission: insights into gender bias in MS susceptibility

OBJECTIVE

Major histocompatibility complex (MHC) genes dominate genetic susceptibility factors in multiple sclerosis (MS). Given the general consensus that incidence and prevalence of MS has been rising and specifically in women, we evaluated MHC-gender interactions.

METHODS

In a large family-based cohort consisting of 7,093 individuals (2,127 affected individuals) from 1,055 MS families, we examined MHC transmission by family structure and gender stratified by genetic distance of affected relatives from the MS proband.

RESULTS

We found that affected individuals with HLA-DRB1*15-positive genotypes have higher female-to-male ratios as compared with affected individuals with HLA-DRB1*15-negative genotypes (χ(2) = 9.97, p = 0.0015) with the exception of multiplex families with 3 or more affected across 2 generations. Transmission disequilibrium test results show that HLA-DRB1*15 transmission was more distorted in collateral families vs nuclear families (χ(2) = 8.030, p = 0.0046), exclusively in affected female-female pairs (χ(2) = 7.81, p = 0.0051), but not in mixed gender pairs (χ(2) = 1.58, p = 0.21) or matched male pairs (Fisher p = 0.21).

CONCLUSIONS

These observations implicate the MHC as the site of interactions and modifications mediating the female-to-male gender ratio in MS and its progressive increase. They further suggest this occurs via gene-environment interactions and epigenetic modifications in this region. The difference between collateral and nuclear families provides some insight into the inheritance, decay, and gender specificity of putative epigenetic marks.

Multiple sclerosis susceptibility and the X chromosome

Multiple sclerosis (MS) is a chronic autoimmune complex trait with strong evidence for a genetic component. A female gender bias is clear but unexplained and a maternal parent-of-origin effect has been described. X-linked transmission of susceptibility has been previously proposed, based on pedigree, association and linkage studies. We genotyped 726 relative pairs including 552 affected sib-pairs for 22 X-chromosome microsatellite markers and a novel dataset of 195 aunt-uncle/niece-nephew (AUNN) affected pairs for 18 markers. Parent-of-origin effects were explored by dividing AUNN families into likely maternal and paternal trait transmission. For the sib-pair dataset we were able to establish exclusion at a lambda s = 1.9 for all markers using an exclusion threshold of LOD < or = -2. Similarly for the AUNN dataset, we established exclusion at lambdaAV = 1.9. For the combined dataset we estimate exclusion of lambda = 1.6. We did not identify significant linkage in either the sib-pairs or the AUNN dataset nor when datasets were stratified for the presence/absence of the HLA-DRB1*15 allele or for paternal or maternal transmission. This comprehensive scrutiny of the X-chromosome suggests that it is unlikely to harbour an independent susceptibility locus or one which interacts with the HLA. Complex interactions including epigenetic ones, and masking by balanced polymorphisms are mechanisms not excluded by the approach taken.

Follow-up investigation of 12 proposed linkage regions in multiple sclerosis

Multiple sclerosis (MS) is an autoimmune disease with overwhelming evidence for genetic determination, and for which a maternal parent-of-origin effect has been reported. As with many complex diseases, multiple suggestive linkage signals have been observed. However, the only unambiguous association and linkage identified to date is with alleles of the human lymphocyte antigen (HLA) class II region. We have now carried out high-density microsatellite genotyping for 12 of the most promising regions (1p, 1q, 2q, 4q, 5p, 9q, 10p, 11p, 12q, 17q, 18p, 19p) from a whole-genome scan in 552 affected sibling pairs. This has been carried out in 194 families containing avuncular pairs. These permit examination of parent-of-origin effects in non-colineal pairs when divided into likely maternal and paternal trait transmission. The results do not confirm any non-major histocompatibility complex linkage in the overall subset nor in the maternal, paternal or HLA-DRB1*1501 subsets. We were able to establish exclusion for a locus with lambda(AV) > or = 1.3 for all the previously suggested regions. These results again raise the possibility that the paradigm of multiple genes of small individual effect used to justify genome searches in MS is incorrect.