Objectives To determine if risk of multiple sclerosis (MS) is associated with month of birth in countries in the northern hemisphere and if factors related to month of birth interact with genetic risk. Design Population based study with population and family based controls and a retrospective cohort identified from death certificates. A post hoc pooled analysis was carried out for large northern datasets including Sweden and Denmark. Setting 19 MS clinics in major cities across Canada (Canadian collaborative project on the genetic susceptibility to multiple sclerosis); incident cases of MS from a population based study in the Lothian and Border regions of Scotland; and death records from the UK Registrar General. Populations 17 874 Canadian patients and 11 502 British patients with multiple sclerosis. Main outcome measure Diagnosis of multiple sclerosis. Results In Canada (n = 17 874) significantly fewer patients with MS were born in November compared with controls from the population census and unaffected siblings. These observations were confirmed in a dataset of British patients (n = 11 502), in which there was also an increase in the number of births in May. A pooled analysis of datasets from Canada, Great Britain, Denmark, and Sweden (n = 42 045) showed that significantly fewer (8.5%) people with MS were born in November and significantly more (9.1%) were born in May. For recent incident data, the effect of month of birth was most evident in Scotland, where MS prevalence is the highest. Conclusions Month of birth and risk of MS are associated, more so in familial cases, implying interactions between genes and environment that are related to climate. Such interactions may act during gestation or shortly after birth in individuals born in the northern countries studied.

Size and ascertainment constraints often limit twin studies to concordance comparisons between identical and fraternal twins. Here we report the final results of a longitudinal, population-based study of twins with multiple sclerosis (MS) in Canada. Bias was demonstrably minimized, and an estimated 75% of all Canadian MS twin pairs were ascertained, giving a sample sufficiently large (n ‫؍‬ 370) to permit additional informative comparisons. Twinning was not found to affect prevalence, and twins with MS did not differ from nontwins for DR15 allele frequency nor for MS risk to their siblings. Probandwise concordance rates of 25.3% (SE ؎ 4.4) for monozygotic (MZ), 5.4% (؎2.8) for dizygotic (DZ), and 2.9% (؎0.6) for their nontwin siblings were found. MZ twin concordance was in excess of DZ twin concordance. The excess concordance in MZ was derived primarily from like-sexed female pairs with a probandwise concordance rate of 34 of 100 (34 ؎ 5.7%) compared with 3 of 79 (3.8 ؎ 2.8%) for female DZ pairs. We did not demonstrate an MZ͞DZ difference in males, although the sample size was small. We observed a 2-fold increase in risk to DZ twins over nontwin siblings of twins, but the difference was not significant. Multiple sclerosis (MS) is one of the most common neurological diseases affecting young adults (1). In Canada, at least 1 in 1,000 individuals has the disease (2-4), and twice as many women are affected compared with men. It is widely believed that susceptibility to MS is determined by a complex interaction between susceptibility genes and environment. Twin studies have played an important role in current concepts of complex traits including MS and have consistently demonstrated an excess of monozygotic (MZ) over dizygotic (DZ) concordance (refs. 5-19 and Table 1).The magnitude of the MZ-to-DZ difference has implied non-Mendelian inheritance (20). However, in more recent data, it seems that the absolute level of concordance in MZ twins also reflects the background population prevalence. Whereas in Canada and northern Europe concordance rates are high, in southern Europe twin concordance rates mirror the lower prevalence (ref. 5 and L. Ristori, S. Cannoni, and M. Salvetti, unpublished data). Uncertainty has remained about final concordance rates, because twins in previously reported studies retained considerable residual risk based on corrections for age of onset. Corrections can be readily calculated for the general population, but applying it to twins may be inappropriate because the age onset correlations are so much larger for concordant MZ pairs than for siblings (sibs) (21).Twin studies in MS and other putative autoimmune diseases have generally been small. Data have been insufficient in any single study to adequately assess the impact of ascertainment bias, a common confounder of twin studies (22). Questions beyond the comparison of MZ with DZ concordance have been left unanswered, and even this comparison shows wide confidence intervals in published data. Furthermore, it has not been possible to study an...

Multiple sclerosis (MS) is a complex trait in which allelic variation in the MHC class II region exerts the single strongest effect on genetic risk. Epidemiological data in MS provide strong evidence that environmental factors act at a population level to influence the unusual geographical distribution of this disease. Growing evidence implicates sunlight or vitamin D as a key environmental factor in aetiology. We hypothesised that this environmental candidate might interact with inherited factors and sought responsive regulatory elements in the MHC class II region. Sequence analysis localised a single MHC vitamin D response element (VDRE) to the promoter region of HLA-DRB1. Sequencing of this promoter in greater than 1,000 chromosomes from HLA-DRB1 homozygotes showed absolute conservation of this putative VDRE on HLA-DRB1*15 haplotypes. In contrast, there was striking variation among non–MS-associated haplotypes. Electrophoretic mobility shift assays showed specific recruitment of vitamin D receptor to the VDRE in the HLA-DRB1*15 promoter, confirmed by chromatin immunoprecipitation experiments using lymphoblastoid cells homozygous for HLA-DRB1*15. Transient transfection using a luciferase reporter assay showed a functional role for this VDRE. B cells transiently transfected with the HLA-DRB1*15 gene promoter showed increased expression on stimulation with 1,25-dihydroxyvitamin D3 (P = 0.002) that was lost both on deletion of the VDRE or with the homologous “VDRE” sequence found in non–MS-associated HLA-DRB1 haplotypes. Flow cytometric analysis showed a specific increase in the cell surface expression of HLA-DRB1 upon addition of vitamin D only in HLA-DRB1*15 bearing lymphoblastoid cells. This study further implicates vitamin D as a strong environmental candidate in MS by demonstrating direct functional interaction with the major locus determining genetic susceptibility. These findings support a connection between the main epidemiological and genetic features of this disease with major practical implications for studies of disease mechanism and prevention.