Age at onset of multiple sclerosis may be influenced by place of residence during childhood rather than ancestry.

Multiple sclerosis (MS) most commonly affects individuals of Northern European descent who live in countries at high latitude. The relative contributions of ancestry, country of birth and residence as determinants of MS risk have been studied in adult MS, but have not been explored in the pediatric MS population. In this study, we compare the demographics of pediatric- and adult-onset MS patients cared for in Toronto, Ontario, Canada, a multicultural region. The country of birth, residence during childhood, and ancestry were compared for 44 children and 573 adults. Our results demonstrate that although both the pediatric and adult cohorts were essentially born and raised in the same region of Ontario, Canada, children with MS were more likely to report Caribbean, Asian or Middle Eastern ancestry, and were less likely to have European heritage compared with individuals with adult-onset MS. The difference in ancestry between the pediatric and adult MS cohorts can be explained by two hypotheses: (1) individuals raised in a region of high MS prevalence, but whose ancestors originate from regions in which MS is rare, have an earlier age of MS onset, and (2) the place of residence during childhood, irrespective of ancestry, determines lifetime MS risk -- a fact that will be reflected in a change in the demographics of the adult MS cohort in our region as Canadian-raised children of recent immigrants reach the typical age of adult-onset MS.