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      Environmental Risk Factors for Multiple Sclerosis: A Review with a Focus on Molecular Mechanisms

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          Abstract

          Multiple sclerosis (MS) is a chronic disabling disease of the central nervous system commonly affecting young adults. Pathologically, there are patches of inflammation (plaques) with demyelination of axons and oligodendrocyte loss. There is a global latitude gradient in MS prevalence, and incidence of MS is increasing (particularly in females). These changes suggest a major role for environmental factors in causation of disease. We have reviewed the evidence and potential mechanisms of action for three exposures: vitamin D, Epstein Barr virus and cigarette smoking. Recent advances supporting gene-environment interactions are reviewed. Further research is needed to establish mechanisms of causality in humans and to explore preventative strategies.

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          Risk alleles for multiple sclerosis identified by a genomewide study.

          Multiple sclerosis has a clinically significant heritable component. We conducted a genomewide association study to identify alleles associated with the risk of multiple sclerosis. We used DNA microarray technology to identify common DNA sequence variants in 931 family trios (consisting of an affected child and both parents) and tested them for association. For replication, we genotyped another 609 family trios, 2322 case subjects, and 789 control subjects and used genotyping data from two external control data sets. A joint analysis of data from 12,360 subjects was performed to estimate the overall significance and effect size of associations between alleles and the risk of multiple sclerosis. A transmission disequilibrium test of 334,923 single-nucleotide polymorphisms (SNPs) in 931 family trios revealed 49 SNPs having an association with multiple sclerosis (P<1x10(-4)); of these SNPs, 38 were selected for the second-stage analysis. A comparison between the 931 case subjects from the family trios and 2431 control subjects identified an additional nonoverlapping 32 SNPs (P<0.001). An additional 40 SNPs with less stringent P values (<0.01) were also selected, for a total of 110 SNPs for the second-stage analysis. Of these SNPs, two within the interleukin-2 receptor alpha gene (IL2RA) were strongly associated with multiple sclerosis (P=2.96x10(-8)), as were a nonsynonymous SNP in the interleukin-7 receptor alpha gene (IL7RA) (P=2.94x10(-7)) and multiple SNPs in the HLA-DRA locus (P=8.94x10(-81)). Alleles of IL2RA and IL7RA and those in the HLA locus are identified as heritable risk factors for multiple sclerosis. Copyright 2007 Massachusetts Medical Society.
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            A brief history of T(H)17, the first major revision in the T(H)1/T(H)2 hypothesis of T cell-mediated tissue damage.

            For over 35 years, immunologists have divided T-helper (T(H)) cells into functional subsets. T-helper type 1 (T(H)1) cells-long thought to mediate tissue damage-might be involved in the initiation of damage, but they do not sustain or play a decisive role in many commonly studied models of autoimmunity, allergy and microbial immunity. A major role for the cytokine interleukin-17 (IL-17) has now been described in various models of immune-mediated tissue injury, including organ-specific autoimmunity in the brain, heart, synovium and intestines, allergic disorders of the lung and skin, and microbial infections of the intestines and the nervous system. A pathway named T(H)17 is now credited for causing and sustaining tissue damage in these diverse situations. The T(H)1 pathway antagonizes the T(H)17 pathway in an intricate fashion. The evolution of our understanding of the T(H)17 pathway illuminates a shift in immunologists' perspectives regarding the basis of tissue damage, where for over 20 years the role of T(H)1 cells was considered paramount.
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              Sex ratio of multiple sclerosis in Canada: a longitudinal study.

              Incidence of multiple sclerosis is thought to be increasing, but this notion has been difficult to substantiate. In a longitudinal population-based dataset of patients with multiple sclerosis obtained over more than three decades, we did not show a difference in time to diagnosis by sex. We reasoned that if a sex-specific change in incidence was occurring, the female to male sex ratio would serve as a surrogate of incidence change. Since environmental risk factors seem to act early in life, we calculated sex ratios by birth year in 27 074 Canadian patients with multiple sclerosis identified as part of a longitudinal population-based dataset. The female to male sex ratio by year of birth has been increasing for at least 50 years and now exceeds 3.2:1 in Canada. Year of birth was a significant predictor for sex ratio (p<0.0001, chi(2)=124.4; rank correlation r=0.84). The substantial increase in the female to male sex ratio in Canada seems to result from a disproportional increase in incidence of multiple sclerosis in women. This rapid change must have environmental origins even if it is associated with a gene-environment interaction, and implies that a large proportion of multiple sclerosis cases may be preventable in situ. Although the reasons why incidence of the disease is increasing are unknown, there are major implications for health-care provision because lifetime costs of multiple sclerosis exceed pound1 million per case in the UK.
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                Author and article information

                Journal
                Int J Mol Sci
                Int J Mol Sci
                ijms
                International Journal of Molecular Sciences
                Molecular Diversity Preservation International (MDPI)
                1422-0067
                2012
                18 September 2012
                : 13
                : 9
                : 11718-11752
                Affiliations
                [1 ]School of Medicine, Griffith University, Gold Coast, Queensland 4222, Australia
                [2 ]National Centre for Epidemiology and Population Health, the Australian National University, Canberra, Australian Capital Territory 0200, Australia; E-Mail: robyn.lucas@ 123456anu.edu.au
                [3 ]Menzies Research Institute Tasmania, University of Tasmania, Hobart, Tasmania 7000, Australia; E-Mail: bruce.taylor@ 123456utas.edu.au
                Author notes
                [* ]Author to whom correspondence should be addressed; E-Mail: c.ogorman@ 123456griffith.edu.au ; Tel.: +61-7-5678-0702; Fax: +61-7-5678-0303.
                Article
                ijms-13-11718
                10.3390/ijms130911718
                3472772
                23109880
                bc89da88-cfff-4f77-b65d-6ad11015b7ee
                © 2012 by the authors; licensee Molecular Diversity Preservation International, Basel, Switzerland.

                This article is an open-access article distributed under the terms and conditions of the Creative Commons Attribution license ( http://creativecommons.org/licenses/by/3.0/).

                History
                : 31 July 2012
                : 31 August 2012
                : 06 September 2012
                Categories
                Review

                Molecular biology
                gene-environment interaction,multiple sclerosis,epstein-barr virus,vitamin d,latitude,epidemiology,smoking,demyelination

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