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      Genetic epidemiology and Mendelian randomization for informing disease therapeutics: Conceptual and methodological challenges

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      PLoS Genetics
      Public Library of Science

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          Abstract

          The past decade has been proclaimed as a hugely successful era of gene discovery through the high yields of many genome-wide association studies (GWAS). However, much of the perceived benefit of such discoveries lies in the promise that the identification of genes that influence disease would directly translate into the identification of potential therapeutic targets, but this has yet to be realized at a level reflecting expectation. One reason for this, we suggest, is that GWAS, to date, have generally not focused on phenotypes that directly relate to the progression of disease and thus speak to disease treatment.

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          Most cited references22

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          'Mendelian randomization': can genetic epidemiology contribute to understanding environmental determinants of disease?

          Associations between modifiable exposures and disease seen in observational epidemiology are sometimes confounded and thus misleading, despite our best efforts to improve the design and analysis of studies. Mendelian randomization-the random assortment of genes from parents to offspring that occurs during gamete formation and conception-provides one method for assessing the causal nature of some environmental exposures. The association between a disease and a polymorphism that mimics the biological link between a proposed exposure and disease is not generally susceptible to the reverse causation or confounding that may distort interpretations of conventional observational studies. Several examples where the phenotypic effects of polymorphisms are well documented provide encouraging evidence of the explanatory power of Mendelian randomization and are described. The limitations of the approach include confounding by polymorphisms in linkage disequilibrium with the polymorphism under study, that polymorphisms may have several phenotypic effects associated with disease, the lack of suitable polymorphisms for studying modifiable exposures of interest, and canalization-the buffering of the effects of genetic variation during development. Nevertheless, Mendelian randomization provides new opportunities to test causality and demonstrates how investment in the human genome project may contribute to understanding and preventing the adverse effects on human health of modifiable exposures.
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            Mendelian randomization studies: a review of the approaches used and the quality of reporting.

            Mendelian randomization (MR) studies investigate the effect of genetic variation in levels of an exposure on an outcome, thereby using genetic variation as an instrumental variable (IV). We provide a meta-epidemiological overview of the methodological approaches used in MR studies, and evaluate the discussion of MR assumptions and reporting of statistical methods.
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              Genome-wide association study identifies distinct genetic contributions to prognosis and susceptibility in Crohn's disease

              James Lee, Kenneth Smith and colleagues report a within-cases genome-wide association analysis for Crohn's disease to identify genetic loci specifically associated with disease severity and outcome. They find four loci associated with prognosis, none of which is associated with susceptibility to Crohn's disease.
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                Author and article information

                Contributors
                Role: Editor
                Journal
                PLoS Genet
                PLoS Genet
                plos
                plosgen
                PLoS Genetics
                Public Library of Science (San Francisco, CA USA )
                1553-7390
                1553-7404
                5 October 2017
                October 2017
                : 13
                : 10
                : e1006944
                Affiliations
                [001]Medical Research Council Integrative Epidemiology Unit, University of Bristol, Bristol, United Kingdom
                Stanford University School of Medicine, UNITED STATES
                Author notes

                The authors have declared that no competing interests exist.

                Author information
                http://orcid.org/0000-0003-2514-0889
                http://orcid.org/0000-0002-1407-8314
                Article
                PGENETICS-D-17-00929
                10.1371/journal.pgen.1006944
                5628782
                28981501
                3d703229-e046-4c3a-a5a7-00fe1f3a559c
                © 2017 Paternoster et al

                This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                History
                Page count
                Figures: 3, Tables: 1, Pages: 9
                Funding
                The authors work in an MRC-funded unit (MC_UU_12013/1, /4, /9). The funders had no role in the preparation of the article.
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