- Front Matter
4
- 10.2217/epi.15.48
Epigenetic epidemiology: is there cause for optimism?
- Aug 01, 2015
- Epigenomics
- Michèle Ramsay
Modulation of gene expression is influenced by environmental exposures, genetic variation and stochastic processes. Epigenetic biomarkers that signal the outcome of their combined impact could be important early predictors of susceptibility to chronic noncommunicable diseases. Can epigenetic epidemiology shed light on causality and mechanism? Common complex traits arise over time and are the end result of lifestyle choices and a combination of environmental exposures during periods of developmental vulnerability and epigenetic plasticity in the presence of a combination of susceptible genetic variants. Genome-wide association studies to identify the genetic contributions to complex traits have identified many high probability allelic associations, but almost invariably each with a small effect, explaining only a modest percentage of the heritability of the trait. Notably, many estimates of heritability are inflated and do not adequately take into account-shared environment and other familial factors. It is not new to suggest that tissue-specific epigenetic modulation plays a major role in the development of complex traits, yet few have managed to support this using a robust experimental approach. Whereas the genome remains constant, gene expression is labile, demonstrates cell type and developmental specificity, age and sex differences, and is variably responsive to changes in the environment. What are the relationships between genetic variation and different environments that influence epigenetic responses? How do we begin to understand causal relationships that influence gene expression? From an experimental perspective, it is challenging to design studies that can address these questions, yet present a feasible and affordable approach. Large international and interdisciplinary studies are beginning to provide a good framework to better understand gene expression regulation in humans. Though open to criticism, their value should not be underestimated. They include ENCODE that examined 147 cell lines to search for functional elements in the genome [1]; the Epigenomics Roadmap [2] that examined 127 tissue biopsies from embryos and adults, both diseased and healthy, with a focus on identifying functional regulatory elements; the International Human Epigenome Consortium [3]; and the Genome Tissue Expression project (GTEx) that explores the relationship between genetic variation and tissue-specific gene expression and alternate splicing by sequencing whole genomes and examining gene expression in over 20,000 tissue samples from 900 individuals [4]. These data are beginning to give insight into cell specific, developmental stage, sex, age and ethnicity influenced, and stochastic gene expression differences at an individual and population level. The layered annotations in genome browsers highlight the importance of integrating transcriptome data (mRNA, miRNA and lncRNA) with DNA methylation, histone modification and chromatin interaction data to more fully understand function. Epigenome-wide association studies (EWASs) to identify regulatory mechanisms Epigenetic epidemiology: is there cause for optimism?
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