Epigenetics and complex diseases, a new era in the assessment of exposure and risk?

Recent advances in the field of epigenetics help us to understand how environmental factors may trigger the development of complex diseases. This insight entails a radical shift in the way that exposure to aggressors should be regarded, as we are now minded to undertake epigenomic studies with high-throughput technology. It would be desirable to add this new methodology to risk assessment, since, as we argue, the current observational approach has a limited capacity to predict disease resulting from the incidence of epigenetic modifiers. Globally, the term epigenetics comprises a group of mechanisms able to imprint a transmittable pattern of gene expression to cells, without altering their genomic sequence. This is done through changes between the so-called open and closed frames of chromatin conformation, which, respectively, lead to increasing or shutting down the transcriptional activity of genes [1]. Both during development and in adult life, cell processes are subject to a tight epigenetic control, and its failure can directly cause disease or increase susceptibility to future conditions [2]. Epigenetic defects, or epimutations, encompass changes in the density of DNA methylation marks, incorrect histone post-translational modifications, or altered function of chromatin modifying enzymes. Typically, a failure of epigenetic control can be observed at early phases of tumorigenesis, during which an accumulation of epimutations alters the cell expression of lineage-associated genes, commonly preceding clonal expansion [3]. Complex diseases arise as a consequence of the interaction between individual factors and the environment. However, in most cases the actual impact of external aggressors on their development remains undeciphered. On one hand, the relatively low prevalence of the effect together with the concurrence of multiple confounders precludes the analysis of attributable risk. On the other, to firmly state a lack of effect is challenging, as it would require long observation periods. Accordingly, epidemiological methodology alone is not conclusive for establishing associations, and this should be acknowledged by both clinicians and experts laying recommendations [4]. The fact that a variety of external stressors, such as pollutants, infectious agents, nutrients or drugs, may act as epigenetic modifiers further hampers traceability of the exposure [1, 5]. It is thought that environmental interactions experienced by our ancestors have greatly contributed to the shape of our current epigenetic marks. This same plasticity permits the appearance of variations in our epigenome as a result of changes in our habits or following exposure to stressors [6]. However, the yield of the exposure is not easy to forecast, as multiple factors are involved in the imprinting of new epigenetic marks and additional ones in rendering an overt phenotype. Although a sustained exposure is probably needed, it does not necessarily take a high concentration of the stressor to induce an epigenetic modification, so it can pass unnoticed. In addition, the outcome relies to a certain extent on the genetic background, as well as on the cell replication rate. Furthermore, once the modification has taken place, the host may keep a long lasting memory of this new cell programme in spite of the cessation of the exposure. For example, tobacco smoking, a prototypic epigenetic modifier, has been shown to alter differentially methylated CpG regions accounting for inter-individual epigenetic variations. Data from prospective cohorts show that, while most of the tobacco-targeted methyl marks slowly revert after quitting, some others persist for years. This prolonged memory of an exposure makes the insult and its visible effect totally dissociated in time [7]. In the always difficult to forecast effect of epigenetic modifiers, the period of intrauterine development should be regarded as particularly hazardous [6, 8]. At the periconceptional stage and during early embryogenesis there is a thorough removal of parental epigenetic marks, which prevents epimutations from being passed on to the next generation. This is followed by intensive epigenetic activity in embryo cells eventually leading to the acquisition by the conceptus of its own pattern of gene expression. Apparently, offspring conceived with assisted reproductive techniques are prone to the occurrence of gene imprinting disorders, this fact reflecting the complexity of artificially reproducing the early embryo reprogramming process [6]. Of note, this epigenetic activity includes germ cells, which follow a separate programme to retain stemness. In this regard, three generations can suffer the effect of exposure to epigenetic modifiers during pregnancy (Fig. 1) [9]. Along with the pregnant woman, or generation G1, the modifier can target foetal somatic cells and lead to epigenetic defects in the offspring (G2), but also G2 germline, which results in epimutations in G3. This type of memory of the exposure makes the origin of the trait almost impossible to track back, particularly considering that the resulting alterations are not necessarily visible at delivery. As an example, maternal smoking has been shown to alter the DNA methylation of genes of the conceptus involved in developmental processes [9, 10]. Interestingly, the children carry the epigenetic defects for some decades and more frequently develop behavioural difficulties, weight problems and elevated blood