Epigenetic understanding of gene-environment interactions in psychiatric disorders: a new concept of clinical genetics

Until recently, in clinical genetics, epigenetics was a minor field, of which two unusual genetic phenomena (genomic imprinting and X-chromosome inactivation (XCI)) were the main aspects under investigation. Based on the findings related to these phenomena, epigenetic disorders were considered to be very rare. However, as epigenetics has become more popular, it has developed into a huge research field that extends beyond genetics, encompassing not only biology and medicine, but also nutrition, education, health and social sciences. It now appears that epigenetics bridges the two major disease-causing factors (environmental and genetic) in medicine. Therefore, it is time to review epigenetics in the light of recent findings.In this review, we explain the epigenetic mechanisms that cause congenital disorders, show examples of environmental factors that can alter the epigenetic status, and discuss recent topics in epigenetics, such as the possibility of its inheritance and the use of epigenetic strategies for the treatment of diseases.It has long been thought that environmental and genetic factors are involved in the pathogenesis of common diseases such as cancer, diabetes, and psychiatric disorders [1-5]. For instance, environmental factors, such as drugs, viral infection, toxins and vaccines were proposed to be associated with the recent increase in the frequency of autism [6-9].In the meantime, a number of genes related to autism have been identified, which are mutated in a subset of autistic children. Most of these genes encode synaptic proteins, including synaptic scaffolding proteins, receptors, transporters, and cell-adhesion molecules [10,11]. A recent comprehensive study confirmed that there were differences between autistic and control brains in the expression levels of genes encoding synaptic proteins and proteins related to inflammation [12]. Based on these findings, autism is now considered as a 'synaptogenesis disorder' [13,14],, and designated 'synaptic