Epigenetics refers to the additional molecular layers and chemical modifications to the genome that control when and where genes are expressed but do not involve a change in the underlying DNA sequence. Such modifications include DNA methylation and histone modification, both of which serve to regulate gene expression without altering the nucleotide sequence of the genome.

Relevance to autism:

Epigenetic mechanisms act at the interface of genetic and environmental factors regulating brain development and determining autism risk. Analysis of postmortem brain tissue, for example, shows that epigenetic markers vary in people with autism compared with controls.

Epigenetic modifications, particularly DNA methylation, can be influenced by chemical exposures in the environment, hormones, the firing patterns of neurons, and diet. Folate, a B vitamin that we absorb from food and is included in prenatal vitamins, is necessary for DNA synthesis and methylation. Preconception use of prenatal vitamins was found to be protective for autism, particularly in mothers with genetic susceptibility in methyl-group metabolism.

Pinpointing epigenetic differences between healthy and diseased cells at progressive stages of development could reveal the roots of neurodevelopmental dysfunction.

Epigenetic changes can occur in adult brain cells and can be ‘activity-dependent’ — that is, triggered by brain activity in response to experience. Such neural responses to changes in the environment are an essential component of adaptive brain function. Therefore, dysregulation of this capacity could lead to maladaptive behavior.

Some studies suggest that when one identical twin has autism, the other will also have it about 40 to 90 percent of the time. A better understanding of how the prenatal environment influences epigenetics could help pinpoint early environmental risk factors for autism and help explain those cases in which only one twin has autism.

Mapping the epigenome in the brain throughout the lifespan is important for understanding epigenomic changes in autism. In 2013, scientists unveiled the first comprehensive maps of human and mouse DNA methylation patterns from fetal development through adulthood. A ten-year effort known as the International Human Epigenome Consortium aims to catalog and examine epigenetic maps for all cell types throughout the course of development.