Epigenetics is the study of how the environment and other factors can change the way that genes are expressed. While epigenetic changes do not alter the sequence of a person's genetic code, they can play an important role in development. Scientists who work in epigenetics explore the mechanisms that affect the activity of genes.
Each person's DNA lays the groundwork for the development of physical and psychological characteristics—providing complex instructions for the creation of proteins and other molecules. But the manner in which these instructions are used can be modified by various factors. The chemical modifications that influence gene activity in this way are collectively called the epigenome.
These modifications occur naturally and help to steer development—for example, they enable cells in the brain and in other parts of the body to perform specialized roles based on the same underlying genetic code. But the epigenome is also susceptible to influence by exposure to toxins and other environmental factors.
Genetics is the study of genes—the units of a person’s genetic code, made from DNA—and the traits that they influence. Epigenetics focuses on physical changes that affect how the genes are “expressed”—whether, for example, a particular gene is active or not, and thus whether particular proteins are produced.
Epigenetic changes do not actually change the underlying DNA sequence of genes. Instead, they involve the attachment of chemical compounds to the DNA. (The prefix “epi-” means “above” or “upon.”) One major type of epigenetic mechanism, called DNA methylation, occurs when molecules called methyl groups attach to certain pieces of DNA. This may render a gene inactive—preventing the creation of proteins based on the gene—by physically blocking off the DNA and through other, less direct effects. Methylation can activate genes as well. Epigenetic changes also include modifications to the histone proteins around which DNA molecules are wound, along with other kinds of alterations.
Epigenetic changes may be caused by health-related factors such as diet, exercise, and smoking or drug use, as well as early stress. For example, research suggests that prenatal exposure to famine may reduce the methylation of a gene associated with growth. Differences in DNA methylation have also been explored in individuals who have experienced other forms of early-life adversity, such as childhood trauma. Such epigenetic differences, if indeed caused by harsh early experiences, could potentially play a role in explaining increased vulnerability to physical and mental illness.
Lifestyle factors in adulthood, such as exercise and substance use, have been linked to epigenetic changes. And epigenetic changes associated with harmful behaviors may not always be permanent. Scientists have found, for example, that a specific epigenetic change tied to smoking can be reversed after a person quits. Research on exercise, psychotherapy, antidepressants, and other interventions has raised the possibility that they exert positive effects on well-being in part by altering the epigenome.
All human characteristics, including psychological ones, are influenced to some extent by genetics. Since the epigenome modulates the effects of an individual's genome—and because it can be influenced by external factors—it is naturally a major area of exploration for those seeking to understand how individual differences, mental illness and other aspects of cognitive life take shape.
Behavioral geneticists and behavioral epigeneticists are scientists who investigate whether epigenetic changes can help account for vulnerability to certain forms of mental illness. Another possibility that has received attention in recent years is that epigenetic changes resulting from traumatic experiences can be passed from one generation to the next, sometimes called intergenerational transmission of trauma.
Evidence that epigenetic differences are associated with mental illness has been found for depression, schizophrenia, bipolar disorder, and other conditions. For example, research participants with depression have shown differences—compared to those without depression—in levels of methylation of a gene responsible for the production of a protein called brain-derived neurotrophic factor, or BDNF. (BDNF plays a key role in the development of the nervous system.) It remains to be determined whether epigenetic differences like these are important for the development of these mental disorders, but in any case, they may serve as biomarkers for the conditions.
Severe stress early in life, some research suggests, may result in epigenetic changes that contribute to a lasting increase in one’s physiological stress response. Such an effect could reflect a mechanism for adapting to a threatening environment based on early experience. But when the source of stress, such as early mistreatment, does not continue indefinitely, a bolstered stress response could prove harmful in the long term. (Studies with rodents make up much of the research on early adversity and epigenetics.)
Some scientists propose that trauma can have a trans-generational impact based on epigenetics. This epigenetic legacy of trauma is hypothesized to play out through the epigenetic impact of maternal trauma on a child in the womb or during infancy, but also through the inheritance of a parent’s epigenetic changes by the child. However, the evidence for transmission of trauma-related epigenetic changes from parents to offspring is still very limited, and further research is required to establish whether and how it occurs.
The epigenetic clock is what researchers call a measurement of epigenetic changes that correspond with aging. The “clock,” which reflects the level of DNA methylation in an individual’s cells, is not only associated with numerical age; research also indicates that this measurement is predictive of life expectancy. Therefore, these epigenetic changes are considered a marker of biological age, or the extent to which age-related biological processes have progressed in a person’s body (rather than the number of years one has lived). Scientists have also identified other epigenetic signs of aging, including the shortening of telomeres—the segments of DNA at the ends of chromosomes.