Epigenetics is a branch of biology that studies how chemical factors around DNA (deoxyribonucleic acid) influence genetic expression—that is, the rate at which DNA produces proteins. Complex organisms comprise a variety of cell types, and although these cells all contain identical DNA, they differ in their structures and functions because of how epigenetic factors differentially influence what their DNA is doing. Specifically, epigenetic factors can increase or decrease activity of particular DNA segments (i.e., genes), so epigenetic factors effectively turn some genes “on” in some cells and “off” in other cells; more precisely, they regulate genetic activity in a way analogous to how a dimmer switch regulates a lamp's brightness. Epigenetics is increasingly of interest to psychologists, because the contexts in which people develop—that is, our experiences—can alter the epigenetic states of some of our cells, thereby influencing our biological and psychological characteristics (i.e., phenotypes). Moreover, some epigenetic characteristics can be transmitted from one generation to the next. Epigenetics is important in the development of normal psychological processes such as learning and remembering, and correlational data suggest that epigenetic factors are involved in several psychopathologies, including schizophrenia and other psychotic disorders.
Although genes are sometime mischaracterized as single-handedly determining biological or psychological phenotypes such as eye color or sexual behaviors, studies of molecular and developmental biology indicate that DNA merely contributes to phenotype development, by working in collaboration with nongenetic molecules in DNA's immediate environment within a cell. Some of these nongenetic molecules are involved in transcribing and translating the potential information coded in DNA strands. Ever since the modern definition of the word epigenetics came into use in the early 2000s, epigenetic factors have been understood to be chemicals that can interfere with or facilitate these processes.
There are a large number of epigenetic factors, but relatively few have been studied extensively. One well-studied factor, DNA methylation, involves a so-called methyl group becoming attached to DNA itself; DNA methylation is often associated with reduced gene expression. Another factor, histone acetylation, involves a so-called acetyl group becoming attached to a histone (a kind of molecule that DNA is normally wrapped around); histone acetylation is often associated with increased gene expression. DNA methylation does not always reduce gene expression, and because some regions of DNA are normally methylated and others are not, there are few absolute statements that can be made about the functional consequences of this (or any) epigenetic “mark.” The study of epigenetics is still in an early stage, but research in this area has reminded scientists that a person's genotype can only partially explain his or her phenotype, because a gene is of no consequence if it is silenced by epigenetic processes.
DNA methylation is relatively stable, which makes it particularly interesting to psychologists. In the 20th century, most biologists considered DNA methylation to be effectively permanent, and although recent data call that conclusion into question, DNA methylation can certainly produce long-term gene silencing. Therefore, an experience early in life that leads to methylation of a DNA segment can have detectable consequences years later. In contrast, histone acetylation is much less stable, so it can be useful in dynamic situations, such as when an organism is learning to adapt to a novel context.
Prior to the late 1990s, biologists believed epigenetic marks were effectively “erased” between generations. In this way, the only information passed across generations would be information contained in DNA itself (i.e., information represented by the sequence of chemical components constituting the DNA). The discovery that epigenetic marks can sometimes avoid erasure between generations means that some gene expression patterns acquired during an organism's life can be transmitted to its descendants. So even without mutations altering the DNA sequence, epigenetic effects might still be able to influence descendant generations.
Epigenetic phenomena are observable in plants and animals, from toadflax flowers and bees to mice, monkeys, and human beings. Epigenetic marks appear to be where they are in the genome for several reasons. Some epigenetic marks reflect experiences, but others reflect the particular cell type they are in (e.g., DNA in muscle cells has distinctive epigenetic patterns that differ from those seen on DNA in neurons). Still other epigenetic marks reflect variations that characterize individuals’ DNA sequences. Finally, some epigenetic marks seem scattered around the genome at random.
Experiments have shown that early-life experiences can have epigenetic effects that alter behavior in adulthood. In a seminal series of studies, Michael Meaney and colleagues first demonstrated that rat pups exposed to particular kinds of mothering often become adults that are highly reactive to stressful situations; rat pups exposed to a different kind of mothering often become less reactive as adults. Next, this research team discovered that the different styles of mothering caused different patterns of DNA methylation in the brains of the pups, patterns that explained the offspring's reactions to stress in adulthood. Independent research revealed similar effects in mice and monkeys, and correlational studies have revealed epigenetic differences between individuals who were or were not abused in childhood, suggesting that these findings likely apply to human populations as well.
Experiments on monkeys have likewise demonstrated epigenetic effects of social status (i.e., of being located in a particular rank in a dominance hierarchy), effects that likely have consequences for immune system functioning. Other studies have demonstrated epigenetic effects of exercise, diet, exposure to environmental toxins, and poverty. Because epigenetic phenomena play roles in the development of diabetes, cancer, and drug addiction, this is a rapidly growing field of study.
Epigenetics amounts to an interface between genes and environments. The stimuli we encounter can alter the milieu inside our cells in ways that mechanistically influence genetic activity, potentially explaining how experiences have long-term effects. So one important insight from epigenetics is that nature and nurture collaborate to build phenotypes, rendering obsolete the idea that there is a competitive relation between the two (i.e., nature vs. nurture). For example, when identical (monozygotic) twins are young, they share similar epigenetic states, but as they accumulate distinctive experiences while aging, their epigenetic states diverge. Thus, the discovery that experiences affect genetic functioning can help explain how identical twins develop distinctive phenotypes. Because organisms with identical DNA sequences can have strikingly different phenotypes that are traceable to epigenetically altered gene expression patterns, a comprehensive understanding of behavior will require attention to genes and environments and to how those factors mutually influence one another.
Experimental studies with rodents have demonstrated important roles for histone acetylation and DNA methylation in the formation, consolidation, and storage of long-term memories; encountering novel situations produces epigenetic effects in the brain that lead to structural changes indicative of learning. These are important discoveries, because several psychopathologies—such as Alzheimer's disease, posttraumatic stress disorder, and addictive disorders—involve learning and memory. Because learning and memory are involved in such conditions, researchers believe epigenetic phenomena might be the key to understanding and treating these disorders. As of the early 2010s, a few drugs have been developed that work by targeting epigenetic processes, but because of their side effects, the U.S. Food and Drug Administration has approved them only for the treatment of certain cancers. Nonetheless, such drugs consistently improve memory in mice, suggesting that additional pharmacological research could yield epigenetic drugs suitable for human use in the future.
Although little experimental work has examined the role of epigenetics in human psychopathology, correlations between epigenetic profiles and mental health conditions have been discovered in many cases, such as bulimia, depression, addiction, suicidal behavior, schizophrenia, bipolar disorder, and others. Mental health professionals are optimistic about this work, in part because experimental studies using animal models of psychopathology suggest that epigenetic effects play causal roles in such abnormalities. For instance, studies of so-called chronic social defeat stress in mice—an animal model of human depression—have revealed epigenetic effects of social stress that appear to cause depression-like symptoms. Corroborating evidence indicates that antidepressant drugs such as imipramine have epigenetic effects and that drugs designed to target epigenetic processes influence gene expression much as antidepressants such as fluoxetine do. Thus, studying epigenetics is likely to illuminate the origins of some psychopathologies and potentially reveal treatments that range from pharmacological to dietary to experiential (e.g., exercise or exposure to “enriched” environments).
Finally, evidence is accruing that prenatal experiences, too, can have significant epigenetic effects on developing mammals. For instance, mothers exposed to domestic violence, famine, or certain nutritional supplements have offspring who show epigenetic (or epigenetically influenced) effects of that exposure, such as altered DNA methylation patterns, obesity, or modified hair colors. Some of these effects can then be transmitted to subsequent generations, as can some epigenetic effects of some experiences in adulthood. Transgenerational inheritance in these cases can occur via transmission of epigenetic marks in sperm or eggs, or via other mechanisms, including mechanisms whereby parental behaviors influence epigenetic states in offspring in ways that foster the replication of the parental behaviors (and hence the epigenetic states) in subsequent generations.
See also Addictive Disorders: Overview; Aging and Neurocognitive Disorders: Overview; Atypical Antidepressants; Biopsychosocial Theoretical Framework; Bipolar I Disorder; Bipolar II Disorder; Diabetes: Psychological and Behavioral Factors; Major Depressive Disorder; Obesity; Posttraumatic Stress Disorder; Schizophrenia Spectrum and Other Psychotic Disorders: Overview; Stress Reactivity
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