Daniel Shen interned at Carrier Clinic this summer, where he worked on clinical trials. He learned under the direction of Andrew Walsh. During some down time, he labored on a series of research projects and articles. This piece on Epigenetics is the first one to be published.
The National Suicide Prevention Lifeline is a 24-hour suicide prevention network that takes 1.5 million calls per year nationally, or roughly 4,000 calls per day. Following the tragic deaths of celebrities Kate Spade and Anthony Bourdain earlier this June, this number rose by 65% for several days, an increase of more than 2,500 calls per day. Such spikes are typical following media coverage of celebrity suicides; similar statistics were reported after musician Chester Bennington’s death in 2017, and in the months following actor Robin Williams’s death in 2014, national suicide morbidity rates increased by nearly 10% according to a recent study from Columbia University.
Regardless of the precise psychological and sociological factors behind the so-called “celebrity-suicide effect”, clearly these high-profile incidents seem to touch a painful hidden nerve within our nation. According to the National Institute of Mental Health, one fifth of all Americans, or nearly 45 million individuals, are currently struggling with some form of mental illness. When news of Ms. Spade and Mr. Bourdain’s deaths broke, conversation within the media focused particularly on an alarming CDC statistic reporting a 30% increase in the national suicide rate since 1999. (For reference, the national population grew by only 16% since then.) Yet despite the prevalence and severity of these illnesses, nearly 60% of suffering adults do not seek treatment, and if the caller volume spikes following Ms. Spade and Mr. Bourdain’s deaths are any indicator, this statistic likely suffers from under-reporting. The minority that do seek treatment are nearly always prescribed medications that often come with a litany of adverse side effects – nausea, drowsiness, weight gain, and muscle tremors, to name a few – that add further stress to an already difficult situation.
Epigenetics is a growing area of biological research which has rapidly gained the attention of medical researchers within the past two decades and is now being considered as a potential strategy for devising effective psychiatric treatments. Promising developments have already begun in other arenas; recently elucidated epigenetic mechanisms behind some cancers have already led to the creation of novel cancer drugs, and similar progress is being made for many other non-communicable diseases.
In basic terms, epigenetics can be defined as the study of the biological mechanisms behind gene expression – that is, how cells can switch certain segments of DNA on or off in order to perform specific functions. For instance, although your skin cells and brain cells all contain the same DNA and thereby the same genes, your skin cells do not sprout dendrites and your brain cells do not secrete oils/sweat because both cell types have switched off the gene segments encoding for inappropriate and unnecessary features. Proper gene expression is essential for an organism’s growth and survival, and many diseases are the result of switches being in the improper state: some gene is turned off when it should be on, or vice versa. For example, many cancers arise when genes encoding for crucial DNA repair/proofreading proteins become switched off, leaving cells more vulnerable to cancer-causing mutations. This leaves medical researchers with a tantalizing prospect: if the onset of a certain disease is due to improperly switching on/off certain genes, could treatment simply be a matter of reversing the switches back to normal?
Psychiatric epigenetics research is based on the hypothesis that psychiatric disorders such as major depressive disorder (MDD) and schizophrenia have epigenetic markers – that is, these diseases are the result of either overexpression or underexpression of certain genes within brain cells, resulting in altered brain activity and behavioral symptoms. Furthermore, these markers can be either hereditary or environmentally triggered, and by reversing the pathological gene switches back to normal, the disease can be mitigated. Encouraging findings from cancer research and other non-psychiatric disciplines have already shown this to be a promising hypothesis8. MDD, schizophrenia, and generalized anxiety disorder (GAD) rank among the most common mental disorders in America, and consequently have been of primary interest for epigeneticists searching for markers. Due to the sheer complexity of the brain, progress has been slower in the psychiatric realm compared to non-psychiatric arenas, as evidenced by the relative dearth of literature on the topic. Nevertheless, studies from animal models and post-mortem tissue studies have already yielded encouraging results; a few of these studies are highlighted below.
Major depressive disorder
Several lines of evidence have implicated brain-derived neurotrophic factor (BDNF), a protein responsible for healthy neuron function, as a key factor in the onset of MDD, with decreased expression of BDNF in the hippocampus being associated with depressive symptoms. In 2009, a team from the University of Alabama used rat models to investigate whether or not the gene encoding for BDNF experienced any epigenetic alterations in depressed individuals. They did so by subjecting a group of rat pups to abusive conditions such as social deprivation and maternal maltreatment to induce depressive behavior, then comparing their brain cells with those of a control group. What they found was that in the depressed rats, the BDNF gene exhibited signs of increased DNA methylation, a chemical modification of DNA that acts as an off-switch for gene expression. Furthermore, experimental treatments with a DNA methylation inhibitor, designed to undo this chemical modification, yielded promising results. Crucially, this epigenetic marker appeared to be heritable between generations as well: when female individuals from the depressed rat group were allowed to mate, their offspring also exhibited the same methylation patterns and the same behavioral symptoms. This suggests that epigenetics may offer a sobering framework by which the long-known hereditary aspects of mental illness can be understood.
γ-aminobutyric acid, or GABA, is a neurotransmitter responsible for reducing neuronal excitability in the brain, among other functions. It has been hypothesized that for individuals suffering from schizophrenia, the mechanisms driving GABA production are defective in certain neurons, resulting in increased neural activity and pathological behavioral symptoms. In 2005, a team of Harvard researchers looked for markers at the RELN gene within GABA-producing neurons, hypothesizing that defects in RELN production are a major factor behind GABA deficiencies. Comparing post-mortem brain tissue between schizophrenic and non-schizophrenic individuals, they indeed found evidence of DNA methylation – an epigenetic off-switch – at this gene within brain cells of the frontal lobe in the schizophrenic samples. Moreover, their results were consistent with a previous study examining RELN expression in patients with bipolar disorder. Given the similarity of psychosis symptoms between these two illnesses, this link suggests that epigenetics may provide a window by which the mechanisms behind these diseases can be better understood, leading to more effective treatments for a multitude of conditions.
The hypothalamic-pituitary-adrenal (HPA) axis is a complex system consisting of the hypothalamus, pituitary gland, and adrenal glands and can be thought of as the “stress-system” of the body, forming the link between brain activity and hormone production. Corticotropin-releasing hormone receptor 1 (CRHR1) is a crucial protein that contributes to activating our bodies’ stress response via this system. A recent 2016 study from the Weizmann Institute of Science in Israel used rat models to investigate whether or not the gene encoding for CRHR1 was affected in rats with anxiety disorders (induced through various means). They found that for anxious rats, the CRHR1 gene was under-methylated compared to healthy rats, suggesting that this protein was being overexpressed in anxious individuals and resulting in overactivation of the stress response. When they treated the anxious mice with an experimental drug designed to restore methylation, they found that many of their symptoms subsided, demonstrating that epigenetics can indeed be a promising avenue for designing new pharmacological therapies.
The above studies all present a positive correlation between specific epigenetic signatures and the onset of psychiatric disorders, suggesting that epigenetics research is indeed a promising direction for future diagnosis and treatment methods for these disorders. Nevertheless, the field of psychiatric epigenetics is still very much in its infancy, as evidenced by the relative dearth of literature on the subject compared to other disease studies like cancer. Moreover, all of the drugs that have been developed thus far (including the cancer drugs), promising though they may be, have been pan-inhibitors – that is, they affect the entire genome rather than specifically targeting one gene, raising concerns about potentially dangerous side-effects that are not fully understood. Reaching the point where epigenetic treatments can act with specificity and accuracy on single genes requires a more extensive understanding of our cells’ epigenetic mechanisms in all of their complexity, which is beyond our current knowledge. Given that our cells regularly erase and re-write epigenetic markers on their own as part of normal functioning, elucidating these mechanisms is certainly possible, and the moment we gain the ability to selectively modify epigenetic markers on single genes will herald a paradigm shift in medicine and human history.
Daniel S. Shen is a member of the Princeton University Class of 2019 and studies within the Department of Music. He is planning on going to Medical School. He can be reached at firstname.lastname@example.org
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