Around 20,000 species of bee currently buzz around the planet, most important for us being the domesticated honey bee. For thousands of years, humans have kept bees to provide a sweetener for food and drinks, and today honey bee colonies help farmers pollinate plants so we can have foods like apple, cantaloupe, cucumber, and squash. But most people don’t know that honey bees have been valuable in helping researchers understand a process called epigenetics. And epigenetics is shedding light on how the environment effects DNA and contributes to mental illness.
Hives contain three bee types, which appear very different but are all genetically identical. A male drone forms if an egg is not fertilized, and after hanging around the hive for some time, eventually the drone will fly off to try and find a female to breed. Fertilized eggs, however, become females, and while most will turn out to be sterile workers, for every 50,000 workers there is one queen bee that can mate and start her own colony. A fertilized egg becomes programmed to make a queen bee because the larva develops in a specific environment. A queen larva grows inside a much larger egg chamber and while normal worker larvae eat pollen and honey, queen larvae only eat a type of bee milk called royal jelly.
Exposure to royal jelly begins to influence the bee’s growth shortly after the egg is fertilized and cells begin dividing. As cells divide and the egg develops into a larva, the cells’ DNA sends signals that make cells act in different ways. For example, some cells will turn into muscle cells, while DNA in other cells can signal them to make an eye. Similarly, unique changes in growth happen if the larva senses that it is growing inside a large egg chamber and never eats honey and pollen. In this case, the queen bee’s cells respond to their unique environment by attaching small chemical markers to their DNA. This process of scarring the DNA with chemical markers is called epigenetics, with the prefix epi coming from the Greek for “outer” or “above.”
The chemical markers on the DNA act as switches that turn on or turn off particular genes. In this case, the genes turn on and off in a pattern that creates a queen bee. She will have the same gene sequence as her sterile sisters, but the epigenetic markers have made her fertile and ready to start her own colony.
So what does bee epigenetics have to do with human behavior, with say depression, schizophrenia or post-traumatic stress disorder (PTSD)?
The same epigenetic process, where the environment alters DNA, also happens in the human brain. The easiest example to explain is post-traumatic stress disorder, or PTSD, where a life-threatening event changes brain function. When a person experiences or witnesses a major traumatic event, the body secretes stress hormones. These hormones create changes to cells as epigenetic chemical markers, attaching to the DNA of cells in certain brain regions. The chemical markers change how those brain regions work and subsequently how the person thinks, feels and behaves. For instance, someone with PTSD may have nightmares or feeling of intense stress if reminded of the traumatic event. These epigenetic changes to the brain can be short-term or they can become chronic and have damaging effects, sometimes making it difficult to handle stressful situations for years afterwards.
And just like in bee larva, the environment starts effecting human epigenetics shortly after fertilization. We now know that the environment can begin altering the brain from the time someone is only a tiny embryo. In one of the largest epigenetic studies of postmortem human brain tissue, investigators at the Lieber institute for Brain Development found that environmental influences during the prenatal period cause brain changes associated with schizophrenia, and that environmental influences in early adulthood, when symptoms of the illness typically emerge, may not be as important as originally thought. On top of that, numerous studies in animals and humans find that stressful pregnancies change the epigenetics in a baby’s brain, making it harder for that person to handle stress as an adult.
The good news is that we also now know that epigenetic effects are potentially reversible. Just as the environment can create epigenetic markets on the DNA, these chemicals can also be pulled off. Medications that target epigenetic modifications are proving effective to treat cancers and neurodegenerative disorders. In psychiatry, we have some evidence that antidepressants work by changing the epigenetic signature of brain cells, essentially undoing the damage caused by past bad experiences that can lead to depression or other psychiatric conditions.
We’ve just gotten started in understanding this process and the growing field of behavioral epigenetics might offer new therapies to heal the brain. Future research should unlock mysteries that explain why our brains don’t recover from particularly stressful experiences and don’t always work the way we would like afterwards. We are beginning to imagine that the emerging field of medicines that target epigenetics may prevent many of the effects of environmental adversity, even from very early in life.