If you think of our DNA as an immense piano keyboard and our genes as keys—each key symbolizing a segment of DNA responsible for a particular note, or trait, and all the keys combining to make us who we are—then epigenetic processes determine when and how each key can be struck, changing the tune being played.One way the study of epigenetics is revolutionizing our understanding of biology is by revealing a mechanism by which the environment directly impacts genes. Studies of animals, for example, have shown that when a rat experiences stress during pregnancy, it can cause epigenetic changes in a fetus that lead to behavioral problems as the rodent grows up. Other epigenetic changes appear to occur randomly—throwing a monkey wrench into the engine of nature versus nurture. Still other epigenetic processes are normal, such as those that guide embryonic cells as they become heart, brain, or liver cells, for example.
In the last decade we have learned by studying very simple organisms such as bacteria that gene expression -- the extent to which a gene is turned on or off -- varies greatly among individuals, even in the absence of genetic and environmental variation. Two cells are not completely identical and sometimes these differences have their origin in random or stochastic processes.
So, while identical twins may appear identical, and while they as genetically close as two humans can be, they remain individuals on the cellular level. One may be more cancer-prone than the other even without differences in the environment.
So, what is it: Nature? Nurture? Both? Neither?
It seems to be a complex mix of all of the above, with our cancers as unique as we are.
Improving our diet and exercise certainly have been shown in multiple studies to reduce our risk of breast cancer. But sometimes the environmental risks that make one twin more prone to illness are no more under their control than their DNA. The way the fetuses develop in the womb, for example, might improve one twin's health while imperiling the other's. And sometimes, that tiny variation in DNA between twins makes all the difference in susceptibility to cancer.
Epigenetics, while not answering the question of what causes cancer, is absolutely getting us closer.
Read more about TNBC in my book, Surviving Triple-Negative Breast Cancer.