by Annie Derry
Since the dawn of genetics, it has been well established that characteristics are passed from parent to offspring through DNA sequences. It was thought until recently that there were rigid rules to which inheritance abided: that traits could only be passed onto offspring if they, themselves, were caused by changes to the amino acid sequence making up DNA.
This was a perfectly rational conclusion based on what we observe in nature, as phenotypes we acquire over our lifetime due to our environment do not usually affect our DNA. How would these be passed on? It seemed sort of impossible.
Enter: Epigenetics.
What had not been realised until (relatively) recently is that the DNA sequence can be altered in more ways than a change to the underlying amino acid (and nucleotide base) sequence.
Epigenetic changes are chemical alterations to the genome that result in the switching ‘on’ or ‘off’ of genes. These chemical modifications can include DNA methylation of certain areas, as well as histone modification. They essentially change how easy or hard it is for that region of DNA to be unravelled, transcribed and translated into a protein (in other words, expressed). Many factors have been found that could potentially cause epigenetic changes in the body, such as stress, physical activity and diet, and it is now thought that these changes can be inherited.
Wiping the slate clean?
It was previously thought that chemical changes to the genome were accumulated over an individual’s lifetime, but that they would be removed in the process of reproduction. It is now believed that this is not the case, as changes to the epigenome seem to be able to jump the generational barrier.
A widely used example of transgenerational epigenetics is the study of the Dutch Hunger Winter (1944-45), a terrible period of starvation for the people of The Netherlands. Based on well-kept medical records, pregnant women and their offspring were studied to understand the health impact of the unique conditions they were subjected to.
The interesting observation was that differences in the timing of malnutrition during pregnancy went on to affect not only the children’s weight at birth, but their health in adulthood too. Some babies (those only affected by malnutrition in early pregnancy) went on to have above-average weight in adulthood, and even to be more prone to obesity and cardiovascular disease. Those born underweight due to malnutrition in later pregnancy remained smaller in adulthood, with lower than average obesity rates. Records even suggested that the grandchildren of those malnourished women were affected similarly. A follow up study indicated that the children – many decades after initial malnutrition in the womb – had less DNA methylation of the IGF2 (insulin-like growth factor) gene than their unaffected siblings. This gene codes for an important protein in growth, thus the results suggest that an epigenetic change had taken place and could have contributed to the growth patterns of those affected children.
This gives us a small insight into how one environmental condition might cause a chemical imprint on the epigenome of a foetus that remains for their entire life and is passed on to their children.
What does this mean for us?
This does not mean to say it is certain that every little thing we do results in a genetic imprint that we pass on to our children and subsequent generations. We don’t know yet whether smoking or eating 5 fruits and vegetables everyday will be detrimental to the health of our unborn children. However, it is now clear that some aspects of our lifestyle will cause an imprint on our genome, and that imprint might not be wiped clean when we reproduce. All this means is that we can decide to take more care of ourselves, knowing that our actions may not only affect our own bodies, but those of our offspring. That being said, the field of epigenetics is a relatively new one and there is much, much more to be understood about modes of inheritance.
Interested in learning more? This blog post was based on:
Nessa Carey – The Epigenetics Revolution
Tim Spector – Identically Different
Epigenetic inheritance and the missing heritability
Transgenerational Epigenetic Inheritance: myths and mechanisms