Can sperm pass on more than just DNA?

As we venture through life, our behaviour, environment and experiences leave subtle traces on our DNA. Enzymes triggered by environmental stimuli create chemical tags that can permanently turn genes ‘on’ or ‘off’, without actually changing the DNA code itself. Denoted epigenetic alterations, they can expose information on our dietary, smoking and drug habits, stress levels, disease, psychiatric illnesses, and even socioeconomic status. But more so intriguingly, recent studies suggest these alterations can be transferred down through generations.

The most common, and most abundantly studied, mechanism by which these epigenetic alterations are made is the addition of a methyl group to DNA, which prevents the production of proteins from the affected gene. It used to be thought that these changes were wiped from the DNA soon after fertilization, unique only to the individual that acquired them. But very recent advances in molecular technology have enabled scientists to research the possibility that our lifestyle may leave lasting effects our children, and perhaps even our grandchildren.

Recent studies suggest these alterations can be transferred down through generations…

Research has found that trauma-induced depression and anxiety in rats can lead to similar conditions being present in their offspring, despite them being reared in a regular, non-traumatic environment. Isabelle Mansuy from the University of Zurich and colleagues subjected rats to maternal deprivation in infancy, and discovered DNA methylation in several of their genes associated with emotional behaviour. The non-maternally deprived pups and grandpups of these rats were found to possess many of the same methylation patterns and displayed similar traits of depression, suggesting these epigenetic changes had somehow been maintained through the generations.

Similar results have been discovered in humans too; research from New York’s Mount Sinai hospital found the offspring of Holocaust survivors bore increased likelihood of stress disorders. The team focused on a gene associated with the regulation of stress hormones and found corresponding methylation tags between child and parent. Further genetic analysis was able to rule out the possibility that the children had acquired the stress within their own lifetimes.

Research has found that trauma-induced depression and anxiety in rats can lead to similar conditions being present in their offspring…

But it’s not just psychological illness that might be epigenetically inherited. A 2015 study implied that acquired physical traits are susceptible too. Romain Barrè and his team from the University of Copenhagen, Denmark, compared obese and lean men and found a striking epigenetic alteration of 300 genes specifically linked to eating behaviour in the obese mens’ sperm. Amongst these was FTO, one of the few genes that has been found to unequivocally correlate with obesity in humans.

Previous studies have implied tiny strands of RNA, initially produced in response to lifestyle factors such as overeating,  remain in the cytoplasm of sperm cells. Upon fertilization, they get passed on to the egg, and as the embryo grows, the RNAs begin to restore the same epigenetic changes that were present in the father’s DNA. Barrè found a higher level of these kinds of short RNA strands in the obese mens’ sperm, including one that is known to target CART, a gene that controls appetite.

But it’s not just psychological illness that might be epigenetically inherited…

Whilst the effects of these altered genes on the children of these men have not yet been investigated, a recent 2016 study by Jerome Jullien and colleagues at the Wellcome Trust CRUK Gurdon Institute, Cambridge, provides promising new evidence. By altering the sperm of a frog to remove epigenetic tags that turn off specific genes, they discovered such tags are actually vital to normal embryonic development. By demonstrating that some tags in sperm are crucial for a healthy embryo, it suggests that environmentally-triggered epigenetic alterations gained throughout a father’s lifetime might affect his offspring in a similar fashion.

So, whilst we may think university is far too young to be considering children, it’s a fascinating, albeit slightly terrifying, thought that the experiences we endure and the choices we make right now might just affect the health of our future family.