Traces of past events written in our genes

Scientists have discovered that changes to DNA which occur during a person's lifetime can be passed on to future generations.

The research shows for the first time that some DNA changes - known as epigenetic modifications - acquired by a parent could be heritable. It was previously thought that, after fertilisation, all such changes were wiped clean from embryo DNA, but the study shows that in two to five percent of the genetic code modifications from the previous generation remain.

'[This] information needs to be reset in every generation before further information is added to regulate development of a newly fertilised egg,' explained Professor Azim Surani, of the Gurdon Institute at the University of Cambridge, UK and lead author on the study. 'It's like erasing a computer disk before you add more data.'

To understand more about how this process works, the researchers collected primordial germ cells from human embryonic tissues and sequenced the RNA to investigate the gene-expression levels of different methylation markers.

The research, published in Cell, found that the reprogramming of DNA to remove epigenetic modifications occurs around gestational weeks two to nine, and that it is not completely effective. Most of the genes which evade this cleaning mechanism are linked to brain and metabolic disorders, such as mental illness and obesity.

The researchers also found that reset-resistant 'escapee' genes can elude further waves of reprogramming that happen later in development, suggesting that epigenetic modifications can cross from parents to offspring.

We know that environmental and lifestyle factors such as smoking, stress and diet, can cause changes to our DNA during our lifetimes through epigenetic mechanisms. The DNA sequence itself does not change, but specific genes can be turned on or off. This happens by the addition or removal of a chemical tag called a methyl group in response to the individual's environment.

This research contradicts the 'Weismann barrier' principle, which states that genetic modifications caused by the environment cannot be passed on to gametes. Emeritus Professor of Paediatric Genetics Marcus Pembrey, of the Institute of Child Health, London, told New Scientist, 'I think we can say the August Weismann barrier has been well and truly breached.'

The study also identified the role of several regulators which form a network that resets the methylation within the embryonic cells. Now the researchers' work will focus on how the 'escapees' manage to evade this resetting network and retain the modifications acquired by their parents, and what the functional consequences of this are.