The July 7 bombings and heritability: carrying trauma to the next generation
The offspring of male survivors of the July 7 bombings may inherit anxiety and depression.
It is often said of a particularly dramatic event – such as the September 11 attacks or the July 7 bombings – that its consequences will "reverberate for generations". It can seem like hyperbole, yet new evidence suggests that traumatic events can affect the genes, and lives, of children as yet unborn. Take the July 7 London bombings. As the harrowing evidence continues to emerge, the psychological impact on the survivors has been all too clear.
As many as 30 per cent of those directly caught up in the atrocities have gone on to develop full post-traumatic stress disorder (PTSD). This is in line with similar incidents: after the Oklahoma City bombing in 1995, 41 per cent of survivors were diagnosed with PTSD after six months, and 26 per cent were still suffering after seven years. Among soldiers returning from Iraq and Afghanistan, the British Armed Forces reckon that 10 per cent develop PTSD. However, an American study gave a figure as high as 30 per cent.
Yet new evidence suggests that the trauma is not just psychological, but biological and even heritable. By altering the chemical mechanisms regulating gene expression, these modifications may become embedded in the male germ line, and can be passed down to the victim's children.
This idea is deeply controversial, not least because it seems to cast doubt on one of the key principles of modern evolutionary theory. The doctrine of natural selection holds that it is our DNA alone that is passed down to our children – and that this remains unaffected by our actual experiences.
Conventional biologists groan with horror at the spectre of the 18th-century French biologist Jean-Baptiste Lamarck rising from the grave.His theory – that characteristics acquired during a creature's lifetime can be passed on to its offspring – is intuitively appealing, but it is rank heresy.
According to the scientific orthodoxy, our only genetic inheritance from our parents is our DNA. Yet this, it now appears, is not entirely the case. Embedded within the DNA sequence are epigenetic regulators, chemical marks that control which genes are expressed and which are not. This is a crucial function, given that every cell in our bodies contains our entire lexicon of DNA. It is the regulators that selectively silence some genes so that particular cells become, say, skin or brain cells, and stay like that when they divide.
The heretical proposition here is that these epigenetic marks can be transmitted along with the DNA. It is the result of intensive research into how these mechanisms work. The best understood is DNA methylation, in which methyl molecules latch on to some areas of the DNA strand and act as switches that render a gene active or inactive.
Too much or too little methylation, and a host of problems occur, from fragile X syndrome to a variety of cancers. The latest findings, however, indicate that psychological conditions, such as trauma and stress, also leave an epigenetic mark. Professor David Sweatt and his colleagues at the University of Alabama have found that maltreating rat pups for just one week is enough to trigger epigenetic changes that deactivate the gene for a protein important in memory formation and emotional balance. This same agent – brain-derived neurotrophic factor – is often abnormally low in schizophrenics and those with bipolar disorder.
In a similar experiment, Professor Eric Richards at Washington University, St Louis, showed that the way rats are nurtured affects the methylation of a crucial receptor in the hippocampus. After a positive nurturing experience, the appropriate gene gets turned on at a vital early stage; after a bad one, the gene remains unused. The same is found in humans. A study of women in Holland who were pregnant during a prolonged famine after the Second World War found that their daughters had twice the normal risk of developing schizophrenia. Again, the causes were epigenetic, the result of changes in the expression of a gene linked with embryonic growth.
But can these effects be inherited? Can nurture in one generation affect nature in the next? It's becoming increasingly clear that the answer is yes. Take Holocaust survivors, for example. A high proportion show abnormally low levels of the hormone cortisol, a deficit of which is associated with PTSD.
What is surprising, though, is that their offspring have equally abnormal levels – and this has been shown to be a biological trait, rather than an effect of the parent-child relationship.
http://www.telegraph.co.uk/science/evolution/8153276/The-July-7-bombings-and-heritability-carrying-trauma-to-the-next-generation.html
