Exposure to early life trauma can lead to poor physical and mental health in some individuals, which can be passed on to their children. Studies in mice show that at least some of the effects of stress can be transmitted to offspring via environmentally-induced changes in sperm miRNA levels.
A new epigenetics study raises the possibility that the same is true in humans. It shows for the first time that the levels of the same two sperm miRNAs change in both men and mice exposed to early life stress. In mice, the negative effects of stress are transmitted to offspring. The study is published in Translational Psychiatry. "The study raises the possibility that some of the vulnerability of children is due to Lamarckian type inheritance derived from their parents' experiences," said Larry Feig.
The new study found that among 28 Caucasian male volunteers, the expression of two highly related sperm miRNAs, miR-449 and miR-34, were inversely proportional to the men's ACE scores. Men with the most extensive early abuse (highest ACE scores) had as much as a 300-fold reduction in the two sperm miRNAs compared to men with the least abuse.
The idea that these changes can affect the next generation is supported by additional findings in the study, e.g.:
- the same sperm miRNA changes that take place in men with high ACE scores also occur in mice exposed to early life social instability stress, which Feig's lab has shown previously leads to anxiety and sociability defects in female offspring of stressed males for at least three generations;
- these two sets of miRNAs are known to work together in mice to allow proper development of the brain and sperm;
- in humans, miR-34c has been implicated in promoting early embryo development;
- the mouse studies showed that the decline in these sperm miRNA levels is transmitted to the next generation; and
- when these embryos mature, these miRNAs are also reduced in the sperm of their male offspring who pass on stress behaviors to their female offspring.
"This is the first study to show that stress is associated with altered levels of sperm miRNAs in humans. We are currently setting up a new, larger study in men, and additional experiments in mice that could yield further support for the idea that changes in these sperm miRNAs do, in fact, contribute to an elevation of stress-related disorders across generations," said David Dickson.
"Looking to the future, we may be able to figure out a way to restore the low miRNA levels found in men exposed to extreme trauma, because epigenetic changes, such as stress-induced decreases in sperm miRNA expression, are reversible, unlike genetic changes that alter the DNA sequence," Dickson added.
"Some people may not answer the ACE survey accurately due to inaccurate recall or because of the sensitive nature of many of the questions, particularly in settings that do not allow anonymity and/or where their answers could affect their future. Thus, discovery of unbiased markers for early trauma, like specific sperm miRNA content, could complement ACE surveys in some clinical settings to bolster preventative medicine," he concluded.
The authors note that the relatively small sample size limits their ability to more deeply explore the association between ACE scores and miRNA expression. In addition, a longitudinal study with information on behavioral and psychological factors throughout adulthood, with repeated measurements of sperm miRNA content, could allow for further exploration on the effect of cumulative exposure to childhood trauma on miRNA.