Life is full of stress. Jobs are lost, divorces ensue, accidents happen…even wars and terrorism are a part of life for some. There is no doubt that these things impact tremendously on children, but could chronic stress in one generation really influence future descendants for generations to come?
It seems daunting to imagine that life works that way or that natural selection gave rise to mechanisms that communicate stressful experiences to future generations. Nonetheless, the evidence is building and I highlight some surprises here.
There has been a major interest in the effects of stress on the development and behavior of offspring for a long time. The work of Michael Meaney and colleagues revealed that patterns of maternal behavior can influence the development of stress responses in rats. This effect establishes a transgenerational form of behavioral inheritance from mother to offspring (Francis et al. 1999). Further work by the Meaney and Szyf groups went on to establish evidence that the transgenerational effects of maternal behavior are correlated with changes in the expression of the glucocorticoid receptor (GCR) in the hippocampus of rats, such that rats exhibiting robust maternal behavior characterized by high licking and grooming of the pups, exhibited elevated GCR expression leading to a feedback effect that dampens the HPA axis response to stress. The increase in GCR expression is proposed to be downstream of increased serotonin signaling and is stabilized in offspring through an epigenetic program in which the methylation status of an alternative promoter for the GCR gene is reduced (Weaver 2004). The loss of this methyl mark is proposed to allow for a stable increase in GCR expression and dampened stress response in the offspring (Weaver 2004). In short, good mothers that groom and care for their pups will trigger an epigenetic program in the offspring that results in those offspring ultimately becoming good mothers themselves and having reduced stress and anxiety-like behaviors. Bad mothers that offer less care to the pups instill a different pattern of epigenetic marks on the DNA of their offspring, such that their pups grow up to be poor mothers and exhibit greater stress responses. Cross fostering of the pups breaks the cycle, revealing that it truly is the early life exposure to maternal behavior that instills this cycle of anxiety and poor mothering.
This work is a striking example of epigenetic inheritance, but has been met with skepticism (see Miller, Science 2010 and Buchen, Nature 2010). A recent study by these groups correlates such processes with early life stress in humans by examining GCR methylation in the brains of 12 suicide victims that were abused as children (McGowan 2009). This is a tough sell for this reader, given that methylation is highly variable in humans and genome wide association studies need thousands of people to detect anything meaningful in a study of this sort. In general, it is likely that the epigenetic story of this one gene is a small piece of a much larger biological picture that involves an epigenetic program that encompasses many genes and other adaptations involving synaptic plasticity, altered cell death, etc. Indeed, new studies are emerging that suggest this is the case. However, the Meaney studies are foundational in that they appear to bring together two disparate fields, namely molecular neuroscience and behavioral neuroscience.
Recently, the evidence for transgenerational effects of stress have become even more astounding. Two papers have suggest evidence for germline transmission of stress effects to offspring. Over the years our awareness of the transgenerational effects of stress has been growing (Matthews and Phillips, Endocrinology 2010). Children born from survivors of the Dutch famine during WWII exhibit elevated stress responses. Children born from mothers at the World Trade Center attacks have depressed cortisol responses, as do those born from Jewish Holocaust survivors. Most believe that these effects are the result of exposure to a “stressful” environment in the womb, and no doubt that many effects are the result of changes to the uterine environment. However, two independent studies in mice have recently suggested that anxiety-like behaviors can be transmitted through the paternal germline, which may indicate that a stress-related epigenetic signature is carried on the DNA packaged in sperm. These studies appear to be remarkable examples of epigenetic transgenerational inheritance.
The first study is from Rene Hen’s group at Columbia (Alter et al. Biol Psychiatry 2009) and is entitled: “Paternal Transmission of Complex Phenotypes in Inbred Mice”
This study used genetically identical BALB/cJ mice and found that males could be separated into those with high anxiety-like (HA) behaviors in an open field test and with low anxiety-like (LA) behaviors. These males were then mated to genetically identical BALB/cJ female mice. Remarkably, correlation matrices and multiple regression models accounting for a variety of variables revealed that the anxiety-like behaviors of fathers were significantly associated with anxiety-like behaviors and hippocampal size of daughters, but with body weight in sons. The authors propose that these surprising and complex associations are suggestive of a nongenetic mode of inheritance through the male germline. The results are very striking. However, from my own experience with mouse genomics, these “genetically identical” inbred mouse strains still have many polymorphic sites drifting through the population. Imposing the behavioral categorization of HA vs LA, may select for subsets of BALB/cJ male mice with sets of polymorphisms that influence anxiety-related behaviors and these polymorphic sites may also influence phenotypes in offspring in a complex and sex specific manner. In this case, the transmission would be genetic. So far the underlying cause(s) of these bizarre effects are not yet clear. It would be nice to know what would happen if the same experiment were performed for mothers.
The second study is from Isabelle Mansuy’s lab (Franklin et al. Biol Psychiatry 2010) and is entitled: “Epigenetic Transmission of the Impact of Early Stress Across Generations”
In this study the authors expose pups to chronic and unpredictable maternal separation, which is extremely stressful to the pups. Not surprisingly, these offspring exhibit depression-like behaviors as adults, but the depression/stress related phenotype was restricted to male mice (F1s). This sex bias effect of maternal separation in rodents was observed previously by others. Remarkably however, the offspring of these depressed males also exhibited depressed behavior, but this time the males were normal and the females were significantly affected (F2s). Finally, the normal behaving F2 males were mated to normal females, and despite the fact that the F2 males seemed normal, their offspring still exhibited altered depression-like phenotypes (F3s). However!!….this time it was only the male F3s that exhibited the phenotype, not the female F3s. Thus, a complex transgenerational epigenetic effect is proposed that is transmitted through the paternal germline and interacts with sex effects in offspring to produce stress-related behavioral phenotypes. Finally, the authors note some modest changes to the expression and/or methylation status of a few genes in F1 sperm and F2 brain. However, it seems unlikely that these small effects play a major role in the observed phenotype, thus the underlying mechanisms remain largely unknown. In summary, the study proposes that exposure to chronic stress during early postnatal development leaves an epigenetic signature that can be propagated through the germline to future offspring for at least 3 generations.
The results are hard to reconcile with our current mechanistic understanding of epigenetic programming in the genome. However, several studies that have found evidence for transgenerational epigenetic inheritance report that complex sex effects are associated with the observed phenotype. These complex effects seem to appear in studies of both humans and mice.
These findings are preliminary and require further investigation by others. However, if this is found to be true, then there is new and fundamental biology to be uncovered.