Several papers have emerged over the last year or so indicating transgenerational effects that influence the behavior and/or physiology of offspring. For example, a recent study in Nature from Margaret Morris's group at the University of New South Wales proposes that obese father's transmit an epigenetic signature through the germ line to female offspring, resulting in impaired beta cell function, impaired insulin secretion and glucose intolerance (Ng et al. Nature 2010). Other recent studies have found similar evidence for paternal inheritance of non-genetic information (for example, see Pentinat et al. Endocrinology 2010; Nelson et al. Epigenomics 2010). However, an outstanding issue relates to the identity of the underlying molecular mechanisms that are involved in these effects. In this blog entry, I highlight some emerging pathways that might potentially contribute to epigenetic inheritance through the germ line.
Two major studies have characterized modified histones in human and mouse sperm (Hammoud et al. Nature 2009; Brykczynska et al. Nat Struc Mol Biol 2010). Previously, it was thought that modified histones were unlikely to be a major component of the highly compact chromatin contained in sperm. However, these two studies indicate that approximately 30% of human promoters contain modified histones. Further, many of these epigenetic signatures are conserved between humans and mice. It has been postulated that these histone signatures are retained in the zygote and play an important role at early stages of development in offspring. However, direct evidence for this model is not yet strong. Interestingly, two noteworthy studies in C. elegans suggest that modified histones established in the parental germ cells transmit essential information to offspring through the germ line (Furuhashi et al. Epigenetics 2010; Rechtsteiner et al. PLoS Genetics 2010).
Both of these studies pickup on an older study by Susan Strome's group, in which she found 6 loci, including the H3K36 methyltransferase MES–4 [an NSD homolog], that are required for normal germ cell development in offspring (Capowski et al. Genetics 1991). Null MES-4 mutant offspring undergo normal germ cell development when MES–4 is expressed by the mother, but not if the mother is homozygous. Thus, a transgenerational maternal effect occurs. In the two most recent studies, it was found that MES–4 establishes H3K36 trimethylated histone marks independent of transcription, and this maternally established epigenetic signature is required for normal germ cell development in offspring. The authors propose that MES–4 transmits a memory of gene expression in the parental germline to offspring.
Taken together, these early observations suggest that epigenetic signatures in the form of modified histones in eggs and/or sperm might impact upon gene expression and the development and physiology of offspring.
This work is the result a major collaborative effort. Jiangwen Zhang at Harvard FAS Computing played a central role in the development of the informatics pipeline. While next generation sequencing data analysis is becoming more mainstream, there was absolutely nothing to help in early 2007 (beyond Eland and a few other aligners) when we started and Jiangwen's work was essential to getting it up and running. David Haig at Harvard played a vital role in the development of the statistical analysis and data interpretation. Gary Schroth and Shujun Luo at Illumina kindly collaborated by sharing their early versions of an RNA-Seq protocol and by carrying out some pilot sequencing studies for us to determine if the approach would succeed (yes, the initiation of the study predated publication of RNA-Seq). Jim Butler worked on the qPCR analysis of Il18 heterozygous mice. The entire study was carried out under the guidance and mentorship of Catherine Dulac in her lab in the Molecular and Cellular Biology Department at Harvard.
Also, thank you to the Jaenisch lab for sharing mice and the outstanding members of the Dulac lab for discussions and ideas.
The work was funded by the Klarman Family Foundation for Eating Disorders, the Howard Hughes Medical Institute, and an award from Merck. I was funded by the Human Frontiers Science Program and the Alberta Heritage Foundation for Medical Research.
Thank you to the Canadian Press for highlighting the prize at home!
The faculty of 1000 (F1000) is a post-publication peer review process performed by leading scientists that ranks publications in a variety of scientific fields. Our paper has the number one spot in Neuroscience! Fantastic.
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.
The evidence that parents pass epigenetic information on to their offspring and that information influences offspring physiology and behavior is continuing to grow. In the field of obesity research, it is now beyond doubt that maternal obesity, and paradoxically calorie restriction, programs the physiology of offspring such they are extremely susceptible to developing metabolic syndrome. Some evidence has indicated that paternal obesity can also have similar effects, which separates out the in utero effects of maternal obesity and indicates that epigenetic programs are being passed on to the next generation through the germ cells.
Paternal germ line transmission of epigenetic programs has been understudied compared to maternal transmission. However, a particularly remarkable study on paternal transmission of metabolic state has recently been published in Human Molecular Genetics by Jo Nadeau and David Buchner (http://www.ncbi.nlm.nih.gov/pubmed/20696673) entitled:
There are several remarkable points that make this paper extremely interesting. The first is the high quality of the work. The samples sizes are large, many different crosses are tested and the read out was body weight, which is a very reliable and simple measure of effect. The authors build on a previous discovery of a mutation that causes resistance to obesity. They find that the obesity resistance phenotype is passed on to offspring even when the mutation itself is not inherited, thus indicating the existence of some heritable epigenetic effect. Astoundingly, the phenotype is only transmitted from fathers and cannot be inherited from mothers. Further, it can be transmitted for at least two generations through the paternal germline, which means that a mouse is obesity-resistant if it's paternal grandfather had the mutation. This work clearly emphasizes the challenges ahead for understanding non-mendelian diseases. We are the sum of many effects: genetic programs, environmental epigenetic programs, transgenerational epigenetic programs, stochastic effects, microbiome effects and lifestyle decisions.
I have been searching for a good way to manage projects and ideas within the lab. I especially want to find a way to motivate creativity and productivity. My belief is that much of what has worked in the IT industry could be used to improve innovation in academic labs (like blogging!). Most of the project management software I am aware of didn't look like a good fit. I am a big fan of google docs and found a very useful platform, called ManyMoon (http://www.manymoon.com/), that interfaces with Google Docs and functions as a personnel and project management platform. I have been using it to set up a structure for the lab and believe that it will work well in the long term with some tinkering....
The company kindly highlighted the Gregg Lab recently, (even though it doesn't quite exist yet) and the manner in which I am attempting to use their platform. Here is the link to the article:
1.) A large flatscreen monitor, which is being designed into the construction of the lab and will allow people to post ideas, papers, findings, art, pictures, etc. from their computers to communicate and promote innovation and interaction with others in the lab.
2.) A small budget (when grants allow) for individual innovation projects in which students/postdocs can attempt to begin to develop high-risk high-reward ideas on their own within a limited budget without fear of failure. Special lab meetings will be set aside for these projects in which we will discuss pig picture problems and emerging technologies and ideas. The idea is inspired by Google, which allows some employees to spend a certina percentage of their time on their own private projects and ideas....this has lead to many great breakthroughs for that company and I hope to experiment with it in different ways in an academic setting.
