By Danielle Gerhard
The idea that transgenerational inheritance of salient life experiences exists has only recently entered the world of experimental research. French scientist Jean-Baptiste Lamarck proposed the idea that acquired traits throughout an organism’s life could be passed along to offspring. This theory of inheritance was originally disregarded in favor of Mendelian genetics, or the inheritance of phenotypic traits isn’t a blending of the traits but instead a specific combination of alleles to form a unique gene encoding the phenotypic trait. However, inheritance is much more complicated than either theory allows for. While Lamarckian inheritance has largely been negated by modern genetics, recent findings in the field of genetics have caused some to revisit l’influence des circonstances, or, the influence of circumstances.
Over the past decade, efforts have shifted towards understanding the mechanisms underlying the non-Mendelian inheritance of experience-dependent information. While still conserving most of the rules of Mendelian inheritance, new discoveries like epigenetics and prions challenge the central dogma of molecular biology. Epigenetics is the study of heritable changes in gene activity as a result of environmental factors. These changes do not affect DNA sequences directly but instead impact processes that regulate gene activity such as DNA methylation and histone acetylation.
Epigenetics has transformed how psychologists approach understanding the development of psychological disorders. The first study to report epigenetic effects on behavior came from the lab of Michael Meany and Moshe Szyf at McGill University in the early 2000s. In a 2004 Nature Neuroscience paper they report differential DNA methylation in pups raised by high licking and grooming mothers compared to pups raised by low licking and grooming mother. Following these initial findings, neuroscientists have begun using epigenetic techniques to better understand how parental life experiences, such as stress and depression, can shape the epigenome of their offspring.
Recent research coming out from the lab of Tracy Bale of the University of Pennsylvania has investigated the heritability of behavioral phenotypes. A 2013 Journal of Neuroscience paper found that stressed males went on to produce offspring with blunted hypothalamic pituitary (HPA) axis responsivity. In simpler terms, when the offspring were presented with a brief, stressful event they had a reduction in the production of the stress hormone corticosterone (cortisol in humans), symptomatic of a predisposition to psychopathology. In contrast, an adaptive response to acute stressors is a transient increase in corticosterone that signals a negative feedback loops to subsequently silence the stress response.
The other key finding from this prior study is the identification of nine small non-coding RNA sperm microRNAs (miRs) increased in stressed sires. These findings begin to delve into how paternal experience can influence germ cell transmission but does not explain how selective increases in these sperm miRs might effect oocyte development in order to cause the observed phenotypic and hormonal deficits seen in adult offspring.
A recent study from the lab published in PNAS builds off of these initial findings to further investigate the mechanisms underlying transgenerational effects of paternal stress. Using the previously identified nine sperm miRs, the researchers performed a multi-miR injection into single-cell mouse zygotes that were introduced into healthy surrogate females. To confirm that all nine of the sperm miRs were required to recapitulate the stress phenotype, another set of single-cell mouse zygotes were microinjected with a single sperm miR. Furthermore, a final set of zygotes received none of the sperm miRs. Following a normal rearing schedule, the adult offspring were briefly exposed to an acute stressor and blood was collected to analyze changes in stress hormones. As hypothesized, male and female adult offspring from the multi-miR group had a blunted stress response relative to both controls.
To further investigate potential effects on neural development, the researchers dissected out the paraventricular nucleus (PVN) of the hypothalamus, a region of the brain that has been previously identified by the group to be involved in regulation of the stress response. Using RNA sequencing and gene set enrichment analysis (GSEA) techniques they found a decrease in genes involved in collagen formation and extracellular matrix organization which the authors go on to hypothesize could be modifying cerebral circulation and blood brain barrier integrity.
The final experiment in the study examined the postfertilization effects of multi-miR injected zygotes. Specifically, the investigators were interested in the direct, combined effect of the nine identified sperm miRs on stored maternal mRNA. Using a similar design as the initial experiment, the zygote mRNA was collected and amplified 24 hours after miR injection in order to examine differential gene expression. The researchers found that microinjection of the nine sperm miRs reduced stored maternal mRNA of candidate genes.
This study is significant as it has never been shown that paternally derived miRs play a regulatory role in zygote miR degradation. In simpler terms, these findings contradict the conventional belief that zygote development is solely maternally driven. Paternal models of transgenerational inheritance of salient life experiences are useful as they avoid confounding maternal influences in development. Studies investigating the effects of paternal drug use, malnutrition, and psychopathology are ongoing.
Not only do early life experiences influence the epigenome passed down to offspring but recent work out of the University of Copenhagen suggests that our diet may also have long-lasting, transgenerational effects. A study that will be published in Cell Metabolism next year examined the effects of obesity on the epigenome. They report differential small non-coding RNA expression and DNA methylation of genes involved in central nervous system development in the spermatozoa of obese men compared to lean controls. Before you start feeling guilty about the 15 jelly donuts you ate this morning, there is hope that epigenetics can also work in our favor. The authors present data on obese men who have undergone bariatric surgery-induced weight loss and they show a remodeling of DNA methylation in spermatozoa.
Although still a nascent field, epigenetics has promise for better understanding intergenerational transmission of risk to developing a psychopathology or disease. The ultimate goal of treatment is to identify patterns of epigenetic alternations across susceptible or diagnosed individuals and develop agents that aim to modify epigenetic processes responsible for regulating genes of interest. I would argue that it will one day be necessary for epigenetics and pharmacogenetics, another burgeoning field, to come into cahoots with one another to not only identify the epigenetic markers of a disease but to identify the markers on an person by person basis. However, because the fields of epigenetics and pharmacogenetics are still in the early stages, the tools and techniques currently available limit them. As a result, researchers are able to extract correlations in many of their studies but unable to determine potential causality. Therefore, longitudinal, transgenerational studies like those from the labs of Tracy Bale and others are necessary to provide insight into the lability of our epigenome in response to lifelong experiences.