What is epigenetic inheritance?

This 2016 review by Eric Nestler, a well-known and well-funded researcher, entitled Transgenerational Epigenetic Contributions to Stress Responses: Fact or Fiction? concluded:

“Further work is needed to understand whether and to what extent true epigenetic inheritance of stress vulnerability adds to the well-established and powerful influence of genetics and environmental exposures in determining an individual’s susceptibility versus resilience to stress throughout life.

There is growing evidence for at least some contribution of epigenetic regulation – perhaps achieved by miRNAs – in mediating part of the ability of parental behavioral experience to influence stress vulnerability in their offspring.”

The reviewer applied the terms involved to exclude behavioral inheritance mechanisms. The extent of what is “epigenetic inheritance” seemed to be lost in the process.

For example, his own 2011 research Paternal Transmission of Stressed-Induced Pathologies was cited for evidence that:

“Adult male mice subjected to chronic social defeat stress generate offspring that are more vulnerable to a range of stressful stimuli than the offspring of control mice.”

Yet that finding was dismissed in the review and in that study as behavioral:

“While epigenetic changes in sperm might be a small factor in transgenerational transmission of stress vulnerability, a large portion of the observed transmission may be behavioral.

The fact that most of the transgenerational transmission of stress vulnerability observed in our experiments was not seen with IVF argues against the preponderance of epigenetic mechanisms. Rather, our data would suggest that the bulk of the vulnerabilities are passed on to subsequent generations behaviorally.”

A few questions:

  1. If the experimental subjects had no more control over their behavioral stress-response effects than they had over their DNA methylation, histone modification, or microRNA stress-response effects, then why was such behavior not included in the “epigenetic mechanisms” term?
  2. How do behavioral inheritance mechanisms fall outside the “true epigenetic inheritance” term when behavioral stress-response effects are shown to be reliably transmitted generation after generation?
  3. Wouldn’t the cessation of behavioral inheritance mechanisms confirm their status by falsifiability? This was done with studies such as the 1995 Adoption reverses the long-term impairment in glucocorticoid feedback induced by prenatal stress.

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