The epigenetic clock theory of aging

My 400th blog post curates a 2018 US/UK paper by two of the coauthors of Using an epigenetic clock to distinguish cellular aging from senescence. The authors reviewed the current state of epigenetic clock research, and proposed a new theory of aging:

“The proposed epigenetic clock theory of ageing views biological ageing as an unintended consequence of both developmental programmes and maintenance programmes, the molecular footprints of which give rise to DNAm [DNA methylation] age estimators.

It is best to interpret epigenetic age estimates as a higher-order property of a large number of CpGs much in the same way that the temperature of a gas is a higher-order property that reflects the average kinetic energy of the underlying molecules. This interpretation does not imply that DNAm age simply measures entropy across the entire genome.

To date, the most effective in vitro intervention against epigenetic ageing is achieved through expression of Yamanaka factors, which convert somatic cells into pluripotent stem cells, thereby completely resetting the epigenetic clock. In vivo, haematopoietic stem cell therapy resets the epigenetic age of blood of the recipient to that of the donor.

Future epidemiological studies should consider other sources of DNA (for example, buccal cells), because more powerful estimates of organismal age can be obtained by evaluating multiple tissues..other types of epigenetic modifications such as adenine methylation or histone modifications may lend themselves for developing epigenetic age estimators.”


I’ve previously curated four other papers which were referenced in this review:


The challenge is: do you want your quality of life to be under or over this curve?

What are you doing to reverse epigenetic processes and realize what you want? Do you have ideas and/or behaviors that interfere with taking constructive actions to change your phenotype?

If you aren’t doing anything, are you honest with yourself about the personal roots of beliefs in fate/feelings of helplessness? Do beliefs in technological or divine interventions provide justifications for inactions?

https://www.nature.com/articles/s41576-018-0004-3 “DNA methylation-based biomarkers and the epigenetic clock theory of ageing” (not freely available)

Advertisements

Manufacturing PTSD evidence with machine learning

What would you do if you were a scientist who had strong beliefs that weren’t borne out by experimental evidence?

Would you be honest with yourself about the roots of the beliefs? Would you attempt to discover why the beliefs were necessary for you, and what feelings were associated with the beliefs?

Instead of the above, the researchers of this 2017 New York human study reworked negative findings of two of the coauthors’ 2008 study until it fit their beliefs:

“The neuroendocrine response contributes to an accurate predictive signal of PTSD trajectory of response to trauma. Further, cortisol provides a stable predictive signal when measured in conjunction with other related neuroendocrine and clinical sources of information.

Further, this work provides a methodology that is relevant across psychiatry and other behavioral sciences that transcend the limitations of commonly utilized data analytic tools to match the complexity of the current state of theory in these fields.”


The limitations section included:

“It is important to note that ML [machine learning]-based network models are an inherently exploratory data analytic method, and as such might be seen as ‘hypotheses generating’. While such an approach is informative in situations where complex relationships cannot be proposed and tested a priori, such an approach also presents with inherent limitations as a high number of relationships are estimated simultaneously introducing a non-trivial probability of false discovery.”


Sex-specific impacts of childhood trauma summarized why cortisol isn’t a reliable biological measurement:

“Findings are dependent upon variance in extenuating factors, including but not limited to, different measurements of:

  • early adversity,
  • age of onset,
  • basal cortisol levels, as well as
  • trauma forms and subtypes, and
  • presence and severity of psychopathology symptomology.”

Although this study’s authors knew or should have known that review’s information, cortisol was the study’s foundation, and beliefs in its use as a biomarker were defended.

What will it take for childhood trauma research to change paradigms? described why self-reports of childhood trauma can NEVER provide direct evidence for trauma during the top three periods when humans are most sensitive to and affected by trauma:

The basic problem prohibiting the CTQ (Childhood Trauma Questionnaire) from discovering likely most of the subjects’ historical traumatic experiences that caused epigenetic changes is that these experiences predated the CTQ’s developmental starting point.

Self-reports were – at best – evidence of experiences after age three, distinct from the experience-dependent epigenetic changes since conception.”

Yet the researchers’ beliefs in the Trauma History Questionnaire’s capability to provide evidence for early childhood traumatic experiences allowed them to make such self-reports an important part of this study’s findings, for example:

“The reduced cortisol response in the ER [emergency room] was dependent on report of early childhood trauma exposure.”

An interview with Dr. Rachel Yehuda on biological and conscious responses to stress was the perspective of one of the study’s coauthors.

https://www.nature.com/articles/tp201738 “Utilization of machine learning for prediction of post-traumatic stress: a re-examination of cortisol in the prediction and pathways to non-remitting PTSD”

The purpose of epigenetic mechanisms

The concluding remarks of this 2018 Chinese review were:

“Using heterochromatin as a model, we have reviewed here the mechanisms behind the establishment and maintenance of silent chromatin domains. We conclude that almost every component of the chromatin environment, including DNA elements, RNAs, histones and other chromatin proteins, plays a role in the process of shaping and maintaining epigenetic states.

Epigenetic mechanisms have evolved..to solve the problem of orchestrating the differentiation of cells with the same genome. Just as any stable system must preserve some degree of flexibility, crosstalk and feedback among all elements in the system are mechanistically required.

We emphasize that:

  1. epigenetic information is inherited [from parent cell to child cell] in a relatively stable but imprecise fashion;
  2. multiple cis and trans factors are involved in the maintenance of epigenetic information during mitosis; and
  3. the maintenance of a repressive epigenetic state requires both recruitment and self-reinforcement mechanisms.”


Studies I’ve curated in 2018 whose methodologies may have benefited from investigating multiple epigenetic mechanisms included:

Only DNA methylation:

Only microRNAs:

A review of studies that investigated DNA methylation and microRNAs but not histone modifications:

https://link.springer.com/article/10.1007/s11427-018-9276-7 “Recruitment and reinforcement: maintaining epigenetic silencing” (not freely available)

Genomic imprinting and growth

This 2018 UK paper reviewed genomic imprinting:

“Since their discovery nearly 30 years ago, imprinted genes have been a paradigm for exploring the epigenetic control of gene expression. Moreover, their roles in early life growth and placentation are undisputed.

However, it is becoming increasingly clear that imprinted gene function has a wider role in maternal physiology during reproduction – both by modulating fetal and placental endocrine products that signal to alter maternal energy homeostasis, and by altering maternal energetic set points, thus producing downstream actions on nutrient provisioning.”

“Imprinted genes in the conceptus produce products that alter maternal resource allocation by:

  1. altering the transport capacity of the placenta;
  2. increasing fetal demand for resources by their action on the intrinsic growth rate; and
  3. signalling to the mother by the production of fetal/placental hormones that modify maternal metabolism.”

Other studies/reviews I’ve curated that covered genomic imprinting are:

http://jeb.biologists.org/content/jexbio/221/Suppl_1/jeb164517.full.pdf “Genomic imprinting, growth and maternal-fetal interactions”


This post has somehow become a target for spammers, and I’ve disabled comments. Readers can comment on other posts and indicate that they want their comment to apply here, and I’ll re-enable comments.

RNA and neurodegenerative diseases

This 2018 Chinese paper reviewed the associations among long non-coding RNA and four neurodegenerative diseases:

“lncRNAs are widely implicated in various physiological and pathological processes, such as epigenetic regulation, cell cycle regulation, cell differentiation regulation, cancer, and neurodegenerative diseases, through their interactions with chromatin, protein, and other RNAs. Numerous studies have suggested that lncRNAs are closely linked with the occurrence and development of a variety of diseases, especially neurodegenerative diseases, of which the etiologies are complicated and the underlying mechanisms remain elusive.

We focus on how lncRNA dysfunctions are involved in the pathogenesis of Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and amyotrophic lateral sclerosis.”


Table 1 showed specific lncRNAs that acted as “bodyguards” in inherited Huntington’s disease, “culprits” in Alzheimer’s disease, and as both in Parkinson’s disease. The table didn’t include lncRNAs associated with amyotrophic lateral sclerosis although the review text mentioned several.

https://www.sciencedirect.com/science/article/pii/S2162253117303104 “Long Non-coding RNAs, Novel Culprits, or Bodyguards in Neurodegenerative Diseases”

An example of researchers changing their field’s paradigms

This 2018 German review subject was retroviruses:

“Initial indications that retroviruses are connected to neoplastic transformation were seen more than a century ago..43% of the human genome is made up of such elements and 8% of the genome is comprised of retroviruses that infected human ancestors, entering cells of the germ line or proliferating thereafter by retrotransposition.

Endogenized retroviruses (ERVs) are abundantly expressed in many transformed cells. In healthy cells, ERV expression is commonly prevented by DNA methylation and other epigenetic control mechanisms.

A recent string of papers has described favorable outcomes of increasing human ERV (HERV) RNA and DNA abundance by treatment of cancer cells with methyltransferase inhibitors. Analogous to an infecting agent, the ERV-derived nucleic acids are sensed in the cytoplasm and activate innate immune responses that drive the tumor cell into apoptosis.”


Some researchers weren’t satisfied with the status quo of this century-old field:

“Chiappinelli et al. (2015) and Roulois et al. (2015) demonstrated a link between DNMTi-induced activation of HERV expression and innate sensing of transcribed viral RNAs and activation of innate immunity signaling pathways leading to an inhibition of tumor cell growth. These results represent a paradigm shift in our comprehension of the antitumor activity of demethylating agents.”

There are opportunities for any researcher whose field can be related to epigenetics to update the way studies are done. Why should researchers settle for mediocrity when they can make a difference?

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5816757/pdf/fmicb-09-00178.pdf “HERVs New Role in Cancer: From Accused Perpetrators to Cheerful Protectors”

What will it take for childhood trauma research to change paradigms?

This 2018 German human study found:

“DNA methylation in a biologically relevant region of NR3C1-1F [the glucocorticoid receptor gene] moderates the specific direction of HPA-axis dysregulation (hypo- vs. hyperreactivity) in adults exposed to moderate-severe CT [childhood trauma].

In contrast, unexposed and mildly-moderately exposed individuals displayed moderately sized cortisol stress responses irrespective of NR3C1-1F DNA methylation. Contrary to some prior work, however, our data provides no evidence for a direct association of CT and NR3C1-1F DNA methylation status.”


The study was an example of why researchers investigating the lasting impacts of human traumatic experiences won’t find causes, effects, and productive therapies until their paradigms change.

1. Limited subject histories

A. Why weren’t the subjects asked for historical information about their parents, grandparents, and great-grandparents?

The researchers had no problem using animal studies to guide the study design, EXCEPT for animal studies of the etiologic bases of intergenerational and transgenerational transmission of biological and behavioral phenotypes. Just the approximate places and dates of three generations of the German subjects’ ancestors’ births, childhoods, adolescences, and early adulthoods may have provided relevant trauma indicators.

B. Why are studies still using the extremely constrained Childhood Trauma Questionnaire? Only one CTQ aspect was acknowledged as a study design limitation:

“Our findings rely on retrospective self-report measures of CT, which could be subject to bias.”

But bias was among the lesser limiting factors of the CTQ.

The study correlated epigenetic changes with what the subjects selectively remembered, beginning when their brains developed sufficient cognitive functionalities to put together the types of memories that could provide CTQ answers – around age four. The basic problem prohibiting the CTQ from discovering likely most of the subjects’ historical traumatic experiences that caused epigenetic changes is that these experiences predated the CTQ’s developmental starting point:

  1. A human’s conception through prenatal period is when both the largest and the largest number of epigenetic changes occur, and is when our susceptibility and sensitivity to our environment is greatest;
  2. Birth through infancy is the second-largest; and
  3. Early childhood through the age of three is the third largest.

CTQ self-reports were – at best – evidence of experiences after age three, distinct from the  experience-dependent epigenetic changes since conception. If links existed between the subjects’ early-life DNA methylation and later-life conditions, they weren’t necessarily evidenced by CTQ answers about later life that can’t self-report relevant early-life experiences that may have caused DNA methylation.

2. Limited subject selection

The researchers narrowed down the initial 622 potential subjects to the eventual 200 subjects aged 18 to 30. An exclusion criteria that was justified as eliminating confounders led to this limitation statement:

“Our results might be based on a generally more resilient sample as we had explicitly excluded individuals with current or past psychopathology.”

Was it okay for the researchers to assert:

“Exposure to environmental adversity such as childhood trauma (CT) affects over 10% of the Western population and ranges among the best predictors for psychopathology later in life.”

but not develop evidence for the statement by letting people who may have been already affected by age 30 and received treatment participate in the study? Was the study design so fragile that it couldn’t adjust to the very people who may be helped by the research findings?

3. Limited consequential measurements

The current study design was very conformant to previous studies’ protocols. The researchers chose cortisol and specific DNA methylation measurements.

A. Here’s what Sex-specific impacts of childhood trauma had to say about cortisol:

“Findings are dependent upon variance in extenuating factors, including but not limited to, different measurements of:

  • early adversity,
  • age of onset,
  • basal cortisol levels, as well as
  • trauma forms and subtypes, and
  • presence and severity of psychopathology symptomology.”

The researchers knew or should have known all of the above since this quotation came from a review.

B. What other consequential evidence for prenatal, infancy, and early childhood experience-dependent epigenetic changes can be measured? One overlooked area is including human emotions as evidence.

There are many animal studies from which to draw inferences about human emotions. There are many animal models of creating measurable behavioral and biological phenotypes of human emotion correlates, with many methods, including manipulating environmental variables during prenatal, infancy, and early childhood periods.

Studies that take detailed histories may arrive at current emotional evidence for human subjects’ earliest experience-dependent changes. It’s not too big a leap to correlate specific historical environments and events, stress measurements, and lasting human emotions expressed as “I’m all alone” and “No one can help me” to better understand causes and effects.

CTQ answers aren’t sufficiently detailed histories.

4. Limited effective treatments and therapies

The current study only addressed this area in the final sentence:

“Given their potential reversibility, uncovering epigenetic contributions to differential trajectories following childhood adversity may serve the long-term goal of delivering personalized prevention strategies.”


Researchers: if your paradigms demonstrate these characteristics, why are you spending your working life in efforts that can’t make a difference? Isn’t your working life more valuable than that? What else could you investigate that could make a difference in your field?

I hope that researchers will value their professions enough to make a difference with their expertise. And that sponsors won’t thwart researchers’ desires for difference-making science by putting them into endless funding queues.

http://www.psyneuen-journal.com/article/S0306-4530(17)31355-0/pdf “Glucocorticoid receptor gene methylation moderates the association of childhood trauma and cortisol stress reactivity” (not freely available)