Non-CpG methylation

Three 2022 papers on methylation epigenetic modifiers, starting with a human study focused on mitochondrial DNA non-CpG methylation involving nucleobases other than guanine (arginine, cytosine, or thymine):

“We collected brain tissue in the nucleus accumbens and prefrontal cortex from deceased individuals without (n = 39) and with (n = 14) drug use, and used whole-genome bisulfite sequencing to cover cytosine sites in the mitochondrial genome. Epigenetic clocks in illicit drug users, especially in ketamine users, were accelerated in both brain regions by comparison with nonusers.

Unlike the predominance of CpG over non-CpG methylation in the nuclear genome, the average CpG and non-CpG methylation levels in the mitochondrial genome were almost equal. The utility of non-CpG methylation was further illustrated by the three indices constructed in this study with non-CpG sites having better distinction between brain areas, age groups, and the presence or absence of drug use than indices consisting of CpG sites only. Results of previous studies on the mitochondrial genome that were solely based on CpG sites should be interpreted cautiously.

The epigenetic clock made up of age-related cytosine sites in mtDNA of the control group was consistently replicated in these two brain regions. One possibility for the correlation is the cycle theory that involves mitochondrial activity, mitochondrial DNA methylation, and alpha-ketoglutarate.

As mitochondrial activity fades with aging, mitochondria gradually lose the ability to eliminate methylation on cytosines through alpha-ketoglutarate. Further investigation of the underlying mechanisms is warranted.

To our knowledge, this is the first report that ketamine might change the mitochondrial epigenetic clock in human brain tissues. We believe this is the first report to elucidate comprehensively the importance of mitochondrial DNA methylation in human brain.”

https://clinicalepigeneticsjournal.biomedcentral.com/articles/10.1186/s13148-022-01300-z “Mitochondrial DNA methylation profiling of the human prefrontal cortex and nucleus accumbens: correlations with aging and drug use”


A second rodent study focused on RNA methylation:

“We investigated the role of RNA N6-methyladenosine (m6A) in improved resilience against chronic restraint stress. A combination of molecular, behavioral, and in vivo recording data demonstrates exercise-mediated restoration of m6A in the mouse medial prefrontal cortex, whose activity is potentiated to exert anxiolytic effects. To provide molecular explanations, it is worth noting that epigenetic regulation, such as histone modification, microRNA, and DNA methylation all participate in mental and cognitive rehabilitation following exercise.

To generalize these rodent data to humans, we recruited a small group of patients with major depressive disorder with prominent anxiety disorders. Compared to age- and sex-matched healthy individuals, patients displayed decreased circulating methyl donor S-adenosyl methionine (SAM) levels. Serum SAM levels were found to be inversely correlated with the Hamilton Anxiety Scale, suggesting the potential value of SAM as a biomarker for depression or anxiety disorders.

Hepatic biosynthesis of methyl donors is necessary for exercise to improve brain RNA m6A to counteract environmental stress. The dependence on hepatic-brain axis suggests the ineffectiveness of exercise training on people with hepatic dysfunctions.

This novel liver-brain axis provides an explanation for brain network changes upon exercise training, and provides new insights into diagnosis and treatment of anxiety disorders. Exercise-induced anxiolysis might be potentiated by further replenishment of RNA methylation donors, providing a strategy of exercise plus diet supplement in preventing anxiety disorders.”

https://onlinelibrary.wiley.com/doi/10.1002/advs.202105731 “Physical Exercise Prevented Stress-Induced Anxiety via Improving Brain RNA Methylation”


A third paper was a review of mitochondrial-to-nuclear epigenetic regulation. I’ll highlight one mitochondrial metabolite, alpha-ketoglutarate (α-KG):

“Apart from established roles in bioenergetics and biosynthesis, mitochondria are signaling organelles that communicate their fitness to the nucleus, triggering transcriptional programs to adapt homeostasis stress that is essential for organismal health and aging. Emerging studies revealed that mitochondrial-to-nuclear communication via altered levels of mitochondrial metabolites or stress signals causes various epigenetic changes, facilitating efforts to maintain homeostasis and affect aging.

Metabolites generated by the tricarboxylic acid (TCA) cycle, the electron transport chain (ETC), or one-carbon cycle within mitochondria can act as substrates or cofactors to control epigenetic modification, especially histone acetylation and methylation and DNA methylation. α-KG produced in the TCA cycle serves as an essential cofactor for the chromatin-modifying Jumonji C (JmjC) domain-containing lysine demethylases (JMJDs) and ten-eleven translocation (TETs) DNA demethylases. Changes in α-KG levels are capable of driving nuclear gene expression by affecting DNA and histone methylation profiles.

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α-KG deficiency in progenitor stem cells increases with age. For example, the level of α-KG is reduced in follicle fluids of aged humans, and supplementation with α-KG preserves ovarian function in mice.

α-KG extends lifespan in Drosophila by activating AMPK signaling and inhibiting the mTOR pathway. Supplementing α-KG in the form of a calcium salt promoted a longer and healthier life associated with decreased levels of inflammatory cytokines in old mice.

A human study showed a nearly 8-year reversal in DNA methylation clock biological ages of 42 individuals taking an α-KG based formulation for 4–10 months. α-KG supplementation leads to both demethylation and hypermethylation of some CpG sites in the genome, suggesting that α-KG may have a broader effect on methylation-based aging, such as metabolic functions.

Outstanding questions:

  1. How is production of mitochondrial metabolites regulated both spatially and temporally to elicit epigenetic changes in response to mitochondrial dysfunction?
  2. What are specific epigenetic factors involved in mitochondrial-to-nuclear communications, and how do they cooperate with transcription factors in response to various external and internal stimuli?
  3. Do various mitochondrial metabolites act alone or in concert on the epigenome to regulate the aging process?
  4. Are some organs or tissues more at risk than others in maintaining mitochondrial-to-nuclear communication during aging?
  5. Can intervention of mitochondrial-to-nuclear communications mimic beneficial epigenetic changes to delay aging or alleviate age-onset diseases?”

https://www.sciencedirect.com/science/article/pii/S0968000422000676 “Mitochondrial-to-nuclear communication in aging: an epigenetic perspective”


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