Hi. Thank you for the questions, and I apologize for the EXTREMELY long delay in responding (a combination of being out of town, along with a wicked head cold).
The main difference between DNA methylation and mtDNA methylation, as far as I’m aware, is that mtDNA methylation has been much less studied. Likely due to its low levels and higher degree of variability, mtDNA methylation wasn’t fully confirmed by the scientific community until the early 2000s. As far as age-related methylation is concerned, I know that several mtDNA methylation sites have shown a linear correlation to chronological age (Mawlood SK, et al. 2016
), but the coefficient of correlation (the tightness of the scatter in the data) was reported as lower than in other methods.
As far as I’m aware, epigenetic methods of chronological age determination are more accurate than telomere length measurements. Telomere length is affected by multiple factors, including oxidative stress. (Global DNA methylation too is affected by a huge number of environmental factors, but multiple specific DNA regions seem to be more affected by chronological age than anything else.) As for y-H2AX, as you stated it is a very reliable marker for double-strand breaks in DNA, and the level of y-H2AX expression has been shown to increase with age until around the age of 57, when it’s increase plateaus (Schurman SH, et al. 2012
). I have not seen y-H2AX levels used as a predictor of chronological age, although I suppose it potentially could be for individuals below 60 years old.
The method I’m using relies on DNA isolated from buccal cells (epithelial cells of the cheek and gums). From my personal experience, these are more than adequate for purposes of DNA methylation analysis for estimating chronological age. I can’t state as to how these cells in general compare to other cell types within the body, but if I had to make an educated guess I would say that they are probably fairly similar in physiology and gene expression to other epithelial cells.
As to your last question: is it possible to reverse this clock biomarker? I certainly hope so. My assumption is that most age-related DNA methylation changes are the result of random epigenetic drift (hypermethylation) and inadequate recovery of epigenetic state after DNA repair (hypomethylation). Some of these age-related epigenetic changes though likely reflect gene regulation that is part of the general developmental inertia that occurs past puberty (there’s an excellent book by Dr. Richard Walker that discusses this topic – he doesn’t get enough attention in my opinion) as well as cellular changes happening in response to body-wide factors such as senescent cell accumulation and immune system decline. Likely, any effective antiaging therapy would at least slow those epigenetic changes, if not reverse them entirely.