Fight Aging!

A growing number of research groups are working on biomarkers of aging based on patterns of DNA methylation, an epigenetic decoration to nuclear DNA that determines the pace at which specific proteins are produced from their genetic blueprints. Quite a few new papers on this topic have caught my attention in the past few weeks, and today I'll point out another one, this time focused on canine species. One of the challenges inherent in this work is that these aging-associated epigenetic patterns are not entirely the same when comparing different mammalian species, yet cost effective life science efforts have to be - at least initially - undertaken in something other than human subjects. The point of building biomarkers of aging is to greatly speed up the development of rejuvenation therapies. Reading the results of a biomarker assay immediately before and after treatment is a very different proposition from having to wait for the entire remaining life span of the study animals in order to determine whether or not a potential therapy actually does in fact extend healthy longevity. Unfortunately, the potential biomarkers of aging must themselves be validated before they can be used, and so we come back to working with shorter-lived animal species for the sake of cost-effectiveness.

In aging research, dogs are a useful intermediary step between mice and humans, when considering the cost to run studies, meaningful differences in cellular biochemistry, and species life span. On that last point, that dogs exhibit such varied life spans between breeds is especially useful. It gives a great deal of flexibility in designing and executing studies that might not otherwise have existed. Studies of approaches to treating aging that are fairly far along, with a good deal of safety data already, and that would be enormously expensive in humans, can even be carried out in companion animals rather than laboratory animals. One research group in the US has set up the Dog Aging Project in order to make some progress on this front, for example. Given this, it isn't surprising to find researchers putting together DNA methylation biomarkers for canine species. As is the case for work on biomarkers of aging in mice, this initiative is a necessary part of making the field of aging research more efficient.

An epigenetic aging clock for dogs and wolves

Technological breakthroughs surrounding genomic platforms have led to major insights about age related DNA methylation changes in humans. In mammals, DNA methylation represents a form of genome modification that regulates gene expression by serving as a maintainable mark whose absence marks promoters and enhancers. During development, germline DNA methylation is erased but is established anew at the time of implantation. Abnormal methylation changes that occur because of aging contribute to the functional decline of adult stem cells. Even small changes of the epigenetic landscape can lead to robustly altered expression patterns, either directly by loss of regulatory control or indirectly, via additive effects, ultimately leading to transcriptional changes of the stem cells.

Several studies describe highly accurate age estimation methods based on combining the DNA methylation levels of multiple CpG dinucleotide markers. We recently developed a multi-tissue epigenetic age estimation method (known as the epigenetic clock) that combines the DNA methylation levels of 353 epigenetic markers known as CpGs. The weighted average of these 353 epigenetic markers gives rise to an estimate of tissue age (in units of years), which is referred to as "DNA methylation age" or as "epigenetic age". DNA methylation age is highly correlated with chronological age across the entire lifespan. We and others have shown that the human epigenetic clock relates to biological age (as opposed to simply being a correlate of chronological age), e.g. the DNA methylation age of blood is predictive of all-cause mortality even after adjusting for a variety of known risk factors.

Many research questions and preclinical studies of anti-aging interventions will benefit from analogous epigenetic clocks in animals. To this end we sought to develop an accurate epigenetic clock for dogs and wolves. Dogs are increasingly recognized as a valuable model for aging studies. Dogs are an attractive model in aging research because their lifespan (around 12 years) is intermediate between that of mice (2 years) and humans (80 years), thus serving as a more realistic model for human aging than most rodents. The maximum lifespan of dogs is known to correlate with the size of their breed. Based on previous studies in human, we expect that the age acceleration (difference between epigenetic age and chronological age) correlates with longevity. We hypothesize that dogs whose epigenetic age is larger than their chronological age are aging more quickly, while those with negative value are aging more slowly. Thus, we would expect to see a correlation between age acceleration and dog breed size. We also sought to build an epigenetic clock for gray wolves because alternative age estimation methods have limitations.

Our study demonstrates that DNA-methylation correlates with age in dogs and wolves as it does in human and related species. This age-dependence of DNA-methylation is conserved at syntenic sites in the respective genomes of these canid species as well for more distantly related mammalian genomes such as human. Overall, our study demonstrates that dogs age in a similar fashion to humans when it comes to DNA methylation changes. Based on our preliminary blood samples of 108 canid specimens, including both dogs and wolves, we accurately measured the methylation status of several hundred thousand CpGs. We demonstrate that these data can produce highly accurate age estimation methods (epigenetic clocks) for dogs and wolves separately. By first removing sites that were variable between dogs and wolves, we could also establish a highly accurate epigenetic clock for all canids (i.e. dogs and wolves combined). This clock allows us to estimate the age of half the canids to within a year.