NHGRI researchers reframe dog-to-human aging comparisons

One of the most common misconceptions is that one human year equals seven dog years in terms of aging. However, this equivalency is misleading and has been consistently dismissed by veterinarians. A recent study, published in the journal Cell Systems, lays out a new framework for comparing dog-to-human aging. In one such comparison, the researchers found the first eight weeks of a dog’s life is comparable to the first nine months of human infancy, but the ratio changes over time. The research used epigenetics, a process by which modifications occur in the genome, as a biological marker to study the aging process. By comparing when and what epigenetic changes mark certain developmental periods in humans and dogs, researchers hope to gain specific insight into human aging as well.    

Researchers performed a comprehensive analysis and quantitatively compared the progression of aging between two mammals, dogs and humans. Scientists at the National Human Genome Research Institute (NHGRI), part of the National Institutes of Health, and collaborators at the University of California (UC) San Diego, UC Davis and the University of Pittsburgh School of Medicine carried out the research.

All mammals experience the same overarching developmental timeline: birth, infancy, youth, puberty, adulthood and death. But researchers have long sought specific biological events that govern when such life stages take place. One means to study such a progression involves epigenetics -- gene expression changes caused by factors other than the DNA sequence itself. Recent findings have shown that epigenetic changes are linked to specific stages of aging and that these are shared among species.

Researchers focused on one type of epigenetic change called methylation, a process in which molecules called methyl groups are attached to particular DNA sequences, usually parts of a gene. Attaching to these DNA regions effectively turns the gene into the "off" position. So far, researchers have identified that in humans, methylation patterns change predictably over time. These patterns have allowed the creation of mathematical models that can accurately gauge the age of an individual--called "epigenetic clocks."

But these epigenetic clocks have only been successful in predicting human age. They do not seem to be valid across species, such as in mice, dogs, and wolves. To see why the epigenetic clocks in these other species differed from the human version, researchers first studied the epigenetic changes over the lifetime of a domestic dog and compared the results obtained with humans.

Dogs are a useful model for such comparisons because much of their environment, diet, chemical exposure, and physiological and developmental patterns are similar to humans.

"Dogs experience the same biological hallmarks of aging as humans, but do so in a compressed period, around 10 to 15 years on average, versus over 70 years in humans. This makes dogs invaluable for studying the genetics of aging across mammals, including humans," said Elaine Ostrander, Ph.D., NIH Distinguished Investigator and co-author of the paper.

Dogs experience the same biological hallmarks of aging as humans, but do so in a compressed period, around 10 to 15 years on average, versus over 70 years in humans. This makes dogs invaluable for studying the genetics of aging across mammals, including humans.

Dr. Ostrander and her colleagues in Trey Ideker's laboratory at UC San Diego took blood samples from 104 dogs, mostly Labrador retrievers, ranging from four weeks to 16 years of age. They also obtained previously published methylation patterns from 320 people, whose ages ranged from 1 to 103 years. The researchers then studied and compared the methylation patterns from both species.

Based on the data, researchers identified similar age-related methylation patterns, specifically when pairing young dogs with young humans or older dogs with older humans. They did not observe this relationship when comparing young dogs to older humans and vice versa.

The study also found that groups of specific genes involved in development can explain much of the similarity, which had similar methylation patterns during aging in dogs and humans.

"These results suggest that aging can, in part, be explained by a continuum of changes beginning in development," said Dr. Ideker. "The programs of development are expressed incredibly strongly at defined periods when the pup is in the womb and childhood. But equally strongly are systems that clamp down to stop it. In a sense, we are looking at aging as the residual 'afterburn' of those powerful forces."

The researchers also attempted to correlate the human epigenetic clock with dogs, using this as a proxy for converting dog years to human years.

A newer formula for comparing human to dog years: human age =​ 16*ln(dog age)​ + 31

The new formula is more complicated than the "multiply by seven" method. When dogs and humans experience similar physiological milestones, such as infancy, adolescence and aging, the new formula provided reasonable estimates of equivalent ages. For example, by using the new formula, eight weeks in dogs roughly translates to nine months in humans, which corresponds to the infant stage in both puppies and babies. The expected lifespan of senior Labrador retrievers, 12 years, correctly translates to 70 years in humans, the worldwide average life expectancy.

The group acknowledges that the dog-to-human years formula is largely based on data from Labrador retrievers alone. Hence, future studies with other dog breeds will be required to test the formula's generalizability. Because dog breeds have different life spans, the formula may be different among breeds.

Dr. Ostrander noted, "It will be particularly interesting to study long-lived breeds, a disproportionate number of which are small in size, versus breeds with a shorter lifespan, which includes many larger breeds. This will help us correlate the well-recognized relationship between skeletal size and lifespan in dogs."

The study also demonstrates that studying methylation patterns may be a useful method to quantitatively translate the age-related physiology experienced by one organism (e.g., humans) to the age at which physiology in a second organism is most similar (e.g., dogs). The group hopes that such translation may provide a useful tool for understanding aging and identifying ways to maximize healthy lifespans.

"This study, which highlights the relevance of canine aging studies, further expands the utility of the dog as a genetic system for studies that inform human health and biology," said Dr. Ostrander.