Our Scotch collie, Buckaroo, is just shy of 14 years old. Following the long-debunked but still popular idea that one dog year equals seven human years, he’s almost a centenarian. (This “formula” may be based on average life spans of 10 and 70 years for dogs and people, respectively.) Now, researchers say they have a new formula (see calculator below) to convert dog years to human years—one with some actual science behind it.
The work is based on a relatively new concept in aging research: that chemical modifications to a person’s DNA over a lifetime create what is known as an epigenetic clock. Scientists have built a case that one such modification, the addition of methyl groups to specific DNA sequences, tracks human biological age—that is, the toll that disease, poor lifestyle, and genetics take on our bodies. As a result, some groups have converted a person’s DNA methylation status to an age estimate—or even a prediction of life expectancy (worrying ethicists, who say the data could be misused by forensic investigators and insurance companies).
Other species also undergo DNA methylation as they age. Mice, chimpanzees, wolves, and dogs, for example, all seem to have epigenetic clocks. To find out how those clocks differ from the human version, geneticist Trey Ideker of the University of California, San Diego, and colleagues started with dogs. Even though man’s best friends diverged from humans early in mammalian evolution, they’re a good group for comparison because they live in the same environments and many receive similar healthcare and hospital treatments.
All dogs—no matter the breed—follow a similar developmental trajectory, reaching puberty around 10 months and dying before age 20. But to increase their chances of finding genetic factors associated with aging, Ideker’s team focused on a single breed: Labrador retrievers.
They scanned DNA methylation patterns in the genomes of 104 dogs, ranging from 4 weeks to 16 years of age. Their analysis revealed that dogs (at least Labrador retrievers) and humans do have similar age-related methylation of certain genomic regions with high mutation rates; those similarities were most apparent when the scientists looked at young dogs and young humans or old dogs and old humans. Most importantly, they found that certain groups of genes involved in development are similarly methylated during aging in both species. That suggests at least some aspects of aging are a continuation of development rather than a distinct process—and that at least some of these changes are evolutionarily conserved in mammals, Ideker and colleagues report in a preprint posted online at bioRxiv.
“We already knew that dogs get the same diseases and functional declines of aging that humans do, and this work provides evidence that similar molecular changes are also occurring during aging,” says Matt Kaeberlein, a biogerontologist at the University of Washington in Seattle, who was not involved with this research. “It’s a beautiful demonstration of the conserved features of the epigenetic age clocks shared by dogs and humans.”
The research team also used the rate of the methylation changes in dogs to match it to the human epigenetic clock, although the resulting dog age conversion is a bit more complex than “multiply by seven.” The new formula, which applies to dogs older than one, says that a canine’s human age roughly equals 16 ln(dog age) + 31. (That’s the natural logarithm of the dog’s real age, multiplied by 16, with 31 added to the total.)
Based on the methylation data, dogs’ and humans’ life stages seem to match up. For example, a 7-week-old puppy would be equivalent roughly to a 9-month-old human baby, both of whom are just starting to sprout teeth. The formula also nicely matches up the average life span of Labrador retrievers (12 years) with the worldwide lifetime expectancy of humans (70 years). Overall, the canine epigentic clock ticks much faster initially than the human one—that 2-year-old Lab may still act like a puppy but it is middle-aged, the methylation-based formula suggests—and then slows down.
“They’ve shown that there’s a gradual increase in DNA methylation in both species with age,” says Steve Austad, an evolutionary biologist and aging expert at the University of Alabama in Birmingham. He doesn’t find that especially surprising, but he thinks the technique could reveal far more interesting results if applied to issues like the different life spans among different dog breeds.
That’s one goal of Kaeberlein, whose group’s new Dog Aging Project (open to all breeds) will include epigenetic profiles of its canine subjects. He hopes to find out why some dogs develop disease at younger ages or die earlier than normal, whereas others live long, disease-free lives.
So, how does our Buckaroo fare? Happily, the epigenetic clock calculation goes in his favor. He’s now only 73 in human years—and a spry 73 at that.
*Update, 16 November, 10:52 a.m.: This story has been updated to include a sentence explaining why young dogs come out middle-aged in human years on the calculator.