It has long been known humans and dogs age differently, but scientists have determined a better way to compare how dogs age compared to humans. For decades, a dog’s age in human years has been determined by a general ratio of one dog year to seven human years, so a three-year-old dog would be the physical equivalent of a 21-year-old human. Where the long-accepted one-to-seven ratio came from is unknown, but a 2020 study by researchers at the University of California San Diego (UC San Diego) School of Medicine, which relied on a comparative analysis of human and canine DNA, determined the equation to be wildly inaccurate. According to lead author Trey Ideker, PhD, professor of medicine at the UC San Diego School of Medicine and Moores Cancer Center, the rate of canine aging is far from linear when compared to human aging, with a one-year-old dog being more similar to a 25-to-30-year-old human. This rate of age, however, does not progress with the same annual equivalence, Ideker notes. “A two-year-old dog is not the equivalent of a 50-year-old human. It levels off because it’s a nonlinear function,” he explains. “However, a 10-year-old dog best matches a 70-year-old person, so at the end of a dog’s life, it does follow the one-to-seven ratio.” Not quite what we thought The key to this new approach to determining canine age are methylation marks, also known as epigenetic marks, which are found on cytosine bases in human and canine DNA, and change over time in a fairly predictable way. By analyzing methylation marks in the DNA of humans at various ages, and comparing the measurements to methylation marks in the DNA of dogs over their lifespan, the researchers were able to find significant comparisons that made determining a dog’s true age much more accurate. Interestingly, accurately determining a dog’s age was not the initial purpose of the DNA research—the hope was to find a way to use methylation marks to confirm the early onset of cancer. “We were trying to use it as a cancer diagnostic, and that project failed,” Ideker says. “But there was an epiphanic moment when the student doing the research blurted out, ‘These damned epigenetic marks…all they track is aging! All I can do is track a patient’s age, and I can’t get past that.’ We sat back and said, ‘Aging, huh? Now that’s interesting!’ So, that was it.” The study, published in the journal Cell Systems,1 compared epigenic changes in 104 Labrador retrievers spanning a 16-year age range with 320 human subjects ages one to 103 years at the time of sample isolation. Labrador retrievers were used exclusively so the researchers would not have to adjust for breed, since there are extensive variations in lifespan across different breeds. “A lot of the work was just correctly collating samples and making sure we knew the actual ages of the animals in the study,” Ideker explains. “We also made sure the ages were spread over a nice range. For a study on aging, you want puppies, adolescent dogs, adults that have moved into fertility, and old age.” Targeted DNA sequencing was used to characterize the methylation marks in DNA found in white blood cells of both dogs and humans. “The big moment was when we realized what we were doing was not just telling someone the age of their dog from a blood sample, it was answering the question: How fast do dogs age relative to people? Once we realized that was the goal, it took about a year to work out the math and write the paper.” Ideker is quick to note the formula will likely undergo some revisions as research continues. “That equation is definitely not the final word,” he says. “In fact, since we published, there have been a lot of good discussions on blogs and elsewhere. However, the equation we published is still more accurate than the one-to-seven ratio.” Refocusing The study’s first author, Tina Wang, was instrumental in redirecting its focus to see if DNA methylation analysis could help track age in dogs, Ideker says. “Her point was, in people you can look at who’s aging fast or slow, but in dogs, most owners have no idea the age of their pets,” he notes. “The vast majority of dogs these days are adopted from shelters, and no one can say with certainty how old they are. Currently, to determine a dog’s relative age, we look at the condition of its teeth. This is not a quantitative approach, but it can tell you if you’re looking at a young dog, a middle-aged dog, or an old dog based on wear and tear and other properties.” The ability to precisely determine a dog’s age carries tremendous value. “Moving forward, I believe it will be a fantastic way to monitor a dog’s state of health, and encourage anti-aging interventions when necessary,” Ideker says. “That parallels what is going on in the human aging community right now. It is emerging there are interventions that will prolong life, such as reducing caloric intake.” Such a measure will also please dog owners, who can become fixated on their pets’ age. “People love their dogs and they obsess over how old their dog is,” Ideker confirms. “Being able to read out the age of your dog is an important accomplishment, and we’re already seeing on pet store shelves kits that use this technology to do just that.” Although Labrador retrievers have been the primary focus of the genetic research, the general idea is translatable to other breeds, and even other species. Study co-author Elaine Ostrander of the National Human Genome Research Institute in Bethesda, Maryland, is working with a consortium of researchers to take the research across canine breeds and even other mammals. “One area of interest is livestock and how this area of research could assist in running a farm and better understanding the health of dairy cows or any animals bred for meat,” Ideker says. “I believe livestock medicine will also be affected by this.” UC San Diego is not the only institution exploring the role of genetics in canine aging. The University of Washington and Texas A&M University, for example, have teamed up with researchers at numerous other institutions across the country to create the Dog Aging Project with the goal of understanding how genes, lifestyle, and environment influence aging. Dog owners across the United States are encouraged to enroll their pets, and if accepted, will report regularly on their dogs’ health and life experiences, engage in special activities, and submit a sample of their dogs’ saliva for DNA testing. A FORMULA FOR ACCURACY The new formula to determine a dog’s age requires the use of an advanced calculator, which is available on most smartphones. (You may have to turn your phone sideways to access it.) It is: 16 x In [advanced calculator code] + 31 = human age. Step-by-step using a seven-year-old dog as an example: (Type 7 and press In. Press x and type 16. Press + and type 31, then press the equal sign. This dog’s “human” age is 27 (rounded up). Don Vaughan is an award-winning writer who frequently writes about veterinary-related topics. Reference Cell Syst. 2020 Aug 26;11(2):176-185. e6. doi: 10.1016/j.cels.2020.06.006. Epub 2020 Jul 2.
