Study shows new method to count dog age in human years

July 11, 2020

THE well-known method to calculate dog age in human years is by seven, that is, a one dog year is the equivalent of 7 years.

But in a new study, scientists said this method is not correct. They said dogs are much older than we think, and thus they devised a more accurate formula to calculate a dog's age based on the chemical changes in the DNA as organisms grow old.

Dogs shared the same environment as their owners and received almost the same standard of health care as humans. This gave scientists at the University of California in San Diego, a unique opportunity for them to understand aging across species.

Dogs follow similar developmental trajectories of humans that lead them to grey and become more susceptible to age-related diseases over time. However, how they age on a molecular level is more complicated -- aging rapidly at first and slowing down later in life.

"In terms of how physiologically mature a 1-year-old dog is, a 9-month-old dog can have puppies. Right away, you know that if you do the math, you don't just times seven," said senior author Trey Ideker. "What's surprising is exactly how old that one-year-old dog is -- it's like a 30-year old human."

Human and dog DNA, which codes who we are, doesn't change much throughout the course of life, but chemical marks on the DNA, called methylation marks, do. Ideker considered these marks like wrinkles in the genome.

The researchers along with canine experts Danika Bannasch of the University of California, Davis, and Elaine Ostrander of the National Institutes of Health studied 104 Labrador retrievers spanning from few-week-old puppies to 16-year-old dogs. They compared the changes in the methylation pattern to humans.

The comparison revealed a new formula that better matches the canine-human life stages: human age = 16 (in dog age) + 31. Based on the new function, an 8-week-old dog is approximately the age of a 9-month-old baby, both being in the infant stage where puppies and babies develop teeth. The average 12-year lifespan of Labrador retrievers also corresponds to the worldwide life expectancy of humans, 70 years.

In both species, they found that the age-driven methylation largely happens in developmental genes that are hotly fired up to create body plans in utero and regulating childhood development. By the time one becomes an adult and stops growing, "you've largely shut off these genes, but they're still smoldering," said Ideker. "If you look at the methylation marks on those developmental genes, they're still changing."

Focusing on the smoldering developmental genes, the team developed a clock that can measure age and physiological states across different species, while other methylation-quantifying age-predicting methods only do well in one species. Ideker noted that future investigation in different dog breeds with various lifespans could provide more insight into the new clock. The clock may not only serve as a tool to understand cross-species aging but also apply as clinical practice for veterinarians to take proactive steps to treat animals. (Cell Press).