Age factor: How old is that polar bear? The answer is in its blood

Susannah Woodruff can’t wait to stop pulling teeth out of polar bears. Dr Woodruff, a wildlife biologist at the U.S. Fish and Wildlife Service, keeps tabs on Alaska’s population of the bears. She needs to know how old they are to estimate how many will soon die of old age, and how many will enter their reproductive years and start producing cubs.

Until recently, the only reliable way to determine the age of a polar bear has been to extract a premolar and inspect its growth rings. “No researcher wants to do it,” Dr. Woodruff said in an interview on Monday, just before embarking on a trip to Alaska’s North Slope.

On this trip, rather than pull teeth, she will merely draw blood. Using a method known as the epigenetic clock, she and her colleagues will be able to estimate the bears’ ages by analyzing chemical tags on their DNA. She and her colleagues have recently found using this method gives an estimate within a year of the bears’ true ages, making it more accurate than examining teeth.

The clock’s implications go far beyond polar bears. On Thursday, an international team of scientists published a study in the journal Nature Aging showing that epigenetic clocks tick inside 185 different species of mammals, including people.“You have a bat, you have a whale — with completely different ecologies — but you can use the same math formula to measuring aging,” said Steve Horvath, who led both of the new studies as a principal investigator at Altos Labs, a biotech company based in San Francisco. “It’s completely stunning.” The epigenetic clock is made possible thanks to millions of small molecules called methyl groups that are bound to our DNA like Christmas lights on a wire. When a cell divides, the DNA in the two new cells typically ends up with the same, distinctive pattern of methyl groups. But our cells also have enzymes that can pry methyl groups off the DNA.

Scientists have known about methylation for decades, but they’re still trying to figure out exactly what its purpose is. It most likely has something to do with keeping genes active or silenced. Adding methyl groups around a gene may be a step in shutting it off, while removing them may be involved in turning the gene back on.

In the 1960s, Soviet scientists noticed that as salmon grew old, their DNA became less methylated. In later years, a few studies found a similar pattern in other species. And other research found that certain regions of DNA get extra methyl groups with age. In the early 2010s, Dr. Horvath, then at the University of California, Los Angeles, wondered whether he could predict the age of an organism from its methylation pattern alone. He fed a computer with methylation data from thousands of human cells. Dr. Horvath then trained the machine to use that data to predict the age of the people from whom the cells came.

Dr. Horvath reported in 2013 that a computer needed to examine just 353 spots in the DNA of a cell to make a guess that was within a few years of a person’s chronological age.

Zimmer writes the “Origins” column for NYT

The New York Times