

What causes humans to age and eventually die? We know that age increases the risk for a host of diseases including cancer, and causes dwindling eyesight and hearing, gray hair and papery skin. But scientists don’t know which biological process triggers those changes.

A study published October 21 in the journal Genome Biology provides a tool that may help future researchers answer that question. It’s a biological clock that can date the age of a cell by measuring methylation, a chemical modification that affects certain parts of DNA.

Using the system, any piece of tissue can be identified with the biological age of its human source.

“It’s surprising that one could develop a predictive tool that reliably keeps time across the human anatomy. My approach really compared apples and oranges, or in this case, very different parts of the body — including brain, heart, lungs, liver, kidney and cartilage,” said author Steve Horvath, a UCLA geneticist and biostatistician.

Methylation increases over time, and Horvath charted its effects on more than 8,000 tissue samples representing 51 types of tissue. He eventually honed in on 353 methylation markers that are present throughout the body, demonstrating that they march nearly in lock step with chronological age.

The finding gives scientists a way to monitor the effects of aging — and, potentially, of anti-aging treatments — on a person’s cells. But it does not suggest that methylation in DNA is one of the causes of aging.

“The general idea that you can read a genome and it reflects the aging process is probably correct. But the weakness is that this study doesn’t provide a mechanism, and without a mechanism it’s just a correlation,” Darryl Shibata, a pathologist at the University of Southern California medical school who has reviewed Horvath’s work, told Forbes.

Horvath agreed but noted, “To fight aging, we first need an objective way of measuring it. My goal in inventing this age-predictive tool is to help scientists improve their understanding of what speeds up and slows down the human aging process.”

The research points to several interesting variations in how cells age. For example, cancer cells appear on average to be 36 years older than other cells. The cells next to a breast cancer tumor appeared 12 years older than healthy cells.

And, apparently helping explain epidemic rates of breast cancer, Horvath found that breast tissue posed a rare exception to the biological clock, appearing 2-3 years older than the rest of a woman’s cells.

The finding is in sync with other studies that suggest that degradation of DNA causes cancer. (One study on a particularly long-lived rodent suggested that with less damage to DNA, aging is slowed and cancer held at bay.)

Horvath’s clock may also help stem cell researchers. The biological clock aged induced pluripotent stem cells — stem cells created in the lab from an adult’s skin cells —at zero. In other words, it is possible to reset the age of a cell back to zero, essentially erasing the aging process.