The search for ways to measure both chronological and biological age from tissue samples is producing interesting early results. Chronological age is how old you are by the clock, but biological age is a measure of how rapidly the processes of degenerative aging are progressing in your case: different people slide down the slippery slope at somewhat different rates, whether because of genes, luck, or lifestyle choices. DNA methylation is one line of research: it occurs across the whole genome and changes with age as metabolism reacts to rising levels of cellular damage. Combining measurements of the methylation of many different genes seems to produce fairly good results when it comes to identifying the age of individuals and tissues:
"To fight aging, we first need an objective way of measuring it. Pinpointing a set of biomarkers that keeps time throughout the body has been a four-year challenge," said Steve Horvath, a professor of human genetics at the David Geffen School of Medicine at UCLA and a professor of biostatistics at the UCLA Fielding School of Public Health. "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."
To create his age predictor, Horvath focused on a naturally occurring process called methylation, a chemical modification of one of the four building blocks that make up our DNA. He sifted through 121 sets of data collected previously by researchers who had studied methylation in both healthy and cancerous human tissue. Gleaning information from nearly 8,000 samples of 51 types of tissue and cells taken from throughout the body, Horvath charted how age affects DNA methylation levels from pre-birth through 101 years. For the age predictor, he zeroed in on 353 markers linked to methylation that change with age and are present throughout the body.
Horvath tested the predictive tool's effectiveness by comparing a tissue's biological age to its chronological age. When the tool repeatedly proved accurate in matching biological to chronological age, he was thrilled - and a little stunned. "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."