How Fast Do We Age?

What Your Body Parts Reveal About Your Rate of Aging

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Could your kidneys, heart or even your breasts be aging faster than the rest of your body? Some research suggests that not all parts of the body age at the same rate. The discovery - if it continues to offer an objective measure of how fast tissues are aging - could eventually help scientists slow down or even reverse the aging process.

The Timekeeper Within Cells

An article published in a 2013 issue of Genome Biology outlines how a natural chemical process that occurs within a cell can be used as a kind of timekeeper or gauge of how quickly that cell is aging.

This process, known as methylation, alters DNA - the genetic building blocks of cells - in a pattern associated with advancing age.

Prior research at the University of California at Los Angeles (UCLA) used saliva samples from 34 pairs of identical twins to demonstrate which DNA sites linked methylation reliably to the age of the subjects. Scientists then tested methylation rates in a broader population and were able to pinpoint each adult's age within five years, using only the person's saliva.

Steve Horvath, UCLA human genetics and biostatistics professor, took this research further and examined more than 8,000 tissue and cell samples donated from other scientists. The human samples were taken from before birth to the age of 101 years, across more than 50 different types of tissues and cells from different parts of the body, including the liver, kidney, brain, lungs and heart. The DNA methylation rates created a consistent gauge of how rapidly the various tissues were aging.

Horvath and his colleagues used the gauge to assess the rate of aging in 6,000 different cancer samples, representing 20 different types of the disease.

Different Parts of the Body Age at Different Rates

Horvath's team found that most tissues age at pretty much the same pace; in effect, their biological age, i.e. how well they're functioning, matches their chronological age, i.e. the number of years represented on the calendar.

Breast tissue, however, was an exception. Healthy breast tissue ages more rapidly and is in effect two to three years older than the rest of a woman's body. This may explain why breast cancer is the most common form of cancer in women since age is a risk factor and more rapidly-aging tissue could therefore be more susceptible to cancer.

In women who already have breast cancer, the accelerated aging is even more pronounced. Testing tissue adjacent to breast cancer tumors, the researchers found it was on average about 12 years older than the rest of the body.

Are Rapidly Aging Cells More Prone to Cancer?

Cancerous tissue was found to be much older than healthy tissue found elsewhere in a body. The research team studies more than 20 different types of cancer and found consistent signs of accelerated aging in the diseased samples. In fact, cancerous tissue was an average of 36 years older than the rest of the body.

This poses an important question: Are cells that are aging faster more vulnerable to cancer, or does cancer age cells more quickly? Horvath explains that both of these things may be true.

For example, in most cancer cases, adjacent tissue looks young, or at least the actual age of the people the samples were from, which suggests cancer itself ages cells.

The fact that even healthy breast tissue appears older according to methylation rates when compared to the rest of a woman's body suggests that more rapidly-aging tissue could be more susceptible to cancer.

"Additional studies will be needed to test this hypothesis," Horvath says. "We would really want to measure the age of healthy, non-cancerous female breast tissue. Further, we would want to test whether age acceleration in breast tissue is predictive of cancer development at a later point."

More testing of healthy breast tissue at various stages in a woman's life - puberty, post-puberty, after pregnancy and after menopause - will help clarify whether breast tissue that is aging more quickly is in fact more likely to develop cancer. If so, testing the degree of methylation within the breasts could one day chart a woman's chance of getting this life-threatening disease.

What About Telomeres?

Telomeres offer another way to measure aging rates. Telomeres are tiny structures that move genetic material around within a cell as the cell gets ready to divide. Each time cell division occurs, the telomeres shorten a bit; once they're too short, the cell can no longer divide, resulting in cell death.

There is research underway to measure telomere length as a kind of objective predictor of how fast cells in your body are aging. In addition, scientists are investigating whether keeping telomeres longer could keep cells young and dividing properly.

See Also

Sources:

Methylation.  US National Cancer Institute Dictionary of Cancer Terms.  Accessed October 25, 2013.
http://www.cancer.gov/dictionary?cdrid=655031

Steve Horvath.  "DNA methylation age of human tissues and cell types."  Genome Biology 2013, 14:R115.
http://genomebiology.com/2013/14/10/R115

Sven Bocklandt, Wen Lin, Mary E. Sehl, Francisco J. Sánchez, Janet S. Sinsheimer, Steve Horvath, and Eric Vilain. "Epigenetic Predictor of Age." PLoS ONE 6(6): e14821.
http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0014821

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