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A Spit Test for Age

Researchers are able to predict individuals’ ages from DNA modifications in their saliva.

Analyzing a few chemical markers in the DNA of saliva can determine a person’s age with surprising accuracy. Researchers at the University of California, Los Angeles, have found that the presence of just two chemical modifications allowed them to predict the ages of members of a sample group within a range of about five years. The technique, if validated, could be a useful tool in forensics. It also points to the possibility that DNA modifications might offer a way to measure aging that’s more medically relevant than counting birthdays.

The discovery, published this week in PLoS One, began as part of different research into the biological basis of sexual orientation. The UCLA team analyzed saliva samples of 34 pairs of identical male twins aged 21 to 55, looking for chemical modifications of DNA called methylation, a common marker of gene regulation, or epigenetics. The researchers, led by geneticist Eric Vilain, noticed that patterns in a subset of methylation sites tend to vary with age, with some sites on the genome becoming more methylated across large groups of cells and others less so.

They identified 88 sites on the genome that correlated strongly with age, and confirmed these findings in a group of 31 men and 29 women aged 18 to 70. Many of the genes associated with these sites are linked to cardiovascular and neurological disease. The researchers chose the two sites that correlated most strongly with age and used them to create a predictive model.

Sven Bocklandt, the study’s lead author, formerly of UCLA and now at biotech company Bioline, says that the model, which can determine a typical person’s age within 5.2 years, “is by far the most accurate predictive tool” of age yet developed. The tool could have clear benefits for forensics, in which the ability to narrow down the age of a suspect would aid investigations. However, the accuracy of the method needs to be validated in other samples, and also in other body fluids and tissues.

Jean-Pierre Issa, a epigenetics expert at University of Texas MD Anderson Cancer Center who was not involved in the study, says the findings are in line with work by him and others demonstrating that DNA methylation patterns change with age. But he adds that certain lifestyle factors, like heavy smoking or drinking, may affect methylation and make people seem to be older than they are. He notes that some study subjects were outliers whose methylation did not correspond as closely to age. In the study, the outliers could be identified because their real age was known, but in forensics applications this wouldn’t be the case.

These outliers also raise the possibility that methylation status might hint at a “biological age” that differs from chronological age. For example, someone with an unhealthy lifestyle might be older biologically than chronologically. Having an objective way to assess this might help predict risks of age-related diseases or better guide the need for age-related screenings and other health recommendations. Issa says this idea has been around since the 1990s but has not been studied enough to verify it.

Scientists have investigated a similar concept based on the length of telomeres—caps on the ends of chromosomes that get shorter with age but that have also been found to respond to stress. Issa says DNA methylation may prove to be a simpler and more accurate measure of biological age.

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