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Old bloodstains divulge the age of the bleeder

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Scientists can estimate a person's age using white blood cell waste products found in blood. This could be useful in solving crimes and identifying disaster victims.

Dexter Morgan analyzes blood spatter to reconstruct the murders he didn’t commit; Catherine Willows used the shape of blood droplets to ultimately uncover motive behind a mass murder of a suburban family.

And now scientists have figured out a way to estimate age from a blood drop.

Molecular biologist Manfred Kayser and an interdisciplinary team from Erasmus MC University Medical Center Rotterdam, the Netherlands, found a way to link DNA with a person’s age – give or take 9 years. They published their findings in Current Biology on Tuesday.

Normally, DNA doesn’t change with age, but some molecular byproducts of white blood cells do. T-cells in particular. They defend against foreign invaders like bacteria, viruses, parasites, and tumor cells. Born in the bone marrow, they develop in the thymus – an organ that is gradually replaced with fat tissue as people get older.

In order to create a surface receptor to recognize these assailants, a T-cell rearranges its DNA as it matures in the thymus. In doing so, they create small circular pieces of DNA molecules as a byproduct. ("We take advantage of this waste product," Kayser says.)

The number of those circles (known as signal joint TCR excision circles, or sjTRECs) declines at a constant rate, because the body produces fewer new T-cells as people age. These loops are present only in newly made T-cells, says Kayser, so they provide a reliable stopwatch to time aging.

Taking advantage of this natural phenomenon, the researchers used a tiny amount of blood collected from 195 Dutch volunteers and calculated the abundance of loops. And then they plotted those against age, which ranged from 0 to 80 years. The researchers found that they could estimate age to within 9 years fairly accurately.

This discovery will be useful in forensic cases where age can provide investigative leads for finding unknown persons, Kayser says. "Our method is applicable in situations where only bloodstains are available, which covers a large proportion of crime cases.”

According to Kayser, the test currently has the highest accuracy of any test designed to estimate a physical characteristic from DNA information.

Teeth and bones and other identifiable body parts (if they’re even left behind) are used to estimate age, but those extraction techniques are invasive, often destroying the evidence. With this technique, a tiny blood drop as small (5 nanograms to be precise) is all that’s needed. And they can be old smears; the researchers found no difference between fresh blood and blood aged 1.5 years.

I spoke with Kayser earlier this year, when he found a way to quantify eye color – from the lightest blue to the darkest brown – from DNA, say, left at a crime scene. But if you can’t find a match with the suspects identified by the police, you’re out of luck.

"Conventional DNA profiling applied in forensics can only identify persons already known to the investigating bodies, because the approach is completely comparative," he says. "Hence, every forensic lab is confronted with cases where the DNA profile obtained from the evidence material does not match that of any known suspect tested, nor anybody in the criminal DNA database, and such cases therefore cannot be solved so far. In such cases, it is expected that appearance information estimated from evidence material will help in finding unknown persons."

However, the new study does have that wide error margin of 9 years and cannot be applied to other leftover bodily fluids. And the researchers don’t know how the technique would apply to people who have compromised immune systems, such as people with HIV/AIDS or leukemia.

Before the technique becomes widely available to forensic teams in the Netherlands, it will need to be verified by two independent labs, according to the head of the forensic laboratory at Leiden University Medical Center in the Netherlands. There are calls for this test already, in cases where the trail has gone cold.

Image by RedGlow82 via Flickr

Janet Fang

Contributing Editor

Janet Fang has written for Nature, Discover and the Point Reyes Light. She is currently a lab technician at Lamont-Doherty Earth Observatory. She holds degrees from the University of California, Berkeley and Columbia University. She is based in New York. Follow her on Twitter. Disclosure