CS-Eye: Copenhagen

Accelerator MS meets forensic pathology.

SHUTTERSTOCK/LOSEVSKY PAVEL

Your eyes can now give away your age.

The poets will tell you that the eyes are the windows to the soul, but according to recent work by forensic pathologist Niels Lynnerup and colleagues at the University of Copenhagen and the University of Aarhus (Denmark), your eyes may also provide scientists with a glimpse of your birth—or more specifically, your birth year. In a situation like the devastating tsunami that struck the coast of Thailand in 2004, says Lynnerup, such a simple piece of information can be critical to identifying the thousands of people who perished.

"We're always looking at new methods to identify people," he explains. "You might say we're trying to develop a forensic toolbox, so that we'll always have something that's appropriate for the specific problem."

Recently, Lynnerup was reading about eye formation and learned that crystallines, proteins found in the eye lens, form almost entirely by the end of the first postnatal year. Furthermore, the structure of the lens is such that the proteins are cut off from the rest of the body and therefore do not exchange over time.

The revelation led him to conclude that if he could correlate some type of molecular signature with an environmental timeline, he could determine the ages of people from the proteins in their eye lenses. Lynnerup and his team eventually realized that high-precision accelerator MS (AMS) and the isotopic echo left behind by the nuclear bomb tests of the 1950s and 1960s might provide the solution.

Before the 1963 Test Ban Treaty, nuclear testing programs produced a dramatic spike in atmospheric ¹⁴C levels. Since then, these levels have declined as the isotope has been absorbed into the environment and food chain. For Lynnerup and colleagues, this spike provided a chronological reference point.

Using AMS, the researchers examined eye lenses from 13 deceased people of varying ages and compared the ¹⁴C levels in the sample proteins with a calibration curve of published atmospheric ¹⁴C levels from the past 90 years (PLoS ONE 2008, 3, e1529). With the exception of the oldest case studied—an 80-year-old person—the researchers were able to identify the age of the test subjects to within 1.5 years with 95% confidence.

Lynnerup is quick to point out that the idea of dating unchanging human tissues is not unique to his group. Other researchers had performed similar experiments using ¹⁴C signatures of dental enamel (Nature 2005, 437, 333–334). "One can immediately understand that dental enamel has an immutable nature," he says. "I mean, it is a hard ceramic-like tissue. That the lens contains molecules just as immutable—even though it changes form whenever [the eye focuses] on something—was for me somewhat surprising."

Because the technique uses only a single radiocarbon reference point, Lynnerup acknowledges that its utility is not unlimited. "The method will still be useful for 100 years or more, but it will become less precise," he says. "Now, we can utilize the very steep [isotopic] increase, so there is a very marked change from year to year. But as atmospheric ¹⁴C levels go back to normal, the differences become less and less."

The researchers also note that sample integrity will be an issue. For these experiments, they extracted lenses up to 3 days postmortem. Beyond this point, they found that lens degradation and putrefaction made it impossible to extract the lens intact.

Although his primary interest is forensics and identifying the dead, Lynnerup also sees a place for his method to help the living by improving our understanding of human disease. "This might be a method to date single tissues," he says—an idea first suggested by the group working on dental enamel (Cell 2005, 122, 133–143). "For instance, if you had a tumor where some of the central bits necrotize and don't change, it might be possible to radiocarbon date some of the earliest parts of the tissue so that you can say more precisely when a given tumor started," he adds.

Perhaps most exciting to Lynnerup, however, is how the research shows the power of an interdisciplinary approach to science. In this case, forensic pathologists, nuclear physicists, and eye pathologists met to share thoughts and goals. "I think the approach is very stimulating and leads to other ideas," he says. "You talk to other people with a completely different mindset." That's the benefit of adding new eyes to an old challenge, one might say.

—Randall C Willis