DNA LOOPHOLE

Something similar to the case below can happen with blood donors and blood recipients, though the effect seems not to be as long-term

It sounds like an open-and-shut case: a clear DNA match is made between semen from a serious sexual assault and a blood sample from a known criminal. Yet in a recent case from Alaska, the criminal in question was in jail when the assault took place. And forensic scientists had already matched the crime sample to the DNA profile of another person who was their prime suspect. It was only after careful detective work that the mystery was solved: the jailed man had received bone marrow from the suspect many years earlier.

This week, at a meeting of the American Society of Human Genetics in Salt Lake City, Utah, Abirami Chidambaram of the Alaska State Scientific Crime Detection Laboratory in Anchorage described the case to highlight the danger of miscarriages of justice. Given the retribution that can be doled out to sex offenders by other jail inmates, the consequences could be severe. "If you implicate the wrong person, they can be killed in prison," says Chidambaram. “As forensic DNA databases grow and more people undergo bone marrow transplants, the risk of a miscarriage of justice increases”

When Chidambaram discovered the perplexing match, she initially thought there had been a sample mix-up. But there was no mistake. What's more, the jailed man and suspect shared the same surname. Because medical records are confidential, a detective had to make further enquiries among family and friends of the two men. That revealed that not only were the convict and suspect brothers, but the inmate had received a bone marrow transplant from his brother. As a result, his blood was populated with cells bearing his brother's DNA profile.

It's an instance of life imitating art: in November 2004, US TV channel NBC broadcast an episode of Law and Order: Special victims unit in which a rapist nearly got away with his crimes because of a similar bone-marrow mix-up. The chances of cases like this arising are very low. But as forensic DNA databases expand, and more people undergo marrow transplants, the risk of a miscarriage of justice will increase. "It makes sense for investigators to be savvy to this," says David Lazer of Harvard University, who studies the policy issues surrounding forensic DNA testing.

Until recently, bone marrow transplants involved destroying the patient's own bone marrow. In such cases their blood will contain the DNA profile of the donor alone. But some treatments in recent years, such as therapies to treat sickle cell disease, retain some of the patient's original bone marrow, so their blood can contain a mixed DNA profile. Mixed profiles can also occur when DNA is collected from swabs taken from the inside of the cheek, rather than blood samples. This practice is already standard in the UK and is increasingly being used by US police.

Cheek cells of a bone marrow recipient will contain mostly their own DNA, but can become contaminated with the donor's DNA over time. So police may have to check both blood and cheek samples to be sure of spotting a transplant recipient. Lazer believes it would cost too much to routinely check both types of sample to confirm whether a suspect has received a bone marrow transplant, and Helen Ng, spokeswoman for the US National Marrow Donor Program, based in Minneapolis, stresses that it would happen only very rarely. "I hope it doesn't prevent people giving patients what they need," she says.

But Chidambaram argues that potential marrow donors should be informed of the small risk of their DNA profile turning up in a crime database if the recipient later commits an offence.

Report here



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