Just 3 months after a bone marrow transplant, Chris Long of Reno, Nevada, learned that the DNA in his blood had changed. It had been replaced by the DNA of his donor, a German man he had never met and only exchanged a few messages with.
Thousands of people get bone marrow transplants every year for blood diseases including cancer, lymphoma, leukemia, and sickle cell anemia. Although it’s unlikely that any of them would end up as the perpetrator or victim of a crime, the idea that they could intrigued Long’s colleagues at the Washoe County Sheriff’s office, where he worked. Long was therefore used as a kind of human guinea pig and encouraged to test his blood.
Renee Romero, who ran the Sheriff’s crime lab, recalls Long telling her that his doctor had found a suitable match on a donor website and he would be undergoing a bone marrow transplant. She remembers telling him: “We need to swab the heck out of you before you have this procedure to see how this DNA takes over your body”. After all, it’s the goal of the procedure: weak blood is replaced by healthy blood, which means the DNA it contains too.
Long agreed. He welcomed an intriguing distraction from his diagnosis of acute myeloid leukemia and myelodysplastic syndromes, both of which impair the production of healthy blood cells.
At the time, he said, “I didn’t even know if I would live.”
So, what are the implications of having someone else’s DNA? Long’s case was presented at an international forensic science conference in September, and has captured the interest of DNA analysts far beyond Nevada.
He was now a chimera
The technical term for a rare person with two sets of DNA is a chimera. The word comes from a Greek mythological fire-breathing creature composed of lion, goat and serpent parts. Doctors and forensic scientists have long known that certain medical procedures turn people into chimeras, but where exactly a donor’s DNA shows up – beyond blood – has rarely been investigated.
DNA chimerism is not likely to be harmful, nor should it change a person. “Their brain and their personality should remain the same”, said Dr Andrew Rezvani, Medical Director of the Inpatient Blood and Marrow Transplant Unit at Stanford University Medical Centre. He added that patients also sometimes ask him what it means for a man to have a woman’s chromosomes in their bloodstream or vice versa. This doesn’t matter to the individual or to their general physician.
However, for a forensic scientist, it’s a different story. The assumption among criminal investigators as they gather DNA evidence from a crime scene is that each victim and each perpetrator leaves behind a single identifying code – not two.
Now, 4 years on, with Long in remission and back at work, Romero’s experiment persisted. Not only had Long’s blood been replaced by his donor’s blood. Swabs collected from his lip, cheek and tongue showed that these also contained his donor’s DNA, with the percentages rising and falling over the years. Of the samples collected, only his chest and head hair were unaffected. The most unexpected part was that the DNA in his semen had been entirely replaced by his donor’s.
“We were kind of shocked that Chris was no longer present at all”, said Darby Stienmetz, a criminalist at the Washoe County Sheriff’s office.
What are the implications of DNA chimeras?
All kinds of questions are now raised. Could another patient in similar circumstances go on to commit a crime and mislead investigators? What happens if Long has a baby? Would he pass on the genes of his German donor or his own to any future offspring?
“There shouldn’t be any way for someone to father someone else’s child”, said Rezvani, the Stanford Medical Director. A donor’s blood cells should not be able to create new sperm cells. One explanation is that Long’s vasectomy could be the reason why his semen came to contain his donor’s DNA. The forensic scientists involved said they plan to investigate further.
It’s impossible to say how many other people will respond to bone marrow transplants the same way Long did. It’s simply one of those curious possibilities that forensic analysts may want to consider when DNA results are not adding up. For his part, Long hopes to meet his donor during an upcoming trip to Germany and to thank him in person for saving his life.
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