Where will the DNA go?

The implications of Mr. Long’s case, which was presented at an international forensic science conference in September, have now captured the interest of DNA analysts far beyond Nevada.

The average doctor does not need to know where a donor’s DNA will present itself within a patient. That’s because this type of chimerism is not likely to be harmful. Nor should it change a person. “Their brain and their personality should remain the same, ” said Andrew Rezvani, the medical director of the inpatient Blood & Marrow Transplant Unit at Stanford University Medical Center.

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. “It doesn’t matter, ” he said.

But 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, including that of a fellow who is 10 years younger and lives thousands of miles away. And so Renee Romero, who ran the crime lab at the Washoe County Sheriff’s Office, saw an opportunity when her friend and colleague told her that his doctor had found a suitable match on a donor website and he would be undergoing a bone marrow transplant.

“We need to swab the heck out of you before you have this procedure to see how this DNA takes over your body, ” she recalled telling him.

Mr. 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. ”

Four years after the bone marrow transplant, Mr. Long’s semen contained 100 percent of his donor’s DNA. Credit... Tiffany Brown Anderson for The New York Times

Four years later, with Mr. Long in remission and back at work, Ms. Romero’s experiment persisted, aided by her crime lab colleagues. Within four months of the procedure, Mr. Long’s blood had been replaced by his donor’s blood.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 four years after the procedure, 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.

If another patient responded similarly to a transplant and that person went on to commit a crime, it could mislead investigators, said Brittney Chilton, a criminalist at the Sheriff’s Office forensic science division.

And it has misled them, Ms. Chilton learned once she began to research chimerism. In 2004, investigators in Alaska uploaded a DNA profile extracted from semen to a criminal DNA database. It matched a potential suspect. But there was a problem: The man had been in prison at the time of the assault. It turned out that he had received a bone marrow transplant. The donor, his brother, was eventually convicted.

Abirami Chidambaram, who presented the Alaska case in 2005, when she worked for the Alaska State Scientific Crime Detection

Laboratory in Anchorage, said she had heard about another disconcerting scenario since then. It involved policeinvestigators who were skeptical of a sexual assault victim’s account because she said there had been one attacker, though DNA analysis showed two. Eventually the police determined that the second profile had come from her bone marrow donor.

Similar scenarios could also create confusion around a victim’s identity — and in fact it has, said Yongbin Eom, a visiting research scholar at the University of North Texas Center for Human Identification. In 2008, he was trying to identify the victim of a traffic accident for the National Forensic Service in Seoul, South Korea. Blood showed that the individual was female. But the body appeared to be male, which was confirmed by DNA in a kidney, but not in the spleen or the lung, which contained male and female DNA. Eventually, he figured out that the victim had received a bone marrow transplant from his daughter.

The Washoe County Sheriff’s Office in Reno, Nev., ran an informal experiment that could have broader criminal implications. Credit... Tiffany Brown Anderson for The New York Times

The specifics of Mr. Long’s situation raise an inevitable question: What happens if he has a baby? Would he pass on the genes of his German donor or his own to future offspring? In this case, the answer will remain untested because Mr. Long had a vasectomy after his second child was born.

But what about everyone else? Three bone marrow transplant experts who were surveyed agreed that it was an intriguing question. They also agreed that passing on someone else’s genes as a result of a transplant like Mr. Long’s was impossible.

“There shouldn’t be any way for someone to father someone else’s child, ” said Dr. Rezvani, the Stanford medical director.

That’s not to say that other forms of chimerism haven’t created comparably confusing scenarios. Fraternal twins sometimes acquire each other’s DNA in the womb; in at least one case that led to unfounded fears of infidelity when a man’s child did not seem to be his. In another case, a mother nearly lost custody of her children after a DNA test.

But a donor’s blood cells should not be able to create new sperm cells, Dr. Rezvani said. Dr. Mehrdad Abedi, the doctor at the University of California, Davis, who treated Mr. Long, agreed: He believed it was Mr. Long’s vasectomy that explained how his semen came to contain his donor’s DNA. The forensic scientists involved say they plan to investigate further.

Everyone who has reviewed Mr. Long’s case agrees on one thing: He is a living, breathing case study of one, and it’s impossible to say how many other people respond to bone marrow transplants the same way he 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, Mr. Long said he hopes to meet his donor during an upcoming trip to Germany and to thank him in person for saving his life.

⇐ Предыдущая 1 2 34