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Forensics find donor DNA in semen of bone marrow transplant patient

16 December 2019
Appeared in BioNews 1028

A US bone marrow transplant recipient found out that the DNA of his German donor was not limited to his blood.

After a successful transplant to treat his acute myeloid leukaemia, Chris Long was encouraged by his colleagues at Washoe County Sheriff's Department in Nevada to see how the transplant affected his DNA. As expected, Long's blood composition changed as his own blood cells were replaced by ones carrying the donor's DNA, created by the donated marrow cells. But the donor DNA was also found in other, unexpected parts of Long's body. 

Four months after the transplant, mouth swabs detected both Long's DNA, and that of his donor. Furthermore, after four years the DNA in his semen had been entirely replaced by his donor's. Of the samples taken, only the DNA in Long's chest and head hair remained unchanged.

'This is not much of a surprise,' said Dr James Davies from the Medical Research Council Weatherall Institute of Molecular Medicine at the University of Oxford. 'We actually struggle to get pure samples of host DNA from these patients because immune cells infiltrate virtually all tissues.'

Long, like most other transplant recipients, had become a chimera: an organism with two or more distinct genomes. His colleagues were inspired to test his DNA to better understand how chimerism could affect interpretation of DNA test results in criminal investigations and other legal contexts. 

Tens of thousands of people each year receive bone marrow transplants for cancers and other blood diseases including leukaemia, lymphoma and sickle cell. There have already been cases in which chimerism has led to mistaken victim and perpetrator identities, including a 2004 Alaskan case where the DNA left at a crime scene matched to a convict who was in prison at the time. It emerged that he had received a bone marrow transplant from his brother, who was eventually arrested and convicted.

Long's case has raised concern about donor's DNA being found in semen, but Dr Davies explained that it would not be present in the sperm, and therefore could not be transmitted to the next generation: 'The finding in sperm superficially seems interesting until you realise that the man had a vasectomy. Thus what is being analysed as "sperm" doesn’t actually contain any sperm and it is contamination with donor lymphocytes that are being detected. It is very important to highlight that it is impossible for the patient to father a child with the donors DNA.'

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