A transplant patient developed the DNA of his donor, it has emerged.
Chris Long, from Reno in Nevada, had a blood test three months after receiving a bone marrow transplant.
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Results revealed his DNA had been “replaced” by that of his donor, a German man 10 years his junior.
Four years later, the donor’s genetic material was found in Mr Long’s cheeks, lips and semen. While levels fluctuated over time, only his chest and head hair were unaffected.
“I thought it was pretty incredible that I can disappear and someone else can appear,” he told The Independent.
Mr Long reportedly sought a transplant over a “donor website” after being diagnosed with acute myeloid leukaemia - blood cancer of certain immune cells - and myelodysplastic syndromes - tumours that stop bone marrow producing healthy cells.
Originally reported at an international forensic science conference in September, the transplant turned Mr Long into a “chimera”. In ancient Greece, this referred to a mythological creature with the head of a lion and body of a goat.
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In medical speak, a chimera is any living being made up of cells from more than one organism, according to Harvard University.
Blood is produced in the bone marrow. When someone gets a transplant, they therefore carry DNA in their blood that is different from the rest of their cells.
Chimeras could one day be used to combat type 1 diabetes, which comes about when the body mistakenly “attacks” pancreatic cells that produce insulin.
Scientists from the University of Tokyo found the pancreas of a mouse functioned perfectly well in a rat. While this may be a long way from proving effectiveness in humans, it suggests chimera tissue could treat disease.
Although promising, chimeras are controversial, with many likening the notion to “Frankenstein-like experiments”. Some also worry how criminal investigations would play out if two people shared DNA.
This was what led Mr Long, a sherif who works alongside Washoe County’s crime laboratory, to have his blood tested post-transplant.
The issue came to light after DNA left at the scene of a crime in Alaska in 2004 matched that of a convict. In prison at the time, the offender had received a bone marrow transplant from his brother, who was later arrested.
Confusion also arose when a man was involved in a traffic accident in Seoul. When trying to identify him, experts found his blood was “female”, kidneys “male”, and spleen and liver a mixture of the two. It was later revealed the patient received a bone marrow transplant from his daughter.
Despite the caveats, transplants that lead to chimeras are not thought to be dangerous.
“Their brain and their personality should remain the same,” Dr Andrew Rezvani, medical director of the inpatient blood and marrow transplant unit at Stanford University Medical Centre, said.
Nonetheless, the controversy means the US National Institutes of Health reportedly only allowed federal money to fund human chimera research in late 2016.
Restrictions also apply to how long a chimera involving human cells can be allowed to grow.
Chimeras can also occur naturally, for instance if a twin dies in the womb and gets “absorbed” by their sibling, Scientific American reported.
“Microchimerism” may also be a common phenomenon in pregnant women if a foetus’ cells migrate to their mother’s tissue.