CHIMERISM – A THREAT TO THE INFALLIBILITY OF DNA TESTING?
I don’t know about any of you, but I am a CSI fiend! I love CSI New York and Las Vegas. I watch CSI NY because I love the characters and the storylines and I love CSI Las Vegas because of the emphasis on the science.
One story in particular really blew my mind. CSI LV a couple of years ago featured the story of a woman who had been brutally attacked and made a positive identification of the man who did it. When the team arrested the suspect and took his DNA, however, it didn’t match the DNA they found on the woman. She was adamant that the man she had identified WAS her attacker, but the crime lab told her that the DNA evidence said otherwise. It turned out that the man was indeed the one that attacked her. But why didn’t the DNA confirm this?
The man in question was a chimera. A chimera is a man or woman who has two completely different DNA strands in different parts of their body; you are supposed to have only one DNA code throughout your system. In the case on CSI the man had a twin brother who had died in his mother’s womb, but the DNA from the dead twin made its way into the living twin’s body. This man knew that he was a chimera and tried to use this to commit the perfect crime; i.e. he could leave all the trace evidence he wanted because the DNA discovered would be that of his dead twin, not his.
On the Discovery Channel in February, they aired a documentary about chimeras which was absolutely fascinating.
The most interesting case was that of a woman who needed a kidney transplant. The doctors tested her immediate family members to see if any of them could be a donor; the doctors told the woman their tests showed her sons were not her biological children. The woman was understandably baffled by this revelation. After ruling out the possibility that all three of her children had been switched at birth, a team of doctors took up the case to find out what was going on. Test after test confirmed that she was not the mother of her children. In fact, DNA tests proved that her husband and his brother were the biological parents of her children!!!!
After two years of testing, the doctors discovered the woman was a chimera. They took DNA from her ovaries, hair, and some of her glands and they finally found the DNA strand that matched that of her children.
There are 40 recorded cases of chimerism world wide. In those cases, there are signs, both subtle and stark, that they have the condition. In one case the person’s skin had very distinct light and dark chequered markings, in another case the body was divided down the middle by a line; one side was light coloured the other side was dark. There are other common indicators such as eyes of two different colours, or multicoloured hair. But in the case of the women on the documentary, they had no such symptoms. NOTHING marked them out as different in any way at all.
A person can be a chimera and have absolutely no clue about it. The woman in this case concurred that had she not needed a transplant, she would have gone to her grave never knowing she was a chimera. This was a point the doctors picked up – simply because there are 40 known cases of chimerism it doesn’t translate that only 40 chimeras exist. Because so little is known about this condition it is impossible to give any kind of indication as to how many other chimeras there may be. Chimerism is so rare that it is not known how to treat it or how to prevent it from happening.
The viability of using DNA as a 100% proof positive or negative catchall was called into question. In the cases on the documentary, DNA evidence was completely wrong. The fallibility of DNA testing was seen for the first time. If a person knew they had the condition they could, as the character in CSI Las Vegas did, use this knowledge in a wicked way thereby making a mockery of a once failsafe system. If a father wanted proof that the children his partner had were his, and he was a chimera and didn’t know it, DNA would say he wasn’t the father, implying that the woman had been unfaithful!! What a nightmare!!
Interesting isn’t it?
I don’t know about any of you, but I am a CSI fiend! I love CSI New York and Las Vegas. I watch CSI NY because I love the characters and the storylines and I love CSI Las Vegas because of the emphasis on the science.
One story in particular really blew my mind. CSI LV a couple of years ago featured the story of a woman who had been brutally attacked and made a positive identification of the man who did it. When the team arrested the suspect and took his DNA, however, it didn’t match the DNA they found on the woman. She was adamant that the man she had identified WAS her attacker, but the crime lab told her that the DNA evidence said otherwise. It turned out that the man was indeed the one that attacked her. But why didn’t the DNA confirm this?
The man in question was a chimera. A chimera is a man or woman who has two completely different DNA strands in different parts of their body; you are supposed to have only one DNA code throughout your system. In the case on CSI the man had a twin brother who had died in his mother’s womb, but the DNA from the dead twin made its way into the living twin’s body. This man knew that he was a chimera and tried to use this to commit the perfect crime; i.e. he could leave all the trace evidence he wanted because the DNA discovered would be that of his dead twin, not his.
On the Discovery Channel in February, they aired a documentary about chimeras which was absolutely fascinating.
The most interesting case was that of a woman who needed a kidney transplant. The doctors tested her immediate family members to see if any of them could be a donor; the doctors told the woman their tests showed her sons were not her biological children. The woman was understandably baffled by this revelation. After ruling out the possibility that all three of her children had been switched at birth, a team of doctors took up the case to find out what was going on. Test after test confirmed that she was not the mother of her children. In fact, DNA tests proved that her husband and his brother were the biological parents of her children!!!!
After two years of testing, the doctors discovered the woman was a chimera. They took DNA from her ovaries, hair, and some of her glands and they finally found the DNA strand that matched that of her children.
There are 40 recorded cases of chimerism world wide. In those cases, there are signs, both subtle and stark, that they have the condition. In one case the person’s skin had very distinct light and dark chequered markings, in another case the body was divided down the middle by a line; one side was light coloured the other side was dark. There are other common indicators such as eyes of two different colours, or multicoloured hair. But in the case of the women on the documentary, they had no such symptoms. NOTHING marked them out as different in any way at all.
A person can be a chimera and have absolutely no clue about it. The woman in this case concurred that had she not needed a transplant, she would have gone to her grave never knowing she was a chimera. This was a point the doctors picked up – simply because there are 40 known cases of chimerism it doesn’t translate that only 40 chimeras exist. Because so little is known about this condition it is impossible to give any kind of indication as to how many other chimeras there may be. Chimerism is so rare that it is not known how to treat it or how to prevent it from happening.
The viability of using DNA as a 100% proof positive or negative catchall was called into question. In the cases on the documentary, DNA evidence was completely wrong. The fallibility of DNA testing was seen for the first time. If a person knew they had the condition they could, as the character in CSI Las Vegas did, use this knowledge in a wicked way thereby making a mockery of a once failsafe system. If a father wanted proof that the children his partner had were his, and he was a chimera and didn’t know it, DNA would say he wasn’t the father, implying that the woman had been unfaithful!! What a nightmare!!
Interesting isn’t it?
2 comments:
Amazing stuff! I heard a podcast on that recently and could only remember the first name of the woman involved - so by googling I found this note
http://abcnews.go.com/Primetime/story?id=2315693
Imagine being your own twin. I think I would be a little sad missing knowing another who might have been a real twin.
the mind wobbles
Thanks for stopping by my blog - it had been a while since I checked that particular one!
This is such an interesting post - I had heard a little bit about this - I can't imagine how frustrating it would have been to be told that a person wasn't the mother of their children! Hopefully this is a rare occurrence as DNA testing has been a tremendous advance in forensics.
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