Human chimeras were once thought to be so rare as to be just a curiosity. But there's a little bit of someone else in all of us, says Claire Ainsworth, and sometimes much more...
EXPLAIN this. You are a doctor and one of your patients, a 52-year- old woman, comes to see you, very upset. Tests have revealed something unbelievable about two of her three grown-up sons. Although
she conceived them naturally with her husband, who is definitely their father, the tests say she isn't their biological mother. Somehow she has given birth to somebody else's children.
This isn't a trick question - it's a genuine case that Margot Kruskall, a doctor at the Beth Israel Deaconess Medical Center in Boston, Massachusetts, was faced with five years ago. The patient, who we will call Jane, needed a kidney transplant, and so her family underwent blood tests to see if any of them would make a suitable donor. When the results came back, Jane was hoping for good news.
Instead she received a hammer blow. The letter told her outright that two of her three sons could not be hers. What was going on?
It took Kruskall and her team two years to crack the riddle. In the end they discovered that Jane is a chimera, a mixture of two individuals - non-identical twin sisters - who fused in the womb and grew into a single body. Some parts of her are derived from one twin, others from the other. It seems bizarre that this can happen at all, but Jane's is not an isolated case. Around 30 similar instances of chimerism have been reported, and there are probably many more out there who will never discover their unusual origins.
While cases like Jane's are the extreme, researchers now think that there's a little bit of chimera in all of us, and what was once seen as a biological oddity may serve a vital function. We may owe our lives to being chimeras.
At first, Jane's case had Kruskall completely puzzled. The original data came from the tests done to "tissue-type" her and her children. Such tests are based on a set of genes called the HLA complex, which encode many different immune proteins, including cell surface proteins that immune cells use to distinguish the body's own tissues from foreign material. There are hundreds of different versions, or alleles, of each HLA gene, and because of this, each person's combination of alleles is almost unique. But because the genes are clustered close together on chromosome 6, they tend to be inherited together in a block known as a haplotype. Everyone inherits two HLA haplotypes, one from each parent.
Transplant doctors know that the closer the match between two people's HLA haplotypes, the lower the risk of a transplant between them being rejected. If you need a transplant, the obvious place to look for people with a similar haplotype is your close family. Your siblings, for example, have a 1-in-4 chance of matching yours exactly, while your children will have at least 50 per cent of your HLA genes.
Confronted with Jane's bizarre test results, Kruskall's team's first line of enquiry was to take another look at Jane's HLA genes and those of her immediate family. They identified Jane's haplotypes and dubbed them 1 and 3. They tested Jane's husband too - he had types 5 and 6. And when they looked at her sons they confirmed that the original tissue-typing was correct. While all three shared a haplotype with their father, only one shared one of Jane's. The other two sons had a haplotype of unknown origin, labelled type 2.
The obvious interpretation was that Jane was not the biological mother of two of her sons, yet they were all conceived naturally, so how could this be? One possibility was that both boys were accidentally swapped at birth, but the chance of this happening twice to the same family is very small. Add in the fact that both sons share a haplotype with their father and it becomes a near impossibility.
Stumped, Kruskall sent her data out to colleagues, asking them if they could make...
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