Human Chimerism: A Rare Phenomenon

[Ivan Seeking]

Chimerism may occur naturally during pregnancy, when two non-identical twins (even of different sexes) combine in the womb at a very early stage of development, such as that of the blastocyst, to form a single organism. Such an organism is called a tetragametic chimera as it is formed from four gametes — two eggs and two sperm.

As the organism develops, the resulting chimera can come to possesses organs that have different sets of chromosomes. For example, the chimera may have a liver composed of cells with one set of chromosomes and have a kidney composed of cells with a second set of chromosomes. This has occurred in humans, though it is considered extremely rare, but since it can only be detected through DNA testing, which itself is currently rare, it may be more common than currently believed. As of 2003, there were about 30 human cases in the literature, according to New Scientist.

http://en.wikipedia.org/wiki/Chimera_(animal)

DHC [Discovery Health Channel] ran a special called "I am my own twin", which is about two women who are both chimeras. One of these nearly lost her two children through the courts because she couldn't prove that she was the mother - the DNA didn't match. These women each have two sets of DNA, with one being most dominant and found in most parts of the body, and the other being much more difficult to find, and in one case, only found in certain organs. Only after extensive testing was the other DNA located and identified; finally proving that these women are the mothers of their children as they claimed. It is a fascinating story that ends by questioning just how rare this really may be.

One of the most unusual aspects of their stories is that they are otherwise completely normal. There is no outward clue that either one has this condition. Photographs of two other chimeras were shown. One was a baby who, between the belly button and pubic region, was dark and mostly female on one side, and white and mostly male on the other, with a distinct line right down the middle. The other chimera shown actually had a black and white checker board pattern on the belly and chest!

Here is another link that popped up to a New Scientist article about this.
http://www.katewerk.com/chimera.html

My schedule shows that this will run again:
Sun 8/14 7:00 PM DHC
Sun 8/21 5:00 PM TLC

 

[honestrosewater]

This was in a CSI episode too. Here's some more interesting info about the color patterns you descibed: http://www.thetech.org/genetics/ask.php?id=75

 

[Moonbear]

I think chimerism has most often been identified when it occurs in the gonads of male-female twins. The reason it is identified in those cases is the obvious one, that you get some degree of hermaphroditism from the unusual hormone patterns produced by an ovo-testis.

Actually, one of my friends just had her cat spayed AND neutered (now she wants a bumper sticker that reads "Get your cat spayed and neutered"), and we suspect some chimerism may have occurred in that litter (though there can be other explanations in this case since the vet only found one testis and a partially developed uterus, and nobody was going to bother sending out the tissue for genetic testing on a pet cat that's never going to be bred).

At the conference I attended recently, there was a really interesting talk about the possibility of cells exchanging between mothers and fetuses (in both directions) as well, and that in some cases, these may underlie some autoimmune diseases in women. No direct causation has been demonstrated yet, but it was a new way of thinking about the disproportionate incidence of autoimmune disease in women vs. men. I had a number of issues with the way the studies were presented, including the lack of explanation of why these cells were crossing the placenta and able to proliferate in the mother (in other words, it wasn't clear if small numbers of cells can always cross and in some women, they are proliferating instead of being immediately destroyed by the immune system, or if there is some placental defect in these women that permits these cells to cross where they don't usually do so).

But, it was still an interesting observation and seems worth consideration of chimerism possibly underlying some autoimmune diseases as well. Is there any greater incidence of autoimmune diseases in twins or other multiples than singletons?

 

[quetzalcoatl9]

i seem to remember reading about ongoing research into how the fetus/placenta evade the female's immune system - maybe this would allow for the transfer of fetal cells without being immediately destroyed.

 

[Moonbear]

i seem to remember reading about ongoing research into how the fetus/placenta evade the female's immune system - maybe this would allow for the transfer of fetal cells without being immediately destroyed.

Well, classically, the idea is that the placenta acts as a barrier between the mother and fetus to prevent mixing of the cells between the mother and fetus and to keep the mother's immune system from attacking the fetal cells. But, if the fetal cells are escaping the placenta and not only entering maternal circulation, but surviving and developing within/as other tissues/organs, that's pretty remarkable. It may be that these women already have a defective immune system that this occurs though. The person presenting the study was reporting the presence of the children's cells in the mother and vice versa long after the children were adults (these were longitudinal studies), not just immediately after birth when it might not be as surprising. Though, I also don't think the study ruled out that the cells could be transferred AT birth similar to the way some blood cells are transferred that lead to an Rh- mother mounting a dangerous immune response to Rh+ factors if she has two Rh+ children.

 

[quetzalcoatl9]

Well, classically, the idea is that the placenta acts as a barrier between the mother and fetus to prevent mixing of the cells between the mother and fetus and to keep the mother's immune system from attacking the fetal cells.

it's not quite that simple since the placenta is derived from the fetal cells.

what has been proposed is that endogenous retroviruses from the fetal placenta play a role in creating immunotolerance.

here is what i was referring to:

http://darwin.bio.uci.edu/~faculty/villarreal/new1/erv-placental.html