The term chimera is usually associated with Greek mythology. It brings to mind a monster with the head of a goat and a lion. Instead of a tail, it has a protruding snake. There is a genetic term that was created from this mixed beast. In humans, a chimera is a person who has cells from two different sources. Since those cells are from different organisms, it results in two sets of DNA. But how could this possibly happen in humans? To explore chimerism in humans, we must first understand how it can happen in the first place.
What is Chimerism?
Chimerism in humans can occur when a mom is pregnant with twins. If one embryo dies, the other one could possibly absorb its deceased twin's cells. Normally, in humans, you have cells with only your DNA, which is a basic understanding of genetics. The body has numerous types of cells, but all the cells contain your exact type of DNA. But with a chimera, this basic understanding is obsolete. A chimera will have two sets of cells with two distinct types of DNA.
The tissues that will be chimeric vary in each circumstance. It will depend upon how the cells travel, which cells come through the placenta, and where they settle. One of the more common outcome types is blood chimerism. Blood cells that form in the bone marrow are sturdy and mobile. They will more than likely survive while traveling through the placenta in the exchange from one of the chimeric twins to the other.
Statistically, about 8% of fraternal twins could end up with blood chimerism. In the case of triplets, the chance for blood chimerism rises up to 21%. But can non-twins be a chimera? Absolutely. Vanishing twin syndrome can be the cause of chimerism in some of these cases. In 5% of pregnancies, multiple embryos occur which can lead to a twin birth, but not all the time. Of those, 25% of the time one may fail to survive a full 9 months. In this case, the embryo is sometimes taken back up into the mom's tissues. Or, it can combine with its twin. Either way, the surviving baby becomes a chimera. There is no statistical information on how many chimeras are born like this. But after examining rates of numerous embryos and vanishing twin syndrome, it is speculated that 1 in 80 pregnancies may result in a vanishing twin pregnancy.
As mentioned above, some people become chimeras before they are born. While still an embryo, they pick up the cells that are not theirs. If they absorb these cells they stay with them for life. Of course, these cells can come from the embryonic twin during the phenomenon of vanishing twin syndrome. It is caused by irregularities in the disappearing twin’s DNA that prevent it from fully developing. These irregularities in that baby's genes can't be prevented or controlled and many times are formed during conception.
Other times, chimerism can occur after a life-saving medical treatment. Think of an organ transplant for example. In this case, the organ from the healthy donor replaces the damaged or diseased tissue in someone that is the recipient. This transplanting of the new organ can make the recipient into a chimera. When placed into the recipient, the transplanted organ does not change and retains the donor's cells and their DNA. Now, the recipient will have two different types of cells. Most of the body will have its original set of cells with its original DNA. However, the new organ will have donor cells and DNA.
Also common are bone marrow transplants. The patient's diseased bone marrow is replaced with a healthy donor's marrow. The new bone marrow will produce new blood cells. The recipient will now have blood that is made from the marrow of the donor. This is being made by someone else's DNA. So, the recipient's blood will now contain new DNA. Even the blood type may be different now.
Chimerism usually will not display signs or symptoms. However, those who go non-diagnosed with the condition may have issues when having children. If a baby is born to a chimera twin, they might be distantly related genetically. Think like a niece or a nephew. We already know that chimerism defines the rule of cells having only one set of DNA. But chimerism can also break the rules of genetics and inheritance. Your kids will inherit half of your DNA. In most cases, these cells are present in the entire body. This is not true mostly for chimeras. Sperm or eggs from a chimera could be different from the cells in the rest of the body. So, the genes that you could pass to your kids could be all mixed up. The kids may be born with characteristics that do not seem similar to the parent. Think back to a bone marrow transplant. The chimera has DNA from its own genes for a certain type of blood, but if the chimera has received a bone marrow transplant, the blood marrow donor may have another blood type. Their kid may have both.
Many people with chimerism have no clue they have a second set of cells. Most can go their whole life without being diagnosed. There is no wide screening that is mandated, and many in the medical community still don’t know about it. Usually, there will not be a diagnosis unless someone fails some type of DNA test.
Is Chimerism Consequential?
If chimerism is not diagnosed, there can be an occurrence of false negative DNA tests in trying to determine paternity. There are numerous documented cases of fathers and mothers with chimerism, potentially losing all custodial rights. The chimerism diagnosis can help with a mental health identity crisis and those that need aid.
Survival chances of a co-twin in vanishing baby syndrome are great if the failure of the twin happens in the first trimester of pregnancy. The mom experiences no health problems, but may only visually have some mild vaginal bleeding as a sign that one of the embryos has been lost. So, the earlier the loss of the vanishing fetus during gestation, the more likely no one will notice, and the chances of survival of both mom and the other fetus are good.