When Taylor Muhl, a California-based singer, asked a doctor to investigate the birthmark that demarcated the left side of her torso, she wondered what role the ruddy patch might play — if any — in a series of seemingly disparate health conditions she had experienced most of her life.
“Everything on the left side of my body is slightly larger than the right side,” Muhl says. “I have a double tooth in the left side of my mouth and many sensitivities and allergies to foods, medications, supplements, jewelry and insect bites.”
While Muhl says she hoped for answers, what she discovered was shocking. Her "birthmark" wasn't a birthmark at all. She actually carried the genetic code of her twin, a sister Muhl had absorbed while still in the womb.
What Is a Chimera?
Muhl is a chimera (pronounced "ky-mer-a"), someone who carries two distinct sets of DNA, each with the genetic code to make a completely separate person. The difference in skin pigmentation on Muhl's abdomen, the size discrepancies between her right and left sides — and a host of autoimmune symptoms — were caused by two warring sets of DNA.
Muhl's condition, tetragametic chimerism, is the most rare of all types of chimerism. It occurs when two eggs (ova) are each fertilized by a separate sperm (spermatozoa). Then, the fertilized eggs, called zygotes, fuse to create a single organism with two genetically distinct types of DNA. As the embryo's cells multiply and grow, so do the two different types of genetic material. The result is a baby who is born a chimera.
Often, babies born with chimerism have patchy skin or eye pigmentation, and sometimes they carry two distinct types of red blood cells. Occasionally, chimeras are born with ambiguous genitalia, or possessing attributes of both male and female sex organs, though this is not the case with Muhl. These symptoms can occur not only in humans, but in other types of mammals, including mice.
A Surprisingly Common Condition
Chimerism was once believed to be extremely rare, but such conditions are "commoner than we realized," Linda Randolph, M.D., told The New York Times in 2013. Randolph is a pediatrician at Children's Hospital in Los Angeles and author of a chimerism review published in The American Journal of Medical Genetics.
In addition to genetic chimerism, the condition is commonly caused when another genetic code is introduced to the body by organ or tissue transplants, as well as blood transfusions. For example, when someone undergoes a bone marrow transplant, they will carry blood cells that are identical to their donor for the rest of their lives, in addition to their own genetically distinct blood cells. A blood transfusion, however, only produces a temporary chimera condition. The donor blood cells will live about 115 days in the host's body, and will eventually be completely replaced by red blood cells carrying their original genetic code. These types of chimerism are commonly known as microchimerism, and one of the most frequent causes is pregnancy.
Trace fetal stem cells often remain in a mother, long after she has given birth. Some of these stem cells have even been found to reside in women's brains. Likewise, a pregnant woman's cells cross the placenta and become part of their child's liver, heart, thymus gland and bloodstream. Many people may not ever suspect microchimerism is part-and-parcel of their existence, but it's probably more frequent than we realize. In fact, chimerism — both tetragemetic and micro — may someday challenge how medicine, forensics and the legal system contend with DNA.
Future Implications of Chimerism
DNA testing, which is done by collecting small amounts of human saliva, semen, hair, bone, blood or skin tissue, relies on the notion that every cell in a single body contains identical DNA — and that each person has a different DNA. Chimerism challenges the notion of a one-DNA/one-person system.
In one complex assault case, for example, evidence at the scene matched a DNA blood sample from a criminal profiled in a law enforcement database. However, this person was in jail when the assault occurred. To make matters more confusing, the DNA blood sample also matched the DNA profile of another person who also could have been implicated in the crime.
After investigating, it was determined the two men were brothers. The man in jail had previously received a bone marrow transplant from his brother. This meant the jailed man was a chimera. His blood DNA profile matched that of his brother's blood DNA, while his saliva DNA profile was distinctly his own. In the end, investigators used a blood sample and cheek swab to make a DNA match from the crime scene, and charged the right brother with the crime.