Women carry fetal DNA long after children’s birth

Pregnancy can be a messy business. Although it was once thought that only mom’s cells could transfer across the placenta to deliver needed supplies to the fetus, we now know that fetal cells make the journey as well. Those fetal cells can be harbored in a mother’s tissues where they become incorporated, lasting decades after the birth. This likely means that every mother’s body contains both her own cells and genetics and tiny, but measurable amounts of her children’s cells and genomes.

Named after the part goat, part dragon, part lion of Greek mythology called Chimera, the modern genetic phenomenon is known by the same term. And just as chimerism was once thought to be an impossibility, scientists are now finding its far more common than they originally thought. Most women, and not just moms, carry someone else’s DNA within them.

Side note here: Chimerism is almost exclusively studied in women because they have two X chromosomes. By looking for the genetic signature of the male Y chromosome in women, researchers can reliably say that DNA does not originate from this woman, thus identifying a chimera.

At least for some diseases, chimerism keeps women healthy and less likely to die, although the mechanisms are not well understood. In one Danish study, chimeric women had a 60 percent lower mortality rate than non-chimeric women, likely because of reduced risk of cancer. These women were first tested for chimerism at age 50 or older, long after their babies were born, which refutes the idea that fetal cells lingered for months or a few years then were cleared from the women’s system. Chimerism has also been associated with reduce risk of Alzheimer’s disease and autoimmune disease.

But chimerism also has some health strikes against it. In research presented at the American Society for Human Genetics meeting scientist showed that fetal cells may increase a mother’s risk of developing the autoimmune disease rheumatoid arthritis later in life. The effect was only found when fetal DNA contained certain genetic variants that increased risk for the arthritis.

And this effect is not only found after pregnancy. Twins often share DNA long after they’ve been delivered because of transfer during gestation. Younger children may also carry the signature of their older siblings due to residual DNA harbored in their mother’s uterus. There are even some reports of chimerism as a result from blood transfusions and organ transplants.

Chimerism must necessarily change how we think about who we are, Carl Zimmer wrote at the New York Times. Our modern conception of identity relies heavily on our DNA. But when you have the DNA of two people, how can forensics or organ transplantation typing necessarily determine the correct matches?

Meredith Knight is editor of the human genetics section for Genetic Literacy Project and a freelance science and health writer in Austin, Texas. Follow her @meremereknight.

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