Male DNA found in female brains

Children live on in their mothers’ brains for decades, and not just as memories. Scientists have found pockets of male DNA, presumably from boy fetuses, in the brain tissue of women who died in their 70s.

Not only is male DNA present in women’s brains, it’s common, researchers report online September 26 in PLOS ONE. J. Lee Nelson of the Fred Hutchinson Cancer Research Center in Seattle and her colleagues found snippets of a male-only gene in the brains of 18 of 26 women who died without neurological disease. The male DNA was spread throughout their brains.

The technique used in the study couldn’t distinguish if the DNA was from intact, functional brain cells, though in a separate test of brain tissue from a different woman, Nelson and colleagues did spot nuclei from male cells in the brain. Earlier studies in mice hinted that these foreign cells can integrate themselves into the brain and start functioning as nerve cells.

So far, cells from fetuses have turned up in women’s blood, livers, lungs, heart and other organs, so finding male DNA in the brain isn’t a complete shock, says geneticist Kirby Johnson of Tufts University in Medford, Mass., who wasn’t involved in the study. “From everything we knew, it’s not really that surprising.”What’s interesting is how the DNA could have gotten there. Male cells from a fetus could have broken through the blood-brain barrier — a wall that protects the fragile brain from pathogens in the blood. But that shouldn’t be possible, Johnson says.

If the male DNA did come from a fetus during pregnancy, then the genetic material stuck around in the brain for decades after that. The average age for these women at the time of their death was 70. “Maybe these are with us for a lifetime,” Nelson says.

Presumably, mothers can also carry a daughter’s genetic material in their brains; the presence of a Y chromosome simply makes it easier to spot male DNA.

Complete medical records, including pregnancy history, weren’t available for the women in the study, which means the researchers couldn’t rule out sources of cellular mingling other than male fetuses. The male DNA could have come from a male twin whose cells ended up moving into his sister’s body during pregnancy, for instance, or they may have come from an organ donation or blood transfusion, or even an older brother who had previously occupied the same uterus as the women.

What’s more, cells from several generations could mingle in a single person. Because cells also flow from mother to fetus, a pregnant woman possesses cells from both her mother and her child, and that child could inherit his grandmother’s cells.

Fetal cells could be beneficial, harmful or innocuous in a mother’s body. In a follow-up experiment, the researchers found that women with Alzheimer’s had less foreign DNA in their brains than women with healthy brains, hinting that these cells might offer protection from the disease. Those results are too preliminary to be conclusive, Nelson says. In tissues outside the brain, there is preliminary evidence that fetal cells may affect risk for cancer and autoimmune diseases.