Brain Food

Biology, evolution, and the underpinnings of neuroscience

How Your Child’s DNA Invades Your Brain

Fetal cells can invade the mother's brain—and may affect brain health

Can a mother give cancer to her unborn child? Yes, she can. In my new book, The Violinist’s Thumb—a collection of spooky and oddball stories buried in human DNA—I devote a chapter to a pregnant woman in Japan whose leukemia infected her fetus. (Cancer can be contagious!) The cancer cells succeeded in invading the fetus by changing their DNA. This change basically made them “invisible” to the baby’s immune system. Overall, the book covers some pretty clever ways that creatures manipulate DNA to invade the human body (e.g., microbes that steal DNA and probably manipulate our minds and emotions). But the cancer that invaded the fetus stands out as pretty insidious.

One side note in that chapter involved a little-known fact—that the human placenta is leaky. Most people, even most doctors, learn that the placenta is a nice, tight seal that prevents anything in the mother’s body from invading the fetus, and vice-versa. That’s mostly true. But the placenta doesn’t seal off the baby perfectly, and every so often something slips across. For a cancer cells to slip across is rare, thankfully. But in the last decade or so scientists have discovered that normal body cells slip across the placenta all the time, in both directions.

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That means that every woman who has given birth (or ever become pregnant, even if the pregnancy ended in abortion or miscarriage) probably bequeathed a few of her cells to her child. Similarly, the child probably donated a few cells to Mom. This mixing of cells is called microchimerism (after the chimera, a mythological beast made of parts of different animals). Scientists had detected evidence of microchimerism in mothers and children before, mostly in the blood and in bone marrow. But the cells can invade other organs, too, and a new study from scientists at the Fred Hutchinson Cancer Center (FHCC) proves that the cells can even invade the brain.

To find microchimeric cells, the FHCC team did DNA testing on the brains of five dozen women who’d died between the ages of 32 and 101. To make things simple, they searched for a gene (called dys14) found only on the Y chromosome. (Women obviously shouldn’t have any Y-chromosome DNA because it’s a male-only chromosome; so finding this gene would provide clear evidence of microchimerism. Scientists did not search for female microchimeric DNA, but it would presumably be there as well.) Overall, the scientists found male DNA in the brains of 63 percent of the women, including one woman who’d died at age 94, a hint that the male cells stuck around a loooong time.

Where did they find the male DNA in the brain? All over. Close to half of the women had male DNA in the parietal and temporal lobes, with lower rates in the occipital and frontal lobes. The scientists found similar rates (a third to a half) in the limbic system and thalamus. They found the highest rates in a part of the brainstem known as the medulla (almost 90 percent). These shouldn’t be taken as absolute numbers, because for some tissues the team had small sample sizes. (They sampled the occipital lobes of only five women, for instance.) But it is proof that male cells can invade just about anywhere in the brain.

As for the effect of these cells, scientists don’t know if they’re good, bad, or neutral. The FHCC scientists actually did the study to look for links to Alzheimer’s disease. Some scientists had previously suggested that invading microchimeric cells might contribute to the disease. The FHCC team found the opposite. Woman in the study had a 60 percent lower chance of having Alzheimer’s if they had male microchimeric cells.

As for the body generally, some scientists have suggested that microchimeric cells (male or female) could affect your odds of having an autoimmune disease, which makes some sense. You could imagine a scenario where fetal cells infiltrate the mother’s organs. Her body would attack them as foreign, but because they’re not completely foreign (they have half her DNA, after all), perhaps her immune cells would get confused and attack her own cells, too. But in truth, women with microchimeric cells suffer from at least one autoimmune disease, rheumatoid arthritis, at lower rates than women without. Similarly, microchimeric women have lower rates of breast cancer (although higher rates of colon cancer). Overall, we have no idea what these microchimeric cells do, if anything. But there are hints that, in a few cases, they might boost a woman’s health.

In The Violinist’s Thumb, I talk about the poignancy of cells leaking across the placenta into both the mother and the child. Microchimeric sharing means that, even if the mother loses a child, she’ll have a small memento of him or her secreted away inside her. Similarly, a bit of our mothers live on in all of us no matter how long ago Mom died. This new work indicates that these cells penetrate even our most sacred space, the brain. And they not only live there, but maybe, just maybe, they make us stronger, too.

Sam Kean is a science writer and the author of The Violinist's Thumb and The Disappearing Spoon.

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