STEVE INSKEEP, HOST:

It's MORNING EDITION from NPR News. I'm Steve Inskeep.

DAVID GREENE, HOST:

And I'm David Greene, good morning.

Today in Your Health, we're going to explore how genes and the environment shape intelligence of adolescents.

INSKEEP: And that's one of two stories we have on how nature and nurture interact. First, pregnant women hear a lot about how things like diet and smoking can affect a developing fetus. Now scientists are beginning to understand why.

NPR's Jon Hamilton reports on the emerging field of epigenetics.

JON HAMILTON, BYLINE: In the early 1990s, Susan Murphy was a mom and a graduate student studying microbiology. Then her older son who was still a toddler died from a rare form of liver cancer.

SUSAN MURPHY: And I made the decision then that following my graduate studies, I'd shift focus and work toward something that would contribute more to cancer research, to honor the memory of my son.

HAMILTON: That led Murphy to Duke University and to epigenetics. It's a field that studies how individual genes in a cell are switched on and off. This epigenetic switching is part of what allows a tiny cluster of identical cells in the womb to grow into a fully formed baby. During pregnancy, epigenetic instructions cause some cells to become heart cells and others to become brain cells.

It's a delicate process that can be disrupted if the cells are exposed to certain chemicals, including some found in cigarette smoke. And Murphy says there appears to be a critical period during the first week or so after conception.

MURPHY: That's usually before a woman even knows she's pregnant. And so, we think that that might be a particularly vulnerable time where environmental influences can directly affect an epigenetic outcome.

HAMILTON: And epigenetic changes have been linked to autism, diabetes, and even the cancer that killed Murphy's son.

Much of what is known about the epigenetics of pregnancy comes from experiments with mice, specifically a group of genetically identical animals known as Agouti mice. When these mice are exposed to certain chemicals or put on a special diet during pregnancy, it switches on the Agouti gene in their offspring. This causes the pups to produce a lot of Agouti protein, which turns their fur a striking yellow. And Murphy says that's just the beginning.

MURPHY: So not only are the mice yellow, but the Agouti protein also affects the satiety response of the mice, so that they never feel full. They continue eating and become very obese and are predisposed to developing diabetes and cancer.

HAMILTON: To learn whether something like that can happen in people, Murphy has been looking at how a mother's environmental exposures and nutrition during pregnancy can cause epigenetic changes in babies. Murphy says her work suggests there's something interesting going on with folic acid, a vitamin many pregnant women take to ward off problems like spina bifida.

MURPHY: At moderate levels, the recommended levels, it's beneficial. At very high levels you actually lose that benefit. So we aren't saying that it's harmful at high levels but the beneficial aspects of folic acid seem to be negated by having too much of it around.

HAMILTON: The results of epigenetic changes don't necessarily appear at birth. Dani Fallin of Johns Hopkins University says that's because things that affect development early in life can show up decades later.

DANI FALLIN: If you think of development as a ball rolling down a creviced hill, there are many different paths that ball can take by the time it gets to the end. It could end up very much to the left of where it started, very much to the right or in the center. And epigenetic mechanisms may help shape that path.

HAMILTON: So what happens in the womb may cause epigenetic changes that contribute to things like mental illness or heart disease in an adult. Fallin is particularly interested in early developmental paths that can lead to autism. So she did a study looking at the epigenetic information in the brain cells of both kids with autism and typical kids.

FALLIN: At specific places, we see differences in the brains from the autistic children than we do from the control children. That's important because those particular genes may give us a clue about what's being turned on and off differently in autistic children.

HAMILTON: Fallin says that's a long way off. In the meantime, she says, epigenetic studies may help get the attention of pregnant women who would otherwise ignore recommendations about diet and behavior.

FALLIN: If you see there is a detectable biological change because of exposure to drinking or because of exposure to smoking that, as a pregnant mom, would convince me that, oh, it matters.

HAMILTON: Fallin, who has two children, hopes epigenetic evidence will eventually make it clear how much exposure during pregnancy is safe. That might finally settle the question of whether it's OK for pregnant woman to have a glass of wine with dinner.

Jon Hamilton, NPR News.

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