Genetic and Environmental Links to Autism
The number of children born with autism is skyrocketing. Now new evidence links the role of genetics to autism. Mark Daly, director of Harvard’s Program in Medical and Population Genetics, has found a genetic mutation that significantly contributes to the disorder. And Irva Hertz-Picciotto, the principal investigator at the center for Childhood Autism Risks from Genetics and the Environment at UC Davis, surveys possible environmental factors. Daly and Hertz-Picciotto speak with host Bruce Gellerman.
GELLERMAN: From the Jennifer and Ted Stanley Studios in Somerville Massachusetts, it’s Living on Earth. I’m Bruce Gellerman. According to the latest figures from the Centers for Disease Control and Prevention, in the United States, one child in 88 has autism. The incidence has been rising steadily and rapidly - up 80% in just a decade - baffling researchers who have long believed that genetics play an important role in autism.
But they lacked hard evidence, until now. Three independent studies in the latest edition of Nature for the first time, link genetic mutations to autism. Professor Mark Daly is lead author of one of the studies. He’s Chief of Analytic and Translational Genetics at Massachusetts General Hospital.
DALY: The types of studies now are things that weren't even possible two to three years ago. So they’ve been made possible by remarkable advances in our ability to sequence the human genome. So in this particular case, we sequenced the genomes of kids with autism and their parents, and we were targeting a very specific type of mutation- that is, those that arise spontaneously, and we were able to detect in each of those kids with autism whether or not they had specific mutations.
GELLERMAN: A mutation, basically, a misspelling in the DNA.
DALY: Exactly, exactly. So this basically is all of your three billion letters of DNA are inherited from your parents - occasionally there is a spelling error. And these errors - if they occur in an important protein causing gene - could disable that copy of the gene.
GELLERMAN: So what did you find specifically?
DALY: So we found that, compared to folks in the general population, kids with autism had about twice as many very severe gene debilitating events. And these are still very rare events - the vast majority of individuals and the majority of kids with autism don’t have these specific mutations, but the fact that there were more of them in the kids with autism allowed us to pinpoint very confidently a small number of genes as genuine risk factors for autism.
GELLERMAN: Autism is considered a spectrum of disorders.
GELLERMAN: What percentage of autism do you think the genes and the mutations that you’ve found do you think are responsible for autism?
DALY: Probably about one percent from these particular genes. So we’ve identified in these studies, three or four genes that have highly confident statistical evidence behind them - these mutations, while they’re very severe - are found only in about one percent of the kids we’ve studied, and, you know, that leads us to extrapolate in a way that’s consistent with previous studies - that there are actually hundreds of genes that contribute to autism risk.
GELLERMAN: Can you test for this genetic mutation?
DALY: There’s not a clinical test that’s been developed yet. I think with some subsequent follow on studies, these tests can ultimately evolve into additional diagnostics.
GELLERMAN: You’re a geneticist…
GELLERMAN: Nature, nurture - when it comes to autism - what percentage do you think is genetic and what percentage is environmental?
DALY: It’s not an easy question to give a single answer to. I think it’s clear that genes by itself do not cause autism in the way that a single gene causes Huntington’s disease or cystic fibrosis. Now it’s hard at this point of our evolution of genetics, since we’ve only discovered a small fraction of the responsible genes, but that doesn't preclude the potential for very, very important environmental factors from also contributing to a large majority of cases. But ultimately both lines of those lines of research - the genetics will be discovering more and more risk factors, the epidemiology will get more confident observations of risk factors - and these two things will then begin to fit together in a much more sensible whole picture.
GELLERMAN: Professor Daly, thank you so much for coming in.
DALY: Hey, it's been a pleasure to be here, Bruce. Thank you.
GELLERMAN: Mark Daly is Chief of Analytic and Translational Genetics at Massachusetts General Hospital. Well, it’s taken decades and billions of dollars to find these first genetic links to autism, but the research only explains a small number of cases. Dr. Irva Hertz-Picciotto is investigating possible environmental causes. She’s principal investigator at the UC Davis Center for Childhood Autism Risks from Genetics and the Environment, or CHARGE. Dr. Hertz-Picciotto says research into autism and the environment needs more attention.
HERTZ-PICCIOTTO: You know, I think people are starting to wake up, that the genetics is not giving us the answers that were promised, and in fact, the money has been around 20-to-one as far as what’s been going to genetics versus the environment, so I think that the small amount of money that’s been invested into investigating environmental factors is starting to really pay off.
GELLERMAN: So you’re the principal investigator for the study called, CHARGE: the Child Autism Risks from Genetics and the Environment. Which proportion do you think is genetics, and which proportion do you think is environment?
HERTZ-PICCIOTTO: Well, let me put it this way- I think that it’s mostly both. I think that most cases of autism have some genetic susceptibility factors, but that those susceptibility factors are not enough to cause outright autism by themselves. That only in combination with environmental factors do you actually get autism.
GELLERMAN: Well, let's look at some of those environmental factors that you studied in this ten-year-long study that you’ve got - and it’s on-going.
HERTZ-PICCIOTTO: OK! Well, we have a few studies that I think are quite important - one of them was that women who reported that they took their prenatal vitamin supplements before they actually conceived - those women appeared to be at almost a 40 percent reduced risk of having a child who later developed autism.
GELLERMAN: What’s in the vitamins - do you think - that may be preventative here?
HERTZ-PICCIOTTO: Well, it may be the folic acid, which is…the amount that you get from diet is much less from those prenatal supplements. Not the regular multivitamins - these are the specifically prenatal vitamins that have 800 milligrams, and that dose may be what is needed at that time period in development, or at least for some women.
What we also saw in that study was that the risk was modified by either the mother’s or the child’s genes that pertains to the metabolism of folic acid. So if they had a high risk, or inefficient metabolism of the folic acid, then they were at substantially higher risk.
GELLERMAN: So, mother’s prenatal nutrition - what’s another environmental factor you studied?
HERTZ-PICCIOTTO: We looked at exposure to air pollution, and the way that we approached it was we looked at where the homes were - where the mom resided at the time that she was pregnant - and we linked that to the roadway system in California. Then we said where’s the closest freeway to that woman’s house - and if she lived close to a freeway - within three quarters of a mile - there was a higher risk that her child would develop autism.
GELLERMAN: Well, so far we’ve been talking about moms. What about dads? Do they play a role in this risk - that’s maybe not genetic?
HERTZ-PICCIOTTO: Well, there’s belief that dads could play a role, for sure. We looked at the occupations of actually both moms and dads and we did find that exposure to solvents may be associated with a higher risk for autism. So there are a lot of chemicals that potentially have neuro-toxicity - pesticides I will cite, because pesticides are designed to damage the nervous system in lower species.
You know, it might be rats and gophers, or it might be insects. But there is no systematic testing starting in the prenatal gestational period to look at neuro-development. And that’s the question - we don’t have a specific system to study those chemicals that get introduced.
GELLERMAN: Dr. Irva Hertz-Picciotto studies autism and the environment at UC Davis.
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