Today’s question came in response to my last blog post. In it, I explained that when scientists talk about the “environmental factors” that increase the risk of a disorder, they’re referring to pretty much any influence beyond genetics.
In the case of autism, the clearest evidence of environmental influence seems to surround very early events such as conception, pregnancy and birth. Those with the strongest link include parental age at time of conception (both mom and dad), maternal nutrition or illness during pregnancy, and certain birth complications.
The commenter’s question is a great one that scientists are actively exploring. The short answer is that inherited genes (DNA) and environmental factors seem to interact to influence whether an infant goes on to develop autism. So if the commenter’s twins are fraternal (meaning they share about half their DNA), the difference in their genetic makeup might explain why only one developed autism.
But what if the boys are identical twins–meaning they share exactly the same DNA? In this case, something beyond genes likely accounts for the different outcomes. Comparing the rates of autism among identical and fraternal twins provides clues.
In July, researchers used our Autism Genetic Resource Exchange (AGRE) to complete the largest autism twin study to date. They found a 70 percent overlap in autism among identical twins and a 35 percent overlap among fraternal twins. That overlap between fraternal twins is much higher than the estimated 19 percent overlap between different-age siblings.
These numbers tell us that it’s not always genes alone that determine whether a child develops autism. If it were, two identical twins would always share the same outcome, and the rate of a shared autism among fraternal twins would look more like that for different-age siblings. So we conclude that shared environmental influences are also at play.
Although twins share very similar pregnancy and birth environments, those environments aren’t exactly the same. For example, twins can have different positions in the womb or different placentas, and this can affect such environmental influences as blood and oxygen flow. Indeed, twins often have different birth weights, a known risk factor for autism.
It’s important to remember that “environmental” influences such as these don’t cause autism by themselves. Rather, if a child has a genetic predisposition for developing autism, these influences may further increase the risk.
Autism Speaks continues to fund and otherwise support research on both genetic and nongenetic risk factors for autism. EARLI is a network of researchers who follow mothers of children with autism beginning at the start of another pregnancy. IBIS is a study of early brain development in the younger siblings of children with autism. These studies depend on the participation and support of the autism community. Please visit our Participate in Research page to learn more.
Importantly, these studies provide insights into the underlying biology of different types of autism. This in turn becomes a basis for developing ways to treat and possibly prevent autism. As always, the goal of the research we support is to improve the lives of all on the autism spectrum.
And thanks for the question. Please keep them coming.
This short film is by Brad Dotson, a student at the University of the Arts in Philadelphia who also has Asperger’s Syndrome. Brad made a very personal film about his twin sister, Jaclyn, who has autism.
Vodpod videos no longer available.
“In Their Own Words” is a series within the Autism Speaks blog which shares the voices of people who have autism, as well as their loved ones. If you have a story you wish to share about your personal experience with autism, please send it to firstname.lastname@example.org. Autism Speaks reserves the right to edit contributions for space, style and content. Because of the volume of submissions, not all can be published on the site.
As reported last week, a large twin study supported by Autism Speaks compared the frequency with which identical and fraternal twins both share a diagnosis of autism. This approach enabled the investigators to use statistical techniques to calculate the degree to which environmental factors shared by twins contribute to their risk of developing autism. Such factors include conditions in the womb and during birth.
The results of the California Autism Twin study were game-changing because they revealed a much larger environmental influence than had previously been estimated—accounting for about 58% of the risk of developing autism. By contrast, much smaller twin studies had previously suggested that genes largely accounted for a child’s risk of autism.
The results underscore the need to investigate the role that non-inherited risk factors play in the development of autism spectrum disorders (ASD). So what’s being done to help speed this research? And what role are Autism Speaks and its donor dollars playing in this effort?
Autism Speaks has funded over $21 million in the study of environmental risk factors, an initiative we call (obviously enough) the Environmental Factors of Autism Initiative. Already, we have a large body of evidence suggesting that it is not any one environmental factor, but many different factors working together, that elevate the risk and severity of autism in individuals with a genetic predisposition for this condition. In other words, autism is seldom caused by any one thing and neither is it an “all or nothing” condition. Furthermore, different combinations of genetic and environmental risk factors contribute to individual cases of autism.
Within the Environmental Factors of Autism Initiative are ongoing studies focusing on environmental exposures that occur before and during pregnancy and throughout the first year of life—crucial periods for human brain development. These studies look at such possible risk factors as maternal and paternal age, socioeconomic status, season of birth, exposure to chemicals or toxic agents, nutrition and exposure to various pharmaceutical drugs during pregnancy, the difficulty of labor and delivery, and various other forms of prenatal stress. The researchers we support are also investigating the mechanisms by which genes and the environment may interact (so-called epigenetics) and the role of the immune system. We are also supporting large scale epidemiological studies that focus on pregnancy and the first year of life. These include the EARLI study and the IBIS study.
Are you interested in learning more about the studies Autism Speaks is funding with donor dollars? We are proud to debut the new Grant Search function on our website. Please use it to explore past and present research studies by topic or location. And if you or your family is affected by autism, please consider participating in one of our clinical studies.
This guest post is by Nancy Miltenberger, Chair of the, 2011 Lehigh Valley Walk.
As any parent or grandparent knows, hearing that a child you love has autism is devastating. I have heard it twice. My twin grandsons, James and Thomas are both on the spectrum. A few years ago, James was also diagnosed with leukemia. Fortunately, when the oncologists told us about his disease, they also handed us a road-map for his cure. Three years and two months later (and many spinal taps, rounds of chemo, transfusions, and more) I am thrilled to report that James is cancer-free. If James had been diagnosed with the same type of leukemia 60 years ago, his story may not have had such a happy ending. In 1950, the cure rate for acute lymphoblastic leukemia was about 4% and in 2010 it is about 94%. What a difference!
As we all know, there is no cure for autism… yet. But the model for leukemia gives me hope. I am so thankful for James’s wonderful doctors and all the funding and research that went into pediatric cancers over the last few decades. Now, we need to turn our attention to finding a cure for autism. It affects far more people than pediatric cancer, diabetes and AIDS combined. I want to do my part to see a cure for my grandchildren and others like them. That is why I Walk and why I am the chair of the 2011 Lehigh Valley Walk Now for Autism Speaks. I recently heard Dr. Geri Dawson, the Chief Science Officer of Autism Speaks, and I now have some hope for finding help for my grandsons and our family. I feel that by supporting our local Walk I am helping to fund the research that is needed to help pull my grandsons and countless others like them from the isolation that is autism, and help them to lead normal lives.