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Unraveling DNA: What Does this Mean for Autism?

Marina Sarris
Interactive Autism Network at Kennedy Krieger Institute
Date Last Revised: 
March 5, 2018
Date Published: 
February 12, 2018

The language of genetics has become part of popular culture. "Musical ability is in his DNA," we say. Television commercials invite us to take DNA tests that will tell us our ethnic heritage, or, as they put it, what we’re "made of." What can DNA tell us about what autism is made of? What are the possibilities and pitfalls of genetics in explaining autism and finding treatments? And how do genes and our environment interact to influence autism? We hope to answer those questions in this and future articles, but first it helps to break down what these concepts really mean.

The science of genetics has advanced exponentially since many of us – particularly those born before 1990 – took high school biology. As an example of this shift, consider one cultural touchstone, the detective story. In recent decades, DNA has become the star of some crime dramas, such as CSI: Crime Scene Investigation, taking the place once occupied by the likes of Sherlock Holmes or Lt. Columbo. We know that DNA is important for catching criminals and finding long-lost relatives. But what exactly is this tiny molecule that contains so much information?

Deciphering DNA

In short, DNA is deoxyribonucleic acid, the molecule that contains the entire instruction manual for making you who you are. Your DNA can be found in every cell in your body, and, unless you have an identical twin, it is essentially unique to you.1

DNA is made up for four chemicals, represented by the letters A, C, G and T. They pair up in a particular order along two strands of a twisted ladder. (This is the double helix you studied in biology class.) The letter A always links with T, and C with G. Blocks of these chemical pairs, from thousands to 2 million pairs long, form a single gene.2

"Our genes are like an instruction manual, or a book, that our body reads to determine things like how we look, how we grow, and how we function," explained Alyssa Blesson, a certified genetic counselor at Kennedy Krieger Institute in Baltimore.

graphic illustration of human chromosomesThink of a gene as a package of DNA with a specific task, such as making a protein your body needs to do or to create something. Genes contain the instructions for making hemoglobin, the protein in red blood cells that carries oxygen from your lungs to other parts of your body. Other genes have the task of making your hair dark brown, your eyes hazel, or your blood type O.

Your genes – more than 20,000 of them – lie along 46 chromosomes. You get half of your chromosomes – 23 to be exact – from your biological father and half from your biological mother. The sets you get from each parent are matched (you have two copies each of chromosome 1 through 22), with the exception of the sex chromosomes. These are known by the letters X and Y. Girls inherit one X chromosome from each parent; boys inherit an X from mother, and a Y from dad.

For many years, most of what we knew about genetic conditions came from analyzing chromosomes or certain genes. Some developmental disorders are caused by having a missing or extra chromosome, which usually occurs at conception. Down Syndrome is caused by having three copies of chromosome 21, instead of the usual two. Fragile X Syndrome is caused by an alteration to a single gene on the X chromosome. Autism affects an unusually high number of people with Down Syndrome (5 to 10 percent3), and Fragile X (almost a third4). As you can see, different genetic conditions have been linked to autism.

Our knowledge of autism genetics – and genetics in general – began exploding in the 21st Century. That is about the time scientists from around the world finished the decade-long task of mapping all the genes that people carry, which is called the human genome.

Laboratories have developed tests that allow them to detect changes, even very tiny ones, in a person's DNA or chromosomes. One such genetic test – and there are many – is the chromosome microarray test. For this test, a lab not only looks for a missing or extra chromosome, but also for a missing or extra piece of chromosome. Depending on the size of the piece, a person may have one or more extra or missing genes.

These changes on a chromosome are called copy number variations or variants, CNVs, for short. A doctor may recommend other genetic tests for someone with autism, as well.5 Changes to a single gene or to a piece of a chromosome are sometimes called mutations.

Does Genetic Mean Inherited?

graphic illustration of DNA double helixA genetic change can be inherited directly from a parent, or it can occur for the first time in a child, Ms. Blesson said. Genetic changes that are not inherited from mom or dad are called "de novo," which is a Latin term meaning "new." It is hard to say exactly why a particular new mutation occurs. These de novo mutations may explain why someone has autism, or another medical condition, when no one else in their family does.

Mutations are not necessarily good or bad, and they can be passed to future generations. Some mutations do not affect health, such as those that gave rise to red hair. Others may provide an advantage, such as a mutation that protects people from Type 2 diabetes.6

Since the 1970s, researchers have believed that autism risk is strongly influenced by genetic factors, based on studies of twins.7 More than three-quarters of the time, if one identical twin boy has autism, his twin does too.8. Interestingly, the number is not 100 percent, which suggests that other factors influence the development of autism in a child. (More on that later.)

The Hunt for Genetic Changes

CNVs, extra or missing pieces of a chromosome, are one focus of autism research. If enough people with autism have the same CNV, researchers may suspect a link to autism. By examining the genes in that location, and what they are supposed to do, scientists hope to discover what causes autism – and possibly, one day, what might treat its symptoms.

Researchers have examined the DNA from the Simons Simplex Collection, which includes 2,600 families who each have one child with autism. The study is so-named because it is funded by the Simons Foundation Autism Research Initiative, or SFARI. Scientists found that new genetic changes appeared to contribute to autism in one out of every 10 children in that Simons project.9

Researchers identified missing or extra genetic material in particular areas on chromosomes 1, 3, 7, 15, 16 and 22. They also identified 65 genes that they believe contribute to the risk of developing autism. Interestingly, these genetic changes were found in children whose IQ ranged from low to high. That suggests that those particular changes affect autism, not intelligence.9

And while boys and girls had mutations in the same areas, the girls had more of them. Researchers say that may be because it takes more mutations to produce autism in girls, who seem otherwise resistant to developing the condition. That is a theory, called the "female protective effect," that tries to explain why only about 20 percent of people with autism are girls or women.9

In the SSC study, researchers believe that the some of the autism-related genes play a role in how nerve cells communicate. Other identified genes play a role in an internal switching system that turns certain genes on or off.

Of course, the majority of the children in the SSC project did not have genetic changes in these high-risk locations for autism, at least as they were defined when that study was published in 2015.

As genetic research continues, the number of suspect CNVs and genes is growing. SPARK for Autism, another research project of SFARI, is looking for CNVs and genes associated with autism across all chromosomes. SPARK is using an "initial list of 11 CNVs and 78 single genes," according to a 2018 article in the scientific journal Neuron.10,11 That list is continually growing, SPARK has explained.

Families that participate in research projects such as SPARK may find out if they and their child with autism have a genetic change associated with autism. (Note: most genetic research studies do not provide participants with their own test results.)

Do I Have an Autism-Related Genetic Change?

A more common way of finding out is to see a doctor, such as a medical geneticist, who can recommend testing. Genetic counselors such as Ms. Blesson help families as they decide whether to undergo testing and what type of testing to have. They also help families and patients understand their test results.

"I try to help families navigate a diagnostic odyssey to figure out why their child has autism," Ms. Blesson said. "We start with testing in the child. We're trying to find that genetic change that has essentially caused him to be at increased risk to develop autism."

photo of genetic counselor Alyssa Blesson of Kennedy Krieger InstituteWhat happens if tests point to a particular genetic change or condition?

Most genetic conditions do not have a specific cure or treatment, but there may be other benefits to having that information. Knowing that a child has a particular genetic change may alert healthcare providers to test for other conditions that may go along with it. Changes in SCN2A, a gene associated with autism, could influence the type of epilepsy medication the person takes, for example, according to SPARK's scientific director, Pamela Feliciano, PhD.12

Also, families also may learn about what to expect in the future, if that information is available.

"Sometimes knowledge is power," Ms. Blesson said. "If we make a diagnosis of something that's genetic, if there are support groups or organizations of other individuals with the same genetic change, it can be helpful to connect with those individuals or families."

Some parents want to know if they also share their child's genetic change, and whether they could pass it to a future baby. They could undergo testing themselves to learn more, Ms. Blesson said.

Of course, many people who undergo testing may find no mutations at all, find changes of unknown significance, or learn unrelated information.

Some people have concerns about discrimination based on their genetic test results, Ms. Blesson said. A U.S. law, the Genetic Information Nondiscrimination Act, seeks to protect people from genetic discrimination in health insurance and employment. Some state laws may provide further protections. (See Additional Resources below.)

Genetics and the Environment

The causes of autism are extremely complicated.

map of a community illustrating possible environmental influences on autism prevalence"The causes of autism are extremely complicated. In most cases, we believe that many factors contribute to its development, not just one thing or another," said epidemiologist Craig Newschaffer, PhD, in a 2017 interview with Autism Speaks.13

Researchers are continuing to study the role of "environment" in autism risk. Environment, in this context, means more than the natural world, although it does include exposure to pollution, pesticides, and hazardous chemicals. It also includes many other things, from the age of parents when they conceive a child; a pregnant woman's medical conditions, illnesses and nutrition; and events that happen at and after a baby's birth.

For example, studies have suggested that exposure to air pollution, farm pesticides, and hazardous chemicals increase autism risk.14-18 Children of older parents, and children whose mothers had problems during pregnancy, such as gestational diabetes and bleeding, also face a higher risk of developing autism.19

Scientists in the field of epigenetics study conditions in a person's environment that turn genes off or on, without actually changing his or her genes themselves. These epigenetic influences can be positive or negative. They include a person's diet, exercise, psychological stress, social interactions, and exposure to disease, drugs of abuse, medicine, and toxic chemicals.20 These exposures can influence the "chemical factors that adhere to the backbone of DNA," explained Dr. Newschaffer, in an online seminar for members of IAN and SPARK for Autism.

Interestingly, these changes in gene functioning can be passed from parent to child. For example, men make sperm throughout their lives, said Dr. Newschaffer, professor of epidemiology and biostatistics, and associate dean for research, at Drexel University. Their exposure to chemicals and other environmental factors may cause epigenetic changes to sperm that may affect their babies, he said.

Stay tuned for future articles that will address the way the environment and genetics interact to influence autism.

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Photo credits: 1-4) iStock, 5) Alyssa Blesson, Kennedy Krieger Institute, 6) iStock.

References: 
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