Posted by jlwile on July 13, 2010
Autism is a poorly-understood condition characterized by problems with social interaction and communication. It is clearly a complex neurological issue, and its symptoms range from quite mild to very severe. As a result, neurologists tend to use the term “autism spectrum disorders” (ASDs), as they suspect autism is made up of a group of disorders with similar features.
I have a good friend with Asperger Syndrome, which is an autism spectrum disorder. While he seems mostly like any other person, he has some obsessive, repetitive rituals, and he sometimes experiences great difficulty in communicating with people, especially those who are unfamiliar with his personality. On the other side of the spectrum, a couple I know fairly well has a son with severe autism. It is difficult for them to communicate with him. It is as if he lives in his own little world. Additionally, he often experiences “meltdowns” in which he slams himself against the ground or the wall and screams at the top of his lungs. His behavior is not the result of “bad parenting.” It is the result of a serious neurological disorder.
What is frustrating for both health-care providers and parents is that so far, medical science has little to offer in terms of explaining what causes autism. In addition, while there are behavioral therapies that have helped many people with ASDs, it is difficult to prescribe a specific therapy for a specific individual. This, of course, leaves doctors and parents rather frustrated.
While there is a lot we don’t know about ASDs, there are things we do know. We know that they are on the rise. Even though there are many different ways to define ASDs, which leads to many different specific numbers, a good overview can be found here. Based on their numbers for the U.S. and outlying areas, for example, ASDs among people age 6-22 have increased 18-fold since 1992!
What are the causes of ASDs? The answer is that we don’t know. However, medical scientists are at least closing in on them.
For a while, it was popular to blame the rise of ASDs on vaccinations. After all, the number of vaccines given to children is rising, and the rate of ASDs is rising. Also, ASDs are generally diagnosed when a child is very young, which is also when a child gets a lot of vaccinations. Over the years, however, lots of studies have been done, and those studies clearly show that vaccination is not related to autism. Indeed, some of the authors on these studies are the same ones who demonstrated a serious side effect of the old rotavirus vaccine. That side effect was bad enough that the vaccine was taken off the market. If scientists who have a proven track record of finding problems with a vaccine couldn’t find a link between vaccination and autism, then it simply doesn’t exist.
More importantly, some very good studies have shown the ASDs are mostly (but not exclusively) inherited. For example, there was an excellent study published in 1995 that analyzed ASDs among British twins.1 It found that among identical twin sets in which one child had autism, 60% of the time the other twin had the same basic kind of autism. However, among fraternal twin sets in which one had autism, there was not a single example of the other fraternal twin having autism. If they expanded the study to include all ASDs, 92% of identical twins shared ASDs, and only 10% of fraternal twins did. Since identical twins have the same DNA and fraternal twins have significantly different DNA, this study shows that genetics is the underlying cause of most ASDs.
But wait a minute. If the underlying cause of most ASDs is genetic, why are ASDs rising so quickly in the United States? Is is really possible that human genetics is changing so rapidly that a genetic disease has increased 18-fold in less than 20 years? Surprisingly, the answer to that question is “Yes.”
People tend to view genetics as relatively stable, but that is just not right. Instead, the human genome changes rather rapidly, in many different ways. For one thing, genes tend to get copied in the human genome, and the number of those copies can be dramatically different from person to person. For example, people in the U.S. have, on average, seven copies of the gene for amylase, a salivary enzyme that helps to break down starch. However, a given individual in the U.S. can have as few as two copies of the gene or as many as fifteen! It turns out that this actually tracks with diet – the average number of amylase copies in a population rises with the amount of starch in the population’s diet.2
The human genome also changes as the average age of a population’s parents changes. Genetic mutations in the germ line (the reproductive cells) accumulate as we age, especially among men.3 As the average age of parents increase, then, they produce children with more mutations.
It turns out that both of these effects play at least some role in autism. For example, several studies have shown that older parents are more likely to have autistic children than are younger parents. One study, for example, analyzed more than 130,000 Jewish people. They found that fathers who were over 40 were almost 6 times more likely to have children with ASDs than were fathers under 30.4 Other studies show that the mother’s age also plays a role in the likelihood of a child having ASD.5 So at least one of the reasons ASDs are increasing in the United States is that the average age of parents is increasing.
More recently, a major genetic study on ASDs was published, and the results indicate that the number of copies of certain genes in a person’s genome dramatically influences the likelihood of a child having ASD.6 The study looked at the genomes of more than 1,000 children with ASDs and compared them to the genomes of almost 2,000 children who did not have ASD but were otherwise similar to the children with ASDs. They found that compared to the non-ASD children, these children had significant variation in the number of copies of certain neurological genes. Interestingly enough, sometimes the number of copies were inherited from a parent. However, some of the variations in number of copies (about 6% of the total) were not inherited from either parent – they were brand new, appearing only in the child.
So there is a lot of evidence that the causes of ASDs are genetic. While it is good to know that scientists are closing in on the causes, it is also a bit disheartening, because a lot of genes were implicated in this study. As a result, there is probably no easy answer in terms of treatment or prevention. Certainly the medical community now knows more than it did before, but there is clearly a lot left to learn.
1. Bailey A, et al., “Autism as a strongly genetic disorder: evidence from a British twin study,” Psychol Med., 25:63-77, 1995.
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2. George H. Perry, et al., “Diet and the evolution of human amylase gene copy number variation,” Nature Genetics, 39:1256-1260, 2007.
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3. A. J. Wyrobek, et al., “Advancing age has differential effects on DNA damage, chromatin integrity, gene mutations, and aneuploidies in sperm,” Proc Natl Acad Sci U S A., 103:9601–9606, 2006.
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