Social Demographic Change and Autism: part 1

3 Oct

I’ve been meaning to blog this for a long time. Ever since it came online, which was months ago. I’ve wanted to do a good job on this paper and so I’ve kept putting it off while I wait for the time to really dig into it. Kev’s recent post about Prof. Bearman got me thinking it is time to get this out. I knew this would be long and it has grown longer than I expected, so I have split the post up. Here are some introductory thoughts. Much as people like to paint me as being in the “genetics” camp, it isn’t really my interest. Someone like Prometheus would do a far better job on an intro and discussion that I can. But in Prom’s absence, I will say what I can.

Prof. Peter Bearman is a researcher at Columbia University. His team has taken a very careful look at the California Department of Developmental Services (CDDS) data and combined this with California birth record data and come up with what are likely some of the best papers to come from those data. The CDDS provides services to the developmentally disabled in California through a series of “Regional Centers”, which are private corporations which administer the state’s funding through largely non-governmental agencies in the state. They have records on the people (consumers) whom they have served over the years and these data include information on how the consumers qualify for services.

There are five eligibility categories for regional center support:

1) Mental Retardation: Significant deficits in general intellectual functioning (generally an IQ of 70 or below) and significant deficits in adaptive functioning.

2) Cerebral Palsy: A neurological condition occurring from birth or early infancy resulting in an inability to voluntarily control muscular activity, and resulting in significant deficits in motor adaptive functioning and or cognitive abilities.

3) Epilepsy: A disorder of the central nervous system in which the major symptoms are seizures. Eligibility is based on a seizure disorder that is uncontrolled or poorly controlled , despite medical compliance and medical intervention.

4) Autism: A syndrome characterized by impairment in social interaction (withdrawal, failure to engage in interaction with peers or adults), delays in both verbal and nonverbal communication skills, deficits in cognitive skills, and impairment in the ability to engage in make-believe play. Individuals may engage in repetitive activities or a limited repertoire of activities.

5) Fifth Condition: This category includes disabling conditions found to be closely related to mental retardation or requiring treatment similar to that required for individuals with mental retardation.

As a side note, a lot of people forget the “Fifth Condition” category. People will say that people with Asperger Syndrome or PDD-NOS don’t qualify for Regional Center services. Well, they don’t under the “autism” category, but they can under the fifth condition if they meet the requirements for a “substantial disability”. But, I am digressing.

The CDDS data have been extensively used to demonstrate the very large increase in autism prevalence that has occurred over the last 20-30 years.
Prof. Bearman’s group has studied the CDDS data and found that some of the increase can be found to attributed to factors such as changes in the way people are diagnosed (diagnostic accretion) and lower ages of identification.

In a recent paper, Social Demographic Change and Autism, Prof. Bearman’s group argues that about 11% of the rise in autism prevalence can be attributed to genetics.

Sorry to give away the conclusion so early but this is going to be long and I know a lot of people won’t read it all.

Genetics is a hot-button issue with a lot of people in the online autism community. Sometimes people will divide the world into two camps: those who believe autism is caused by vaccines and those who believe autism is caused by genetics. It is a major oversimplification but it happens.

Another oversimplification is to confuse genetics and heritability. As in, “I’m not autistic and my wife isn’t autistic, genetics doesn’t account for my kid being autistic”. This is wrong on so many counts. Heritability implies genetics, but not all genetics is heritable.

In high school or even earlier you probably learned about a monk and pea plants and later studies on fruit flies and the color of their eyes. This is Mendelian inheritance. You learned that some traits are recessive and some are dominant.

From this framework, you can’t get a genetic epidemic.

Whenever the argument about genes and changing prevalence comes up, you can be sure someone will eventually bring up Down Syndrome. Down Syndrome is a developmental disability (possibly an example of the sort that comprise the “fifth category” in the DDS). Down Syndrome is genetic. Not always Mendelian inheritance genetic, but genetic all the same.

The risk factors for having a child with Down Syndrome are

1) Advancing maternal age. A woman’s chances of giving birth to a child with Down syndrome increase with age because older eggs have a greater risk of improper chromosome division. By age 35, a woman’s risk of conceiving a child with Down syndrome is 1 in 400. By age 45, the risk is 1 in 35. However, most children with Down syndrome are actually born to women under age 35 because younger women have far more babies.
2) Having had one child with Down syndrome. Typically, a woman who has one child with Down syndrome has about a 1 percent chance of having another child with Down syndrome.
3) Being carriers of the genetic translocation for Down syndrome. Both men and women can pass the genetic translocation for Down syndrome on to their children.

Part 2 and 3 are what we usually think of as “genetic”, as in “Mendalian”. But what about (1) advancing maternal age? A 10 times greater risk for older mothers? Keep in mind, there is a clear genetic difference behind Down Syndrome.

In humans, the egg cells and sperm cells have 23 chromosomes. The rest of your cells normally contain 23 pairs of chromosomes — one from your father and one from your mother. Kids with Down syndrome usually have three copies of chromosome 21 — called trisomy 21 — instead of two copies.

There is a difference, some might call it an error, in the genetic sequence which leads to Down Syndrome. The parents don’t need to have it. It can be genetic and not heritable. Or, at least, not heritable in the way most people think.

Parental age is increasing. We would be seeing an epidemic of Down Syndrome if it weren’t for the genetic test that is available and offered to most pregnant women.

There are already studies out discussing increased risk for having an autistic child with parental age. If parental age is increasing (and it is), why don’t we see an epidemic of autism from this?

Add to this the recent study from the Autism Genome Project (which came out after this paper by Prof. Bearman’s group). That study, and others, are showing that rather than an autism “gene”, that copy number variations (CNVs) may be one source of genetic risk for autism. These are not heritable in the usual sense as usually they exist in the child and not the parent.

According to Prof. Bearman, we are seeing it. It accounts for about 11% of the increase in autism prevalence in the CDDS data. It is a big effect, but small compared to the other factors going on (the other 89%). So without a careful look, one can’t show it.

Prof. Bearman’s group *is* taking a careful look. The result is their paper Social Demographic Change and Autism. There are a lot of very interesting results, like twin concordance being much smaller than has been previously reported. Another recent paper confirms that. Strangely, no one seems to have noticed.

I’ll try to rectify that in the next installment when we look closer at the paper. Until then, here is the abstract:

Parental age at child’s birth—which has increased for U.S. children in the 1992-2000 birth cohorts—is strongly associated with an increased risk of autism. By turning a social demographic lens on the historical patterning of concordance among twin pairs, we identify a central mechanism for this association: de novo mutations, which are deletions, insertions, and duplications of DNA in the germ cells that are not present in the parents’ DNA. Along the way, we show that a demographic eye on the rising prevalence of autism leads to three major discoveries. First, the estimated heritability of autism has been dramatically overstated. Second, heritability estimates can change over remarkably short periods of time because of increases in germ cell mutations. Third, social demographic change can yield genetic changes that, at the population level, combine to contribute to the increased prevalence of autism

4 Responses to “Social Demographic Change and Autism: part 1”

  1. passionlessDrone October 4, 2010 at 17:47 #

    Hi Sullivan –

    I’m completely on board with the older parents theory and the accumulation of genetic problems in the germ line as the mechanism (or one of them), but I thought I should point out that other researchers have found similar trends in the California data, but the effect was far less pronounced.

    Independent and dependent contributions of advanced maternal and paternal ages to autism risk

    Based on the first examination of heterogeneity in parental age effects, it appears that women’s risk for delivering a child who develops autism increases throughout their reproductive years whereas father’s age confers increased risk for autism when mothers are <30, but has little effect when mothers are past age 30. We also calculated that the recent trend towards delayed childbearing contributed approximately a 4.6% increase in autism diagnoses in California over the decade

    Very difficult to know which set of findings is more accurate. (?)

    I also think that some other things that tend to go along with an older parent might be able to influence the outcome in ways other than genetic disruption; things like being fatter, or having more time to accumlate persistent pollutants come to mind.

    – pD

  2. Roger Kulp October 4, 2010 at 18:31 #

    One question.How might this effect the theory,proven in some autistics,that autism requires “hits” on genes on more than one chromosome?

  3. dr treg October 4, 2010 at 21:34 #

    Are there any studies into how the incidence of autism
    relates to the increasing geographical distance between the origin of the parental genes?
    With the advent of cheap travel, more parents consist of couples from different parts of the world. It would be of interest to know if this is associated with a higher risk of autism as well as parental age.
    Children derived from genes of different genetic pools may have a tendency to a greater immunogenic response to their own auto-immunogens or after exposure of their genes to new environmental immunogens.


  1. Tweets that mention Autism Blog - Social Demographic Change and Autism: part 1 « Left Brain/Right Brain -- - October 4, 2010

    […] This post was mentioned on Twitter by Brandon Blietz, Catherina+ScienceMom. Catherina+ScienceMom said: Sullivan doing a great job explaining: Social Demographic Change and Autism: part 1 via @kevleitch […]

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