I understand the evidence that suggests that autism is "genetic". I understand that twin studies suggest that something about being a twin greatly increases the risk of autism and that the risks are greater for identical than fraternal twins. But what I don't understand is why the presumption is that genetics is the cause of the greater risk. I don't understand why the other things that twins have in common - such as a shared prenatal and early childhood environments - are often ignored or overlooked.
For example, the existing evidence suggests that a fraternal twin has a greater risk of autism if their twin has autism than other, non-twin siblings do. Fraternal twins are no closer genetically than other other siblings so the increased risk over non-twin siblings has to come from shared environmental factors.
And then there is the little fact that the genetics of even identical twins isn't as straight forward as you might think. Yes identical twins start off with identical genetic material at the moment of conception, but after that point things can get a little murky.
As I think I have mentioned in the past, my identical twin daughters who both have autism each have their own set distinct set of mutations. Under the standard assumption that identical twins are always genetically identical that shouldn't happen. But clearly someone forgot to tell them that they shouldn't be like that.
Just because identical twins start out with the same genetic code when they split from each other a few days after conception doesn't mean they will still be identical when they are born. But if my little anecdote doesn't make you say hmm, then consider the following abstract of a study that was just published -
Monozygotic twins discordant for submicroscopic chromosomal anomalies in 2p25.3 region detected by array CGH.
Although discordant phenotypes in monozygotic twins with developmental disorder are not an exception, underlying genetic discordance is rarely reported. Here, we report on the clinical and cytogenetic details of 4-year-old female monozygotic twins with discordant phenotypes. Twin 1 exhibited global developmental delay, overweight and hyperactivity. Twin 2 had an autistic spectrum disorder. Molecular karyotyping in twin 1 identified a 2p25.3 deletion, further confirmed by FISH analysis on leukocytes. Interestingly, array-CGH was normal in twin 2 but FISH analysis using the same probe as twin 1 showed mosaicism with 1/3 of cells with a 2p25.3 deletion, 1/3 of cells with a 2p25.3 duplication, and 1/3 of normal cells. Genotyping with microsatellite markers confirmed the monozygosity of the twins. We propose that the chromosome imbalance may be due to a mitotic non-allelic recombination occurring during blastomeric divisions of a normal zygote. Such event will result in three distinct cell populations, whose proportion in each embryo formed after separation from the zygote may differ, leading to discordant chromosomal anomalies between twins. We also discuss that the MYTL1L and the SNTG2 genes within the reported region could probably relate to the phenotypic discordance of the monozygotic twins.Makes perfect sense, right?
OK, the text is a little bit dense, so lets break it down a little bit. In this study there is a pair of four year old identical twin girls who have different but related developmental disorders -
Twin 1 -
- has global developmental delay (intellectual disability)
- is overweight
- is hyperactive
- has a deletion at 2.25p3
- has autism
- did not have an overall mutation at 2.25p3
- but did show signs of mosaicism, meaning that roughly -
- 1/3 of her cells had a 2.25p3 deletion
- 1/3 of her cells had a 2.25p3 duplication
- 1/3 of her cells had a normal 2.25p3 gene
I haven't read the full text of the study but there are a few things that jumped out at me.
First, genetics - even in the case of identical twins - is not simple. Most people hear the word "identical" and jump to the conclusion that identical means exactly the same. But as this abstract and other results have shown, identical twins aren't exactly the same. You cannot assume that because one twin has a certain genetic mutation that her twin will have the same.
Second, just because you can point to developmental differences between identical twins and can find a mutation in one twin does not mean that the mutation is automatically the cause. There has to be a mechanism related to the mutation that could cause the differences.
Third, when you find differences in the genetics of identical twins, the fact that there are differences might be more important than what the actual differences are. Any mutation that is not shared has to happen after conception but before the fetus gets large enough that a mutation wouldn't be able to spread to the entire body.
Since these mutations cannot be inherited there has to be something in the prenatal environment that causes them (and please don't say random chance). It seems, at least to me, that any factor that is strong enough to cause permanent genetic change would also be able to play havoc with the delicate developmental process of the fetus.
Finally, you have to look at the overall picture of the twins to see if the mutation is important. In the study the twins both have different yet related developmental disorders - global developmental delay and autism. Since these conditions are so closely related and since it can be difficult to tell the difference between the two (especially in four year olds), I think the fact that both twins are both developmentally delayed is more important than the subtle differences between the two.
The bottom line here is that presumption that autism is largely genetic rests heavily on twin studies but the genetics of twins isn't as straight forward as you would think. When you add in the facts that there isn't an "autism gene" or even a small set of "autism genes" but rather hundreds of mutations that might each account for a very small number of cases and that even identical twins don't always share these rare yet presumed causal mutations, the genetic presumption starts to look a little weak.
Rio M, Royer G, Gobin S, de Blois M, Ozilou C, Bernheim A, Nizon M, Munnich A, Bonnefont JP, Romana S, Vekemans M, Malan CT. Monozygotic twins discordant for submicroscopic chromosomal anomalies in 2p25.3 region detected by array CGH. Clin Genet. 2012 Oct 15. doi: 10.1111/cge.12036. [Epub ahead of print] PubMed PMID: 23061379.