Tuesday, 5 July 2011

Genes overestimated, environment underestimated in autism?

Only a short post this one in response to an article which has just appeared in the Archives of General Psychiatry titled: Genetic heritability and shared environmental factors among twin pairs with autism. A copy of the full-text paper can be found here.

For many years, genes have perhaps received the lion's share of attention when it comes to autism and autism research. Studies conducted in the late to mid 1990's and early noughties estimated a significant genetic loading for autism based on studies of the rates of autism in monozygotic and dizygotic twin pairs; that is twins derived from one egg and those derived from separate eggs. A few choice papers detailing such observations can be found here and here.

Whilst genes are important for autism (and nearly every other condition), there has been a bit of a sea-change in recent years away from notions of an 'autism gene' or genes to something a little more complex with regards to genes and environment. If you don't believe me, have a look at my various posts on CNVs and mutations studied with autism in mind.

This recent paper from Hallmayer and colleagues, corroborates the gene-environment interaction and indeed, perhaps swings the pendulum from genes to more of an environmental effect in terms of risk of developing autism. The collaborative study is a detailed one based in our old autism research favourite place California, where data from a very well-defined autism group, autism and broader autism using ADI-R and ADOS, were examined based on their monozygotic or dizygotic membership in order to ascertain whether autism rates were higher in identical twins than fraternal twins.

The study and findings: of the 202 twin pairs looked at (N=404), 242 twins reached the criteria for autism spectrum disorder (60%), of whom 171 twins met the stricter criteria for autism. Based on these data, 192 twin pairs were submitted for genetic analysis to ascertain zygosity, that is whether they were identical or fraternal twins based on various markers. Fifty-four pairs were found to be monozygotic (28%) and 138 were dizygotic (72%). There are other findings based on the breakdown of specific diagnosis and gender, but I perhaps need a little longer to go into those.

What this means is that if autism was a strongly genetic condition, one would expect that there would be a lot more twins with autism derived from the same egg, the same genetic material, than those derived from separate eggs. The fact that those from separate eggs showed a high-ish rate of autism implies that whilst genes might be important, there is perhaps also a significant contribution from some environmental factor/s in connection to a diagnosis of autism or autism spectrum disorder.

The authors do caution about the findings and issues such as their applicability to all diagnoses of autism across the various geographies and ethnicities. I have posted entries about some of these potential environmental factors previously (here and here for example) but as yet no-one has been able to specifically tie down what they might be and the specific temporal window of exposure. I do wonder also if there might be some influence of time in terms of the results found; is the autism of today, the same as the autism of yester-year?

Replication is the next stage of the process, replication in the same geographical area (or thereabouts) and replication overseas, like here in Northern Europe where many of the original genetic studies on autism were carried out. One would perhaps also like to see a little more information about whether the 'type' of autism might also show any effect and also covering the issue of the broader autism phenotype.

For now though, the sea-change continues as environment steps up to share centre-stage alongside genes in the very complicated world of autism research. The question is: will as much money be directed to the study of environmental factors and autism as has been spent on looking at genes?