Most people with some interest in science will know of the saying ‘correlation is not the same as causation’. The idea being that just because factor X is linked to, or appears alongside condition Y does not mean that X causes Y (correlation ≠ causation). It could merely represent that X just happens to be present at the same time as Y in an epiphenomenal sense, and hence no relationship exists between the two.
The correlation ≠ causation statement has quite frequently been used in an autism research context. Given the multitude of factors which have been suggested to be linked to either autism risk or autism onset, one might get the impression that just about everything can ‘cause’ or elevate the risk of receiving a diagnosis of autism. Such is the wealth of autism research areas coupled with the diversity of autism.
Several factors have however cropped up more than once when it comes to autism risk and to some extent invite a rethink of the correlation ≠ causation cast off statement. Outside of the increasingly interesting evidence linking pregnancy exposure to antiepileptics such as valproate as potentially elevating offspring autism risk, the issue of air pollution as a risk factor for autism continues to gain scientific traction.
The paper by Heather Volk and colleagues* adds a new flavour to the debate on air pollution and autism risk via their introduction of an interactive effect of genes and environmental exposure elevating risk. Whereas previous research in this area has relied solely on the number of cases of diagnosed autism statistically being linked to exposure or proximity to exposure to various air pollutants, the Volk study suggested that a certain types of genetics may be more vulnerable to developing autism as a function of air pollution exposure.
Based on data derived from CHARGE (Childhood Autism Risk from Genetics and the Environment Study), researchers looked at the genetics of a particular gene called MET – MET receptor tyrosine kinase. Issues with the MET gene have previously been linked to autism as a function of its important role in brain development. Variations in the MET gene have also been associated with the presence of maternal autoantibodies to foetal brain protein; another very interesting avenue of autism research. Volk reported that a particular form of the MET gene seemed to confer a greater risk of having a diagnosis of autism when combined with higher exposure to air pollution.
With all the talk about genetics and environment (variably) meeting with autism in mind, the Volk study provides an intriguing example of this mixed model being applied in research practice. Whilst interesting, the data are crying out for replication. That also the focus of the study was on one particular gene and offered little in the way of information about the expression of that gene outside of detailing genetic structural changes, means that caution still needs to be applied to these findings. Values for air pollution were also based on geographical measurements so don’t necessarily provide data on individuals.
That being said, the Volk paper does invite much greater research scrutiny into how the gene-environmental relationship may have an effect on autism risk. Further, it asks whether clean air strategies may yet have more far reaching consequences than just other, more traditional links between air pollution and health.
* Volk HE. et al. Autism Spectrum Disorder: Interaction of Air Pollution with the MET Receptor Tyrosine Kinase Gene. Epidemiology. 2013 Nov 14.
Further commentary on this study can be found at: http://questioning-answers.blogspot.com/2013/12/even-more-air-pollution-and-autism-risk.html