Environmental exposures and autism: decoding brain transcriptional patterns

Recent evidence indicates that individuals with autism exhibit characteristic gene expression changes in the brain. Specifically, they often have increased expression of genes related to immune and microglial function and decreased expression of genes related to synaptic transmission. Recognizing such patterns, researchers at the University of North Carolina-Chapel Hill asked an intriguing question: can these distinctive changes be used to identify chemicals that might contribute to risk of Autism Spectrum Disorder (ASD)?

For this work (recently published in Nature Communications), the researchers screened 294 chemicals from the US EPA ToxCast Phase I library in mouse cortical neuron-enriched cultures. While such 2D cultures do not represent the complexity of a fully functioning brain, it should be noted that gene expression profiles of their cultures revealed that their system was highly reflective of a whole embryonic brain during mid to late gestation (a critical period of development – the disruption of which has been linked to ASD).

After treatment, they monitored gene expression and created six clusters of chemicals based on the patterns of changes observed. While all of the chemical clusters represent potentially consequential biological changes, the researchers focused in particular on cluster 2, which up-regulated expression of immune and cytoskeletal-related genes and down-regulated expression of ion channel and synaptic genes. These patterns mirror changes observed in post-mortem ASD brains. (Interestingly, the gene expression changes also correlated with patterns observed in Alzheimer’s disease and Huntington’s disease, which suggests that neurodevelopmental and neurodegenerative diseases may share common pathways and pathology.) In addition to the observed gene expression changes, the cluster 2 chemicals also led to oxidative stress and microtubule disruption – effects that are implicated in neurodevelopmental and neurodegenerative disorders.

The results of this work are as unsettling as they are groundbreaking. The chemicals in cluster 2 are EPA-approved pesticides and fungicides: famoxadone, fenamidone, fenpyroximate, fluoxastrobin, pyraclostrobin, pyridaben, rotenone, and trifloxystrobin. (Rotenone might sound familiar to you; it has already been linked to Parkinson’s disease.) Several of these chemicals can be found at high concentrations on conventionally grown food crops, such as leafy green vegetables – yet another reason to buy organic. And, given that usage of some of these chemicals seems to be increasing (see Figure 7 of their paper), exposure across the population is a concern.

Correlation does not mean causation, however, and therefore this study does not prove that these chemicals trigger autism. Furthermore, as noted above, this study was conducted in cell culture using mouse neurons and therefore is not fully representative of what would happen in an actual human brain. But, we can use these findings as an important warning and should now prioritize these chemicals for further evaluation in animal studies (and also, perhaps, develop relevant epidemiological studies to monitor population-level effects given existing exposures).

While we lack general hazard information on most of the thousands of chemicals in commerce, the absence of information about potential developmental neurotoxicity is a particular problem. This study demonstrates that evaluation of gene expression changes could provide a screening-level assessment that might help to fill some of these concerning gaps. In addition, the authors suggest that their approach could be used to help identify therapeutics that could counter these disease-related gene expression changes – perhaps the first step towards treatment for this increasingly common condition.

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