Not a single SNP was found to influence the development of autism spectrum disorders in an ample GWAS study:
Richard Anney et al., Individual common variants exert weak effects on the risk for autism spectrum disorderspi. Human Molecular Genetics 2012. Open access ··> LINK [doi: 10.1093/hmg/dds301]
AbstractWhile it is apparent that rare variation can play an important role in the genetic architecture of autism spectrum disorders (ASDs), the contribution of common variation to the risk of developing ASD is less clear. To produce a more comprehensive picture, we report Stage 2 of the Autism Genome Project genome-wide association study, adding 1301 ASD families and bringing the total to 2705 families analysed (Stages 1 and 2). In addition to evaluating the association of individual single nucleotide polymorphisms (SNPs), we also sought evidence that common variants, en masse, might affect the risk. Despite genotyping over a million SNPs covering the genome, no single SNP shows significant association with ASD or selected phenotypes at a genome-wide level. The SNP that achieves the smallest P-value from secondary analyses is rs1718101. It falls in CNTNAP2, a gene previously implicated in susceptibility for ASD. This SNP also shows modest association with age of word/phrase acquisition in ASD subjects, of interest because features of language development are also associated with other variation in CNTNAP2. In contrast, allele scores derived from the transmission of common alleles to Stage 1 cases significantly predict case status in the independent Stage 2 sample. Despite being significant, the variance explained by these allele scores was small (Vm< 1%). Based on results from individual SNPs and their en masse effect on risk, as inferred from the allele score results, it is reasonable to conclude that common variants affect the risk for ASD but their individual effects are modest.
A synthetic report on this paper may be found at The Spitoon.
This study seems to effectively discard any major genetic influence in autism and forces us to look again for environmental clues like environmental pollutants, pregnancy conditions, early parental care (such as breastfeeding), vaccines, etc. in order to understand the causes behind these problems. Per Wikipedia:
Environmental factors that have been claimed to contribute to autism or exacerbate its symptoms, or may be important to consider in future research, include certain foods, infectious disease, heavy metals, solvents, diesel exhaust, PCBs, phthalates and phenols used in plastic products, pesticides, brominated flame retardants, alcohol, smoking, illicit drugs, and vaccines.
Common pollutants, notably many common plastics, pesticides, etc., are high among the likely causes of the disorders, as well as many other modern diseases.