Although elevation of blood and platelet serotonin has been documented in autism, genetic analyses of serotonin transporter gene have given conflicting results. Tryptophan 2,3 dioxygenase (TDO2) is the rate-limiting enzyme in the catabolism of tryptophan, the precursor of serotonin. A mutation that results in decreased activity of the TDO2 can decrease catabolism of tryptophan and increase the level of whole body serotonin. As such it is a potential candidate gene for autism. We have investigated five single nucleotide polymorphisms in the TDO2 gene for association with autistic disorder. One hundred and ninety six multiplex autistic disorder families were tested using transmission disequilibrium test. There was a significant difference in the transmission of a promoter variant to autistic subjects (P = 0.0006). Haplotype analysis also demonstrated significant difference in the transmission of TDO2 haplotypes to autistic subjects (P = 0.0027). Our results suggest the presence of a susceptibility mutation in the TDO2 or a nearby gene, but may also represent a chance finding.

Autism is a neurodevelopmental syndrome defined by deficits in social reciprocity and communication and by unusual repetitive behaviors. Although there is an underlying genetic predisposition, the etiology of autism is currently unknown. A recent increase in prevalence suggests that genetically determined vulnerability to environmental exposure might contribute to the causation of autism. We performed family-based association studies of polymorphisms in metal-regulatory transcription factor 1(MTF1), a multispecific organic anion transporter (ABCC1), proton-coupled divalent metal ion transporters (SLC11A3 and SLC11A2), paraoxonase 1 (PON1), and glutathione S-transferase (GSTP1) genes in 196 autistic disorder families. There was deviation from the expected pattern of transmission for polymorphisms in MTF1 (Single nucleotide polymorphism database reference identification number, dbSNP rs3790625, P = .02) and divalent metal ion transporter SLC11A3 (dbSNP rs2304704, P = .07) genes. Although these results might represent chance finding, further investigations of genetic variations of metal metabolism in autism are warranted.

Several independent genome scans have revealed excess allele sharing in an overlapping 40 cM region of 7q21-34 in autism. DLX6 and Piccolo (PCLO) at 7q21-q22 are two positional and functional candidate genes in autism. We have investigated a single nucleotide polymorphism (SNP) in exon 4 of the PCLO gene and a SNP in intron 1 of the DLX6 gene for linkage and association in autistic disorder using both qualitative and quantitative analyses. One hundred ninety-six multiplex autistic disorder families were tested using transmission disequilibrium and two-point affected sib pair linkage analysis. We found no evidence of association or linkage with the two intragenic markers. In addition, there was also no linkage or association between language and stereotypic behavior quantitative traits in autism and the SNPs. In conclusion, our studies suggest that these two SNPs in DLX6 and PCLO genes are not in linkage disequilibrium with autism.