Autism spectrum disorders (ASDs) are characterized by repetitive behaviours and interests, as well as deficiencies in communication and social interaction. They are believed to be complex, polygenic disorders predominantly characterized by multifactorial inheritance, although Zhao et al. (2007) suggested that Mendelian inheritance may apply to autism risk in a subgroup of families with affected males. To address the significant genetic heterogeneity and phenotypic variation seen among affected individuals, which has confounded the conclusive identification of candidate genes for the majority of cases, we have been testing genes for evidence of association with specific ASD endophenotypes in an effort to identify a subgroup within the ASD population whose members share an underlying pathophysiology.
Abnormalities in neurotransmitter pathways can account for the deficits seen in persons with ASDs. In contrast to the attention which has been directed to the study of genes involved in the glutamate, GABA and serotonin pathways, genes related to the synthesis, function and metabolism of dopamine (DA) have received little attention.
We have argued that genes in the dopaminergic (DAergic) pathway are excellent candidates based on their affect on ASD behaviours. DA modulates motor functions, cognitive processes (including executive functions and learning), and emotional regulation - all of which are abnormal in individuals with autism[10–13]. DA also plays a role in social interactions and the pathophysiology of stereotypies; impairments in social interaction and the presence of increased stereotypies are core features of autism. Furthermore, there is decreased DAergic activity in the medial prefrontal cortex (PFC) in children with autism, and increased levels of the major metabolite of DA, homovanillic acid (HVA), in cerebrospinal fluid from affected children compared to controls, indicating altered DAergic function in these individuals.
Based on our earlier findings on the dopamine β-hydroxylase (DBH) gene, which encodes the enzyme that converts DA to norepinephrine, in mothers from male-only affected sib-pair families, we have pursued a comprehensive study of DA-related genes in mothers and sons with ASDs. Since our initial study with DBH, in which we found an increased frequency of the 19-bp deletion in mothers from male-only affected sib-pair families, we identified a 3-marker risk haplotype in the dopamine D1 receptor (DRD1) gene in our family cohort having only affected sons. Here we report our findings on two other genes affecting DA levels and function, the dopamine D2 receptor (DRD2) and protein phosphatase 1, regulatory subunit 1B (PPP1R1B) genes, and results of tests for gene-gene interactions.
The DRD2 gene comprises eight exons and maps to 11q22-q23. It encodes the dopamine D2 receptor which, in addition to its role in postsynaptic neurons, acts as an autoreceptor mediating DA synthesis and neurotransmission in DAergic neurons. The dopamine D2 receptor is involved in the DAergic modulation of executive functions, reversal learning and emotional processing. Drd2−/− mice have abnormal gait similar to that of individuals with Parkinson disease, and the administration of antipsychotic medications (e.g., risperidone, a dopamine D2 receptor antagonist) has proven efficacious in treating symptoms associated with ASDs.
The PPP1R1B gene, located at chromosome 17q12 and comprising 7 exons (http://www.ncbi.nlm.nih.gov; GeneID 84152), encodes DARPP-32, which is expressed in dopaminoceptive (DAceptive) neurons and mediates the effects of both D1 and D2 dopamine receptor classes. For example, dopamine D2 receptor antagonist-induced catalepsy in rats is attenuated in Ppp1r1b−/− mice and knockout mice are impaired in reversal learning. Genetic[32, 33] and immunoblot studies showed an association of PPP1R1B with altered PFC DARPP-32 protein levels in schizophrenia and bipolar disorder, two conditions for which DA dysfunction is evident and which exhibit comorbidity with autism.
There are two previous studies which examined the DRD2 gene as a candidate gene for autism. The first reported an increased frequency of the TaqI A1 allele in persons with autism (N = 33) compared to controls (N = 314), whereas the second found no evidence for transmission disequilibrium of an intragenic microsatellite in 39 affected sib-pair families. No association studies have examined the role of PPP1R1B as a candidate gene for ASDs.
Based on our hypothesis that DA-related genes are important in male-only affected sib-pair families, we examined four markers at the DRD2 locus that are commonly used to investigate possible associations between DAergic function and behavioural abnormalities, and three polymorphisms at the PPP1R1B locus to determine whether there was an association of these DA-related genes with autism.