Unraveling genetic mechanisms in autism spectrum disorders

Autism Spectrum Disorders (ASDs) are a group of neurodevelopmental disorders characterized by impairments in 
socialization and communication accompanied by repetitive and stereotypic behaviors. ASDs are highly heritable 
and heterogeneous with a complex genetic etiology. Numerous candidate genes have been suggested by linkage, 
association and candidate gene studies and recurrent submicroscopic deletions and duplications have been identified 
using array technology. In order to screen for deletions and duplications in ASD candidate genes and regions, we 
developed a Multiplex Ligation-dependent Probe Amplification (MLPA) assay (paper I). Screening of 26 genes in 
148 individuals we found a 15q11-13 interstitial duplication, which had  escaped detection by conventional 
karyotyping, in 1.3% of the patients. Synthetic MLPA showed to be an easy, reliable and cost-effective method for 
the identification of Copy Number Variants (CNVs) in selected candidate regions. In order to screen the whole 
genome for CNVs in ASD  patients and identify alterations associated with susceptibility for ASDs we used high 
resolution array-based comparative genomic hybridization (array-CGH).  In 4-year-old girl we identified a 7.1 Mb 
interstitial deletion of chromosome band 6p22.3 (paper II). The patient had cognitive delay, specific language 
impairments and dysmorphic features. The deletion overlapped with six previously reported cases with a 6p22–24 
interstitial deletion. Developmental delay was present in all cases, while heart defects, short neck and/or redundant 
skin folds, eye abnormalities, and ear anomalies were present in the majority of cases. By our finding we could 
narrow down the critical region for the 6p22 deletion phenotype to 2.2 Mb comprising twelve genes including the 
ATXN1 gene previously reported susceptibility gene for learning difficulties. In the whole genome screening of 223 
ASD patients by array-CGH (paper III), clinically significant CNVs were identified in 18 individuals (8%) and 
CNVs of unclear clinical relevance in 20 individuals (9%). Among the latter cases, 13 individuals carried rare 
inherited CNVs, while parental samples were unavailable in the remaining seven cases. All patients were classified 
into different phenotype and inheritance subgroups. Rare inherited CNVs were present in a higher proportion of 
ASD cases having first- or second-degree relatives with an ASD related neuropsychiatric phenotype in comparison 
with cases without reported heredity (P=0.0096). We concluded that rare CNVs, encompassing potential candidate 
regions for ASDs, increase the susceptibility for ASDs and related neuropsychiatric disorders. In 514 ASD patients 
screened by array-CGH (paper IV), an exonic PARK2 deletion was found in three cases (~0.6%). No such deletion 
was identified in 149 control subjects. In a summary of comparable CNVs reported in the Database of Genomic 
Variants (DGV), 9/5141 controls had a deletion within the PARK2 gene (~0.2%). Compared with the DGV controls, 
deletions in the  PARK2 gene were significantly more common in our ASD patient cohort (p=0.019).  PARK2
deletions have previously been reported in autism and our results further support that PARK2 deletions may be a risk 
factor for the development of ASDs. PARK2 encodes for the E3 ubiquitin-protein ligase Parkin, which belongs to 
the Ubiquitin proteasome system (UPS). UPS operate pre- and postsynaptic compartments demonstrating a direct 
link between these two major systems that may be important in the pathophysiology of autism.  
The future challenge will be to, in combination with the increased usage of high resolution array-CGH and whole 
genome sequencing identifying genetic alterations, create useful analysis systems in which the co-occurring 
pathways and gene-gene interactions in ASDs can be linked together and the different genes involved identified.