The genetics of autism spectrum disorders
Jeanette J.A. Holden and Xudong Liu
It is now widely accepted that Asperger Syndrome (AS) belongs to the group of the autism spectrum disorders (ASDs), which are characterized by a triad of characteristics including problems with reciprocal social interaction and communication impairments, and the presence of restricted, repetitive or stereotyped behaviours. Based on family and twin studies, the spectrum is often perceived as even broader, including individuals with two or even one of these characteristics. The term “broader phenotype” has been used by Bailey and colleagues (Bailey et al. 1995; Bailey et al. 1998) to describe persons with social and/or language abnormalities, often found in families with a proband with autistic disorder or even in monozygotic co-twins with autism. The results of genetic studies, including genome scans that include the broad spectrum of phenotypes, suggest that at least some of the genes underlying susceptibility to AS are shared by the wider group of persons with autistic disorder and Pervasive Developmental Disorder – Not Otherwise Specified (PDD-NOS). Therefore, this chapter reviews genetic findings on this spectrum of disorders. When available, studies exclusively on individuals with AS are presented, but few such studies have been undertaken to date.
autism spectrum disorders
The results of family and twin studies indicate that ASDs are a group of highly genetic disorders, meaning that genes play a significant role in the susceptibility of a person to developing an ASD. Based on the findings from epidemiological studies that indicate a prevalence of ASD of about 3 to 6 per 1000 (Fombonne 2003; Yeargin-Allsopp et al. 2003), the recurrence risk to siblings and increased