Is autistic spectrum disorder genetic?
Autistic spectrum disorder (ASD) is a group of neurodevelopmental disorders, characterised by varying degrees of social, imaginative and communicative deficits (APA, 2013). ASD includes autism, pervasive developmental disorder, Rett’s disorder and childhood disintegrative disorder (Volkmar and Wiesner, 2009).
Some argue that ASD is genetic, with de novo mutations, copy number variations and chromosomal abnormalities, influencing an ASD individual’s behaviour (Marshall et al, 2008; Levy et al, 2011; Gilberg, 1998; Pinto et al, 2010). Others argue it may be caused by the environment (Hallmayer et al, 2011) and specific experiences, such as maternal stress during postpartum and prenatal development, may be the underlying cause (Claassen et al, 2008; Hao et al, 2012).
This essay will explore these arguments, looking at twin studies, at broader phenotype and at prenatal/postpartum development, specifically in relational to maternal stress. It will conclude that ASD is not solely genetic, but can also be influenced by environmental factors.
Reviewing autism’s heritability can shed light on the disorder’s nature. Heritability is the extent to which an individual’s genetics influence the behavioural variations or observed traits.
Twin studies give an indication of how genetics or environmental factors could be reflected in ASD. Early clinical studies found higher concordance rates for autism in MZ than DZ twins, ranging from 36% to 91% for MZ, yet 0% for DZ twins. There was also an 82-92% concordance rate in MZ twins for related cognitive and social impairments whilst, on average, only 20% for DZ twins (Bailey et al, 1995; Folstein and Rutter, 1977; Steffenburg et al, 1989). These studies suggest a genetic component is involved.
However, the sample sizes for all three studies were extremely small; the Steffenburg study involved only 35 autistic children. Furthermore, the majority suffered from brain damage or dysfunction, meaning the actual cause of ASD could not be conclusively identified. Foldstein and Rutter (1977) also concluded that brain damage during early childhood, together with a possible genetic predisposition, may cause autism. These findings lend support to the theory that, although ASD may have a genetic component, factors such as brain damage may interact, triggering that genetic predisposition.
More recent studies have incorporated all types of autistic spectrum disorders and used larger samples. In a population based study, Lichtenstein et al (2010) studied over 10,000 Swedish twins with ASD. They found higher concordance rates for DZ twins (14%) than in previous studies. Similarly, Hallmayer et al (2011) differentiated between narrowly defined autism and broader definitions of ASD. The MZ vs. DZ concordance rate for narrowly defined autism was, on average, 59% and 24%. With broader definitions, rates increased, except for female MZ twins.