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.
These twin studies suggest four findings. Firstly ASD is not solely genetic, but occurs in combination with environmental factors. Secondly, the choice of definition can explain (some of the) inconsistencies in the literature, and finally earlier studies have underestimated DZ twins’ concordance rates.
Early studies placed the recurrence risk for siblings at around 3% (Smalley et al, 1988; Piven et al; 1990; Bolton et al, 1994). This low percentage casts doubt on the theory of genetic involvement in autism. Those studies looked at both younger and older siblings however stoppage factors may have had an effect and skewed results. Stoppage is where families stop having children, following the birth of an autistic child. Ozonoff et al (2011) avoided stoppage factors and looked at later siblings, finding that 18.7% of younger siblings went on to develop ASD. This suggests that the risk of recurrence may have been underestimated. Constantino found that siblings affected with ASD from multiple-incidence autistic families reported significantly more, and a wider range of, autistic traits than siblings from single-incidence families. They were also more likely to have ASD as a result of genetics causes rather than non-genetic causes (Piven and Folstein et al, 1994).
It is possible that not enough importance has been placed on the risk of autistic-like traits in these families. Although not officially recognised as ASD through DSM V, studies show an increased risk for broad phenotype in relatives of ASD individuals (Bailey et al, 1998; Piven et al, 2001; Pickles et al, 2000), especially in first-degree relatives (Bolton et al, 1994; Ben-Yizhak et al, 2011). These traits, such as behavioural or social deficits, are called broader autism phenotypes (BAP), and are associated with ASD (Wheelwright et al, 2010). They have been found in non-autistic siblings.
Whilst 10% of siblings of ASD individuals went on to develop the disorder, 20% of non-autistic siblings displayed delayed language development (Constantino et al, 2010) and their cognitive and social abilities were similar to children with ASD (Valentina et al, 2014; Gamliel et al, 2007; Bolton et al, 1994). This provides strong evidence that BAP may aggregate amongst relatives, perhaps indicating a lesser variation of autism, which is inherited by relatives of ASD individuals. One explanation could be that BAP and ASD may share a similar etiology (Ronald et al, 2011; Lundström et al, 2012).
This leads to the conclusion that, firstly, current restrictive boundaries and definitions of ASD should consider these lesser variations (Volkmar et al, 2005) and secondly, rather than ASD, it may be a susceptibility to social, cognitive and language deficits that is genetic.
The prenatal and postpartum periods are particularly sensitive times for infants, and are also when abnormalities/disorders may surface. For example, prenatal stress can reduce the circulation volume of foetal blood and affect brain structure and development (Kinney et al, 2008b). Stott and Latchford (1976) found that infants, classed as displaying behavioural disturbances, were significantly more likely to have mothers who’d experienced emotional stress in prenatal/postpartum. They were also more likely to suffer neurobehavioural difficulties (Ronald et al, 2011).
Ward (1990) found that infants, whose mothers had suffered stressors in prenatal/postpartum stages, went on to develop early infantile autism. (Ward, 1990). The timing of the occurrence of the stressors was also important. Beversforf et al (2005) found a significantly higher number of stressors experienced between 21-32 weeks gestation, peaking between 25-28 weeks in ASD. This was not seen with Down's Syndrome mothers or controls. Although the effect was small, the findings were significant.
Kinney et al (2008) reviewed birth rates, ASD prevalence and the weather in Louisiana. Infants exposed to stormy weather during either their fifth, sixth or final prenatal month had increased susceptibility to ASD than those exposed to the same factors during the other months of gestation. This further supports the notion that environmental factors contribute to ASD.
Though environmental factors should be given more importance (Chaste and Leboyer, 2012; Yu et al, 2015), as with the MZ and DZ twins, results are not conclusive. Not all infants whose mothers experienced stress developed ASD. Therefore, maternal stress cannot account for ASD, by itself (Jones et al, 2010).
Although MZ concordance rates indicate it, heritability and genetics cannot fully explain ASD. BAP shows traits associated with relatives of ASD, although it may not meet the DSM-V criteria. This suggests that in addition to ASD, a lesser type of ASD may be caused by genetics. Recent evidence points towards the involvement of environmental factors explaining some, if not all, incidences of ASD.
To conclude, no single theory can account for ASD and it is not a solely a genetic disorder. Rather, multiple factors contribute (genetic, epigenetic, and environmental) and are involved in influencing ASD.
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- Quote paper
- Sanum Ghafoor (Author), 2014, Is Autistic Spectrum Disorder Genetic? Exploring the Arguments, Munich, GRIN Verlag, https://www.grin.com/document/377438