The Heritability of Autism Spectrum Disorder in Twins

Investigation into factors that affect the onset and development of autism spectrum disorder is ongoing. The sheer number of autistic traits that suffer demonstrate, however, makes this process very difficult and to date, there have been a great number of both genetic and environmental factors that have been linked to this illness. However, until recently, these studies had only been carried out using individuals as test samples and many medical professionals are of the opinion that this limits the level of understanding about autism that we are able to achieve and also makes it likely that the more subtle cases of autism may be missed, and that the findings of such studies may not a good representation of the wider population.

In order to try and address this, a study was recently conducted in the UK that used samples of twins, as opposed to individuals. Leading the study was Beata Tick (M.Sc.) of Kings College London and the findings of this study were published by JAMA Psychiatry, which you can access here: 

The study investigated both genetic and environmental factors that could potentially increase the likelihood of a person suffering from autism spectrum disorder. You can read about it in more depth below.

Importance  Most evidence to date highlights the importance of genetic influences on the liability to autism and related traits. However, most of these findings are derived from clinically ascertained samples, possibly missing individuals with subtler manifestations, and obtained estimates may not be representative of the population.

Objectives  To establish the relative contributions of genetic and environmental factors in liability to autism spectrum disorder (ASD) and a broader autism phenotype in a large population-based twin sample and to ascertain the genetic/environmental relationship between dimensional trait measures and categorical diagnostic constructs of ASD.

Design, Setting, and Participants  We used data from the population-based cohort Twins Early Development Study, which included all twin pairs born in England and Wales from January 1, 1994, through December 31, 1996. We performed joint continuous-ordinal liability threshold model fitting using the full information maximum likelihood method to estimate genetic and environmental parameters of covariance. Twin pairs underwent the following assessments: the Childhood Autism Spectrum Test (CAST) (6423 pairs; mean age, 7.9 years), the Development and Well-being Assessment (DAWBA) (359 pairs; mean age, 10.3 years), the Autism Diagnostic Observation Schedule (ADOS) (203 pairs; mean age, 13.2 years), the Autism Diagnostic Interview–Revised (ADI-R) (205 pairs; mean age, 13.2 years), and a best-estimate diagnosis (207 pairs).

Main Outcomes and Measures  Participants underwent screening using a population-based measure of autistic traits (CAST assessment), structured diagnostic assessments (DAWBA, ADI-R, and ADOS), and a best-estimate diagnosis.

Results  On all ASD measures, correlations among monozygotic twins (range, 0.77-0.99) were significantly higher than those for dizygotic twins (range, 0.22-0.65), giving heritability estimates of 56% to 95%. The covariance of CAST and ASD diagnostic status (DAWBA, ADOS and best-estimate diagnosis) was largely explained by additive genetic factors (76%-95%). For the ADI-R only, shared environmental influences were significant (30% [95% CI, 8%-47%]) but smaller than genetic influences (56% [95% CI, 37%-82%]).

Conclusions and Relevance  The liability to ASD and a more broadly defined high-level autism trait phenotype in this large population-based twin sample derives primarily from additive genetic and, to a lesser extent, nonshared environmental effects. The largely consistent results across different diagnostic tools suggest that the results are generalizable across multiple measures and assessment methods. Genetic factors underpinning individual differences in autismlike traits show considerable overlap with genetic influences on diagnosed ASD.

While this study by no means reveals a cure or a definite way to prevent autism spectrum disorder, it does shed some light on how significant a role genetics play in the development of this condition, with ASD measurements among identical twins considerably higher than for fraternal twins.

This study is the first step to building more of a complete picture of the causes of autism, covering a wider demographic so that the more subtle manifestations are not overlooked.

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