Summary: Data analysis from previous studies reveals if one twin is on the autism spectrum, there is a 96% chance the other twin will also be diagnosed with ASD. However, symptom severity varies greatly between the twins. Researchers estimate genetic factors contribute to only 9% of the cause of trait variation.
Identical twins with autism spectrum disorder (ASD) often experience large differences in symptom severity even though they share the same DNA, according to an analysis funded by the National Institutes of Health. The findings suggest that identifying the causes of this variability may inform the treatment of ASD-related symptoms. The study was conducted by John Constantino, M.D., of Washington University School of Medicine in St. Louis, and colleagues. Funding was provided by NIH’s Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD). The study appears in Behavior Genetics.
ASD is a developmental disorder that affects how a person behaves, interacts with others and learns. Previous studies have found that when one identical twin has ASD, chances are extremely likely that the other twin has it, too.
The authors analyzed data from three previous studies comprising a total of 366 identical twin pairs with and without ASD. The severity of autism traits and symptoms in the twins was measured by a clinician’s assessment or by parents’ ratings on a standardized questionnaire. Some cases were diagnosed by both methods. The researchers determined a 96% chance that if one twin has ASD, the other has it, too. However, symptom scores varied greatly between twins diagnosed with ASD. The researchers estimated that genetic factors contributed to only 9% of the cause of trait variation among these twins. In contrast, among pairs of identical twins without ASD, the scores for traits were very similar.
The study authors do not know the reasons for differences in symptom severity, but they rule out genetic and most environmental causes because the twins share the same DNA and were raised in the same environment. Additional studies are needed to determine the cause.
About this neuroscience research article
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On the Nature of Monozygotic Twin Concordance and Discordance for Autistic Trait Severity: A Quantitative Analysis
The characterizing features of autism spectrum disorder (ASD) are continuously distributed in nature; however, prior twin studies have not systematically incorporated this knowledge into estimations of concordance and discordance. We conducted a quantitative analysis of twin–twin similarity for autistic trait severity in three existing data sets involving 366 pairs of uniformly-phenotyped monozygotic (MZ) twins with and without ASD. Probandwise concordance for ASD was 96%; however, MZ trait correlations differed markedly for pairs with ASD trait burden below versus above the threshold for clinical diagnosis, with R2s on the order of 0.6 versus 0.1, respectively. Categorical MZ twin discordance for ASD diagnosis is rare and more appropriately operationalized by standardized quantification of twin–twin differences. Here we provide new evidence that although ASD itself is highly heritable, variation-in-severity of symptomatology above the diagnostic threshold is substantially influenced, in contrast, by non-shared environmental factors which may identify novel targets of early ASD amelioration.