A study has found that identical twins with autism spectrum disorder (ASD), although they have the same genes, frequently suffer major differences in symptoms. The findings also suggested that the origins of this variation may affect the management of symptoms associated with ASD.
The study that appeared in 'Behavior Genetics' was conducted by John Constantino, MD, of Washington University School of Medicine in St Louis, and colleagues.
ASD is a developmental disorder that affects the behaviour, and interaction with and learning of a person. Previous studies have found that if one twin is identical with ASD, it is highly likely that the other twin will have it.
Evidence from three previous studies, composed of 366 identical twin pairs with and without ASD, was reviewed by the scientists.
The severity of autism traits and symptoms in the twins was measured by a clinician's assessment or by parents' ratings on a standardised questionnaire.
Some cases were diagnosed by both methods. The researchers determined a 96 per cent 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 per cent 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 authors of the study do not know why the signs vary, yet they do exclude genetic and most environmental factors because the twins have the same genes and have been born in the same community.
To determine the cause, further studies are needed.