Lower birth weight indicates higher risk of autistic traits in discordant twin pairs
Posted Dec 07 2011 1:54am
Twin studies have shown that there is a strong genetic component to autism. When one “identical” twin has autism, the odds are high that the other twin does as well. But, what about those cases where only one twin has autism? The pair is “discordant”.
One of the major twin studies ongoing is the “Child and Adolescent Twin Study of Sweden” (CATSS). The study is not just an autism study, as their website notes:
The aim of this study is to investigate how both genetic and environmental effects influence health and behavior in children and adolescents. In this study parents to all Swedish twins turning 9 or 12 years are asked to complete a telephone interview concerning the health and behavior of their twins. The interview screens for several different health (e.g., asthma, allergies, diabetes) and behavior (e.g., attention, social interaction) problems. Some of the families will be followed up with additional questionnaires, as well as with genotyping and clinical interviews.
By studying discordant pairs, they are able to look for other risk factors. In this case, low birth weight. They found that low birth weight confers a significant risk for autism. Three times higher risk for a discordant autism pair for low birth weights.
They conclude ” a non-genetic influence associated with birth weight may contribute to the development of ASD”
Roxelyn and Richard Pepper Department of Communication Sciences and Disorders, Northwestern University, Evanston, IL, USA.
Autism spectrum disorder (ASD) is a neurodevelopmental disorder of complex etiology. Although strong evidence supports the causal role of genetic factors, environmental risk factors have also been implicated. This study used a co-twin-control design to investigate low birth weight as a risk factor for ASD.
We studied a population-based sample of 3715 same-sex twin pairs participating in the Child and Adolescent Twin Study of Sweden (CATSS). ASD was assessed using a structured parent interview for screening of ASD and related developmental disorders, based on DSM-IV criteria. Birth weight was obtained from medical birth records maintained by the Swedish Medical Birth Registry.
Twins lower in birth weight in ASD-discordant twin pairs (n=34) were more than three times more likely to meet criteria for ASD than heavier twins [odds ratio (OR) 3.25]. Analyses of birth weight as a continuous risk factor showed a 13% reduction in risk of ASD for every 100 g increase in birth weight (n=78). Analysis of the effect of birth weight on ASD symptoms in the entire population (most of whom did not have ASD) showed a modest association. That is, for every 100 g increase in birth weight, a 2% decrease in severity of ASD indexed by scores on the Autism – Tics, attention-deficit hyperactivity disorder (AD/HD), and other Comorbidities (A-TAC) inventory would be expected in the sample as a whole.
The data were consistent with the hypothesis that low birth weight confers risk to ASD. Thus, although genetic effects are of major importance, a non-genetic influence associated with birth weight may contribute to the development of ASD.
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