25489292 - Journal of Child Psychology and Psychiatry...

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Genetic influences on mechanically-assessed activity level in children Alexis C. Wood, 1 Kimberly J. Saudino, 2 Hannah Rogers, 1 Philip Asherson, 1 and Jonna Kuntsi 1 1 MRC Social, Genetic and Developmental Psychiatry Centre, Institute of Psychiatry, Kings College London, UK; 2 Psychology Department, Boston University, USA Background: Activity level is an important component of children’s temperament, as well as being part of the core symptom domain of hyperactivity-impulsivity within attention deficit hyperactivity disorder (ADHD). Yet it is poorly understood, due partly to limitations on parent and teacher ratings, which are typically used as measurements of these symptoms. Methods: We aimed to study the etiology of objectively-measured activity level across two situations, using actigraphs. A population-based sample of 463 7–9-year-old twin pairs were assessed individually both when apart undergoing laboratory-based cognitive testing and when together during a break in testing. Results: Heritability of activity level was estimated as 24% during the test session and at 30% during the break in testing. Shared environmental influences accounted for 27% of the variance in activity level during the test session and 42% of activity level measured during the break. A genetic correlation of 1.0 indicated that the same genes influenced activity level across the two situations, justifying the use of a composite measure of the two situations. This produced a heritability estimate of 36%. Conclusions: Objectively-measured activity level shows a moderate degree of genetic influence, with a common set of genes influencing activity level across situations. This supports the use of actigraphs as an additional source of information in studies that aim to improve phenotype definition for molecular genetic studies of activity level and ADHD. Key- words: Activity level, actigraph, etiology, heritability, twin study, genetic effects, attention deficit hyperactivity disorder. Abbreviations: ADHD: attention deficit hyperactivity disorder; CI: confidence interval; MZ: monozygotic; DZ: dizygotic; A: variance due to additive genetic effects; C: variance due to shared environmental effects; E: variance due to child-specific environmental effects; r A : genetic correlation; r C : shared environment correlation; r E : child-specific environment correlation. Activity level is an important component of children’s temperament, as well as being part of the core symptom domain of hyperactivity-impulsivity within attention deficit hyperactivity disorder (ADHD). Par- ent and teacher ratings suggest moderately strong genetic influences on activity level or symptoms of overactivity, with heritability estimates in the range of 42–72% (Thapar, Hervas, & McGuffin, 1995). The diagnosis of ADHD is similarly highly heritable, with a recent meta-analysis converging at a heritability estimate of 76% (Faraone et al., 2005). Molecular genetic studies on ADHD indicate an association with several candidate genes, including the dopa- mine D4 and D5 receptor genes, although the effect sizes are
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