Catherina E. M. van Beijsterveldt

Effect of shared environmental factors on exercise behavior from age 7 to 12 years.

INTRODUCTION The aim of this study was to investigate the relative influence of genetic and environmental factors on childrens leisure time exercise behavior through the classic twin design. METHODS Data were taken from The Netherlands Twin Register. The twins were 7 (n = 3966 subjects), 10 (n = 3562), and 12-yr-olds (n = 8687), with longitudinal data for 27% of the sample. Parents were asked to indicate the childrens regular participation in leisure time exercise activities, including frequency and duration. Resemblance between monozygotic and dizygotic twins for weekly MET-hours spent on exercise activities was analyzed as a function of their genetic relatedness. RESULTS Average weekly MET-hours increased with age for both boys (age 7 yr: 14.0 (SD = 11.8); age 10 yr: 22.6 (SD = 18.7); age 12 yr: 28.4 (SD = 24.9)) and girls (age 7 yr: 9.7 (SD = 9.5); age 10 yr: 15.3 (SD = 15.1); age 12 yr: 19.3 (SD = 19.8)). Around 13% of boys and girls across all age groups did not participate in any regular leisure time exercise activities. Tracking of exercise behavior from age 7 to 12 yr was modest (0.168 < r < 0.534). For boys, genetic effects accounted for 24% (confidence interval, 18%-30%) of the variance at age 7 yr, 66% (53%-81%) at age 10 yr, and 38% (32%-46%) at age 12 yr. For girls, this was 22% (15%-30%), 16% (9%-24%), and 36% (30%-43%), respectively. Environmental influences shared by children from the same family explained 71%, 25%, and 50% of the variance in boys (age 7, 10, and 12 yr) and 67%, 72%, and 53% in girls. The shared environment influencing exercise behavior was partially different between boys and girls. CONCLUSION Our results stress the important role of shared environment for exercise behavior in young children.

Twin Specific Risk Factors in Primary School Achievements

The main aim of this study was to examine twin specific risk factors that influence educational achievement in primary school. We included prenatal factors that are not unique to twins, except for zygosity, but show a higher prevalence in twins than in singletons. In addition, educational achievement was compared between twins and their nontwin siblings in a within-family design. Data were obtained from parents and teachers of approximately 10,000 twins and their nontwin siblings registered with the Netherlands Twin Register. Teachers rated the proficiency of the children on arithmetic, language, reading, and physical education, and reported a national educational achievement test score (CITO). Structural equation modeling showed that gestational age, birth weight, and sex were significant predictors of educational achievement, even after correction for socioeconomic status. Mode of delivery and zygosity did not have an effect, while parental age only influenced arithmetic. Mode of conception, incubator time, and birth complications negatively affected achievement in physical education. The comparison of educational achievement of twins and singletons showed significantly lower ratings on arithmetic, reading, and language in twins, compared to their older siblings, but not compared to their younger siblings. Low gestational age and low birth weight were the most important risk factors for lower educational achievement of twins in primary school. It seems that the differences observed between twins and their nontwin siblings in educational achievement can largely be explained by birth order within the family.

Intelligence: shared genetic basis between Mendelian disorders and a polygenic trait

Multiple inquiries into the genetic etiology of human traits indicated an overlap between genes underlying monogenic disorders (eg, skeletal growth defects) and those affecting continuous variability of related quantitative traits (eg, height). Extending the idea of a shared genetic basis between a Mendelian disorder and a classic polygenic trait, we performed an association study to examine the effect of 43 genes implicated in autosomal recessive cognitive disorders on intelligence in an unselected Dutch population (N=1316). Using both single-nucleotide polymorphism (SNP)- and gene-based association testing, we detected an association between intelligence and the genes of interest, with genes ELP2, TMEM135, PRMT10, and RGS7 showing the strongest associations. This is a demonstration of the relevance of genes implicated in monogenic disorders of intelligence to normal-range intelligence, and a corroboration of the utility of employing knowledge on monogenic disorders in identifying the genetic variability underlying complex traits.

Genetic overlap between schizophrenia and developmental psychopathology: a longitudinal approach applied to common childhood disorders between age 7 and 15 years

Various non-psychotic psychiatric disorders in childhood and adolescence can precede the onset of schizophrenia, but the nature of this relationship remains unclear. We investigated to what extent the association between schizophrenia and psychiatric disorders in childhood is explained by shared genetic risk factors. Polygenic risk scores (PRS), reflecting an individual’s genetic risk for schizophrenia, were constructed for participants in two birth cohorts (2,588 children from the Netherlands Twin Register (NTR) and 6,127 from the Avon Longitudinal Study of Parents And Children (ALSPAC)). The associations between schizophrenia PRS and measures of anxiety, depression, attention deficit hyperactivity disorder (ADHD), and oppositional defiant disorder/conduct disorder (ODD/CD) were estimated at age 7, 10, 12/13 and 15 years in the two cohorts. Results were then meta-analyzed, and age-effects and differences in the associations between disorders and PRS were formally tested in a meta-regression. The schizophrenia PRS was associated with childhood and adolescent psychopathology Where the association was weaker for ODD/CD at age 7. The associations increased with age this increase was steepest for ADHD and ODD/CD. The results are consistent with a common genetic etiology of schizophrenia and developmental psychopathology as well as with a stronger shared genetic etiology between schizophrenia and adolescent onset psychopathology. A multivariate meta-analysis of multiple and repeated observations enabled to optimally use the longitudinal data across diagnoses in order to provide knowledge on how childhood disorders develop into severe adult psychiatric disorders.