Genetic and Environmental Influences on Intellectual Disability in Childhood

Abstract

Intellectual disability is characterized as a limitation both in cognition and adaptive behavior. Its prevalence reaches 10–20 per 1,000, but lower and higher estimates could also be found depending on the populations surveyed and methods used (nationality and age of the population, national registry or not, cross-sectional data on children in mainstream public schools, data from special education schools, etc.). Moreover inconsistency in data collected may be largely attributable to the classifications system revisions. The main causes of intellectual disability are presented (genetic and environmental). It is assumed that in approximately half of intellectual disability cases, there is no known cause, but more requests are being made to screen for genetic defects in cases of moderate to severe intellectual disability. Environmental factors are numerous (intrauterine and neonatal insults, severe malnutrition, acute and chronic psychological stress, physical abuse, exposure to family violence, and institutional deprivation, etc.). The etiology of intellectual disability is complex and gene-environment correlations and/or interactions have been illustrated. Two genetic disorders linked to intellectual deficiency (Phenylketonuria and Fragile X) are selected to present both the research methodologies and the type of findings, before discussing the contribution of cross-syndrome comparisons. To uncover a causal link between genetic events and a behavioral phenotype, it is often essential to use model organisms. The advantage of such models, plus the requirements and limitations involved in their use, are presented. The conclusion argues that pathological model contributes in the understanding of the genetic mechanisms underlying cognitive differences within the normal range of variation