In this review, we summarize the findings regarding genetic, epigenetic, and environmental risk factors identified in autism, and discuss the issue of gene x environment interactions (GxE). Genetic risk factors Genetic epidemiology Heritability The recurrence risk of pervasive developmental disorder in siblings of children with autism is 2% to 8%;4 and

it rises to 12% to 20% if one takes into account the siblings showing impairment in one or two of the three domains impaired in autism respectively.5 Moreover, several twin Inhibitors,research,lifescience,medical studies have suggested that this aggregation within families is best explained by shared genes as opposed to shared environment.6-8 Interestingly, the variation of autistic traits in the general population has been shown to be highly heritable, at a similar level of genetic influence to autism itself, even though the results are heterogeneous (heritability 40% to 80%).9,10 These results have

led to a huge effort in research to try to unravel Inhibitors,research,lifescience,medical Inhibitors,research,lifescience,medical the genetic factors underlying the disorder. However, two recent twin studies have provided intriguing results. One study showed that monozygotic twins had higher concordance rates than dizygotic twins for ASDs, attention deficit hyperactivity disorder (ADHD), developmental coordination disorder, and tic disorder with differences in cross-disorder effects between monozygotic and dizygotic twins, raising the question of the specificity of the underlying genetic factors.8 Another study recently challenged the high heritability Inhibitors,research,lifescience,medical model of autism, estimating the heritability of autism to be 55 %.3 This study generated considerable discussion, the main criticisms concerning the very large confidence interval of the odds ratio (9% to 81%) and the

low participation rate. However, this study is the largest population-based twin study of autism that used contemporary standards for the diagnosis of autism. The independent Inhibitors,research,lifescience,medical heritability of each of the domains of autistic LY2835219 in vitro symptomatology is still a matter of debate. While some argue that different autistic symptoms, to a considerable extent, have separate genetic influences,11,12 others argue that there is strong evidence in favor below of the hypothesis that symptom domains represent correlated behavioral manifestations of a single underlying quantitative neurodevelopmental impairment.13 Transmission in simplex and multiplex families According to two studies, the prevalence of de novo chromosomal rearrangements is higher in subjects from simplex families (one affected individual) compared with subjects from multiplex families,14,15 which is consistent with the high rate of notable de novo mutations identified in probands from simplex families.