The behavioral phenotype of the idic(15) syndrome

Idic(15) syndrome is a neurogenetic disorder clinically delineated by early central hypotonia, developmental delay and intellectual disability (ID), epilepsy, absent or very poor speech, and autistic or autistic-like behavior. It is due to the presence of a supernumerary marker chromosome formed by the inverted duplication of proximal chromosome 15, resulting in tetrasomy 15p and partial tetrasomy 15q, and containing the Prader-Willi/Angelman syndrome critical region (PWS/ASCR). The vast majority of these idic(15) derives from the two homologous maternal chromosomes at meiosis. To better define the behavior profile, we studied 22 idic(15) children (15 males and 7 females) observed at our institute between 1986 and 2010, and present, in detail, case studies of five of them. We have been able to perform standardized and semi-standardized measures of intelligence, and psychopathology in only 13 of our 22 patients, due to the limitations of chronological age, and to the severity of ID (ranging from mild-moderate, in 15%, to severe-profound, in 85% of our sample). The results show a distinct developmental profile in idic(15) patients, that may provide a behavioral signature for autism spectrum disorder (ASD)/ASD-like arising from the susceptibility locus on proximal 15q; and suggest that idic(15) individuals are not "true autistic," but distinct "autistic-like" persons with high score in the third ADOS-G and ADI-R area.