Down syndrome: genes, model systems, and progress towards pharmacotherapies and clinical trials for cognitive deficits.

Down syndrome (DS) is caused by an extra copy of all or part of the long arm of human chromosome 21 (HSA21). While the complete phenotype is both complex, involving most organs and organ systems, and variable in severity among individuals, intellectual disability (ID) is seen in all people with DS and may have the most significant impact on quality of life. Because the worldwide incidence of DS remains at approximately 1 in 1,000 live births, DS is the most common genetic cause of ID. In recent years, there have been important advances in our understanding of the functions of genes encoded by HSA21 and in the number and utility of in vitro and in vivo systems for modeling DS. Of particular importance, several pharmacological treatments have been shown to rescue learning and memory deficits in one mouse model of DS, the Ts65Dn. Because adult mice were used in the majority of these experiments, there is considerable interest in extending the studies to human clinical trials, and a number of trials have been completed, are in progress or are being planned. A recent conference brought together researchers with a diverse array of expertise and interests to discuss (1) the functions of HSA21 genes with relevance to ID in DS, (2) the utility of model systems including Caenorhabditis elegans, zebrafish and mouse, as well as human neural stem cells and induced pluripotent stems cells, for studies relevant to ID in DS, (3) outcome measures used in pharmacological treatment of mouse models of DS and (4) outcome measures suitable for clinical trials for cognition in adults and children with DS.