ABSTRACT: Down syndrome (DS), the chromosomal disorder caused by trisomy of chromosome 21 (Tri21), presents many neurological conditions, including intellectual disability (ID), autism spectrum disorder (ASD), epilepsy, and Alzheimer’s disease (AD), and is the most common genetic cause of intellectual disability worldwide. Although some human chromosome 21 (HSA21) genes have been identified as key contributors to some of these pathologies, the mechanisms that link the triplication of HSA21 genes to most DS-related diseases remain largely unknown. Many DS-related neurological conditions are caused by a dysregulated GABAergic system. Excessive GABAergic functions impair synaptic plasticity and memory formation in DS mouse models. DS mouse models exhibit synaptic plasticity deficits, which are rescued by GABA receptor antagonists. Our primary goal is to discover the molecular underpinnings that cause human GABAergic dysregulation, which will provide critical knowledge for understanding the pathogenesis of DS-related human brain disorders, and for developing effective human treatments. Down syndrome cell adhesion molecule (DSCAM) is a major cause of the increased GABAergic synapses and enhanced GABAergic synaptic transmission in the neocortex of DS mouse models. Normalization of DSCAM dosage rescued the excessive GABAergic innervation on pyramidal neuron soma and axon initial segments and prevented the increase in postsynaptic inhibitory current frequency. DSCAM is expressed in GABAergic neurons as well as other neuronal types and is overexpressed in the brains of DS patients and DS mouse models. Human genetics studies have shown strong associations between DSCAM variants and cognitive abilities. These findings raise the possibility that DSCAM overexpression might explain the dysregulation of GABAergic synapses in DS. However, whether the HSA21 dosage and overexpression of DSCAM is causative of GABAergic neuron dysfunction in humans remains to be studied. We hypothesize that DSCAM triplication, resulting from trisomy 21, causes dysregulated GABAergic interneuron development and synaptogenesis in human trisomy 21 cell models. To test this hypothesis, we will use GABAergic neurons derived from control and trisomy 21 human embryonic stem cells. The combined expertise of the Smith and Ye labs is ideally suited for the proposed study. The proposed study is significant because its successful accomplishment will identify the molecular regulators of the GABAergic system in human neurons. This knowledge will offer insights into the development of therapeutic treatments that target DS-related conditions that are caused by dysregulated GABAergic synapses. It will also provide molecular and cellular insights into many neurological disorders, such as other intellectual disabilities, epilepsy, ASD, and neurodegenerative disorders such as AD. 1