53. A Neuro-Specific Gene Therapy Approach to Treat Cognitive Impairment in Down Syndrome by RNA Interference

Abstract

Down syndrome (DS) is a genetic disorder caused by the presence of a third copy of chromosome 21. DS affects multiple organs, resulting in characteristic facial features, muscular hypotonia, heart defects, brain development impairment, and varying degrees of intellectual disability. Trisomic mouse models of DS reproduce the main cognitive disabilities of the human syndrome. In particular, DS mice show structural and functional synaptic impairment as well as learning and memory deficits, largely determined by altered GABAergic transmission through chloride-permeable GABAa receptors (GABAaR). In particular, we have recently found that intracellular chloride accumulation shifts GABAAR-mediated signaling from inhibitory to excitatory in the adult brain of the Ts65Dn mouse model of DS. Accordingly, intracellular chloride accumulation was paralleled by increased expression of the chloride importer NKCC1 (Na-K-Cl cotransporter) in the brains of both trisomic mice and DS patients. Our findings on NKCC1 as a pivotal molecular target for the rescue of cognitive deficits in DS opens the possibility of a gene therapy approach to treat the disease. Here, to normalize NKCC1 expression and rescue synaptic dysfunctions as well as cognitive deficits in Ts65Dn mice we have developed and characterized a knock-down approach to normalize NKCC1 activity. Reducing the expression of the chloride importer NKCC1 by RNA interference restored GABAAR-mediated inhibition and also rescued the structural dendritic deficits found in trisomic neurons in vitro. Most importantly, focal administration of an AAV expressing a silencing RNA under the transcriptional control of a neuron-specific promoter in the hippocampus of Ts65Dn animals mediated NKCC1 knockdown in vivo and rescued behavioral performance on different learning and memory tests at levels undistinguishable from those of WT mice. Our findings demonstrate that NKCC1 overexpression drives excitatory GABAAR signaling in trisomic cells, leading to structural neuronal abnormalities and behavioral impairments in DS mice. Moreover, our study identifies a new gene therapy target for treatments aimed at rescuing cognitive disabilities in individuals with DS.