368. Advances in Fragile X Gene Therapy Using Adeno-Associated Viral Vectors Coding for the Fragile X Mental Retardation Protein

Fragile X syndrome (FXS) is a severe debilitating neurodevelopmental disorder characterized by a loss of proper translational control at the synapse. In FXS, an aberrant CGG trinucleotide expansion upstream of the Fragile X Mental Retardation Protein (FMRP) gene causes drastic downregulation of this translational modulator. The absence of FMRP impairs synaptic plasticity and leads to mental retardation and autistic spectrum-related phenotypes. To correct this neurological disorder we recently devised an adeno-associated viral (AAV) vector encoding FMRP where its cellular tropism was controlled by the neuron-specific synapsin promoter. Following intracerebroventricular (i.c.v.) injection in neonatal Fmr1 KO mice, transgene expression remained stable for over 7 months in terminally differentiated neurons. The FMRP transgene corrected PSD-95 protein hypo-expression in the cortex of Fmr1 KO animals, as well as lowered MeCP2 protein over-expression. Behavioral endophenotypes including hyperactivity, non-social anxiety, pre-pulse inhibition, repetitive stereotypies, and social dominance were fully or partially corrected using this viral construct. We combined i.c.v. injection with additional injections into the parenchyma of refractory brain regions to achieve wider transduction, while avoiding potentially pathological transgene over expression effects. This ongoing translational study enables us to determine the proper cellular tropism and the range of FMRP expression required for rescue, as well as the brain region dependent correlation of autistic behaviors implicated in FXS neuropathology. These findings are relevant to gene therapy strategies for treating human FXS, as well as other neurodevelopmental disorders.