Gene Therapy in Fragile X Syndrome

Background:  Fragile X Syndrome (FXS) is a genetic disorder causing cognitive impairment and is one of the leading known genetic causes of autism.  The genetic aberration is caused by an expanded CGG repeat in the 5’ untranslated region of the FMR1 gene on the X chromosome.  The expanded CGG repeat causes an elimination or severe reduction in the level of Fragile X Mental Retardation Protein (FMRP), an mRNA binding protein that controls translation of its bound mRNA substrates.  

Objectives:  To determine if FMRP transgene expression in the CNS could reverse phenotypic deficits in the Fmr1 knockout mouse model of fragile X, we used a single-stranded adeno-associated viral (AAV) vector that contained a major isoform of FMRP.  

Methods:  The vector was delivered to the brain via a single bilateral intra-cerebroventricular injection into neonatal Fmr1 knockout mice.  

Results:  Treated Fmr1 KO mice displayed prominent FMRP transgene expression in most regions of the forebrain but little or no expression more caudal regions.  Reduced levels of the synaptic protein PSD-95, and abnormal motor activity, anxiety, and acoustic startle responses observed in Fmr1 KO mice, were fully or partially rescued by treatment with AAV-FMRP.  We also demonstrate that mRNA for MeCP2, an epigenetic modulator mutated in Rett Syndrome, is substrate for FMRP and that MeCP2 protein levels are elevated in the Fmr1 mouse brain. Further analysis revealed a correlation between motor activity and levels of MeCP2, suggesting that motor hyperactivity in fragile X could be linked to elevated brain MeCP2 expression.

Conclusions:  Our results provide proof-of-principle that gene therapy using a viral vector can correct specific behavioral abnormalities in a mouse model of fragile X syndrome.  The results indicate that early postnatal administration of AAV-FMRP facilitates vector diffusion and distribution within the CNS.  Early postnatal treatment may promote normal neurogenesis and maturation of the brain and might have contributed to the behavioral rescue subsequently seen in adult mice.  Finally, our findings also raise the intriguing possibility that MeCP2 over-expression could contribute to the fragile X phenotype.