FMR1 gene therapy restores translationally relevant phenotypes in a mouse model for fragile X syndrome

Fragile X Syndrome (FXS) is the most common inherited form of intellectual disability. It is caused by a trinucleotide expansion in the 5' UTR of the Fragile X messenger ribonucleoprotein 1 (FMR1) gene leading to loss of expression of Fragile X messenger ribonucleoprotein (FMRP). There is currently no cure for FXS. We developed an FMR1 gene therapy based on an adeno-associated viral vector designed with strong translational potential for future clinical testing. The viral vector was tested in Fmr1 knockout mice using two translationally relevant delivery routes and ages corresponding to in utero, toddler, and adolescent ages in humans. Functional studies showed that the FMR1 gene therapy improved select translational FXS phenotypes spanning three critical domains: sensory hyperexcitability, adaptation to change, and altered brain activity. Expression after intracerebroventricular injection was most prominent in the forebrain, whereas intravenous delivery predominantly led to expression across midbrain and brainstem, suggesting that a dual route may be needed to achieve full brain coverage. Biodistribution analyses further suggested that FMRP expression must be titrated carefully for optimal rescue. In summary, we show that FMR1 gene therapy using delivery routes and vehicles approved for clinical use improves core phenotypes in a mouse model for FXS.