In vivo base editing extends lifespan of a humanized mouse model of prion disease

Prion disease is a fatal neurodegenerative disease caused by the misfolding of prion protein (PrP) encoded by the PRNP gene. While there is currently no cure for the disease, depleting PrP in the brain is an established strategy to prevent or stall templated misfolding of PrP. Here we developed in vivo cytosine base editing and adenine base editing strategies delivered by dual- or single-vector AAVs to permanently modify the PRNP locus to achieve PrP knockdown in the brain of mice expressing human PRNP. Systemic injection of dual-AAV PHP.eB encoding BE3.9max and sgRNA installing PRNP R37X edit resulted in 37% average installation of the desired edit, 50% reduction of PrP in the mouse brain, and 52% extension of lifespan in transgenic human PRNP mice inoculated with pathogenic human brain isolates representing the most common sporadic and genetic subtypes of prion disease. We further engineered base editing systems to achieve improved in vivo potency and reduced base editor expression in non-targeting tissues, resulting in 63% average PrP reduction in the mouse brain with 6.7-fold lower total viral dose, with no detected off-target editing of anticipated clinical significance. These findings support the potential of in vivo base editing as one-time treatment for prion disease.