BI-TfR1 CapX rapidly transits the blood-brain barrier for efficient, low-dose gene delivery throughout the CNS

Safe, efficient gene delivery throughout the CNS remains a central obstacle to treating genetic diseases of the brain and spinal cord. We describe BI-TfR1 CapX, a human transferrin receptor (TfR1)-binding AAV capsid that, after a low intravenous dose in adult humanized TFRC mice, transduced more than 80% of cortical and spinal neurons and ∼60% of cortical astrocytes, with reduced distribution and expression in several peripheral tissues relative to AAV9. Unlike other engineered BBB-crossing capsids, CapX bound the CNS vasculature within minutes and a large fraction of its genomes reached parenchymal nuclei within 24 hours. Delivering a Prnp -targeted epigenetic silencer ( Prnp -CHARM), CapX reduced brain Prnp mRNA by over 95% requiring 18-fold fewer vector genomes per brain cell than AAV-PHP.eB for 50% lowering. A CapX delivered dual-vector cytosine base editor installed a protein-truncating PRNP stop codon lowering brain prion protein by 80%. These properties make CapX an attractive vehicle for human CNS gene therapy.