The Template-Jumping Editing Approach in F9-Associated Hemophilia B Gene Therapy

Hemophilia B is a hereditary bleeding disorder caused by mutations localized throughout the F9 gene. Existing gene therapy products containing AAV vectors have significant limitations. Replacement therapy with coagulation factor FIX infusions is not an optimal way of treatment, as patients still have periodic bleeding and require frequent transfusions. Moreover, approximately 5% of adult patients with hemophilia B develop inhibitory antibodies to recombinant forms of FIX. Therefore, it is important to develop universal CRISPR/Cas gene therapy approaches for F9 editing using non-viral delivery systems to enable gene reversion to a functional sequence at an early stage of disease development and establishment of the patients’ immune system. In this study, a unique approach of F9 prime-editing was tested for the first time. This method is estimated to edit 7.3% of pathogenic F9 mutation types. Specifically, it targets the gene region encoding amino acids 374 V to 408 Q, which accounts for approximately 9.35% of patients with hemophilia B. An advantage of this gene therapy approach is the absence of the need to change Primer Binding Site (PBS) or Reverse Transcriptase Template (RTT) sequences until going from preclinical to clinical trials, as well as the introduction of gain of function mutations in order to compensate for the low prime-editing frequencies and enhance the effect of treatment in vivo.