Advancements in CRISPR-based in vivo gene therapy for hemophilia

Hemophilia is an X-linked hereditary bleeding disorder caused by loss-of-function mutations in the genes encoding coagulation factors, leading to excessive bleeding and potentially being life-threatening. Currently, regular treatment for hemophilia is the infusion of recombinant blood coagulation factors. This approach is not only costly but can also give rise to complications such as the development of neutralizing antibodies (Nabs), which impede the therapeutic outcome. Hemophilia is a monogenic disease, making gene therapy a promising curative measure that includes gene addition and gene editing approaches. Adeno-associated virus (AAV) vectors have gained massive attention as the premier delivery vehicle for clinical gene therapy due to their diverse tissue tropisms dictated by the natural and engineered AAV capsids, their excellent safety profile since AAVs do not cause any known human diseases, low immunogenicity, and long-lasting gene expression. Several AAV-based gene addition therapies have been approved for hemophilia B and hemophilia A, and dozens of similar AAV-based clinical trials are underway. Gene editing technologies, like Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/CRISPR-associated protein 9 (Cas9), have shown immense potential in the treatment of genetic diseases. CRISPR/Cas9-mediated transgene integration provides critical advantages over regular gene insertion therapy by enabling site-specific, targeted genomic integration rather than random integration. Clinical trials focusing on AAV-CRISPR/Cas9-mediated knock-in of human genes represent the frontier of in vivo genetic medicine, and one such prominent ongoing clinical trial for hemophilia B is designed to insert the human Factor IX gene into hepatocytes. This review outlines recent advances and feasible strategies of CRISPR/Cas9-based genome editing for hemophilia therapy, highlights the potential applications of next-generation gene editing and vector delivery technologies, and offers new insights for the advantages, limitations, and future directions for these novel treatment modalities.