Application of in-vitro-cultured primary hepatocytes to evaluate species translatability and AAV transduction mechanisms of action

Deciphering conundrums of adeno-associated virus liver-directed gene therapy: focus on hemophilia

Adeno-associated virus gene therapy has been the subject of intensive investigation for monogenic disease gene addition therapy for more than 25 years, yet few therapies have been approved by regulatory agencies. Most have not progressed beyond phase 1/2 due to toxicity, lack of efficacy, or both. The liver is a natural target for adeno-associated virus since most serotypes have a high degree of tropism for hepatocytes due to cell surface receptors for the virus and the unique liver sinusoidal geometry facilitating high volumes of blood contact with hepatocyte cell surfaces. Recessive monogenic diseases such as hemophilia represent promising targets since the defective proteins are often synthesized in the liver and secreted into the circulation, making them easy to measure, and many do not require precise regulation. Yet, despite initiation of many disease-specific clinical trials, therapeutic windows are often nonexistent, resulting in excess toxicity and insufficient efficacy. Iterative progress built on these attempts is best illustrated by hemophilia, with the first regulatory approvals for factor IX and factor VIII gene therapies eventually achieved 25 years after the first gene therapy studies in humans. Although successful gene transfer may result in the production of sufficient transgenic protein to modify the disease, many emerging questions on durability, predictability, reliability, and variability of response have not been answered. The underlying biology accounting for these heterogeneous responses and the interplay between host and virus is the subject of intense investigation and the subject of this review.

A humanized mouse model for adeno-associated viral gene therapy

Clinical translation of AAV-mediated gene therapy requires preclinical development across different experimental models, often confounded by variable transduction efficiency. Here, we describe a human liver chimeric transgene-free Il2rg-/-/Rag2-/-/Fah-/-/Aavr-/- (TIRFA) mouse model overcoming this translational roadblock, by combining liver humanization with AAV receptor (AAVR) ablation, rendering murine cells impermissive to AAV transduction. Using human liver chimeric TIRFA mice, we demonstrate increased transduction of clinically used AAV serotypes in primary human hepatocytes compared to humanized mice with wild-type AAVR. Further, we demonstrate AAV transduction in human teratoma-derived primary cells and liver cancer tissue, displaying the versatility of the humanized TIRFA mouse. From a mechanistic perspective, our results support the notion that AAVR functions as both an entry receptor and an intracellular receptor essential for transduction. The TIRFA mouse should allow prediction of AAV gene transfer efficiency and the study of AAV vector biology in a preclinical human setting.