AAV Engineering for Improving Tropism to the Central Nervous System

SIMPLE SUMMARY: Adeno-associated virus (AAV) is a small, non-pathogenic, and replication-defective virus that mainly infects primates. AAV has demonstrated great success in pre-clinical and clinical applications, including central nervous system (CNS), ocular, muscular, and liver diseases. We have encountered a variety of obstacles, such as delivery efficiency, packaging optimization, and immunogenicity that have hindered the therapeutic potential of AAV gene delivery. Much progress has been made to enhance AAV trophism to the CNS while de-targeting peripheral organs such as the liver, to minimize toxicity. However, the blood-brain barrier (BBB), remains a significant challenge for clinical applications, complicating vector delivery into and within various CNS compartments. Here, we outline the key studies utilizing AAV engineering methods including directed evolution, rational design, and in silico design that have been developed to accelerate the discovery and translation of novel CNS capsids. ABSTRACT: Adeno-associated virus (AAV) is a non-pathogenic virus that mainly infects primates with the help of adenoviruses. AAV is being widely used as a delivery vector for in vivo gene therapy, as evidenced by five currently approved drugs and more than 255 clinical trials across the world. Due to its relatively low immunogenicity and toxicity, sustained efficacy, and broad tropism, AAV holds great promise for treating many indications, including central nervous system (CNS), ocular, muscular, and liver diseases. However, low delivery efficiency, especially for the CNS due to the blood-brain barrier (BBB), remains a significant challenge for more clinical application of AAV gene therapy. Thus, there is an urgent need for utilizing AAV engineering to discover next-generation capsids with improved properties, e.g., enhanced BBB penetrance, lower immunogenicity, and higher packaging efficiency. AAV engineering methods, including directed evolution, rational design, and in silico design, have been developed, resulting in the discovery of novel capsids (e.g., PhP.B, B10, PAL1A/B/C). In this review, we discuss key studies that identified engineered CNS capsids and/or established methodological improvements. Further, we also discussed important issues that need to be addressed, including cross-species translatability, cell specificity, and modular engineering to improve multiple properties simultaneously.