Xun Zhou,1,2,* Jingzhou Liu,2,* Shuang Xiao,2,3,* Xiaoqing Liang,1,2 Yi Li,2 Fengzhen Mo,3 Xin Xin,2 Yang Yang,2 Chunsheng Gao1,2 

1School of Pharmacy, Henan University, Kaifeng, People’s Republic of China; 2State Key Laboratory of Toxicology and Medical Countermeasures, Beijing Institute of Pharmacology and Toxicology, Beijing, People’s Republic of China; 3School of Pharmacy, Guangxi Medical University, Nanning, People’s Republic of China

*These authors contributed equally to this work

Correspondence: Fengzhen Mo; Xin Xin, Email mofzhen@163.com; 13511014091@163.com

Abstract: Gene therapy aims to add, replace or turn off genes to help treat disease. To date, the US Food and Drug Administration (FDA) has approved 14 gene therapy products. With the increasing interest in gene therapy, feasible gene delivery vectors are necessary for inserting new genes into cells. There are different kinds of gene delivery vectors including viral vectors like lentivirus, adenovirus, retrovirus, adeno-associated virus et al, and non-viral vectors like naked DNA, lipid vectors, polymer nanoparticles, exosomes et al, with viruses being the most commonly used. Among them, the most concerned vector is adeno-associated virus (AAV) because of its safety, natural ability to efficiently deliver gene into cells and sustained transgene expression in multiple tissues. In addition, the AAV genome can be engineered to generate recombinant AAV (rAAV) containing transgene sequences of interest and has been proven to be a safe gene vector. Recently, rAAV vectors have been approved for the treatment of various rare diseases. Despite these approvals, some major limitations of rAAV remain, namely nonspecific tissue targeting and host immune response. Additional problems include neutralizing antibodies that block transgene delivery, a finite transgene packaging capacity, high viral titer used for per dose and high cost. To deal with these challenges, several techniques have been developed. Based on differences in engineering methods, this review proposes three strategies: gene engineering-based capsid modification (capsid modification), capsid surface tethering through chemical conjugation (surface tethering), and other formulations loaded with AAV (virus load). In addition, the major advantages and limitations encountered in rAAV engineering strategies are summarized.

Keywords: AAV engineering, capsid modification, surface tethering, virus load, rational design, directed evolution, machine learning