Chemical modification of AAV9 capsid with N-ethyl maleimide alters vector tissue tropism

Chemical approaches to probe and engineer AAV vectors

Adeno-associated virus (AAV) has emerged as the most promising vector for in vivo human gene therapy, with several therapeutic approvals in the last few years and countless more under development. Underlying this remarkable success are several attractive features that AAV offers, including lack of pathogenicity, low immunogenicity, long-term gene expression without genomic integration, the ability to infect both dividing and non-dividing cells, etc. However, the commonly used wild-type AAV capsids in therapeutic development present significant challenges, including inadequate tissue specificity and the need for large doses to attain therapeutic effectiveness, raising safety concerns. Additionally, significant preexisting adaptive immunity against most natural capsids, and the development of such anti-capsid immunity after the first treatment, represent major challenges. Strategies to engineer the AAV capsid are critically needed to address these challenges and unlock the full promise of AAV gene therapy. Chemical modification of the AAV capsid has recently emerged as a powerful new approach to engineer its properties. Unlike genetic strategies, which can be more disruptive to the delicate capsid assembly and packaging processes, "late-stage" chemical modification of the assembled capsid-whether at natural amino acid residues or site-specifically installed noncanonical amino acid residues-often enables a versatile approach to introducing new properties to the capsid. This review summarizes the significant recent progress in AAV capsid engineering strategies, with a particular focus on chemical modifications in advancing the next generation of AAV-based gene therapies.

The infectivity of AAV9 is influenced by the specific location and extent of chemically modified capsid residues

BACKGROUND

Several treatments for genetic diseases utilizing recombinant adeno-associated viruses (AAVs) have recently gained approval. However, the development of a greater number of therapeutic AAVs is constrained by certain limitations. While extensive efforts have concentrated on screening AAV genetic libraries, an alternative strategy involves modifying the AAV capsid by attaching various moieties. The capsid of AAV plays a pivotal role in transducing target cells and evading immune responses, making modifications a key avenue for engineering improved variants.

RESULTS

In our study, we replaced specific AAV9 capsid residues with an unnatural amino acid bearing a bioorthogonal group, identifying four positions with no adverse impact on production. Utilizing click chemistry, we attached varying proportions of Cy5.5 to these positions, allowing us to assess the impact of these modifications on AAV9 infectivity in cultured cells. Our findings reveal that both the position and degree of capsid modification significantly affect AAV transduction. While higher amounts of attached molecules lead to an increased number of AAV genomes within cells, this does not positively impact transgene expression. Conversely, a negative impact on transgene expression is observed when the AAV capsid is highly modified, with the degree of this effect associated with the modified residue.

CONCLUSION

Careful control of both the degree and specific position of capsid modifications is crucial for optimizing transduction efficiency and minimizing undesired effects on transgene expression. These results underscore the importance of precision in AAV capsid modification to achieve optimal transduction efficiency while mitigating potential drawbacks on transgene expression.

Next-generation biological vector platforms for in vivo delivery of genome editing agents

CRISPR-based genome editing holds promise for addressing genetic disease, infectious disease, and cancer and has rapidly advanced from primary research to clinical trials in recent years. However, the lack of safe and potent in vivo delivery methods for CRISPR components has limited most ongoing clinical trials to ex vivo gene therapy. Effective CRISPR in vivo genome editing necessitates an effective vehicle ensuring target cell transduction while minimizing off-target effects, toxicity, and immune reactions. In this review, we examine promising biological-derived platforms to deliver DNA editing agents in vivo and the engineering thereof, encompassing potent viral-based vehicles, flexible protein nanocages, and mammalian-derived particles.