Preventing large deletions and chromosome loss in engineered human primary T cells by CasPlus with optimized guide RNAs

Abstract

Genetically engineered T-cell therapies rely heavily on genome editing tools, such as the CRISPR/Cas9 system. However, unintended on-target chromosomal alterations, including large deletions and chromosome loss can occur and pose significant risks including tumorigenesis. Here we combined CasPlus and optimized guide RNAs to reduce these issues in CRISPR/Cas9 engineering human primary T cells. CasPlus, which integrates an engineered T4 DNA polymerase with Cas9 nuclease and guide RNA, promotes favorable small insertions (1-2 bp) while reducing large deletions and chromosome loss in T cells. Our optimized guide RNAs favoring small insertions reduced large deletions and chromosome loss by two- to five-fold versus those favoring small deletions. Moreover, combining optimized guide RNA with T4 DNA polymerase further synergistically reduced large deletions and chromosome loss by additional two-fold. Notably, replacing currently used guide RNA pairs in clinically applications with optimized pairs biased towards small insertions, along with CasPlus instead of Cas9, for editing greatly reduced large deletions and chromosome loss in gene-edited human primary T cells. These findings demonstrated that pre-selecting target sites favoring small insertions via guide RNA optimization coupled with CasPlus editing is a safer and more effective strategy to improve genome stability in T-cell engineering and other gene-editing applications.