Editing of highly homologous gamma-globin genes by nickase deficient Base Editor mitigates large intergenic deletions

Base editing in gamma-globin promoter is a promising approach for reactivation of fetal-hemoglobin. Recent studies have shown that base editing could result in genotoxic events at the gamma globin locus including 4.9 kb large deletion of intervening region due to nicking in the paralogous HBG 1 and HBG 2 genes. Although the deletion frequency is less than what is observed with Cas9, it could diminish the therapeutic potential. We sought to evaluate if large deletion could be overcome while maintaining the editing efficiency by replacing the nCas9 of ABE8e with a catalytically inactive deadCas9 (dCas9). Using 3 therapeutically relevant gRNAs targeting the gamma globin promoter, we performed a comprehensive evaluation of the editing outcome and frequency of large deletion using dCas9, nCas9, dCas9-ABE8e and nCas9-ABE8e. Our findings indicate that while nicking in itself induced large deletions, the frequency reduced upon efficient base editing. Notably, there was no appreciable deletion with the use of dCas9-ABE8e making it a safer approach, in terms of genome integrity, for therapeutic genome editing in the gamma-globin locus. Further, we also demonstrate that dCas9 ABE8e can edit efficiently in primary human CD34+ hematopoietic stem and progenitor cells (HSPCs) to achieve therapeutic benefits.