338. Evaluation of TALENs and the CRISPR/Cas9 Nuclease System To Correct the Sickle Cell Disease Mutation

Abstract

Targeted genome editing technology can correct the sickle cell disease mutation of the beta-globin gene in hematopoietic stem and progenitor cells (HSCs). The correction induces production of red blood cells that synthesize normal hemoglobin proteins. Transcription Activator-Like Effector Nucleases (TALENs) and the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)/Cas9 nuclease systems have been developed to target the sickle mutation in the beta-globin gene for site-specific cleavage to facilitate precise correction of the sickle mutation by a co-delivered homologous donor template. K562 cells were electroporated with TALEN and CRISPR/Cas9 expression plasmids and, using the Surveyor Nuclease Assay (Cel-1), cleavage rates were quantified and compared between TALEN- and CRISPR/Cas9-treated cells. Of the six CRISPR/Cas9 guides tested, each of them led to target disruption of the beta-globin locus with the highest cleavage rates upwards of 35% of alleles. Of 4 distinct TALEN pairs generated, only 2 demonstrated targeted cleavage at rates nearing 10% of alleles. In addition to on-target cleavage at beta-globin, nuclease off-target cleavage at other beta-globin family genes was evaluated for each technology by Cel-1 of nucleofected K562 cells. Here the two TALEN pairs demonstrate cleavage in the highly-homologous delta-globin gene with the optimal TALEN pair cleaving 11% of alleles. In this assay, of the 6 CRISPR guides tested, none showed off-target disruption of delta-globin or any of the other beta-globin cluster genes. Of note, in each of the tested guides, at least one base differed from the target site in beta-globin to the respective sequence in delta-globin in the 10bp PAM proximal region. Further experiments are being conducted to determine the genome-wide off-target effects of each of these nucleases.Upon co-delivery of a plasmid donor template containing the corrective base at the sickle site as well as a restriction fragment length polymorphism (RFLP) for rapid assessment of targeted gene modification, both nuclease technologies led to gene modification. Gene modification rates were assayed by qPCR with primers specific to the modified base. TALENs drove gene modification rates of 18%, while the optimal CRISPR guides resulted in 37% modification in K562 cells without sorting for transfected cells. These results provide the basis for pursuing the use of the CRISPR/Cas9 nuclease system for targeted correction of the sickle mutation in human HSCs.