Design Principles of a Novel Construct for HBB Gene-Editing and Investigation of Its Gene-Targeting Efficiency in HEK293 Cells

Beta-thalassemia is one of the most common monogenic inherited disorders worldwide caused by different mutations in the hemoglobin subunit beta (HBB) gene. Genome-editing based on clustered regularly interspaced short palindromic repeats/ CRISPR-associated protein 9 system (CRISPR/Cas9) has raised the hope for life-long gene therapy of beta-thalassemia. In a proof-of-concept study, we describe the detailed design and assess the efficacy of a novel homology-directed repair (HDR)based CRISPR construct for targeting the HBB locus. The selected sgRNAs were designed and cloned into an optimized CRISPR plasmid. The HDR donor templates containing a reporter and a selection marker flanked by the piggyBac Inverted Tandem Repeat (ITRs), the homology arms and the delta thymidine kinase (Delta TK) gene for negative selection were constructed. The efficiency of on-target mutagenesis by the eSpCas9/sgRNAs was assessed by mismatch assays. HDR- positive cells were isolated by treatment with G418 or selection based on truncated Neuron Growth Factor Receptor (tNGFR) expression using the Magnetic Activated Cell Sorting (MACS) method followed by ganciclovir (GCV) treatment to eliminate cells with random genomic integration of the HDR donor template. In-out PCR and sanger sequencing confirmed HDR in the isolated cells. Our data showed similar to 50% efficiency for co-transfection of CRISPR/donor template plasmids in HEK293 cells and following G418 treatment, the HDR efficiency was detected at similar to 37.5%. Moreover, using a clinically- relevant strategy, HDR events were validated after selection for tNGFR+ cells followed by negative selection for Delta TK by GCV treatment. Thus, our HDR-based gene-editing strategy could efficiently target the HBB locus and enrich for HDR-positive cells.