557. Targeted CYBB Minigene Insertion into the CYBB Locus for Correction of X-CGD iPSCs Requires Intronic Elements for Expression

Abstract

X-linked chronic granulomatous disease (X-CGD) is an immune deficiency characterized by defective phagocyte production of microbicidal reactive oxygen species (ROS), resulting in recurring, life-threatening infections and hyper-inflammation. Mutations causing X-CGD span the entire 13 exons or intronic splice sites of the >30-kb CYBB gene encoding gp91phox, resulting in a loss of gp91phox protein expression. We previously tested a TALEN-mediated targeted gene therapy approach to insert a codon-optimized CYBB minigene into the start site of endogenous CYBB. Although targeted insertion into the endogenous start site was achieved in X-CGD patient iPSCs, little or no gp91phox expression or ROS activity was observed upon granulocyte differentiation, suggesting that downstream intronic or regulatory elements may be necessary for efficient gene expression from the CYBB promoter. To test this hypothesis, we tested CRISPR-mediated targeted insertion of a codon-optimized CYBB cDNA consisting of exons 2 through 13 (CYBB2-13) together with a puromycin-resistance gene cassette into exon 2 of the CYBB locus. In iPSCs from X-CGD patients with a CYBB mutation in exon 5, exon 7, or intron 10, the efficiency of targeted insertion of the CYBB2-13 plasmid donor without random inserts in puromycin-selected clones was 50-66%. Upon granulocyte differentiation of CYBB2-13 corrected X-CGD iPSCs, gp91phox expression and ROS production were restored to levels 64-100% (gp91phox) and 68-76% (DHR) of normal healthy donor controls. As expected for expression from the endogenous CYBB promoter, expression of gp91phox was specific to CD13+ granulocytes, and was undetected in undifferentiated iPSCs. This targeted gene therapy approach should allow correction of ~90% of X-CGD patient mutations (those involving mutations in exons 2 through 13), to restore ROS activity while maintaining normal regulation of CYBB expression. Further, these findings demonstrate a key issue for the design of targeted gene insertion to capture expression from an endogenous promoter: for some endogenous promoters, the inclusion of intronic elements is necessary for efficient expression of the insert.