123. Gene Editing as a Therapeutic Approach to Treat IPEX Syndrome

Abstract

Site-specific gene editing combined with autologous stem cell transplantation holds tremendous potential to treat both inherited and acquired diseases. However, the translational applications of gene editing are limited by the challenge of achieving sufficient levels of gene targeting in human primary cells. Recent advances in gene editing, including use of the CRISPR-Cas9 system, nuclease-resistant chemically modified small guide RNAs (sgRNAs), and serotype-optimized AAV delivery of donor repair DNA templates, are enabling clinically relevant levels of gene editing in human primary cells. Here, we build upon these advances to develop a gene editing strategy to treat the immune-dysregulation polyendocrinopathy-enteropathy-X-linked (IPEX) syndrome, a severe primary immune deficiency manifesting with life-threatening multi-organ autoimmunity in children. IPEX is caused by mutations in the forkhead box protein 3 gene (FOXP3), resulting in dysfunction of T regulatory cells (Tregs) and T effector cell lymphoproliferation. Due to the presence of disease causative mutations throughout the entire FOXP3 gene and the complex nature of FOXP3 regulatory elements, we designed a strategy to functionally correct the gene in IPEX patient cells using on-target, homology directed repair (HDR)-mediated insertion of the FOXP3 coding sequence into the gene locus. By targeting the FOXP3 gene using a CRISPR system comprised of Cas9 mRNA and chemically modified sgRNAs, we were able to attain high targeting frequencies, reaching 70-80% in human primary CD4+ T cells. We use this CRISPR system in combination with a repair donor DNA template delivered with AAV to demonstrate FOXP3 gene correction in human CD34+ hematopoietic stem and progenitor cells (HSPCs). In addition to gene correction of FOXP3, we use a similar strategy in parallel to knockout FOXP3 in wild-type human CD34+ HSPCs. We use the knockout HSPCs to generate a humanized mouse model of FOXP3 dysfunction, which enables us to study the pathophysiology of IPEX and to ultimately test the efficacy of gene-corrected Treg cell-based therapies. These results will help demonstrate the feasibility of FOXP3 gene editing, which we propose for the translational application of autologous transplant of Tregs and HSPCs as a therapy for IPEX syndrome.