Cystic fibrosis (CF) is caused by mutations in a single gene called the cystic fibrosis transmembrane conductance regulator (CFTR), which codes for an ion channel that transports Cl- ions. CF affects many organs but lung failure caused by repeated respiratory infections is the leading cause of death. When CF was first described, most patients died as children. Therapeutic interventions that do not restore CFTR function still enabled patients to survive into adulthood. Recently developed small molecule modulators restore CFTR function modestly and have been shown to reduce pulmonary exacerbations. However, these therapies are expensive, have side- effects and cannot treat all CF patients. As a monogenic disease, CF has been the target of several in vivo gene therapy studies that aimed to restore CFTR function. However, these studies were unsuccessful due to challenges in delivery and immunity mediated against viral vectors. Ex-vivo gene therapy in which corrected airway stem cells are transplanted into patients have been proposed. However, these are limited by our failure to efficiently correct CFTR mutations in a readily accessible airway stem cell type. W e have identified the sinuses as a readily accessible source of sinus basal stem cells and optimized the use of Cas9 and adeno-associated virus (AAV) to gene edit these basal stem cells. We have corrected the most common CFTR mutation (F508del mutation) in >40% alleles. However, several other mutations which cannot be treated using current therapies remain. Moreover, the safety of gene editing using Cas9 and the long-term differentiation potential of edited sinus basal stem cells to regenerate the sinus and bronchial epithelia need to be characterized. Here, we propose to use Cas9/AAV to insert the CFTR cDNA with a truncated CD19 (tCD19) enrichment marker at exon 1 of the endogenous CFTR locus to achieve universal correction of CFTR mutations. We will characterize the safety of genome editing using next-generation sequencing and the differentiation potential of edited sinus basal stem cells using single-cell RNA seq and immunohistochemistry. Preliminary results show that we can obtain an enriched population of tCD19+ sinus basal stem cells after gene editing. These corrected sinus basal stem cells retain their differentiation potential to produce epithelial sheets that show >90% CFTR function (response to CFTR inhibitor or activator) relative to wild-type samples in an Ussing chamber electrophysiological assay. However, further work is needed to determine if the corrected sinus basal stem cells generate all the other airway cell types. Over the period of this grant, we will further validate the safety and efficacy of this platform and evaluate the long-term differentiation potential of these cells to generate all the cell types present in both the sinus and bronchial epithelia. These experiments are an important first step to optimize the autologous transplantation of edited airway stem cells to treat CF. The knowledge developed in this process can be applied to other airway diseases in the future.
Efficient genome editing of airway stem cells is a critical barrier that has limited the development of ex-vivo gene therapies to treat cystic fibrosis. Our project addresses that critical barrier by developing a universal gene correction strategy in airway stem cells that may be used to treat most CF patients. This knowledge can be further used to develop airway stem cell therapies to treat other airway diseases.
