UCSF

Background

Glioblastoma (GBM) carries a poor prognosis, and new therapeutic strategies are necessary to improve outcomes for patients with this disease. Alkylating chemotherapies including temozolomide (TMZ) and lomustine (CCNU) are critical for treating GBM, but resistance mechanisms, including hypomethylation of O6-methylguanine-DNA methyltransferase (MGMT) promoter, undermine treatment. CRISPRoff is a programmable epigenetic memory editor that can induce stable and heritable gene silencing after transient delivery, and we hypothesize that CRISPRoff could potentiate the activity of TMZ and CCNU through long term suppression of target genes.

Methods

We transiently delivered CRISPRoff mRNA along with sgRNAs against target genes using both electroporation and lipid nanoparticles (LNPs) into established GBM cell lines, patient-derived primary GBM cultures, and orthotopic GBM xenografts. Gene repression, specificity, and stability was measured by RT-qPCR, Western blot, bisulfite sequencing, and RNA-sequencing. Sensitivity to chemotherapies was measured by cell viability dose response, microscopy, and bioluminescence imaging. Genome-wide mapping of CCNU sensitizers was performed using CRISPRi screens.

Results

CRISPRoff induced complete suppression of MGMT and sensitization to TMZ that were stable for over 8 months of continuous cell propagation. GBM orthotopic tumors treated with CRISPRoff against MGMT demonstrated sensitivity to TMZ in vivo, and CRISPRoff delivery resulted in chemosensitivity in patient-derived primary GBM. Genome-wide CRISPRi screens identified combinatorial genetic vulnerabilities (BRIP1, FANCE) that were targetable by multiplexed CRISPRoff to achieve sensitization to CCNU.

Conclusion

Transient delivery of a site-specific epigenetic memory can induce stable, complete, and multiplexed suppression of target genes for therapeutic application in GBM.