(316e) An Oncogene Activity-Dependent “Suicide Gene” Vector System for Selective Targeting of Cancer Cells

The aim of suicide gene therapy is to introduce to mammalian cells exogenous genes that drive the selective conversion of non-toxic molecules to lethal substances. The HSV-tk/GCV (Herpes simplex virus thymidine kinase/ganciclovir) prodrug system is one common suicide gene strategy. Integration of the HSV-tk gene into the host cell genome allows for phosphorylation of GCV, an acyclic analog of the 2’-deoxyguanosine nucleotide. Phosphorylated GCV competes with guanine for DNA integration during synthesis. Due to its acyclic structure, GCV is a poor substrate for DNA chain elongation and causes DNA damage and cell death¹. Thus, insertion of the HSV-tk gene in host cancer cell genomes, typically through viral vectors, and subsequent administration of GCV, enables selective cell killing. While this system has been studied extensively as a therapeutic strategy in cancer, discovering techniques to localize gene delivery to cancerous tissues, rather than healthy cells, has been challenging². This work outlines a new strategy in which HSV-tk expression is directly regulated by the activity of the extracellular regulated kinase (ERK), an oncogenic kinase that is commonly hyper-activated in cancer cells with mutation or upregulation of oncogenes such as RAF, RAS, and EGFR, among others. Consequently, while both cancerous and non-cancerous cells may be transduced by a viral delivery system, this system can be tuned such that only cancer cells displaying elevated ERK activity will have sufficient HSV-tk expression to induce cell death. By modifying an existing fluorescence-based reporter for ERK activity³, we engineered a viral vector that expresses a fusion of HSV-tk with a protein domain from the transcription factor fos-related antigen 1 (FRA1). ERK phosphorylation of specific sites within the FRA1 fragment slows the turnover of the HSV-tk fusion protein. Therefore, cells with high ERK activity express more of the suicide gene product and selectively die upon GCV administration. We have begun to explore the efficacy of this new system in glioblastoma multiforme cell lines to demonstrate its selectivity in killing cells with high ERK activity. Additional ongoing work is focused on demonstrating the vector’s usefulness in a heterogenous population of cancerous and healthy cells, with an ultimate goal of applying the strategy in vivo. The outlined suicide gene strategy is novel example of targeting an aberrant signaling process without perturbing the signal directly. This approach could prove beneficial because strategies that target ERK directly eventually result in resistant subpopulations that are highly aggressive and lead to recurrence.

1 - Fillat, C. et al. Current Gene Therapy, 3: 13-26, 2003.
2 – Karjoo, Z. et al. Advanced Drug Delivery Reviews, 99: 113-128, 2016.
3 – Albeck, J. et al. Mol Cell, 49: 249-61, 2013.