Introduction: The transcription regulatory protein, inhibitor of DNA-binding 1 (ID1), has been implicated as a key regulator of cell phenotype in glioblastoma (GBM). High ID1 expression has been shown to confer chemoresitancy and be associated with poorer outcome in multiple solid tumors. In our study, we investigated the role of ID1 in tumor initiation, tumor growth, and chemoresistance, in glioblastoma and other solid tumors.
Methods: We utilized CRISPR-Cas9 to knockout ID1 in GBM (U251), breast cancer (MDA-MB-231) and melanoma (A375) cell lines. We preformed in vitro studies and mouse xenograft experiments to characterize the role of ID1 in tumor initiation, tumor growth, and chemoresistance.
Results: We identified a clinically available inhibitor of ID1, the FDA approved drug pimozide, and studied pimozide in combination with temozolomide (TMZ) in vitro and in vivo. ID1 is upregulated in response to TMZ in various glioma cancer stem cell and GBM cell lines. ID1 knockout U251 cells displayed significantly lower instance of cell survival following TMZ chemotherapy in comparison to control. Concurrent treatment with pimozide inhibited ID1 expression and enhanced the cytotoxic effect of TMZ.
ID1 knockout in U251 cells resulted in a three-fold increase in survival compared to control U251 in a mouse intracranial xenograft model due to a substantial delay in tumor formation. Moreover, ID1 knockout in breast cancer and melanoma resulted in significantly slowed tumor growth and smaller tumors. Concurrent treatment of a U251 mouse xenograft with pimozide and TMZ resulted in significantly lower tumor weight and volume compared to treatment with TMZ alone. Further, combined therapy resulted in significantly delayed tumor recurrence and increased survival.
Conclusions: Our results suggest that ID1 is responsible for tumor initiation and tumor growth in GBM, breast cancer and melanoma. Furthermore, ID1 acts as a pro-survival protein in GBM following chemotherapy. ID1 inhibition using pimozide enhances the cytotoxic effect of TMZ in glioblastoma and delays tumor recurrence following chemotherapy. Our findings identify ID1 as a therapeutic target in patients with glioblastoma.
Patient Care: We have identified a therapeutic target that could enhance the effect of temozolomide therapy in patients with GBM.
Learning Objectives: 1. To understand the role of ID1 in temozolomide resistancy and tumor progression in glioblastoma; 2. To understand the effect of chemical and genetic inhibition of ID1 in glioblastoma.