8038 Targeting Breast Cancer Stem Cells With Oncolytic Mammalian Orthoreovirus (MRV)
Authors: Roman Ortiz Natasha Ivelisse , Davydova Julia , Danthi Pranav , Hanson Ostrander Julie 
Published: October 2024

Disclosure: N.I. Roman Ortiz: None. J. Davydova: None. P. Danthi: None. J.H. Ostrander: None.

Breast cancer is the most frequently diagnosed malignant cancer in women and is the second leading cause of cancer deaths among women in the United States. Patients with ER+ breast cancer can experience recurrence for more than 10 years after initial diagnosis. Breast cancer stem-like cells (BCSC) are slowly proliferative and exist as a minority sub-population (0-5%) within the tumor. Existing treatments target rapidly dividing cells, and BCSC are unresponsive to these therapies. Due to their self-renewal capabilities, BCSCs can fuel recurrence, metastasis, and resistance to therapies. Notably, no FDA-approved therapeutics selectively target BCSCs. Mammalian orthoreovirus (MRV) is an oncolytic virus tested in clinical trials for many cancer types including metastatic breast cancer. MRV was deemed safe but lacked efficacy as standalone treatment. We tested various laboratory MRV strains (T1L, R2) with T3D, a similar strain to the one evaluated in clinical trials. Our studies suggest that T3D is less effective in killing ER+ breast cancer cells and BCSCs compared to T1L and R2. To understand how T1L and R2 are more effective at killing ER+ breast cancer we aim to test the stages of MRV infection and how MRV induces cell death in ER+ therapy sensitive and resistant cells. MRV can enter cells by receptor mediated endocytosis through attachment with Junctional Adhesion Molecule A (JAM-A) and sialic acids as co-receptors. We have found that T1L attaches to cells in a JAM-A-dependent manner in ER+ sensitive cancer cells, while ER+ therapy resistant cells have increased resistance to JAM-A mediated endocytosis. Previous work has shown that apoptosis is involved in MRV-induced cell death. We found that apoptosis inhibitors in MRV-treated cells increase cell viability, suggesting that MRV-induces apoptotic cell death in ER+ breast cancer cells. Our current studies involve using RNA sequencing as a tool to assess gene expression of ER+ therapy sensitive and resistant cells infected with MRV at early and late stages of infection to determine key genes and pathways associated with cell death.We also aim to generate novel MRV strains with enhanced BCSC killing. We used a forward genetic approach and serially passaged MRV strains in BCSC enriched tumorsphere cultures of MCF-7 paclitaxel resistant (TaxR) cells to generate a selective oncolytic virus that can infect and kill BCSCs. Preliminary data has shown that serially passaged (SP) MRV strains (T1L SP, R2 SP) are more effective in decreasing cell viability and inhibiting tumorsphere formation compared to the parental strains (T1L and R2). Current studies are focused on sequencing SP clones to identify mutations that enhance BCSC death. We expect these studies to enhance MRV virotherapeutics for combined use with clinically relevant inhibitors (i.e. CDK4/6 inhibitors), anti-estrogens, or immunotherapy to prevent and treat metastatic ER+ breast cancer.

Presentation: 6/2/2024