Restriction of γ34.5-Deleted Oncolytic Herpes Simplex Virus-1 in Glioblastoma Stem Cells and Therapies to Treat Them

Abstract

Glioblastoma (GBM) is a lethal brain tumor which frequently recurs after treatment. A stem cell like population of tumor cells, termed glioblastoma stem cells (GSCs), is theorized to cause recurrence. GSCs populate tumors with a heterogeneous mixture of transformed cells which make up the bulk of the tumor. Therapy removes the bulk of the tumor but fails to remove the GSCs, leading to recurrence. GSCs are a critical target for developing new glioblastoma therapies to cure patients. Our lab studies the effect of oncolytic herpes simplex virus-1 (oHSV) on bulk GBM tumor cells and GSCs. OHSV kills bulk GBM cells while sparing normal tissue due to the deletion of the HSV1 γ34.5 genes. Here we show that GSCs restrict γ34.5-deleted oHSV replication regardless of their genetic background or primary/recurrent status of the patient tumor they were isolated from. Interestingly, the patient-matched bulk tumor lines (SCCs) were permissive despite their similar genetics to the nonpermissive GSCs. We show that reprogramming SCCs into GSCs reconstitutes the stem cell characteristics of the cells as well as the restriction of γ34.5-deleted HSV1 replication. GSCs restrict γ34.5-deleted oHSV replication late into the infection by repressing the translation of viral true-late proteins. We show viral de novo DNA replication and transcription occur in GSCs, suggesting restriction occurs via repression of true-late HSV1 transcript translation. Expression of Us11 as an immediate-early protein complements the deletion of γ34.5 when expressed ectopically via lentivirus or through recombinant oHSV. Our observations suggest Us11 complementation of γ34.5-deleted HSV1 only occurs in GSCs if Us11 is robustly expressed. We further engineer an IE-Us11 expressing γ34.5-deleted oHSV which grows in GSCs, without altering its natural Us11 locus. Finally, We observe differences between GSCs and SCCs in response to CDK7 inhibitor THZ1. GSCs are more sensitive to THZ1 than patient-matched SCCs in vitro. Orthotopic implantation of GSCs into athymic mice causes a breakdown of the blood brain barrier which THZ1 counteracts. In summary, we demonstrate where GSCs restrict oHSV replication and provide insight into novel ways to develop new γ34.5-deleted oHSVs as well as the benefits of CDK7 inhibition for GBM.