(505b) Single-Cell Analysis of Drug-Induced Proneural-to-Mesenchymal Transition in Patient-Derived GBM Stem-like Cells

Non-genetic cell phenotype plasticity can lead to the formation of aggressive tumor cell phenotypes without the acquisition of mutations. An unfortunate consequence of such plasticity is treatment-induced state transitions of tumor cells into drug-resistant states. In the context of glioblastoma (GBM), the most lethal primary brain tumor in adults, one way in which phenotypic plasticity manifests both experimentally and clinically is the transition of GBM tumor cells from a proneural subtype to the more aggressive and drug-resistant mesenchymal subtype. Such transitions have also been observed in GBM stem-like cells (GSCs), a rare subpopulation within the tumor that exhibit stem cell-like properties like initiating capabilities, making them a clinically relevant target for preventing GBM recurrence. Here, we investigate at the single-cell level the sequence of transcriptional regulatory mechanisms underlying treatment-induced subtype transitions in two patient-derived GSC (PD-GSC) populations that display distinct sensitivity profiles to the drug pitavastatin, a statin previously shown to have anti-proliferative effects on glioma cells. Our results show that treatment triggers a coordinated sequence of regulatory events that drives a small subpopulation of GSCs into the mesenchymal phenotype, which results in a corresponding increase in drug resistance. By applying single-cell level network inference, we identified specific transcription factors driving the treatment-induced mesenchymal transition and verified their role via siRNA knock-down experiments. The sequence of regulator activity is distinct from previously reported master regulators involved in mesenchymal transitions. Our results highlight the need to understand the non-genetic mechanisms associated with cell state transitions and may open an avenue toward the development of new therapeutic strategies aimed at preventing treatment-induced resistance in GSCs.