41Chromatin accessibility discriminates stem from mature cell populations, enabling the 42 identification of primitive stem-like cells in primary tumors, such as Glioblastoma (GBM) where 43 self-renewing cells driving cancer progression and recurrence are prime targets for therapeutic 44 intervention. We show, using single-cell chromatin accessibility, that primary GBMs harbor a 45 heterogeneous self-renewing population whose diversity is captured in patient-derived 46 glioblastoma stem cells (GSCs). In depth characterization of chromatin accessibility in GSCs 47 identifies three GSC states: Reactive, Constructive, and Invasive, each governed by uniquely 48 essential transcription factors and present within GBMs in varying proportions. Orthotopic 49 xenografts reveal that GSC states associate with survival, and identify an invasive GSC signature 50 predictive of low patient survival. Our chromatin-driven characterization of GSC states improves 51 prognostic precision and identifies dependencies to guide combination therapies. 52 53 56only an additional 2.5 months in the small subset of responsive patients [2]. Despite extensive 57 characterization and stratification of the bulk primary tumors, no targeted therapies have been 58 successfully developed [1,3]. GBM tumors are rooted in self-renewing tumor-initiating cells 59 commonly referred to as glioblastoma stem cells (GSCs)[4] that drive disease progression in 60 vivo [5,6] and display resistance to chemo-and radiotherapy leading to disease recurrence [7]. 61 The promise of therapeutically targeting self-renewing tumor-initiating cancer cells depends on 62 3 our capacity to capture the full range of heterogeneity within this population from individual 63 tumors. Intratumoral heterogeneity within primary GBM has recently been documented 64 through single cell RNA-seq experiments and revealed a continuum between four cellular 65 states[8]: neural-progenitor-like (NPC), oligodendrocyte-progenitor-like (OPC), astrocyte-like 66 (AC), and mesenchymal-like (MES)[8]. A subsequent study[9] using single-cell gene-centric 67 enrichment analysis placed GBM cells along a single axis of variation from proneural to 68 mesenchymal transcriptional profiles, with cells expressing stem-associated genes lying at the 69 extremes of this axis. Hence, primary GBM consists of distinct states, across which stem-like 70 cells appear to be found. Whether these stem-like cells found across GBM states represent 71 functionally distinct GSC populations with tumor-initiating properties and unique dependencies 72 remains to be established to guide therapeutic progress. To address this issue, we combined 73 single-cell technologies to define GSC composition in primary GBM with functional assays to 74 reveal the unique dependencies across GSCs, reflective of invasive, constructive and reactive 75 states that relate to patient outcome. 76 77
RESULTS
78Chromatin accessibility readily discriminates stem from mature cell populations [10], 79 which can be resolved at the single cell level thro...