ASCL1 Promotes Differentiation of Glioblastoma Stem Cells

Published OnlineFirst July 28, 2017; DOI: 10.1158/2159-8290.CD-RW2017-143

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kinase Inhibitors

Major finding: Sequential targeting of RAF, Concept: Concurrent RAF/MEK/ERK target- Impact: Combination therapy targeting MEK, or ERK in BRAF-mutant PDXs pro- ing prevents emergence of BRAF-amplified RAF, MEK, and ERK may improve outcomes motes BRAF amplification and resistance . subclones to suppress tumor growth . in patients with BRAF-mutant tumors .

siMuLtAnEous tArgEting of rAf, MEK, And ErK LiMits drug rEsistAncE BRAF mutations that hyperactivate ERK sign- ber occurred. Further, although treatment-naïve aling are found in a subset of patients with a patients responded to ERKi, patients who had variety of tumor types. Targeting RAF alone or in previously progressed on RAFi–MEKi combina- combination with the downstream MEK kinase tion therapy failed to respond to ERKi, altogether slows the growth BRAF-mutant tumors, but resist- suggesting that sequential therapy may promote ance eventually develops. ERK inhibitors (ERKi) resistance due to selection for increased BRAF copy have been developed to potentially treat these number. In contrast, combined treatment with patients. Xue, Martelotto, and colleagues used RAFi, MEKi, and ERKi resulted in suppression of single-cell DNA sequencing to track the evolution of resist- BRAF-amplifi ed tumor growth, and tumors that regrew after ance to ERKi in patient-derived xenograft (PDX) models of treatment discontinuation did not exhibit increased BRAF BRAFV600E-driven lung cancer and melanoma. ERKi initially copy number. Moreover, combined treatment on an intermit- suppressed the growth of 3 of 6 lung cancer PDXs, but tent schedule resulted in minimal toxicity and suppressed the resistance emerged within several weeks. Single-cell copy- growth of 11 of 11 lung cancer and melanoma PDXs. These number analysis revealed high-level BRAF amplifi cation in fi ndings suggest that concurrent targeting of RAF, MEK, and resistant PDXs that resulted in BRAFV600E overexpression. ERK may be more effective than sequential targeting in pre- BRAF amplifi cation emerged independently in multiple sub- venting the emergence of resistant BRAF-amplifi ed subclones, clones through parallel evolution and provided a growth supporting further investigation of this regimen in patients advantage in the presence of ERKi. Thus, ERKi selected for with BRAF-mutant tumors. n BRAF-amplifi ed cells, but intratumoral heterogeneity was maintained due to parallel evolutionary trajectories. When Xue Y, Martelotto L, Baslan T, Vides A, Solomon M, Mai TT, PDXs from patients treated with RAFi were subsequently et al. An approach to suppress the evolution of resistance in BRAFV600E- treated with ERKi, a progressive increase in BRAF copy num- mutant cancer. Nat Med 2017;23:929–37.

Glioblastoma

Major finding: ASCL1 promotes neuro- Mechanism: ASCL1 acts as a pioneer Impact: Patients with ASCL1hi glioblas- nal gene expression to induce differenti- factor to promote expression of neuro- toma stem cells may be sensitive to ation and suppress glioblastoma growth . genic program target genes . therapies that induce differentiation .

AscL1 ProMotEs diffErEntiAtion of gLiobLAstoMA stEM cELLs Glioblastoma is driven by a subpopulation of glioblas- resulted in tumor growth that was suppressed by the induc- toma stem cells (GSC) with enhanced tumor propagating tion of ASCL1 overexpression, thereby extending survival. potential. Thus, therapeutic strategies to promote GSC RNA-sequencing of ASCL1 wild-type and knockout GSCs differentiation may potentially suppress tumorigenesis. after GSI treatment revealed that ASCL1 upregulates neu- Notch signaling is responsible for maintaining neural stem rogenic program genes including TNR, MAP2, DLX2, and cells and inhibiting neuronal differentiation by suppressing DCX, and similar ASCL1 targets were identifi ed by ASCL1 target genes including the proneural transcription factor induction in knockout cells. Mechanistically, ASCL1 acted Achaete-scute homolog 1 (ASCL1). Park and colleagues as a pioneer factor, binding to closed chromatin at promot- found that ASCL1 has higher relative expression in a subset ers and enhancers of neuronal target genes to facilitate of differentiation-competent GSCs. ASCL1 was expressed in chromatin opening and target gene expression. In addition a binary pattern with 35 primary glioblastoma-derived GSC to demonstrating that ASCL1 promotes neuronal differ- cultures segregating into an ASCL1hi and an ASCL1lo sub- entiation of GSCs to suppress tumor growth, these fi nd- population. The ASCL1hi population was sensitive to Notch ings suggest the possibility for therapies that induce GSC signaling disruption with a gamma-secretase inhibitor differentiation, such as Notch inhibitors, in glioblastoma (GSI), exhibiting reduced sphere-forming ability, whereas patients with ASCL1hi GSCs. n the ASCL1lo population was unaffected. Further, ASCL1 was required for neuronal differentiation in response to GSI Park NI, Guilhamon P, Desai K, McAdam RF, Langille E, treatment, and induction of ASCL1 expression in ASCL1lo O’Connor M, et al. ASCL1 reorganizes chromatin to direct neuronal GSC cultures was suffi cient to induce neuronal differentia- fate and suppress tumorigenicity of glioblastoma stem cells. Cell Stem tion. In vivo, intracranial transplantation of ASCL1lo GSCs Cell 2017;21:209–24.e7.

September 2017 CANCER DISCOVERY | 931

00-CD-16-FM_Sep.indd 931 8/18/17 8:11 AM Published OnlineFirst July 28, 2017; DOI: 10.1158/2159-8290.CD-RW2017-143

ASCL1 Promotes Differentiation of Glioblastoma Stem Cells

Cancer Discov 2017;7:931. Published OnlineFirst July 28, 2017.

Updated version Access the most recent version of this article at: doi:10.1158/2159-8290.CD-RW2017-143

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