RBPJ May Be a More Effective Therapeutic Target Than NOTCH in GBM

Activation of stem cell pathways maintains brain tumor–initiating cells (BTIC) and contributes to glioblastoma (GBM) initiation and resistance. The NOTCH pathway is an attractive target in BTICs given its known role in maintaining stemness, but NOTCH inhibition has achieved only transient effects in glioma clinical trials. NOTCH activity is largely mediated by a central transcriptional mediator, recombining binding protein suppressor of hairless (RBPJ), prompting Xie and colleagues to investigate the role of RBPJ in BTICs. Unexpectedly, BTICs exhibited elevated expression and activation of RBPJ compared with differentiated cells, but NOTCH was not differentially activated. Consistent with these findings, NOTCH inhibition did not significantly affect BTIC proliferation, whereas RBPJ knockdown suppressed BTIC growth and self-renewal and inhibited tumor growth in vivo, altogether indicating the RBPJ is specifically required for BTIC maintenance. Targeting RBPJ had little effect on the expression of canonical NOTCH target genes, but reduced expression of genes regulating BTIC stemness and cell-cycle progression. The majority of RBPJ binding sites in the genome were within active promoters or enhancers, supporting a role for RBPJ as a transcriptional enhancer. Proteomic analysis of RBPJ binding proteins to identify downstream effectors identified cyclin-dependent kinase 9 (CDK9), a core component of the P-TEFb transcriptional elongation complex, which was also shown to promote BTIC growth and self-renewal. A high level of RBPJ/CDK9 co-occupancy was observed at gene promoters, and RBPJ knockdown reduced CDK9 occupancy at co-occupied sites and reduced RNA polymerase II binding at 3′ ends of genes, indicating a role for RBPJ in transcriptional elongation. The finding that RBPJ has NOTCH-independent functions in maintaining BTICs suggests that targeting RBPJ may be beneficial even when NOTCH inhibitors are ineffective.

Xie Q, Wu Q, Kim L, Miller TE, Liau BB, Mack SC, et al. RBPJ maintains brain tumor-initiating cells through CDK9-mediated transcriptional elongation. J Clin Invest 2016;126:2757–72.

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