UBC Theses and Dissertations
A novel role for the Notch effector RBPJ in tumorigenesis Kulic, Iva
The Notch signaling pathway, which converges on RBPJ, is deregulated in a number of malignancies. Following pathway activation, RBPJ, the DNA-binding component of the pathway, associates with Notch to activate transcription of target genes. In the absence of Notch activity, RBPJ acts as a transcriptional repressor by recruiting a co-repressor complex that must be displaced to reinitiate the cycle of activation. As RBPJ is a key regulator of Notch signaling and is constitutively expressed in normal cells, we set out to evaluate the effect of RBPJ loss in the context of human cancer. Frequent RBPJ loss was detected in human breast and lung tumors. Moreover, depletion of RBPJ in a human breast cancer cell line accelerated xenograft tumor growth, whereas over-expression of a mutated version of RBPJ (which allows retained function as a transcriptional repressor but prevents activation via Notch) reduced tumor growth in a mouse model. These findings were confirmed in a lymphoma knock-out cell line, where a complete loss of RBPJ strikingly increased tumor growth in mice. RBPJ-deficient tumor xenografts showed up-regulated expression of HEY family genes, which represent direct canonical RBPJ targets. Blockade of Notch activation had no effect on the magnitude of HEY gene derepression in the absence of RBPJ, indicating that Notch does not participate in deregulated signal activation resulting from loss of transcriptional repression. To identify other aberrantly induced genes that contribute to the oncogenic phenotype with RBPJ loss, we performed a global analysis. RBPJ removal led to enrichment of acetylated histone H4 at induced gene promoters. We therefore used this epigenetic mark as an indirect measure of promoter activity to identify processes that were differentially active in the RBPJ-depleted breast cancer cells compared to RBPJ-containing controls. RBPJ loss enriched for a Notch-like signal and increased acetyl marks at genes associated with cell survival. Indeed, resistance to cell death was observed in RBPJ-deficient breast cancer and lymphoma tumors both in vitro and in vivo. This work defines a new role for RBPJ as a tumor suppressor, the loss of which represents an alternate mechanism for deregulating Notch signaling in cancer.
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