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The transcriptional network of the zinc-finger factor ZEB1 and its function in the embryonic nervous system and glioma development

Final Report Summary - ZEB1 (The transcriptional network of the zinc-finger factor ZEB1 and its function in the embryonic nervous system and glioma development)

Current interest in neural stem/progenitor cells derives from the prospect of using them in brain repair strategies, but also to understand neurodevelopmental pathologies. This will require, however, a significant improvement of our understanding of the gene expression programs associated with their maintenance and differentiation, and how these are regulated. Recently, several lines of evidence suggest an important role for the zinc-finger transcription factor ZEB1, a classical inducer of epithelial-to-mesenchymal transition (EMT), in embryonic neural stem/progenitor cells, and in tumors of neural origin. The goal of the current project was to use a multidisciplinary approach, with a strong component of functional genomics, to gather novel insights into the function of ZEB1 in neural development and cancer.

In the developing mammalian brain, differentiating neurons mature morphologically via neuronal polarity programs. Despite discovery of polarity pathways acting concurrently with differentiation, it is unclear how neurons traverse complex polarity transitions or how neuronal progenitors delay polarization during development. By using gain- and loss-of-function approaches in the mouse cerebellum, we found that ZEB1 is necessary and sufficient to maintain Granule Neuron Progenitors (GNPs) in an undifferentiated, unpolarized, transiently amplifying state and to control the onset of their exit from the germinal layer. We performed a genome-wide characterization of ZEB1 target genes in GNPs, and found Zeb1 directly binds to and represses the transcription of multiple polarity and cell adhesion genes, such as Pard6a, Pard3A and Chl1. By using a functional screen, we show that restored expression of these genes rescues the defects in GNP differentiation, neurite extension, and exit from the germinal layer. Our results reveal unexpected parallels between the process of neurogenesis and EMT transition and identify a mechanism whereby ZEB1 controls the rate of neuronal differentiation by regulating the exit of neural stem/progenitor cells from the germinal layer.

GBM is a devastating and highly invasive brain tumor. GBM harbors a population of Cancer Stem Cells (CSCs) with the capacity to seed an entire new tumor, meaning that even the smallest number of such cells left after surgery will likely cause tumor recurrence. In addition, these cells have enhanced DNA repair programs, rendering them more resistant to radiation and conventional therapy. Thus, GBM CSCs are believed to play an essential role in treatment resistance and relapse of the disease, and are currently the focus of intense research in the field. We found ZEB1 to be highly expressed in GBM CSCs, and addressed its function in this cellular context. Recent evidence suggests ZEB1 promotes invasion, chemoresistance and tumorigenesis of GBM, however little is known on the molecular basis for its activities, and in particular on the identity of its target genes. We combined ZEB1 location analysis with transcriptional profiling upon gene knock-down, in order to characterize the ZEB1 transcriptional program in GBM CSCs. Surprisingly, we found that ZEB1 binding associates with both activation and repression of gene expression, and that this dual role results from two distinct modes of recruitment to regulatory regions. Expected binding to E-box sequences mediates target gene repression, while indirect recruitment via Lef1/Tcf factors occurs at promoters of genes activated by ZEB1. The latest include putative regulators of cell invasion/migration, such as Prex1. Moreover, transcriptional assays confirm ZEB1 strongly potentiates Lef1/Tcf-mediated activation of target gene promoters, and that this does not require active Wnt signaling. Finally, using transcriptomic data from publically available data bases, we show that Zeb1 and Prex1 expression are highly correlated in GBM tumors, and that elevated Prex1 expression correlates with poor GBM prognosis. Overall, our study reveals a novel mechanism whereby Zeb1 can activate and repress gene expression in the same cellular context, while providing an important molecular basis for the cellular activities of this important transcriptional regulator in GBM.