Bmi1 Controls Tumor Development in an Ink4a/Arf-Independent Manner in a Mouse Model for Glioma

SummaryThe Polycomb group and oncogene Bmi1 is required for the proliferation of various differentiated cells and for the self-renewal of stem cells and leukemic cancer stem cells. Repression of the Ink4a/Arf locus is a well described mechanism through which Bmi1 can exert its proliferative effects. However, we now demonstrate in an orthotopic transplantation model for glioma, a type of cancer harboring cancer stem cells, that Bmi1 is also required for tumor development in an Ink4a/Arf-independent manner. Tumors derived from Bmi1;Ink4a/Arf doubly deficient astrocytes or neural stem cells have a later time of onset and different histological grading. Moreover, in the absence of Ink4a/Arf, Bmi1-deficient cells and tumors display changes in differentiation capacity

A new transgenic mouse model for conditional overexpression of the Polycomb Group protein EZH2

The Polycomb Group protein EZH2 is upregulated in most prostate cancers, and its overexpression is associated with poor prognosis. Most insights into the functional role of EZH2 in prostate cancer have been gained using cell lines and EZH2 inactivation studies. However, the question remains whether overexpression of EZH2 can initiate prostate tumourigenesis or drive tumour progression. Appropriate transgenic mouse models that are required to answer such questions are lacking. We developed one such transgenic mouse model for conditional overexpression of Ezh2. In this transgene, Ezh2 and Luciferase are transcribed from a single open reading frame. The latter gene enables intravital bioluminescent imaging of tissues expressing this transgene, allowing the detection of tumour outgrowth and potential metastatic progression over time. Prostate-specific Ezh2 overexpression by crossbreeding with Probasin-Cre mice led to neoplastic prostate lesions at low incidence and with a long latency. Compounding a previously described Bmi1-transgene and Pten-deficiency prostate cancer mouse model with the Ezh2 transgene did not enhance tumour progression or drive metastasis formation. In conclusion, we here report the generation of a wildtype Ezh2 overexpression mouse model that allows for intravital surveillance of tissues with activated transgene. This model will be an invaluable tool for further unravelling the role of EZH2 in cancer

Ink4a and Arf differentially affect cell proliferation and neural stem cell self-renewal in Bmi1-deficient mice

The Polycomb group (PcG) gene Bmi1 promotes cell proliferation and stem cell self-renewal by repressing the Ink4a/Arf locus. We used a genetic approach to investigate whether Ink4a or Arf is more critical for relaying Bmi1 function in lymphoid cells, neural progenitors, and neural stem cells. We show that Arf is a general target of Bmi1, however particularly in neural stem cells, derepression of Ink4a contributes to Bmi1(-/-) phenotypes. Additionally, we demonstrate haploinsufficient effects for the Ink4a/Arf locus downstream of Bmi1 in vivo. This suggests differential, cell type-specific roles for Ink4a versus Arf in PcG-mediated (stem) cell cycle control

GFAP-Cre-Mediated Transgenic Activation of Bmi1 Results in Pituitary Tumors

Bmi1 is a member of the polycomb repressive complex 1 and plays different roles during embryonic development, depending on the developmental context. Bmi1 over expression is observed in many types of cancer, including tumors of astroglial and neural origin. Although genetic depletion of Bmi1 has been described to result in tumor inhibitory effects partly through INK4A/Arf mediated senescence and apoptosis and also through INK4A/Arf independent effects, it has not been proven that Bmi1 can be causally involved in the formation of these tumors. To see whether this is the case, we developed two conditional Bmi1 transgenic models that were crossed with GFAP-Cre mice to activate transgenic expression in neural and glial lineages. We show here that these mice generate intermediate and anterior lobe pituitary tumors that are positive for ACTH and beta-endorphin. Combined transgenic expression of Bmi1 together with conditional loss of Rb resulted in pituitary tumors but was insufficient to induce medulloblastoma therefore indicating that the oncogenic function of Bmi1 depends on regulation of p16(INK4A)/Rb rather than on regulation of p19(ARF)/p53. Human pituitary adenomas show Bmi1 overexpression in over 50% of the cases, which indicates that Bmi1 could be causally involved in formation of these tumors similarly as in our mouse mode

Akt-mediated phosphorylation of Bmi1 modulates its oncogenic potential, E3 ligase activity, and DNA damage repair activity in mouse prostate cancer

Prostate cancer (PCa) is a major lethal malignancy in men, but the molecular events and their interplay underlying prostate carcinogenesis remain poorly understood. Epigenetic events and the upregulation of polycomb group silencing proteins including Bmi1 have been described to occur during PCa progression. Here, we found that conditional overexpression of Bmi1 in mice induced prostatic intraepithelial neoplasia, and elicited invasive adenocarcinoma when combined with PTEN haploinsufficiency. In addition, Bmi1 and the PI3K/Akt pathway were coactivated in a substantial fraction of human high-grade tumors. We found that Akt mediated Bmi1 phosphorylation, enhancing its oncogenic potential in an Ink4a/Arf-independent manner. This process also modulated the DNA damage response and affected genomic stability. Together, our findings demonstrate the etiological role of Bmi1 in PCa, unravel an oncogenic collaboration between Bmi1 and the PI3K/Akt pathway, and provide mechanistic insights into the modulation of Bmi1 function by phosphorylation during prostate carcinogenesis

IHC analysis of pituitary hormones in Gcre; Bm1^LSL induced tumors.

<p>n.d., not done, n.o., not observed.</p

Transgenic expression of Bmi1 is sufficient to induce intermediate and anterior lobe pituitary tumors but does not induce medulloblastoma or glioma.

<p>(A) Kaplan Meier survival curves of mice carrying tumors because of GCre induced transgenic expression of Bmi1 complemented with loss of Rb. These data show that Bmi1 transgenic mice develop pituitary tumors after about one year. Pituitary tumors are also observed in Rb^Lox/Lox and Bmi1^LSL;Rb^Lox/Lox transgenic mice. (B) Histograms showing the relative frequency and penetrance of tumors generated by the individual genotypic groups. All genotypes shown are GCre positive. Total cohort size: GCre;Bmi1^LSL n = 14, GCre;Rb^Lox/Lox n = 20, GCre;Rb^Lox/Lox;Bmi1^LSL n = 12, GCre;p53^Lox/Lox;Rb^Lox/Lox n = 6, GCre;p53^Lox/Lox;Rb^Lox/Lox; Bmi1^LSL n = 4, WT mice did not develop tumors, n = 7. (C) IHC results showing transgenic Bm1 expression in tumors raised on a GCre;Bmi1^LSL background (5/5), no expression in tumors raised on a Gcre;Rb^lox/lox background (0/5) while some of the GCre;Bmi1^LSL; Rb^lox/lox mice were positive (2/5, 40%). These results are summarized in a histogram (D). Bar is 50 µm.</p