Within the adult mammalian brain the asymmetric distribution of cell-fate determinants during mitosis maintains a balance between differentiated and self-renewing cells. We have previously shown that disruption of asymmetric divisions in oligodendrocyte progenitor cells (OPC) causes cells to aberrantly self-renew and fail to differentiate, giving rise to oligodendroglioma (Sugiarto et al. Cancer Cell; 2011; 20; 320-40).
Here we proposed to address whether asymmetric divisions are also disrupted in precursors of malignant astrocytoma, a highly aggressive and therapy resistant type of glioma.
The constitutively active BRaf mutant BRafV600E is mutated in a subset of malignant astrocytoma alongside deletion of Cdkn2a, and can transform NSCs into astrocytoma precursors (Huillard et al. PNAS; 2012; 109; 8710-5). We used BRafCA mice (Dankort et al. Genes Dev. 2007; 21; 379-84) with Cre recombinase-activated expression of BRafV600E and deletion of Ink4a/Arf to generate an in situ model of NSC-derived malignant astrocytoma. In vivo studies to assess asymmetric division and related defects were complemented by studies of BRafV600E expressing, Ink4a/Arf-deleted neurosphere cells in cell-based assays of asymmetric division, self-renewal and differentiation.
Our results provide unprecedented evidence that BRafV600E, although uniformly expressed in NSC and their progeny, disrupts asymmetric divisions in a cell-type specific manner. NSCs continue to divide asymmetrically but their progeny, including OPC, divide more symmetrically and exhibit defects in differentiation and proliferation. These defects are reversible with BRafV600E-specific inhibitor PLX4720, and are associated with increased expression of the mitotic kinase polo-like kinase 1, increased pERK and decreased pAKT.
These data describe a mechanism by which NSC-targeted expression of BRafV600E may generate numerous subpopulations of tumor cells with distinct characteristics. This heterogeneity may mirror that seen in malignant astrocytoma, which has often been associated with high incidence of therapy resistance (Burger et al. Cancer. 1985; 56; 1106-11).
Citation Format: Robin G. Lerner, Yuichiro Ihara, Brian Reichholf, Dian Qu, Sista Sugiarto, Amelie Griveau, David James, Martin McMahon, Claudia Petritsch. Cell-specific effects of BRafV600E expression disrupt gliogenesis and lead to astrocytoma formation. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 2298. doi:10.1158/1538-7445.AM2013-2298