Sources of CNS tumor heterogeneity

Central nervous system tumors are marked with high intra-and inter-tumoral heterogeneity. For instance, an integrative large scale gene expression study performed in 2010 revealed 4 subtypes glioblastoma multiforme (GBM) with differential responses to treatment and prognosis [1]. A more comprehensive GBM classification based on combination of epigenetics, copy number variation, gene expression and genetic mutation analysis has led to identification of as many as 6 GBM subgroups [2]. Two, not mutually exclusive, general models have been proposed to explain tumor heterogeneity [3]. The genetic mutation model proposes that different genetic mutations lead to different tumor formation, while the cell of origin model explains different tumors as arising from different cell types. There is experimental evidence supporting both models. For example, 3 distinctly different CNS tumor types can be induced by infection of postnatal mouse neural stem cells with virus containing V12HRAS and c-MYC depending on the combination and sequence in which oncogenes are introduced [4]. Similarly, RNA interference (RNAi) knock down of NF1 and p53 in GFAP+ or SynI+ cells induces mesenchymal GBM, whereas the same RNAi in Nestin+ cells induced neural GBM[5]. GMB gene expression analysis also indicates that different GBM subtypes have transcipt profiles similar to different cell types [1]. To fully explore the causes of tumor diversity, it is desirable 2 to have an animal model in which both the cell of origin and genetic insult can be conveniently and independently manipulated. To achieve this, we have recently developed a central nervous system tumor model in the rat in which multiple oncogenes can be expressed in selected cell populations at different times in brain development [6]. In this model, we used the piggyBac transposon system [7] to stably integrate oncogenes into defined cell populations by in utero electroporation (IUE). Using this model, we evaluated the contribution of cell of origin and genetic mutation in tumor heterogeneity. To test whether the same oncogenic event in different, but closely related, cell population gives rise to same or different tumors, we directed HRasV12/AKT expression in disparate cell populations in the radial glia lineage with promoters that are ubiquitously active (CAG promoter), astrocyte selective GFAP (glial fibrillary acidic protein) promoter or oligodendrocytes selective MBP (myelin basic protein) promoter. We showed that HRasV12/AKT expression under CAG or GFAP promoter induced similar tumors, glioblastoma multiforme (WHO grade 4). However, HRasV12/AKT expression controlled by MBP promoter induced anaplastic oligoastrocytoma (WHO grade 3). We further …