From stem cell to brain tumor: a genetic analysis

Stem cells have a unique property: They can generate daughter cells that remain stem cells and undergo multiple rounds of self-renewal. At the same time, they give rise to more specialized cell types that ultimately replace cells in the target tissue. While this property makes stem cells an ideal source of cells for regenerative medicine, it also means that the balance between those cell types needs to be precisely controlled. If stem cells are defective and generate only daughter cells, this leads to uncontrolled cellular amplification and it has been proposed that defects like this can contribute to the formation of certain tumors. This project uses the fruitfly Drosophila to understand the mechanisms that control this balance in stem cells of the brain. We focus on the so-called type II neuroblasts, neural stem cells that divide asymmetrically into one cell that remains a neuroblast while the other becomes an intermediate neural progenitor (INP). The INP again divides asymmetrically but now, one cell becomes a ganglion mother cell (GMC) which divides once more into two differentiating neurons Defects in lineage progression lead to the formation of a brain tumor. In wild type adult flies, all neural stem cells have differentiated into neurons. In tumor bearing flies, however, the brain is full of proliferating stem cells. In this project, we use type II neuroblasts as a model for stem cell biology and to understand, how stem cells can contribute to tumor formation. In a previous project, we have used an RNAi approach to identify 620 genes that control the multiple differentiation steps that occur in this lineage. We also have succesfully developed a method to isolate Drosophila neuroblasts and differentiating neurons and to determine all genes that are active in those cells. In this project, we build upon those previous results and characterize the most interesting hits from the RNAi screen. In particular, we use the new ability to do transcriptome analysis to characterize the many epigenetic regulators that were identified in our screen. We have evidence that the crucial steps in tumor formation are regulated by purely epigenetic means and want to analyze, how the SWI/SNF chromatin remodeling complex prevents tumor formation in type II neuroblasts. Three members of this complex were identified in our screen, indicating an essential function for this important chromatin regulator. We will identify the key target genes and determine, how this complex regulates their activity. For this, we will establish a cell culture system to characterize the most important pathway regulating type II neuroblasts, the Notch/Delta pathway. In a complementary approach, we will determine the precise stages of tumor formation and ask, when the various tumor suppressors exert their function. Together, we hope that these experiments will allow us to understand the mechanisms of stem cell derived tumor formation at a level of detail not possible in other model organisms.

The recent years have seen a major change in our view of tumor development. While it was traditionally thought that tumors are assemblies of cells that have lost the ability to control growth and division but otherwise do not show any cellular hierarchies, it is now becoming increasingly clear that at least some tumors contain stem cells just like normal organs do. In this so-called tumor stem cell hypothesis, all cells in a tumor arise from those stem cells in a cellular hierarchy that is similar to many of the organs in a healthy body. This has strong implications for tumor therapy as any successful cancer therapy needs to target those tumor stem cells to avoid that they regenerate the tumor mass after successful surgery. This project continues a collaboration with the laboratory of Prof. Heinrich Reichert (University of Basel) use the fruitfly Drosophila melanogaster as a model system to analyze tumor stem cells, how they arise and what distinguishes them different from normal stem cells. In fruitflies, all neurons in the adult brain arise from stem cells called neuroblasts. These neuroblasts generate a precisely defined number of neurons, but also more stem cells to maintain the stem cell pool. When the balance between those cell types is disrupted, brain tumors arise that resemble human cancer in a striking manner. In particular, pieces of tumors can be transplanted into other flies where they grow in an uncontrolled manner and will kill the host. The sophisticated genetic tools available in fruitflies have allowed us to investigate mechanisms responsible for tumor development in flies.Within this project, we could demonstrate that the so-called SWI/SNF complex plays a crucial role for tumorigenesis. When it is absent, the cellular hierarchy within a neural stem cell lineage is inverted: Cells already destined to become neurons will revert back into additional stem cells, leading to uncontrolled expansion of the stem cell pool and ultimately to tumorigenesis. In a second set of experiments, we uncovered the mechanism by which stem cells within the fly brain exit the cell cycle and differentiate at a given time point during development. We could show that characteristic modifications of energy metabolism mediated by the Mediator complex within neural stem cells lead to a reduction in cellular growth. They uncouple cell growth from cell division and cause a progressive reduction in cell size and ultimately induce terminal differentiation. Ongoing investigations analyze the functional conservation of those mechanisms in mammals and have the potential to reveal further insights into the early stages of human tumorigenesis.