Researches stem cells, with a focus on brain cancer stems cell that contribute to the initiation and maintenance of malignant brain tumors.
The discovery of stem cell-like cells (CSCs) in human tumors opens the possibility for a new generation of anti-cancer therapies that target this critical population of cancer cells responsible for therapy resistance and tumor recurrence. CSCs are a subpopulation of cancer cells that are endowed with the defining characteristics of stem cells (self-renewal and multipotentiality) and the unique ability to initiate a tumor when transplanted. In theory, a single CSC can re-initiate tumor formation or form metastatic tumors. Accumulating evidence indicates that CSCs are more resistant to chemo- and radiation-therapies compared to non-stem cancer cells from the same tumor through multiple molecular mechanisms, supporting the idea that targeting CSCs will improve long-term outcomes for cancer patients.
To discover unique vulnerabilities of glioma stem cells, we identified a gene signature, comprising 45 genes, that showed differential expression patterns in glioma stem cells when compared to normal neural stem cells and non-stem cancer cells. Many of these genes are independent prognostic indicators of glioma patient survival and are expressed in a subset of cells in human GBM tissues. We performed in-depth analysis of one of these genes, S100A4, and valiated that it is a novel marker and a regulator of glioma stem cells. We are currently pursuing its role in promoting glioma stem cell survival and self-renewal by elucidating its molecular mechanisms of action and its function in glioma stem cell interaction with its perivascular niche.
One of the confounding factors for developing therapies that target CSCs is the heterogeneity of CSCs even within the same clinical tumor type. We discovered that a major contributor to CSC heterogeneity in the Ptch+/- mouse model of medulloblastoma is the cell of origin. Using CSC culture phenotype as an initial identifier of CSC subtypes in Ptch+/- medulloblastomas, modeling the SHH subgroup of human medulloblastoma, we discovered that there are three distinct tumor subtypes in the Ptch+/- model. The three subtypes of tumors are molecularly distinct at the bulk tumor level and at the CSC level. We showed through cell type-specific activation of the SHH (sonic hedgehog) pathway in vivo that transformation of neural stem cells (NSCs) and neural progenitor cells (NPCs) generate CSCs that are molecularly and cellularly distinct, particularly in their sensitivity to SHH inhibitors. Our work shows that CSCs retain molecular (epigenetic) memories of their cell of origin and that the mitogenic pathway that drives CSC proliferation is the same pathway that drove its cell of origin (until tumor progression, which can render CSCs independent of exogenous growth factor signaling).
These observations have multiple implications for understanding molecular mechanisms of therapy resistance in CSCs. First, it demonstrates a novel mechanism of targeted therapy resistance in CSCs that does not require de novo mutations upon treatment. Second, it suggests that it may be possible to predict the types of mutations that will occur upon targeted therapy treatment, based on the cellular phenotype of CSCs in each tumor. We are currently testing this hypothesis.
Most driver oncogenes that transform normal cells were identified through cell culture studies that were highly oncogene-centric in their design. However, recent studies in our laboratory suggest that in living organisms, the ability of certain oncogenes to function is dependent on the cellular context. Using cell type-specific Cre drivers, we activated multiple oncogenes in the developing brain, either in NSCs or committed NPCs. Surprisingly, while some oncogenes could induce transformation of both cell types (such as SHH pathway activation described above), certain oncogenes such as Id2 and Notch1 could transform the most primitive cells (NSCs) but not more mature cells (NPCs). In fact, there was no observable phenotype in NPCs, indicating that in this cellular context, Id2 and Notch1 are not oncogenic at all. These studies indicate that the epigenetic state of the target cell may play a siginificant, and sometimes dominant, role in modulating oncogene function.
We are currently investigating the nature of epigenetic differences between NSCs and NPCs using purified populations and single NSC and NPC cells from developng mouse brain. The goal of this project is to elucidate differential vulnerability of cells in different states of maturation to oncogenic insults, DNA damage response, and cellular transformation so that we can better design and implement intervention strategies for preventing tumor formation or progression in the brain.
Recent studies indicate that classical developmental signaling pathways, such as the SHH, Notch, Wnt, and Hippo pathways, cross-regulate each other and play critical roles in CSC maintenance. We showed that Notch1 promotes NSC self-renewal in vivo in part by elevating the basal levels of limiting transcription factors in each of these pathways, including YAP1. YAP1 is a transcriptional effector of the Hippo pathway and is elevated in many human cancers, including brain tumors. We showed that YAP1 expression is sufficient to bypass the need for Notch pathway activation in NSC self-renewal in vitro. Our anlaysis of Yap1-/- mouse brain indicates that Yap1 is also required for normal brain development. Currently, we are testing its role in brain tumor initiation and progression. Furthermore, we are studying its molecular mechanism of function by identifying its binding partners and downstream targets in normal NSCs and brain tumor cells.