Approaching DIPG from Neural Stem Cell Biology
Diffuse intrinsic pontine gliomas (DIPG) occur strictly in the ventral pons and typically during a relatively specific period during mid-childhood, peaking between ages 6 and 8. The age and location-specific nature of DIPG suggests that the underlying pathophysiology may involve dysregulation of a developmental process. In this context, it makes sense to approach DIPG from the vantage of neural stem and precursor cell biology.
Normal Neural Stem Cells
Neural stem cells – cells that can renew themselves and also can make all types of neural cells (neurons, oligodendrocytes and astrocytes) – are well-recognized in the brain and spinal cord of both children and adults. Two populations of neural stem cells are very well studied. These two populations reside in the hippocampus, a brain structure important in memory function, and in what is called the subventricular zone (i.e. just below the ventricular walls) of the lateral ventricles. It is known that, at least in mice and rats, stem cells exist throughout the ventricular system of the brain and spinal cord, but little attention has been paid to those in the subventricular zone of the third and fourth ventricles. The fourth ventricle sits immediately behind the pons. The term “neural precursor cell” includes both true stem cells and cells that are somewhat further along the path of differentiation but still give rise to daughter cells. Both types of cell—stem and “precursor” are important to developmental processes in the brain both before and after birth.
Cancer Stem Cells
Cancer stem cells (CSCs) represent a subpopulation of cells that can generate all cell types found within a tumor and are thought to be responsible for tumor growth and spread. Like normal stem cells, cancer stem cells possess the capacity for self-renewal and multi-potency. (Multi-potent cells can make all the cell types in a tissue. In this case the tissue is the tumor.) The first cancer stem cells were described in acute myeloid leukemia, and have now been shown in many solid tumors, including many brain tumors such as glioblastoma and ependymoma. CSCs isolated from primary brain tumors possess many of the characteristics of normal neural stem cells, and can recapitulate the tumor in vitro and in vivo, whereas other cell types from the tumor cannot. CSCs are thus a small proportion of a tumor, but are solely responsible for tumor propagation.
The relationship of normal neural stem cells to cancer stem cells is somewhat controversial, but there is an emerging consensus that many brain tumors arise from stem or precursor cell populations in both children and adults. Excellent examples of this point include “radial glia” cells (a type of stem cell) giving rise to ependymoma and subventricular zone neural stem cells giving rise to central neurocytomas. With respect to more lineage-restricted precursors, Shh-responsive granule cell precursor cells of the cerebellum give rise to medulloblastoma in many cases, and recent animal model data indicate that oligodendrocyte precursors give rise to periventricular low grade gliomas in a mouse model of platelet-derived growth factor (PDGF) overexpression. Brain tumor stem cells exhibit many of the same marker proteins and utilize many of the same signaling pathways as normal neural stem cells. Understanding normal neural stem or precursor cells in the brainstem may thus shed light on brainstem tumor pathogenesis.
What Stem Cells Need to Thrive—the “Stem Cell Niche”
Normal Stem Cell Niche
Neural stem cells in the childhood and adult nervous system reside in a niche of signaling factors, extracellular matrix composition and specialized cell types that support neural stem cell function for that brain region. Perhaps best studied is the stem cell niche that supports forebrain neurogenesis in the hippocampus. This specific microenvironment necessary for stem cell production of new neurons is referred to as the neurogenic niche. Transplantation experiments demonstrate that neurogenesis is restricted in the postnatal brain to regions in which it occurs naturally, namely the subventricular zone (SVZ) and the subgranular zone (SGZ) of the hippocampus. In general, microenvironmental determinants of neurogenesis include the presence of the trophic signals required for progenitor cell proliferation, differentiation and survival, and the absence of inhibitory factors. Neural stem/precursor cells form a close anatomical relationship with the small vessels in the neurogenic region, and this neurovascular relationship—the so-called “vascular niche”—is believed to be crucial not only for nutritional but also for growth factor support. Vessel cells (endothelial cells, pericytes) and glial cells (astrocytes) all contribute to the stem cell niche. Hippocampal astrocytes play key roles in creating and maintaining the neurogenic niche. As noted above, many of the signaling pathways central to prenatal neural development are conserved in postnatal neurogenesis, including pathways called Wnt, Shh, and Notch. Additional molecules with potent pro-neurogenic effects include fibroblast growth factor (FGF), vascular endothelial growth factor (VEGF) and certain neurotransmitters. An important negative regulator of the neurogenic microenvironment is microglial cell inflammation, particularly in disease states. Pro-inflammatory cytokines elaborated by microglial cells in certain states of activation, including IL-6 and TNF-alpha, inhibit neurogenesis via a specific blockade in neuronal differentiation mediated by Notch signaling, as well as a non-specific increase in precursor cell death. The effects of inflammatory cells on neurogenesis are complex and depend on the microglial phenotype involved; microglia stimulated by cranial irradiation or systemically-administered lipopolysaccaride (LPS, also known as endotoxin) inhibit neurogenesis, while microglia stimulated by IL-4 or interferon gamma promote neurogenesis.
Cancer Stem Cell (CSC) Niche
Just as cancer stem cells share many properties with normal stem cells, so the cancer stem cell niche is similar to the normal stem cell niche. The vascular niche appears to be recapitulated in human brain tumors. Cancer stem cells, defined molecularly by expression of the proteins CD133 and Nestin, are localized in close proximity with tumor microvessels in human medulloblastoma, glioblastoma, oligodendroglioma and ependymoma. The relationship between cancer stem cells and tumor microvessels is bidirectional: glioblastoma cells induce angiogenesis (new vessel cell growth) via VEGF elaboration, and vascular endothelial cells supports glioblastoma cell tumoriogenicity. Treatment of a mouse orthotopic glioblastoma model with the VEGF blocking agent bevacizumab (Avastin) depletes CD133+ cells, decreases tumor vascularity and reduces tumor growth rate. Accordingly, bevacizumab has shown modest clinical efficacy in glioblastoma, at least in adult glioblastoma of the forebrain. Highlighting the differences between DIPG and adult glioblastoma, bevacizumab is not efficacious for DIPG. Important determinants of the DIPG cancer stem cell niche are yet to be defined.
Stem Cells in DIPG
Cell of Origin
Presently, intense research is underway to identify the cell type in the normal childhood pons from which DIPGs originate. The cell type that transforms and gives rise to DIPG could be a neural stem cell, a neural precursor cell type (that is destined to give rise to glial cells or neuronal cells) or a differentiated cell type (glia or neurons). Lessons learned from other pediatric brain tumors teach us that the most likely candidate would be a neural stem or precursor cell. Neural stem and precursor cells are not well described in the brainstem, but current research will soon shed light on candidate cells of origin for DIPG. Understanding the cell of origin for DIPG is of fundamental importance to elucidate mechanisms by which DIPG may form, and thus potential targets for treatment.
DIPG Cancer Stem Cell
Researchers are working to identify and characterize a cancer stem cell in DIPG. This research requires fresh tumor samples for cell culture, and scarcity of tissue for research has limited progress in this area until very recently. Donation of tumor in the early post-mortem period after the loss of a child allows for successful cell culture of both normal brain and brain tumor tissue. This strategy can allow crucial research to be done without putting a child through an additional procedure such as a biopsy. Identifying and studying the cancer stem cell of DIPG may elucidate new targets for therapy that are at the core of DIPG growth and propagation.
A Few Words on Hematopoietic Stem Cell and Bone Marrow Transplant
Hematopoietic stem cell transplant (HSCT) and bone marrow transplant are designed to rescue the bone marrow after intensive chemotherapy or to provide cell replacement therapy for certain genetic diseases. At present, there is no role for either HSCT or bone marrow transplant for DIPG.