Identification of molecules that modulate adult neural and brain tumor stem cell biology

Funding Type: 
Basic Biology I
Grant Number: 
RB1-01385
ICOC Funds Committed: 
$0
Public Abstract: 
Tissue repair and regeneration critically depend on the recruitment and function of stem cells that reside within adult tissues. For example, neural stem cells are localized in distinct locations in the adult brain, the so called neurogenic niches. They are able to reproduce (self-renew) throughout life and to differentiate into functional cell types of the brain such as neurons, astrocytes and oligodendrocytes. Therefore, it may be possible to pharmacologically mobilize these stem cells to compensate for damage in the central nervous system, for example after stroke. Moreover, functional similarities exist between normal stem cells and so called cancer stem cells that can initiate aggressive brain tumors and fuel tumor growth. However, little is known about signals and factors controlling the balance between stem cell self-renewal and differentiation, and how normal stem cells can develop into a source of cancer. Taking the biological complexity of mechanisms regulating stem cells into account, unbiased large-scale screens are a promising tool for revealing key mechanisms controlling cell fate decisions in normal and cancerous brain stem cells in culture and ultimately in animal models. To probe these interrelated aspects of brain stem cell biology, we will use unbiased cell-based screens that identify small molecule modulators and genes which control the balance between human adult neural progenitor replication, growth and differentiation, and that induce differentiation or programmed cell death of patient-derived brain cancer stem cells. We will investigate the effects of newly identified lead compounds or genes on stem cell behavior and we will elucidate their specific mechanisms of action. Chemically optimized molecules will also be studied in animal models for example upon transplantation of human cancer stem cells into the brains of rodents or upon pharmacological treatment of mice showing neurological symptoms. This approach should provide new insights into those factors that control stem cell biology in normal and disease states, and may accelerate the development of novel and more effective therapeutic options for the treatment of aggressive brain tumors, as well as of neurodegenerative diseases.
Statement of Benefit to California: 
In the United States approximately 1.2 million people aged 18 years and older are diagnosed annually with adult onset brain disease or disorders including brain tumors (~35,000 people diagnosed annually), epilepsy (~135,000 people diagnosed annually), Huntington's disease (~30,000 people are currently living with the disorder), stroke (~600,000 people diagnosed annually), and traumatic brain injury (~80,000 people diagnosed annually). An estimated 13% - 16% of the United States and California households may currently be dealing with the burden of long-term care (medical and non-medical care) to a family member suffering from a brain disease. Hence, there is a pressing need for non-invasive treatment strategies and one such example is regenerative medicine in which new cells such as neurons and oligodendrocytes are generated to replace tissues lost to degenerative diseases or aging. So far, the pharmaceutical industry has been reluctant to invest into this approach and without government funding of basic research in regenerative medicine, progress is likely to be hindered. A regenerative therapy approach requires both an understanding of the genes and pathways that control stem cell biology and the identification and chemical optimization of drug-like small molecules promoting the mobilization of tissue stem cells, for example neural stem cells residing in the adult brain. To this end, we will use unbiased cell-based screens that provide a powerful strategy for identifying small molecule modulators and genes which control stem cell fate decisions such as self-renewal and differentiation. Given the overlapping developmental pathways shared by normal and cancer-initiating stem cells, we will pursue screens of both in parallel as they share common pathways, screening strategies and methodologies. The chemical and biological screens also complement each other well, are synergistic and have historically often provided distinct insights. Thus, our proposed strategy may shed new light on the link between stem cell biology, tumorigenesis, and chemical strategies for intervention in disease. By using biological and chemical approaches synergistically, we hope to accelerate the development of novel and more effective therapeutic options for treatment of brain diseases such as brain cancer and neurodegenerative disorders.
Progress Report: 
  • Induced pluripotent stem cells (iPSCs) hold great promise in regenerative medicine: these cells are similar to embryonic stem cells (ESCs) but can be derived upon “reprogramming” of any mature cell type isolated from a patient. Thus, tissue-specific stem cells derived from iPSCs and re-injected into the same patient may not trigger immune rejection. However, before the full potential of iPSCs is achieved, we need to learn how to better generate these cells, control their maturation into tissue-specific stem cells and progenitors, and harness their tumorigenic potential. Interestingly, ESCs and iPSCs share many characteristics of cancer cells, including their unlimited proliferation potential, and emerging evidence suggests that the mechanisms underlying the infinite proliferation of cancer cells and ESCs are intimately intertwined. Similarly, the progressions stages of tumorigenesis and cellular reprogramming to iPSCs share several characteristics, including changes in the packaging of the chromosomes.
  • Based on these observations, we proposed to directly study the function of a major cancer pathway, the RB pathway, in cellular reprogramming and iPSCs. RB is a key tumor suppressor in humans. RB acts as a cellular brake that restricts cell division but has several other cellular functions, including in the control of cellular maturation. When RB is mutated, cells divide faster and become more immature, two features of cancer cells, but also of cells undergoing reprogramming. We hypothesized that RB is an important regulator of cellular reprogramming and will test this idea using mouse and human cell types in culture. In the last year, we have performed experiments that largely support this hypothesis. We have found that, similar to its role in normal cell cycle, RB acts as a brake to normally restrict the reprogramming of cells into iPSCs. We have also found that RB is regulated in cells by enzymes that normally control the coating structure of chromosomes; these enzymes are thought to play a role in reprogramming, suggesting that RB may be a critical regulator of reprogramming by controlling the ability of reprogramming factors to modify the structure of the DNA. These experiments now provide a powerful system to analyze the molecular mechanisms underlying cellular reprogramming.
  • Our general goal is to better understand the differences and similarities between cancer cells and embryonic stem cells, to prevent tumor formation following stem cell transplantation but also to gain novel insights into the mechanisms of tumorigenesis and into the biology of embryonic stem cells. To this end, we have been studying how a tumor suppressor named Rb controls the dedifferentiation (or "reprogramming") of cells into induced pluripotent stem cells (iPS cells), which are similar to embryonic stem cells. (ES cells).
  • We have found that, similar to other tumor suppressors such as p53, Rb normally restricts the reprogramming process, both in human and mouse cells. We have also found that loss of RB does not change the proliferation rate of cells during reprogramming, suggesting that the enhanced efficiency of reprogramming observed in the absence of Rb is not due to a simple increase cell number. We are currently investigating the mechanisms by which Rb normally restricts the reprogramming process.
  • Our overarching goal is to understand the mechanisms controlling the balance between stem cell pluripotency, self-renewal, and tumorigenesis, to harness the full therapeutic promise of human embryonic stem cells (hESCs). To this end, we study the function of the RB gene family in stem cells. Our initial hypothesis was that RB family genes may control the reprogramming of somatic cells into iPSCs by interacting with chromatin remodeling factors to induce specific changes in the chromatin structure and control the expression of a specific program of genes. We found that loss of RB, but not of its family members p107 and p130, results in enhanced reprogramming of fibroblasts to iPS cells. In the past year, we have investigated this unique function of RB. In particular, we have performed high throughput RNA-seq and ChIP-seq experiments for RB early in the differentiation process to explore the mechanisms by which loss of RB may enhance reprogramming. We have also performed ChIP-Seq experiments with various chromatin marks to explore the relationship between RB loss and change sof the chromatin structure of cells early in reprogramming.
  • During the reporting period, we have pursued our work on the role of the retinoblastoma tumor suppressor during the reprogramming of mouse and human cells into induced pluriptoent stem cells (iPS cells). We have performed and analyzed genome-wide RNA-seq and ChIP-seq experiments to investigate how loss of RB promotes reprogramming. We have also tested candidate downstream mediators of RB in reprogramming using mouse genetics in vivo.

© 2013 California Institute for Regenerative Medicine