Cancer stem cell targeted therapy: Removing the sword of Damocles (2007)

SY26-01 A growing body of evidence suggests that a primitive population of cancer stem cells (CSC) with unique self-renewal, proliferation, differentiation and survival potential is responsible for therapeutic resistance and relapse thus, providing a constant source of peril or a metaphorical Damocles’ sword for patients with cancer. The first compelling support for the existence of CSC stemmed from research that revealed that only a rare subset of acute myelogenous leukemia (AML) CD34+CD38- marrow or blood cells (1/104) transplanted leukemia in immunocompromised mice while the predominant more differentiated blast population did not. Similarly, studies performed with human breast cancer demonstrated that only CD44+CD24- cells, a small fraction (1-5%) of cells within the tumor, could transplant the cancer in cleared mouse mammary fat pads. Another report revealed that as few as 100 CD133+ brain tumor cells initiated human brain tumor formation in NOD/SCID mice thereby providing direct evidence for the existence of CSC in brain tumors. Recently, colon CSC were discovered. Although CSC have been identified in a number of malignancies, the precise developmental stage and sequence of molecular events involved in the initiation and propagation of human CSC remains an area of intense investigation that may help to elucidate novel therapeutic targets. To investigate the hematopoietic developmental stage and number of mutations required to give rise to CSC, we studied chronic myelogenous leukemia (CML). CML is a myeloproliferative disorder that progresses inexorably from a relatively well-tolerated chronic phase to accelerated phase and then to a blast crisis phase that is almost uniformly recalcitrant to therapy. As a result of a number of landmark discoveries, CML represents an important paradigm for understanding the multi-step pathogenesis of cancer. In a study involving 100 chronic myelogenous leukemia (CML) blood and marrow samples, we found that progression to blast crisis was typified by a relative expansion of the granulocyte-macrophage progenitor (GMP) pool. These blast crisis phase CML GMP had increased BCR-ABL transcripts endowing them with a proliferative advantage and had activated the Wnt/β-catenin pathway resulting in enhanced self-renewal capacity. As a result, these aberrantly self-renewing CML GMP established clonal dominance in bioluminescent immunocompromised mouse models and gave rise to BCR-ABL-positive human cells in serial transplantation experiments suggesting that they were enriched for CSC. Recently, we discovered that 3 of 5 blast crisis CML samples and 1 accelerated phase CML sample with enhanced in vivo self-renewal potential harbored misspliced transcripts of one of the major negative regulators of the Wnt/β-catenin pathway - GSK3β. In addition, there was a commensurate decrease in GSK3β protein expression with progression from chronic phase (n=3) to blast crisis (n=3) suggesting that epigenetic dysregulation of the Wnt/β-catenin pathway may endow CML GMP with the capacity to self-renew and thus, behave like CSC. Currently, extracellular and intracellular Wnt/β-catenin pathway antagonists are being tested in the bioluminescent CML CSC model to determine their capacity to inhibit CML GMP self-renewal at doses that spare normal HSC self-renewal capacity. Results gained from this research may be more broadly applicable to other cancers that have activated the Wnt/β-catenin pathway. Another key feature of CSC is their aberrant differentiation potential. While CML is typified by BCR-ABL driven myeloid differentiation another myeloproliferative disorder (MPD) polycythemia vera (PV) has enhanced erythroid differentiation potential as a result of JAK2 overexpression. Although CML progresses to acute leukemia, PV rarely progresses to leukemia and thus, a comparative analysis of the molecular pathogenesis of both diseases may provide further insight into the events that initiate leukemogenesis. Preliminary studies suggest that transcription factors involved in myeloid and eythroid cell fate decisions such as GATA-1 and PU.1 are differentially expressed at the stem cell level in myeloproliferative disorders and that availability of these factors may be determined in part by FLIP expression thereby providing a connection between cell fate decisions and stem and progenitor cell survival that may set the stage for the development of CSC. Finally, bcl-2 overexpression in mouse transgenic models as well as CML and AML patient samples has been identified as a key CSC survival pathway. In addition, the ability of CSC to evade innate immunity may also be important for CSC survival. We recently found that CD47, integrin associated protein, is over-expressed in CML CSC and may render these cells impervious to phagocytosis by macrophages. Thus, a detailed molecular understanding of the hematopoietic developmental stage at which mutations arise that give rise to CSC may provide the basis for developing highly active anti-CSC therapy with combinations of inhibitors that target vital CSC pathways.