1 INTRODUCTION TO CANCER BIOLOGY
1.3 TUMOR HETEROGENEITY AND CANCER STEM CELLS
As previously mentioned, tumors often exhibit heterogeneous features in such as morphology, gene expressions, metabolism, motility, proliferative and metastatic potential18, 40. This heterogeneity can arise from the clonal evolution, phenotypic plasticity of cells and differentiation of cancer stem cells (CSCs).
As discussed above (see 1.2), tumor clonal evolution is one of the sources heterogeneity in which distinct subclones emerge among the tumor due to genetic or epigenetic alterations, hence resulting in phenotypical heterogeneity. Clonal heterogeneity has been demonstrated in many cancer types such as breast, leukemia, prostate, colon, brain18. However, genetic studies for clonal differentiation are still limited by the detection methods (e.g. low resolution, high cost and labor intensive) or by sampling quantity and/or quality.
Another source of heterogeneity is the phenotypic plasticity of cells which is one of the basic properties of organisms. These phenotypic changes could stem from experiencing different microenvironment such as variable composition of extracellular matrix or blood vasculature densities. Gene expression variabilities could also cause the phenotypic changes in cells even though cells are in an homogenous environment and have the same genetics41. EMT is one of the examples for reversible phenotypic change and cells in mesenchymal state are more prone to form tumors than those in epithelial state in breast cancer42.
The most intriguing concept for heterogeneity is the cancer stem cell concept. This model postulates that tumors are comprised of tumorigenic cancer stem cell (CSC) subpopulation and their non-tumorigenic progenitor cells43, CSC being the cells driving tumor growth, therapy resistance44 and metastasis42.
Cancer stem cells model has been developed after a collection of observations that only a rare subpopulation of cancer cells could form metastasis when transplanted in mice. This was tested for many cancer types such as acute myeloid leukemia45, breast cancer46, pancreatic cancer47. However there are still uncertainties about the origin of CSC, such as whether they are derived from normal stem cell with cancerous phenotype, or they are previously differentiated progenitor cells with oncogenic mutations and regain the self-renewability34. Lastly, it is hypothesized that a rare fusion event between stem cells and other cells may generate CSCs however there is still no direct evidence for this, especially in human conditions48.There are evidences both supporting or against this model and some uncertainties how this concept can be tested in mouse models.
24
Cancer stem cells have been suggest to have unique properties according to the general reviews49, 50: expressing a catalog of distinctive surface markers, selectively enhanced tumorigenic capacity and sustain heterogeneous tumor growth with generating a hierarchy of differentiated progenitor with non-tumorigenic capacity. The only way to test the tumorigenic capacity is in permissive environment43 where cancer cells are transplanted into mouse;
tumor growth and disease progression being followed. For example, minor population of CD34+CD38- human acute myeloid leukemia (AML) cells has greater capability of forming tumor than CD34+CD38+ and CD34- fractions when transplanted into non-obese diabetic/severe combined immunodeficient mice (NOD-SCID) 45. However the true tumorigenic potential might be hampered by several reasons and one of them is the lack of key adhesion molecules or growth factor due to inability of mouse ligand when human cells are transplanted into mouse. Though, the model is also supported by the syngeneic mouse43 models (a mouse tumor growing in mice of the strain in which the tumor originated51).
Other property that non-tumorigenic progeny stem from the tumorigenic cancer cell forming heterogeneous tumor is supported similarly by AML studies in which tumorigenic CD34+CD38- cell population gave rise to non-tumorigenic CD34-CD38+ cells on transplantation52. This is also similar to normal stem cell differentiation under epigenetic changes. Nonetheless, there are inconsistencies between studies on melanoma cells one suggesting that only CD271+ cells can form tumor in immunocompromised mice, other that CD271- cells also form tumors in NSG (NOD/SCID IL-2 NOD/SCID IL-2Rγ-null) immunodeficient mice. Another study reports that both CD271+ and CD271- cells formed tumors regardless of transplanted into NSG mice or to NOD-SCID mice43.
This fact raises the question of universality of the markers identifying CSC. Several markers are already defined for some cancers and they are not common for all the cancer types.
For example CD133 surface marker is commonly used to isolate CSCs from brain, lung, liver 53. However, when brain tumors were tested for tumorigenic property, both CD133+
and CD133- cells were shown to be tumorigenic in several studies 54. Moreover, the tumorigenic markers expressed among the patients may vary due to phenotypic changes resulting from different mutations or cells of origin. These evidences points out the necessity of more specific markers.
Taking account altogether, tumor heterogeneity may come from various sources; plasticity clonal evolution and CSC model. It is not yet clear which type of source leads to different phenotypical and functional properties. Nonetheless a hierarchical organization of tumorigenic and non-tumorigenic cells is obvious. Furthermore, clonal evolution and tumorigenic cell differentiation can independently or together contribute to heterogeneity43.Yet It is necessary
25
to extend the studies to all cancer types and subtypes and have a better idea which fractions of cells follow the CSC model55. Validity and specificity of surface markers on tumorigenicity should be evaluated on more patient samples. Mouse model assays should be better designed to eliminate the differences between human and mouse environment. All these aspects will be essential for comprehending biology of metastasis and designing/testing new therapies for a better diagnosis and treatment56.
26