Stem Cells, Cell Differentiation, and Cancer
Summary of Key Points
• Many tumors originate in organs and tissues that undergo a continuous process of cell turnover, which is sustained by a minority population of stem cells (e.g., the colon, breast, lung, prostate, brain, and bone marrow).
• Stem cells have four fundamental properties: the ability to give rise to new stem cells with intact and unlimited growth potential (self-renewal), the ability to give rise to a progeny of specialized cells (differentiation), the ability to migrate into new tissue locations and establish tissue growth (migration and tissue repair), and the ability to balance the previous three properties according to a genetic program that places constraints on their numerical expansion (homeostatic control).
• In many tissues, stem cells are the only long-lived cells. This observation suggests that early transforming events, either genetic mutations or epigenetic modifications, are likely to accumulate in stem cells.
• In addition to oncogenes that control cell survival and proliferation, there is a class of oncogenes that regulate self-renewal. In some cancers, tumor growth might be sustained by progenitor cells, which do not naturally self-renew but have aberrantly acquired this ability during disease progression.
• Experimental data suggest that, in many leukemias and solid tumors (e.g., breast, colon, head and neck, pancreas, bladder, and prostate carcinomas), only a specific, phenotypically distinct subset of cancer cells is able to form tumors when transplanted in mice.
• Currently, most antitumor drugs are evaluated on the basis of their capacity to rapidly reduce tumor size. However, because self-renewing cancer cells are often minority populations, treatments that simply reduce tumor size may not effectively target these cells.
• To result in cure, therapies must eradicate self-renewing cancer cells. The ability to identify these cells should allow the identification of new diagnostic markers and therapeutic targets.
A The capacity of selected cells to naturally undergo a process of full epigenetic reprogramming, which causes loss of differentiation identity and reversal to a totipotent stem cell state
B A fundamental property of stem cells that consists of the capacity to divide and give rise to functionally identical stem cells, with intact long-term expansion, proliferation, and differentiation potential
C The capacity possessed by stem cells to induce, when damaged, the dedifferentiation of neighboring daughter cells, thus creating new stem cells and renewing the stem cell pool
D A unique property possessed by mesenchymal stem cells, defined by the capacity to actively migrate out of a specific tissue and relocate into a new one, where they can change their epigenetic identity and acquire the function of other types of resident stem cells
2. What is the definition of “cancer stem cells?”
A Normal hematopoietic stem cells that migrated inside the tumor mass as the result of the strong inflammatory processes usually observed in cancer
B Embryonic stem cells, artificially transformed in vitro by means of transduction with retroviral vectors encoding the NRAS oncogene, and endowed with high tumorigenic capacity in immunodeficient mice
C A subpopulation of cancer cells, often a minority subset, identified by a specific surface marker phenotype and defined by the capacity to self-renew and differentiate, as assessed by serial transplantation experiments in immunodeficient mice
D Fusion hybrids between mesenchymal stem cells and cancer cells, endowed with tumorigenic capacity and high metastatic potential in immunodeficient mice
3. To what kind of normal cell type do CSCs correspond to, in terms of surface marker phenotype?
A They are always characterized by a surface marker profile found exclusively in normal stem cells.
B They are always characterized by a surface marker profile found exclusively in a specific lineage of terminally differentiated, effector cells.
C Their surface marker profile is usually typical of immature/progenitor cells and can correspond to that of stem cells and/or immature multipotent, oligopotent, or lineage-restricted progenitors, depending on the tumor subtype and the disease stage.
D Their surface marker profile is usually typical of terminally differentiated effector cells and can correspond to that of a specialized cellular lineage, depending on the tumor subtype and the disease stage.
4. Which important implication might CSCs have for the future development of novel diagnostic and/or prognostic assays in patients with cancer?
A The analysis by gene-expression array of a bulk tumor’s transcriptional profile and the identification of a high degree of similarity with a CSC gene-expression signature could be used to identify patients characterized by a more aggressive disease, associated to poor prognosis and worse clinical outcomes.
B The presence of a low percentage of CSCs within cancer tissues (<30%), as evaluated by immunohistochemistry for CD44, could be used to discriminate between benign and malignant tumors across different tissue types (e.g., between a teratoma and a teratocarcinoma, a benign and a malignant mole, and an adenoma and a carcinoma).
C The development of a diagnostic bone-scan test, based on immunoscintigraphy using anti-CD44 monoclonal antibodies labeled with radioactive isotopes, could aid in the early identification on bone metastases.
D The development of a diagnostic assay based on polymerase chain reaction and aimed at measuring CD44 mRNA expression could aid in the early identification of lymph node metastases.
5. Which important implication might CSCs have for the future development of antitumor drugs?
A The development of novel antitumor drugs could be based on finding agents able to arrest differentiation processes within the tumor mass.
B To overcome tumor resistance to radiotherapy, new radio-sensitizing agents should be able to selectively protect nontumorigenic cancer cells during irradiation, thus creating a Darwinian competition between CSCs and their nontumorigenic progeny.
C The development of novel antitumor drugs could be based exclusively on in vitro experiments performed on cell lines similar to CSCs, thus eliminating the need of in vivo animal models.
D To permanently eradicate cancer tissues and to abolish the risk of tumor relapse, novel antitumor agents must be able to eliminate self-renewing CSCs.
1. Answer: B. Self-renewal is a defining and intrinsic property of all types of stem cells. As opposed to most other cell types, which progressively exhaust their long-term expansion potential upon sequential rounds of cell division (i.e., they undergo “senescence”), stem cells are able to maintain their capacity for unlimited proliferation (i.e., they “self-renew”). The capacity to self-renew is dependent on the capacity of stem cells to preserve a specific epigenetic identity over time rather than to change or transform it.
2. Answer: C. The term “cancer stem cell” (CSC) is an operational definition (i.e., based on the experimental fulfillment of a specific set of functional properties). It identifies a specific subset of cancer cells that is able to self-renew (i.e., form tumors that can be serially passaged in vivo) and differentiate (i.e., form tumors that also contain other types of nontumorigenic cancer cells). These cells are usually identified on the basis of a specific repertoire of surface markers and frequently represent a minority subset of the neoplastic clone. Although they display stem-cell properties (i.e., self-renewal and differentiation), their molecular and phenotypic identity does not necessarily correspond to that of normal stem cells.
3. Answer: C. It is important to remember that CSC is an operational definition (see explanation of Question 2). As such, this term does not necessarily define a cell population whose phenotypic or molecular identity is always the same as that of normal stem cell populations. As a general rule, the surface marker phenotype of CSCs corresponds to that of immature/progenitor cells and can encompass both stem cells and various types of multipotent, oligopotent, or lineage-restricted progenitors, depending on the tumor subtype and the stage of the disease.
4. Answer: A. Several studies have shown that malignant tumors characterized by a gene-expression profile similar to that of CSCs are associated with worse clinical outcomes. A high degree of similarity between a whole tumor’s gene-expression profile and that of CSCs is likely the consequence of a high CSC content in the tumor. In many epithelial carcinomas, CD44 can be used, in combination with a panel of other markers, to isolate a subpopulation of cancer cells enriched in CSCs. This is likely because, within many epithelial tissues, CD44 is expressed in a gradient, with the highest levels observed in the most basal layers, where immature stem/progenitor cells usually reside. CD44, however, is not specific to epithelial stem/progenitor cells and is expressed abundantly across many other cell types, including stromal and immune cells (e.g., fibroblasts, endothelial cells, monocytes, and lymphocytes). Expression of CD44 cannot be used to purify and/or identify CSCs across all tumors, especially nonepithelial tumors. Even within epithelial tumors, expression of CD44 alone (e.g., without careful exclusion of the contribution of stromal cells) cannot be used to accurately define CSCs.
5. Answer: D. Because they are able to self-renew, CSCs are able to propagate themselves indefinitely. If not eliminated, they hold the potential to lead to tumor relapse. To permanently eradicate tumor tissues, antitumor therapies must be able to kill or otherwise eliminate CSCs. From a theoretical perspective, a possible strategy to deplete tumor tissues of CSCs could be to promote their differentiation into terminally mature cells, not to inhibit or prevent it. As a general rule, nontumorigenic cancer cells are the progeny of CSCs, not a different genetic clone of the cancer tissue. Thus nontumorigenic cancer cells originate from CSCs and are not in “Darwinian competition” with them. The dynamic equilibrium between CSCs and other cellular components of cancer tissues is extremely difficult to model in vitro using traditional cell lines grown as two-dimensional monolayers. Currently, CSCs can be best, if not exclusively, studied using solid tissue xenografts in animal models.
