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Targeting Cancer Stemness in the Clinic: From Hype to Hope

2018; Elsevier BV; Volume: 24; Issue: 1 Linguagem: Inglês

10.1016/j.stem.2018.11.017

1934-5909

Caner Saygin, Daniela Matei, Ravindra Majeti, Ofer Reizes, Justin D. Lathia,

Cancer Research and Treatments

Tumors are composed of non-homogeneous cell populations exhibiting varying degrees of genetic and functional heterogeneity. Cancer stem cells (CSCs) are capable of sustaining tumors by manipulating genetic and non-genetic factors to metastasize, resist treatment, and maintain the tumor microenvironment. Understanding the key traits and mechanisms of CSC survival provides opportunities to improve patient outcomes via improved prognostic models and therapeutics. Here, we review the clinical significance of CSCs and results of potential CSC-targeting therapies in various cancers. We discuss barriers to translating cues from pre-clinical models into clinical applications and propose new strategies for rational design of future anti-CSC trials. Tumors are composed of non-homogeneous cell populations exhibiting varying degrees of genetic and functional heterogeneity. Cancer stem cells (CSCs) are capable of sustaining tumors by manipulating genetic and non-genetic factors to metastasize, resist treatment, and maintain the tumor microenvironment. Understanding the key traits and mechanisms of CSC survival provides opportunities to improve patient outcomes via improved prognostic models and therapeutics. Here, we review the clinical significance of CSCs and results of potential CSC-targeting therapies in various cancers. We discuss barriers to translating cues from pre-clinical models into clinical applications and propose new strategies for rational design of future anti-CSC trials. The modern era of anti-cancer drug discovery began after World War II with the observation that cytotoxic agents could be used to treat cancers with a high proliferative index (Farber et al., 1948Farber S. Diamond L.K. Mercer R. Sylvester R. Wolff J. Temporary remissions in acute leukemia in children produced by folic acid antagonist, 4-aminopteroyl-glutamic acid.N. Engl. J. Med. 1948; 238: 787-793Crossref PubMed Google Scholar, Gilman, 1963Gilman A. The initial clinical trial of nitrogen mustard.Am. J. Surg. 1963; 105: 574-578Abstract Full Text PDF PubMed Google Scholar). Since then, our understanding of human tumor biology has increased substantially, translating into the development of numerous cytotoxic agents, targeted small molecules and monoclonal antibodies, and immunotherapies. However, despite achieving considerable improvements in survival, cures are rare and the majority of patients relapse. Furthermore, treatment options are still limited for many cancers with high mortality, including acute myeloid leukemia (AML), glioblastoma, ovarian cancer, pancreatic cancer, and hepatocellular carcinoma (HCC). The major barrier against curing cancer is the intratumoral heterogeneity of cancer cells. Tumors are not homogeneous collections of malignant cells but rather harbor phenotypic and functional diversity, which is proposed to account for treatment failure and relapse (Meacham and Morrison, 2013Meacham C.E. Morrison S.J. Tumour heterogeneity and cancer cell plasticity.Nature. 2013; 501: 328-337Crossref PubMed Scopus (822) Google Scholar). Early studies analyzing genetic mutations across different tumor stages demonstrated that tumor heterogeneity and progression result from sequential acquisition of mutations (Nowell, 1976Nowell P.C. The clonal evolution of tumor cell populations.Science. 1976; 194: 23-28Crossref PubMed Google Scholar, Vogelstein et al., 1988Vogelstein B. Fearon E.R. Hamilton S.R. Kern S.E. Preisinger A.C. Leppert M. Nakamura Y. White R. Smits A.M. Bos J.L. Genetic alterations during colorectal-tumor development.N. Engl. J. Med. 1988; 319: 525-532Crossref PubMed Google Scholar). Recently, high-throughput sequencing of tumor genomes supported these findings and provided evidence that cancer within a single patient is characterized by genetically distinct subclones that arise through branching evolution (Burrell et al., 2013Burrell R.A. McGranahan N. Bartek J. Swanton C. The causes and consequences of genetic heterogeneity in cancer evolution.Nature. 2013; 501: 338-345Crossref PubMed Scopus (784) Google Scholar, Greaves and Maley, 2012Greaves M. Maley C.C. Clonal evolution in cancer.Nature. 2012; 481: 306-313Crossref PubMed Scopus (1228) Google Scholar, McGranahan and Swanton, 2017McGranahan N. Swanton C. Clonal heterogeneity and tumor evolution: Past, present, and the future.Cell. 2017; 168: 613-628Abstract Full Text Full Text PDF PubMed Scopus (212) Google Scholar). Even though this concept clearly documents how mutational landscape of individual clones contribute to cellular heterogeneity, studies have also shown functional variations within a single genetic clone (Kreso et al., 2013Kreso A. O’Brien C.A. van Galen P. Gan O.I. Notta F. Brown A.M. Ng K. Ma J. Wienholds E. Dunant C. et al.Variable clonal repopulation dynamics influence chemotherapy response in colorectal cancer.Science. 2013; 339: 543-548Crossref PubMed Scopus (340) Google Scholar). Strong evidence is emerging that within a subclone, differential ability of cells to survive therapeutic targeting and promote tumor progression is determined by nongenetic determinants, largely related to epigenetic modifications and the activity of developmental pathways (Visvader and Lindeman, 2012Visvader J.E. Lindeman G.J. Cancer stem cells: Current status and evolving complexities.Cell Stem Cell. 2012; 10: 717-728Abstract Full Text Full Text PDF PubMed Scopus (626) Google Scholar). To add an additional layer of complexity, tumors can be modeled as multicellular ecosystems containing neoplastic cells, as well as various infiltrating endothelial, immune, and stromal cells, including cancer-associated fibroblasts (CAFs). The cells constituting tumor microenvironment (TME) enable and sustain the hallmarks of cancer through reciprocal communications with cancer cells, which contributes to the functional diversity (Hanahan and Coussens, 2012Hanahan D. Coussens L.M. Accessories to the crime: Functions of cells recruited to the tumor microenvironment.Cancer Cell. 2012; 21: 309-322Abstract Full Text Full Text PDF PubMed Scopus (1329) Google Scholar). Collectively, these three contributors of intratumor heterogeneity—genetics, epigenetics, and the TME—have been postulated to govern therapeutic resistance and tumor progression via impinging on stemness properties of cancer cells (Kreso and Dick, 2014Kreso A. Dick J.E. Evolution of the cancer stem cell model.Cell Stem Cell. 2014; 14: 275-291Abstract Full Text Full Text PDF PubMed Scopus (740) Google Scholar). The cancer stem cell (CSC) hypothesis posits that cellular heterogeneity within a tumor is organized in hierarchical manner with a subpopulation of cells endowed with an enhanced capacity for self-renewal and elevated ability to seed new tumors upon experimental implantation in new hosts (Pattabiraman and Weinberg, 2014Pattabiraman D.R. Weinberg R.A. Tackling the cancer stem cells—what challenges do they pose?.Nat. Rev. Drug Discov. 2014; 13: 497-512Crossref PubMed Scopus (361) Google Scholar). Depending on the biological system in which they are studied, CSCs have also been referred to as “tumor-initiating cells” or “sphere-forming cells,” as outlined in the guidelines by 2011 Working Conference on CSCs (Valent et al., 2012Valent P. Bonnet D. De Maria R. Lapidot T. Copland M. Melo J.V. Chomienne C. Ishikawa F. Schuringa J.J. Stassi G. et al.Cancer stem cell definitions and terminology: The devil is in the details.Nat. Rev. Cancer. 2012; 12: 767-775Crossref PubMed Scopus (326) Google Scholar). In vitro tumorsphere formation is a surrogate assay for self-renewal, which is widely used in preclinical studies and has also been utilized in clinical trials testing anti-CSC therapeutics. After the first prospective identification of a CSC in AML by Dick and colleagues (Lapidot et al., 1994Lapidot T. Sirard C. Vormoor J. Murdoch B. Hoang T. Caceres-Cortes J. Minden M. Paterson B. Caligiuri M.A. Dick J.E. A cell initiating human acute myeloid leukaemia after transplantation into SCID mice.Nature. 1994; 367: 645-648Crossref PubMed Scopus (2873) Google Scholar), CSCs have been isolated from multiple solid tumors, including but not limited to breast, head and neck, brain, lung, colon, pancreatic, hepatic, prostate, bladder, and ovarian cancers, as well as sarcomas (Visvader and Lindeman, 2008Visvader J.E. Lindeman G.J. Cancer stem cells in solid tumours: Accumulating evidence and unresolved questions.Nat. Rev. Cancer. 2008; 8: 755-768Crossref PubMed Scopus (2199) Google Scholar). Notably, certain tumors with very high proliferative index (e.g., melanoma, Burkitt lymphoma) may not obey the CSC model at all and instead are characterized by lack of hierarchic organization among tumor cells (Kreso and Dick, 2014Kreso A. Dick J.E. Evolution of the cancer stem cell model.Cell Stem Cell. 2014; 14: 275-291Abstract Full Text Full Text PDF PubMed Scopus (740) Google Scholar). The gold standard for defining CSCs has been serial in vivo transplantation, but several cell-surface markers (e.g., CD133, CD44, epithelial cell-adhesion molecule [EpCAM]), and ALDH enzyme activity have proved useful to study CSCs in patient specimens and experimental systems (Schulenburg et al., 2015Schulenburg A. Blatt K. Cerny-Reiterer S. Sadovnik I. Herrmann H. Marian B. Grunt T.W. Zielinski C.C. Valent P. Cancer stem cells in basic science and in translational oncology: Can we translate into clinical application?.J. Hematol. Oncol. 2015; 8: 16Crossref PubMed Scopus (0) Google Scholar). Similarly, CSCs could be isolated based on CD34 and CD38 expression in AML (i.e., leukemia stem cells [LSCs]) (Thomas and Majeti, 2017Thomas D. Majeti R. Biology and relevance of human acute myeloid leukemia stem cells.Blood. 2017; 129: 1577-1585Crossref PubMed Scopus (27) Google Scholar). However, none of these markers are exclusive to CSC compartment of tumors, and CSCs could also be identified in marker negative populations (Zhou et al., 2009Zhou B.B. Zhang H. Damelin M. Geles K.G. Grindley J.C. Dirks P.B. Tumour-initiating cells: Challenges and opportunities for anticancer drug discovery.Nat. Rev. Drug Discov. 2009; 8: 806-823Crossref PubMed Scopus (557) Google Scholar). This indicates the need to use multiple markers to identify CSCs from different subclones. Reporter-based approaches to functionally define CSCs based on their expression of core pluripotency genes (e.g., NANOG) or CSC-specific signaling pathways (e.g., Wnt pathway) provide another opportunity to study CSCs in pre-clinical models (Saygin et al., 2016Saygin C. Samour M. Chumakova A. Jarrar A. Lathia J.D. Reizes O. Reporter systems to study cancer stem cells.Methods Mol. Biol. 2016; 1516: 319-333Crossref PubMed Scopus (1) Google Scholar). For two decades, these markers have been valuable tools for clinical correlative and prognostic analyses, and their longitudinal assessment during clinical tumor progression improved our understanding of CSC evolution dynamics. Despite having pre-clinically validated surface CSC markers and other CSC-specific molecules, identification and isolation of CSCs in clinical specimens is a challenging task. The extent to which these marker-designated populations are true CSCs remains to be answered. Emerging novel technologies such as live-cell RNA detection and single-cell DNA and RNA sequencing methods may advance CSC identification; however, it is unlikely that these methods can yield a universal CSC marker. Therefore, precise characterization of different CSC subclones in individual patient samples depends on tumor-specific validated markers, high-resolution single-cell analysis techniques, and functional assays (e.g., sphere formation, in vivo tumor generation at low cell densities) to validate these signatures. The cell of origin for a CSC varies in individual tumors. Normal tissue stem cells can give rise to CSCs upon somatic alterations, but recent studies have also demonstrated a pool of intermediary cells, called “transient amplifying cells,” which are mitotically active and accumulate mutations to dedifferentiate and sustain CSC pool (Chaffer and Weinberg, 2015Chaffer C.L. Weinberg R.A. How does multistep tumorigenesis really proceed?.Cancer Discov. 2015; 5: 22-24Crossref PubMed Scopus (0) Google Scholar). In addition to stem and progenitor cells, CSCs were shown to arise from more differentiated cells as a consequence of constitutive nuclear factor κB (NF-κB) activation, or APC deletion and chemically induced inflammation in colorectal cancer (Schwitalla et al., 2013Schwitalla S. Fingerle A.A. Cammareri P. Nebelsiek T. Göktuna S.I. Ziegler P.K. Canli O. Heijmans J. Huels D.J. Moreaux G. et al.Intestinal tumorigenesis initiated by dedifferentiation and acquisition of stem-cell-like properties.Cell. 2013; 152: 25-38Abstract Full Text Full Text PDF PubMed Scopus (475) Google Scholar, Westphalen et al., 2014Westphalen C.B. Asfaha S. Hayakawa Y. Takemoto Y. Lukin D.J. Nuber A.H. Brandtner A. Setlik W. Remotti H. Muley A. et al.Long-lived intestinal tuft cells serve as colon cancer-initiating cells.J. Clin. Invest. 2014; 124: 1283-1295Crossref PubMed Scopus (127) Google Scholar). In parallel, studies in AML models revealed that early driver mutations (e.g., DNMT3A) accumulate in pre-leukemic hematopoietic stem cells, which are still capable of contributing to lymphoid and myeloid progeny in patients, and these cells can undergo clonal evolution resulting in full-blown AML (Jan et al., 2012Jan M. Snyder T.M. Corces-Zimmerman M.R. Vyas P. Weissman I.L. Quake S.R. Majeti R. Clonal evolution of preleukemic hematopoietic stem cells precedes human acute myeloid leukemia.Sci. Transl. Med. 2012; 4: 149ra118Crossref PubMed Scopus (345) Google Scholar, Abelson et al., 2018Abelson S. Collord G. Ng S.W.K. Weissbrod O. Mendelson Cohen N. Niemeyer E. Barda N. Zuzarte P.C. Heisler L. Sundaravadanam Y. et al.Prediction of acute myeloid leukaemia risk in healthy individuals.Nature. 2018; 559: 400-404Crossref PubMed Scopus (13) Google Scholar). Additionally, there is accumulating evidence on the role of inflammation in inducing a CSC state. In a skin cancer model, microenvironmental stress signals were shown to trigger stem cell fate switching through the activation of stress-induced regulatory elements and silencing of homeostatic signals (Ge et al., 2017Ge Y. Gomez N.C. Adam R.C. Nikolova M. Yang H. Verma A. Lu C.P. Polak L. Yuan S. Elemento O. Fuchs E. Stem cell lineage infidelity drives wound repair and cancer.Cell. 2017; 169: 636-650.e14Abstract Full Text Full Text PDF PubMed Scopus (24) Google Scholar). This process created a persistent state of lineage infidelity independent of stem cell origin and laid the ground for emergence of CSCs. Regardless of their cell of origin, multiple CSC pools exist within individual tumors, which might have biologically distinct properties, including quiescence, therapeutic resistance, and the ability to undergo epithelial-mesenchymal transition, invasion, and metastasis (Visvader and Lindeman, 2012Visvader J.E. Lindeman G.J. Cancer stem cells: Current status and evolving complexities.Cell Stem Cell. 2012; 10: 717-728Abstract Full Text Full Text PDF PubMed Scopus (626) Google Scholar). The CSC state is dynamic. Cancer cells can transition between stem and differentiated states in response to therapeutic insults or other stimuli within the microenvironment (Plaks et al., 2015Plaks V. Kong N. Werb Z. The cancer stem cell niche: How essential is the niche in regulating stemness of tumor cells?.Cell Stem Cell. 2015; 16: 225-238Abstract Full Text Full Text PDF PubMed Scopus (310) Google Scholar, Rich, 2016Rich J.N. Cancer stem cells: Understanding tumor hierarchy and heterogeneity.Medicine (Baltimore). 2016; 95: S2-S7Crossref PubMed Scopus (16) Google Scholar, Wiechert et al., 2016Wiechert A. Saygin C. Thiagarajan P.S. Rao V.S. Hale J.S. Gupta N. Hitomi M. Nagaraj A.B. DiFeo A. Lathia J.D. Reizes O. Cisplatin induces stemness in ovarian cancer.Oncotarget. 2016; 7: 30511-30522Crossref PubMed Scopus (15) Google Scholar). For example, CSCs can be enriched in vivo by treatment with radiation, which suggests radiation-induced phenotypic conversion to CSC state (Plaks et al., 2015Plaks V. Kong N. Werb Z. The cancer stem cell niche: How essential is the niche in regulating stemness of tumor cells?.Cell Stem Cell. 2015; 16: 225-238Abstract Full Text Full Text PDF PubMed Scopus (310) Google Scholar). We have also reported direct evidence for this phenotypic conversion in cisplatin-treated ovarian cancer cells (Wiechert et al., 2016Wiechert A. Saygin C. Thiagarajan P.S. Rao V.S. Hale J.S. Gupta N. Hitomi M. Nagaraj A.B. DiFeo A. Lathia J.D. Reizes O. Cisplatin induces stemness in ovarian cancer.Oncotarget. 2016; 7: 30511-30522Crossref PubMed Scopus (15) Google Scholar). Leveraging a NANOG-promoter driven GFP reporter system and real-time imaging of treated cells, we demonstrated that cisplatin-treated non-CSCs can activate their self-renewal program and convert from NANOG-GFP negative to positive cells with the latter having a higher self-renewal capacity. Moreover, CSCs from an individual tumor can exist in different phenotypic states. Liu et al. demonstrated that breast cancer CSCs can have distinct mesenchymal-like and epithelial-like states with differences in proliferation, invasiveness, and localization within tumor mass (Liu et al., 2013Liu S. Cong Y. Wang D. Sun Y. Deng L. Liu Y. Martin-Trevino R. Shang L. McDermott S.P. Landis M.D. et al.Breast cancer stem cells transition between epithelial and mesenchymal states reflective of their normal counterparts.Stem Cell Reports. 2013; 2: 78-91Abstract Full Text Full Text PDF PubMed Scopus (304) Google Scholar). The conversion between these states is mediated by epigenetic alterations and increases the capacity for invasion and metastasis. These examples of phenotypic interconversion, also known as plasticity, contribute to the complexity of the cancer ecosystem and represent a challenge against tumor eradication. The CSC model has gained widespread attention as it incorporates the three facets of cellular heterogeneity (i.e., genetics, epigenetics, and TME) within the tumor, explains treatment failure and tumor recurrence, and promises new targets for anti-cancer drug discovery. In this review, we discuss clinical and prognostic significance of CSCs, discuss some of the promising approaches to target them, and highlight the agents that are currently in clinical trials. Although we focus on trials that utilized stem cell assays to investigate anti-CSC effect of these drugs, we caution the reader that some clinical trials testing these promising agents lack robust CSC assays. However, there is emerging evidence that these agents are also relevant for treating CSCs. We further provide a perspective on rational design of clinical trials incorporating CSC-directed therapeutics and discuss the challenges of translating the accumulating knowledge on CSC biology to the clinic. This article is not meant to be an exhaustive review of all anti-CSC approaches. Instead, we summarize evolving treatment strategies with promising results in clinical development, hence will potentially emerge as new anti-cancer agents. CSCs possess intrinsic resistance to chemotherapy and radiotherapy, largely due to their quiescence. Similar to normal stem cells, CSCs from a variety of tumors are known to be slow cycling and maintained in G0 phase through intricate cell-cycle regulatory mechanisms (Pattabiraman and Weinberg, 2014Pattabiraman D.R. Weinberg R.A. Tackling the cancer stem cells—what challenges do they pose?.Nat. Rev. Drug Discov. 2014; 13: 497-512Crossref PubMed Scopus (361) Google Scholar). However, CSC populations in more aggressive tumors may have a higher proliferative index and utilize additional resistance mechanisms, such as high-level expression of drug efflux pumps (e.g., ATP binding cassette transporters), anti-apoptotic proteins, reactive oxygen species scavengers, more efficient DNA repair, and a protective microenvironment created by interactions with the niche (Schulenburg et al., 2015Schulenburg A. Blatt K. Cerny-Reiterer S. Sadovnik I. Herrmann H. Marian B. Grunt T.W. Zielinski C.C. Valent P. Cancer stem cells in basic science and in translational oncology: Can we translate into clinical application?.J. Hematol. Oncol. 2015; 8: 16Crossref PubMed Scopus (0) Google Scholar, Skvortsova et al., 2015Skvortsova I. Debbage P. Kumar V. Skvortsov S. Radiation resistance: Cancer stem cells (CSCs) and their enigmatic pro-survival signaling.Semin. Cancer Biol. 2015; 35: 39-44Crossref PubMed Scopus (37) Google Scholar). Moreover, epigenetic mechanisms can play a role in mediating therapeutic resistance in CSCs. In estrogen-receptor-positive breast cancer, methylation events have been linked to resistance to endocrine therapies (Magnani et al., 2013Magnani L. Stoeck A. Zhang X. Lánczky A. Mirabella A.C. Wang T.-L. Gyorffy B. Lupien M. Genome-wide reprogramming of the chromatin landscape underlies endocrine therapy resistance in breast cancer.Proc. Natl. Acad. Sci. USA. 2013; 110: E1490-E1499Crossref PubMed Scopus (0) Google Scholar). Epigenetic silencing is also involved in maintaining plasticity and transition of non-CSCs to CSCs, as evidenced by Zeb1 activation in basal breast cancers (Chaffer et al., 2013Chaffer C.L. Marjanovic N.D. Lee T. Bell G. Kleer C.G. Reinhardt F. D’Alessio A.C. Young R.A. Weinberg R.A. Poised chromatin at the ZEB1 promoter enables breast cancer cell plasticity and enhances tumorigenicity.Cell. 2013; 154: 61-74Abstract Full Text Full Text PDF PubMed Scopus (372) Google Scholar). Another example for epigenetically driven resistance to targeted therapies was observed in lung cancer (Sharma et al., 2010Sharma S.V. Lee D.Y. Li B. Quinlan M.P. Takahashi F. Maheswaran S. McDermott U. Azizian N. Zou L. Fischbach M.A. et al.A chromatin-mediated reversible drug-tolerant state in cancer cell subpopulations.Cell. 2010; 141: 69-80Abstract Full Text Full Text PDF PubMed Scopus (1002) Google Scholar). Upon exposure to lethal doses of EGFR tyrosine kinase inhibitor therapy, a reversible drug tolerant state was attained by an altered chromatin state requiring histone demethylases. Similarly, treatment with targeted kinase inhibitors can induce transition of glioblastoma (GBM) CSCs to a slow-cycling persistent state until more permanent resistance mechanisms can evolve (Liau et al., 2017Liau B.B. Sievers C. Donohue L.K. Gillespie S.M. Flavahan W.A. Miller T.E. Venteicher A.S. Hebert C.H. Carey C.D. Rodig S.J. et al.Adaptive chromatin remodeling drives glioblastoma stem cell plasticity and drug tolerance.Cell Stem Cell. 2017; 20: 233-246Abstract Full Text Full Text PDF PubMed Scopus (41) Google Scholar). These results highlight the mechanisms by which CSCs hijack epigenetic regulators to sustain their pool and resist targeted therapies. However, given the reversible nature of epigenetic alterations, these pathways hold great promise as therapeutic targets. The frequency of CSCs has been reported to be <1% in various solid tumors (Ishizawa et al., 2010Ishizawa K. Rasheed Z.A. Karisch R. Wang Q. Kowalski J. Susky E. Pereira K. Karamboulas C. Moghal N. Rajeshkumar N.V. et al.Tumor-initiating cells are rare in many human tumors.Cell Stem Cell. 2010; 7: 279-282Abstract Full Text Full Text PDF PubMed Scopus (137) Google Scholar) and ranges between 1 in 103 and 106 cells in AML (Eppert et al., 2011Eppert K. Takenaka K. Lechman E.R. Waldron L. Nilsson B. van Galen P. Metzeler K.H. Poeppl A. Ling V. Beyene J. et al.Stem cell gene expression programs influence clinical outcome in human leukemia.Nat. Med. 2011; 17: 1086-1093Crossref PubMed Scopus (455) Google Scholar). In addition, poorly differentiated (i.e., higher grade) solid tumors were shown to have higher CSC content than their well-differentiated counterparts (Lathia et al., 2015Lathia J.D. Mack S.C. Mulkearns-Hubert E.E. Valentim C.L. Rich J.N. Cancer stem cells in glioblastoma.Genes Dev. 2015; 29: 1203-1217Crossref PubMed Google Scholar, Pece et al., 2010Pece S. Tosoni D. Confalonieri S. Mazzarol G. Vecchi M. Ronzoni S. Bernard L. Viale G. Pelicci P.G. Di Fiore P.P. Biological and molecular heterogeneity of breast cancers correlates with their cancer stem cell content.Cell. 2010; 140: 62-73Abstract Full Text Full Text PDF PubMed Scopus (547) Google Scholar, Roudi et al., 2014Roudi R. Madjd Z. Korourian A. Mehrazma M. Molanae S. Sabet M.N. Shariftabrizi A. Clinical significance of putative cancer stem cell marker CD44 in different histological subtypes of lung cancer.Cancer Biomark. 2014; 14: 457-467Crossref PubMed Scopus (26) Google Scholar). For example, the colorectal CSC pool expands during tumor development from adenomas as a result of genetic mutations and environmental pressures, ultimately resulting in the evolution of metastatic CSCs at advanced stages (Zeuner et al., 2014Zeuner A. Todaro M. Stassi G. De Maria R. Colorectal cancer stem cells: From the crypt to the clinic.Cell Stem Cell. 2014; 15: 692-705Abstract Full Text Full Text PDF PubMed Scopus (108) Google Scholar). Moreover, distinct molecular subtypes of AML have different LSC frequencies with higher proportion of LSCs observed in high-risk AML (Heuser et al., 2009Heuser M. Sly L.M. Argiropoulos B. Kuchenbauer F. Lai C. Weng A. Leung M. Lin G. Brookes C. Fung S. et al.Modeling the functional heterogeneity of leukemia stem cells: Role of STAT5 in leukemia stem cell self-renewal.Blood. 2009; 114: 3983-3993Crossref PubMed Scopus (55) Google Scholar, Somervaille et al., 2009Somervaille T.C. Matheny C.J. Spencer G.J. Iwasaki M. Rinn J.L. Witten D.M. Chang H.Y. Shurtleff S.A. Downing J.R. Cleary M.L. Hierarchical maintenance of MLL myeloid leukemia stem cells employs a transcriptional program shared with embryonic rather than adult stem cells.Cell Stem Cell. 2009; 4: 129-140Abstract Full Text Full Text PDF PubMed Scopus (213) Google Scholar). These observations highlight the association between the higher CSC content and adverse clinicopathological characteristics of tumors at the time of diagnosis. Additionally, since CSC populations are resistant to conventional therapies aimed at the bulk of proliferative cells, they can be enriched in post-treatment setting. In a study of 52 breast cancer patients treated in two phase 2 trials, residual tumor tissue after endocrine therapy (letrozole) or chemotherapy (docetaxel) demonstrated enrichment of CD44+/CD24– CSC signature when compared to pre-treatment samples (Creighton et al., 2009Creighton C.J. Li X. Landis M. Dixon J.M. Neumeister V.M. Sjolund A. Rimm D.L. Wong H. Rodriguez A. Herschkowitz J.I. et al.Residual breast cancers after conventional therapy display mesenchymal as well as tumor-initiating features.Proc. Natl. Acad. Sci. USA. 2009; 106: 13820-13825Crossref PubMed Scopus (805) Google Scholar). Similarly, CD133 expression was evaluated in pre-treatment tumor biopsies and the corresponding post-chemoradiotherapy surgical specimens from 126 rectal cancer patients. CD133 expression was significantly increased after therapy, and patients who had an increased fraction of CD133+ cells had significantly lower histopathologic tumor regression, disease-free survival (DFS), and overall survival (OS) (Sprenger et al., 2013Sprenger T. Conradi L.C. Beissbarth T. Ermert H. Homayounfar K. Middel P. Rüschoff J. Wolff H.A. Schüler P. Ghadimi B.M. et al.Enrichment of CD133-expressing cells in rectal cancers treated with preoperative radiochemotherapy is an independent marker for metastasis and survival.Cancer. 2013; 119: 26-35Crossref PubMed Scopus (22) Google Scholar). Treatment in AML patients led to a 10- to 100-fold expansion of the LSC compartment, and these LSCs were detectable in immunophenotypic compartments not containing LSCs at initial diagnosis (Ho et al., 2016Ho T.C. LaMere M. Stevens B.M. Ashton J.M. Myers J.R. O’Dwyer K.M. Liesveld J.L. Mendler J.H. Guzman M. Morrissette J.D. et al.Evolution of acute myelogenous leukemia stem cell properties after treatment and progression.Blood. 2016; 128: 1671-1678Crossref PubMed Scopus (37) Google Scholar). CSCs might be refractory to targeted treatments as well. Patients with chronic myeloid leukemia (CML) are effectively treated with tyrosine kinase inhibitors (TKI) against BCR-ABL1 transcript, which is the driver mutation in this disease. However, among patients who undertake TKI therapy for a 5-year period, two-thirds demonstrate evidence of LSC persistence, even in deep molecular remission with no detectable BCR-ABL transcripts (Holyoake and Vetrie, 2017Holyoake T.L. Vetrie D. The chronic myeloid leukemia stem cell: Stemming the tide of persistence.Blood. 2017; 129: 1595-1606Crossref PubMed Scopus (45) Google Scholar). These findings suggest that CSCs may persist after targeted or non-targeted therapeutic blockade and respond by increasing in number or adopting a more aggressive phenotype. It is also likely that non-CSCs convert into CSCs, expanding the pool to sustain tumorigenesis (Harless, 2011Harless W.W. Cancer treatments transform residual cancer cell phenotype.Cancer Cell Int. 2011; 11: 1Crossref PubMed Scopus (0) Google Scholar). In addition to these observations, several studies investigated the prognostic value of assessing CSC content in tumors at the time of diagnosis. In AML patients, a high percentage of CD34+/CD38– LCSs at diagnosis was associated with higher rates of engraftment in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice, as well as high minimal residual disease (MRD) after chemotherapy, and poor survival (van Rhenen et al., 2005van Rhenen A. Feller N. Kelder A. Westra A.H. Rombouts E. Zweegman S. van der Pol M.A. Waisfisz Q. Ossenkoppele G.J. Schuurhuis G.J. High stem cell frequency in acute myeloid leukemia at diagnosis predicts high minimal residual disease and poor survival.Clin. Cancer Res. 2005