The Open Biomarkers Journal




ISSN: 1875-3183 ― Volume 8, 2018

Precision Medicine Approaches to Cancer Diagnosis and Treatment: Focus on Cancer Stem Cell Biomarkers



Katarzyna Rygiel*
Department of Environmental Medicine and Epidemiology, Medical University of Silesia (SUM), Zabrze, Poland

Abstract

Background:

Recent research evidence has revealed that cancer cells contain a subpopulation of cancer stem cells (CSCs) that can remain even after traditional oncology therapies (e.g.: surgical resection of a tumor, radiation therapy (RT), and chemotherapy (ChT)), and can subsequently regenerate the original tumor or metastases, which are resistant to standard anticancer treatments. Such a resistance can be activated in various CSC populations, via different signal transduction pathways.

Conclusion:

The signaling pathways (e.g.: NANOG, Wnt/β-catenin, Hedgehog, Notch, signal transducer and activator of transcription 3 (STAT 3), and phosphoinositide 3-kinase (PI3K)) play a crucial role in the CSCs, leading to tumorigenesis and metastatic spread. Therefore, their detailed analysis, including innovative biomarkers, is necessary to develop the effective, novel therapies that will specifically target CSCs, in patients with aggressive cancers. This review briefly outlines the concept of CSCs, and key components of CSC dysregulation in the signaling pathways. Furthermore, it describes some innovative strategies, such as: Single-Cell Sequencing (SCS), Circulating Tumor Cells (CTCs), Disseminated Tumor Cells (DTCs), cell-free DNA (cfDNA), and circulating tumor DNA (ctDNA) that may have critical importance in the detection, early diagnosis, prognosis and monitoring of patients with various, difficult to treat malignancies (e.g.: breast or gastrointestinal cancers). It also focuses on some barriers to achieving the clinical management goals (for both patients with cancers and the interdisciplinary treatment teams), as well as suggests some solutions, how to overcome them, in personalized oncology approaches.

Keywords: Cancer, Biomarkers, Cancer Stem Cells (CSCs), Circulating Tumor Cells (CTCs), Precision medicine, Disseminated Tumor Cells (DTCs).


Article Information


Identifiers and Pagination:

Year: 2018
Volume: 8
First Page: 9
Last Page: 16
Publisher Id: TOBIOMJ-8-9
DOI: 10.2174/1875318301808010009

Article History:

Received Date: 11/9/2017
Revision Received Date: 11/12/2017
Acceptance Date: 2/1/2018
Electronic publication date: 08/02/2018
Collection year: 2018

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© 2018 Katarzyna Rygiel.

open-access license: This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 International Public License (CC-BY 4.0), a copy of which is available at: (https://creativecommons.org/licenses/by/4.0/legalcode). This license permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.


* Address correspondence to this author at the Department of Environmental Medicine and Epidemiology, Medical University of Silesia (SUM), Zabrze, Poland, Tel: 33 814 1426; 692 576 729; E-mails: kasiaalpha@yahoo.co.uk; medsrod@sum.edu.pl




1. INTRODUCTION

The concept that cancer develops from normal cells via the accumulation of genetic alterations, which activate carcinogenic pathways or inhibit tumor suppressor genes, has been well-known [1Polyak K, Hahn WC. Roots and stems: Stem cells in cancer. Nat Med 2006; 12: 296-300.]. It has been established that adult stem cells (SCs) can survive in an undifferentiated state for long periods of time, and have the ability to unlimited self-renewal and differentiation [2Al-Hajj M, Becker MW, Wicha M, Weissman I, Clarke MF. Therapeutic implications of cancer stem cells. Curr Opin Genet Dev 2004; 14: 43-7.]. Furthermore, the evidence exists that the SCs can acquire some carcinogenesis-initiating mutations that alter the genome stability, cellular growth inhibition, normal cell differentiation, proliferative potential, and resistance to apoptosis. Subsequently, these SCs can turn into cancer stem cells (CSCs), leading to genetic changes in clones of cells, or in primary tumors, and metastatic sites [3Boman BM, Wicha MS. Cancer stem cells: A step toward the cure. J Clin Oncol 2008; 26: 2795-9.].

This review addresses key components of CSC dysregulation in the signaling pathways, and focuses on their possible clinical (diagnostic and therapeutic) implications in personalized (precision medicine) oncology approaches.

Also, this article describes some emerging strategies, such as: Single-Cell Sequencing (SCS), Circulating Tumor

Cells (CTCs), Disseminated Tumor Cells (DTCs), cell-free DNA (cfDNA), and circulating tumor DNA (ctDNA) that may have critical importance in the detection, early diagnosis, prognosis and monitoring of patients with various, difficult to treat malignancies (e.g.: breast or gastrointestinal cancers). It also outlines some barriers to achieving the clinical management goals, for both patients with cancers and the interdisciplinary treatment teams, as well as indicates some ways to overcome them.

2. CANCER STEM CELL (CSC) PATHWAYS: A ‘DOUBLE EDGE SWORD’

It has been determined that the undesirable phenomenon of resistance to conventional anticancer therapies can be activated in various CSC populations, within the same tumor, via different mechanisms [3Boman BM, Wicha MS. Cancer stem cells: A step toward the cure. J Clin Oncol 2008; 26: 2795-9.]. Since the activation of stemness-signaling pathways seems to play a crucial role in these processes, it is essential to analyze them in order to develop the innovative therapies that will specifically target CSCs [3Boman BM, Wicha MS. Cancer stem cells: A step toward the cure. J Clin Oncol 2008; 26: 2795-9.]. It has been determined that a strict regulation of the signaling pathways, such as: NANOG, Wnt/β-catenin, Hedgehog, Notch, signal transducer and activator of transcription 3 (STAT 3), and phosphoinositide 3-kinase (PI3K) (Table 1) [4Sun AX, Liu CJ, Sun ZQ, Wei Z. NANOG: A promising target for digestive malignant tumors. World J Gastroenterol 2014; 20: 13071-8.-13Xia P, Xu XY. PI3K/Akt/mTOR signaling pathway in cancer stem cells: From basic research to clinical application. Am J Cancer Res 2015; 5: 1602-9.] is critical for SCs to maintain the ability to self-renewal and differentiation. On the other hand, however, an abnormal inhibition or activation or of these pathways can adversely upregulate cell proliferation [14Reya T, Morrison SJ, Clarke MF, Weissman IL. Stem cells, cancer, and cancer stem cells. Nature 2001; 414: 105-11.]. Moreover, some similarities have been found between SCs and cancer cells. For instance, tumors can originate from the transformation of normal SCs, and almost identical signaling pathways can regulate self-renewal in SCs, and in cancer cells. In addition, cancer cells may contain a subpopulation of cancer stem cells (CSCs) that are characterized by the potential for both self-renewal and tumorigenesis [14Reya T, Morrison SJ, Clarke MF, Weissman IL. Stem cells, cancer, and cancer stem cells. Nature 2001; 414: 105-11.]. As a consequence, the remaining CSCs even after traditional oncology therapies, such as surgical resection of a tumor, radiation therapy (RT), and chemotherapy (ChT), can regenerate the original tumor, at the primary location, or at the metastatic sites. Since the tumor recurrence originates from the CSCs, which evaded conventional anticancer treatments, such relapsed tumors are usually very aggressive, and resistant to standard anticancer therapies [15Verga Falzacappa MV, Ronchini C, Reavie LB, Pelicci PG. Regulation of self-renewal in normal and cancer stem cells. FEBS J 2012; 279: 3559-72.].

Table 1
Key components of Cancer Stem Cells (CSCs) dysregulation in the stemness signaling pathways and their possible clinical implications in oncology.


3. CSC BIOMARKERS: NEW HOPES AND CHALLENGES

CSCs possess a unique self-renewal activity, together with an ability to mediate tumor initiation and propagation [15Verga Falzacappa MV, Ronchini C, Reavie LB, Pelicci PG. Regulation of self-renewal in normal and cancer stem cells. FEBS J 2012; 279: 3559-72.]. A process of the identification and isolation of CSCs, using biomarkers (such as cell-surface molecular markers) is a basic step for the development, and subsequent implementation of innovative therapies that can specifically target CSCs. Research in this area has been growing over the last decade. However, many challenges still remain [16Chen K, Huang YH, Chen JL. Understanding and targeting cancer stem cells: Therapeutic implications and challenges. Acta Pharmacol Sin 2013; 34: 732-40.]. For instance, several studies were performed on the surface biomarkers for a possible identification and isolation of CSCs. However, lack of universal expression of these surface biomarkers limits their usage. Also, there is no consensus with regard to the most appropriate combination of cell biomarkers for identification of CSCs. In addition, no specific combination of biomarkers has yet been determined to identify the CSCs that can cause initiation and metastatic spread of particular neoplastic tumors. Moreover, many of the presently available biomarkers can also be expressed in non-CSCs [16Chen K, Huang YH, Chen JL. Understanding and targeting cancer stem cells: Therapeutic implications and challenges. Acta Pharmacol Sin 2013; 34: 732-40.]. Currently, the most promising biomarker molecules include: CD133, CD44, and the epithelial cell adhesion molecule (EpCAM) (Table 2) [17Chen S, Song X, Chen Z, et al. CD133 expression and the prognosis of colorectal cancer: A systematic review and meta-analysis. PLoS One 2013; 8: e56380.-19Trzpis M, McLaughlin PM, de Leij LM, et al. Epithelial cell adhesion molecule: More than a carcinoma marker and adhesion molecule. Am J Pathol 2007; 171: 386-95.].

4. CLINICAL IMPLICATIONS OF CSC BIOMARKERS: FOCUS ON GASTROINTESTINAL CANCERS

Gastrointestinal (GI) malignancies represent a group of common cancers, worldwide, often characterized by poor prognosis and high mortality rates [20De Angelis R, Sant M, Coleman MP, et al. Cancer survival in Europe 1999-2007 by country and age: Results of EUROCARE-5--a population-based study. Lancet Oncol 2014; 15: 23-34.]. According to the results of recent clinical trials, such as EUROCARE-5, a population-based study, the most common GI cancers, in Europe and in North America, include: colorectal, gastric, and pancreatic cancer. Current therapies of these GI malignancies could be successful, only upon early disease detection (e.g., in localized stages). Otherwise, due to metastasis or relapse, the outcomes of patients with GI cancers are poor. In addition, the recurrent cancer is frequently resistant to conventional ChT or RT, probably due to the CSC properties [21Singh A, Settleman J EMT, et al. Cancer stem cells and drug resistance: An emerging axis of evil in the war on cancer. Oncogene 2010; 29: 4741-51.]. In this context, a combination therapy with medications that target CSCs, together with standard anticancer therapy could have a beneficial impact on the outcomes of GI neoplasms. In fact, some investigational agents, for targeting CSCs in GI cancers, are currently in different phases of clinical development. It should be underscored that two oral medications: napabucasin and vismodegib have already been approved by the US Food and Drug Administration (FDA) for the therapy of skin basal cell cancer. Recently, these two agents have also been examined in randomized, clinical trials (RCTs) for targeting CSCs, in patients with GI cancers (in which the Hedgehog signaling pathway plays an important role in CD44(+) gastric cancer cells) [22Zhang Y, Jin Z, Zhou H, et al. Suppression of prostate cancer progression by cancer cell stemness inhibitor napabucasin. Cancer Med 2016; 5: 1251-8., 23Yoon C, Park DJ, Schmidt B, et al. CD44 expression denotes a subpopulation of gastric cancer cells in which Hedgehog signaling promotes chemotherapy resistance. Clin Cancer Res 2014; 20: 3974-88.]. Napabucasin is a STAT3 inhibitor, targeting the STAT3, β-catenin, and NANOG signaling pathways. It inhibits the main genes that are mandatory for maintaining cancer stemness. Napabucasin has revealed antitumor and antimetastatic activity in colorectal, gastric, and gastroesophageal cancer [22Zhang Y, Jin Z, Zhou H, et al. Suppression of prostate cancer progression by cancer cell stemness inhibitor napabucasin. Cancer Med 2016; 5: 1251-8.]. Vismodegib is an antagonist of the hedgehog pathway, which was studied in a randomized, double-blind, phase 2 clinical trial, in patients with advanced gastric and gastroesophageal junction malignancies, in combination with folinic acid (leucovorin), fluorouracil, and oxaliplatin (FOLFOX) chemotherapy In the vismodegib group, two patients had a complete response, and they revealed enhanced median CD44 expression rates. In contrast, in the chemotherapy-alone group, high CD44 expression was correlated with reduced survival. These findings suggest that the patients who have GI cancers with high CD44 expression can have improved survival upon receiving a combination of FOLFOX chemotherapy and vismodegib [23Yoon C, Park DJ, Schmidt B, et al. CD44 expression denotes a subpopulation of gastric cancer cells in which Hedgehog signaling promotes chemotherapy resistance. Clin Cancer Res 2014; 20: 3974-88.].

Table 2
The main biomarkers of Cancer Stem Cells (CSCs).


5. SINGLE-CELL SEQUENCING (SCS): POSSIBLE ADVANTAGES AND DIFFICULTIES

In general, malignant carcinomas are characterized by different types of heterogeneity, including: (1) population heterogeneity or differences between tumors from different patients, (2) intratumor or spatial heterogeneity, within a single tumor mass, and (3) temporal heterogeneity reflecting variability over time, during the tumor growth and development, or in response to treatment [24Ellsworth RE, Blackburn HL, Shriver CD, Soon-Shiong P, Ellsworth DL. Molecular heterogeneity in breast cancer: State of the science and implications for patient care. Semin Cell Dev Biol 2017; 64: 65-72.]. In particular, the intratumor heterogeneity (e.g., in breast cancer), is characterized by molecular and genomic variability within carcinomas, and also, among disseminated cells and cell-free nucleic acids. Usually, targeted anticancer treatment agents are designed to focus on actionable mutations that are detected via a biopsy of the primary tumor. However, such “actionable” mutations may no longer cause disease progression, especially when the tumor cells were disseminated from the primary carcinoma and underwent genomic transformation. At this point, the capability of single-cell sequencing (SCS) to reveal the genomic “make-up” of circulating and disseminated cancer cells appears very promising for improving the diagnostic accuracy in patients with cancer. After applying SCS mostly in the research area, there are some potential clinical applications, related to diagnostic work-up, therapeutic decision making, monitoring, and outcome prediction [25Navin NE. The first five years of single-cell cancer genomics and beyond. Genome Res 2015; 25: 1499-507.]. In fact, the SCS could be particularly useful in an early step of disease diagnosis, via the analysis of blood or urine. During such a noninvasive monitoring of high-risk patients, single disseminated cancer cells or very small tumors can be identified at an early stage, before a malignant lesion can be detected by conventional methods.(e.g., on CT scan). Furthermore, an assessment of genomic heterogeneity within the primary tumor or among disseminated cells would have a prognostic value (e.g., a lower intratumor heterogeneity is usually related with more beneficial outcomes) [24Ellsworth RE, Blackburn HL, Shriver CD, Soon-Shiong P, Ellsworth DL. Molecular heterogeneity in breast cancer: State of the science and implications for patient care. Semin Cell Dev Biol 2017; 64: 65-72., 26Burrell RA, McGranahan N, Bartek J, Swanton C. The causes and consequences of genetic heterogeneity in cancer evolution. Nature 2013; 501: 338-45.].

SCS may also be used to optimize treatment strategies. For instance, the ability to identify common mutations, throughout a malignant lesion, could permit use of single agent that targets the tumor’s bulk, while assaying heterogeneous actionable mutations, could lead to implementing novel approaches that simultaneously target sub-clonal populations of cells [25Navin NE. The first five years of single-cell cancer genomics and beyond. Genome Res 2015; 25: 1499-507.]. For cancer treatment, the most promising clinical use of SCS is the analysis of circulating tumor cells (CTCs), which may provide a non-invasive method for clinicians to monitor individual response to therapy, before tumors become symptomatic or detectable through traditional diagnostic methods. Serial analysis of individual CTCs, isolated from blood samples taken over the course of treatment, may be used to identify new mutations that emerge in response to therapy, which can impact disease progression or therapeutic resistance, enabling oncologists to promptly adjust the treatment, accordingly [27Aparicio S, Caldas C. The implications of clonal genome evolution for cancer medicine. N Engl J Med 2013; 368: 842-51.]. In addition, targeted elimination of circulating tumor cells, with stem-cell-like expression profiles, could prevent the colonization of secondary sites and formation of metastases [27Aparicio S, Caldas C. The implications of clonal genome evolution for cancer medicine. N Engl J Med 2013; 368: 842-51.].

Despite the potential utility of SCS in clinical cancer care, several current limitations need to be addressed before SCS can be used routinely in practice. In the clinical environment, cancerous tissues excised from the body have traditionally been prepared for pathological examination by fixing the tissue in formalin and embedding in paraffin. However, most single-cell isolation and sequencing methods have been designed for use with suspensions of live cells acquired from fresh tissues [28Baslan T, Kendall J, Rodgers L, et al. Genome-wide copy number analysis of single cells. Nat Protoc 2012; 7: 1024-41.]. Although the nuclear membrane is resistant to freezing and thawing, allowing individual nuclei to be isolated from nuclear suspensions derived from frozen tissues for DNA sequencing fresh tissue is currently needed for single-cell RNA-seq. To implement SCS in the clinic, new tissue collection and handling protocols will have to be established and validated at specialized medical centers and treatment facilities. Single-cell whole-genome amplification (WGA) and whole-transcriptome amplification (WTA) techniques, currently being used, have some technological limitations. A big challenge to implementing SCS in the clinic is overcoming errors that can be introduced by amplifying the minute amount of DNA or RNA in a single cell, and validating the sequencing results [28Baslan T, Kendall J, Rodgers L, et al. Genome-wide copy number analysis of single cells. Nat Protoc 2012; 7: 1024-41., 29Mato Prado M, Frampton AE, Stebbing J, Krell J. Single-cell sequencing in cancer research. Expert Rev Mol Diagn 2016; 16: 1-5.]. Future improved technologies, as well as new computational methods will be necessary before SCS can reliably distinguish technical errors from true biological variability, and generate valid results to inform the patient care [29Mato Prado M, Frampton AE, Stebbing J, Krell J. Single-cell sequencing in cancer research. Expert Rev Mol Diagn 2016; 16: 1-5.]. Currently, the cost of SCS also prohibits its wide implementation in the clinical setting. This is partially due to the fact that many single cells need to be sequenced, depending on a variety of factors (e.g.: the disease stage and tumor heterogeneity), in order to add important medical information to existing baseline. Of course, further large-scale studies, evaluating clinical validity are needed, prior to implementation of SCS into standard diagnostic work-up [30Niu F, Wang DC, Lu J, Wu W, Wang X. Potentials of single-cell biology in identification and validation of disease biomarkers. J Cell Mol Med 2016; 20: 1789-95.].

6. CURRENT BARRIERS TO IMPLEMENTING SCS TECHNOLOGY IN THE CLINICAL PRACTICE

There are some barriers to introducing SCS into the clinical practice, including the cost of SCS, the time necessary for isolation of single cells, DNA amplification, NGS, and data interpretation, and the lack of experts (e.g.: oncologists or pathologists, who will process the sequencing results, and combine them with clinical decision making processes). Some key issues that need to be clarified involve: the interpretation and application of SCS findings in context of individual patients, the translation of DNA or RNA variation within single cells into specific clinical phenotypes, and the methods to use SCS results for prediction of patient response to anticancer therapy [30Niu F, Wang DC, Lu J, Wu W, Wang X. Potentials of single-cell biology in identification and validation of disease biomarkers. J Cell Mol Med 2016; 20: 1789-95., 31Laskin J, Jones S, Aparicio S, et al. Lessons learned from the application of whole-genome analysis to the treatment of patients with advanced cancers. Cold Spring Harb Mol Case Stud 2015; 1: a000570.[http://dx.doi.org/10.1101/mcs.a000570] ]. Recently, the Individualized Molecular Pancreatic cancer Therapy (IMPaCT) trial, designed to improve patient outcomes, applying genomic information to direct therapeutic decisions for patients with advanced pancreatic cancer, revealed that a multidisciplinary team (including a pathologist, oncologist, geneticist, genetic counselor, and research coordinator), working in well-equipped setting (e.g., able conduct genomic analyses), and returning the results in an acceptable timeframe (e.g., approximately two weeks) would be required [32Chantrill LA, Nagrial AM, Watson C, et al. Precision medicine for advanced pancreas cancer: The individualized molecular pancreatic cancer therapy (IMPaCT) trial. Clin Cancer Res 2015; 21: 2029-37.]. In the near future, progress in the isolation of single cells, WGA, next-generation sequencing (NGS), and computation methods will be mandatory to improve the clinical utility of SCS. In particular, the ability to amplify and sequence RNA molecules (e.g.: long non-coding RNAs and micro RNAs) can provide important data on gene regulation. Furthermore, innovative methods to amplify and sequence genomic DNA and full-length mRNA, from the same cell, can provide precise tools for evaluating the effects of genomic variation on gene expression profiles [33Dey SS, Kester L, Spanjaard B, Bienko M, Van OA. Integrated genome and transcriptome sequencing of the same cell. Nat Biotechnol 2015; 33: 285-9., 34Macaulay IC, Teng MJ, Haerty W, Kumar P, Ponting CP, Voet T. Separation and parallel sequencing of the genomes and transcriptomes of single cells using G&T-seq. Nat Protoc 2016; 11: 2081-103.] Similarly, the ability to couple genome-wide methylation and proteomic analysis, with single-cell DNA- and RNA-sequencing from of individual cells, may reveal some important mechanisms, by which genetic and epigenetic modifications regulate the transcriptional heterogeneity in cancer [35Hou Y, Guo H, Cao C, et al. Single-cell triple omics sequencing reveals genetic, epigenetic, and transcriptomic heterogeneity in hepatocellular carcinomas. Cell Res 2016; 26: 304-19., 36Darmanis S, Gallant CJ, Marinescu VD, et al. Simultaneous multiplexed measurement of RNA and proteins in single cells. Cell Reports 2016; 14: 380-9.]. Moreover, fluidic systems to simultaneously isolate and analyze millions of cells in parallel can provide a detailed ‘portrait’ of cancer development and response to therapy, for each individual patient. Finally, localizing the spatial organization of gene and protein expression, within a single cell, can serve as a clue, allowing to determine the behavior and survival of cancer cells during a targeted therapy [37Lee JH, Daugharthy ER, Scheiman J, et al. Highly multiplexed subcellular RNA sequencing in situ. Science 2014; 343: 1360-3.]. Since heterogeneity in patients with malignancies is very dynamic, it can evolve unpredictably during cancer progression, and create many new challenges for anticancer treatments. As a potential solution to these problems, SCS can facilitate precision (personalized) treatment approach, in which the heterogeneity will be well-characterized prior to, and during therapy. In addition, integrated SCS approaches can provide new insights into malignancy evolution, and outline new directions for activation of signaling pathways that cause heterogeneous cellular responses to therapy [38Wang Y, Navin NE. Advances and applications of single-cell sequencing technologies. Mol Cell 2015; 58: 598-609.].

7. CIRCULATING TUMOR CELLS (CTCS) AND DISSEMINATED TUMOR CELLS (DTCS)

It has been known that malignant neoplasms are heterogeneous, and this heterogeneity affects clinical management and patient outcomes (e.g.: recurrence and therapeutic resistance). Consequently, functional significance of the cancer genome of every individual patient is crucial for the development of novel treatments that can overcome difficulties created by molecular heterogeneity [3Boman BM, Wicha MS. Cancer stem cells: A step toward the cure. J Clin Oncol 2008; 26: 2795-9., 14Reya T, Morrison SJ, Clarke MF, Weissman IL. Stem cells, cancer, and cancer stem cells. Nature 2001; 414: 105-11., 39Clevers H. The cancer stem cell: Premises, promises and challenges. Nat Med 2011; 17: 313-9.]. For instance, in breast cancer, intratumor heterogeneity for genetic changes and activated signaling pathways (e.g.: the Notch, Hedgehog, and Wnt) has been noted in populations of putative breast CSCs. This genetic heterogeneity can lead to phenotypic heterogeneity that significantly impacts clinical outcomes, including treatment resistance and metastatic spread [39Clevers H. The cancer stem cell: Premises, promises and challenges. Nat Med 2011; 17: 313-9.].

Since rare de novo mutations, and transcriptional changes, in individual cells, usually cannot be detected during assessment of larger parts of the malignant tumors, “single-cell genomics” is being applied to explore individual cells from primary tumors, metastatic lesions, circulating tumor cells (CTCs) or disseminated tumor cells (DTCs), in order to guide subsequent,personalized diagnostic and therapeutic process [40Zhang X, Marjani SL, Hu Z, Weissman SM, Pan X, Wu S. Single-cell sequencing for precise cancer research: Progress and prospects. Cancer Res 2016; 76: 1305-12.]. It should be highlighted that the cell-free DNA (cfDNA) is composed of nucleic acid parts that have been released to the blood stream from cells during necrosis, apoptosis, or macrophage phagocytosis. The cfDNA can be detected in healthy persons, however, in cancer patients, cfDNA levels in serum are much higher (e.g., especially in patients with metastatic disease) [41Shaw JA, Guttery DS, Hills A, et al. Mutation analysis of cell-free DNA and single circulating tumor cells in metastatic breast cancer patients with high CTC counts. Clin Cancer Res 2017; 23: 88-96.]. A fraction of the cfDNA, known as a circulating tumor DNA (ctDNA), is more abundant in the bloodstream than CTCs. Moreover, ctDNA profiles, in patients with metastatic breast cancer, can precisely illustrate the mutations of individual CTCs [41Shaw JA, Guttery DS, Hills A, et al. Mutation analysis of cell-free DNA and single circulating tumor cells in metastatic breast cancer patients with high CTC counts. Clin Cancer Res 2017; 23: 88-96.].

8. CLINICAL IMPLICATIONS OF CTCS FOR THE PATIENT MANAGEMENT

In clinical practice, a single tumor biopsy is likely to contain only a minority of genetic aberrations present in the entire carcinoma. This often results in underestimation of the mutational burden of heterogeneous tumors, as well as inaccurate prediction of mandatory therapy [42Gerlinger M, Rowan AJ, Horswell S, et al. Intratumor heterogeneity and branched evolution revealed by multiregion sequencing. N Engl J Med 2012; 366: 883-92.]. For instance, presently, patients with breast cancer rarely undergo a metastatic site biopsy. It should be highlighted that biopsies of metastatic lesions can be clinically required for majority of patients, to make sure that the applied therapies accurately target genomic heterogeneity between the primary carcinoma and metastases [43Foukakis T, Åström G, Lindström L, Hatschek T, Bergh J. When to order a biopsy to characterise a metastatic relapse in breast cancer. Ann Oncol 2012; 23(Suppl. 10): x349-53.]. In the future, in order to improve this situation, monitoring of breast cancer heterogeneity (during malignancy progression, or in response to treatment) via CTCs, DTCs, and ctDNA will help overcome sampling bias. Many traditional anticancer medications have been developed to target rapidly proliferating cells of the primary tumor [44Miller SJ, Lavker RM, Sun TT. Interpreting epithelial cancer biology in the context of stem cells: Tumor properties and therapeutic implications 1756.]. These agents usually produce clinically beneficial results in initial phases of treatment, illustrated by decreases in size of the primary tumor. However, such a clinical remission is often temporary, since initially quiescent, but genetically diverse CSCs can survive, and lead to recurrence, once a standard therapy is completed. Although the choice of targeted therapy is often based on mutations present in an initial biopsy specimen, these “actionable” mutations may no longer drive cancer progression, when tumor cells disseminate from the primary tumor. Similarly, the predominant clones, in the primary malignant lesion, may not be prevalent in the metastases or CTCs, due to clonal selection that occurs with various therapies. Therefore, it is crucial to identify which clones, within a given cancer patient are the most relevant to cancer progression or treatment resistance [45Aparicio S, Caldas C. The implications of clonal genome evolution for cancer medicine. N Engl J Med 2013; 368: 842-51.]. Unquestionably, large clinical trials are needed to assess the value of matching patients to specific, targeted therapies, and to determine whether or not modern genetic profiling could significantly improve patient care [46Bedard PL, Hansen AR, Ratain MJ, Siu LL. Tumour heterogeneity in the clinic. Nature 2013; 501: 355-64.].

CONCLUSION

In summary, the discovery of CSCs and related signaling pathways outlined some new research directions in oncology. However, the role of CSC regulatory mechanisms in nonmalignant tissues continues to be a significant challenge to the practical application of CSC-targeted therapies, in patients with different types of malignancies. In the future, the identification and validation of specific biomarkers, which can more accurately detect patients with upregulated CSCs, is one of the priorities. In consequence, an incorporation of such biomarkers into clinical trials, and then, possibly into oncology practice, could help identify patient subpopulations that are most likely to benefit from the combination of standard anticancer treatments (e.g., chemotherapy) and CSC-inhibiting agents. In addition, further understanding of the communication between various CSC signaling pathways is necessary to reveal mechanisms of resistance to therapy. Finally, it should be highlighted that personalized strategies, in which the cancer heterogeneity is precisely characterized (prior to the initiation of therapy) are needed, and deserve exploration, in a large scale clinical trials (investigating the use of different agents targeting CSCs). Subsequent translating of these research findings into clinical settings will be useful for guiding precise treatment, in individual patients with the most aggressive malignancies

CONSENT FOR PUBLICATION

Not applicable.

CONFLICT OF INTEREST

The author declares no conflict of interest, financial or otherwise.

ACKNOWLEDGEMENTS

Declared none.

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