Cancer Stem Cell Markers

Among all cancerous cells, a few act as stem cells that reproduce and sustain the tumor, much like stem cells normally renew and sustain our organs and tissues. Cancer stem cells (CSCs) are generated by a combination of DNA mutations, epigenetic events, and tissue microenvironment factors from normal stem cells or precursor/progenitor cells, to which they are closely related and share many of the behaviors and features¹. 

One crucial ability of the CSCs is to form tumors already at low cell numbers. Therefore, targeting CSCs is of primary importance. Many new anti-cancer therapies are evaluated based on their ability to shrink tumors. However, if these therapies are not destroying CSCs, the tumor will soon grow back, often with high resistance to treatments². In addition, CSCs might give rise to metastases, acting as a reservoir of cancer cells, causing relapse after the surgery or treatment has eliminated the visible signs of cancer. 

CSCs were first identified in hematological cancer such as acute myeloid leukemia in the late ’90s when scientists isolated a subpopulation of leukemia cells that expressed the surface marker CD34 but not CD38. The authors established that this CD34+/ CD38− subpopulation was capable of initiating tumors³. 

The existence of stem cells in hematological tissue prompted research in other tissue cancer types. Since 2003, CSCs have been identified in several solid tumors, including brain⁴, breast⁵, colon⁶, ovary^7,8, pancreas⁹, prostate^10,11, melanoma^12,13, multiple myeloma¹⁴, and non-melanoma skin cancer^15,16. 

The term CSCs itself was, however, only coined in 2001¹⁷. 

Examples of CSC markers

The most frequently used markers for CSCs isolation include cell surface markers, such as CD133 (PROM1), CD44, ALDH1A1, CD34, CD24, and EpCAM (ESA). 

In addition, the expression of stemness genes/proteins is also used to identify CSCs. For instance, OCT4, SOX2, and NANOG are the transcription factors analyzed in most studies. They are commonly expressed in pluripotent embryonic stem cells, germ cells, certain committed progenitors, and cancer cells¹⁸. However, different markers can be used depending on the tissue where the tumor is generated. CSCs isolated in breast cancer, for example, are enriched in CD44+CD24−, SP, and ALDH+ subpopulations¹⁹. On the other hand, in brain tumors, such as glioma and glioblastoma, CSCs have instead been identified using cell surface markers, including SSEA-1²⁰, EGFR²¹, and CD44²². 

The use of CD133 (PROM1) to identify CSCs in glioma is still in question because tumorigenic cells are equally found in both CD133+ and CD133− cells in some gliomas, while some CD133+ brain tumor cells may not possess tumor-initiating capacity ²³. 

Using CD133 (PROM1) as a positive marker for colon CSCs also generated conflicting results²⁴. 

For lung cancers, CSC markers include CD44 and CD133 (PROM1), but also CD117 (KIT), CD90 (or THY1), CD166, EpCAM for non-small-cell lung carcinoma (NSCLC), and PODXL-1, PTCH, and CD87 for small-cell lung carcinoma (SCLC)²⁵. 

CSC markers for solid and hematological tumors

In the next tables, we have selected our CSC markers for solid (Table 1) and hematological (Table 2) tumors. You can also find a list of our extracellular CSC/CD markers (Table 3), extracellular CSC/not- CD markers (Table 4), and intracellular CSC markers (Table 5).

Tumors of the hematopoietic and lymphoid tissues: leukemia and lymphoma

Hematological cancers (blood cancers) arise in the blood-forming tissues, such as bone marrow, lymph nodes, and lymphatic system. Examples of hematologic cancer include leukemia, lymphoma, and multiple myeloma. 

Since these tissues are all intimately connected through both the circulatory and the immune system, a disease affecting one system will often affect the other as well, making myeloproliferation (leukemias) and lymphoproliferation (lymphomas) closely related and often overlapping conditions. 

The more aggressive forms of hematopoietic and lymphoid tissue diseases require treatment with chemotherapy, radiotherapy, immunotherapy and, in some cases, a bone marrow transplant. 

Acute (AML) and chronic (CML) myeloid leukemia are cancers of the myeloid line of blood cells, characterized by the rapid growth of abnormal cells that build up in the bone marrow and blood and interfere with normal blood cell production. AML progresses rapidly into CML, becoming fatal within weeks or months if left untreated. 

1. CSC markers in leukemia

Unlike solid tumors, a common cause of hematological cancers is chromosomal translocations. Interestingly, studies show that there is a sequential order for the acquisition of mutations during leukemogenesis: a) somatic mutations in epigenetic modifiers that regulate cytosine methylation, such as DNMT3A, IDH1/2^26, and TET2²⁷, occur early in pre-leukemic hematopoietic stem cells; b) mutations in signaling pathways that drive proliferation, such as NPM1, FLT3-ITD, and KRAS/NRAS, are later events in AML transformation^28,29. 

A combinatorial approach is often used when determining which markers to select for leukemia. Common CSCs markers in leukemia include CD34, CD38, CD123, TIM3, CD25, CD32 and CD96. 

Figure 3 Left: Western blot analysis in human cell lines A-549 and A-431 using the anti-ALDH1A1 antibody (HPA002123). Loading control: Anti-PPIB.
Right: orthogonal validation of protein expression by comparison to RNA-seq data of corresponding ALDH1A1 RNA-seq data for the same cell lines. 

2. CSC markers in lymphomas

Lymphomas are a heterogeneous group of lymphoid malignancies with varied clinical behavior and responses to treatment. The two main categories of lymphomas are the non- Hodgkin lymphoma and Hodgkin lymphoma. Non-Hodgkin lymphoma (about 90% of cases) is further subdivided into different subtypes. Some examples are mantle cell lymphoma and diffuse large B cell lymphoma. 

Mantle cell lymphoma (MCL) is a rare but unique subtype of B-cell non-Hodgkin lymphoma. The presence of CD45+/CD19- cell population was first demonstrated in 2014 in bone marrow aspirates of patients with MCL. These CD45+/ CD19- cells, resistance to treatment, might also be associated with treatment failure^30,31. 

Diffuse large B cell lymphoma (DLBCL) accounts for about 40% of non-Hodgkin’s lymphoma (NHL) cases. Since CD45+CD19- is a marker of CSCs in MCL, a recent study explored the possibility of CD45+CD19- as a potential marker of DLBCL CSCs in vivo and in vitro. However, the results show that in DLBCL, CSCs are not enriched in the CD45+CD19- but ALDH high cells³². 

Extracellular and intracellular CSC markers

* Products with enhanced validation for the indicated application 

* Products with enhanced validation for the indicated application 

References

1. Lia Walcher et al., (2020) Cancer Stem Cells-Origins and Biomarkers: Perspectives for Targeted Personalized Therapies. Review - eCollection Frontiers Immunology

2. Arnold CR, et al., (2020) The Role of Cancer Stem Cells in Radiation Resistance. Front Oncol. 10:164

3. Bonnet D, Dick JE (1997) “Human acute myeloid leukemia is organized as a hierarchy that originates from a primitive hematopoietic cell.” Nature Medicine. 3 (7): 730–7

4. Singh SK, et al., (2003) “Identification of a cancer stem cell in human brain tumors.” Cancer Research. 63 (18): 5821–8

5. Al-Hajj M, et al., (2003) “Prospective identification of tumorigenic breast cancer cells.” PNAS USA 100 (7): 3983–8

6. O’Brien CA, et al., (2007) “A human colon cancer cell capable of initiating tumor growth in immunodeficient mice.” Nature. 445 (7123) 106–10

7. Zhang S, et al., (2008) “Identification and characterization of ovarian cancer-initiating cells from primary human tumors.” Cancer Research. 68 (11): 4311–20

8. Alvero AB, et al., (2009) “Molecular phenotyping of human ovarian cancer stem cells unravels the mechanisms for repair and chemoresistance.” Cell Cycle. 8 (1): 158–66

9. Li C, et al.,(2007). “Identification of pancreatic cancer stem cells” Cancer Research. 67 (3): 1030–7

10. Maitland NJ, Collins AT (2008) “Prostate cancer stem cells: a new target for therapy.” Journal of Clinical Oncology. 26 (17): 2862–70

11. Lang SH, et al., (2009) “Prostate cancer stem cells.” The Journal of Pathology. 217 (2): 299–306

12. Schatton T, et al., (2008) “Identification of cells initiating human melanomas.” Nature. 451 (7176): 345–9

13. Schmidt P, et al., (2011) “Eradication of melanomas by targeted elimination of a minor subset of tumor cells.” PNAS USA 108 (6): 2474–9

14. Matsui W, et al.,(2004) “Characterization of clonogenic multiple myeloma cells.” Blood. 103 (6): 2332–6

15. Colmont CS, et al.,(2013) “CD200-expressing human basal cell carcinoma cells initiate tumor growth”. PNAS USA 110 (4): 1434–9

16. Patel GK, et al., (2012) “Identification and characterization of tumor-initiating cells in human primary cutaneous squamous cell carcinoma.” The Journal of Investigative Dermatology. 132 (2): 401–9

17. Reya T, et al., (2001) “Stem cells, cancer, and cancer stem cells.” Nature. 414 (6859): 105–11