Prognostic significance of stem cell markers: In triple-negative breast cancers

Abstract

Triple-negative breast cancer (TNBC) is defined by the lack of expression of estrogen receptor, progesterone receptor, and human epidermal growth factor receptor-2. Investigating the role of cancer stem cell (CSC) markers in TNBC carcinogenesis and tumor progression is of increasing interest. TNBC cells have been reported to exhibit CSC characteristics at functional, molecular, and transcriptional levels. Several stemness markers have been described across different histological subtypes of breast cancer, including CD44, CD24, CD133, EpCAM, CD166, Lgr5, CD47, ALDH1, and ABCG2. Recent studies have demonstrated that CD133 positivity enables the identification of CSC populations in breast cancer. These markers may have enhanced prognostic value if TNBC tumors are further stratified into molecular subtypes, particularly through evaluation of their expression in the mesenchymal stem-like subtype. In this article, we comment on the article by Depar et al published in the World Journal of Clinical Oncology.

Key Words: Triple-negative breast cancer; Cancer stem cell; Bio-markers; Prognostic Significance

Core Tip: Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer characterized by absence of estrogen, progesteron and human epidermal growth factor receptor-2 expressions. TNBC exhibit cancer stem cell (CSC)-like properties at functional, molecular, and transcriptional levels. Stemness markers described across different histological subtypes include CD44, CD24, CD133, ALDH1 and ABCG2. These markers have prognostic and predictive value in TNBC, as their expression has been associated with increased tumor aggressiveness, chemoresistance, poor prognosis, lack of effective targeted therapies and unfavorable clinical outcomes. Recent studies on CSC markers in TNBC highlight their potential as new prognostic biomarkers and targeted therapies.

This editorial refers to “Cancer stem cell markers, chemotherapy response, and survival in triple-negative breast cancer” by Depar et al, 2026; https://doi.org/10.5306/wjco.v17.i2.116251.

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INTRODUCTION

Breast cancer remains one of the most common malignancies affecting women worldwide[1]. Triple-negative breast cancer (TNBC) refers to breast cancer with immunohistochemically negative expression of estrogen receptor, progesterone receptor (PR) and human epidermal growth factor receptor-2[2]. TNBC accounts for 15%-20% of global breast cancers, characterized by high aggression, poor prognosis, and limited targeted therapies, with 25%-30% prevalence in developing countries, representing a major disparity. Also TNBC frequently diagnosed in younger women, premenopausal women, and those with BRCA1 mutations.

It is more likely that TNBC is related to hereditary conditions than other breast cancer subtypes. Beyond BRCA1/2, other hereditary gene mutations are associated with an increased risk of developing TNBC, many of which are involved in DNA repair, such as: Mutations in TP53, BARD1, RAD51C, RAD51D and PALB2. TNBC is a highly aggressive malignancy associated with poor prognosis and limited therapeutic options. It is often diagnosed at advanced stages and is marked by a high propensity for recurrence and metastasis, substantial resistance to chemotherapy, and low overall survival rates[3]. John et al[4] study showed that, considering the correlation between BRCA1 mutations and TNBCs, genetic screens can be potentially useful to distinguish individuals at high risk of developing TNBCs. Triple negative breast cancers showed 29.2% germline positivity[4].

THE POTENTIAL MECHANISM OF CANCER STEM CELLS IN TUMOR PROGRESSION IN TNBC

Cancer stem cells (CSC) are key drivers of tumor initiation, relapse, and recurrence[5]. Carcinomas contain a distinct population of stem-like cells from which tumors arise. These cells reside in specific microenvironmental condition, possess self-renewal capacity, and are thought to play a key role in tumor relapse and disease recurrence. Based on transcriptome and pathway research, TNBC is separated into four primary actionable subtypes: Mesenchymal/mesenchymal stem-like, epithelial-to-mesenchymal transition (EMT/immune suppression), luminal androgen receptor (steroid metabolism), basal-like 1 (DNA damage/proliferation), and basal-like 2 (growth factor signalling). Through functional axes such self-renewal signalling [notch, Wnt/β-catenin, hedgehog, PI3K/Akt, mTOR, transforming growth factor-β (TGF-β)], EMT, and immune evasion, CSC mechanically drive tumor aggressiveness, recurrence, and therapy resistance through high adaptability[6].

The recent evidence shows that HOXC8 is aberrantly expressed across multiple malignancies, including cervical, breast, non-small cell lung, liver, and epithelial ovarian cancers[7]. Functionally, HOXC8 has been shown to promote cancer cell invasion and metastasis, EMT, CSC self-renewal and therapeutic resistance. HOXC8 regulates self renewal, differentiation and transformation of breast CSC[8].

Accumulating evidence indicates that the proliferation, progression, and metastasis of breast cancer are likewise driven by a small subset of cells with stem-like characteristics, known as breast CSC. Targeting these stem cells is currently being explored as a promising long-term therapeutic strategy for patients with TNBCs.

BREAST CSC IN TNBC

CSCs can be identified using stem cell markers; however, the antibodies used to detect these markers lack specificity and frequently label additional cell populations. Therefore, immunostaining with stem cell marker antibodies may reflect phenotypic changes associated with prognosis. TNBC cells have been reported to exhibit CSCs properties at functional, molecular, and transcriptional levels. The biomarkers serve as the critical link between molecular status and clinical decision-making, to include genetic alterations, immune microenvironment, epigenetic modifications, protein expression patterns, metabolic signatures, etc.[9].

Recently, the identification of CSCs niches within tumor tissues has gained significant prognostic importance, largely because breast carcinogenesis may arise from the deregulation of molecular pathways that govern the self-renewal of mammary epithelial cells. Numerous stemness markers have been reported for the identification of breast CSC across different cancer subtypes, including CD44, CD24, CD133, EpCAM, CD166, Lgr5, CD47, ALDH1, and ABCG2. TNBC, particularly basal-like and claudin-low subtypes, is significantly enriched with both CD44+/CD24-/low and ALDH1+ cells compared to other molecular subtypes, which contributes to its poor prognosis. The CD44+/CD24- phenotype and high ALDH activity have become the characteristics of breast CSCs[10,11].

ALDH1 is a recognized marker of both normal and CSCs in the breast and colon. In breast cancer, ALDH1 immunostaining identifies stem cell populations within both normal tissue and tumors. Notably, ALDH1 expression correlates with basal cytokeratin expression, a characteristic feature commonly observed in TNBCs.

The ALDH1 expression rate was significantly higher in TNBCs than in non-TNBCs and was associated with overall survival in both subsets; it turned out to be an independent prognostic factor reflecting poor prognosis[12]. ALDH1-positive cases had shorter relapse-free survival and overall survival. ALDH1 expression was an independent prognostic indicator for both with overall survival and recurrence-free survival in TNBCs[13].

ANXA1 expression in cancer is not only tissue-specific but also varies among different cancer subtypes arising from the same tissue. For instance, its expression differs between primary invasive breast cancer, ductal carcinoma in situ, and metastatic breast cancer. Few studies have noted that ANXA1 expression is particularly correlated with poor outcome in the basal-like 2 subtype rather than other subtypes. Compared to other subtypes, basal-like TNBC has higher levels of ANXA1 expression, which drives drug resistance, cell proliferation, metastasis, and the EMT, all of which contribute to carcinogenesis.

ANXA1 contributes to tumor cell invasiveness by activating the TGF-β and nuclear factor kappa B signaling pathways. Activation of the TGF-β pathway subsequently induces epithelial-to-mesenchymal transition in cancer cells.

Wang et al[14] analyzed the prognostic value of AnnexinA1 expression in TMAs of 135 invasive breast carcinomas; lack of AnnexinA1 expression (0 to < 5% staining) correlated with pathological TNM stage and especially with lymph node metastases[14].

CD44 is a cell-surface glycoprotein receptor that primarily binds hyaluronic acid, a process that contributes to cell invasiveness. CD44 also interacts with integrins, which function as cell adhesion molecules involved in intercellular signaling, cell proliferation, and differentiation. Notably, CD44 expression on the surface of breast cancer cells is elevated compared with that of normal breast epithelial cells[15]. CD24-/Low/CD44⁺ cells have been shown to have CSC properties in TNBC[16]. The recent research by Depar et al[17] published in the World Journal of Clinical Oncology, analyzed 256 women treated between 2015 and 2022, focusing on the CSC phenotype defined by the markers CD44 high, CD24 low, and ALDH1 positive. Study demonstrated that a positive CSC phenotype-defined by high CD44, low CD24, and ALDH1 positivity is associated with markedly reduced pathological complete response to neoadjuvant chemotherapy[17].

CD117, also known as c-Kit, is a proto-oncogene that functions as a stem cell factor receptor. It is a transmembrane receptor tyrosine kinase that binds stem cell factor as its ligand. CD117 is expressed in various progenitor cells, including those in the breast, hematopoietic system, myocardium, lung, and testis, and is also present in luminal epithelial cells of normal adult breast tissue. CD117 plays a crucial role in regulating cell proliferation, differentiation, apoptosis, and cell motility. CD117 positivity was associated with tumor recurrence and some markers of poor prognosis (vascular invasion, proliferation)[18]. CD117 is associated with the basal-like phenotype, but CK5 and EGFR are superior immunohistochemical markers.

CD133 serves as a marker for these cells and is involved in tumor migration and invasion. It is also a target for various therapies, including monoclonal antibodies, antibody-drug conjugates, CAR T-cell therapies, and nanoparticle-based drug delivery systems directed at CD133+ cells. In the TNBC group, CD133 expression was significantly higher in terms of intensity, proportion of positive cells, and overall expression. CD133+ cells have CSC characteristics in early-onset breast tumors linked to BRCA1 mutations. More recently, the use of this tumor stemness marker in breast malignancies has gained popularity.

CD133 expression was significantly elevated in TNBCs and correlated with larger tumor sizes and more advanced stages. Stromal TILs were associated with patient age in TNBC, while high Ki67 expression was linked to Grade 3 histology. These findings suggest that CD133 could serve as a potential target for immunotherapies, including monoclonal antibodies, antibody–drug conjugates, CAR T-cell therapies, and nanoparticle-based drug delivery in TNBC[19].

CD166, also referred to as activated leukocyte cell adhesion molecule, is expressed in various invasive cancers, including breast, lung, bladder, prostate, liver, pancreatic, head and neck cancers, as well as epithelial ovarian carcinomas. CD166 plays a role in mediating cell–cell interactions, such as those between epithelial or endothelial cells and lymphocytes, and is also involved in angiogenesis. In breast tumors and endothelial tumor cells, CD166 interacts with ligands including CD6, CD9, S100B, and galectin-8[20].

The distribution of CD166 differs between normal breast epithelial cells and breast cancer cells. In TNBCs, CD166 staining intensity is lower compared to other breast cancer subtypes, and reduced membranous expression of CD166 has been associated with more aggressive tumor behavior.

Nanog is a stem cell transcription factor involved in cell differentiation, apoptosis, and determination of cell fate. In breast cancer, it predominantly shows nuclear rather than cytoplasmic localization[21]. Nanog expression positively correlates with cell proliferation and is associated with higher tumor stage, poorer patient outcomes, and poor differentiation. According to recent studies, its expression may also be linked to therapy resistance.

This manuscript emphasizes that subclassifying TNBC into molecular subtypes could enhance the prognostic relevance of CSC markers. Overall mesenchymal stem-like subtypes show the highest enrichment of CSC markers like CD44+/CD24- and ALDH1. The most aggressive molecular subtypes are represented by mesenchymal stem-like and basal-like immune-activated subtypes, which have upregulated genes governing B cell, T cell, natural killer cell, and inflammatory cytokine activities. The presence of these markers is strongly linked to EMT, high metastatic potential, and the poorest overall prognosis. The subtype basal-like 2 is defined by growth factor signaling (EGFR, Wnt/β-catenin), this demonstrates elevated stemness and typically presents a poorer prognosis in TNBC compared to the basal-like 1 subtype, which is more proliferative yet less stem-like. Molecular subtypes vary significantly in their sensitivity to standard and targeted therapies such as neoadjuvant chemotherapy, immunomodulatory androgen receptor antagonists, checkpoint inhibitors, and molecular targeted inhibitors. Table 1 summarizes the key findings of stem cell markers in TNBCs and its prognostic significance.

Table 1 Stem cell markers in triple-negative breast cancer and its prognostic significance.

Stem cell markers in TNBC	Roles of CSCs in TNBC	Prognostic significance in TNBC
ALDH1+	Involved in cellular detoxing, self-renewal, and strong driver of cell migration	ALDH1 expression rate was significantly higher in TNBCs. Independent predictor of poor clinical outcomes; linked to early metastasis and chemotherapy resistance
Oct-4, SOX2	Core transcription factors that maintain pluripotency and self-renewal. They positively regulate the epithelial-mesenchymal transition process	High expression (especially Oct-4) is an independent predictor of shorter survival and higher histological grade
Nanog	Nanog expression positively correlates with cell proliferation and is resistance	associated with higher tumor stage, poorer patient outcomes, and poor differentiation. Expression may also be linked to therapy
ABCG2	A ‘hub gene’ involved in drug efflux and autophagy-promoting drug resistance	Associated with triple-negative status and increased resistance to conventional chemotherapy
CD44+/CD24-/low	Cell adhesion molecules involved in intercellular signaling, cell proliferation, and differentiation. Maintains multipotency, promotes cell proliferation, and initiates tumorigenesis	Associated with inferior disease-free survival and overall survival; correlated with higher tumor grade and metastasis
CD117 (c-kit)	Role in regulating cell proliferation, differentiation, apoptosis, and cell motility Receptor tyrosine kinase involved in cell survival and proliferation	CD117 positivity was associated with tumor recurrence and some markers of poor prognosis Diffuse expression is as an independent prognosticator for overall survival
Combined phenotype (CD44+/CD24-/ALDH1+)	Represents the most tumorigenic and metastatic cell subpopulation	Highest value for predicting metastatic risk and significantly reduced survival rates

TNBC: Triple-negative breast cancer; CSC: Cancer stem cell.

CHALLENGES AND FUTURE DIRECTIONS IN TNBC

Breast CSC, which possess tumor-initiating potential and self-renewal capabilities, are thought to contribute to poor clinical outcomes by promoting treatment resistance, metastasis, and disease recurrence. A hallmark of CSCs is their plasticity-the ability to transition dynamically between distinct cellular states in response to microenvironmental cues such as hypoxia, inflammation, or therapy-induced stress. The tumor microenvironment plays pivotal roles in each stage of tumor development[22]. The tumor microenvironment has emerged as a critical target in cancer research due to its role in regulating tumor growth, metastasis and treatment response[23].

This remarkable adaptability allows CSCs to survive and thrive under changing conditions, contributing to intratumoral heterogeneity and playing a pivotal role in tumor progression, metastasis, and therapeutic resistance, particularly in TNBC. Supporting this, recent studies on tumor heterogeneity and stemness have shown that multiple CSC subtypes can coexist within a single breast tumor. Mutations in BRCAs are responsible for causing genetic instability and worsening the prognosis[24]. Consequently, a variety of treatment strategies are being investigated, with particular focus on therapies targeting the CSC population. Yadav et al[25], observed that, biomarkers may be useful as prognostic or predictive indicators as well as suggest possible targets for novel therapies. Despite the successes of emerging targeted therapies, relapse, recurrence, and therapy failure rates in TNBC significantly surpass in speed than other subtypes of breast cancer[26].

Recent advances in the treatment of TNBCs have expanded the standard of care beyond chemotherapy alone, with growing incorporation of immunotherapy and targeted therapies[27]. These strategies have shown promise in improving survival outcomes, particularly for patients with advanced or metastatic breast cancers[28].

CSCs are emerging as potential key regulators driving the aggressiveness TNBCs[29]. Consequently, targeting and manipulating CSCs may offer a promising therapeutic strategy for controlling TNBC progression and improving patient outcomes in the future. CSC-targeted strategies repeatedly face relapse and adaptive resistance and also non-traditional interventions may modulate stem-like populations[30]. To achieve this goal and make significant advancements in TNBC management, new research, clinical trials, and interdisciplinary approach are crucial[31].

CONCLUSION

Stem cell markers serve as key indicators of tumor aggressiveness, chemoresistance, and poor prognosis in breast cancer. In TNBC, multiple CSC-related markers have been identified, including ALDH1, CD133, CD44, CD24, EpCAM, CD166, Lgr5, CD47, and ABCG2. The expression of these markers is closely linked to increased tumor initiation, metastatic potential, and resistance to standard therapies. The highly aggressive and heterogeneous nature of TNBC presents significant treatment challenges, complicating the development of an optimal therapeutic approach. Beyond their prognostic value, stem cell markers are increasingly being investigated as therapeutic targets, aiming to eliminate the stem-like tumor cells that often evade standard chemotherapy and drive disease recurrence. For TNBC treatment, future research should focus on identifying novel targets and developing novel therapeutic strategies to target CSC’s plasticity.