Correlation between epidermal growth factor receptor and tumor stem cell markers CD44/CD24 and their relationship with prognosis in breast invasive ductal carcinoma

Breast cancer is a heterogeneous disease with different molecular profiles, clinical behaviors, and responses to therapy. Modern genomic and immunohistochemical techniques have enabled the classification of breast cancers into distinct subsets, including hormone receptor positive (luminal A and luminal B), human epidermal growth receptor 2 (HER2)-positive, and basal-like type [1]. Triple-negative breast cancer (TNBC) accounts for 90 % of the basal-like type and is characterized by its biological aggressiveness, worse prognosis, and lack of therapeutic target in contrast with hormone receptor-positive and HER2-positive breast cancer [2, 3]. Immunohistochemically, TNBC is typically negative for estrogen receptor (ER), progesterone receptor (PR), and HER2, but positive for basal cytokeratins (CK5/6/14/17), epidermal growth factor receptor (EGFR), and/or c-kit [4, 5].

EGFR is a cell surface receptor, and its expression has been implicated in multiple biological processes, including proliferation, apoptosis, invasion, and angiogenesis, which are the hallmarks of cancer [6]. Using gene and protein expressions, previous studies have reported high EGFR expression in most TNBC [7, 8]. High expression of EGFR is reported to be associated with poor clinical outcome in breast cancer, while its prognostic value remains debated [9, 10].

The stem cell-like phenotype of tumor-initiating cells and their limited number within the bulk of a tumor may account for their ability to escape conventional therapies, leading to disease relapse although the primary lesion has been removed [11]. In breast cancer, Al-Hajj et al. [12] were the first to isolate a highly tumorigenic subpopulation of tumor cells with the CD44+/CD24− phenotype. They demonstrated that CD44+/CD24− tumor cells resembled normal stem/progenitor cells with respect to their ability to self-renew, proliferate, and differentiate [11]. Although only one-third of human breast cancers have the CD44+/CD24− phenotype, this tumor cell population appears most commonly in TNBC [13].

Research on the correlation between EGFR expression and CD44/CD24 and their prognostic value in breast invasive ductal carcinoma (BIDC) is limited. In this study, we aimed to evaluate the correlation between EGFR expression and CD44/CD24 and determine their relationship with BIDC clinicopathological parameters and their prognostic value in BIDC.

IDC is the most common type of breast cancer. BIDC can be divided into triple-negative and non-triple-negative based on the hormone receptor status. TNBC accounts for 10–17 % of all breast cancers [15]. As we studied IDC in the present study, TNBC only accounted for 8.5 % of all breast cancers.

TNBC is relatively large, high grade, has a high rate of node positivity at diagnosis, and is biologically more aggressive. Despite the higher rates of clinical response to neoadjuvant chemotherapy, TNBC patients have a higher rate of distant recurrence and poorer prognosis compared with patients with other breast cancer subtypes [2, 3]. Consistent with the literature, our findings demonstrated that TNBC patients had significantly worse DFS and OS compared with non-TNBC patients (Fig. 2a, e).

EGFR is expressed in all types of breast cancer, especially in TNBC [5, 16]. EGFR was first reported as a significant prognostic factor in breast cancer by Sainsbury et al. [17]. Since then, many researchers have reported differing findings on the prognostic value of EGFR in breast cancer, and there is still no agreement on the relationship between EGFR and clinical outcome [18, 19]. In the present study, EGFR expression was inversely correlated with ER status (p = 0.023). Although EGFR was frequently expressed in TNBC, there was no statistically significant difference between TNBC and non-TNBC patients (p = 0.108). EGFR-positive patients were more likely to develop distant metastases (p = 0.018), which indicated that EGFR may be an important prognostic factor for distant metastasis. Survival analysis showed that EGFR positive indicated significantly shorter survival time in the lymph node- and HR-negative group. These data strongly indicate that EGFR-positive status is related to breast cancer progression and is a pivotal prognostic factor for specific subgroup.

Although EGFR-positive status is correlated with poor clinical prognosis, Tang et al. [20] found that EGFR overexpression predicted better response to neoadjuvant chemotherapy in patients with TNBC. This is a possible reason that TNBC is more chemosensitive.

Numerous studies have reported that CD44+/CD24− epithelial tumor cells are the most common in TNBC [14, 21, 22], but the relationship between CD44+/CD24− cells and prognosis in breast cancer is debated. Lee et al. reported that a high proportion of CD44+/CD24− tumor cells in prechemotherapy tissue was correlated with higher histological grade, ER negativity, and high Ki-67 proliferation index. After primary systemic therapy, the proportions of CD44+/CD24− tumor cells were significantly increased [23]. Wu et al. [14] demonstrated that the CD44+/CD24− and CD24+/CD44− phenotypes associated with decreased DFS but were not independent predictors for DFS. Giatromanolaki et al. [22] reported that the CD44+/CD24− and CD44−/CD24− phenotypes indicated unfavorable prognosis in TNBC. However, Kim and colleagues reported that the CD44+/CD24− group is considered a favorable prognostic subgroup in breast cancer. CD24 expression was a poor prognostic marker in HR-positive breast cancer, and CD44 expression was a good prognostic marker in the HR-negative group [24]. Another study showed that there was no correlation between the CD44+/CD24− phenotype and outcome in HR-positive breast cancer [25]. In the present study, 76.9 and 50 % of TNBC and non-TNBC patients, respectively, showed positive CD44 expression (p < 0.05). However, CD24 did not show statistically different expression levels between the two groups (p = 0.131). CD44-positive expression was inversely associated with HER2-positive status and HR-positive status, while CD24-positive expression was correlated with HER2-positive status but had nothing to do with HR-positive status. This study showed that CD44 positive was significantly correlated with survival time only in lymph node- and HR-negative patients while CD24 positive was significantly correlated with survival time in lymph node-negative and HR-positive patients.

Although the rate of EGFR expression is high in breast cancer, especially in TNBC, EGFR inhibitors do not have an ideal curative effect in breast cancer. Previous studies have suggested that synthetic lethal cross-linking of inhibitors of the mitogen-activated protein kinase (MAPK) kinase (MEK)/MAPK pathway, EGFR, and poly(ADP-ribosyl) transferase (PARP) may be related [26, 27]. In this study, EGFR and CD44 expressions were significantly positively correlated (p = 0.017). The CD44+/CD24− tumor cell phenotype is recognized as a cancer stem cell characteristic [11]. It is believed that CD44+/CD24− breast cancer cells are highly invasive and radioresistant and chemoresistant [28]. The close correlation between CD44 and EGFR expressions may be another reason for the resistance to EGFR inhibitors in breast cancer. Combining EGFR inhibitors with CD44 inhibitors may be a novel method for breast cancer treatment, especially TNBC.

In conclusion, our study indicates that in combination, EGFR and CD44/CD24 expression status are powerful identifiers of breast cancer patient subgroups with different clinical behavior. Although our study has some limitations, such as its retrospective design and relatively small number of studied patients, the data obtained indicate that EGFR and CD44 could serve as useful biomarkers for better determination of the prognosis of invasive breast cancer. These findings may have therapeutic significance and may improve the management of breast cancer patients.