This is the largest and most comprehensive study to date analyzing EGFR expression in breast cancer patients. It demonstrates that EGFR expression is associated with an aggressive phenotype with distinct clinical and biologic characteristics. EGFR expressing tumors occurred more often in young and minority women. These tumors were larger and slightly more likely to metastasize to the lymph nodes. They had a substantially higher proliferation rate and greater genomic instability. They were also more likely to co-express HER2 but much less likely to express hormone receptors, especially PR.
EGFR has been extensively studied in the preclinical and clinical settings in different types of cancer. In breast cancer, preclinical evidence has indicated its clear role in increasing proliferation, resistance to apoptosis, and ligand-independent activation of the estrogen receptor, and in mediating hormone therapy resistance [4
]. Additionally, activating mutations of pathway elements downstream of EGFR in cancer cells may render them resistant to EGFR inhibition [37
]. This was recently shown in colon and lung cancers [38
In the clinical setting, multiple studies have examined EGFR expression in breast cancer, but results have varied possibly due to relatively small sample sizes and non-uniform methodologies for the measurement of EGFR [31
]. These studies reported that EGFR is expressed in 15%– 45% of breast tumors, is inversely related to hormone receptor expression, and they inconsistently linked EGFR expression to poor prognosis [6
]. A recent study utilized a quantitative immunofluorescence-based technology to measure EGFR in a subset of premenopausal women treated with adjuvant tamoxifen [43
]. The authors found EGFR in 39% of the tumors they tested, and linked its expression to higher tumor grade and worse clinical outcome in patients treated with tamoxifen but not in untreated patients. However, that study found no correlation with tumor size, lymph node status or co-expression of HER2. This technology is promising and warrants further study.
The present study sought to overcome these deficiencies by analyzing frozen tumors from a large number of patients. EGFR expression was objectively quantified by ligand binding assay and all measurements were performed at a central lab with strict quality control.
Only a minority of breast cancer specimens (18%) expressed EGFR and apparently only low levels are needed to exert an effect on biologic and clinical characteristics. The association with a different biology, such as down-regulation of ER or PR, was strong– 37% of EGFR-positive tumors were negative for both ER and PR compared to only 9% of EGFR-negative tumors, representing a fourfold difference. b
This study provides further evidence of the inverse relation between hormone receptor expression and growth factor receptor expression and signaling, in this case EGFR. This relationship was most pronounced with PR expression, where only a quarter of EGFR-positive tumors expressed PR compared to almost two thirds of the EGFR-negative tumors.
In recent years, DNA microarray profiling studies on breast tumors defined distinct subtypes of breast carcinomas and linked them to varying clinical outcomes [44
]. Basal-type breast cancer is one such subtype that has been associated with EGFR expression [46
] and a poorer prognosis [44
]. The triple-negative phenotype (ER-negative, PR-negative, HER2-negative) is considered a surrogate for this basal type breast cancer. In our study, nearly half (48%) of these triple-negative tumors were EGFR-positive compared to 18% of the overall study tumors. These findings are also consistent with other studies of triple-negative or basal tumors which also found a higher incidence of EGFR expression [46
EGFR expression was positively correlated with HER2 over-expression, which has been associated with worse prognosis as well. The two molecules may be potent signaling partners, and based on animal models, effective EGFR and HER2 double blockade may result in improved clinical therapies and thus warrants further study in the clinical setting.
One of our most intriguing findings is the association of EGFR expression with clinical outcome. There were notable associations between EGFR expression and worse DFS and OS in patients who received systemic treatment (chemotherapy, tamoxifen, or both) but not in those who did not receive systemic therapy.
These findings have two possible explanations. First, the two groups were unbalanced in regards to a number of factors. As expected, the untreated patients tended to have tumors with more favorable clinical and biologic features (). If the event rate in this group is relatively low, the effect of EGFR expression would need even larger numbers to become evident. In a higher risk population with greater event rates, the adverse prognostic effect of EGFR would be more clearly discernable.
Comparison of tumor characteristics by treatment group
Alternatively, EGFR may result in poorer outcome by causing resistance to systemic treatments. In support of this, EGFR expression has been associated with resistance to hormone therapy and chemotherapy in a number of studies [9
]. These two possible possibilities may also explain the significant difference in post-relapse survival between patients with EGFR-positive and EGFR-negative tumors.
Targeting EGFR therapeutically resulted in limited efficacy with the use of small molecules such as gefitinib and erlotinib [52
], or with monoclonal antibodies such as cetuximab [55
]. Reasons for this may be the complexity and redundancy of its signaling network, and therefore, multiple points in the pathway may need to be blocked [56
]. Unlike lung cancer, there is no evidence that altered forms of the receptor influence the efficacy of current therapeutics [57
]. However, there may be activating mutations of downstream components of the signaling axis. Recent evidence shows that downstream mutations in K-RAS limits response to EGFR targeted therapies in colon and lung cancer [38
] and warrants investigation in breast cancer. Additionally, there is a clear need for technologies that functionally or globally assess the activated status of EGFR and related downstream pathways.
This study has several limitations: It is a retrospective review of a non-randomized database collected over a long period of time. Treatment was not controlled or specified, and chemotherapy was heterogeneous, although most patients received CMF or a CMF-like regimen. However, this remains the largest and most comprehensive study of EGFR in breast cancer. It provides further insight into this so-far elusive biomarker, its effect on tumor biology, and its clinical impact.
EGFR needs to be studied as part of a robust signaling network with multiple signaling partners, intricate downstream components, and crosstalk with other pathways. Only small amounts of this receptor may need to be expressed to exert an effect on tumors. Assessment of the EGFR pathway may also need a functional assay to discern its signaling activity. This may help identify subgroups of patients in whom EGFR or related pathways play an important role, and may enable specific and effective inhibition of these active signaling elements by targeting EGFR, its signaling partners, or downstream pathways. This may be the best way to realize the promise of EGFR as a biomarker and therapeutic target, a promise that is earnestly awaited and long overdue.