HR and HER2 statuses are two well-established prognostic factors in breast cancer, and the prognosis of patients with HR+, HER2-, early breast cancer is favorable[23, 24]. Our population-based study found that the non-CSD accounted for 41.92% of all causes of death, indicating that non-CSD should be emphasized and clinicians should be cautious when performing adjuvant chemotherapy for this cohort considering the side effects brought by chemotherapy.
In our survival analyses, we found that the entire cohort failed to benefit from the addition of chemotherapy, indicating some patients may be spared the use of chemotherapy. In our further subgroup analyses, N stage was associated with chemotherapy benefits, which was consistent with guidelines that lymph nodal status was an indicator for choosing adjuvant chemotherapy. We concluded from our analyses that, in patients with HR+, HER2-, early breast cancer, most patients with N0 breast cancer might be spared the use of adjuvant chemotherapy and patients with N2 and N3 cancer should be arranged to receive the adjuvant chemotherapy, while the chemotherapy benefits were uncertain for patients with N1 cancer just according to the clinical characteristics.
There has been a tendency to equate factors associated with prognosis with those related to chemotherapy benefits. However, in our study, we found these two types of factors were not entirely same. In the multivariable analyses, age, marital status, race, histological type, differentiated grade, T stage and N stage were all associated with CSD, while only race, T stage and N stage were significant when considering the interactions between adjuvant chemotherapy and demographic and clinicopathological characteristics. Therefore, we concluded that only race, T stage and N stage were associated with chemotherapy benefits, and factors like histological type and differentiated grade were not (even though histological type and differentiated grade were associated with prognosis). In addition, we established a score for predicting chemotherapy benefits with the 3 factors (race, T stage and N stage). Different from previous predictive scores, which were transformed from models for predicting prognosis, we established our score using a totally new method.
The 21-gene recurrence score assay was the only genetic assay recommended for predicting chemotherapy benefits in the NCCN guidelines, while its predictive ability in HR+, HER2-, early breast cancer is doubtful. The 21-gene assay is initially established for predicting prognosis of estrogen receptor–positive (ER+), N0 breast cancer rather than chemotherapy benefits of HR+, HER2-, early breast cancer.
Evidences supporting the 21-gene assay for predicting chemotherapy benefits were mainly from the NSABP B20 trial, the TAILORx trial and the SWOG-8814 trial. The NSABP B20 trial enrolled 2363 patients with ER+, N0 breast cancer, who were randomly assigned to chemotherapy or not. Among the 2363 patients, a total of 651 patients were assessed using the 21-gene recurrence score assay and were classified into three groups according to the score: low-RS (RS < 18), intermediate-RS (18 < RS < 30) and high-RS (RS ≥ 31). Results of the study suggested that patients with low RSs failed to receive chemotherapy benefits and patients with high RSs benefited from chemotherapy, and chemotherapy benefits were uncertain for patients with intermediate RSs. Note that the study included patients with both HER2- and HER2 + breast cancer, therefore it was doubtful to conclude that patients with HR+, HER2-, N0 breast cancer benefited from chemotherapy because the HER2 score accounts for a certain proportion of the 21-gene RS.
In a secondary analysis of the NSABP B20 trial, gene expression results of the 21-gene assay were obtained in the 651 patients, and 569 of them were identified as HER2- according to the HER2-gene expression from the RT-PCR assay. Then chemotherapy benefits for the 569 patients were assessed using the Kaplan–Meier plots, and patients were reported to benefit from the addition of chemotherapy in the high-RS group (whether high-RS was set as 31 or more or as greater than 25), while no significant benefits in the low- and intermediate-RS groups. Considering the retrospective trait and the relatively low sample size in the high-RS group (97 patients in the RS ≥ 31 group or 121 patients in the RS > 25 group), results of this secondary analysis need further validation. In addition, the NSABP B20 trial was conducted in an early era when the effectiveness of endocrine therapy is limited, and this effectiveness has increasingly improved nowadays.
The TAILORx trial considered the impact of HER2 status on survival and chemotherapy benefits, and enrolled patients with HR+, HER2-, N0 breast cancer. The 6711 patients with a midrange RS of 11 to 25 by the 21-gene assay were randomly assigned to receive endocrine therapy alone or endocrine therapy plus chemotherapy. Although some patients aged 50 or younger seemed to benefit from the chemotherapy, the entire cohort failed to benefit from the chemotherapy (endocrine therapy alone versus endocrine therapy plus chemotherapy: invasive disease recurrence or death, P = 0.26; recurrence at a distant site, P = 0.48). The 1389 patients with a RS of 26 or higher were all assigned to receive chemoendocrine therapy, thus the study did not directly demonstrate that patients with HR+, HER2-, N0 breast cancer who had a high RS benefited from chemotherapy.
In a secondary analysis of the TAILORx trial, 1389 patients with HR+, HER2-, N0 breast cancer who had a high RS of 26 or more were identified and 89 of the patients had no chemotherapy. The study reported that patients without chemotherapy had a worse prognosis in invasive disease-free survival (IDFS) than patients with chemotherapy (HR with 95% CI: 0.48, 0.29–0.80), rather than freedom from recurrence at a distant site (HR with 95% CI: 0.74, 0.32–1.69). However, in the non-chemotherapy group, 57 of the 89 patients underwent breast conservation surgery while fewer patients (29 patients) received postoperative radiation therapy, indicating these non-chemotherapy patients might be of poor adherence. The radiation therapy has been validated to reduce local recurrence after breast conservation and been suggested for patients treated with conservation surgery. Therefore, results of this secondary analysis that chemotherapy rendered benefits in IDFS for patients with HR+, HER2-, N0 breast cancer who had a high RS need to be further validated.
According to the ASCO guidelines, the 70-gene assay is also recommended for treatment decision making, and the MINDACT trial is the highest level of evidence supporting its predictive values[28, 29]. In this randomized, phase 3 study, patients with early breast cancer were classified into four groups according to their clinical risk (using a modified version of Adjuvant! Online) and genomic risk (using the 70-gene signature), and the groups with discordant risk results (high clinical and low genomic risk or low clinical and high genomic risk) were randomly assigned to receive chemotherapy or not. Results of this study showed that there were no statistically significant chemotherapy benefits with respect to survival without distant metastasis, disease-free survival or overall survival, indicating that high genomic risk may not be an accurate indicator for chemotherapy benefits. Additionally, subgroup analyses of the MINDACT trial of patients with ER+, HER2-, N- showed this subgroup failed to benefit from the addition of chemotherapy (DMFS for chemotherapy versus no chemotherapy in the two discordant risk groups, in the HR+, HER2-, N- subgroup, P = 0.456 in high clinical risk/low genomic risk; P = 0.333 in low clinical risk/high genomic risk), which was consistent with our results.
The other multiple gene assays are also not suitable for predicting chemotherapy benefits in HR+, HER2-, early breast cancer because they all did not take the interaction between chemotherapy and gene expression level into consideration. Results that genes from previous multiple gene assays (including genes in the Oncotype Dx, MammaPrint, PAM 50, 28-gene signature, EndoPredict and BCI) failed to cluster patients with similar pathological stages also proved that these previous genes were not suitable for the prediction of HER2-, HR+, early breast cancer[12, 18–22]. Therefore, we adopted a new method, which considered the interaction, and identified that SOWAHA、RP11-205M3.3、IRX6、PPBP and EMX1 were genes most closely related to chemotherapy benefits in the HER2-, HR+, early breast cancer, and we established a chemotherapy predictive genetic score based on the 5 genes.
In colorectal cancer, the Iroquois Homeobox 6 (IRX6) expression has been reported to be lower in tumor samples and was a prognostic predictor. Pro-platelet basic protein (PPBP) was also known as C-X‐C motif chemokine ligand 7 (CXCL7), the overexpression of which has been reported to promote cell proliferation in vivo and in vitro, and it has been proposed as a novel biomarker for the diagnosis of lung cancer and renal cell carcinoma[31–33]. Zhou et al. reported that Empty spiracles homeobox 1 (EMX1) was a potential target of miR-497, which was recognized as a tumor suppressor in many cancers, and the high expression of EMX1 was related to advanced clinicopathologic characteristics in endometrial cancer. Our study confirmed that the high expression of EMX1 was associated with chemotherapy benefits in breast cancer, indicating the inconsistency between prognostic and predictive values of EMX1. The role of SOWAHA、RP11-205M3.3 in HER2-, HR+, early breast cancer needs further investigation.
There are several limitations in our study. Firstly, our study was a retrospective study and biases such as selection bias and treatment bias were inevitable in our study. In addition, our established CPCS and CPGS need further external validations to prove their effectiveness in predicting chemotherapy benefits. Finally, some important information such as regimens and courses of chemotherapy are not available in the SEER and TCGA database, which limits our further analyses.