Differentiating HER2-low and HER2-zero tumors with 21-gene multigene assay in 2,295 h + HER2- breast cancer: a retrospective analysis

Abstract

Background: HER2-positivity is an essential marker for therapeutic decisions, while HER2 expression is heterogenous. In recent years, there has been increasing recognition of a subgroup of breast cancer patients who have low levels of HER2 expression, also known as HER2-low because trastuzumab deruxtecan offers clinical benefit for patients with HER2-low metastatic breast cancer. Despite the growing interest in HER2-low breast cancer, there is limited research on how multigene assays can help differentiate between HER2-low and HER2-negative breast cancer. Among HR + HER2- breast cancer, we compared genomic characteristics between HER2-low and HER2-zero using the 21-gene assay.

Methods: A retrospective review of clinical records was performed in 2,295 patients who underwent Oncotype DX^® test in two hospitals between 2013 and 2020. Patients were classified into two groups as the HER2-zero and HER2-low based on HER2 immunohistochemistry. In cases with HER2 2+, no amplification of HER2 gene was confirmed by silver in situ hybridization. High genomic risk was defined as cases with 21-gene recurrence score (RS) > 25. Multivariable binary logistic-regression analysis was performed.

Results: Of these, 944 (41.1%) patients were assigned to the HER2-zero group, while 1351 (58.9%) patients were assigned to the HER2-low group. The average Recurrence Score (RS) was found to be 17.802 in the HER2-zero breast cancer group and 18.503 in the HER2-low group, respectively (p-value < 0.005). When comparing the proportion of high RS between the two groups, the HER2-zero group had a high RS rate of 12.4% (117 out of 944), while the HER2-low group had a high RS rate of 17.0% (230 out of 1351) (p = 0.002). The HER2 score identified by qRT-PCR was 8.912 in the HER2-zero group and 9.337 in the HER2-low group (p < 0.005). In multivariable analysis, HER2-low status was found to be an independent factor for high RS, with an odds ratio of 1.517 (1.172-1.964), independent of ER, PR, and Ki67. Within the subgroup of patients with invasive ductal carcinoma, the high RS rates were 19% in the HER2-low group and 14% in the HER2-zero group. However, when considering all patients, there were no significant differences observed in recurrence-free survival and overall survival between the HER2-low and HER2-zero groups.

Conclusion: Within HR + HER2- breast cancer, HER2-low tumors are associated with high RS, especially for histologically invasive ductal carcinoma. A prognostic influence of HER2-low expression among HR + HER2- breast cancer remains as an area that requires further study.

Keywords: 21-gene multigene assay; Breast cancer; HER2-low.

Fig. 1

Strobe Diagram of Enrolled Patients. ER indicates estrogen receptor; PR progesterone receptor. ERBB2 + ERBB2-positive; ERBB2- ERBB2-negative

Fig. 2

Comparisons of 21-gene recurrence score (RS) and HER2 Status. (a) Mean RS is significantly higher in the HER2-low than in the HER2-zero (17.8 vs. 18.5; P < 0.001, Student’s T-test) (b) Higher proportions of high RS patients in HER2-low patients compared to HER2-zero patients (P = 0.002, Chi-square test) (c) Gene Expression ERBB-2, ESR1, and PGR were compared between HER2-zero and HER2-low. Significant difference was observed for ERBB-2 (8.91 vs. 9.33; P < 0.01) and PGR (7.54 vs. 7.26; P < 0.01) expression, while there was no difference in ESR1 expression (9.84 vs. 9.76; P = 0.15) by Student’s T-test. (d) Comparison of ERBB-2 expression scores among different IHC scores of HER2 (8.91 in HER2-zero, 9.29 in HER2-1+, 9.39 in HER2-2+; One-way ANOVA test, p < 2e-16). Post-hoc analysis was conducted with the Benjamini-Hochberg method. (HER2-zero vs. HER2-1+, p < 2e-16; HER2-zero vs. HER2-2+, p < 2e-16; HER2-1 + vs. HER2-2+, p = 0.0025)

Fig. 3

Exploratory Survival Analysis. a. Kaplan-Meier Survival analysis for RFS and DRFS showing no difference in survival outcomes between HER2-zero and HER2-low patient groups; b. The RFS and DRFS of the RS-low group (RS ≤ 25) were significantly superior to those of the RS-high group (RS > 25); c. Ratio of Adjuvant chemotherapy receipt is significantly higher among HER2-low patients compared to HER2-zero patients. (16.9% vs. 20.6%; P = 0.026). DRFS, distant recurrence-free survival; RFS, recurrence-free survival

Fig. 4

RS Distribution by Histologic Type. Statistically significant higher RS was sustained in IDC (17.4 vs. 18.8; P < 0.01) but not in ILC (15.9 vs. 16.9; P = 0.19)