Biologic and prognostic significance of HER2-low and ER-low expressions in de novo metastatic breast cancer.
Authors: Ren Chongxi , Sun Jianna , Kong Lingjun , Yang Yunmei , Liu Jiabin , Zhou Tong , Yin Ruisheng , Li Fengying , Wang Xin , Wang Hongqiao , Gu Xiaoying , Li Sheng , Ren Haixia , Yu Hongjun , Di Shuping , Zhao Jing 
Published: June 2024

e13170 Background: It is unclear whether ER-low-positive (ER-low) and HER2-low-positive (HER2-low) biomarkers, as new biological subtypes, have different biological and clinicopathological characteristics in de novo metastatic breast cancer (dnMBC). Per ASCO/CAP guidelines, tumors were defined as HER2-low-positive (HER2-low) if they had an HER2 immunohistochemical (IHC) score of 1+ or 2+ with negative in situ hybridization assay and HER2-0 if they had an HER2 IHC score of 0; ER-low refers to the tumor IHC staining of 1%-10%, if the staining is less than 1%, it is defined as ER-0. This study aimed to investigate whether ER-low and HER2-low expressions is associated with clinicopathologic characteristics and prognosis among patients with de novo metastatic breast cancer. The study was approved by the Ethics Committee of Hebei Cangzhou Hospital of Integrated Traditional Chinese and Western Medicine (2017-AF29-058). Methods: Using hospital-based database, we identified patients with dnMBC diagnosed between 2010 and 2017, retrospectively collecting demographic data, tumor characteristics, treatment types, histopathological report and survival data. According to ASCO/CAP guideline recommendations, patient-specific histopathological results were classified, and IHC-based breast cancer markers, mainly including ER-low and HER2-low, were evaluated and analyzed. Median survival time (MST) was estimated by Kaplan-Meier method and log rank test. Univariable and multivariable analyses were performed using the Cox proportional hazard model to identify statistically significant prognostic factors. All statistical analyses were performed using Stata 18.0. Results: A total of 518 patients (median age of 53 years) were included in this study, including 194 patients with HER2-low, 201 patients with HER2-low, 36 patients with ER-low, and 179 patients with ER-0. Expression of HER2-low was significantly more common among ER-positive tumors (excluding the ER-low) compared with HER2-0 tumors (71.1% vs. 62.2%; P < .001). The MST in the HER2-low cohort was longer than that in the HER2-0 cohort (40 vs. 30 months; HR,0.65; 95% CI, 0.52-0.82, p=.0001); while the MST of ER-low cohort was slightly longer than that of the ER-0 cohort (27 vs. 20 months; HR,0.75; 95% CI, 0.51-1.11, p=.231), but there was no statistical difference. Notably, in the metastasis pattern cohort, HER2-low showed a significantly higher MST compared with HER2-0 (P<.001), with a MST of up to 68 months for bone-only metastases. There was no significant difference in MST between ER-low and ER-0 regardless of whether patients received endocrine therapy (P=.135). Conclusions: In the present study, for dnMBC patients, ER-low expression has similar biological and clinical outcome characteristics as ER-0. However, unlike HER2-0, HER2-low expression may predict a survival benefit in bone-only metastases.