Histological and Molecular Characterization of Breast Carcinomas: Predictive Role of HER2-low and HER2 3+ in Response to Neoadjuvant Chemotherapy

Introduction: Breast cancer represents the most prevalent malignancy among women and has therefore been subject of extensive research. Neoadjuvant chemotherapy (NACT) is a relatively recent advancement, indicated especially in patients with early and locally advanced breast cancer. The primary aim of NACT is to achieve a pathological complete response (pCR), which represents a strong positive predictive factor. However, NACT response is very heterogeneous, prompting numerous research groups to identify new predictive factors. Well-established predictors include tumor histology, TNM stage, tumor cell proliferative markers, and molecular subtype. Recently, HER2-low tumors (HER2 1+ and HER2 2+ FISH-) have emerged as a distinct category within HER2-negative breast cancer, raising questions about their responsiveness to systemic therapies. Similarly, HER2 3+ tumors exhibit unique molecular characteristics that may impact their response to NACT. Objective: This study aims to evaluate the predictive role of HER2-low and HER2 3+ subcategories in response to NACT, assessing their impact on pCR rates, overall survival (OS), and disease-free survival (DFS). Methods: A retrospective multicenter study was conducted on a cohort of 510 patients with breast cancer who underwent NACT and subsequent surgery between 2010 and 2014 at two hospitals in Turin. Patients were categorized into three groups based on HER2 status. Tumor grade, histotype, and molecular characteristics were analyzed. Response to therapy was assessed through radiological and pathological evaluations. Statistical analyses were performed to determine correlations between HER2 subcategories and treatment outcomes. The study cohort was consistent, in terms of clinical and histopathological characteristics of the tumor, with the population typically referred for NACT. Most patients presented with high-grade, cT2 tumors, positive lymph nodes at diagnosis, infiltrative ductal histotype and high Ki-67 index. Nearly all patients received 4 cycles of anthracycline and cyclophosphamide, followed by 12 cycles of taxane. HER2-positive patients also received monoclonal anti-HER2 antibodies, such as trastuzumab and pertuzumab, as monotherapy or in combination. Results: The association between known predictive factors and pCR rates aligned with existing literature: ductal histotype, high grade, HER2 receptor positivity, absence of hormonal receptors, and HER2+ and TNBC molecular subtypes were linked to higher pCR rates. Among HER2-negative tumors, HER2 0 and HER2-low showed no significant difference in pCR rates, whereas HER2 3+ tumors showed better pCR rates compared to HER2 2+ FISH+ tumors (p=0.001). DFS was significantly lower in HER2 0 compared to HER2-low tumors (p=0.017); this may be attributed to the higher prevalence of TNBC among HER2 0 cases, given that hormone receptor negativity is a well-recognized negative prognostic factor. There was no significant difference between HER2-positive subcategories (p=0.66). OS curves showed no significant differences across HER2 subcategories. Conclusions: Introducing additional subcategories could help identify subpopulations with distinct biological behavior within these groups. Among HER2-negative tumors, patients with HER2-low expression may potentially benefit from new target anti-HER2 therapies. Within the HER2-positive group, HER2 2+ FISH+ tumors may, in some cases, not derive the expected benefit from a standard target anti-HER2 treatment, potentially benefiting from new target therapies.