Abstract 7208: PLK1 inhibition as a novel therapeutic strategy in HER2+ breast cancer with germline METN375S mutation

Abstract Introduction: Germline MET polymorphism (N375S), a variant common in Asians but rare in Caucasians (7.9% in South Asians vs 1-2% in Caucasians), has been previously reported to enhance the aggressiveness of squamous cell carcinoma. Germline METN375S polymorphism was more prevalent in metastatic compared to early-stage hormone receptor negative (HR-) HER2+ breast cancer (BC) patients in Singapore (19.2% vs 9.2%; p=0.058; n=201) and is significantly associated with poorer relapse-free (p=0.015) and overall survival (p<0.001). In HER2+ BC cell lines, METN375S enhances an aggressive malignant phenotype in vitro compared to METWT. METN375S expressing HER2+ clones were not sensitive to MET inhibitors but displayed higher sensitivity to HER2 inhibitors. In this study, we aim to (1) Identify drug classes that are effective against METN375S HR-/HER2+ BC through high-throughput anticancer drug screening; and (2) establish the molecular mechanism that maintains the vital pro-survival signals in METN375S HR-/HER2+ BC. Methods: We generated isogenic HR-/HER2 +BC cell models (SKBR3, AU565) expressing either wild-type METWT or mutant N375S METN375S and carried out high-throughput drug screening comprising 414 compounds. Bliss scores were calculated using Synergy Finder to determine the interaction of drugs in combination studies. Immunoprecipitation and immunoblotting analysis evaluated interaction partner and expression of protein in the cell lines. Results: High-throughput drug screening identified 6 compounds with selective sensitivity to METN375S cells. Intriguingly, 4 out of 6 compounds target PLK1 (Polo-Like Kinase1), a key mitotic kinase downstream of HER2. The IC50 of the four PLK1 inhibitors was (range is for METWT 0.2 to 1.1 μM and for METN375S 0.02 to 0.1 μM) 10-fold reduction in METN375S cells. Western blot analysis revealed that the N375S mutation led to increased phosphorylation of both MET and HER2, which in turn activated downstream signaling pathways, including SRC and MAPK kinases. METN375S cells demonstrated elevated PLK-1 protein expression. Co-immunoprecipitation confirmed the interaction between METN375S and HER2, resulting in hyperactivation of HER2 signaling and enhanced MAPK pathway activation. Notably, HER2 knockdown disrupted this signaling, reduced PLK1 levels, and decreased the cells' sensitivity to PLK1 inhibitors. The combination of the PLK1 inhibitor volasertib with HER2 inhibitor lapatinib was highly synergistic in METN375S cells, significantly enhancing apoptosis at nanomolar concentrations. Conclusions: Our findings suggest that METN375S variant promotes tumorigenesis by hyperactivating HER2 signaling and enhancing PLK1 pathway. These findings highlight the potential of PLK1 inhibitors, as a novel therapeutic option for METN375S mutant HER2+ BC and supports further evaluation of PLK-1 in HER2+ BC. Citation Format: Arpita Datta, Calista Jia Ow, Charlie Marvalim, Natasha Gandasasmita, Joline Si Lim, Soo Chin Lee. PLK1 inhibition as a novel therapeutic strategy in HER2+ breast cancer with germline METN375S mutation [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 7208.