Meta-Analysis and Experimental Studies Reveal Mitotic Network Activity Index (MNAI) as Breast Cancer Metastasis and Treatment Biomarker

<b>Objective:</b> Identifying biomarkers that predict metastatic potential or guide treatment selection is critical for improving breast cancer (BC) management. Previously, we established the Mitotic Network Activity Index (MNAI) as a prognostic marker in BC. Here, we bioinformatically and experimentally evaluated MNAI as a biomarker for metastasis risk and therapeutic response. <b>Methods:</b> We used Kaplan-Meier and Cox proportional hazard regression analyses to assess the association between MNAI and distant metastasis-free survival (DMFS) across 14 published BC datasets. A total of 16 publicly available clinical trial datasets, including the I-SPY trials, were used to evaluate the predictive value of MNAI for treatment response. Additionally, wound-healing and transmembrane assays were conducted to determine the effects of PLK1, CHEK1, and BUB1 inhibition on BC cell migration and invasion. <b>Results:</b> High MNAI levels were strongly associated with shorter DMFS. Multivariate analysis further confirmed MNAI as an independent risk factor for DMFS, beyond estrogen receptor status and PAM50-based molecular subtypes. Functionally, pharmacologic disruption of the mitotic network using PLK1, CHEK1, or BUB1 inhibitors significantly reduced cell migration and invasion in MDA-MB-231 and BT-549 BC cell lines. Moreover, BC cells with high MNAI increased sensitivity to microtubule-targeting agents such as docetaxel, paclitaxel, and ixabepilone but increased resistance to tamoxifen, AKT1/2 inhibitors, and mTOR inhibitors. Consistent with these findings, analysis of 16 clinical trial cohorts revealed that patients with high MNAI achieved higher pathological complete response rates to taxane-containing and ixabepilone-based therapies. <b>Conclusions:</b> Our findings demonstrate the MNAI as a clinically actionable biomarker that can refine risk stratification and guide the selection of targeted or chemotherapy regimens, advancing precision medicine in BC management.