SMARCA4 mutations drive metastatic progression and predict poor survival in breast cancer based on a real-world cohort study.

To investigate the incidence, clinicopathological characteristics, and prognostic impact of SMARCA4 mutations in breast cancer, we conducted a retrospective cohort study using the MSK-CHORD database, enrolling 4,032 patients categorized into SMARCA4 mutant (1.22%, 49/4032) and wild-type groups. We analyzed overall survival (OS) via the Kaplan-Meier method and log-rank test, performed subgroup analyses by HER2/hormone receptor (HR) status, and explored associations with tumor mutational burden (TMB), microsatellite instability (MSI), and co-mutations. Results showed the mutant group had significantly higher rates of bone (75.5% vs. 55.3%, p = 0.005) and lymph node metastasis (81.6% vs. 60.9%, p = 0.003), shorter OS (p = 0.0087), and attenuated prognostic advantages of HER2-positive (p = 0.25) and HR-positive (p = 0.72) status. Additionally, the mutant group exhibited higher TMB (p < 0.05), poorer prognosis in high-MSI patients (p = 0.04), and frequent co-mutations in TP53 and PIK3CA. SMARCA4 mutation is an independent adverse prognostic factor in breast cancer (HR = 1.6, p = 0.02), linked to enhanced metastasis, altered HER2/HR prognostic value, and genomic instability, with potential as a prognostic biomarker.