High expression reflects mTOR pathway activity and cancer cell proliferation and is a biomarker for poor breast cancer prognosis.

Eukaryotic translation initiation factor 4E binding protein 1 (EIF4EBP1) is regulated by the mTOR (mammalian target of rapamycin) signaling pathway. Phosphorylated EIF4EBP1 protein leads to pathway activation and correlates with aggressive breast cancer features. However, the clinical relevance of gene expression as a prognostic biomarker in bulk breast tumors is not understood. In this study, expression was analyzed in over 5000 breast cancers from three large independent cohorts, TCGA, METABRIC, and SCAN-B (GSE96058), and expression was dichotomized into low and high groups by the median. We also performed gene set enrichment analysis (GSEA) and cell cybersorting via the xCell algorithm to investigate biology and expression patterns within the tumor microenvironment (TME). We additionally confirmed expression location in the TME via single cell RNA sequencing. expression was highest in both triple negative and high-grade tumors (both P<0.001), and tumor mutational burden scores were highest in the high -expression groups (all P<0.001). High expression significantly correlated to worse overall survival in all three cohorts (hazard ratios (HR) 1.4-1.9), and worse distant relapse-free survival in patients treated with neoadjuvant taxane-anthracycline chemotherapy (HR 2.4). GSEA demonstrated enriched mTOR and cell proliferation-related gene sets, including, MYC, G2M checkpoint, and E2F targets across all three bulk tumor and single cell RNA sequencing cohorts. Phenotypically, these pathways were reflected by increased Ki67 gene expression and signaling via pharmacologically-activated mTOR gene sets in high-expressing tumors (all P<0.001). expression was increased in whole breast tumors compared to normal breast tissue (P<0.001), and was expressed predominantly in cancer epithelial cells, particularly in basal epithelial cell subclasses. expression did not correlate to a consistent immune system phenotype across all three cohorts. Overall, these findings support that high gene expression in bulk breast tumors could represent a poor prognostic marker via mTOR signaling pathways activation and upregulation of cell cycling, ultimately leading to increased tumorigenesis.