Clinical and Molecular Characteristics of ER-Positive Breast Cancer Tumors Identified as Ultralow Risk by the 70-Gene Signature in a Randomized Clinical Trial

BackgroundBreast cancer tumors are heterogenous, including their metastatic potential and time to distant metastasis. Patients with estrogen receptor (ER)-positive tumors have a continuous long-term risk of fatal disease decades after diagnosis. Recently, ER-positive breast cancer patients classified as having ultralow risk tumors by the 70-gene expression signature (MammaPrint) were associated with a minimal risk of fatal disease. However, the underlying tumor characteristics of ultralow risk tumors are unknown.MethodsSecondary analysis of the Stockholm tamoxifen randomized trial (STO-3, 1976-1990) enrolling postmenopausal lymph node-negative breast cancer patients. Immunohistochemistry of the clinically used breast cancer markers (n=727 patients) and gene expression profiling by Agilent microarrays (n=652 patients) were performed in 2014. Ultralow risk tumors, identified by the 70-gene signature, were compared to other ER-positive tumors (of low/high risk) and Luminal A and B PAM50 subtype tumors (ER-positive, low/high risk) by the clinical markers and multi-gene modules, representative of specific biological processes and pathways, using Fisher’s exact test. Furthermore, differential gene expression analysis was performed contrasting ultralow risk tumors to other ER-positive tumors (of low/high risk) using t-statistics and false discovery rate (FDR).ResultsUltralow risk tumors were significantly (P