Abstract 3650: PAM50 breast cancer subtyping and risk of recurrence in a population-based cohort.

BACKGROUND The PAM50 gene expression signature for identifying intrinsic subtypes of breast cancer has been shown to add significant clinical information beyond standard molecular biomarkers (ER, PR, Her2). Previous studies with the PAM50 have focused primarily on retrospective clinical trials in which the drug regimens and enrollment criteria are relatively homogenous. We assess the prognostic significance of subtyping within the community setting using therapies appropriate at the time of diagnosis. METHODS Two prospective studies of breast cancer survivors, Life After Cancer Epidemiology (LACE) and Pathways, were subtyped by the PAM50 using quantitative RT-PCR. LACE participants were diagnosed from 1997-2000 with tumors ≥1 cm; while Pathways enrolled subjects at time of diagnosis (2006-2008) with tumors ≥0.5 cm. Both studies included AJCC stages I-III. LACE participants were enrolled on average 2 years post-diagnosis, such that early recurrences were excluded. In addition, women in Pathways were diagnosed after 2006 when Herceptin treatment was available. Formalin-fixed, paraffin-embedded tumor blocks were centrally reviewed by a pathologist and 1mm directed punches of invasive cancer were taken for RNA extraction and expression profiling. A published algorithm was used for subtyping and provided a continuous gene expression score for proliferation, ESR1/ER, PGR/PR, and ERBB2/Her2 (Bastien et al, BMC Med Genomics. 2012 Oct 4;5:44). Subtype classification was correlated with disease-free survival, estimated using Kaplan-Meier plots and log-rank testing. Multivariable delayed entry Cox regression analyses determined the prognostic significance of subtypes in the context of standard clinical indicators of survival. RESULTS The PAM50 subtype distribution for the cohort was Luminal A (52%), Luminal B (20%), HER2-E (14%), Basal-like (10%), and Normal-like (4%). Approximately 7% of Luminal tumors (A and B) were called ER negative by immunohistochemistry (IHC). Luminal A tumors were more likely to be PR positive (82%) compared to those classified as Luminal B (61%). Only 2% of Luminal B tumors were low proliferation. The HER2-E subtype contained 66% clinically Her2+ tumors and the Basal-like tumors were 90% triple negative. In univariate analyses, women with non-Luminal A subtypes were all at higher risk of recurrence. Controlling for tumor size and positive nodes, those with Basal-like subtype still had a significantly higher risk of recurrence (HR=2.34, 95% CI 1.37, 3.98). CONCLUSIONS There are proportionally more Luminal A patients within the population-based studies compared to clinical trials that have focused on higher risk patients. The PAM50 subtype remains a significant prognostic indicator of recurrence in the context of standard therapies used in community practice today. Citation Format: Philip Bernard, Erin Weltzien, Inge Stijleman, Candyce H. Kroenke, Carole Davis, Marilyn L. Kwan, Charles Quesenberry, Adrienne Castillo, Laurel Habel, Rachel Factor, Larry Kushi, Carol Sweeney, Bette Caan. PAM50 breast cancer subtyping and risk of recurrence in a population-based cohort. [abstract]. In: Proceedings of the 104th Annual Meeting of the American Association for Cancer Research; 2013 Apr 6-10; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2013;73(8 Suppl):Abstract nr 3650. doi:10.1158/1538-7445.AM2013-3650