Abstract 1179: PAM50 subtype admixture in individual breast cancers and the relationship of this intratumoral heterogeneity to clinical variables

Abstract

Abstract Classification of breast cancers by PAM50 gene profiling assigns each cancer to one of five intrinsic subtypes. However, individual cancers vary in their adherence to the assigned prototype, and some may exhibit expression patterns that indicate admixture with other subtypes. The prevalence of this intratumoral heterogeneity phenomenon and the relationship of subtype admixture to clinical endpoints has not been extensively studied. We obtained microarray data on 1,460 cases from the public Gene Expression Omnibus and processed the data to normalize and log-transform expression levels and control for batch effects. For each case, we extracted levels for PAM50 genes and computed the Mahalanobis distance in multidimensional space for each case from all five centroids, including the one assigned. This metric obtains the standardized distance from a centroid for a single case while accounting for correlation between genes. We then discriminated pure from admixed cases by setting criteria for distance from the assigned centroid as well as distance from nonassigned centroids. Using the criteria that pure cases are those within the 75th percentile of distance from the assigned centroid, while being greater than the 75th percentile from any other centroid, we found that 59% of basal cases were categorized as pure, versus luminal A 31%, luminal B 35%, HER2 34% and normal 27%. We used t-Distributed Stochastic Neighbor Embedding (t-SNE) plots to visualize these admixtures in two dimensions. This confirmed that basal cancers were more heterogeneous than the other types, and that LumA, LumB and normal were more frequently admixed than other combinations. Pure LumA cases were less likely to be high-grade than LumA cases admixed with LumB, basal, HER2 or null cases not near any other centroid (18.5% vs. 7.2%, P < 0.01). Pure basal cases were more likely to be high-grade than basal cases admixed with any other subtype. Kaplan-Meier curves for metastasis-free survival for pure LumA were very similar to admixed cases for the first 5 years, but then diverged, with worse survival when LumA was admixed with worse subtypes. These results support the hypothesis that subtype admixture, or at least the tendency for some breast cancers to exhibit this form of intratumoral heterogeneity, can be identified and that it should be explored for clinical consequences, including failure to respond to subtype-specific therapy. Citation Format: Peter H. Gann, Neeraj Kumar, Dan Zhao, Amit Sethi. PAM50 subtype admixture in individual breast cancers and the relationship of this intratumoral heterogeneity to clinical variables [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 1179.