Correlation of intratumor gene expression heterogeneity with chemotherapy sensitivity in breast cancer.

Abstract

1013 Background: The goal of this study was to develop a method to quantify intratumor heterogeneity of cancers using gene expression data. We compared gene expression heterogeneity between different molecular subtypes of breast cancer and between basal like cancers with or without pathologic complete response (pCR) to neoadjuvant chemotherapy. Methods: Affymetrix U133A gene expression data of 335 stage I-III breast cancers were analyzed. Molecular class was assigned using the PAM50 predictor. All patients received neoadjuvant chemotherapy. We measured tumor heterogeneity by the Gini index (GI) calculated individually for each case over the expression of all probe sets and random subsets. The GI was used as a metric of inequality of gene expression distributions between cases. The higher the GI, the greater the inequality of the expression distribution. Results: Basal like cancers (n=138) had greater heterogeneity than luminal cancers (n=197) (mean GI values 24.51 vs 23.05, p<0.001) and luminal B (n=71) cancers had greater heterogeneity compared to Luminal A (n=126) cancers (24.49 vs 22.25, p<0.001). Among the basal-like cancers, those with pCR (n=44) had significantly higher heterogeneity compared to cancers with residual disease (RD, n=94) (26.10 vs 23.77, p<0.001). Significant differences in GI between cancer subtypes remained for as low 2500 randomly selected probe sets. Conclusions: Breast cancer subtypes differ in intratumor gene expression heterogeneity. Greater degree of heterogeneity correlate with greater chemotherapy sensitivity. Importantly, among basal-like cancers only the heterogeneity metric differed significantly between cases with pCR or RD but not individual genes expression values or gene signatures.