Abstract
Intratumoral genetic heterogeneity leads to tumor progression and therapeutic resistance. However, due to the difficulty associated with its assessment, the use of this heterogeneity as a prognostic or predictive marker remains limited. To investigate the significance of the Shannon diversity index of gene copy number variation as a tool for measuring genetic heterogeneity in breast cancer, we performed fluorescence in situ hybridization of c-MYC in two sets of invasive breast cancer samples and correlated the Shannon index of c-MYC copy number variation with clinicopathologic features and patient survival. The Shannon index was correlated with average c-MYC copy number and was higher in tumors in which c-MYC was amplified and in those with c-MYC genetic or regional heterogeneity. A high Shannon index was associated with adverse pathologic features including high histologic grade, lymphovascular invasion, p53 overexpression, high Ki-67 proliferation index and negative hormone receptor status. It was also associated with poor disease-free survival in the whole group, in a subgroup excluding c-MYC-amplified cases, and in the hormone receptor-positive subgroup of both a test and a validation set. A high Shannon index for FGFR1 gene copy number variation was also an independent adverse prognostic factor. Our findings suggest that the Shannon diversity index is a measure of intratumoral heterogeneity and can be used as a prognostic factor in breast cancer.