Advances in genomic signatures have begun to dissect breast cancer heterogeneity and application of these signatures will allow the prediction of which pathways are important in tumor development. Here we used genomic signatures to predict involvement of specific E2F transcription factors in Myc-induced tumors. We genetically tested this prediction by interbreeding Myc transgenics with mice lacking various activator E2F alleles. Tumor latency decreased in the E2F1 mutant background and significantly increased in both the E2F2 and E2F3 mutants. Investigating the mechanism behind these changes revealed a reduction in apoptosis in the E2F1 knockout strain. E2F2 and E2F3 mutant backgrounds alleviated Myc proliferative effects on the pregnant mammary gland, reducing the susceptible tumor target population. Gene expression data from tumors revealed that the E2F2 knockout background resulted in fewer tumors with EMT, corresponding with a reduction in probability of Ras activation. In human breast cancer we found that a low probability of E2F2 pathway activation was associated with increased relapse-free survival time. Together these data illustrate the predictive utility of genomic signatures in deciphering the heterogeneity within breast cancer and illustrate the unique genetic requirements for individual E2Fs in mediating tumorigenesis in both mouse models and human breast cancer.

Expression of c-Myc is highly prevalent in human breast cancer and stability of the oncoprotein is regulated through Ras-regulated phosphorylation at serine 62 and threonine 58. Previous studies have illustrated the importance of accumulation of KRas mutations in Myc-mediated tumor formation. To examine Myc dependence upon Ras mutations we have generated MMTV regulated Myc and Myc T58A transgenic mice. Expression of the more stable T58A Myc allele resulted in a reduction in KRas-activating mutations. However, in a low-level expression T58A Myc transgenic, the majority of the tumors were squamous or epithelial-to-mesenchymal in nature and accumulated KRas mutations at a higher frequency. Interestingly, we show that these mice develop similar gene expression patterns and signaling pathway utilization as a subtype of human claudin-low breast cancer. Indeed, our results demonstrate a clear division in human claudin-low tumors based on Myc pathway activation and target genes. Together, our results demonstrate that Myc expression and stability has critical effects on molecular heterogeneity in mouse mammary tumors that parallel subtypes of human breast cancer.

In human breast cancer, mortality is associated with metastasis to distant sites. Therefore, it is critical to elucidate the biological mechanisms that underlie tumor progression and metastasis. Using signaling pathway signatures we previously predicted a role for E2F transcription factors in Myc induced tumors. To test this role we interbred MMTV-Myc transgenic mice with E2F knockouts. Surprisingly, we observed that the loss of E2F2 sharply increased the percentage of lung metastasis in MMTV-Myc transgenic mice. Examining the gene expression profile from these tumors, we identified genetic components that were potentially involved in mediating metastasis. These genes were filtered to uncover the genes involved in metastasis that also impacted distant metastasis free survival in human breast cancer. In order to elucidate the mechanism by which E2F2 loss enhanced metastasis we generated knockdowns of E2F2 in MDA-MB-231 cells and observed increased migration in vitro and increased lung colonization in vivo. We then examined genes that were differentially regulated between tumors from MMTV-Myc, MMTV-Myc E2F2−/−, and lung metastases samples and identified PTPRD. To test the role of PTPRD in E2F2-mediated breast cancer metastasis, we generated a knockdown of PTPRD in MDA-MB-231 cells. We noted that decreased levels of PTPRD resulted in decreased migration in vitro and decreased lung colonization in vivo. Taken together, these data indicate that E2F2 loss results in increased metastasis in breast cancer, potentially functioning through a PTPRD dependent mechanism.

Supplementary Figure 1 from Prediction and Genetic Demonstration of a Role for Activator E2Fs in Myc-Induced Tumors

Supplementary Figure Legends 1-4, Table Legends 1-2 from Prediction and Genetic Demonstration of a Role for Activator E2Fs in Myc-Induced Tumors