Genetic Analysis of Triple-Negative Breast Cancer and Pineoblastoma

Metastatic triple-negative breast cancer (TNBC) is a devastating disease affecting relatively young women. The tumor suppressor genes TP53 and RB1 are commonly lost together in a subset of TNBC with exceedingly poor prognosis. The identification of oncogenic drivers that cooperate with p53/Rb-loss in TNBC may uncover novel therapeutic modalities. Here, I employed sleeping beauty (SB) mutagenesis screens to identify oncogenic drivers that cooperate with p53- versus p53 plus Rb-deletion to promote primary mammary tumors and lung metastases, using a WAP-Cre deleter line to induce transposon mobilization. SB mutagenesis accelerated tumorigenesis in the absence of functional p53 or p53 plus Rb. 66.7% of metastases formed in p53/Rb-deficient mice had transposon insertions in genes involved in the Ras pathway (Hras1 and Sos2) or cell motility/migration (Rock2, Fam19a5, and Ntn1). In accordance, bioinformatic analysis revealed that the Ras pathway is enriched in TNBC samples and cell lines. Inhibition of ERK1/2 via ulixertinib suppressed growth of TNBC cell lines with high Ras activity; suppression was more pronounced in cells with stable RB1 knocked-down relative to parental RB1 wild-type cells. These results suggest that alterations in genes involved in Ras pathway activation and motility promote metastatic p53/Rb-deficient TNBC, and point to ERK1/2 inhibitors as potential therapeutic intervention for this subgroup of metastatic TNBC patients. I also used WAP-Cre to generate a mouse model for Pineoblastoma (PB), a lethal pediatric brain cancer. Germline mutations in RB1 and DICER1 have been implicated in the etiology of PB, and deletion of Rb and p53 via WAP-Cre has been shown to induce pineal tumors that resemble human PB with 100% penetrance. Here, I inactivated Dicer1 plus p53 or Dicer1 plus Rb and p53 in the pineal gland via WAP-Cre. Dicer1/p53 and Dicer1/Rb/p53-deficient mice developed PB with 33.3% and 93.8% penetrance, respectively. These tumors clustered close to human PB, and exhibited sensitivity to the tricyclic antidepressant drug nortriptyline in vitro. This study describes the first mouse model for Dicer1-deficient pineoblastoma, demonstrates that WAP-Cre can be used to target the cell-of-origin of distinct PB subtypes, and identifies nortriptyline as a potential new therapeutic for children with DICER1-deficient pineoblastoma.Ph.D