Abstract P1-08-05: Discovering drug targets for aggressive breast cancer with TP53 missense mutations by a genome-wide screen
Authors: Pal A , Park J , Gonzalez-Malerva L , Eaton S , LaBaer J 
Published: May 2019

Abstract
Breast cancer is a heterogeneous disease and has subtypes with distinct phenotypic and molecular characteristics. Genetically, 30% of all breast tumors and 80% of the basal-like breast cancer (BLBC) subtype harbor mutations in TP53 and numerous sporadic somatic mutations in other genes. This molecular heterogeneity has posed a challenge in developing safe and effective therapeutic regimens to treat a broad spectrum of breast cancer patients, and discovery of intra-subtype along with inter-subtype heterogeneity has added another layer of complexity for targeted therapies. Based on the analysis of somatic mutation profiles in the BLBC subtype, we identified a wide variety of TP53 missense mutations and thousands of co-existing mutations, which led us to hypothesize that intra-subtype heterogeneity is derived from combinatorial effects of neo-morphic (gain-of-function) activities of different types of missense mutant p53 proteins and complex interplay between specific driver mutations in TP53 and a distinct subset of functionally important co-mutations (or “co-drivers”). We ectopically overexpressed the ten most prevalent missense mutations in TP53 found in breast cancer tumors in non-transformed mammary epithelial cells and examined their cellular functions associated with the hallmarks of cancer. The results showed widely different spectrums of phenotypic changes in cell proliferation, resistance to apoptosis, cell invasion, anoikis resistance and cell polarity. As a proof of concept for the 'co-drivers', we knocked-out PTEN in a non-invasive TP53 mutant cell using the CRISPR and shRNA systems. Inactivation of PTEN in non-invasive TP53-mutant cell increased its invasiveness. TP53 mutations and PTEN deletions are frequent in BLBC patients, who have significantly higher rates of metastasis, recurrence and a lower survival rate than the other subtypes. The absence of three signaling hormone receptors, which can be targeted by specific inhibitors, significantly limits available therapies for the majority of BLBC patients to surgery and cytotoxic chemo/radiotherapies. Combination therapies targeting the driver and co-driver gene associated pathways could be quite promising solutions to treat aggressive breast cancer cases. To address this, we have developed a genome-wide CRISPR based gene knock-out screening approach to test the effect of loss-of-function of individual human genes in clinically important TP53 mutant backgrounds that can promote cancer-like behaviors and identify the combinations of 'TP53 mutation and co-driver pathways' that can be targeted by specific inhibitors.
Citation Format: Pal A, Park J, Gonzalez-Malerva L, Eaton S, LaBaer J. Discovering drug targets for aggressive breast cancer with TP53 missense mutations by a genome-wide screen [abstract]. In: Proceedings of the 2016 San Antonio Breast Cancer Symposium; 2016 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2017;77(4 Suppl):Abstract nr P1-08-05.