Targeted cancer therapy has pioneered the concept of precision medicine by integrating genetic profiling of each patient’s tumor biopsy with targeting of tumor-specific molecular alterations (cancer vulnerabilities) for a superior therapeutic efficacy and fewer side effects on normal tissues. However, cancer is a highly heterogeneous genetic disease. Being the most aggressive type of breast cancer, triple-negative breast cancer (TNBC) lacks Food and Drug Administration (FDA)-approved targeted therapies due to its genome complexity. Therefore, identifying additional targetable vulnerabilities of TNBC through basic and preclinical mechanistic research remains an unmet need for TNBC patients. In this proposal, we aim to address two overarching challenges: (1) Can we identify what drives breast cancer growth; determine how to stop it? (2) Can we identify molecular mechanisms and exploit these unique mechanisms to revolutionize treatment regimens to TNBC patients by replacing current chemotherapy with more effective, less toxic combinatorial targeted therapy and impact survival?

In searching for the TNBC-specific vulnerability, we conducted a whole genome loss-of-function RNAi screen with the goal of systematic identification of synthetic lethal events under the context of mitogen signaling inhibition (anti-EGFR treatment). Surprisingly, the top hits derived from this “drop-out” screen converged on a cluster of genes at 1q23-42 loci, which is prevalently amplified among TNBC tumors. Within this loci, we validated death-effector domain (DED)-containing protein (DEDD) is essential to drive mitogen-independent TNBC proliferation.

DEDD belongs to a large family of DED-containing protein. Without known enzymatic activity, DEDD executes it biological function primarily through protein-protein interactions via its DED domain. Limited studies have demonstrated a facilitator role of nuclear DEDD in promoting cell death apoptosis and controlling cell size. Paradoxically, DEDD is significantly upregulated in >70% TNBC tumors. These perplexing observations raise two intriguing questions to be pursued in this proposal: (1) How does the seemingly cell death gene DEDD reprogram cell cycle regulatory machinery to render a proliferative advantage for cancer transformation? (2) Can the dysregulated cell cycle machinery driven by abnormal DEDD expression be therapeutically targeted as a TNBC vulnerability? In this proposed study, we will systemically examine the role of DEDD, from functional phenotype (Aim 1) to molecular insights (Aim 2). Ultimately, we will preclinically test a novel combinatorial strategy using TNBC xenograft and PDX models (Aim 3).

The proposed study is an innovative attempt to unravel the perplexing genome vulnerability of TNBC. The success of this project will provide preclinical mechanistic rationale for immediate clinical translation to TNBC management. The genome of TNBC has long been deemed to be chaotic due to its genome instability. Thus, the mechanistic underpinnings for rationally designed targeted therapies for TNBC patients are still largely lacking. This project represents a significant pursuit, contributing to the increasing compendium of TNBC vulnerability for therapeutic targeting. Mechanistically, we refute the current dogma that anti-cancer CDK4/6 inhibitor’s clinical efficacy solely depends on the presence of wild-type RB. Based on our preliminary studies, we propose a more holistic view of a non-oncogene addition model of G1/S cell cycle transition reprogrammed by the gain of DEDD. Building upon this concept, our extensive preclinical testing (Aim 3) of a combinatorial regimen containing a CDK4/6 inhibitor will be the first proof-of-concept study to demonstrate the potential of CDK4/6 inhibitors in RB-deficient tumors. Lastly, as CDK4/6 inhibitors and EGFR/HER2 inhibitors have been FDA approved and widely used in the clinic for breast cancer patient, novel clinical indications based on our study can be swiftly translated into TNBC clinical trials, with patient selection guided by DEDD expression.