NRF2 is a transcription factor serving as a master regulator of the expression of many genes involved in cellular responses to oxidative and other stresses. In the absence of stress, NRF2 is constantly synthesized but maintained at low levels as it is targeted by KEAP1 for ubiquitination and proteasome-mediated degradation. NRF2 binds KEAP1 mainly through a conserved “ETGE” motif that has also been found in several other proteins, such as DPP3, which has been shown to bind KEAP1 and enhance NRF2 function upon overexpression. Here we demonstrate the interaction between endogenous DPP3 and endogenous KEAP1. We further show that the DPP3–KEAP1 interaction is strongly induced by hydrogen peroxide and that DPP3 is required for timely NRF2 induction and nuclear accumulation in the estrogen receptor (ER)-positive MCF7 breast cancer cells. Moreover, we present evidence that the binding of DPP3 to KEAP1 stabilizes the latter. Finally, we show that DPP3 is overexpressed in breast cancer and that elevated levels of DPP3 mRNA correlate with increased NRF2 downstream gene expression and poor prognosis, particularly for ER-positive breast cancer. Our studies reveal novel insights into the regulation of NRF2 and identify DPP3 and an NRF2 transcriptional signature as potential biomarkers for breast cancer prognosis and treatment. Cancer Res; 77(11); 2881–92. ©2017 AACR.

Mapping protein interactions driving cancer: Cancer is a genetic disease, and much cancer research is focused on identifying carcinogenic mutations and determining how they relate to disease progression. Three papers demonstrate how mutations are processed through networks of protein interactions to promote cancer (see the Perspective by Cheng and Jackson). Swaney et al . focus on head and neck cancer and identify cancer-enriched interactions, demonstrating how point mutant–dependent interactions of PIK3CA, a kinase frequently mutated in human cancers, are predictive of drug response. Kim et al . focus on breast cancer and identify two proteins functionally connected to the tumor-suppressor gene BRCA1 and two proteins that regulate PIK3CA. Zheng et al . developed a statistical model that identifies protein networks that are under mutation pressure across different cancer types, including a complex bringing together PIK3CA with actomyosin proteins. These papers provide a resource that will be helpful in interpreting cancer genomic data. —VV

The BRCA1–PALB2–BRCA2 axis plays essential roles in the cellular response to DNA double-strand breaks (DSB), maintenance of genome integrity, and suppression of cancer development. Upon DNA damage, BRCA1 is recruited to DSBs, where it facilitates end resection and recruits PALB2 and its associated BRCA2 to load the central recombination enzyme RAD51 to initiate homologous recombination (HR) repair. In recent years, several BRCA1-independent mechanisms of PALB2 recruitment have also been reported. Collectively, these available data illustrate a series of hierarchical, context-dependent, and cooperating mechanisms of PALB2 recruitment that is critical for HR and therapy response either in the presence or absence of BRCA1. Here, we review these BRCA1-dependent and independent mechanisms and their importance in DSB repair, cancer development, and therapy. As BRCA1-mutant cancer cells regain HR function, for which PALB2 is generally required, and become resistant to targeted therapies, such as PARP inhibitors, targeting BRCA1-independent mechanisms of PALB2 recruitment represents a potential new avenue to improve treatment of BRCA1-mutant tumors.

BRCA1 plays a critical role in homology-directed repair (HDR) of DNA double strand breaks, and the repair defect of BRCA1-mutant cancer cells is being targeted with platinum drugs and poly (ADP-ribose) polymerase (PARP) inhibitors. We have employed relatively simple and sensitive assays to determine the function of BRCA1 variants or mutants in two HDR mechanisms, homologous recombination (HR) and single strand annealing (SSA), and in conferring resistance to cisplatin and olaparib in human cancer cells. Our results define the functionality of the top 22 patient-derived BRCA1 missense variants and the contribution of different domains of BRCA1 and its E3 ubiquitin ligase activity to HDR and drug resistance. Importantly, our results also demonstrate that the BRCA1-PALB2 interaction dictates the choice between HR and SSA. These studies establish functional and mutational landscapes of BRCA1 for HDR and therapy resistance, while revealing novel insights into BRCA1 regulatory mechanisms and HDR pathway choice.

The major breast cancer suppressor proteins BRCA1 and BRCA2 play essential roles in homologous recombination (HR)-mediated DNA repair, which is thought to be critical for tumor suppression. The two BRCA proteins are linked by a third tumor suppressor, PALB2, in the HR pathway. While truncating mutations in these genes are generally pathogenic, interpretation of missense variants remains a challenge. To date, patient-derived missense variants that disrupt PALB2 binding have been identified in BRCA1 and BRCA2; however, there has not been sufficient evidence to prove their pathogenicity in humans, and no variants in PALB2 that disrupt either its BRCA1 or BRCA2 binding have been reported. Here we report on the identification of a novel PALB2 variant, c.104T>C (p.L35P), that segregates in a family with a strong history of breast cancer. Functional analyses showed that L35P abrogates the PALB2–BRCA1 interaction and completely disables its abilities to promote HR and confer resistance to platinum salts and PARP inhibitors. Whole-exome sequencing of a breast cancer from a c.104T>C carrier revealed a second, somatic, truncating mutation affecting PALB2, and the tumor displays hallmark genomic features of tumors with BRCA mutations and HR defects, cementing the pathogenicity of L35P. Parallel analyses of other germline variants in the PALB2 N-terminal BRCA1-binding domain identified multiple variants that affect HR function to varying degrees, suggesting their possible contribution to cancer development. Our findings establish L35P as the first pathogenic missense mutation in PALB2 and directly demonstrate the requirement of the PALB2-BRCA1 interaction for breast cancer suppression.

Background and Aims: BRCA1 (BRCA1 DNA repair associated) and PALB2 (partner and localizer of BRCA2) interact with each other to promote homologous recombination and DNA double‐strand breaks repair. The disruption of this interaction has been reported to play a role in tumorigenesis. However, its precise function in HCC remains poorly understood. Approach and Results: We demonstrated that mice with disrupted BRCA1‐PALB2 interaction were more susceptible to HCC than wild‐type mice. HCC tumors arising from these mice showed plenty of T‐lymphocyte infiltration and a better response to programmed cell death 1 (PD‐1) antibody treatment. Mechanistically, disruption of the BRCA1‐PALB2 interaction causes persistent high level of DNA damage in HCC cells, leading to activation of the cyclic GMP‐AMP synthase (cGAS)–stimulator of interferon genes (STING) signaling pathway in both malignant hepatocytes and M1 macrophages in the tumor microenvironment. The activated cGAS‐STING pathway induces programmed cell death 1 ligand 1 expression via the STING–interferon regulatory factor 3 (IRF3)–signal transducer and activator of transcription 1 pathway, causing immunosuppression to facilitate tumorigenesis and tumor progression. Meanwhile, M1 macrophages with an activated cGAS‐STING pathway could recruit T lymphocytes through the STING‐IRF3 pathway, leading to T‐lymphocyte infiltration in tumors. After normalizing immune responses by PD‐1 antibody treatment, the infiltrating T lymphocytes attack tumor cells rapidly and effectively. Conclusions: This study reveals that persistent DNA damage caused by a defective BRCA pathway induces tumor immunosuppression and T‐lymphocyte infiltration in HCC through the cGAS‐STING pathway, providing insight into tumor immune microenvironment remodeling that may help improve HCC response to PD‐1 antibody treatment.