The triple-negative breast cancer (TNBC) subtype represents a cancer that is highly aggressive with poor patient outcome. Current preclinical success has been gained through synthetic lethality, targeting genome instability with PARP inhibition in breast cancer cells that harbor silencing of the homologous recombination (HR) pathway. Histone deacetylase inhibitors (HDACi) are a class of drugs that mediate epigenetic changes in expression of HR pathway genes. Here, we compare the activity of the pan-HDAC inhibitor suberoylanilide hydroxamic acid (SAHA), the class I/IIa HDAC inhibitor valproic acid (VPA), and the HDAC1/2–specific inhibitor romidepsin (ROMI) for their capability to regulate DNA damage repair gene expression and in sensitizing TNBC to PARPi. We found that two of the HDACis tested, SAHA and ROMI, but not VPA, indeed inhibit HR repair and that RAD51, BARD1, and FANCD2 represent key proteins whose inhibition is required for HDACi-mediated therapy with PARP inhibition in TNBC. We also observed that restoration of BRCA1 function stabilizes the genome compared with mutant BRCA1 that results in enhanced polyploid population after combination treatment with HDACi and PARPi. Furthermore, we found that overexpression of the key HR protein RAD51 represents a mechanism for this resistance, promoting aberrant repair and the enhanced polyploidy observed. These findings highlight the key components of HR in guiding synthetic lethality with PARP inhibition and support the rationale for utilizing the novel combination of HDACi and PARPi against TNBC in the clinical setting. Mol Cancer Ther; 14(10); 2321–31. ©2015 AACR.

Epidermal growth factor receptor (EGFR) supports colorectal cancer progression via oncogenic signaling. Anti‐EGFR therapy is being investigated as a clinical option for colorectal cancer, and an observed interaction between EGFR and Prion protein has been detected in neuronal cells. We hypothesized that PrP^C expression levels may regulate EGFR signaling and that detailed understanding of this signaling pathway may enable identification of resistance mechanisms and new actionable targets in colorectal cancer. We performed molecular pathway analysis following knockdown of PrP^C or inhibition of EGFR signaling via gefitinib to identify changes in expression of key signaling proteins that determine cellular sensitivity or resistance to cisplatin. Expression of these proteins was examined in matched primary and metastatic patient samples and was correlated for resistance to therapy and progression of disease. Utilizing three colorectal cancer cell lines, we observed a correlation between high expression of PrP^C and resistance to cisplatin. Investigation of molecular signaling in a resistant cell line revealed that PrP^C contributed to signaling via colocalization with EGFR, which could be overcome by targeting p38 mitogen‐activated protein kinases (p38 MAPK). We revealed that the level of Krüppel‐like factor 5 (KLF5), a target downstream of p38 MAPK, was predictive for cell line and patient response to platinum agents. Further, high KLF5 expression was observed in BRAF‐mutant colorectal cancer. Our study indicates that the EGFR to KLF5 pathway is predictive of patient progression on platinum‐based therapy.

Anthracyclines are among the most effective chemotherapeutics ever developed, but they produce grueling side effects and serious adverse events, and resistance often develops over time. We found that these compounds can be sequestered by secreted cellular prion protein (PrP ^C ), which blocks their cytotoxic activity. This effect was dose dependent using either cell line–conditioned medium or human serum as a source of PrP ^C . Genetic depletion of PrP ^C or inhibition of binding via chelation of ionic copper prevented the interaction and restored cytotoxic activity. This was more pronounced for doxorubicin than its epimer, epirubicin. Investigating the relevance to breast cancer management, we found that the levels of PRNP transcript in pretreatment tumor biopsies stratified relapse-free survival after neoadjuvant treatment with anthracyclines, particularly among doxorubicin-treated patients with residual disease at surgery. These data suggest that local sequestration could mediate treatment resistance. Consistent with this, tumor cell expression of PrP ^C protein correlated with poorer response to doxorubicin but not epirubicin in an independent cohort analyzed by IHC, particularly soluble isoforms released into the extracellular environment by shedding. These findings have important potential clinical implications for frontline regimen decision making. We suggest there is warranted utility for prognostic PrP ^C / PRNP assays to guide chemosensitization strategies that exploit an understanding of PrP ^C -anthracycline-copper ion complexes.

Chemotherapy is used as a standard-of-care against cancers that display high levels of inherent genome instability. Chemotherapy induces DNA damage and intensifies pressure on the DNA repair pathways that can lead to deregulation. There is an urgent clinical need to be able to track the emergence of DNA repair driven chemotherapy resistance and tailor patient staging appropriately. There have been numerous studies into chemoresistance but to date no study has elucidated in detail the roles of the key DNA repair components in resistance associated with the frontline clinical combination of anthracyclines and taxanes together. In this study, we hypothesized that the emergence of chemotherapy resistance in triple negative breast cancer was driven by changes in functional signaling in the DNA repair pathways. We identified that consistent pressure on the non-homologous end joining pathway in the presence of genome instability causes failure of the key kinase DNA-PK, loss of p53 and compensation by p73. In-turn a switch to reliance on the homologous recombination pathway and RAD51 recombinase occurred to repair residual double strand DNA breaks. Further we demonstrate that RAD51 is an actionable target for resensitization to chemotherapy in resistant cells with a matched gene expression profile of resistance highlighted by homologous recombination in clinical samples.