Journal of Pathology J Pathol 2017; 241: 661–670 Published online 1 March 2017 in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/path.4877 ORIGINAL PAPER MDM4 is a rational target for treating breast cancers with mutant p53 Panimaya Jeffreena Miranda 1,2 , Daniel Buckley 1,2 , Dinesh Raghu 1,2 , Jia-Min B Pang 1,3 , Elena A Takano 1,3 , Reshma Vijayakumaran 1,2 , Amina FAS Teunisse 4 , Atara Posner 1,2 , Tahlia Procter 1,2 , Marco J Herold 5,6 , Cristina Gamell 1,2 , Jean-Christophe Marine 7,8 , Stephen B Fox 1,3 , Aart Jochemsen 4 , Sue Haupt 1,2 * ,† and Ygal Haupt 1,2,9,10† 1 Tumour Suppression Laboratory, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia 2 The Sir Peter MacCallum Department of Oncology, The University of Melbourne, Victoria, Australia 3 Department of Pathology, Peter MacCallum Cancer Centre, Melbourne, Victoria, Australia 4 Department of Molecular Cell Biology, University Medical Centre, Leiden, The Netherlands 5 Molecular Genetics of Cancer, The Walter and Eliza Hall Institute, Parkville, Victoria, Australia 6 Department of Medical Biology, University of Melbourne, Parkville, Victoria, Australia 7 Laboratory for Molecular Cancer Biology, Center for Cancer Biology, VIB, Leuven, Belgium 8 Laboratory for Molecular Cancer Biology, Department of Oncology, KULeuven, Leuven, Belgium 9 Department of Pathology, The University of Melbourne, Parkville, Victoria, Australia 10 Department of Biochemistry and Molecular Biology, Monash University, Clayton, Australia *Correspondence to: S Haupt, Tumour Suppression Laboratory, Peter MacCallum Cancer Centre, 305 Grattan Street, Melbourne, Victoria, Australia 3000. E-mail: Sue.Haupt@petermac.org † Equal contributions. Abstract Mutation of the key tumour suppressor p53 deines a transition in the progression towards aggressive and metastatic breast cancer (BC) with the poorest outcome. Speciically, the p53 mutation frequency exceeds 50% in triple-negative BC. Key regulators of mutant p53 that facilitate its oncogenic functions are potential therapeutic targets. We report here that the MDM4 protein is frequently abundant in the context of mutant p53 in basal-like BC samples. Importantly, we show that MDM4 plays a critical role in the proliferation of these BC cells. We demonstrate that conditional knockdown (KD) of MDM4 provokes growth inhibition across a range of BC subtypes with mutant p53, including luminal, Her2 + and triple-negative BCs. In vivo, MDM4 was shown to be crucial for the establishment and progression of tumours. This growth inhibition was mediated, at least in part, by the cell cycle inhibitor p27. Depletion of p27 together with MDM4 KD led to recovery of the proliferative capacity of cells that were growth-inhibited by MDM4 KD alone. Consistently, we identiied low levels of p27 expression in basal-like tumours corresponding to high levels of MDM4 and p53. This predicts a signature for a subset of tumours that may be amenable to therapies targeted towards MDM4 and mutant p53. The therapeutic potential of MDM4 as a target in BC with mutant p53 was shown in vitro by use of a small-molecule inhibitor. Overall, our study supports MDM4 as a novel therapeutic target for BC expressing mutant p53. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd. Keywords: MDM4; breast cancer; mutant p53; p27; TNBC Received 19 August 2016; Revised 20 December 2016; Accepted 8 January 2017 No conlicts of interest were declared. Introduction Breast cancer (BC) is the most commonly diagnosed cancer in women worldwide and a major cause of female cancer morbidity [1]. BC is a heterogeneous disease that is divided into subtypes according to molecular sig- natures [2]. Relapse of drug-resistant, metastatic dis- ease poses the greatest survival risk for BC patients [3]. Aggressive and metastatic BC is driven by muta- tion of the tumour suppressor p53 [4]. Wild-type (WT) p53 is a pivotal orchestrator of cellular stress responses and a vital tumour suppressor [5]. WT p53 preserves genomic integrity and prevents cancer by promoting DNA repair or eliminating damaged cells, through elic- itation of growth inhibition by cell cycle arrest, apop- tosis, and senescence [6,7]. Failure of p53 to function properly is a frequent outcome of its mutation, lead- ing to cancer development. Mutation in p53 occurs with an overall incidence of ∼23% in all BCs [8], but is more prevalent in triple-negative BC (TNBC) (nega- tive for oestrogen and progesterone receptors, and with- out ampliied Her2 + ). The p53 mutation incidence has been reported to be >50% in TNBCs [9] and in 84% of basal-like BCs, of which 80% were TNBCs [2]. Copyright © 2017 Pathological Society of Great Britain and Ireland. J Pathol 2017; 241: 661–670 Published by John Wiley & Sons, Ltd. www.pathsoc.org www.thejournalofpathology.com