Overexpression of EZH2 in conjunctival melanoma offers a new therapeutic target.

Malignant melanoma of the conjunctiva (CM) is an uncommon but potentially deadly disorder. Many malignancies show an increased activity of the epigenetic modifier enhancer of zeste homolog 2 (EZH2). We studied whether EZH2 is expressed in CM, and whether it may be a target for therapy in this malignancy. Immunohistochemical analysis showed that EZH2 protein expression was absent in normal conjunctival melanocytes and primary acquired melanosis, while EZH2 was highly expressed in 13 (50%) of 26 primary CM and seven (88%) of eight lymph node metastases. Increased expression was positively associated with tumour thickness (p =0.03). Next, we targeted EZH2 with specific inhibitors (GSK503 and UNC1999) or depleted EZH2 by stable shRNA knockdown in three primary CM cell lines. Both pharmacological and genetic inactivation of EZH2 inhibited cell growth and colony formation and influenced EZH2-mediated gene transcription and cell cycle profile in vitro. The tumour suppressor gene p21/CDKN1A was especially upregulated in CM cells after EZH2 knockdown in CM cells. Additionally, the potency of GSK503 against CM cells was monitored in zebrafish xenografts. GSK503 profoundly attenuated tumour growth in CM xenografts at a well-tolerated concentration. Our results indicate that elevated levels of EZH2 are relevant to CM tumourigenesis and progression, and that EZH2 may become a potential therapeutic target for patients with CM. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

MDM4 is a rational target for treating breast cancers with mutant p53.

Mutation of the key tumour suppressor p53 defines a transition in the progression towards aggressive and metastatic breast cancer (BC) with the poorest outcome. Specifically, 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 identified 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.

High levels of Hdmx promote cell growth in a subset of uveal melanomas.

The p53 tumor suppressor pathway is inactivated in cancer either via direct mutation or via deregulation of upstream regulators or downstream effectors. P53 mutations are rare in uveal melanoma. Here we investigated the role of the p53 inhibitor Hdmx in uveal melanoma. We found Hdmx over-expression in a subset of uveal melanoma cell lines and fresh-frozen tumor samples. Hdmx depletion resulted in cell-line dependent growth inhibition, apparently correlating with differential Hdm2 levels. Surprisingly, p53 knockdown hardly rescued cell cycle arrest and apoptosis induction upon Hdmx knockdown, whereas it effectively prevented growth suppression induced by the potent p53 activator Nutlin-3. In addition, two compounds inhibiting Hdmx function or expression, SAH-p53-8 and XI-011, also elicited a growth inhibitory effect in a partly p53-independent manner. These findings suggest a novel, growth-promoting function of Hdmx that does not rely on its ability to inhibit p53. We provide evidence for a contribution of p27 protein induction to the observed p53-independent G1 arrest in response to Hdmx knockdown. In conclusion, our study establishes the importance of Hdmx as an oncogene in a subset of uveal melanomas and widens the spectrum of its function beyond p53 inhibition.