BAP1 deficiency causes loss of melanocytic cell identity in uveal melanoma

BMC Cancer, Aug 2013

Background Uveal melanoma is a highly aggressive cancer with a strong propensity for metastasis, yet little is known about the biological mechanisms underlying this metastatic potential. We recently showed that most metastasizing uveal melanomas, which exhibit a class 2 gene expression profile, contain inactivating mutations in the tumor suppressor BAP1. The aim of this study was to investigate the role of BAP1 in uveal melanoma progression. Methods Uveal melanoma cells were studied following RNAi-mediated depletion of BAP1 using proliferation, BrdU incorporation, flow cytometry, migration, invasion, differentiation and clonogenic assays, as well as in vivo tumorigenicity experiments in NOD-SCID-Gamma mice. Results Depletion of BAP1 in uveal melanoma cells resulted in a loss of differentiation and gain of stem-like properties, including expression of stem cell markers, increased capacity for self-replication, and enhanced ability to grow in stem cell conditions. BAP1 depletion did not result in increased proliferation, migration, invasion or tumorigenicity. Conclusions BAP1 appears to function in the uveal melanocyte lineage primarily as a regulator of differentiation, with cells deficient for BAP1 exhibiting stem-like qualities. It will be important to elucidate how this effect of BAP1 loss promotes metastasis and how to reverse this effect therapeutically.

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BAP1 deficiency causes loss of melanocytic cell identity in uveal melanoma

BMC Cancer BAP1 deficiency causes loss of melanocytic cell identity in uveal melanoma Katie A Matatall 1 Olga A Agapova 1 Michael D Onken 1 Lori A Worley 1 Anne M Bowcock J William Harbour 0 1 0 Bascom Palmer Eye Institute, University of Miami Miller School of Medicine , 900 S.W. 17th Street, Miami, FL , USA 1 Department of Ophthalmology & Visual Sciences, Washington University School of Medicine , St. Louis, Missouri , USA Background: Uveal melanoma is a highly aggressive cancer with a strong propensity for metastasis, yet little is known about the biological mechanisms underlying this metastatic potential. We recently showed that most metastasizing uveal melanomas, which exhibit a class 2 gene expression profile, contain inactivating mutations in the tumor suppressor BAP1. The aim of this study was to investigate the role of BAP1 in uveal melanoma progression. Methods: Uveal melanoma cells were studied following RNAi-mediated depletion of BAP1 using proliferation, BrdU incorporation, flow cytometry, migration, invasion, differentiation and clonogenic assays, as well as in vivo tumorigenicity experiments in NOD-SCID-Gamma mice. Results: Depletion of BAP1 in uveal melanoma cells resulted in a loss of differentiation and gain of stem-like properties, including expression of stem cell markers, increased capacity for self-replication, and enhanced ability to grow in stem cell conditions. BAP1 depletion did not result in increased proliferation, migration, invasion or tumorigenicity. Conclusions: BAP1 appears to function in the uveal melanocyte lineage primarily as a regulator of differentiation, with cells deficient for BAP1 exhibiting stem-like qualities. It will be important to elucidate how this effect of BAP1 loss promotes metastasis and how to reverse this effect therapeutically. BAP1; Uveal melanoma; Differentiation; Stem cell; Metastasis; Tumor suppressor - Background Uveal melanoma is a highly aggressive cancer that arises from melanocytes within the uveal tract of the eye. Uveal melanomas can be classified according to their transcriptomic signature into two prognostically significant subtypes. Class 1 uveal melanomas are less aggressive and rarely metastasize, whereas class 2 uveal melanomas are highly aggressive and commonly give rise to fatal metastatic disease [1,2]. We recently showed that inactivating mutations in the tumor suppressor BAP1 occur almost exclusively in class 2 tumors and are strongly associated with metastasis, suggesting that BAP1 may function as a metastasis suppressor in uveal melanoma [3]. One patient in this report carried a germline BAP1 mutation, indicating that BAP1 mutations can give rise to a familial cancer syndrome. Since this report, somatic and germline BAP1 mutations have been identified in a variety of other tumors, including mesothelioma, cutaneous melanoma, atypical cutaneous melanocytic tumors, lung adenocarcinoma, meningioma and renal cell carcinoma [4-9]. BAP1 (BRCA1-associated protein-1) is an ubiquitin carboxy-terminal hydrolase that was identified in a screen for proteins that interact with BRCA1 [10]. It was initially found to be mutated in a few breast and lung cancer cell lines, where it exhibited tumor suppressor activity upon re-introduction [10]. BAP1 has been suggested to function in several pathways, including DNA damage repair, cell proliferation and development [11-14]. In Drosophila the BAP1 homolog Calypso is a component of the PR-DUB Polycomb repressive complex, and its loss results in a developmental phenotype characterized by deregulated HOX gene expression [14]. This study showed that both Calypso and human BAP1 catalyze the removal of monoubiquitin moieties from histone H2A when in the presence of Asx or ASXL1, respectively. This activity of BAP1 opposes the H2A ubiquitinating activity of the PRC1 complex, which contains BMI1. Interestingly BMI1 is an oncogene involved in stem cell maintenance, and its over-expression leads to a loss of cell identity in multiple cancers [15]. We recently showed that BAP1 loss causes increased histone H2A ubiquitination in melanoma cells and melanocytes, and this hyperubiquitination was reversed by treatment with HDAC inhibitors, which inhibit BMI1 [16]. Another recent study found that BAP1 loss leads to a myelodysplastic syndrome (MDS) in mouse [17]. They found that the predominant BAP1-interacting proteins in the hematopoietic lineage are HCF-1, OGT, ASXL1/2, and FOXK1/2, which is consistent with other studies [12-14,18]. In contrast to the findings in Drosophila, however, BAP1 loss in mouse did not effect HOX gene expression, suggesting that BAP1 may have divergent roles across species. Despite the recently renewed interest in BAP1, the precise cellular impact of BAP1 loss during tumorigenesis remains unclear. In this study, we wished to determine the function of BAP1 in uveal melanoma, where BAP1 loss appears to play a specific role in tumor progression and acquisition of metastatic capacity. O (...truncated)


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Katie A Matatall, Olga A Agapova, Michael D Onken, Lori A Worley, Anne M Bowcock, J Harbour. BAP1 deficiency causes loss of melanocytic cell identity in uveal melanoma, BMC Cancer, 2013, pp. 371, 13, DOI: 10.1186/1471-2407-13-371