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
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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)