New strategies in pleural mesothelioma: BAP1 and NF2 as novel targets for therapeutic development and risk assessment

Abstract

Malignant pleural mesothelioma (MPM) is a highly lethal cancer with limited therapeutic options. Recent work has focused on the frequent somatic inactivation of two tumor suppressor genes in MPM-NF2 (Neurofibromatosis type 2) and the recently identified BAP1 (BRCA associated protein 1). In addition, germline mutations in BAP1 have been identified that define a new familial cancer syndrome, which includes MPM, ocular melanoma, and other cancers. These recent advances may allow screening of high-risk individuals and the development of new therapies that target key pathways in MPM.

Figure 1. Mechanisms of NF2(Merlin)-mediated inhibition of proliferation

Merlin exists in an open, inactive form and a closed, active form. Matrix adhesion and the consequent activation of joint integrin-receptor tyrosine kinase signaling activate the serine-threonine kinase PAK. PAK in turn phosphorylates the C-terminus of Merlin, disrupting the intramolecular association that maintains the protein in a closed conformation. The resulting inactivation of Merlin removes a block to cell cycle progression in normal cells. Conversely, engagement of E-cadherin and the ensuing assembly of adherens junctions (AJs) inactivate PAK, leading to an accumulation of the closed, active form of Merlin, which migrates into the nucleus to inhibit CRL4^DCAF1. When it is not inhibited by Merlin, CRL4^DCAF1 positively regulates a broad oncogenic program of gene expression, which includes mitogenic signaling components, anti-apoptotic proteins, and Hippo pathway target genes. CRL4, cullin-ring E3 ligase 4; DCAF1, DDB1- and CUL4-associated factor 1; PAK, p21-activated kinase. Figure adapted and modified from reference .

Figure 2. A. BAP1 functional domains and mapped interacting regions

BAP1 is a nuclear deubiquitinating enzyme (DUB). DUBs catalyze the removal of single ubiquitin moieties from ubiquitin chains or cleavage of the isopeptide bond between ubiquitin and the substrate protein. BAP1 is composed of an N-terminal UCH domain (blue; a.a. 1-250), an HCF1-binding domain (HBM)-like motif (a.a. 363-366), a motif that shares conservation with UCH37 (ULD: UCH37-like domain; green, a.a. 634-693), and a bipartite nuclear localization signal (NLS, a.a.656-661, a.a.717-722). BAP1 has been reported to interact with BARD1 (a.a. 182-365), HCF1 (a.a. 365-385), BRCA1 (a.a. 596-721) and YY1 (a.a. 642-686). B. BAP1 functions: role of BAP1 in transcription factor regulation (left). Host Cell factor 1(HCF1) is detected as major binding partner of BAP1 by mass spectrum analysis. HCF1 interacts with specific transcription factors, including OCT1, E2F1, Kox20, Sp1 and GA binding protein (GABP). HCF1 also associates with several chromatin methyltransferases (Set1, MLL1, MLL5), chromatin acetyltransferases (tMOF) and deacetylases (HDAC1, HDAC2). HCF1 is known to recruit LSD1 to demethylate histone H4 lysine 9 and to promote the trimethylation of histone H3 lysine 4.^, YY1 is a transcriptional factor which binds to BAP1 and HCF1. BAP1 and HCF1 are recruited by YY1 to various promoters to upregulate gene expression. Role of BAP1 in chromatin modifications (middle). The trimethylation of lysine 27 of histone H3 (H3K27me3) is mediated the histone methyltransferase EZH1/2, a component of the PRC2 complex. This triggers recruitment of the PRC1 multiprotein complex through recognition of the H3K27me3 mark by the CBX subunit of the PRC1 complex. Another subunit of the PRC1 complex, the RING1 E3 ligase, then ubiquitylates lysine 119 of histone H2A (H2AK119ub1), which fixes chromatin in a repressed state, silencing gene expression. The H2AK119ub1 mark may be in a dynamic, continuously regulated state, as another part of the PRC1 complex, containing BAP1 and ASXL 1, displays an opposing, de-ubiquitylating activity (PR-DUB). A balance between ubiquitination and deubiquitination may be needed for appropriate PRC target gene repression. Possible role of BAP1 in DNA repair (right). Other binding partners of BAP1 include BRCA1 and its partner BARD1, which have important roles in the double strand DNA (dsDNA) repair process. Presently, the possible function of BAP1 in this process is not clear. The Rad51-dependent DNA repair pathway is highly regulated and includes many proteins, some of which may be potential substrates for BAP1-mediated ubiquitin hydrolysis.