BAP1/ASXL1 recruitment and activation for H2A deubiquitination

O-GlcNAc Transferase Congenital Disorder of Glycosylation (OGT-CDG): Potential mechanistic targets revealed by evaluating the OGT interactome

O-GlcNAc transferase (OGT) is the sole enzyme responsible for the post-translational modification O-GlcNAc on thousands of target nucleocytoplasmic proteins. To date, nine variants of OGT that segregate with OGT Congenital Disorder of Glycosylation (OGT-CDG) have been reported and characterized. Numerous additional variants have been associated with OGT-CDG, some of which are currently undergoing investigation. This disorder primarily presents with global developmental delay and intellectual disability (ID), alongside other variable neurological features and subtle facial dysmorphisms in patients. Several hypotheses aim to explain the etiology of OGT-CDG, with a prominent hypothesis attributing the pathophysiology of OGT-CDG to mutations segregating with this disorder disrupting the OGT interactome. The OGT interactome consists of thousands of proteins, including substrates as well as interactors that require noncatalytic functions of OGT. A key aim in the field is to identify which interactors and substrates contribute to the primarily neural-specific phenotype of OGT-CDG. In this review, we will discuss the heterogenous phenotypic features of OGT-CDG seen clinically, the variable biochemical effects of mutations associated with OGT-CDG, and the use of animal models to understand this disorder. Furthermore, we will discuss how previously identified OGT interactors causal for ID provide mechanistic targets for investigation that could explain the dysregulated gene expression seen in OGT-CDG models. Identifying shared or unique altered pathways impacted in OGT-CDG patients will provide a better understanding of the disorder as well as potential therapeutic targets.

Structural and mechanistic basis for nucleosomal H2AK119 deubiquitination by single-subunit deubiquitinase USP16

Epigenetic regulators have a crucial effect on gene expression based on their manipulation of histone modifications. Histone H2AK119 monoubiquitination (H2AK119Ub), a well-established hallmark in transcription repression, is dynamically regulated by the opposing activities of Polycomb repressive complex 1 (PRC1) and nucleosome deubiquitinases including the primary human USP16 and Polycomb repressive deubiquitinase (PR-DUB) complex. Recently, the catalytic mechanism for the multi-subunit PR-DUB complex has been described, but how the single-subunit USP16 recognizes the H2AK119Ub nucleosome and cleaves the ubiquitin (Ub) remains unknown. Here we report the cryo-EM structure of USP16-H2AK119Ub nucleosome complex, which unveils a fundamentally distinct mode of H2AK119Ub deubiquitination compared to PR-DUB, encompassing the nucleosome recognition pattern independent of the H2A-H2B acidic patch and the conformational heterogeneity in the Ub motif and the histone H2A C-terminal tail. Our work highlights the mechanism diversity of H2AK119Ub deubiquitination and provides a structural framework for understanding the disease-causing mutations of USP16.

Potential roles of UCH family deubiquitinases in tumorigenesis and chemical inhibitors developed against them

Deubiquitinating enzymes (DUBs) are a large group of proteases that reverse ubiquitination process and maintain protein homeostasis. The DUBs have been classified into seven subfamilies according to their primary sequence and structural similarity. As a small subfamily of DUBs, the ubiquitin C-terminal hydrolases (UCHs) subfamily only contains four members including UCHL1, UCHL3, UCHL5, and BRCA1-associated protein-1 (BAP1). Despite sharing the deubiquitinase activity with a similar catalysis mechanism, the UCHs exhibit distinctive biological functions which are mainly determined by their specific subcellular localization and partner substrates. Besides, growing evidence indicates that the UCH enzymes are involved in human malignancies. In this review, the structural information and biological functions of the UCHs are briefly described. Meanwhile, the roles of these enzymes in tumorigenesis and the discovered inhibitors against them are also summarized to give an insight into the cancer therapy with the potential alternative strategy.

ASXLs binding to the PHD2/3 fingers of MLL4 provides a mechanism for the recruitment of BAP1 to active enhancers

The human methyltransferase and transcriptional coactivator MLL4 and its paralog MLL3 are frequently mutated in cancer. MLL4 and MLL3 monomethylate histone H3K4 and contain a set of uncharacterized PHD fingers. Here, we report a novel function of the PHD2 and PHD3 (PHD2/3) fingers of MLL4 and MLL3 that bind to ASXL2, a component of the Polycomb repressive H2AK119 deubiquitinase (PR-DUB) complex. The structure of MLL4 PHD2/3 in complex with the MLL-binding helix (MBH) of ASXL2 and mutational analyses reveal the molecular mechanism which is conserved in homologous ASXL1 and ASXL3. The native interaction of the Trithorax MLL3/4 complexes with the PR-DUB complex in vivo depends solely on MBH of ASXL1/2, coupling the two histone modifying activities. ChIP-seq analysis in embryonic stem cells demonstrates that MBH of ASXL1/2 is required for the deubiquitinase BAP1 recruitment to MLL4-bound active enhancers. Our findings suggest an ASXL1/2-dependent functional link between the MLL3/4 and PR-DUB complexes.

To Ub or not to Ub: The epic dilemma of histones that regulate gene expression and epigenetic cross-talk

A dynamic array of histone post-translational modifications (PTMs) regulate diverse cellular processes in the eukaryotic chromatin. Among them, histone ubiquitination is particularly complex as it alters nucleosome surface area fostering intricate cross-talk with other chromatin modifications. Ubiquitin signaling profoundly impacts DNA replication, repair, and transcription. Histones can undergo varied extent of ubiquitination such as mono, multi-mono, and polyubiquitination, which brings about distinct cellular fates. Mechanistic studies of the ubiquitin landscape in chromatin have unveiled a fascinating tapestry of events that orchestrate gene regulation. In this review, we summarize the key contributors involved in mediating different histone ubiquitination and deubiquitination events, and discuss their mechanism which impacts cell transcriptional identity and DNA damage response. We also focus on the proteins bearing epigenetic reader modules critical in discerning site-specific histone ubiquitination, pivotal for establishing complex epigenetic crosstalk. Moreover, we highlight the role of histone ubiquitination in different human diseases including neurodevelopmental disorders and cancer. Overall the review elucidates the intricate orchestration of histone ubiquitination impacting diverse cellular functions and disease pathogenesis, and provides insights into the current challenges of targeting them for therapeutic interventions.

ASXL3-related disorder: Molecular phenotyping and comprehensive review providing insights into disease mechanism

ASXL3-related disorder, sometimes referred to as Bainbridge-Ropers syndrome, was first identified as a distinct neurodevelopmental disorder by Bainbridge et al. in 2013. Since then, there have been a number of case series and single case reports published worldwide. A comprehensive review of the literature was carried out. Abstracts were screened, relevant literature was analysed, and descriptions of common phenotypic features were quantified. ASXL3 variants were collated and categorised. Common phenotypic features comprised global developmental delay or intellectual disability (97%), feeding problems (76%), hypotonia (88%) and characteristic facial features (93%). The majority of genetic variants were de novo truncating variants in exon 11 or 12 of the ASXL3 gene. Several gaps in our knowledge of this disorder were identified, namely, underlying pathophysiology and disease mechanism, disease contribution of missense variants, relevance of variant location, prevalence and penetrance data. Clinical information is currently limited by patient numbers and lack of longitudinal data, which this review aims to address.

DoUBLing up: ubiquitin and ubiquitin-like proteases in genome stability

Maintaining stability of the genome requires dedicated DNA repair and signalling processes that are essential for the faithful duplication and propagation of chromosomes. These DNA damage response (DDR) mechanisms counteract the potentially mutagenic impact of daily genotoxic stresses from both exogenous and endogenous sources. Inherent to these DNA repair pathways is the activity of protein factors that instigate repair processes in response to DNA lesions. The regulation, coordination, and orchestration of these DDR factors is carried out, in a large part, by post-translational modifications, such as phosphorylation, ubiquitylation, and modification with ubiquitin-like proteins (UBLs). The importance of ubiquitylation and UBLylation with SUMO in DNA repair is well established, with the modified targets and downstream signalling consequences relatively well characterised. However, the role of dedicated erasers for ubiquitin and UBLs, known as deubiquitylases (DUBs) and ubiquitin-like proteases (ULPs) respectively, in genome stability is less well established, particularly for emerging UBLs such as ISG15 and UFM1. In this review, we provide an overview of the known regulatory roles and mechanisms of DUBs and ULPs involved in genome stability pathways. Expanding our understanding of the molecular agents and mechanisms underlying the removal of ubiquitin and UBL modifications will be fundamental for progressing our knowledge of the DDR and likely provide new therapeutic avenues for relevant human diseases, such as cancer.

B-cell intrinsic regulation of antibody mediated immunity by histone H2A deubiquitinase BAP1

Introduction

BAP1 is a deubiquitinase (DUB) of the Ubiquitin C-terminal Hydrolase (UCH) family that regulates gene expression and other cellular processes, through its direct catalytic activity on the repressive epigenetic mark histone H2AK119ub, as well as on several other substrates. BAP1 is also a highly important tumor suppressor, expressed and functional across many cell types and tissues. In recent work, we demonstrated a cell intrinsic role of BAP1 in the B cell lineage development in murine bone marrow, however the role of BAP1 in the regulation of B cell mediated humoral immune response has not been previously explored.

Methods and results

In the current study, we demonstrate that a B-cell intrinsic loss of BAP1 in activated B cells in the Bap1 fl/fl Cγ1-cre murine model results in a severe defect in antibody production, with altered dynamics of germinal centre B cell, memory B cell, and plasma cell numbers. At the cellular and molecular level, BAP1 was dispensable for B cell immunoglobulin class switching but resulted in an impaired proliferation of activated B cells, with genome-wide dysregulation in histone H2AK119ub levels and gene expression.

Conclusion and discussion

In summary, our study establishes the B-cell intrinsic role of BAP1 in antibody mediated immune response and indicates its central role in the regulation of the genome-wide landscapes of histone H2AK119ub and downstream transcriptional programs of B cell activation and humoral immunity.

Beyond the tail: the consequence of context in histone post-translational modification and chromatin research

The role of histone post-translational modifications (PTMs) in chromatin structure and genome function has been the subject of intense debate for more than 60 years. Though complex, the discourse can be summarized in two distinct - and deceptively simple - questions: What is the function of histone PTMs? And how should they be studied? Decades of research show these queries are intricately linked and far from straightforward. Here we provide a historical perspective, highlighting how the arrival of new technologies shaped discovery and insight. Despite their limitations, the tools available at each period had a profound impact on chromatin research, and provided essential clues that advanced our understanding of histone PTM function. Finally, we discuss recent advances in the application of defined nucleosome substrates, the study of multivalent chromatin interactions, and new technologies driving the next era of histone PTM research.

The role of ASXL1 mutations and ASXL1 CircRNAs in cancer

BACKGROUND

Mutations in the Additional Sex Combs Like 1 (ASXL1) gene were first reported in myelodysplastic syndromes. Recent studies have clarified the relationship between ASXL1 mutations and the development of cancers.

OBJECTIVE

This study aims to review the roles of ASXL1 and ASXL1 CircRNAs, such as epigenetic regulation, chromatin modification, and transcription factor function in malignancies.

METHOD

This study is a review of articles related to the role of ASXL1 and ASXL1 CircRNAs in malignancies, retrieved from PubMed and Scopus.

RESULTS

ASXL1 plays a role in malignancies and is also related to poor overall survival and cancer metastasis. ASXL1 encodes conserved and abundant Circular RNAs (circRNAs) that act as post-transcriptional regulators, regulating tumorigenesis and progression in cancer. ASXL1 circRNA was identified in the top 10% of differentially expressed circRNAs in clinically relevant tissues. Additionally, the role of ASXL1 gene circRNAs in cancer development is reviewed in this study.

CONCLUSION

ASXL1 and ASXL1circRNA have dual functions in combination with different proteins, being involved in both transcriptional activation and repression in a context-dependent manner. Moreover, studies indicate these genes play an important role in epithelial-mesenchymal transition (EMT) and metastasis. Ongoing research is aimed at determining this gene family's function in biological events.

Morphological characteristics of SETD2-mutated locally advanced clear cell renal cell carcinoma: Comparison with BAP1-mutated clear cell renal cell carcinoma

SET-domain containing 2 (SETD2) and BRCA1-associated protein 1 (BAP1), both chromatin remodeling genes, are frequently mutated in clear cell renal cell carcinoma (ccRCC) and involved in tumor progression and metastasis. Herein, we studied clinicopathologic features of 7 cases of locally advanced ccRCC with single SETD2 mutation, and compared to 7 cases of locally advanced ccRCC with single BAP1 mutation. SETD2-mutated ccRCC showed high-grade transformation, comprising of enlarged tumor cells with voluminous clear cytoplasm, enlarged irregular nuclei with prominent nucleoli, eosinophilic cytoplasmic granules, arranged in various architectural patterns such as large nested, tubular, tubulopapillary and solid. 71 % (5 of 7 cases) of SETD2-mutated ccRCC showed a rhabdoid morphology. SETD2-mutated ccRCC have striking propensity for invasive growth; all cases have vascular invasion and perirenal (extracapsular) adipose tissue invasion. After nephrectomy, distant metastasis was found in 67 % (4 of 7 cases) of patients with SETD2-mutated ccRCC. The most common metastatic site was the lung (3 cases), followed by precaval lymph nodes (1 case). BAP1-mutated ccRCC also showed a similar high-grade morphology, with rhabdoid and/or sarcomatoid features. Their high-grade features mostly overlapped with those of SETD2-mutated ccRCC, which makes difficult to predict the presence of BAP1 or SETD2 mutation solely from morphology. These findings justify the use of molecular testing to detect these mutations, especially when we encounter high-grade ccRCC. Detecting SETD2 and BAP1 mutation in ccRCC is useful for risk stratification and proper therapeutic strategy.

The UBP5 histone H2A deubiquitinase counteracts PRCs-mediated repression to regulate Arabidopsis development

Polycomb Repressive Complexes (PRCs) control gene expression through the incorporation of H2Aub and H3K27me3. In recent years, there is increasing evidence of the complexity of PRCs' interaction networks and the interplay of these interactors with PRCs in epigenome reshaping, which is fundamental to understand gene regulatory mechanisms. Here, we identified UBIQUITIN SPECIFIC PROTEASE 5 (UBP5) as a chromatin player able to counteract the deposition of the two PRCs' epigenetic hallmarks in Arabidopsis thaliana. We demonstrated that UBP5 is a plant developmental regulator based on functional analyses of ubp5-CRISPR Cas9 mutant plants. UBP5 promotes H2A monoubiquitination erasure, leading to transcriptional de-repression. Furthermore, preferential association of UBP5 at PRC2 recruiting motifs and local H3K27me3 gaining in ubp5 mutant plants suggest the existence of functional interplays between UBP5 and PRC2 in regulating epigenome dynamics. In summary, acting as an antagonist of the pivotal epigenetic repressive marks H2Aub and H3K27me3, UBP5 provides novel insights to disentangle the complex regulation of PRCs' activities.

RNA methylation, homologous recombination repair and therapeutic resistance

Homologous recombination (HR) repair of DNA double-strand breaks (DSBs) is critical for maintaining genomic integrity and stability. Defects in HR increase the risk of tumorigenesis. However, many human tumors exhibit enhanced HR repair capabilities, consequently endowing tumor cells with resistance to DNA-damaging chemotherapy and radiotherapy. This review summarizes the role of RNA methylation in HR repair and therapeutic resistance in human tumors. We also analyzed the interactions between RNA methylation and other HR-modulating modifications including histone acetylation, histone deacetylation, ubiquitination, deubiquitination, protein arginine methylation, and gene transcription. This review proposes that targeting RNA methylation is a promising approach to overcoming HR-mediated therapeutic resistance.

Homology modeling of human BAP1 and analysis of its binding properties through molecular docking and molecular dynamics simulation

BRCA1-associated protein 1 (BAP1) is a nuclear-localized Ubiquitin C-terminal hydrolase (UCH) that functions as a tumour suppressor, and although BAP1 has been linked to cancer, the molecular mechanism by which BAP1 regulates cancer and its crystal structure have not been elucidated. In this study, computational approaches were used to identify the protein model of BAP1 and its potential inhibitors. The structure of the BAP1 model was constructed through homology modeling and the generated BAP1 model was observed to exhibit good quality protein model as the distribution of its amino acids in the Ramachandran's plot corresponded to 87.7% in the most favoured regions. Docking and simulating of the ubiquitin on the BAP1 model structure revealed the rearrangement of F228, F50, and H169 residues of the BAP1 and switching of BAP1's conformation into a productive state. Our screening results of potential BAP1 inhibitors against the FDA approved drugs shortlisted two potential inhibitors, which are FDA1065 and FDA755. We then performed molecular dynamics simulations and Molecular mechanics Poisson-Boltzmann surface area (MMPBSA) analysis on both inhibitors and found that only the BAP1-FDA755 formed a stable complex and the FDA755 ligand remained its position inside the active site of the BAP1 with a total binding energy of (-51.77 ± 3.49 kcal/mol). We speculate that the presence of methyl group in FDA755 play an important role in stabilizing the BAP1-FDA755 complex.Communicated by Ramaswamy H. Sarma.

Structural basis of histone H2A lysine 119 deubiquitination by Polycomb repressive deubiquitinase BAP1/ASXL1

Histone H2A lysine 119 (H2AK119Ub) is monoubiquitinated by Polycomb repressive complex 1 and deubiquitinated by Polycomb repressive deubiquitinase complex (PR-DUB). PR-DUB cleaves H2AK119Ub to restrict focal H2AK119Ub at Polycomb target sites and to protect active genes from aberrant silencing. The PR-DUB subunits (BAP1 and ASXL1) are among the most frequently mutated epigenetic factors in human cancers. How PR-DUB establishes specificity for H2AK119Ub over other nucleosomal ubiquitination sites and how disease-associated mutations of the enzyme affect activity are unclear. Here, we determine a cryo-EM structure of human BAP1 and the ASXL1 DEUBAD in complex with a H2AK119Ub nucleosome. Our structural, biochemical, and cellular data reveal the molecular interactions of BAP1 and ASXL1 with histones and DNA that are critical for restructuring the nucleosome and thus establishing specificity for H2AK119Ub. These results further provide a molecular explanation for how >50 mutations in BAP1 and ASXL1 found in cancer can dysregulate H2AK119Ub deubiquitination, providing insight into understanding cancer etiology.

Integrative modeling of lncRNA-chromatin interaction maps reveals diverse mechanisms of nuclear retention

BACKGROUND

Many long non-coding RNAs, known to be involved in transcriptional regulation, are enriched in the nucleus and interact with chromatin. However, their mechanisms of chromatin interaction and the served cellular functions are poorly understood. We sought to characterize the mechanisms of lncRNA nuclear retention by systematically mapping the sequence and chromatin features that distinguish lncRNA-interacting genomic segments.

RESULTS

We found DNA 5-mer frequencies to be predictive of chromatin interactions for all lncRNAs, suggesting sequence-specificity as a global theme in the interactome. Sequence features representing protein-DNA and protein-RNA binding motifs revealed potential mechanisms for specific lncRNAs. Complementary to these global themes, transcription-related features and DNA-RNA triplex formation potential were noted to be highly predictive for two mutually exclusive sets of lncRNAs. DNA methylation was also noted to be a significant predictor, but only when combined with other epigenomic features.

CONCLUSIONS

Taken together, our statistical findings suggest that a group of lncRNAs interacts with transcriptionally inactive chromatin through triplex formation, whereas another group interacts with transcriptionally active regions and is involved in DNA Damage Response (DDR) through formation of R-loops. Curiously, we observed a strong pattern of enrichment of 5-mers in four potentially interacting entities: lncRNA-bound DNA tiles, lncRNAs, miRNA seed sequences, and repeat elements. This finding points to a broad sequence-based network of interactions that may underlie regulation of fundamental cellular functions. Overall, this study reveals diverse sequence and chromatin features related to lncRNA-chromatin interactions, suggesting potential mechanisms of nuclear retention and regulatory function.

Secondary-Type Mutations in Acute Myeloid Leukemia: Updates from ELN 2022

The characterization of the molecular landscape and the advent of targeted therapies have defined a new era in the prognostication and treatment of acute myeloid leukemia. Recent revisions in the European LeukemiaNet 2022 guidelines have refined the molecular, cytogenetic, and treatment-related boundaries between myelodysplastic neoplasms (MDS) and AML. This review details the molecular mechanisms and cellular pathways of myeloid maturation aberrancies contributing to dysplasia and leukemogenesis, focusing on recent molecular categories introduced in ELN 2022. We provide insights into novel and rational therapeutic combination strategies that exploit mechanisms of leukemogenesis, highlighting the underpinnings of splicing factors, the cohesin complex, and chromatin remodeling. Areas of interest for future research are summarized, and we emphasize approaches designed to advance existing treatment strategies.

Multiomics of Bohring-Opitz syndrome truncating ASXL1 mutations identify canonical and noncanonical Wnt signaling dysregulation

ASXL1 (additional sex combs-like 1) plays key roles in epigenetic regulation of early developmental gene expression. De novo protein-truncating mutations in ASXL1 cause Bohring-Opitz syndrome (BOS; OMIM #605039), a rare neurodevelopmental condition characterized by severe intellectual disabilities, distinctive facial features, hypertrichosis, increased risk of Wilms tumor, and variable congenital anomalies, including heart defects and severe skeletal defects giving rise to a typical BOS posture. These BOS-causing ASXL1 variants are also high-prevalence somatic driver mutations in acute myeloid leukemia. We used primary cells from individuals with BOS (n = 18) and controls (n = 49) to dissect gene regulatory changes caused by ASXL1 mutations using comprehensive multiomics assays for chromatin accessibility (ATAC-seq), DNA methylation, histone methylation binding, and transcriptome in peripheral blood and skin fibroblasts. Our data show that regardless of cell type, ASXL1 mutations drive strong cross-tissue effects that disrupt multiple layers of the epigenome. The data showed a broad activation of canonical Wnt signaling at the transcriptional and protein levels and upregulation of VANGL2, which encodes a planar cell polarity pathway protein that acts through noncanonical Wnt signaling to direct tissue patterning and cell migration. This multiomics approach identifies the core impact of ASXL1 mutations and therapeutic targets for BOS and myeloid leukemias.

Epigenetic regulation by ASXL1 in myeloid malignancies

Myeloid malignancies are clonal hematopoietic disorders that are comprised of a spectrum of genetically heterogeneous disorders, including myelodysplastic syndromes (MDS), myeloproliferative neoplasms (MPN), chronic myelomonocytic leukemia (CMML), and acute myeloid leukemia (AML). Myeloid malignancies are characterized by excessive proliferation, abnormal self-renewal, and/or differentiation defects of hematopoietic stem cells (HSCs) and myeloid progenitor cells hematopoietic stem/progenitor cells (HSPCs). Myeloid malignancies can be caused by genetic and epigenetic alterations that provoke key cellular functions, such as self-renewal, proliferation, biased lineage commitment, and differentiation. Advances in next-generation sequencing led to the identification of multiple mutations in myeloid neoplasms, and many new gene mutations were identified as key factors in driving the pathogenesis of myeloid malignancies. The polycomb protein ASXL1 was identified to be frequently mutated in all forms of myeloid malignancies, with mutational frequencies of 20%, 43%, 10%, and 20% in MDS, CMML, MPN, and AML, respectively. Significantly, ASXL1 mutations are associated with a poor prognosis in all forms of myeloid malignancies. The fact that ASXL1 mutations are associated with poor prognosis in patients with CMML, MDS, and AML, points to the possibility that ASXL1 mutation is a key factor in the development of myeloid malignancies. This review summarizes the recent advances in understanding myeloid malignancies with a specific focus on ASXL1 mutations.

Basis of the H2AK119 specificity of the Polycomb repressive deubiquitinase

Repression of gene expression by protein complexes of the Polycomb group is a fundamental mechanism that governs embryonic development and cell-type specification1-3. The Polycomb repressive deubiquitinase (PR-DUB) complex removes the ubiquitin moiety from monoubiquitinated histone H2A K119 (H2AK119ub1) on the nucleosome4, counteracting the ubiquitin E3 ligase activity of Polycomb repressive complex 1 (PRC1)5 to facilitate the correct silencing of genes by Polycomb proteins and safeguard active genes from inadvertent silencing by PRC1 (refs. 6-9). The intricate biological function of PR-DUB requires accurate targeting of H2AK119ub1, but PR-DUB can deubiquitinate monoubiquitinated free histones and peptide substrates indiscriminately; the basis for its exquisite nucleosome-dependent substrate specificity therefore remains unclear. Here we report the cryo-electron microscopy structure of human PR-DUB, composed of BAP1 and ASXL1, in complex with the chromatosome. We find that ASXL1 directs the binding of the positively charged C-terminal extension of BAP1 to nucleosomal DNA and histones H3-H4 near the dyad, an addition to its role in forming the ubiquitin-binding cleft. Furthermore, a conserved loop segment of the catalytic domain of BAP1 is situated near the H2A-H2B acidic patch. This distinct nucleosome-binding mode displaces the C-terminal tail of H2A from the nucleosome surface, and endows PR-DUB with the specificity for H2AK119ub1.

The Pleiotropic Ubiquitin-Specific Peptidase 16 and Its Many Substrates

Ubiquitin-specific peptidase 16 (USP16) is a deubiquitinase that plays a role in the regulation of gene expression, cell cycle progression, and various other functions. It was originally identified as the major deubiquitinase for histone H2A and has since been found to deubiquitinate a range of other substrates, including proteins from both the cytoplasm and nucleus. USP16 is phosphorylated when cells enter mitosis and dephosphorylated during the metaphase/anaphase transition. While much of USP16 is localized in the cytoplasm, separating the enzyme from its substrates is considered an important regulatory mechanism. Some of the functions that USP16 has been linked to include DNA damage repair, immune disease, tumorigenesis, protein synthesis, coronary artery health, and male infertility. The strong connection to immune response and the fact that multiple oncogene products are substrates of USP16 suggests that USP16 may be a potential therapeutic target for the treatment of certain human diseases.

Structural basis of histone H2A lysine 119 deubiquitination by Polycomb Repressive Deubiquitinase BAP1/ASXL1

The maintenance of gene expression patterns during metazoan development is achieved by the actions of Polycomb group (PcG) complexes. An essential modification marking silenced genes is monoubiquitination of histone H2A lysine 119 (H2AK119Ub) deposited by the E3 ubiquitin ligase activity of the non-canonical Polycomb Repressive Complex 1. The Polycomb Repressive Deubiquitinase (PR-DUB) complex cleaves monoubiquitin from histone H2A lysine 119 (H2AK119Ub) to restrict focal H2AK119Ub at Polycomb target sites and to protect active genes from aberrant silencing. BAP1 and ASXL1, subunits that form active PR-DUB, are among the most frequently mutated epigenetic factors in human cancers, underscoring their biological importance. How PR-DUB achieves specificity for H2AK119Ub to regulate Polycomb silencing is unknown, and the mechanisms of most of the mutations in BAP1 and ASXL1 found in cancer have not been established. Here we determine a cryo-EM structure of human BAP1 bound to the ASXL1 DEUBAD domain in complex with a H2AK119Ub nucleosome. Our structural, biochemical, and cellular data reveal the molecular interactions of BAP1 and ASXL1 with histones and DNA that are critical for remodeling the nucleosome and thus establishing specificity for H2AK119Ub. These results further provide a molecular explanation for how >50 mutations in BAP1 and ASXL1 found in cancer can dysregulate H2AK119Ub deubiquitination, providing new insight into understanding cancer etiology.

One Sentence Summary

We reveal the molecular mechanism of nucleosomal H2AK119Ub deubiquitination by human BAP1/ASXL1.

Targeting BAP1 with small compound inhibitor for colon cancer treatment

BRCA1-associated protein-1 (BAP1) is a ubiquitin C-terminal hydrolase domain-containing deubiquitinase. The gene encoding BAP1 is mutated in various human cancers, including mesothelioma, uveal melanoma and renal cell carcinoma. BAP1 plays roles in many cancer-related cellular functions, including cell proliferation, cell death, and nuclear processes crucial for genome stability, such as DNA repair and replication. While these findings suggest that BAP1 functions as a tumor suppressor, recent data also suggest that BAP1 might play tumor-promoting roles in certain cancers, such as breast cancer and hematopoietic malignancies. Here, we show that BAP1 is upregulated in colon cancer cells and tissues and that BAP1 depletion reduces colon cancer cell proliferation and tumor growth. BAP1 contributes to colon cancer cell proliferation by accelerating DNA replication and suppressing replication stress and concomitant apoptosis. A recently identified BAP1 inhibitor, TG2-179-1, which seems to covalently bind to the active site of BAP1, exhibits potent cytotoxic activity against colon cancer cells, with half-maximal inhibitory concentrations of less than 10 μM, and inhibits colon tumor growth. TG2-179-1 exerts cytotoxic activity by targeting BAP1, leading to defective replication and increased apoptosis. This work therefore shows that BAP1 acts oncogenically in colon cancer and is a potential therapeutic target for this cancer. Our work also suggests that TG2-179-1 can be developed as a potential therapeutic agent for colon cancer.

YAP/TAZ activation predicts clinical outcomes in mesothelioma and is conserved in in vitro model of driver mutations

The Hippo signalling pathway is dysregulated across a wide range of cancer types and, although driver mutations that directly affect the core Hippo components are rare, a handful is found within pleural mesothelioma (PM). PM is a deadly disease of the lining of the lung caused by asbestos exposure. By pooling the largest-scale clinical datasets publicly available, we here interrogate associations between the most prevalent driver mutations within PM and Hippo pathway disruption in patients, while assessing correlations with a variety of clinical markers. This analysis reveals a consistent worse outcome in patients exhibiting transcriptional markers of YAP/TAZ activation, pointing to the potential of leveraging Hippo pathway transcriptional activation status as a metric by which patients may be meaningfully stratified. Preclinical models recapitulating disease are transformative in order to develop new therapeutic strategies. We here establish an isogenic cell-line model of PM, which represents the most frequently mutated genes and which faithfully recapitulates the molecular features of clinical PM. This preclinical model is developed to probe the molecular basis by which the Hippo pathway and key driver mutations affect cancer initiation and progression. Implementing this approach, we reveal the role of NF2 as a mechanosensory component of the Hippo pathway in mesothelial cells. Cellular NF2 loss upon physiological stiffnesses analogous to the tumour niche drive YAP/TAZ-dependent anchorage-independent growth. Consequently, the development and characterisation of this cellular model provide a unique resource to obtain molecular insights into the disease and progress new drug discovery programs together with future stratification of PM patients.

Acyl azide modification of the ubiquitin C-terminus enables DUB capture

Deubiquitinating enzyme (DUB) abnormalities are associated with many diseases. Previous attempts have been made to introduce various chemical groups such as alkynes, unsaturated olefins and alkyl halides to the C-terminus of ubiquitin (Ub) to capture the active-site cysteine residue in DUBs for structural and biochemical studies. Here, we find that a Ub C-terminal acyl azide can capture DUBs, thereby forming thioester bonds in buffers and cell lysates. This finding not only makes ubiquitin acyl azide a chemical probe for capturing DUBs, but also extends the utility of azide groups in biological applications.

Mechanisms of myeloid leukemogenesis: Current perspectives and therapeutic objectives

Acute myeloid leukemia (AML) is a heterogeneous hematopoietic neoplasm which results in clonal proliferation of abnormally differentiated hematopoietic cells. In this review, mechanisms contributing to myeloid leukemogenesis are summarized, highlighting aberrations of epigenetics, transcription factors, signal transduction, cell cycling, and the bone marrow microenvironment. The mechanisms contributing to AML are detailed to spotlight recent findings that convey clinical impact. The applications of current and prospective therapeutic targets are accentuated in addition to reviews of treatment paradigms stratified for each characteristic molecular lesion - with a focus on exploring novel treatment approaches and combinations to improve outcomes in AML.

BAP1 loss induces mitotic defects in mesothelioma cells through BRCA1-dependent and independent mechanisms

The tumour suppressor BRCA1-associated protein 1 (BAP1) is the most frequently mutated cancer gene in mesothelioma. Here we report novel functions for BAP1 in mitotic progression highlighting the relationship between BAP1 and control of genome stability in mesothelioma cells with therapeutic implications. Depletion of BAP1 protein induced proteasome-mediated degradation of BRCA1 in mesothelioma cells while loss of BAP1 correlated with BRCA1 loss in mesothelioma patient tumour samples. BAP1 loss also led to mitotic defects that phenocopied the loss of BRCA1 including spindle assembly checkpoint failure, centrosome amplification and chromosome segregation errors. However, loss of BAP1 also led to additional mitotic changes that were not observed upon BRCA1 loss, including an increase in spindle length and enhanced growth of astral microtubules. Intriguingly, these consequences could be explained by loss of expression of the KIF18A and KIF18B kinesin motors that occurred upon depletion of BAP1 but not BRCA1, as spindle and astral microtubule defects were rescued by re-expression of KIF18A and KIF18B, respectively. We therefore propose that BAP1 inactivation causes mitotic defects through BRCA1-dependent and independent mechanisms revealing novel routes by which mesothelioma cells lacking BAP1 may acquire genome instability and exhibit altered responses to microtubule-targeted agents.

BAP1 promotes the repair of UV-induced DNA damage via PARP1-mediated recruitment to damage sites and control of activity and stability

BRCA1-associated protein-1 (BAP1) is a ubiquitin C-terminal hydrolase domain-containing deubiquitinase with tumor suppressor activity. The gene encoding BAP1 is mutated in various human cancers, with particularly high frequency in kidney and skin cancers, and BAP1 is involved in many cancer-related cellular functions, such as DNA repair and genome stability. Although BAP1 stimulates DNA double-strand break repair, whether it functions in nucleotide excision repair (NER) is unknown. Here, we show that BAP1 promotes the repair of ultraviolet (UV)-induced DNA damage via its deubiquitination activity in various cell types, including primary melanocytes. Poly(ADP-ribose) polymerase 1 (PARP1) interacts with and recruits BAP1 to damage sites, with BAP1 recruitment peaking after the DDB2 and XPC damage sensors. BAP1 recruitment also requires histone H2A monoubiquitinated at Lys119, which accumulates at damage sites. PARP1 transiently poly(ADP-ribosyl)ates (PARylates) BAP1 at multiple sites after UV damage and stimulates the deubiquitination activity of BAP1 both intrinsically and via PARylation. PARP1 also promotes BAP1 stability via crosstalk between PARylation and ubiquitination. Many PARylation sites in BAP1 are mutated in various human cancers, among which the glutamic acid (Glu) residue at position 31, with particularly frequent mutation in kidney cancer, plays a critical role in BAP1 stabilization and promotes UV-induced DNA damage repair. Glu31 also participates in reducing the viability of kidney cancer cells. This study therefore reveals that BAP1 functions in the NER pathway and that PARP1 plays a role as a novel factor that regulates BAP1 enzymatic activity, protein stability, and recruitment to damage sites. This activity of BAP1 in NER, along with its cancer cell viability-reducing activity, may account for its tumor suppressor function.

CARM1-mediated methylation of ASXL2 impairs tumor-suppressive function of MLL3/COMPASS

An imbalance in the activities of the Polycomb and Trithorax complexes underlies numerous human pathologies, including cancer. The BRCA1 associated protein-1 (BAP1) deubiquitinase negatively regulates Polycomb activity and recruits the Trithorax histone H3K4 methyltransferase, mixed-lineage leukemia protein 3 (MLL3) within Complex Proteins Associated with Set1 (COMPASS), to the enhancers of tumor suppressor genes. We previously demonstrated that the BAP1-MLL3 pathway is mutated in several cancers, yet how BAP1 recruits MLL3 to its target loci remains an important unanswered question. We demonstrate that the ASXL2 subunit of the BAP1 complex mediates a direct interaction with MLL3/COMPASS. ASXL2 loss results in decreased MLL3 occupancy at enhancers and reduced BAP1-MLL3 target gene expression. Interaction between ASXL2 and MLL3 is negatively regulated by protein arginine methyltransferase 4 (PRMT4/CARM1), which methylates ASXL2 at R639/R641. ASXL2 methylation blocks binding to MLL3 and impairs the expression of MLL3/COMPASS-dependent genes. This previously unidentified transcriptional repressive function of CARM1 provides insight into the BAP1/MLL3-COMPASS axis and reveals a potential cancer therapeutic target.

MBD5 and MBD6 stabilize the BAP1 complex and promote BAP1-dependent cancer

Background

BRCA1-associated protein 1 (BAP1) is an ubiquitin carboxy-terminal hydrolase, which forms a multi-protein complex with different epigenetic factors, such as ASXL1-3 and FOXK1/2. At the chromatin level, BAP1 catalyzes the removal of mono-ubiquitination on histone H2AK119 in collaboration with other subunits within the complex and functions as a transcriptional activator in mammalian cells. However, the crosstalk between different subunits and how these subunits impact BAP1’s function remains unclear.

Results

We report the identification of the methyl-CpG-binding domain proteins 5 and 6 (MBD5 and MBD6) that bind to the C-terminal PHD fingers of the large scaffold subunits ASXL1-3 and stabilize the BAP1 complex at the chromatin. We further identify a novel Drosophila protein, the six-banded (SBA), as an ortholog of human MBD5 and MBD6, and demonstrate that the core modules of the BAP1 complex is structurally and functionally conserved from Drosophila (Calypso/ASX/SBA) to human cells (BAP1/ASXL/MBD). Dysfunction of the BAP1 complex induced by the misregulation/mutations in its subunit(s) are frequent in many human cancers. In BAP1-dependent human cancers, such as small cell lung cancer (SCLC), MBD6 tends to be a part of the predominant complex formed. Therefore, depletion of MBD6 leads to a global loss of BAP1 occupancy at the chromatin, resulting in a reduction of BAP1-dependent gene expression and tumor growth in vitro and in vivo.

Conclusions

We characterize MBD5 and MBD6 as important regulators of the BAP1 complex and maintain its transcriptional landscape, shedding light on the therapeutic potential of targeting MBD5 and MBD6 in BAP1-dependent human cancers.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13059-022-02776-x.

ASXL1/2 mutations and myeloid malignancies

Myeloid malignancies develop through the accumulation of genetic and epigenetic alterations that dysregulate hematopoietic stem cell (HSC) self-renewal, stimulate HSC proliferation and result in differentiation defects. The polycomb group (PcG) and trithorax group (TrxG) of epigenetic regulators act antagonistically to regulate the expression of genes key to stem cell functions. The genes encoding these proteins, and the proteins that interact with them or affect their occupancy at chromatin, are frequently mutated in myeloid malignancies. PcG and TrxG proteins are regulated by Enhancers of Trithorax and Polycomb (ETP) proteins. ASXL1 and ASXL2 are ETP proteins that assemble chromatin modification complexes and transcription factors. ASXL1 mutations frequently occur in myeloid malignancies and are associated with a poor prognosis, whereas ASXL2 mutations frequently occur in AML with t(8;21)/RUNX1-RUNX1T1 and less frequently in other subtypes of myeloid malignancies. Herein, we review the role of ASXL1 and ASXL2 in normal and malignant hematopoiesis by summarizing the findings of mouse model systems and discussing their underlying molecular mechanisms.

Know when to fold 'em: Polycomb complexes in oncogenic 3D genome regulation

Chromatin is spatially and temporally regulated through a series of orchestrated processes resulting in the formation of 3D chromatin structures such as topologically associating domains (TADs), loops and Polycomb Bodies. These structures are closely linked to transcriptional regulation, with loss of control of these processes a frequent feature of cancer and developmental syndromes. One such oncogenic disruption of the 3D genome is through recurrent dysregulation of Polycomb Group Complex (PcG) functions either through genetic mutations, amplification or deletion of genes that encode for PcG proteins. PcG complexes are evolutionarily conserved epigenetic complexes. They are key for early development and are essential transcriptional repressors. PcG complexes include PRC1, PRC2 and PR-DUB which are responsible for the control of the histone modifications H2AK119ub1 and H3K27me3. The spatial distribution of the complexes within the nuclear environment, and their associated modifications have profound effects on the regulation of gene transcription and the 3D genome. Nevertheless, how PcG complexes regulate 3D chromatin organization is still poorly understood. Here we glean insights into the role of PcG complexes in 3D genome regulation and compaction, how these processes go awry during tumorigenesis and the therapeutic implications that result from our insights into these mechanisms.

BAP1 maintains HIF-dependent interferon beta induction to suppress tumor growth in clear cell renal cell carcinoma

BRCA1-associated protein 1 (BAP1) is a deubiquitinase that is mutated in 10-15% of clear cell renal cell carcinomas (ccRCC). Despite the association between BAP1 loss and poor clinical outcome, the critical tumor suppressor function(s) of BAP1 in ccRCC remains unclear. Previously, we found that hypoxia-inducible factor 2α (HIF2α) and BAP1 activate interferon-stimulated gene factor 3 (ISGF3), a transcription factor activated by type I interferons and a tumor suppressor in ccRCC xenograft models. Here, we aimed to determine the mechanism(s) through which HIF and BAP1 regulate ISGF3. We found that in ccRCC cells, loss of the von Hippel-Lindau tumor suppressor (VHL) activated interferon beta (IFN-β) expression in a HIF2α-dependent manner. IFN-β was required for ISGF3 activation and suppressed the growth of Ren-02 tumors in xenografts. BAP1 enhanced the expression of IFN-β and stimulator of interferon genes (STING), both of which activate ISGF3. Both ISGF3 overexpression and STING agonist treatment increased ISGF3 activity and suppressed BAP1-deficient tumor growth in Ren-02 xenografts. Our results indicate that BAP1 loss reduces type I interferon signaling, and reactivating this pathway may be a novel therapeutic strategy for treating ccRCC.

Histone Mono-Ubiquitination in Transcriptional Regulation and Its Mark on Life: Emerging Roles in Tissue Development and Disease

Epigenetic regulation plays an essential role in driving precise transcriptional programs during development and homeostasis. Among epigenetic mechanisms, histone mono-ubiquitination has emerged as an important post-transcriptional modification. Two major histone mono-ubiquitination events are the mono-ubiquitination of histone H2A at lysine 119 (H2AK119ub), placed by Polycomb repressive complex 1 (PRC1), and histone H2B lysine 120 mono-ubiquitination (H2BK120ub), placed by the heteromeric RNF20/RNF40 complex. Both of these events play fundamental roles in shaping the chromatin epigenetic landscape and cellular identity. In this review we summarize the current understandings of molecular concepts behind histone mono-ubiquitination, focusing on their recently identified roles in tissue development and pathologies.

Mechanisms orchestrating the enzymatic activity and cellular functions of deubiquitinases

Deubiquitinases (DUBs) are required for the reverse reaction of ubiquitination and act as major regulators of ubiquitin signaling processes. Emerging evidence suggests that these enzymes are regulated at multiple levels in order to ensure proper and timely substrate targeting and to prevent the adverse consequences of promiscuous deubiquitination. The importance of DUB regulation is highlighted by disease-associated mutations that inhibit or activate DUBs, deregulating their ability to coordinate cellular processes. Here, we describe the diverse mechanisms governing protein stability, enzymatic activity, and function of DUBs. In particular, we outline how DUBs are regulated by their protein domains and interacting partners. Intramolecular interactions can promote protein stability of DUBs, influence their subcellular localization, and/or modulate their enzymatic activity. Remarkably, these intramolecular interactions can induce self-deubiquitination to counteract DUB ubiquitination by cognate E3 ubiquitin ligases. In addition to intramolecular interactions, DUBs can also oligomerize and interact with a wide variety of cellular proteins, thereby forming obligate or facultative complexes that regulate their enzymatic activity and function. The importance of signaling and post-translational modifications in the integrated control of DUB function will also be discussed. While several DUBs are described with respect to the multiple layers of their regulation, the tumor suppressor BAP1 will be outlined as a model enzyme whose localization, stability, enzymatic activity, and substrate recognition are highly orchestrated by interacting partners and post-translational modifications.

Tumor suppressor BAP1 nuclear import is governed by transportin-1

Subcellular localization of the deubiquitinating enzyme BAP1 is deterministic for its tumor suppressor activity. While the monoubiquitination of BAP1 by an atypical E2/E3-conjugated enzyme UBE2O and BAP1 auto-deubiquitination are known to regulate its nuclear localization, the molecular mechanism by which BAP1 is imported into the nucleus has remained elusive. Here, we demonstrated that transportin-1 (TNPO1, also known as Karyopherin β2 or Kapβ2) targets an atypical C-terminal proline-tyrosine nuclear localization signal (PY-NLS) motif of BAP1 and serves as the primary nuclear transporter of BAP1 to achieve its nuclear import. TNPO1 binding dissociates dimeric BAP1 and sequesters the monoubiquitination sites flanking the PY-NLS of BAP1 to counteract the function of UBE2O that retains BAP1 in the cytosol. Our findings shed light on how TNPO1 regulates the nuclear import, self-association, and monoubiquitination of BAP1 pertinent to oncogenesis.

DNA methylation signature associated with Bohring-Opitz syndrome: a new tool for functional classification of variants in ASXL genes

The additional sex combs-like (ASXL) gene family-encoded by ASXL1, ASXL2, and ASXL3-is crucial for mammalian development. Pathogenic variants in the ASXL gene family are associated with three phenotypically distinct neurodevelopmental syndromes. Our previous work has shown that syndromic conditions caused by pathogenic variants in epigenetic regulatory genes show consistent patterns of genome-wide DNA methylation (DNAm) alterations, i.e., DNAm signatures in peripheral blood. Given the role of ASXL1 in chromatin modification, we hypothesized that pathogenic ASXL1 variants underlying Bohring-Opitz syndrome (BOS) have a unique DNAm signature. We profiled whole-blood DNAm for 17 ASXL1 variants, and 35 sex- and age-matched typically developing individuals, using Illumina's Infinium EPIC array. We identified 763 differentially methylated CpG sites in individuals with BOS. Differentially methylated sites overlapped 323 unique genes, including HOXA5 and HOXB4, supporting the functional relevance of DNAm signatures. We used a machine-learning classification model based on the BOS DNAm signature to classify variants of uncertain significance in ASXL1, as well as pathogenic ASXL2 and ASXL3 variants. The DNAm profile of one individual with the ASXL2 variant was BOS-like, whereas the DNAm profiles of three individuals with ASXL3 variants were control-like. We also used Horvath's epigenetic clock, which showed acceleration in DNAm age in individuals with pathogenic ASXL1 variants, and the individual with the pathogenic ASXL2 variant, but not in individuals with ASXL3 variants. These studies enhance our understanding of the epigenetic dysregulation underpinning ASXL gene family-associated syndromes.

Deubiquitinases in cell death and inflammation

Apoptosis, pyroptosis, and necroptosis are distinct forms of programmed cell death that eliminate infected, damaged, or obsolete cells. Many proteins that regulate or are a part of the cell death machinery undergo ubiquitination, a post-translational modification made by ubiquitin ligases that modulates protein abundance, localization, and/or activity. For example, some ubiquitin chains target proteins for degradation, while others function as scaffolds for the assembly of signaling complexes. Deubiquitinases (DUBs) are the proteases that counteract ubiquitin ligases by cleaving ubiquitin from their protein substrates. Here, we review the DUBs that have been found to suppress or promote apoptosis, pyroptosis, or necroptosis.

A cryptic K48 ubiquitin chain binding site on UCH37 is required for its role in proteasomal degradation

Degradation by the 26 S proteasome is an intricately regulated process fine tuned by the precise nature of ubiquitin modifications attached to a protein substrate. By debranching ubiquitin chains composed of K48 linkages, the proteasome-associated ubiquitin C-terminal hydrolase UCHL5/UCH37 serves as a positive regulator of protein degradation. How UCH37 achieves specificity for K48 chains is unclear. Here, we use a combination of hydrogen-deuterium mass spectrometry, chemical crosslinking, small-angle X-ray scattering, nuclear magnetic resonance (NMR), molecular docking, and targeted mutagenesis to uncover a cryptic K48 ubiquitin (Ub) chain-specific binding site on the opposite face of UCH37 relative to the canonical S1 (cS1) ubiquitin-binding site. Biochemical assays demonstrate the K48 chain-specific binding site is required for chain debranching and proteasome-mediated degradation of proteins modified with branched chains. Using quantitative proteomics, translation shutoff experiments, and linkage-specific affinity tools, we then identify specific proteins whose degradation depends on the debranching activity of UCH37. Our findings suggest that UCH37 and potentially other DUBs could use more than one S1 site to perform different biochemical functions.

Effects of HMGA2 on the epithelial-mesenchymal transition-related genes in ACHN renal cell carcinoma cells-derived xenografts in nude mice

Background

The architectural transcriptional regulator high-mobility group AT-hook 2 (HMGA2) is an oncofetal protein which has been reported to be ectopically expressed in a variety of cancers. A high expression of HMGA2 in human renal cell carcinoma (RCC) is related with tumor invasiveness and poor prognosis. Recent in vitro studies have shown that HMGA2 knockdown was able to decrease cell proliferation and migration, and regulate the gene expression related to epithelial-mesenchymal transition (EMT).

Methods

To understand the HMGA2’s effect in vivo, HMGA2 expression was knocked down in ACHN cells using small hairpin RNA (shRNA), then the HMGA2-deficient ACHN cells were xenografted into the BALB/c nude mice. Tumor growth was monitored and the expression of EMT-related genes was analyzed.

Results

HMGA2 expression was confirmed to be knocked down in the cultured and xenografted ACHN cells. The xenograft tumor of HMGA2-deficient cells demonstrated a retarded growth pattern compared with the control. The expression of E-cadherin was increased, whereas N-cadherin and Snail were decreased in the HMGA2-deficient xenograft tumors.

Conclusions

In conclusion, to the best of our knowledge, for the first time, we have successfully developed an in vivo experiment using HMGA2-silencing ACHN cells to be grown as xenografts in nude mice. Our findings show that HMGA2 deficiency was sufficient to suppress the xenograft tumor growth in vivo, which support our hypothesis that HMGA2-induced renal carcinogenesis occurs at least in part through the regulation of tumor associated EMT genes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-09537-w.

Deubiquitinases: From mechanisms to their inhibition by small molecules

Deubiquitinases (DUBs) are specialized proteases that remove ubiquitin from substrates or cleave within ubiquitin chains to regulate ubiquitylation and therefore play important roles in eukaryotic biology. Dysregulation of DUBs is implicated in several human diseases, highlighting the importance of DUB function. In addition, many pathogenic bacteria and viruses encode and deploy DUBs to manipulate host immune responses and establish infectious diseases in humans and animals. Hence, therapeutic targeting of DUBs is an increasingly explored area that requires an in-depth mechanistic understanding of human and pathogenic DUBs. In this review, we summarize the multiple layers of regulation that control autoinhibition, activation, and substrate specificity of DUBs. We discuss different strategies to inhibit DUBs and the progress in developing selective small-molecule DUB inhibitors. Finally, we propose a classification system of DUB inhibitors based on their mode of action.

Oncogenic Truncations of ASXL1 Enhance a Motif for BRD4 ET-Domain Binding

Proper regulation of gene-expression relies on specific protein-protein interactions between a myriad of epigenetic regulators. As such, mutation of genes encoding epigenetic regulators often drive cancer and developmental disorders. Additional sex combs-like protein 1 (ASXL1) is a key example, where mutations frequently drive haematological cancers and can cause developmental disorders. It has been reported that nonsense mutations in ASXL1 promote an interaction with BRD4, another central epigenetic regulator. Here we provide a molecular mechanism for the BRD4-ASXL1 interaction, demonstrating that a motif near to common truncation breakpoints of ASXL1 contains an epitope that binds the ET domain within BRD4. Binding-studies show that this interaction is analogous to common ET-binding modes of BRD4-interactors, and that all three ASX-like protein orthologs (ASXL1-3) contain a functional ET domain-binding epitope. Crucially, we observe that BRD4-ASXL1 binding is markedly increased in the prevalent ASXL1^Y591X truncation that maintains the BRD4-binding epitope, relative to full-length ASXL1 or truncated proteins that delete the epitope. Together, these results show that ASXL1 truncation enhances BRD4 recruitment to transcriptional complexes via its ET domain, which could misdirect regulatory activity of either BRD4 or ASXL1 and may inform potential therapeutic interventions.

Histone lysine modifying enzymes and their critical roles in DNA double-strand break repair

Cells protect the integrity of the genome against DNA double-strand breaks through several well-characterized mechanisms including nonhomologous end-joining repair, homologous recombination repair, microhomology-mediated end-joining and single-strand annealing. However, aberrant DNA damage responses (DDRs) lead to genome instability and tumorigenesis. Clarification of the mechanisms underlying the DDR following lethal damage will facilitate the identification of therapeutic targets for cancer. Histones are small proteins that play a major role in condensing DNA into chromatin and regulating gene function. Histone modifications commonly occur in several residues including lysine, arginine, serine, threonine and tyrosine, which can be acetylated, methylated, ubiquitinated and phosphorylated. Of these, lysine modifications have been extensively explored during DDRs. Here, we focus on discussing the roles of lysine modifying enzymes involved in acetylation, methylation, and ubiquitination during the DDR. We provide a comprehensive understanding of the basis of potential epigenetic therapies driven by histone lysine modifications.

MYSM1 inhibits human colorectal cancer tumorigenesis by activating miR-200 family members/CDH1 and blocking PI3K/AKT signaling

Background

Histone epigenetic modification disorder is an important predisposing factor for the occurrence and development of many cancers, including colorectal cancer (CRC). The role of MYSM1, a metalloprotease that deubiquitinates monoubiquitinated histone H2A, in colorectal cancer was identified to evaluate its potential clinical application value.

Methods

MYSM1 expression levels in CRC cell lines and tumor tissues were detected, and their associations with patient survival rate and clinical stage were analyzed using databases and tissue microarrays. Gain- and loss-of-function studies were performed to identify the roles of MYSM1 in CRC cell proliferation, apoptosis, cell cycle progression, epithelial-mesenchymal transition (EMT) and metastasis in vitro and in vivo. ChIP, rescue assays and signal pathway verification were conducted for mechanistic study. Immunohistochemistry (IHC) was used to further assess the relationship of MYSM1 with CRC diagnosis and prognosis.

Results

MYSM1 was significantly downregulated and was related to the overall survival (OS) of CRC patients. MYSM1 served as a CRC suppressor by inducing apoptosis and inhibiting cell proliferation, EMT, tumorigenic potential and metastasis. Mechanistically, MYSM1 directly bound to the promoter region of miR-200/CDH1, impaired the enrichment of repressive H2AK119ub1 modification and epigenetically enhanced miR-200/CDH1 expression. Testing of paired CRC patient samples confirmed the positive regulatory relationship between MYSM1 and miR-200/CDH1. Furthermore, silencing MYSM1 stimulated PI3K/AKT signaling and promoted EMT in CRC cells. More importantly, a positive association existed between MYSM1 expression and a favorable CRC prognosis.

Conclusions

MYSM1 plays essential suppressive roles in CRC tumorigenesis and is a potential target for reducing CRC progression and distant metastasis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13046-021-02106-2.

Proteomic analysis identifies novel binding partners of BAP1

BRCA1-associated protein 1 (BAP1) is a tumor suppressor and its loss can result in mesothelioma, uveal and cutaneous melanoma, clear cell renal cell carcinoma and bladder cancer. BAP1 is a deubiquitinating enzyme of the UCH class that has been implicated in various cellular processes like cell growth, cell cycle progression, ferroptosis, DNA damage response and ER metabolic stress response. ASXL proteins activate BAP1 by forming the polycomb repressive deubiquitinase (PR-DUB) complex which acts on H2AK119ub1. Besides the ASXL proteins, BAP1 is known to interact with an established set of additional proteins. Here, we identify novel BAP1 interacting proteins in the cytoplasm by expressing GFP-tagged BAP1 in an endogenous BAP1 deficient cell line using affinity purification followed by mass spectrometry (AP-MS) analysis. Among these novel interacting proteins are Histone acetyltransferase 1 (HAT1) and all subunits of the heptameric coat protein complex I (COPI) that is involved in vesicle formation and protein cargo binding and sorting. We validate that the HAT1 and COPI interactions occur at endogenous levels but find that this interaction with COPI is not mediated through the C-terminal KxKxx cargo sorting signals of the COPI complex.

BAP1 enhances Polycomb repression by counteracting widespread H2AK119ub1 deposition and chromatin condensation

BAP1 is mutated or deleted in many cancer types, including mesothelioma, uveal melanoma, and cholangiocarcinoma. It is the catalytic subunit of the PR-DUB complex, which removes PRC1-mediated H2AK119ub1, essential for maintaining transcriptional repression. However, the precise relationship between BAP1 and Polycombs remains elusive. Using embryonic stem cells, we show that BAP1 restricts H2AK119ub1 deposition to Polycomb target sites. This increases the stability of Polycomb with their targets and prevents diffuse accumulation of H2AK119ub1 and H3K27me3. Loss of BAP1 results in a broad increase in H2AK119ub1 levels that is primarily dependent on PCGF3/5-PRC1 complexes. This titrates PRC2 away from its targets and stimulates H3K27me3 accumulation across the genome, leading to a general chromatin compaction. This provides evidence for a unifying model that resolves the apparent contradiction between BAP1 catalytic activity and its role in vivo, uncovering molecular vulnerabilities that could be useful for BAP1-related pathologies.