STAG2 loss-of-function affects short-range genomic contacts and modulates the basal-luminal transcriptional program of bladder cancer cells

Untangling the loops of STAG2 mutations in myelodysplastic syndrome

Myelodysplastic syndrome (MDS) is a heterogeneous myeloid neoplasm that is hallmarked by the acquisition of genetic events that disrupt normal trilineage hematopoiesis and results in bone marrow dysfunction. Somatic genes involving transcriptional regulation, signal transduction, DNA methylation, and chromatin modification are often implicated in disease pathogenesis. The cohesin complex, composed of SMC1, SMC3, RAD21, and either STAG1 or STAG2, has been identified as a recurrent mutational target with STAG2 mutations accounting for more than half of all cohesin mutations in myeloid malignancies. In the last decade, STAG2 cohesin biology has been of great interest given its role in transcriptional activation, association with poorer prognosis, and lack of mutation-specific therapies. This review discusses the clinical landscape of cohesin mutant myeloid malignancies, particularly STAG2 mutant MDS, including molecular features of STAG2 mutations, clinical implications of cohesin mutant neoplasms, and the current understanding of the pathophysiological function of STAG2 mutations in MDS.

STAG2 mutations reshape the cohesin-structured spatial chromatin architecture to drive gene regulation in acute myeloid leukemia

Cohesin shapes the chromatin architecture, including enhancer-promoter interactions. Its components, especially STAG2, but not its paralog STAG1, are frequently mutated in myeloid malignancies. To elucidate the underlying mechanisms of leukemogenesis, we comprehensively characterized genetic, transcriptional, and chromatin conformational changes in acute myeloid leukemia (AML) patient samples. Specific loci displayed altered cohesin occupancy, gene expression, and local chromatin activation, which were not compensated by the remaining STAG1-cohesin. These changes could be linked to disrupted spatial chromatin looping in cohesin-mutated AMLs. Complementary depletion of STAG2 or STAG1 in primary human hematopoietic progenitors (HSPCs) revealed effects resembling STAG2-mutant AML-specific changes following STAG2 knockdown, not invoked by the depletion of STAG1. STAG2-deficient HSPCs displayed impaired differentiation capacity and maintained HSPC-like gene expression. This work establishes STAG2 as a key regulator of chromatin contacts, gene expression, and differentiation in the hematopoietic system and identifies candidate target genes that may be implicated in human leukemogenesis.

STAG2 mutations regulate 3D genome organization, chromatin loops, and Polycomb signaling in glioblastoma multiforme

Inactivating mutations of genes encoding the cohesin complex are common in a wide range of human cancers. STAG2 is the most commonly mutated subunit. Here we report the impact of stable correction of endogenous, naturally occurring STAG2 mutations on gene expression, 3D genome organization, chromatin loops, and Polycomb signaling in glioblastoma multiforme (GBM). In two GBM cell lines, correction of their STAG2 mutations significantly altered the expression of ∼10% of all expressed genes. Virtually all the most highly regulated genes were negatively regulated by STAG2 (i.e., expressed higher in STAG2-mutant cells), and one of them-HEPH-was regulated by STAG2 in uncultured GBM tumors as well. While STAG2 correction had little effect on large-scale features of 3D genome organization (A/B compartments, TADs), STAG2 correction did alter thousands of individual chromatin loops, some of which controlled the expression of adjacent genes. Loops specific to STAG2-mutant cells, which were regulated by STAG1-containing cohesin complexes, were very large, supporting prior findings that STAG1-containing cohesin complexes have greater loop extrusion processivity than STAG2-containing cohesin complexes and suggesting that long loops may be a general feature of STAG2-mutant cancers. Finally, STAG2 mutation activated Polycomb activity leading to increased H3K27me3 marks, identifying Polycomb signaling as a potential target for therapeutic intervention in STAG2-mutant GBM tumors. Together, these findings illuminate the landscape of STAG2-regulated genes, A/B compartments, chromatin loops, and pathways in GBM, providing important clues into the largely still unknown mechanism of STAG2 tumor suppression.

Bladder cancer

Bladder cancer is a global health issue with sex differences in incidence and prognosis. Bladder cancer has distinct molecular subtypes with multiple pathogenic pathways depending on whether the disease is non-muscle invasive or muscle invasive. The mutational burden is higher in muscle-invasive than in non-muscle-invasive disease. Commonly mutated genes include TERT, FGFR3, TP53, PIK3CA, STAG2 and genes involved in chromatin modification. Subtyping of both forms of bladder cancer is likely to change considerably with the advent of single-cell analysis methods. Early detection signifies a better disease prognosis; thus, minimally invasive diagnostic options are needed to improve patient outcomes. Urine-based tests are available for disease diagnosis and surveillance, and analysis of blood-based cell-free DNA is a promising tool for the detection of minimal residual disease and metastatic relapse. Transurethral resection is the cornerstone treatment for non-muscle-invasive bladder cancer and intravesical therapy can further improve oncological outcomes. For muscle-invasive bladder cancer, radical cystectomy with neoadjuvant chemotherapy is the standard of care with evidence supporting trimodality therapy. Immune-checkpoint inhibitors have demonstrated benefit in non-muscle-invasive, muscle-invasive and metastatic bladder cancer. Effective management requires a multidisciplinary approach that considers patient characteristics and molecular disease characteristics.

Multiomic analysis of cohesin reveals that ZBTB transcription factors contribute to chromatin interactions

One bottleneck in understanding the principles of 3D chromatin structures is caused by the paucity of known regulators. Cohesin is essential for 3D chromatin organization, and its interacting partners are candidate regulators. Here, we performed proteomic profiling of the cohesin in chromatin and identified transcription factors, RNA-binding proteins and chromatin regulators associated with cohesin. Acute protein degradation followed by time-series genomic binding quantitation and BAT Hi-C analysis were conducted, and the results showed that the transcription factor ZBTB21 contributes to cohesin chromatin binding, 3D chromatin interactions and transcriptional repression. Strikingly, multiomic analyses revealed that the other four ZBTB factors interacted with cohesin, and double degradation of ZBTB21 and ZBTB7B led to a further decrease in cohesin chromatin occupancy. We propose that multiple ZBTB transcription factors orchestrate the chromatin binding of cohesin to regulate chromatin interactions, and we provide a catalog of many additional proteins associated with cohesin that warrant further investigation.

Network models of chromatin structure

Increasing numbers of datasets and experimental assays that capture the organization of chromatin inside the nucleus warrant an effort to develop tools to visualize and analyze these structures. Alongside polymer physics or constraint-based modeling, network theory approaches to describe 3D epigenome organization have gained in popularity. Representing genomic regions as nodes in a network enables visualization of 1D epigenomics datasets in the context of chromatin structure maps, while network theory metrics can be used to describe 3D epigenome organization and dynamics. In this review, we summarize the most salient applications of network theory to the study of chromatin contact maps, demonstrating its potential in revealing epigenomic patterns and relating them to cellular phenotypes.

Full circle: a brief history of cohesin and the regulation of gene expression

The cohesin complex has a range of crucial functions in the cell. Cohesin is essential for mediating chromatid cohesion during mitosis, for repair of double-strand DNA breaks, and for control of gene transcription. This last function has been the subject of intense research ever since the discovery of cohesin's role in the long-range regulation of the cut gene in Drosophila. Subsequent research showed that the expression of some genes is exquisitely sensitive to cohesin depletion, while others remain relatively unperturbed. Sensitivity to cohesin depletion is also remarkably cell type- and/or condition-specific. The relatively recent discovery that cohesin is integral to forming chromatin loops via loop extrusion should explain much of cohesin's gene regulatory properties, but surprisingly, loop extrusion has failed to identify a 'one size fits all' mechanism for how cohesin controls gene expression. This review will illustrate how early examples of cohesin-dependent gene expression integrate with later work on cohesin's role in genome organization to explain mechanisms by which cohesin regulates gene expression.

Whole exome sequencing analysis of canine urothelial carcinomas without BRAF V595E mutation: Short in-frame deletions in BRAF and MAP2K1 suggest alternative mechanisms for MAPK pathway disruption

Molecular profiling studies have shown that 85% of canine urothelial carcinomas (UC) harbor an activating BRAF V595E mutation, which is orthologous to the V600E variant found in several human cancer subtypes. In dogs, this mutation provides both a powerful diagnostic marker and a potential therapeutic target; however, due to their relative infrequency, the remaining 15% of cases remain understudied at the molecular level. We performed whole exome sequencing analysis of 28 canine urine sediments exhibiting the characteristic DNA copy number signatures of canine UC, in which the BRAF V595E mutation was undetected (UDV595E specimens). Among these we identified 13 specimens (46%) harboring short in-frame deletions within either BRAF exon 12 (7/28 cases) or MAP2K1 exons 2 or 3 (6/28 cases). Orthologous variants occur in several human cancer subtypes and confer structural changes to the protein product that are predictive of response to different classes of small molecule MAPK pathway inhibitors. DNA damage response and repair genes, and chromatin modifiers were also recurrently mutated in UDV595E specimens, as were genes that are positive predictors of immunotherapy response in human cancers. Our findings suggest that short in-frame deletions within BRAF exon 12 and MAP2K1 exons 2 and 3 in UDV595E cases are alternative MAPK-pathway activating events that may have significant therapeutic implications for selecting first-line treatment for canine UC. We developed a simple, cost-effective capillary electrophoresis genotyping assay for detection of these deletions in parallel with the BRAF V595E mutation. The identification of these deletion events in dogs offers a compelling cross-species platform in which to study the relationship between somatic alteration, protein conformation, and therapeutic sensitivity.

Different NIPBL requirements of cohesin-STAG1 and cohesin-STAG2

Cohesin organizes the genome through the formation of chromatin loops. NIPBL activates cohesin's ATPase and is essential for loop extrusion, but its requirement for cohesin loading is unclear. Here we have examined the effect of reducing NIPBL levels on the behavior of the two cohesin variants carrying STAG1 or STAG2 by combining a flow cytometry assay to measure chromatin-bound cohesin with analyses of its genome-wide distribution and genome contacts. We show that NIPBL depletion results in increased cohesin-STAG1 on chromatin that further accumulates at CTCF positions while cohesin-STAG2 diminishes genome-wide. Our data are consistent with a model in which NIPBL may not be required for chromatin association of cohesin but it is for loop extrusion, which in turn facilitates stabilization of cohesin-STAG2 at CTCF positions after being loaded elsewhere. In contrast, cohesin-STAG1 binds chromatin and becomes stabilized at CTCF sites even under low NIPBL levels, but genome folding is severely impaired.

Identification of STAG2-Mutant Bladder Cancers by Immunohistochemistry

Bladder cancer is the fifth most common cancer in the United States. Most bladder cancers are early-stage lesions confined to the mucosa or submucosa and are therefore classified as non-muscle-invasive bladder cancer (NMIBC). A minority of tumors are diagnosed after they have invaded the underlying detrusor muscle and are classified as muscle-invasive bladder cancer (MIBC). Mutational inactivation of the STAG2 tumor suppressor gene is common in bladder cancer, and we and others have recently demonstrated that STAG2 mutation status can be used as an independent prognostic biomarker to predict whether NMIBC will recur and/or progress to MIBC. Here we describe an immunohistochemistry-based assay for identifying the STAG2 mutational status of bladder tumors.

Nuclear envelope, chromatin organizers, histones, and DNA: The many achilles heels exploited across cancers

In eukaryotic cells, the genome is organized in the form of chromatin composed of DNA and histones that organize and regulate gene expression. The dysregulation of chromatin remodeling, including the aberrant incorporation of histone variants and their consequent post-translational modifications, is prevalent across cancers. Additionally, nuclear envelope proteins are often deregulated in cancers, which impacts the 3D organization of the genome. Altered nuclear morphology, genome organization, and gene expression are defining features of cancers. With advances in single-cell sequencing, imaging technologies, and high-end data mining approaches, we are now at the forefront of designing appropriate small molecules to selectively inhibit the growth and proliferation of cancer cells in a genome- and epigenome-specific manner. Here, we review recent advances and the emerging significance of aberrations in nuclear envelope proteins, histone variants, and oncohistones in deregulating chromatin organization and gene expression in oncogenesis.

Contribution of variant subunits and associated factors to genome-wide distribution and dynamics of cohesin

BACKGROUND

The cohesin complex organizes the genome-forming dynamic chromatin loops that impact on all DNA-mediated processes. There are two different cohesin complexes in vertebrate somatic cells, carrying the STAG1 or STAG2 subunit, and two versions of the regulatory subunit PDS5, PDS5A and PDS5B. Mice deficient for any of the variant subunits are embryonic lethal, which indicates that they are not functionally redundant. However, their specific behavior at the molecular level is not fully understood.

RESULTS

The genome-wide distribution of cohesin provides important information with functional consequences. Here, we have characterized the distribution of cohesin subunits and regulators in mouse embryo fibroblasts (MEFs) either wild type or deficient for cohesin subunits and regulators by chromatin immunoprecipitation and deep sequencing. We identify non-CTCF cohesin-binding sites in addition to the commonly detected CTCF cohesin sites and show that cohesin-STAG2 is the preferred variant at these positions. Moreover, this complex has a more dynamic association with chromatin as judged by fluorescence recovery after photobleaching (FRAP), associates preferentially with WAPL and is more easily extracted from chromatin with salt than cohesin-STAG1. We observe that both PDS5A and PDS5B are exclusively located at cohesin-CTCF positions and that ablation of a single paralog has no noticeable consequences for cohesin distribution while double knocked out cells show decreased accumulation of cohesin at all its binding sites. With the exception of a fraction of cohesin positions in which we find binding of all regulators, including CTCF and WAPL, the presence of NIPBL and PDS5 is mutually exclusive, consistent with our immunoprecipitation analyses in mammalian cell extracts and previous results in yeast.

CONCLUSION

Our findings support the idea that non-CTCF cohesin-binding sites represent sites of cohesin loading or pausing and are preferentially occupied by the more dynamic cohesin-STAG2. PDS5 proteins redundantly contribute to arrest cohesin at CTCF sites, possibly by preventing binding of NIPBL, but are not essential for this arrest. These results add important insights towards understanding how cohesin regulates genome folding and the specific contributions of the different variants that coexist in the cell.

A global high-density chromatin interaction network reveals functional long-range and trans-chromosomal relationships

BACKGROUND

Chromatin contacts are essential for gene-expression regulation; however, obtaining a high-resolution genome-wide chromatin contact map is still prohibitively expensive owing to large genome sizes and the quadratic scale of pairwise data. Chromosome conformation capture (3C)-based methods such as Hi-C have been extensively used to obtain chromatin contacts. However, since the sparsity of these maps increases with an increase in genomic distance between contacts, long-range or trans-chromatin contacts are especially challenging to sample.

RESULTS

Here, we create a high-density reference genome-wide chromatin contact map using a meta-analytic approach. We integrate 3600 human, 6700 mouse, and 500 fly Hi-C experiments to create species-specific meta-Hi-C chromatin contact maps with 304 billion, 193 billion, and 19 billion contacts in respective species. We validate that meta-Hi-C contact maps are uniquely powered to capture functional chromatin contacts in both cis and trans. We find that while individual dataset Hi-C networks are largely unable to predict any long-range coexpression (median 0.54 AUC), meta-Hi-C networks perform comparably in both cis and trans (0.65 AUC vs 0.64 AUC). Similarly, for long-range expression quantitative trait loci (eQTL), meta-Hi-C contacts outperform all individual Hi-C experiments, providing an improvement over the conventionally used linear genomic distance-based association. Assessing between species, we find patterns of chromatin contact conservation in both cis and trans and strong associations with coexpression even in species for which Hi-C data is lacking.

CONCLUSIONS

We have generated an integrated chromatin interaction network which complements a large number of methodological and analytic approaches focused on improved specificity or interpretation. This high-depth "super-experiment" is surprisingly powerful in capturing long-range functional relationships of chromatin interactions, which are now able to predict coexpression, eQTLs, and cross-species relationships. The meta-Hi-C networks are available at https://labshare.cshl.edu/shares/gillislab/resource/HiC/ .

Stromal Antigen 2 Deficiency Induces Interferon Responses and Restricts Porcine Deltacoronavirus Infection

Porcine deltacoronavirus (PDCoV) is a recently discovered enteropathogenic coronavirus and has caused significant economic impacts on the pork industry. Although studies have partly uncovered the molecular mechanism of PDCoV-host interaction, it requires further research. In this study, we explored the roles of Stromal Antigen 2 (STAG2) in PDCoV infection. We found that STAG2-deficient cells inhibited infection with vesicular stomatitis virus (VSV) and PDCoV, whereas restoration of STAG2 expression in STAG2-depleted (STAG2-/-) IPEC-J2 cells line restored PDCoV infection, suggesting that STAG2 is involved in the PDCoV replication. Furthermore, we found that STAG2 deficiency results in robust interferon (IFN) expression. Subsequently, we found that STAG2 deficiency results in the activation of JAK-STAT signaling and the expression of IFN stimulated gene (ISG), which establish an antiviral state. Taken together, the depletion of STAG2 activates the JAK-STAT signaling and induces the expression of ISG, thereby inhibiting PDCoV replication. Our study provides new insights and potential therapeutic targets for unraveling the mechanism of PDCoV replication.

STAG2 regulates interferon signaling in melanoma via enhancer loop reprogramming

The cohesin complex participates in the organization of 3D genome through generating and maintaining DNA loops. Stromal antigen 2 (STAG2), a core subunit of the cohesin complex, is frequently mutated in various cancers. However, the impact of STAG2 inactivation on 3D genome organization, especially the long-range enhancer-promoter contacts and subsequent gene expression control in cancer, remains poorly understood. Here we show that depletion of STAG2 in melanoma cells leads to expansion of topologically associating domains (TADs) and enhances the formation of acetylated histone H3 lysine 27 (H3K27ac)-associated DNA loops at sites where binding of STAG2 is switched to its paralog STAG1. We further identify Interferon Regulatory Factor 9 (IRF9) as a major direct target of STAG2 in melanoma cells via integrated RNA-seq, STAG2 ChIP-seq and H3K27ac HiChIP analyses. We demonstrate that loss of STAG2 activates IRF9 through modulating the 3D genome organization, which in turn enhances type I interferon signaling and increases the expression of PD-L1. Our findings not only establish a previously unknown role of the STAG2 to STAG1 switch in 3D genome organization, but also reveal a functional link between STAG2 and interferon signaling in cancer cells, which may enhance the immune evasion potential in STAG2-mutant cancer.

STAG2 Protein Expression in Non–muscle-invasive Bladder Cancer: Associations with Sex, Genomic and Transcriptomic Changes, and Clinical Outcomes

Background

Mutations in STAG2 cause complete loss of STAG2 protein in approximately one-third of non–muscle-invasive bladder cancers (NMIBCs). STAG2 protein expression is easily determined via immunohistochemistry (IHC) and published data suggest that loss of STAG2 expression is a good prognostic indicator in NMIBC.

Objective

To confirm the relationship between STAG2 protein expression and clinical outcomes and tumour characteristics in NMIBC.

Design, setting, and participants

IHC was used to determine STAG2 expression in 748 incident urothelial bladder cancers (UBCs) and recurrence-free, progression-free, and disease-specific survival were compared for patients with and without STAG2 loss. Exome and RNA sequencing were used to explore links between STAG2 loss and tumour molecular characteristics.

Results and limitations

STAG2 loss was observed in 19% of UBC patients and was 1.6-fold more common among female patients. Loss was frequent among grade 1 pTa tumours (40%), decreasing with stage and grade to only 5% among grade 3 pT2+ tumours. Loss was associated with fewer copy-number changes and less aggressive expression subtypes. In UBC, STAG2 loss was a highly significant prognostic indicator of better disease-free survival but was not independent of stage and grade. STAG2 loss was not a statistically significant predictor of NMIBC recurrence. STAG2 loss was significantly associated with better progression-free survival in NMIBC and appeared to be more prognostic for males than for females.

Conclusions

A simple IHC-based STAG2 test shows promise for identifying NMIBC patients at lower risk of progression to MIBC for whom more conservative treatments may be suitable.

Patient summary

A protein called STAG2 is frequently lost in early bladder cancers, most often in less aggressive tumours. STAG2 loss is easily measured and could be used as a biomarker to help guide treatment decisions.