RNA interacts with topoisomerase I to adjust DNA topology.

Topoisomerase I (TOP1) is an essential enzyme that relaxes DNA to prevent and dissipate torsional stress during transcription. However, the mechanisms underlying the regulation of TOP1 activity remain elusive. Using enhanced cross-linking and immunoprecipitation (eCLIP) and ultraviolet-cross-linked RNA immunoprecipitation followed by total RNA sequencing (UV-RIP-seq) in human colon cancer cells along with RNA electrophoretic mobility shift assays (EMSAs), biolayer interferometry (BLI), and in vitro RNA-binding assays, we identify TOP1 as an RNA-binding protein (RBP). We show that TOP1 directly binds RNA in vitro and in cells and that most RNAs bound by TOP1 are mRNAs. Using a TOP1 RNA-binding mutant and topoisomerase cleavage complex sequencing (TOP1cc-seq) to map TOP1 catalytic activity, we reveal that RNA opposes TOP1 activity as RNA polymerase II (RNAPII) commences transcription of active genes. We further demonstrate the inhibitory role of RNA in regulating TOP1 activity by employing DNA supercoiling assays and magnetic tweezers. These findings provide insight into the coordinated actions of RNA and TOP1 in regulating DNA topological stress intrinsic to RNAPII-dependent transcription.

It's a DoG-eat-DoG world-altered transcriptional mechanisms drive downstream-of-gene (DoG) transcript production.

The past decade has revolutionized our understanding of regulatory noncoding RNAs (ncRNAs). Among the most recently identified ncRNAs are downstream-of-gene (DoG)-containing transcripts that are produced by widespread transcriptional readthrough. The discovery of DoGs has set the stage for future studies to address many unanswered questions regarding the mechanisms that promote readthrough transcription, RNA processing, and the cellular functions of the unique transcripts. In this review, we summarize current findings regarding the biogenesis, function, and mechanisms regulating this exciting new class of RNA molecules.

H3K4me3 interactions with TAF3 regulate preinitiation complex assembly and selective gene activation.

Histone modifications regulate chromatin-dependent processes, yet the mechanisms by which they contribute to specific outcomes remain unclear. H3K4me3 is a prominent histone mark that is associated with active genes and promotes transcription through interactions with effector proteins that include initiation factor TFIID. We demonstrate that H3K4me3-TAF3 interactions direct global TFIID recruitment to active genes, some of which are p53 targets. Further analyses show that (1) H3K4me3 enhances p53-dependent transcription by stimulating preinitiation complex (PIC) formation; (2) H3K4me3, through TAF3 interactions, can act either independently or cooperatively with the TATA box to direct PIC formation and transcription; and (3) H3K4me3-TAF3/TFIID interactions regulate gene-selective functions of p53 in response to genotoxic stress. Our findings indicate a mechanism by which H3K4me3 directs PIC assembly for the rapid induction of specific p53 target genes.

The basal transcription complex component TAF3 transduces changes in nuclear phosphoinositides into transcriptional output.

Phosphoinositides (PI) are important signaling molecules in the nucleus that influence gene expression. However, if and how nuclear PI directly affects the transcriptional machinery is not known. We report that the lipid kinase PIP4K2B regulates nuclear PI5P and the expression of myogenic genes during myoblast differentiation. A targeted screen for PI interactors identified the PHD finger of TAF3, a TATA box binding protein-associated factor with important roles in transcription regulation, pluripotency, and differentiation. We show that the PI interaction site is distinct from the known H3K4me3 binding region of TAF3 and that PI binding modulates association of TAF3 with H3K4me3 in vitro and with chromatin in vivo. Analysis of TAF3 mutants indicates that TAF3 transduces PIP4K2B-mediated alterations in PI into changes in specific gene transcription. Our study reveals TAF3 as a direct target of nuclear PI and further illustrates the importance of basal transcription components as signal transducers.

Mutant p53 regulates enhancer-associated H3K4 monomethylation through interactions with the methyltransferase MLL4.

Monomethylation of histone H3 lysine 4 (H3K4me1) is enriched at enhancers that are primed for activation and the levels of this histone mark are frequently altered in various human cancers. Yet, how alterations in H3K4me1 are established and the consequences of these epigenetic changes in tumorigenesis are not well understood. Using ChIP-Seq in human colon cancer cells, we demonstrate that mutant p53 depletion results in decreased H3K4me1 levels at active enhancers that reveal a striking colocalization of mutant p53 and the H3K4 monomethyltransferase MLL4 following chronic tumor necrosis factor alpha (TNFα) signaling. We further reveal that mutant p53 forms physiological associations and direct interactions with MLL4 and promotes the enhancer binding of MLL4, which is required for TNFα-inducible H3K4me1 and histone H3 lysine 27 acetylation (H3K27ac) levels, enhancer-derived transcript (eRNA) synthesis, and mutant p53-dependent target gene activation. Complementary in vitro studies with recombinant chromatin and purified proteins demonstrate that binding of the MLL3/4 complex and H3K4me1 deposition is enhanced by mutant p53 and p300-mediated acetylation, which in turn reflects a MLL3/4-dependent enhancement of mutant p53 and p300-dependent transcriptional activation. Collectively, our findings establish a mechanism in which mutant p53 cooperates with MLL4 to regulate aberrant enhancer activity and tumor-promoting gene expression in response to chronic immune signaling.

Exploring new roles for RNA-binding proteins in epigenetic and gene regulation.

A significant portion of the human proteome comprises RNA-binding proteins (RBPs) that play fundamental roles in numerous biological processes. In the last decade, there has been a staggering increase in RBP identification and classification, which has fueled interest in the evolving roles of RBPs and RBP-driven molecular mechanisms. Here, we focus on recent insights into RBP-dependent regulation of the epigenetic and transcriptional landscape. We describe advances in methodologies that define the RNA-protein interactome and machine-learning algorithms that are streamlining RBP discovery and predicting new RNA-binding regions. Finally, we present how RBP dysregulation leads to alterations in tumor-promoting gene expression and discuss the potential for targeting these RBPs for the development of new cancer therapeutics.

Downstream-of-gene (DoG) transcripts contribute to an imbalance in the cancer cell transcriptome.

Downstream-of-gene (DoG) transcripts are an emerging class of noncoding RNAs. However, it remains largely unknown how DoG RNA production is regulated and whether alterations in DoG RNA signatures exist in major cancers. Here, through transcriptomic analyses of matched tumors and nonneoplastic tissues and cancer cell lines, we reveal a comprehensive catalog of DoG RNA signatures. Through separate lines of evidence, we support the biological importance of DoG RNAs in carcinogenesis. First, we show tissue-specific and stage-specific differential expression of DoG RNAs in tumors versus paired normal tissues with their respective host genes involved in tumor-promoting versus tumor-suppressor pathways. Second, we identify that differential DoG RNA expression is associated with poor patient survival. Third, we identify that DoG RNA induction is a consequence of treating colon cancer cells with the topoisomerase I (TOP1) poison camptothecin and following TOP1 depletion. Our results underlie the significance of DoG RNAs and TOP1-dependent regulation of DoG RNAs in diversifying and modulating the cancer transcriptome.

Tissue-specific reprogramming leads to angiogenic neutrophil specialization and tumor vascularization in colorectal cancer.

Neutrophil (PMN) tissue accumulation is an established feature of ulcerative colitis (UC) lesions and colorectal cancer (CRC). To assess the PMN phenotypic and functional diversification during the transition from inflammatory ulceration to CRC we analyzed the transcriptomic landscape of blood and tissue PMNs. Transcriptional programs effectively separated PMNs based on their proximity to peripheral blood, inflamed colon, and tumors. In silico pathway overrepresentation analysis, protein-network mapping, gene signature identification, and gene-ontology scoring revealed unique enrichment of angiogenic and vasculature development pathways in tumor-associated neutrophils (TANs). Functional studies utilizing ex vivo cultures, colitis-induced murine CRC, and patient-derived xenograft models demonstrated a critical role for TANs in promoting tumor vascularization. Spp1 (OPN) and Mmp14 (MT1-MMP) were identified by unbiased -omics and mechanistic studies to be highly induced in TANs, acting to critically regulate endothelial cell chemotaxis and branching. TCGA data set and clinical specimens confirmed enrichment of SPP1 and MMP14 in high-grade CRC but not in patients with UC. Pharmacological inhibition of TAN trafficking or MMP14 activity effectively reduced tumor vascular density, leading to CRC regression. Our findings demonstrate a niche-directed PMN functional specialization and identify TAN contributions to tumor vascularization, delineating what we believe to be a new therapeutic framework for CRC treatment focused on TAN angiogenic properties.

Clinical and histopathologic characterization of SETD2-mutated colorectal cancer.

With the advent of next-generation sequencing (NGS), identifying and better understanding genetic mutations in cancer pathways has become more feasible. A mutation now commonly reported in NGS panels is the SETD2 gene (H3K36 trimethyltransferase). However, its contributions to colorectal cancer (CRC) are not well described. In this study, we describe the clinicopathologic characteristics of SETD2-mutated CRC, determine common mutation sites on the SETD2 gene, and correlate these mutations with the loss of H3K36 trimethylation and the aberrant expression of beta-catenin. By searching pathology reports at our institution which included the 161-gene NGS panel from 2019 to 2021, we identify 24 individuals with SETD2-mutated CRC. All samples were evaluated for microsatellite status, H3K36 trimethylation, and beta-catenin via immunohistochemistry. In this cohort of 24 SETD2-mutated CRC individuals (a median age of 62.4 years [interquartile range: 49.1-73.6]), 10 (41.7%) patients presented at American Joint Committee on Cancer (AJCC) tumor stage II, seven (29.2%) at stage III, six (25%) at stage IV, and one (4.2%) at stage I. Most tumors studied were adenocarcinomas with no further specification (22, 92%), and most tumors were microsatellite stable (18, 82.5%). Thirty-three mutation locations were represented by 24 patients, with one patient having six mutations in the SETD2 gene and two patients having three mutations. The dominant mutation type is missense mutations (N = 29, 87.9%), and no mutation hotspots were found. Only two samples lost trimethylation of histone H3K36, both from individuals with multiple SETD2 mutations and aberrant nuclear beta-catenin expression. SETD2-mutated CRC is similar in clinical and histologic presentation to other commonly reported CRC. SETD2 mutations were missense dominantand showed no hotspots, and multiple mutations are likely necessary for loss of H3K36 trimethylation. These results warrant further study on determining a role of SETD2-histone H3K36 pathway in CRC.

Enhancer RNAs are an important regulatory layer of the epigenome.

Noncoding RNAs (ncRNAs) direct a remarkable number of diverse functions in development and disease through their regulation of transcription, RNA processing and translation. Leading the charge in the RNA revolution is a class of ncRNAs that are synthesized at active enhancers, called enhancer RNAs (eRNAs). Here, we review recent insights into the biogenesis of eRNAs and the mechanisms underlying their multifaceted functions and consider how these findings could inform future investigations into enhancer transcription and eRNA function.

The role of enhancer RNAs in epigenetic regulation of gene expression.

Since the discovery that enhancers can support transcription, the roles of enhancer RNAs have remained largely elusive. We identified that enhancer RNAs interact with and augment bromodomain engagement with acetylated chromatin. Here, we discuss our recent findings and the potential mechanisms underlying the regulation and functions of enhancer RNA-bromodomain associations.

RNAs interact with BRD4 to promote enhanced chromatin engagement and transcription activation.

The bromodomain and extra-terminal motif (BET) protein BRD4 binds to acetylated histones at enhancers and promoters via its bromodomains (BDs) to regulate transcriptional elongation. In human colorectal cancer cells, we found that BRD4 was recruited to enhancers that were co-occupied by mutant p53 and supported the synthesis of enhancer-directed transcripts (eRNAs) in response to chronic immune signaling. BRD4 selectively associated with eRNAs that were produced from BRD4-bound enhancers. Using biochemical and biophysical methods, we found that BRD4 BDs function cooperatively as docking sites for eRNAs and that the BDs of BRD2, BRD3, BRDT, BRG1, and BRD7 directly interact with eRNAs. BRD4-eRNA interactions increased BRD4 binding to acetylated histones in vitro and augmented BRD4 enhancer recruitment and transcriptional cofactor activities. Our results suggest a mechanism by which eRNAs are directly involved in gene regulation by modulating enhancer interactions and transcriptional functions of BRD4.

Mutant p53 shapes the enhancer landscape of cancer cells in response to chronic immune signaling.

Inflammation influences cancer development, progression, and the efficacy of cancer treatments, yet the mechanisms by which immune signaling drives alterations in the cancer cell transcriptome remain unclear. Using ChIP-seq, RNA-seq, and GRO-seq, here we demonstrate a global overlap in the binding of tumor-promoting p53 mutants and the master proinflammatory regulator NFκB that drives alterations in enhancer and gene activation in response to chronic TNF-α signaling. We show that p53 mutants interact directly with NFκB and that both factors impact the other's binding at diverse sets of active enhancers. In turn, the simultaneous and cooperative binding of these factors is required to regulate RNAPII recruitment, the synthesis of enhancer RNAs, and the activation of tumor-promoting genes. Collectively, these findings establish a mechanism by which chronic TNF-α signaling orchestrates a functional interplay between mutant p53 and NFκB that underlies altered patterns of cancer-promoting gene expression.Inflammation is known to affect cancer development, yet the mechanisms by which immune signaling drives transformation remain unclear. Here, the authors provide evidence that chronic TNF-α signaling promotes the enhancer binding and transcriptional interplay between mutant p53 and NFκB.

A phosphomimetic mutation in the Sall1 repression motif disrupts recruitment of the nucleosome remodeling and deacetylase complex and repression of Gbx2.

The multizinc finger transcription factor Sall1 is a critical developmental regulator that mediates repression through the recruitment of the nucleosome remodeling and deacetylase (NuRD) complex. Although a short conserved peptide motif in Sall1 is sufficient to recruit NuRD, its ability to regulate native Sall1 target genes in vivo has not been demonstrated. In this report, we demonstrate an in vivo role for the Sall1 repression motif and describe a novel direct target gene of Sall1, Gbx2, that is directly repressed in a NuRD-dependent fashion. The ability of Sall1 to repress Gbx2 was impaired in Xenopus embryos expressing mutant forms of Sall1 that are defective for NuRD binding. Finally, we demonstrate that protein kinase C phosphorylates serine 2 of the Sall1 repression motif and reveal that a phosphomimetic mutation of serine 2 disrupts the ability of Sall1 to repress Gbx2 in cell culture and Xenopus embryos. Together, these studies establish that Sall1 recruits NuRD via the Sall1 repression motif to mediate repression of a native target gene and suggest a model in which dynamic control of gene expression by Sall1 is modulated by serine phosphorylation of the Sall1 repression motif.

A conserved 12-amino acid motif in Sall1 recruits the nucleosome remodeling and deacetylase corepressor complex.

Sall1 is a multi-zinc finger transcription factor that represses gene expression and regulates organogenesis. In this report, we further characterize the domain of Sall1 necessary for repression. We show that endogenous Sall1 binds to the nucleosome remodeling and deacetylase corepressor complex (NuRD) and confirm the functionality of the Sall1-associating macromolecular complex by showing that the complex possesses HDAC activity. NuRD is involved in global transcriptional repression and regulation of specific developmental processes. The mechanism by which sequence-specific DNA-binding proteins associate with NuRD is not well understood. We have identified a highly conserved 12-amino acid motif in the transcription factor Sall1 that is sufficient for the recruitment of NuRD. Single amino acid substitutions defined the critical amino acid peptide motif as RRKQXK-PXXF. This motif probably exhibits a more general role in regulating gene expression, since other proteins containing this domain, including all Sall family members and an unrelated zinc finger protein Ebfaz, mediate transcriptional repression and associate with NuRD. These results also have important implications for the pathogenesis of Townes-Brocks, a syndrome caused by SALL1 mutations.