Chromatin regulation critically influences gene expression and cancer progression, yet the functions of chromatin adaptors remain incompletely defined. Using focused CRISPR screening, we identified TRIM28, a multi-domain chromatin adaptor, as a dependency in acute leukemia, where its depletion impaired leukemia cell proliferation in vitro and in vivo , while activating neutrophil differentiation programs. Integrative transcriptomic and chromatin profiling revealed that TRIM28 acts as a co-repressor of neutrophil-associated loci independently of H3K9 methylation, and that TRIM28 loss drives terminal differentiation of leukemia cells into functionally mature neutrophil-like cells with reduced leukemic potential. We developed a selective small-molecule TRIM28 inhibitor that binds the TRIM28 PHD-bromodomain, phenocopies TRIM28 loss across biochemical and cellular assays, exhibits low micromolar anti-leukemia activity, induces neutrophil differentiation, and synergizes with Menin inhibition. Together, these findings, spanning target discovery, mechanism of action, and chemical probe development, establish TRIM28 as a regulator of myeloid cell fate and a promising pro-differentiation therapeutic target in acute leukemia.

ABSTRACT:Perturbations in intermediary metabolism contribute to the pathogenesis of acute myeloid leukemia (AML) and can produce therapeutically actionable dependencies. Here, we probed whether α-ketoglutarate (αKG) metabolism represents a specific vulnerability in AML. Using functional genomics, metabolomics, and mouse models, we identified the αKG dehydrogenase complex, which catalyzes the conversion of αKG to succinyl coenzyme A, as a molecular dependency across multiple models of adverse-risk AML. Inhibition of 2-oxoglutarate dehydrogenase (OGDH), the E1 subunit of the αKG dehydrogenase complex, impaired AML progression and drove differentiation. Mechanistically, hindrance of αKG flux through the tricarboxylic acid (TCA) cycle resulted in rapid exhaustion of aspartate pools and blockade of de novo nucleotide biosynthesis, whereas cellular bioenergetics was largely preserved. Additionally, increased αKG levels after OGDH inhibition affected the biosynthesis of other critical amino acids. Thus, this work has identified a previously undescribed, functional link between certain TCA cycle components and nucleotide biosynthesis enzymes across AML. This metabolic node may serve as a cancer-specific vulnerability, amenable to therapeutic targeting in AML and perhaps in other cancers with similar metabolic wiring.

Abstract Tumor genomes often harbor a complex spectrum of single nucleotide mutations, small indels, and large chromosome rearrangements that can perturb coding and non-coding regions of the genome in ways that remain poorly understood. The mutational processes responsible for the genesis of these events are also not static; instead, they continue to operate throughout disease evolution and further diversify the genetic and phenotypic landscape of cancer cells. Mounting evidence suggests that tumor genotype can be an important determinant of disease progression and therapy response, such as by modulating the sensitivity or resistance of cancer cells to mutant-specific drugs. These types of observations have motivated efforts to treat cancers with genome-informed therapies, highlighting the need to understand how different genetic variants precisely affect gene function and overall tumor phenotypes. Variant-function maps that provide a mechanistic understanding of the biology driven by cancer-associated mutations are needed to design these types of treatment strategies. Precision genome editing technologies like base and prime editing are uniquely suited to tackle this problem. Nevertheless, deploying these methods for systematic variant-function studies and disease modeling in vivo has not been straightforward due to lack of robust and scalable platforms capable of assessing editing efficiency and precision, particularly at endogenous loci. With this goal in mind, we previously developed and applied high-throughput base editing ‘sensor’ approaches that link endogenous genome editing outcomes with synthetic DNA-based readouts and cellular fitness measurements. Using these approaches, we showed that several uncharacterized mutant p53 alleles drive cancer cell proliferation and in vivo tumor development. Building upon this work, we recently developed new prime editing guide RNA design tools and sensor-based approaches that similarly couple quantitative editing outcomes to cellular fitness, allowing us to significantly expand the breadth of cancer-associated mutations that can be interrogated using these precision genome editing technologies. In this talk, I will describe ongoing work using base and prime editing sensor libraries to probe the biological impact of thousands of functionally-distinct genetic variants in diverse types of protein-coding genes and families to learn how these influence various cancer phenotypes. I will also discuss how the implementation of these modular technologies will allow researchers to functionally disentangle the impact of endogenous and exogenous mutational processes on functional selection and tumor evolution with close to single base pair resolution. This generalizable precision genome editing framework will facilitate the functional interrogation of genetic variants across diverse biological contexts, providing much needed insight into cancer variant-function relationships that could be leveraged to develop more precise cancer treatment paradigms, including synthetic lethal therapies that exploit tumor genotype. Citation Format: Francisco J. Sánchez-Rivera, Samuel I. Gould, Alexandra N. Wuest, Kexin Dong, Grace A. Johnson, Alvin Hsu, Varun K. Narendra, Ondine Atwa, Stuart S. Levine, David R. Liu. Functional studies of genetic variation using precision genome editing [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Expanding and Translating Cancer Synthetic Vulnerabilities; 2024 Jun 10-13; Montreal, Quebec, Canada. Philadelphia (PA): AACR; Mol Cancer Ther 2024;23(6 Suppl):Abstract nr IA020.

Tumor genomes often harbor a complex spectrum of single nucleotide alterations and chromosomal rearrangements that can perturb protein function. Prime editing has been applied to install and evaluate genetic variants, but previous approaches have been limited by the variable efficiency of prime editing guide RNAs. Here we present a high-throughput prime editing sensor strategy that couples prime editing guide RNAs with synthetic versions of their cognate target sites to quantitatively assess the functional impact of endogenous genetic variants. We screen over 1,000 endogenous cancer-associated variants of TP53-the most frequently mutated gene in cancer-to identify alleles that impact p53 function in mechanistically diverse ways. We find that certain endogenous TP53 variants, particularly those in the p53 oligomerization domain, display opposite phenotypes in exogenous overexpression systems. Our results emphasize the physiological importance of gene dosage in shaping native protein stoichiometry and protein-protein interactions, and establish a framework for studying genetic variants in their endogenous sequence context at scale.

CCCTC-binding factor (CTCF) is critical to three-dimensional genome organization. Upon differentiation, CTCF insulates active and repressed genes within Hox gene clusters. We conducted a genome-wide CRISPR knockout (KO) screen to identify genes required for CTCF-boundary activity at the HoxA cluster, complemented by biochemical approaches. Among the candidates, we identified Myc-associated zinc-finger protein (MAZ) as a cofactor in CTCF insulation. MAZ colocalizes with CTCF at chromatin borders and, similar to CTCF, interacts with the cohesin subunit RAD21. MAZ KO disrupts gene expression and local contacts within topologically associating domains. Similar to CTCF motif deletions, MAZ motif deletions lead to derepression of posterior Hox genes immediately after CTCF boundaries upon differentiation, giving rise to homeotic transformations in mouse. Thus, MAZ is a factor contributing to appropriate insulation, gene expression and genomic architecture during development.

Corticosteroids, anti-CD20 agents, immunotherapies, and cytotoxic chemotherapy are commonly used in the treatment of patients with cancer. It is unclear how these agents affect patients with cancer who are infected with SARS-CoV-2. We retrospectively investigated associations between SARS-CoV-2-associated respiratory failure or death with receipt of the aforementioned medications and with pre-COVID-19 neutropenia. The study included all cancer patients diagnosed with SARS-CoV-2 at Memorial Sloan Kettering Cancer Center until June 2, 2020 (N = 820). We controlled for cancer-related characteristics known to predispose to worse COVID-19 as well as level of respiratory support during corticosteroid administration. Corticosteroid administration was associated with worse outcomes prior to use of supplemental oxygen; no statistically significant difference was observed in sicker cohorts. In patients with metastatic thoracic cancer, 9 of 25 (36%) and 10 of 31 (32%) had respiratory failure or death among those who did and did not receive immunotherapy, respectively. Seven of 23 (30%) and 52 of 187 (28%) patients with hematologic cancer had respiratory failure or death among those who did and did not receive anti-CD20 therapy, respectively. Chemotherapy itself was not associated with worse outcomes, but pre-COVID-19 neutropenia was associated with worse COVID-19 course. Relative prevalence of chemotherapy-associated neutropenia in previous studies may account for different conclusions regarding the risks of chemotherapy in patients with COVID-19. In the absence of prospective studies and evidence-based guidelines, our data may aid providers looking to assess the risks and benefits of these agents in caring for cancer patients in the COVID-19 era.

INTRODUCTION: Patients (pts) with blood disorders are at particular risk for severe infection and death from COVID-19. Factors that contribute to this risk, including cancer treatment, have not been clearly delineated. The ASH RC COVID-19 Registry for Hematology is a public-facing, volunteer registry reporting outcomes of COVID-19 infection in pts with underlying blood disorders. We report a multivariable analysis of the impact of cancer treatment and other key variables on COVID-19 mortality and hospitalization among pts with blood cancer. METHODS: Data were collected between April 1, 2020, and July 2, 2021. All analyses were performed using R version 4.0.2. Multivariable logistic regression explored associations between mortality and seven patient/disease factors previously reported as important to COVID-19 outcome. Independent variables included: age (>60); sex; presence of a major comorbidity (defined as any of heart disease, hypertension, pulmonary disease and/or diabetes); type of hematologic malignancy; estimated prognosis of < 6 months prior to COVID-19; deferral of ICU care; and administration of cancer treatment in the previous year (excluding single agent hydroxyurea). A secondary multivariable logistic regression explored associations between the same variables and hospitalization with COVID-19. RESULTS: We included all pts in the registry with a malignant diagnosis except for 3 patients excluded based on a data sharing agreement (N=1029). Median age category was 50-59y (range <5y to > 90y). The sample was 42% female and 28% had major comorbidities. Types of hematologic malignancies were 354 (34%) acute leukemia/MDS, 255 (25%) lymphoma, 206 (20%) plasma cell dyscrasia (myeloma/amyloid/POEMS), 116 (11%) CLL, 98 (10%) myeloproliferative neoplasm (MPN). Most pts (73%) received cancer treatment during the previous year, 9% had a pre-COVID-19 prognosis of <6months, and 10% deferred ICU care. COVID-19 mortality in the entire cohort was 17%. In multivariable analyses, age > 60 (OR 2.03, 1.31-3.18), male sex (OR 1.69, 1.11 - 2.61), estimated pre-COVID-19 prognosis of less than 6 months (OR 6.16, 3.26 - 11.70) and ICU deferral (OR 10.87, 6.36 - 18.96) were all independently associated with an increased risk of death. Receiving cancer treatment in the year prior to COVID-19 diagnosis and type of hematologic malignancy were not significantly associated with death. In multivariable analyses, age > 60 (OR 2.46, 1.83 - 3.31), male sex (OR 1.34, 1.02 - 1.76), estimated pre-COVID-19 prognosis of < 6 months (OR 4.81, 2.45 - 10.50), presence of a major comorbidity (OR 1.57, 1.15 - 2.16), and cancer treatment in the previous year (OR 1.50, 1.10 - 2.06) were all independently associated with an increased risk of a severe COVID-19 requiring hospitalization. Pts with a MPN or plasma cell dyscrasia and COVID-19 were less likely to require hospitalization for COVID-19 compared to patients with CLL, leukemia/MDS, or lymphoma. CONCLUSIONS: These analyses confirm the negative impact of age > 60, male sex, pre-COVID-19 prognosis of < 6 months, and deferral of ICU care on mortality among patients with hematologic malignancy and COVID-19. We did not observe an increased risk of COVID-19 mortality among pts with COVID-19 who received blood cancer treatment in the previous year, although rate of hospitalization was higher. Pts with some hematologic malignancies (MPN, plasma cell dyscrasias), may experience less severe COVID-19 infections than others. Disclosures Anderson: Celgene: Membership on an entity's Board of Directors or advisory committees; Millenium-Takeda: Membership on an entity's Board of Directors or advisory committees; Gilead: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees; Sanofi-Aventis: Membership on an entity's Board of Directors or advisory committees; Bristol Myers Squibb: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees; Scientific Founder of Oncopep and C4 Therapeutics: Current equity holder in publicly-traded company, Current holder of individual stocks in a privately-held company; AstraZeneca: Membership on an entity's Board of Directors or advisory committees; Mana Therapeutics: Membership on an entity's Board of Directors or advisory committees. Desai: Janssen R&D: Research Funding; Astex: Research Funding; Kura Oncology: Consultancy; Agios: Consultancy; Bristol Myers Squibb: Consultancy; Takeda: Consultancy. Goldberg: Celularity: Research Funding; Genentech: Consultancy, Membership on an entity's Board of Directors or advisory committees; Astellas: Consultancy, Membership on an entity's Board of Directors or advisory committees; Aptose: Consultancy, Research Funding; Prelude Therapeutics: Research Funding; DAVA Oncology: Honoraria; Pfizer: Research Funding; Arog: Research Funding; Aprea: Research Funding; AbbVie: Consultancy, Membership on an entity's Board of Directors or advisory committees, Research Funding. Neuberg: Madrigal Pharmaceuticals: Other: Stock ownership; Pharmacyclics: Research Funding. Radhakrishnan: Janssen India: Honoraria; Dr Reddy's Laboratories: Honoraria, Membership on an entity's Board of Directors or advisory committees; Aurigene: Speakers Bureau; Novartis: Honoraria; Johnson and Johnson: Honoraria; Pfizer: Consultancy, Honoraria; Astrazeneca: Consultancy, Honoraria; Emcure Pharmaceuticals: Other: payment to institute; Cipla Pharmaceuticals: Honoraria, Other: payment to institute; Bristol Myers Squibb: Other: payment to institute; Roche: Honoraria, Other: payment to institute; Intas Pharmaceutical: Other: payment to institute; NATCO Pharmaceuticals: Research Funding. Sehn: Genmab: Consultancy; Debiopharm: Consultancy; Novartis: Consultancy. Sekeres: Novartis: Membership on an entity's Board of Directors or advisory committees; Takeda/Millenium: Membership on an entity's Board of Directors or advisory committees; BMS: Membership on an entity's Board of Directors or advisory committees. Tallman: Kura: Membership on an entity's Board of Directors or advisory committees; Syros: Membership on an entity's Board of Directors or advisory committees; Innate Pharma: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Biosight: Membership on an entity's Board of Directors or advisory committees; Roche: Membership on an entity's Board of Directors or advisory committees; Jazz Pharma: Membership on an entity's Board of Directors or advisory committees; Oncolyze: Membership on an entity's Board of Directors or advisory committees; KAHR: Membership on an entity's Board of Directors or advisory committees; Orsenix: Membership on an entity's Board of Directors or advisory committees; Daiichi-Sankyo: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Amgen: Research Funding; Rafael Pharmaceuticals: Research Funding; Glycomimetics: Research Funding; Biosight: Research Funding; Orsenix: Research Funding; Abbvie: Research Funding; NYU Grand Rounds: Honoraria; Mayo Clinic: Honoraria; UC DAVIS: Honoraria; Northwell Grand Rounds: Honoraria; NYU Grand Rounds: Honoraria; Danbury Hospital Tumor Board: Honoraria; Acute Leukemia Forum: Honoraria; Miami Leukemia Symposium: Honoraria; New Orleans Cancer Symposium: Honoraria; ASH: Honoraria; NCCN: Honoraria.

Abstract Patients with hematological malignancies are at risk for poor outcomes when diagnosed with coronavirus disease 2019 (COVID-19). It remains unclear whether cytopenias and specific leukemia subtypes play a role in the clinical course of COVID-19 infection. Here, we report outcomes and their clinical/laboratory predictors for 65 patients with acute and chronic leukemias diagnosed with COVID-19 between 8 March 2020 and 19 May 2020 at Memorial Sloan Kettering Cancer Center in New York City. Most patients had CLL (38%) or AML (26%). A total of 14 (22%) patients required high flow nasal cannula or were intubated for mechanical ventilation and 11 patients (17%) died. A diagnosis of AML (OR 4.7, p=.028), active treatment within the last 3 months (OR 5.22, p=.047), neutropenia within seven days prior and up to 28 days after SARS-CoV-2 diagnosis (11.75, p=.001) and ≥3 comorbidities (OR 6.55, p=.019) were associated with increased odds of death.

Background: Predictors of severe infection and outcomes with COVID-19 in patients (pts) with acute myeloid leukemia (AML), acute lymphocytic leukemia (ALL) and myelodysplastic syndromes (MDS) are lacking. Pts with active disease may experience worse outcomes due to overall prognosis and cytopenias. Here we identify risk factors for severe COVID-19 infection and mortality in pts with AML, MDS, and ALL using the ASH RC COVID-19 Registry for Hematology.

The essential CCCTC-binding factor (CTCF) is critical to three-dimensional (3D) genome organization. CTCF binding insulates active and repressed genes within the Hox clusters upon differentiation, but such binding does not participate in boundary formation in all cell types, such as embryonic stem cells. We conducted a genome-wide CRISPR knockout screen to identify genes required for CTCF boundary activity at the HoxA cluster, complemented by novel biochemical approaches. This screen identified Myc-associated zinc finger protein (MAZ) as a CTCF insulator co-factor, among other candidates listed herein. MAZ depletion or specific deletion of MAZ motifs within the Hox clusters led to de-repression of posterior Hox genes immediately after CTCF boundaries upon differentiation, which phenocopied deletion of the proximal CTCF motifs. Similar to CTCF, MAZ interacted with the cohesin subunit, RAD21. Upon loss of MAZ, local contacts within topologically associated domains (TADs) were disrupted, as evidenced by HiC analysis. Thus, MAZ is a novel factor sharing insulation properties with CTCF and contributing to the genomic architectural organization. One Sentence Summary MAZ is identified as an insulator functioning at CTCF boundaries delimiting active and repressed genes at Hox clusters ### Competing Interest Statement D.R. was a cofounder of Constellation Pharmaceuticals and Fulcrum Therapeutics. Currently, D.R has no affiliation with either company. The authors declare that they have no other competing interests.

PURPOSE Coronavirus-2019 (COVID-19) mortality is higher in patients with cancer than in the general population, yet the cancer-associated risk factors for COVID-19 adverse outcomes are not fully characterized. PATIENTS AND METHODS We reviewed clinical characteristics and outcomes from patients with cancer and concurrent COVID-19 at Memorial Sloan Kettering Cancer Center until March 31, 2020 (n = 309), and observed clinical end points until April 13, 2020. We hypothesized that cytotoxic chemotherapy administered within 35 days of a COVID-19 diagnosis is associated with an increased hazard ratio (HR) of severe or critical COVID-19. In secondary analyses, we estimated associations between specific clinical and laboratory variables and the incidence of a severe or critical COVID-19 event. RESULTS Cytotoxic chemotherapy administration was not significantly associated with a severe or critical COVID-19 event (HR, 1.10; 95% CI, 0.73 to 1.60). Hematologic malignancy was associated with increased COVID-19 severity (HR, 1.90; 95% CI, 1.30 to 2.80). Patients with lung cancer also demonstrated higher rates of severe or critical COVID-19 events (HR, 2.0; 95% CI, 1.20 to 3.30). Lymphopenia at COVID-19 diagnosis was associated with higher rates of severe or critical illness (HR, 2.10; 95% CI, 1.50 to 3.10). Patients with baseline neutropenia 14-90 days before COVID-19 diagnosis had worse outcomes (HR, 4.20; 95% CI, 1.70 to 11.00). Findings from these analyses remained consistent in a multivariable model and in multiple sensitivity analyses. The rate of adverse events was lower in a time-matched population of patients with cancer without COVID-19. CONCLUSION Recent cytotoxic chemotherapy treatment was not associated with adverse COVID-19 outcomes. Patients with active hematologic or lung malignancies, peri-COVID-19 lymphopenia, or baseline neutropenia had worse COVID-19 outcomes. Interactions among antineoplastic therapy, cancer type, and COVID-19 are complex and warrant further investigation.

Polycomb repressive complex 2 (PRC2) maintains gene silencing by catalyzing methylation of histone H3 at lysine 27 (H3K27me2/3) within chromatin. By designing a system whereby PRC2-mediated repressive domains were collapsed and then reconstructed in an inducible fashion in vivo, a two-step mechanism of H3K27me2/3 domain formation became evident. First, PRC2 is stably recruited by the actions of JARID2 and MTF2 to a limited number of spatially interacting “nucleation sites”, creating H3K27me3-forming polycomb foci within the nucleus. Second, PRC2 is allosterically activated via its binding to H3K27me3 and rapidly spreads H3K27me2/3 both in cis and in far-cis via long-range contacts. As PRC2 proceeds further from the nucleation sites, its stability on chromatin decreases such that domains of H3K27me3 remain proximal, and those of H3K27me2 distal, to the nucleation sites. This study demonstrates the principles of de novo establishment of PRC2-mediated repressive domains across the genome.

Developmental specification of germ cells lies at the heart of inheritance, as germ cells contain all of the genetic and epigenetic information transmitted between generations. The critical developmental event distinguishing germline from somatic lineages is the differentiation of primordial germ cells (PGCs), precursors of sex-specific gametes that produce an entire organism upon fertilization. Germ cells toggle between uni- and pluripotent states as they exhibit their own 'latent' form of pluripotency. For example, PGCs express a number of transcription factors in common with embryonic stem (ES) cells, including OCT4 (encoded by Pou5f1), SOX2, NANOG and PRDM14 (refs 2, 3, 4). A biochemical mechanism by which these transcription factors converge on chromatin to produce the dramatic rearrangements underlying ES-cell- and PGC-specific transcriptional programs remains poorly understood. Here we identify a novel co-repressor protein, CBFA2T2, that regulates pluripotency and germline specification in mice. Cbfa2t2(-/-) mice display severe defects in PGC maturation and epigenetic reprogramming. CBFA2T2 forms a biochemical complex with PRDM14, a germline-specific transcription factor. Mechanistically, CBFA2T2 oligomerizes to form a scaffold upon which PRDM14 and OCT4 are stabilized on chromatin. Thus, in contrast to the traditional 'passenger' role of a co-repressor, CBFA2T2 functions synergistically with transcription factors at the crossroads of the fundamental developmental plasticity between uni- and pluripotency.

The genome is organized into repeating topologically associated domains (TADs), each of which is spatially isolated from its neighbor by poorly understood boundary elements thought to be conserved across cell types. Here, we show that deletion of CTCF (CCCTC-binding factor)-binding sites at TAD and sub-TAD topological boundaries that form within the HoxA and HoxC clusters during differentiation not only disturbs local chromatin domain organization and regulatory interactions but also results in homeotic transformations typical of Hox gene misregulation. Moreover, our data suggest that CTCF-dependent boundary function can be modulated by competing forces, such as the self-assembly of polycomb domains within the nucleus. Therefore, CTCF boundaries are not merely static structural components of the genome but instead are locally dynamic regulatory structures that control gene expression during development.

USP7 is a protein deubiquitinase with an essential role in development. Here, we provide evidence that USP7 regulates the activity of Polycomb repressive complex 1 (PRC1) in coordination with SCML2. There are six versions of PRC1 defined by the association of one of the PCGF homologues (PCGF1 to PCGF6) with the common catalytic subunit RING1B. First, we show that SCML2, a Polycomb group protein that associates with PRC1.2 (containing PCGF2/MEL18) and PRC1.4 (containing PCGF4/BMI1), modulates the localization of USP7 and bridges USP7 with PRC1.4, allowing for the stabilization of BMI1. Chromatin immunoprecipitation (ChIP) experiments demonstrate that USP7 is found at SCML2 and BMI1 target genes. Second, inhibition of USP7 leads to a reduction in the level of ubiquitinated histone H2A (H2Aub), the catalytic product of PRC1 and key for its repressive activity. USP7 regulates the posttranslational status of RING1B and BMI1, a specific component of PRC1.4. Thus, not only does USP7 stabilize PRC1 components, its catalytic activity is also necessary to maintain a functional PRC1, thereby ensuring appropriate levels of repressive H2Aub.

Polycomb and Trithorax group proteins encode the epigenetic memory of cellular positional identity by establishing inheritable domains of repressive and active chromatin within the Hox clusters. Here we demonstrate that the CCCTC-binding factor (CTCF) functions to insulate these adjacent yet antagonistic chromatin domains during embryonic stem cell differentiation into cervical motor neurons. Deletion of CTCF binding sites within the Hox clusters results in the expansion of active chromatin into the repressive domain. CTCF functions as an insulator by organizing Hox clusters into spatially disjoint domains. Ablation of CTCF binding disrupts topological boundaries such that caudal Hox genes leave the repressed domain and become subject to transcriptional activation. Hence, CTCF is required to insulate facultative heterochromatin from impinging euchromatin to produce discrete positional identities.

Accurate and sensitive detection of protein–protein and protein–RNA interactions is key to understanding their biological functions. Traditional methods to identify these interactions require cell lysis and biochemical manipulations that exclude cellular compartments that cannot be solubilized under mild conditions. Here, we introduce an in vivo proximity labeling (IPL) technology that employs an affinity tag combined with a photoactivatable probe to label polypeptides and RNAs in the vicinity of a protein of interest in vivo. Using quantitative mass spectrometry and deep sequencing, we show that IPL correctly identifies known protein–protein and protein–RNA interactions in the nucleus of mammalian cells. Thus, IPL provides additional temporal and spatial information for the characterization of biological interactions in vivo.

The multifunctional CCCTC-binding factor (CTCF) protein exhibits a broad range of functions, including that of insulator and higher-order chromatin organizer. We found that CTCF comprises a previously unrecognized region that is necessary and sufficient to bind RNA (RNA-binding region [RBR]) and is distinct from its DNA-binding domain. Depletion of cellular CTCF led to a decrease in not only levels of p53 mRNA, as expected, but also those of Wrap53 RNA, an antisense transcript originated from the p53 locus. PAR-CLIP-seq (photoactivatable ribonucleoside-enhanced cross-linking and immunoprecipitation [PAR-CLIP] combined with deep sequencing) analyses indicate that CTCF binds a multitude of transcripts genome-wide as well as to Wrap53 RNA. Apart from its established role at the p53 promoter, CTCF regulates p53 expression through its physical interaction with Wrap53 RNA. Cells harboring a CTCF mutant in its RBR exhibit a defective p53 response to DNA damage. Moreover, the RBR facilitates CTCF multimerization in an RNA-dependent manner, which may bear directly on its role in establishing higher-order chromatin structures in vivo.