Project description:IDH1 mutation impacts CTCF binding and function through methylating cytosine in the CTCF binding motif. We hypothesize that the impairment of CTCF-associated DNA repair process may be relevant to the loss of CTCF chromatin association and shifts in chromatin conformation. Chromatin immunoprecipitation-sequencing confirmed the loss of CTCF coverage in the proximity sites of AsisI sites in IDH1 mutant cells.
Project description:Catalytic activity of the ISWI family of remodelers is critical for nucleosomal organization and transcription factor binding, including the insulator protein CTCF. To define which subcomplex mediates these diverse functions we phenotyped a panel of isogenic mouse stem cell lines each lacking one of six ISWI accessory subunits. Individual deletions of either CERF, RSF1, ACF, WICH or NoRC subcomplexes only moderately affect the chromatin landscape, while removal of the NURF-specific subunit BPTF leads to drastic reduction in chromatin accessibility and Snf2h ATPase localization around CTCF sites. While this reduces distances to the adjacent nucleosomes it only modestly impacts CTCF binding itself. In absence of accessibility, the insulator function of CTCF is nevertheless impaired resulting in lower occupancy of cohesin and cohesin-loading factors, and reduced insulation at these sites, highlighting the need of NURF-mediated remodeling for open chromatin and proper CTCF function. Our comprehensive analysis reveals a specific role for NURF in mediating Snf2h localization and chromatin opening at bound CTCF sites showing that local accessibility is critical for cohesin binding and insulator function.
Project description:CGGBP1-dependent CTCF-binding sites identified in Patel et. al. 2019 [PMID 31547883] serve as barrier elements consistent with asymmetrical levels of H3K9me3 in the flanks and asymmetrical RNA levels at these sites in the genome. In this study we have characterised the function of such CGGBP1-depenedent CTCF binding sites which are usually repeat rich in nature. By cloning one such CTCF-binding site in an episomal system, we have studied the barrier activity of this CGGBP1-dependent CTCF-binding sites in the prsesnce and absence of CGGBP1 in HEK293T cells through various molecular assays and RNA-sequencing. Our results show that these sites act as barrier for the ectopic transcription as the depletion of CGGBP1 lead to starnd-specific bidirectional transcription as a result of loss of barrier activity concomitant with the loss of CTCF-binding. Further, analysing the RNA-seq revealed that weakly transcribed sites are flanked by the CTCF-binding at transcription start and end sites in the presence of CGGBP1. However, CGGBP1 depletion leads to loss of barrier activity maintained by CGGBP1 with dispersed CTCF binding and a loss of transcription restriction. Such CGGBP1-dependent CTCF-binding sites prevents ectopic transcription.
Project description:Pervasive usage of alternative promoters leads to deregulation of gene expression in carcinogenesis and may drive the emergence of new genes in spermatogenesis. However, little is known regarding the mechanisms underpinning the activation of alternative promoters. In our present study, we uncovered a novel mechanism by which alternative cancer-testis-specific transcription is activated from the intergenic and intronic clustered CTCF binding sites, which are transcriptionally inert in normal somatic cells. BORIS/CTCFL, a paralog of CTCF with cancer-testis-specific expression forms a heterodimer with CTCF at the clustered binding sites thus triggering epigenetic reprogramming of these sites into units of active transcription. BORIS binding to CTCF sites leads to the recruitment of chromatin-remodeling factor SRCAP, with subsequent replacement of H2A histone with H2A.Z, therefore creating a more relaxed chromatin state in the nucleosomes flanking the clustered binding sites. This facilitates opening of chromatin beyond CTCF/BORIS binding sites and paves the way to the recruitment of multiple additional transcription factors, thereby activating transcription from a given binding site. We demonstrate that the CTCF binding sites, epigenetically reprogrammed by ectopic BORIS expression can drive the expression of cancer-testis genes, long-noncoding RNAs, retro-pseudogenes, and dormant transposable elements harbored by activated long transcripts. Taking together, our results reveal that BORIS functions as a transcription factor that epigenetically reprograms clustered CTCF binding sites into transcriptional start sites, promoting transcription from alternative promoters in both germ cells and cancer cells.
Project description:Mammalian chromosomes are partitioned into topologically associating domains (TADs) by the loop-extrusion activity of cohesin that is blocked at specific DNA sites bound by CTCF. Chromosome structure inside TADs is highly variable in single cells, yet little is known about its temporal dynamics, how it influences the rates and durations of chromosomal contacts, and how it depends on CTCF and cohesin. To address these questions we combine two quantitative live-cell imaging strategies that minimize locus-specific confounding effects. We show that loop extrusion by cohesin globally reduces the mobility of the chromatin fiber in living cells, while also increasing the rates of formation and durations of contacts between sequences inside the same TAD. Quantitative analysis of high-resolution microscopy data reveals that contacts assemble and disassemble frequently in the course of the cell cycle, and become substantially more frequent and longer in the presence of convergent CTCF sites. Comparison with polymer modeling additionally reveals that cohesin-mediated CTCF loops last around 10 minutes on average. Our data support the notion that chromosome structure within TADs is highly dynamic and provide a quantitative framework for understanding the principles that link chromosome structure to biological function.