Biology subjects

Pollex, T.

Publications and source records attributed to Pollex, T..

4 recordsLinked to original sources

bioRxiv2026

X-inactivation escapee domains are CTCF-cohesin independent chromatin compartments

X-chromosome inactivation involves chromosome-wide gene silencing accompanied by extensive chromatin changes, as well the loss of topologically associating domains. Yet discrete regions of the inactive X chromosome retain activity within localised 3D domains, which contain active genes that variably escape from X inactivation. The transcription factor and architectural protein CTCF has been proposed to be implicated in escape by insulating escape domains or sustaining their topology via cohesin-mediated loop extrusion. Here, we test the role of CTCF and cohesin in escape using acute degron-mediated depletion of CTCF and RAD21 in neural progenitor cells with established escape profiles. Although CTCF occupancy correlates with escape status on the inactive X chromosome, its removal - together with loss of loop extrusion - does not disrupt escapee gene expression, or domain organization, nor does it result in spreading of silencing or activation of genes in cis. Rather, we show that facultative escape regions are self-sustaining compartments of active chromatin enriched in H3K27 acetylation and depleted in H3K27 methylation, with the magnitude of compartment strength scaling up with the degree of transcriptional activity on the inactive X chromosome. These active escapee compartments are propagated independently of CTCF and RAD21-dependent 3D architecture. Our findings identify chromatin compartmentalization as the primary feature of facultative escapee domains.

genetics↗

bioRxiv2022

New enhancer-promoter interactions are gained during tissue differentiation and reflect changes in E/P activity

To regulate gene expression, enhancers must come into proximity with their target gene. At some loci the timing of enhancer-promoter proximity is uncoupled from gene activation, while at others it is tightly linked. Here, we assessed this more globally for 600 characterized enhancers or promoters (E/P) with tissue-specific activity in Drosophila embryos, by performing Capture-C and insulator ChIP in FACS-purified myogenic or neurogenic cells at different stages of embryogenesis. This high-resolution view enabled direct comparison between E/P interactions and activity across 5 developmental conditions. This revealed largely invariant E/P contacts between the blastoderm and cell fate specification stages, despite changes in activity. However, E/P interactions diverge during terminal tissue differentiation when many tissue-specific interactions are gained on top of a pre-existing topology. Changes in E/P proximity reflect changes in enhancer activity and gene activation, and are generally not accompanied by changes in insulator binding. Using transgenes and deletions, we show that many tissue-specific interactions represent functional E-P pairs. Our results reveal a shift in E-P landscapes as embryogenesis proceeds, from largely pre-formed topologies at early stages to more distal tissue-specific loops during differentiation, when E/P proximity appears coupled to activation.

developmental biology↗

bioRxiv2022

CTCF, BEAF-32 and CP190 are not required for the initial establishment of TADs in early Drosophila embryos, but have locus specific roles

The boundaries of Topologically-Associating Domains (TADs) are delimited by insulators and active promoters, however how they are initially established during embryogenesis remains unclear. Here, we examined this during the first hours of Drosophila embryogenesis. DNA-FISH on individual embryos indicates that domains form during zygotic genome activation (ZGA), but have extensive cell-to-cell heterogeneity compared to later stages. Most newly formed boundaries are occupied by combinations of CTCF, BEAF- 32 and/or CP190. Depleting each insulator from chromatin revealed that TADs can still establish during ZGA, although with lower insulation, with particular boundaries being more sensitive. Some weakened boundaries have aberrant gene expression, however the majority of mis-expressed genes have no obvious relationship to changes in domain-boundary insulation. Deletion of an active promoter (thereby blocking transcription) at one boundary had a greater impact compared to deleting the insulator-bound region itself. These results suggest cross-talk between insulators and transcription might reinforce domain formation during embryogenesis.

developmental biology↗

bioRxiv2018

Modeling of DNA methylation in cis reveals principles of chromatin folding in vivo in the absence of crosslinking and ligation

Mammalian chromosomes are folded into an intricate hierarchy of structural domains, within which topologically associating domains (TADs) and CTCF-associated loops partition the physical interactions between regulatory sequences. Current understanding of chromosome folding largely relies on chromosome conformation capture (3C)-based experiments, where chromosomal interactions are detected as ligation products after crosslinking of chromatin. To measure chromosome structure in vivo, quantitatively and without relying on crosslinking and ligation, we have implemented a new method named damC. DamC combines DNA-methylation based detection of chromosomal interactions with next-generation sequencing and a biophysical model of methylation kinetics. DamC performed in mouse embryonic stem cells provides the first in vivo validation of the existence of TADs and CTCF loops, confirms 3C-based measurements of the scaling of contact probabilities within TADs, and provides evidence that mammalian chromatin in vivo is essentially rigid below 5 kilobases. Combining damC with transposon-mediated genomic engineering shows that new loops can be formed between ectopically introduced and endogenous CTCF sites, which alters the partitioning of physical interactions within TADs. This establishes damC as a crosslinking-and ligation-free framework to measure and modify chromosome interactions combined with a solid theoretical background for rigorous data interpretation. This orthogonal approach to 3C validates the existence of key structural features of mammalian chromosomes and provides novel insights into how chromosome structure within TADs can be manipulated.

genomics↗