PcG proteins, DNA methylation, and gene repression by chromatin looping

Area of Science:

• Epigenetics and Gene Regulation
• Cancer Biology
• Chromatin Structure

Background:

• Many genes silenced in adult cancers are marked by Polycomb group (PcG) proteins in embryonic stem (ES) cells.
• The GATA-4 gene, a target of PcG silencing, serves as a model to study higher-order chromatin organization.

Purpose of the Study:

• To investigate the three-dimensional chromatin conformation of the GATA-4 locus in embryonic carcinoma (EC) cells and colon cancer cells.
• To determine the role of PcG proteins, H3K27me3, and DNA methylation in establishing and maintaining these chromatin structures.
• To explore the relationship between chromatin looping, PcG occupancy, and gene transcription states.

Main Methods:

• Utilized small interfering RNA (siRNA) to deplete EZH2 (a PcG protein) in Tera-2 EC cells.
• Analyzed changes in long-range chromatin associations and H3K27me3 enrichment.
• Examined chromatin conformation and gene expression during Tera-2 cell differentiation.
• Investigated chromatin structure and DNA methylation in colon cancer cells, including MBD2 occupancy.
• Assessed the impact of DNA methyltransferase disruption (DKO cells) on chromatin conformation and MBD2 binding.

Main Results:

• In undifferentiated EC cells, the GATA-4 locus forms complex multi-loop structures enriched for PcG proteins and H3K27me3, associated with low GATA-4 transcription.
• EZH2 depletion reduced long-range contacts and H3K27me3, leading to increased GATA-4 transcription.
• Differentiation of EC cells dissolved these loops, removed PcG marks, and significantly upregulated GATA-4.
• In colon cancer cells, GATA-4 locus exhibited increased long-range interactions, DNA hypermethylation, and MBD2 occupancy, with no basal transcription.
• Disruption of DNA methylation in DKO cells reduced MBD2 occupancy and altered long-range contacts towards an EC-like state.

Conclusions:

• PcG-occupied and H3K27me3-enriched regions can form cis-acting chromatin loops around single genes in mammalian cells.
• These loops are linked to a poised, low-transcription state in EC cells and a repressed state with DNA methylation in cancer cells.
• Similarities in higher-order chromatin conformation exist between stem/precursor cells and adult cancers, suggesting conserved regulatory mechanisms.