# Investigating crosstalk between H3K27 acetylation and H3K4 trimethylation in CRISPR/dCas-based epigenome editing and gene activation

## TL;DR

This paper explores how two histone modifications, H3K27ac and H3K4me3, interact during gene activation using CRISPR/dCas-based epigenome editing.

## Contribution

It reveals that H3K27ac induces H3K4me3 and gene activation, while H3K4me3 alone does not, and identifies BRD2 as the mediator of this crosstalk.

## Key findings

- Ectopic H3K27ac in promoters leads to H3K4me3 enrichment and gene activation.
- H3K4me3 installation alone does not induce H3K27ac or gene activation.
- BRD2, not BRD4, mediates H3K4me3 installation and gene activation upon H3K27ac writing.

## Abstract

Epigenome editing methods enable the precise manipulation of epigenetic modifications, such as histone posttranscriptional modifications (PTMs), for uncovering their biological functions. While histone PTMs have been correlated with certain gene expression status, the causalities remain elusive. Histone H3 Lysine 27 acetylation (H3K27ac) and histone H3 Lysine 4 trimethylation (H3K4me3) are both associated with active genes, and located at active promoters and enhancers or around transcriptional start sites (TSSs). Although crosstalk between histone lysine acetylation and H3K4me3 has been reported, relationships between specific epigenetic marks during transcriptional activation remain largely unclear. Here, using clustered regularly interspaced short palindromic repeats (CRISPR)/dCas-based epigenome editing methods, we discovered that the ectopic introduction of H3K27ac in the promoter region lead to H3K4me3 enrichment around TSS and transcriptional activation, while H3K4me3 installation at the promoter cannot induce H3K27ac increase and failed to activate gene expression. Blocking the reading of H3K27ac by BRD proteins using inhibitor JQ1 abolished H3K27ac-induced H3K4me3 installation and downstream gene activation. Furthermore, we uncovered that BRD2, not BRD4, mediated H3K4me3 installation and gene activation upon H3K27ac writing. Our studies revealed the relationships between H3K27ac and H3K4me3 in gene activation process and demonstrated the application of CRISPR/dCas-based epigenome editing methods in elucidating the crosstalk between epigenetic mechanisms.