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Distinct Chromatin Scanning Modes Lead to Targeting of Compacted Chromatin by Pioneer Factors FOXA1 and SOX2 [Mnase-seq]

ABSTRACT: Pioneer transcription factors, by interacting with nucleosomes, scan silent, compact chromatin to target regulatory sequences, enabling cooperative binding events that modulate local chromatin structure and gene activity. However, not all cognate motifs are targeted by pioneer factors and dynamic scanning parameters required to scan compact chromatin are unknown. A combined genomics and single-molecule tracking approach shows that to target DNase-resistant, low-histone turnover sites, pioneer factors FOXA1 and SOX2 display opposite dynamics of chromatin scanning: slow, with low nucleoplasmic diffusion and stable interactions, versus fast, with high nucleoplasmic diffusion and transient interactions, respectively. Despite such differences, the ability of FOXA1 and SOX2 to scan low-mobility chromatin, mediated by protein domains outside of the respective DNA binding domains, leads to targeting silent chromatin. By contrast, non-pioneer HNF4A predominantly targets DNase-sensitive, nucleosome-depleted regions. We conclude that the targeting of compact chromatin sites by pioneer factors can be performed through diverse dynamic processes. Micrococcol nuclease digestion and sequencing (MNase-seq) of human BJ fibroblasts

ORGANISM(S): Homo sapiens

PROVIDER: GSE220569 | GEO | 2023/06/07

REPOSITORIES: GEO

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Project description:FOXA1 is a pioneer factor that is important in hormone dependent cancer cells to stabilise nuclear receptors, such as estrogen receptor (ER) to chromatin. FOXA1 binds to enhancers regions that are enriched in H3K4mono- and dimethylation (H3K4me1, H3K4me2) histone marks and evidence suggests that these marks are requisite events for FOXA1 to associate with enhancers to initate subsequent gene expression events. However, exogenous expression of FOXA1 has been shown to induce H3K4me1 and H3K4me2 signal at enhancer elements and the order of events and the functional importance of these events is not clear. We performed a FOXA1 Rapid Immunoprecipitation Mass Spectrometry of Endogenous Proteins (RIME) screen in ERα-positive MCF-7 breast cancer cells in order to identify FOXA1 interacting partners and we found histone-lysine N-methyltransferase (MLL3) as the top FOXA1 interacting protein. MLL3 is typically thought to induce H3K4me3 at promoter regions, but recent findings suggest it may contribute to H3K4me1 deposition, in line with our observation that MLL3 associates with an enhancer specific protein. We performed MLL3 ChIP-seq in breast cancer cells and unexpectedly found that MLL3 binds mostly at non-promoter regions enhancers, in contrast to the prevailing hypothesis. MLL3 was shown to occupy regions marked by FOXA1 occupancy and as expected, H3K4me1 and H3K4me2. MLL3 binding was dependent on FOXA1, indicating that FOXA1 recruits MLL3 to chromatin. Motif analysis and subsequent genomic mapping revealed a role for Grainy head like protein-2 (GRHL2) which was shown to co-occupy regions of the chromatin with MLL3. Regions occupied by all three factors, namely FOXA1, MLL3 and GRHL2, were most enriched in H3K4me1. MLL3 silencing decreased H3K4me1 at enhancer elements, but had no appreciable impact on H3K4me3 at enhancer elements. We identify a complex relationship between FOXA1, MLL3 and H3K4me1 at enhancers in breast cancer and propose a mechanism whereby the pioneer factor FOXA1 can interact with a chromatin modifier MLL3, recruiting it to chromatin to facilitate the deposition of H3K4me1 histone marks, subsequently demarcating active enhancer elements.

Dataset Information

Distinct Chromatin Scanning Modes Lead to Targeting of Compacted Chromatin by Pioneer Factors FOXA1 and SOX2 [Mnase-seq]

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