Polycomb-repressed genes have permissive enhancers that initiate reprogramming

Area of Science:

• Epigenetics and Gene Regulation
• Cellular Reprogramming
• Molecular Biology

Background:

• Polycomb group proteins and H3K27me3 methylation suppress key regulatory genes.
• These genes are activated during normal differentiation and induced cell reprogramming.
• Enhancers are crucial regulatory elements in gene expression.

Purpose of the Study:

• To investigate the role of enhancers in cell fate reprogramming using the MYOD1 gene as a model.
• To understand the initial molecular events at enhancers during reprogramming.
• To elucidate how different transcription factors influence chromatin states at promoters.

Main Methods:

• Analysis of the enhancer/promoter pair of the MYOD1 gene.
• Chromatin analysis, including nucleosome depletion and histone modification profiling (H3K4me1, H3K27me3).
• Studies involving exogenous Myod1 and OCT4 to observe their binding and effects on gene transcription and chromatin.

Main Results:

• An H3K4me1-enriched enhancer with a nucleosome-depleted region (NDR) was identified.
• Transcriptional regulators initially bind to the enhancer, inducing chromatin changes at the promoter.
• Exogenous Myod1 binding to the enhancer activates its own transcription via NDR and permissive promoter chromatin.
• Exogenous OCT4 binding to the enhancer leads to a bivalent H3K27me3 state at the promoter, characteristic of stem cells.

Conclusions:

• Enhancers are critical for initiating cell fate reprogramming by facilitating the binding of transcriptional regulators.
• The MYOD1 enhancer model demonstrates how enhancers control promoter chromatin states and gene activation.
• A significant proportion of Polycomb targets are linked to permissive enhancers, suggesting a broad mechanism for reprogramming initiation.