The histone chaperone Spt6 is required for activation-induced cytidine deaminase target determination through H3K4me3...

Area of Science:

• Epigenetics
• Molecular Biology
• Immunology

Background:

• Histone H3 lysine 4 trimethylation (H3K4me3) is crucial for activation-induced cytidine deaminase (AID)-mediated DNA cleavage in immunoglobulin heavy chain (IgH) switch regions during class-switch recombination (CSR).
• The Facilitates Chromatin Transcription (FACT) complex is known to establish H3K4me3 at AID target sites.

Purpose of the Study:

• To investigate the role of the histone chaperone suppressor of Ty6 (Spt6) in regulating H3K4me3 for CSR and somatic hypermutation.
• To elucidate the mechanisms by which Spt6 influences AID target loci and B cell gene expression.

Main Methods:

• Analysis of H3K4me3 levels in relation to AID-induced DNA breakage and mutation frequencies in IgH and non-IgH loci.
• Global gene expression analysis in B cells to identify Spt6-regulated genes.
• Investigation of Spt6's interaction with histone methyltransferases and its role in specific histone modification pathways.

Main Results:

• Spt6 deficiency led to H3K4me3 loss, impaired AID-induced DNA breakage, and reduced mutation frequencies in IgH and other loci.
• Spt6 acts as both a positive and negative transcriptional regulator in B cells, affecting genes including Spt4 and AID.
• Spt6 utilizes distinct pathways, H3K4me3 and H3K36me3, to regulate CSR and AID expression, respectively.
• The tandem SH2 domain of Spt6 is critical for CSR and H3K4me3 regulation via the Set1 histone methyltransferase.

Conclusions:

• Spt6 is a key histone chaperone involved in regulating H3K4me3 epigenetic modifications at AID target loci, impacting CSR and somatic hypermutation.
• Spt6 plays a dual role in transcriptional regulation within B cells.
• Spt6 uniquely controls the epigenetic state of both AID targets and the AID locus itself through distinct histone modification pathways.