Bivalent chromatin instructs lineage specification during hematopoiesis.

Area of Science:

• Epigenetics
• Hematopoiesis
• Molecular Biology

Background:

• Developmental gene expression relies on histone H3 lysine 4 (H3K4) and H3 lysine 27 (H3K27) methylation.
• The precise physiological functions of these epigenetic marks are not fully elucidated.

Purpose of the Study:

• To investigate the physiological roles of H3K4 methylation in mammalian development.
• To understand the interplay between H3K4 and H3K27 methylation in hematopoietic stem cells (HSCs) and progenitor cells.

Main Methods:

• Utilized a dominant histone H3-lysine-4-to-methionine (H3K4M) mutation in mice to deplete all forms of H3K4 methylation.
• Analyzed HSC maintenance, progenitor cell commitment, and maturation.
• Investigated the deposition of H3K27 methylation at differentiation-associated genes.
• Concomitantly suppressed H3K27 methylation in H3K4-methylation-depleted mice to assess rescue effects.

Main Results:

• Mice with H3K4 methylation depletion exhibited severe loss of all major blood cell types.
• H3K4 methylation is dispensable for HSC maintenance and commitment but essential for progenitor cell maturation.
• H3K4 methylation antagonizes H3K27 methylation deposition at bivalent chromatin states in HSCs and progenitors.
• Suppression of H3K27 methylation rescued lethality, hematopoietic failure, and gene dysregulation in H3K4-depleted mice.

Conclusions:

• H3K4 methylation is critical for hematopoietic progenitor cell maturation and opposes repressive H3K27 methylation.
• The functional antagonism between H3K4 and H3K27 methylation is vital for mammalian tissue homeostasis, particularly in hematopoiesis.