MacroH2A histone variants limit chromatin plasticity through two distinct mechanisms

Area of Science:

• Epigenetics and chromatin biology
• Molecular oncology
• Histone modifications

Background:

• Macrohistone variants (MacroH2A) are crucial epigenetic regulators involved in tumor suppression and maintaining cellular identity.
• Their precise mechanisms in modulating chromatin plasticity and response to DNA damage remain incompletely understood.

Purpose of the Study:

• To elucidate the structural and functional contributions of different macroH2A protein domains to chromatin structure and dynamics.
• To investigate the role of macroH2A variants in regulating chromatin relaxation following DNA damage.

Main Methods:

• X-ray crystallography to determine the structure of the macrodomain of human macroH2A2.
• Quantitative binding assays to assess ADP-ribose binding specificity across macroH2A isoforms.
• Cellular assays to analyze macroH2A-mediated effects on chromatin relaxation and heterochromatin architecture.

Main Results:

• The macrodomain of macroH2A2 exhibits structural differences from macroH2A1.1, preventing ADP-ribose binding, a specificity conserved across vertebrate isoforms.
• MacroH2A histones mitigate transient, PARP1-dependent chromatin relaxation upon DNA damage through two mechanisms: isoform-specific PARP1 activity suppression (MacroH2A1.1) and a common repressive effect from the unstructured linker region.
• The macroH2A linker region alone can restore heterochromatin architecture in macroH2A-deficient cells, even without DNA damage.

Conclusions:

• MacroH2A variants employ distinct domain-specific and common mechanisms to control chromatin plasticity and maintain genome stability.
• The structural basis for ADP-ribose binding specificity among macroH2A isoforms is elucidated, highlighting functional divergence.
• MacroH2A's role in suppressing DNA damage-induced chromatin relaxation and maintaining heterochromatin integrity is confirmed.