HIRA-dependent boundaries between H3 variants shape early replication in mammals

Area of Science:

• Cell Biology
• Epigenetics
• Molecular Biology

Background:

• Mammalian DNA replication origins lack a defined DNA sequence, prompting investigation into chromatin-based mechanisms.
• The dynamic nature of chromatin, with nucleosome disruption during replication and transcription, poses challenges to stable origin definition.
• Understanding how chromatin specifies and maintains replication origins is crucial for comprehending genome stability.

Purpose of the Study:

• To investigate the role of histone variant deposition in defining mammalian replication origins.
• To elucidate the mechanism by which chromatin structure specifies and maintains early replication initiation zones.
• To determine the involvement of histone variant H3.3 chaperone HIRA in this process.

Main Methods:

• Genome-wide mapping of de novo histone variant (H3.1 and H3.3) deposition during S phase in human cells.
• Analysis of H3.1/H3.3 boundary formation and its correlation with replication initiation zones.
• Assessment of the impact of HIRA depletion on histone variant deposition patterns and origin activity.

Main Results:

• Identified a dual deposition mode of histone variants H3.1 and H3.3, forming stable H3.1/H3.3 boundaries.
• Demonstrated that these H3.1/H3.3 boundaries precisely correspond to the initiation zones of early replication origins.
• Showed that loss of HIRA disrupts these boundaries and impairs early origin initiation.

Conclusions:

• Propose a model where HIRA-dependent H3.3 deposition maintains H3.1/H3.3 boundaries, protecting them from H3.1 invasion.
• Suggest that these chromatin boundaries contribute to a stable, chromatin-based definition of early replication zones.
• Highlight the critical role of histone variant dynamics in specifying replication origins in mammals.