Differential chromatin structure encompassing replication origins in transformed and normal cells

Area of Science:

• Epigenetics and Molecular Biology
• Cellular Biology
• Genomics

Background:

• Chromatin structure plays a crucial role in regulating DNA replication.
• Epigenetic modifications influence the accessibility of replication origins.
• Cellular transformation alters gene expression and chromatin organization.

Purpose of the Study:

• To investigate the differences in chromatin structure at replication origins between transformed and normal cells.
• To determine the role of epigenetic factors, such as histone modifications and chromatin remodeling proteins, in regulating origin activity.
• To compare the activity of episomal and chromosomal replication origins in different cellular states.

Main Methods:

• Analysis of histone modifications (acetylation and trimethylation) as markers of open and closed chromatin.
• Chromatin immunoprecipitation (ChIP) assays to assess protein binding (pre-RC, Brg-1, Bmi-1) at origins.
• Quantitative analysis of replication origin activity and nascent DNA abundance.
• MNase-Southern blot assay for nucleosomal positioning analysis.

Main Results:

• Transformed cells exhibit a higher ratio of open to closed chromatin at replication origins.
• Overexpression and increased chromatin binding of Brg-1 (trithorax group) and Bmi-1 (polycomb group) proteins in transformed cells.
• Chromosomal replication origins (20mer1, 20mer2, c-myc) show significantly higher activity and pre-RC association in transformed cells.
• Episomal origins and the lamin B2 housekeeping origin display similar activity and chromatin states in both cell types.
• Nucleosomal regions around origins are loosely packed and spaced further apart before S phase onset.

Conclusions:

• Cellular transformation is associated with differential epigenetic regulation of chromatin structure.
• More open chromatin at replication origins in transformed cells enhances accessibility to initiator proteins.
• This increased accessibility leads to elevated replication origin activity during cellular transformation.