Chromatin modifiers Mdm2 and RNF2 prevent RNA:DNA hybrids that impair DNA replication

Area of Science:

• Cell Biology
• Molecular Biology
• Epigenetics

Background:

• The p53-Mdm2 pathway is a critical tumor suppressor mechanism.
• Mdm2 is known to regulate p53 and also functions in chromatin modification.
• Polycomb Repressor Complex (PRC) proteins are involved in epigenetic regulation and gene silencing.

Purpose of the Study:

• To investigate the role of Mdm2 and PRC components in DNA replication.
• To determine if Mdm2 and PRC1 share functions in supporting DNA synthesis.
• To elucidate the mechanism by which Mdm2 and PRC1 influence DNA replication fork progression.

Main Methods:

• Depletion of Mdm2 and PRC members using knockdown techniques.
• DNA fiber assays to measure DNA synthesis and replication fork progression.
• Analysis of histone modifications, ubiquitination, and RNA/DNA hybrid formation (R-loops).

Main Results:

• Depletion of PRC members, particularly RNF2/Ring1B, impairs DNA synthesis similar to Mdm2 knockdown.
• Mdm2 and RNF2 exhibit overlapping functions in supporting DNA replication, with each capable of rescuing the other's depletion.
• Mdm2 and RNF2 regulate H2A ubiquitination at K118/K119, which is essential for fork progression.
• Mdm2 depletion leads to R-loop accumulation, hindering DNA replication; this is rescued by RNase H or CDK9 inhibition.
• Chromatin modification by Mdm2 and PRC1 prevents R-loop formation, ensuring smooth DNA replication.

Conclusions:

• Mdm2 and PRC1 components, including RNF2, play essential, overlapping roles in maintaining DNA replication fork stability.
• The ubiquitin ligase activity of Mdm2 and its interaction with PRC1 are critical for DNA synthesis.
• Proper H2A ubiquitination and deubiquitination, along with the avoidance of R-loop formation, are key mechanisms regulated by Mdm2 and PRC1 to support DNA replication.