Two replication fork maintenance pathways fuse inverted repeats to rearrange chromosomes

Area of Science:

• Genetics
• Molecular Biology
• Genomic Instability

Background:

• Replication fork maintenance pathways are crucial for chromosomal integrity.
• Dysfunctional pathways, particularly at nonallelic repeats, can lead to cancer and genomic disorders.
• Homologous recombination and error-free postreplication repair (EF-PRR) are key mechanisms involved.

Purpose of the Study:

• To delineate the distinct pathways responsible for spontaneous fusion of inverted repeats in mouse embryonic stem cells.
• To investigate the roles of Bloom syndrome mutated (BLM) helicase and RAD18 in these fusion pathways.
• To elucidate the involvement of TREX2 exonuclease in repeat fusion and its relationship with EF-PRR.

Main Methods:

• Utilizing wild-type mouse embryonic stem cells.
• Inducing DNA damage with gamma radiation and ultraviolet light.
• Analyzing repeat fusion events and their regulation by specific proteins like BLM, RAD18, and TREX2.
• Assessing proliferating cell nuclear antigen (PCNA) ubiquitination and replication fork stalling.

Main Results:

• Gamma radiation induced a BLM-regulated pathway that fused identical repeats.
• Ultraviolet light induced a RAD18-dependent pathway that fused mismatched repeats.
• TREX2 suppressed identical repeat fusion but enhanced mismatched repeat fusion, associating it with EF-PRR and UBC13.

Conclusions:

• Two distinct pathways, differentially regulated by BLM and RAD18, mediate spontaneous inverted repeat fusion.
• TREX2 acts as a novel component of the EF-PRR pathway, distinguishing between identical and mismatched repeat fusion.
• Replication fork stalling is implicated as a causal mechanism for repeat fusion in both pathways, highlighting the importance of replication fork maintenance.