Bloom helicase mediates formation of large single-stranded DNA loops during DNA end processing

Area of Science:

• Molecular Biology
• Genetics
• Cancer Research

Background:

• Bloom syndrome (BS) is a genetic disorder linked to a high cancer risk.
• Mutations in the Bloom helicase (BLM) gene cause BS.
• BLM protein is crucial for DNA double-strand break (DSB) repair via homologous recombination (HR).

Purpose of the Study:

• To investigate the role of Bloom helicase (BLM) in DNA double-strand break (DSB) processing.
• To elucidate the mechanism by which BLM facilitates homologous recombination (HR).
• To understand how BLM mutations impact DNA repair and cancer predisposition.

Main Methods:

• Single-molecule imaging techniques were employed to visualize BLM's activity on DNA.
• In vitro biochemical assays were used to assess the DNA end processing activity of BLM and its mutants.
• Cellular assays were performed to examine DNA repair foci formation (RAD51) and repair pathway choice (HR vs. SSA).

Main Results:

• BLM mediates the formation of large single-stranded DNA (ssDNA) loops during DSB end processing.
• A BLM mutant lacking the N-terminal domain (NTD) processed DNA ends but failed to form ssDNA loops.
• Cells with the NTD-mutant BLM showed defective RAD51 foci formation and favored synthesis-dependent strand annealing (SSA) over HR.

Conclusions:

• The N-terminal domain of BLM is critical for generating ssDNA loops during DSB processing.
• BLM-mediated ssDNA loop formation is essential for RAD51 filament assembly and HR-dependent repair.
• Defects in BLM's loop-forming activity impair HR and may contribute to the increased cancer risk in Bloom syndrome.