The BLM helicase is necessary for normal DNA double-strand break repair

Experiments with the supF20 mutagenesis system demonstrate that extracts from Bloom's syndrome (BS) cells are unable to use microhomology elements within the supF20 gene to restore supF function after the induction of a double-strand break (DSB). Additional experiments with the pUC18 mutagenesis system demonstrate that although the efficiency and fidelity of DSB repair by BS extracts are comparable with those of normal extracts when ligatable ends are present, a significant 5-fold increase in mutation rate with BS extracts is observed when terminal phosphates are removed from the DNA substrate that needs repair. Mutant plasmids recovered after DSB repair by BS extracts contain smaller deletions within the lacZalpha gene not commonly recovered from normal extracts. This work demonstrates that BS cells, lacking the BLM helicase, process DSBs differently than normal cells and strongly suggests a role for the BLM helicase in aligning microhomology elements during recombinational events in DSB repair.