Regulation of intrachromosomal homologous recombination in human cells by BLM helicase

Mutation of BLM helicase causes Blooms syndrome, an autosomal recessive disorder associated with genome instability and predisposition to cancer. In vitro studies of BLM have suggested that BLM may modulate homologous recombination (HR). We studied the influence of BLM on intrachromosomal HR events by stably transfecting normal human fibroblast cell line GM637 with pLB4 which contains a thymidine kinase (tk)‐neo fusion gene disrupted by insertion of the 18 bp recognition site for endonuclease I‐SceI. Also contained on pLB4 is a functional tk gene. A double‐strand break (DSB) was introduced within the tk‐neo gene by expression of I‐SceI endonuclease. DSB repair events were recovered by selection for cells that gained resistance to G418 either via HR or by nonhomologous end‐joining (NHEJ). When DSB repair was examined following transfection of siRNA directed against BLM expression, 89 out of 123 HR events recovered had resolved as crossovers. In controls, 43 out of 123 HR events were crossovers. This difference was highly statistically significant (p=5.8 X 10 −9 by a Fisher exact test). Our data also suggested that a greater portion of DSB repair occurred by HR (versus NHEJ) when BLM expression was knocked down. Our work provides the first direct evidence that BLM deficiency may contribute to genome instability by promoting the resolution of bona fide HR events as crossovers. Supported by NIH grant R01GM081472 to ASW.