Intrachromosomal recombination between highly diverged DNA sequences is enabled in human cells deficient in Bloom helicase

Mutation of Bloom helicase (BLM) causes Bloom syndrome (BS), a rare human genetic disorder associated with genome instability, elevation of sister chromatid exchanges, and predisposition to cancer. Deficiency in BLM homologs in Drosophila and yeast brings about significantly increased rates of recombination between imperfectly matched sequences (“homeologous recombination,” or HeR). To assess whether BLM deficiency provokes an increase in HeR in human cells, we transfected an HeR substrate into a BLM-null cell line derived from a BS patient. The substrate contained a thymidine kinase (tk)-neo fusion gene disrupted by the recognition site for endonuclease I-SceI, as well as a functional tk gene to serve as a potential recombination partner for the tk-neo gene. The two tk sequences on the substrate displayed 19% divergence. A double-strand break was introduced by expression of I-SceI and repair events were recovered by selection for G418-resistant clones. Among 181 events recovered, 30 were accomplished via HeR with the balance accomplished by nonhomologous end-joining. The frequency of HeR events in the BS cells was elevated significantly compared to that seen in normal human fibroblasts or in BS cells complemented for BLM expression. We conclude that BLM deficiency enables HeR in human cells.