Interactions with topoisomerase IIalpha enhance the unwinding activity of the BLM helicase on recombination substrates and are necessary for preventing chromosome breakage

The BLM helicase is a member of the RecQ-like family of 3'-5' ATP- and Mg2+-dependent helicases. Cells deficient in BLM are characterized by increased sister chromatid exchanges, quadriradial structures and chromosome breaks, chromosome structures that suggest the disruption of normal mechanisms that resolve recombination intermediates and maintain chromosome stability. Previously identified interactions between yeast RecQ-like helicases and topoisomerases suggest cooperation in DNA transactions such as recombination repair and chromosome segregation. This work demonstrates that human BLM and topoisomerase IIalpha interact directly. BLM and topoisomerase IIalpha co-immunoprecipitate from human cells and colocalize in a cell cycle-specific manner. Their association and co-localization increases in S- and G2/M- and is predominant in M-phase. In vitro binding assays demonstrate that amino acids 489-587 within the N-terminus of BLM are required for this interaction. In vitro biochemical analysis of the decatenation activity of topoisomerase IIalpha in the presence of BLM revealed no effect by BLM. On the other hand, BLM helicase activity is enhanced approximately three- and five-fold by topoisomerase IIalpha on a 3' overhang duplex and bubble substrates, respectively, but not an X-junction substrate. These data suggest that BLM and topoisomerase IIalpha may be involved in the processing of early homologous replication intermediates, but not structures that form later. In vivo, siRNA mediated knock-down of either BLM, topoisomerase IIalpha, or both in combination increased chromosome breakage to the same extent compared to a scrambled control knock-down, suggesting a common pathway that requires BLM and topoisomerase IIalpha for protecting the integrity of genomic DNA.;In order to determine the importance of the BLM-topoisomerase IIalpha interaction, a mutant BLM protein lacking the topoisomerase IIalpha interaction domain (aa 489-587) was generated, EGFP-BLMDelta489-587. The mutant protein was unable to co-immunoprecipitate topoisomerase IIalpha, although it still localized to nuclear PML foci and the nucleolus as wild-type BLM. To determine the significance of their interaction on chromosome breakage, the ability of this mutant to reduce the high endogenous levels of gammaH2AX foci of BS cells was tested. Transfection BS cells line with the EGFP-BLM Delta489-587 mutant protein did not reduce gammaH2AX foci formation to the levels of transfection with a wild-type BLM protein, but exacerbated the number of gammaH2AX foci in cells compared to untransfected cells. We propose that in the presence of BLM, cells commit to a BLM dependent pathway of double strand break repair and the inability of EGFP-BLM Delta489-587 mutant to bind topoisomerase IIalpha locks the cells in such pathways without being able to correctly finish the repair. Taken together, these data suggest an involvement of BLM and topoisomerase IIalpha in a pathway that prevents chromosome breakage. The in vitro biochemical data suggest that this pathway may involve the regulation of homologous recombination through the processing of DNA intermediates occurring at early stages of repair.