Identification of RecQL1 as a Holliday junction processing enzyme in human cell lines

Bloom helicase and DNA topoisomerase IIIalpha are involved in the dissolution of sister chromatids

Bloom's syndrome (BS) is an autosomal disorder characterized by predisposition to a wide variety of cancers. The gene product whose mutation leads to BS is the RecQ family helicase BLM, which forms a complex with DNA topoisomerase IIIalpha (Top3alpha). However, the physiological relevance of the interaction between BLM and Top3alpha within the cell remains unclear. We show here that Top3alpha depletion causes accumulation of cells in G2 phase, enlargement of nuclei, and chromosome gaps and breaks that occur at the same position in sister chromatids. The transition from metaphase to anaphase is also inhibited. All of these phenomena except cell lethality are suppressed by BLM gene disruption. Taken together with the biochemical properties of BLM and Top3alpha, these data indicate that BLM and Top3alpha execute the dissolution of sister chromatids.

Rqh1 blocks recombination between sister chromatids during double strand break repair, independent of its helicase activity

Many questions remain about the process of DNA double strand break (DSB) repair by homologous recombination (HR), particularly concerning the exact function played by individual proteins and the details of specific steps in this process. Some recent studies have shown that RecQ DNA helicases have a function in HR. We studied the role of the RecQ helicase Rqh1 with HR proteins in the repair of a DSB created at a unique site within the Schizosaccharomyces pombe genome. We found that DSBs in rqh1(+) cells, are predominantly repaired by interchromosomal gene conversion, with HR between sister chromatids [sister-chromatid conversion (SCC)], occurring less frequently. In Deltarqh1 cells, repair by SCC is favored, and gene conversion rates slow significantly. When we limited the potential for SCC in Deltarqh1 cells by reducing the length of the G2 phase of the cell cycle, DSB repair continued to be predominated by SCC, whereas it was essentially eliminated in wild-type cells. These data indicate that Rqh1 acts to regulate DSB repair by blocking SCC. Interestingly, we found that this role for Rqh1 is independent of its helicase activity. In the course of these studies, we also found nonhomologous end joining to be largely faithful in S. pombe, contrary to current belief. These findings provide insight into the regulation of DSB repair by RecQ helicases.