Molecular mechanism of the high incidence of cancer of Bloom's syndrome

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Bloom's syndrome (BS) is a rare genetic disorder and the cells from BS patients show genomic instability and an increased level of sister chromatid exchange (SCE). We analyzed the functions of yeast Sgs 1, which is the yeast homologue for BLM, as well as those of BLM in higher eukaryotic cells and obtained the following results.1. We made mutated SGS1 genes coding a protein having equivalent missense mutations of BS patients and found that none of the mutated genes could suppress the higher sensitivity to MMS and HU and the elevated SCE of sgs 1 disruptants. Analyses of Sgs 1 functions indicated that the two different mechanisms are operated in Sgs 1 functions, one requiring DNA helicase activity and one not. In addition, genetic analyses indicated that Sgs 1 functions in the down stream of Mec 1 and the upstream of Rad 51.2. We generated a monoclonal antibody specifically recognizing BLM protein and using this antibody, found that BLM is localized in speckled structures in the nucleus. We also obtained a piece of evidence indicating that BLM is associated with SUMO-1. Deletion experiments indicated that the amino acid region 238-586 of BLM is required for the localization in speckled structures.3. We generated BLM^<-/-> and BLM^<-/->/RAD54^<-/-> DT40 cells from the chicken B lymphocyte line DT40 and found that SCE and targeted integration frequencies were increased remarkably in BLM^<-/-> cells. The results obtained with BLM^<-/->/RAD54^<-/-> cells indicated that a large portion of the SCE in BLM^<-/-> cells occurs via homologous recombination and more double strand breaks occur during DNA replication in the absence of BLM and some of these breaks are repaired by homologous recombination.