The Bloom's syndrome (BS) gene, BLM, plays an important role in the maintenance of genomic stability in somatic cells. A candidate for BLM was identified by direct selection of a cDNA derived from a 250 kb segment of the genome to which BLM had been assigned by somatic crossover point mapping. In this novel mapping method, cells were used from persons with BS that had undergone intragenic recombination within BLM. cDNA analysis of the candidate gene identified a 4437 bp cDNA that encodes a 1417 amino acid peptide with homology to the RecQ helicases, a subfamily of DExH box-containing DNA and RNA helicases. The presence of chain-terminating mutations in the candidate gene in persons with BS proved that it was BLM.

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Chromosomal double-strand breaks (DSBs) in mammalian cells are repaired by either homology-directed repair (HDR), using a homologous sequence as a repair template, or nonhomologous end-joining (NHEJ), which often involves sequence alterations at the DSB site. To characterize the interrelationship of these two pathways, we analyzed HDR of a DSB in cells deficient for NHEJ components. We find that the HDR frequency is enhanced in Ku70 Double-strand breaks (DSBs) are potentially catastrophic lesions that if not repaired will lead to loss of genetic information and mutagenesis or cell death. In mammalian cells, two major pathways exist to repair DSBshomologous recombination and nonhomologous endjoining (NHEJ;Liang et al. 1998). NHEJ, the rejoining of DNA ends with the use of little or no sequence homology, involves the processing of ends such that nucleotides are often deleted or inserted at the break site prior to ligation (Jeggo 1998). Such modifications are likely central to the ability of mammalian cells to rejoin DNA ends with a variety of structures. Homology-directed repair (HDR) of a DSB, in contrast, requires significant lengths of sequence homology so that a DNA end from one molecule can invade a homologous sequence and prime repair synthesis (Pâques and Haber 1999). Processing of DNA ends also occurs with HDR; however, repair is typically precise, because a homologous sequence templates the repair event.Several processes exist in which repair of a DSB is restricted to either NHEJ or HDR. For example, DSBs introduced by the RAG proteins to generate antigen receptor diversity during V(D)J rearrangement are repaired by the NHEJ pathway (Jeggo 1998), whereas those introduced during meiosis by the Spo11 protein are repaired by the HDR pathway (Keeney 2001). The restriction in type of DSB repair raises the question as to how pathway choice is regulated. Several studies point to cell cycle phase as one factor that modulates repair pathway choice. In chicken cells, HDR has been found to play a dominant role in repairing radiation-induced DSBs in late S/G 2 phase, whereas NHEJ is preferentially used during G 1 /early S phase (Takata et al. 1998). Consistent with this in mammalian cells, the preferred homologous template for HDR-the sister chromatid (Johnson and Jasin 2001)-is present only during the S/G 2 phase of the cell cycle. Despite a cell cycle preference, HDR and NHEJ can nevertheless be coupled for the repair of a single DSB in mammalian cells (Richardson and Jasin 2000), indicating that the two repair pathways are not completely restricted to different cell cycle phases and that other factors influence pathway choice.Based on in vitro studies of DNA end-binding proteins such as RAD52, it has been suggested that end-binding proteins may direct entry into alternative DSB repair pathways (Van Dyck et al. 1999). However, no evidence yet exists in vivo to support this model, and mutation of the Rad52 gene in mouse does not confer a cellular DSB repair phenotype (Rijkers et al. 1998). NHEJ proteins, that is, th...

Bloom syndrome (BS) is an autosomal recessive disorder characterized by a high incidence of cancer and genomic instability. BLM, the protein defective in BS, is a RecQ-like helicase, presumed to function in DNA replication, recombination, or repair. BLM localizes to promyelocytic leukemia protein (PML) nuclear bodies and is expressed during late S and G2. We show, in normal human cells, that the recombination/repair proteins hRAD51 and replication protein (RP)-A assembled with BLM into a fraction of PML bodies during late S/G2. Biochemical experiments suggested that BLM resides in a nuclear matrix–bound complex in which association with hRAD51 may be direct. DNA-damaging agents that cause double strand breaks and a G2 delay induced BLM by a p53- and ataxia-telangiectasia mutated independent mechanism. This induction depended on the G2 delay, because it failed to occur when G2 was prevented or bypassed. It coincided with the appearance of foci containing BLM, PML, hRAD51 and RP-A, which resembled ionizing radiation-induced foci. After radiation, foci containing BLM and PML formed at sites of single-stranded DNA and presumptive repair in normal cells, but not in cells with defective PML. Our findings suggest that BLM is part of a dynamic nuclear matrix–based complex that requires PML and functions during G2 in undamaged cells and recombinational repair after DNA damage.

MMRpro is a broadly applicable, accurate prediction model that can contribute to current screening and genetic counseling practices in a high-risk population. It is more sensitive and more specific than existing clinical guidelines for identifying individuals who may benefit from MMR germline testing. It is applicable to individuals for whom tumor samples are not available and to individuals in whom germline testing finds no mutation.