Functional link between BLM defective in Bloom's syndrome and the ataxia-telangiectasia-mutated protein, ATM.
(41/188)
Chromosome aberrations, genomic instability, and cancer predisposition are hallmarks of a number of syndromes in which the defective genes recognize and/or repair DNA damage or are involved in some aspect of DNA processing. We report here direct interaction between BLM, mutated in Bloom's Syndrome (BS), and ATM, mutated is ataxia-telangiectasia, and we have mapped the sites of interaction. Full-length BLM cDNA corrected sister chromatid exchange (SCE) and radiosensitivity in BS cells. Mitotic phosphorylation of BLM was partially dependent on ATM, and phosphorylation sites on BLM were identified. A phosphospecific antibody against one of these sites (Thr-99) revealed radiation-induced phosphorylation, which was defective in ataxia-telangiectasia cells. Stable cell lines expressing phosphorylation site mutants failed to correct radiosensitivity in BS cells but corrected SCE. These mutants also sensitized normal control cells to radiation and increased radiation-induced chromosome aberrations but did not cause SCE numbers to increase. These data suggest that ATM and BLM function together in recognizing abnormal DNA structures by direct interaction and that these phosphorylation sites in BLM are important for radiosensitivity status but not for SCE frequency. (+info)
Enhanced tumor formation in mice heterozygous for Blm mutation.
(42/188)
Persons with the autosomal recessive disorder Bloom syndrome are predisposed to cancers of many types due to loss-of-function mutations in the BLM gene, which encodes a recQ-like helicase. Here we show that mice heterozygous for a targeted null mutation of Blm, the murine homolog of BLM, develop lymphoma earlier than wild-type littermates in response to challenge with murine leukemia virus and develop twice the number of intestinal tumors when crossed with mice carrying a mutation in the Apc tumor suppressor. These observations indicate that Blm is a modifier of tumor formation in the mouse and that Blm haploinsufficiency is associated with tumor predisposition, a finding with important implications for cancer risk in humans. (+info)
Cleavage of BLM and sensitivity of Bloom's syndrome cells to hydroxurea and UV-C radiation.
(43/188)
Patients with Bloom's syndrome (BS) show a strong genetic instability and a predisposition to all types of cancer. Here, we report that the Bloom's syndrome protein (BLM) is cleaved in response to hydroxyurea (HU)- or UVC-induced apoptosis. The appearance and solubility of BLM proteolytic products differed according to whether proteolysis occurred in response to HU or UVC. One BS cell line homozygous for a null mutation in BLM was resistant to both UVC- and HU-induced apoptosis, while another one expressing a mutated BLM protein was resistant to HU-induced apoptosis but displayed normal sensitivity to UVC. Thus, UVC and HU appear to induce apoptosis through distinct pathways. (+info)
The Bloom syndrome helicase BLM interacts with TRF2 in ALT cells and promotes telomeric DNA synthesis.
(44/188)
Telomerase-negative immortalized human cells maintain telomeres by alternative lengthening of telomeres (ALT) pathway(s), which may involve homologous recombination. We find that endogenous BLM protein co-localizes with telomeric foci in ALT human cells but not telomerase positive immortal cell lines or primary cells. BLM interacts in vivo with the telomeric protein TRF2 in ALT cells, as detected by FRET and co-immunoprecipitation. Transient over-expression of green fluorescent protein (GFP)-BLM results in marked, ALT cell-specific increases in telomeric DNA. The association of BLM with telomeres and its effect on telomere DNA synthesis require a functional helicase domain. Our results identify BLM as the first protein found to affect telomeric DNA synthesis exclusively in human ALT cells and suggest that BLM facilitates recombination-driven amplification of telomeres in ALT cells. (+info)
BLM helicase-dependent transport of p53 to sites of stalled DNA replication forks modulates homologous recombination.
(45/188)
Diverse functions, including DNA replication, recombination and repair, occur during S phase of the eukaryotic cell cycle. It has been proposed that p53 and BLM help regulate these functions. We show that p53 and BLM accumulated after hydroxyurea (HU) treatment, and physically associated and co-localized with each other and with RAD51 at sites of stalled DNA replication forks. HU-induced relocalization of BLM to RAD51 foci was p53 independent. However, BLM was required for efficient localization of either wild-type or mutated (Ser15Ala) p53 to these foci and for physical association of p53 with RAD51. Loss of BLM and p53 function synergistically enhanced homologous recombination frequency, indicating that they mediated the process by complementary pathways. Loss of p53 further enhanced the rate of spontaneous sister chromatid exchange (SCE) in Bloom syndrome (BS) cells, but not in their BLM-corrected counterpart, indicating that involvement of p53 in regulating spontaneous SCE is BLM dependent. These results indicate that p53 and BLM functionally interact during resolution of stalled DNA replication forks and provide insight into the mechanism of genomic fidelity maintenance by these nuclear proteins. (+info)
Heterozygosity for the BLM(Ash) mutation and cancer risk.
(46/188)
Bloom syndrome is an autosomal recessive disorder whose characteristics include an increased risk for many types of cancers. In contrast to the homozygous mutations of Bloom syndrome, heterozygous carriers of BLM mutations may be at increased risk for developing colorectal cancer. We have screened 2,333 Jewish individuals, including 497 individuals with colorectal cancer, 125 with adenomatous polyps, 767 with noncolorectal cancers and 944 controls for the truncating BLM(Ash) founder mutation. The BLM(Ash) mutation was carried by 0.80% of individuals with colorectal neoplasia, 0.87% of those with any type of cancer and 0.85% of controls. In addition to case-control data, we found no evidence to support a significant relationship between increased cancer risk and heterozygous BLM(Ash) mutations with respect to age of cancer diagnosis, tumor multiplicity or family cancer history. (+info)
A multiprotein nuclear complex connects Fanconi anemia and Bloom syndrome.
(47/188)
Bloom syndrome (BS) is a genetic disorder associated with dwarfism, immunodeficiency, reduced fertility, and an elevated risk of cancer. To investigate the mechanism of this disease, we isolated from human HeLa extracts three complexes containing the helicase defective in BS, BLM. Interestingly, one of the complexes, termed BRAFT, also contains five of the Fanconi anemia (FA) complementation group proteins (FA proteins). FA resembles BS in genomic instability and cancer predisposition, but most of its gene products have no known biochemical activity, and the molecular pathogenesis of the disease is poorly understood. BRAFT displays a DNA-unwinding activity, which requires the presence of BLM because complexes isolated from BLM-deficient cells lack such an activity. The complex also contains topoisomerase IIIalpha and replication protein A, proteins that are known to interact with BLM and could facilitate unwinding of DNA. We show that BLM complexes isolated from an FA cell line have a lower molecular mass. Our study provides the first biochemical characterization of a multiprotein FA complex and suggests a connection between the BLM and FA pathways of genomic maintenance. The findings that FA proteins are part of a DNA-unwinding complex imply that FA proteins may participate in DNA repair. (+info)
The human Bloom syndrome gene suppresses the DNA replication and repair defects of yeast dna2 mutants.
(48/188)
Bloom syndrome is a disorder of profound and early cancer predisposition in which cells become hypermutable, exhibit high frequency of sister chromatid exchanges, and show increased micronuclei. BLM, the gene mutated in Bloom syndrome, has been cloned previously, and the BLM protein is a member of the RecQ family of DNA helicases. Many lines of evidence suggest that BLM is involved either directly in DNA replication or in surveillance during DNA replication, but its specific roles remain unknown. Here we show that hBLM can suppress both the temperature-sensitive growth defect and the DNA damage sensitivity of the yeast DNA replication mutant dna2-1. The dna2-1 mutant is defective in a helicase-nuclease that is required either to coordinate with the crucial Saccharomyces cerevisiae (sc) FEN1 nuclease in Okazaki fragment maturation or to compensate for scFEN1 when its activity is impaired. We show that human BLM interacts with both scDna2 and scFEN1 by using coimmunoprecipitation from yeast extracts, suggesting that human BLM participates in the same steps of DNA replication or repair as scFEN1 and scDna2. (+info)