Bloom's Syndrome Gene Product Research Articles

TFIP11 promotes replication fork reversal to preserve genome stability

Replication fork reversal, a critical protective mechanism against replication stress in higher eukaryotic cells, is orchestrated via a series of coordinated enzymatic reactions. The Bloom syndrome gene product, BLM, a member of the highly conserved RecQ helicase family, is implicated in this process, yet its precise regulation and role remain poorly understood. In this study, we demonstrate that the GCFC domain-containing protein TFIP11 forms a complex with the BLM helicase. TFIP11 exhibits a preference for binding to DNA substrates that mimic the structure generated at stalled replication forks. Loss of either TFIP11 or BLM leads to the accumulation of the other protein at stalled forks. This abnormal accumulation, in turn, impairs RAD51-mediated fork reversal and slowing, sensitizes cells to replication stress-inducing agents, and enhances chromosomal instability. These findings reveal a previously unidentified regulatory mechanism that modulates the activities of BLM and RAD51 at stalled forks, thereby impacting genome integrity.

Nanocarrier Composed of Magnetite Core Coated with Three Polymeric Shells Mediates LCS-1 Delivery for Synthetic Lethal Therapy of BLM-Defective Colorectal Cancer Cells.

Synthetic lethality is a molecular-targeted therapy for selective killing of cancer cells. We exploited a lethal interaction between superoxide dismutase1 inhibition and Bloom syndrome gene product (BLM) defect for the treatment of colorectal cancer (CRC) cells (HCT 116) with a customized lung cancer screen-1-loaded nanocarrier (LCS-1-NC). The drug LCS-1 has poor aqueous solubility. To overcome its limitations, a customized NC, composed of a magnetite core coated with three polymeric shells, namely, aminocellulose (AC), branched poly(amidoamine), and paraben-PEG, was developed for encapsulating LCS-1. Encapsulation efficiency and drug loading were found to be 74% and 8.2%, respectively. LCS-1-NC exhibited sustained release, with ∼85% of drug release in 24 h. Blank NC (0.5 mg/mL) exhibited cytocompatibility toward normal cells, mainly due to the AC layer. LCS-1-NC demonstrated high killing selectivity (104 times) toward BLM-deficient HCT 116 cells over BLM-proficient HCT 116 cells. Due to enhanced efficacy of the drug using NC, the sensitivity difference for BLM-deficient cells increased to 1.7 times in comparison to that with free LCS-1. LCS-1-NC induced persistent DNA damage and apoptosis, which demonstrates that LCS-1-NC effectively and preferentially killed BLM-deficient CRC cells. This is the first report on the development of a potential drug carrier to improve the therapeutic efficacy of LCS-1 for specific killing of CRC cells having BLM defects.

Accumulation and Phosphorylation of RecQ-Mediated Genome Instability Protein 1 (RMI1) at Serine 284 and Serine 292 during Mitosis.

Chromosome instability usually leads to tumorigenesis. Bloom syndrome (BS) is a genetic disease associated with chromosome instability. The BS gene product, BLM, has been reported to function in the spindle assembly checkpoint (SAC) to prevent chromosome instability. BTR complex, composed of BLM, topoisomerase IIIα (Topo IIIα), RMI1 (RecQ-mediated genome instability protein 1, BLAP75) and RMI2 (RecQ-mediated genome instability protein 2, BLAP18), is crucial for maintaining genome stability. Recent work has demonstrated that RMI2 also plays critical role in SAC. However, little is know about RMI1 regulation during the cell cycle. Here we present that RMI1 protein level does not change through G1, S and G2 phases, but significantly increases in M phase. Moreover, phosphorylation of RMI1 occurs in mitosis. Upon microtubule-disturbing agent, RMI1 is phosphorylated primarily at the sites of Serine 284 and Serine 292, which does not interfere with the formation of BTR complex. Additionally, this phosphorylation is partially reversed by roscovitine treatment, implying cycling-dependent kinase 1 (CDK1) might be one of the upstream kinases.

Scaffolding protein SPIDR/KIAA0146 connects the Bloom syndrome helicase with homologous recombination repair

The Bloom syndrome gene product, BLM, is a member of the highly conserved RecQ family. An emerging concept is the BLM helicase collaborates with the homologous recombination (HR) machinery to help avoid undesirable HR events and to achieve a high degree of fidelity during the HR reaction. However, exactly how such coordination occurs in vivo is poorly understood. Here, we identified a protein termed SPIDR (scaffolding protein involved in DNA repair) as the link between BLM and the HR machinery. SPIDR independently interacts with BLM and RAD51 and promotes the formation of a BLM/RAD51-containing complex of biological importance. Consistent with its role as a scaffolding protein for the assembly of BLM and RAD51 foci, cells depleted of SPIDR show increased rate of sister chromatid exchange and defects in HR. Moreover, SPIDR depletion leads to genome instability and causes hypersensitivity to DNA damaging agents. We propose that, through providing a scaffold for the cooperation of BLM and RAD51 in a multifunctional DNA-processing complex, SPIDR not only regulates the efficiency of HR, but also dictates the specific HR pathway.

PICH: A DNA Translocase Essential for Resolving Anaphase Bridged DNA

PICH (PLK1-Interacting Checkpoint Helicase) is a recently identified member of the RAD54 subgroup of SNF2 family proteins. PICH localizes to so-called ultra-fine anaphase bridges (UFBs) in mitosis alongside a multi-protein complex of DNA repair proteins including BLM, the Bloom's syndrome gene product. Very little is known about the function of PICH or how it is recruited selectively to UFBs. Nevertheless, depletion of PICH results in genomic instability, including an elevated frequency of sister chromatid exchanges, micronuclei and loss of heterozygosity. using a combination of microfluidics, single-molecule fluorescence microscopy and optical tweezers, we have defined the properties of PICH in an in vitro model of an anaphase bridge. We show that PICH binds with a remarkably high affinity to dsDNA and displays ATP-dependent dsDNA translocase activity. The application of stretching forces to the DNA, which mimics the effects of the mitotic spindle on a UFB, enhances the binding of PICH to dsDNA, but also serves to diminish stretching-induced DNA melting. Based on our findings, we suggest that PICH plays several roles in the development and processing of UFBs: (i) to recognize and bind to dsDNA exposed by the mitotic spindle force-induced unwrapping of nucleosomes, (ii) to help expel exclusively these unwrapped nucleosomes, (iii) to stabilize stretched dsDNA, and (iv) to recruit the DNA repair machinery required for UFB resolution in anaphase.

Developing T lymphocytes are uniquely sensitive to a lack of topoisomerase III alpha

All organisms possess at least one type IA DNA topoisomerase. These topoisomerases function as part of a DNA structure-specific "dissolvasome," also known as the RTR complex, which has critical functions in faithful DNA replication, recombination, and chromosome segregation. In humans, the heteromeric RTR complex consists of RMI1, RMI2, the Bloom's syndrome gene product (BLM), and topoisomerase 3A (TOP3A) proteins. Here, we describe the identification and characterization of two deleterious mutations in the zebrafish top3a gene that reveal an unexpected tissue-specific requirement of top3a function in developing thymocytes. Deficiency in top3a activates a p53-dependent check-point but does not affect VDJ recombination. Our results suggest that TOP3A could be a candidate gene involved in human primary immunodeficiency syndromes.

MPS1-dependent mitotic BLM phosphorylation is important for chromosome stability

Spindle assembly checkpoint (SAC) ensures bipolar attachment of chromosomes to the mitotic spindle and is essential for faithful chromosome segregation, thereby preventing chromosome instability (CIN). Genetic evidence suggests a causal link between compromised SAC, CIN, and cancer. Bloom syndrome (BS) is a genetic disorder that predisposes affected individuals to cancer. BS cells exhibit elevated rates of sister chromatid exchange, chromosome breaks, and CIN. The BS gene product, BLM, is a member of the RecQ helicases that are required for maintenance of genome stability. The BLM helicase interacts with proteins involved in DNA replication, recombination, and repair and is required for the repair of stalled-replication forks and in the DNA damage response. Here we present biochemical evidence to suggest a role of BLM phosphorylation during mitosis in maintaining chromosome stability. BLM is associated with the SAC kinase MPS1 and is phosphorylated at S144 in a MPS1-dependent manner. Phosphorylated BLM interacts with polo-like kinase 1, a mitotic kinase that binds to phosphoserine/threonine through its polo-box domain (PBD). Furthermore, BS cells expressing BLM-S144A show normal levels of sister chromatid exchange but fail to maintain the mitotic arrest when SAC is activated and exhibit a broad distribution of chromosome numbers. We propose that MPS1-dependent BLM phosphorylation is important for ensuring accurate chromosome segregation, and its deregulation may contribute to cancer.

Physical and functional interaction between the Bloom's syndrome gene product and the largest subunit of chromatin assembly factor 1.

Bloom's syndrome (BS) is a genomic instability disorder characterized by cancer susceptibility. The protein defective in BS, BLM, belongs to the RecQ family of DNA helicases. In this study, we found that BLM interacts with hp150, the largest subunit of chromatin assembly factor 1 (CAF-1), in vitro and in vivo. Colocalization of a proportion of the cellular complement of these two proteins is found at specific nuclear foci coinciding with sites of DNA synthesis in the S phase. This colocalization increases in the presence of agents that damage DNA or inhibit DNA replication. In support of a functional interaction between BLM and CAF-1, we show that BLM inhibits CAF-1-mediated chromatin assembly during DNA repair in vitro. Although CAF-1 activity is not altered in BLM-deficient cells, the absence of BLM does impair the ability of CAF-1 to be mobilized within the nucleus in response to hydroxyurea treatment. Our results provide the first link between BLM and chromatin assembly coupled to DNA repair and suggest that BLM and CAF-1 function in a coordinated way to promote survival in response to DNA damage and/or replication blockade.

The mismatch DNA repair heterodimer, hMSH2/6, regulates BLM helicase.

The human MSH2/6 complex is essential for mismatch recognition during the repair of replication errors. Although mismatch repair components have been implicated in DNA homologous recombination repair, the exact function of hMSH2/6 in this pathway is unclear. Here, we show that the recombinant hMSH2/6 protein complex stimulated the ability of the Bloom's syndrome gene product, BLM, to process Holliday junctions in vitro, an activity that could also be regulated by p53. Consistent with these observations, hMSH6 colocalized with BLM and phospho-ser15-p53 in hydroxyurea-induced RAD51 nuclear foci that may correspond to the sites of presumed stalled DNA replication forks and more likely the resultant DNA double-stranded breaks. In addition, we show that hMSH2 and hMSH6 coimmunoprecipitated with BLM, p53, and RAD51. Both the number of RAD51 foci and the amount of the BLM-p53-RAD51 complex are increased in hMSH2- or hMSH6-deficient cells. These data suggest that hMSH2/6 formed a complex with BLM-p53-RAD51 in response to the damaged DNA forks during double-stranded break repair.

Stimulation of Flap Endonuclease-1 by the Bloom's Syndrome Protein

Bloom's syndrome (BS) is a rare autosomal recessive genetic disorder associated with genomic instability and an elevated risk of cancer. Cellular features of BS include an accumulation of abnormal replication intermediates and increased sister chromatid exchange. Although it has been suggested that the underlying defect responsible for hyper-recombination in BS cells is a temporal delay in the maturation of DNA replication intermediates, the precise role of the BS gene product, BLM, in DNA metabolism remains elusive. We report here a novel interaction of the BLM protein with the human 5'-flap endonuclease/5'-3' exonuclease (FEN-1), a genome stability factor involved in Okazaki fragment processing and DNA repair. BLM protein stimulates both the endonucleolytic and exonucleolytic cleavage activity of FEN-1 and this functional interaction is independent of BLM catalytic activity. BLM and FEN-1 are associated with each other in human nuclei as shown by their reciprocal co-immunoprecipitation from HeLa nuclear extracts. The BLM-FEN-1 physical interaction is mediated through a region of the BLM C-terminal domain that shares homology with the FEN-1 interaction domain of the Werner syndrome protein, a RecQ helicase family member homologous to BLM. This study provides the first evidence for a direct interaction of BLM with a human nucleolytic enzyme. We suggest that functional interactions between RecQ helicases and Rad2 family nucleases serve to process DNA substrates that are intermediates in DNA replication and repair.

Functional Interaction between the Bloom's Syndrome Helicase and the RAD51 Paralog, RAD51L3 (RAD51D)

Bloom's syndrome (BS) is a genetic disorder associated with short stature, fertility defects, and a predisposition to the development of cancer. BS cells are characterized by genomic instability; in particular, a high rate of reciprocal exchanges between sister-chromatids and homologous chromosomes. The BS gene product, BLM, is a helicase belonging to the highly conserved RecQ family. BLM is known to form a complex with the RAD51 recombinase, and to act upon DNA intermediates that form during homologous recombination, including D-loops and Holliday junctions. Here, we show that BLM also makes a direct physical association with the RAD51L3 protein (also known as RAD51D), a so-called RAD51 paralog that shows limited sequence similarity to RAD51 itself. This interaction is mediated through the N-terminal domain of BLM. To analyze functional interactions between BLM and RAD51L3, we have purified a heteromeric complex comprising RAD51L3 and a second RAD51 paralog, XRCC2. We show that the RAD51L3-XRCC2 complex stimulates BLM to disrupt synthetic 4-way junctions that model the Holliday junction. We also show that a truncated form of BLM, which retains helicase activity but is unable to bind RAD51L3, is not stimulated by the RAD51L3-XRCC2 complex. Our data indicate that the activity of BLM is modulated through an interaction with the RAD51L3-XRCC2 complex, and that this stimulatory effect on BLM is dependent upon a direct physical association between the BLM and RAD51L3 proteins. We propose that BLM co-operates with RAD51 paralogs during the late stages of homologous recombination processes that serve to restore productive DNA replication at sites of damaged or stalled replication forks.

Interaction between the C-Terminal Region of the Bloom's Syndrome Gene Product, BLM, and RAD51

Interaction between the C-Terminal Region of the Bloom's Syndrome Gene Product, BLM, and RAD51

The Bloom's syndrome helicase interacts directly with the human DNA mismatch repair protein hMSH6.

Bloom's syndrome (BS) is a rare genetic disorder characterised by genome instability and cancer susceptibility. BLM, the BS gene product, belongs to the highly-conserved RecQ family of DNA helicases. Although the exact function of BLM in human cells remains to be defined, it seems likely that BLM eliminates some form of homologous recombination (HR) intermediate that arises during DNA replication. Similarly, the mismatch repair (MMR) system also plays a crucial role in the maintenance of genomic stability, by correcting DNA errors generated during DNA replication. Recent evidence implicates components of the MMR system also in HR repair. We now show that hMSH6, a component of the heterodimeric mismatch recognition complex hMSH2/hMSH6 (hMutS(alpha)), interacts with the BLM protein both in vivo and in vitro. In agreement with these findings, BLM and hMSH6 co-localise to discrete nuclear foci following exposure of the cells to ionising radiation. However, the purified recombinant MutS(alpha) complex does not affect the helicase activity of BLM in vitro. As BLM has previously been shown to interact with the hMLH1 component of the hMLH1/hPMS2 (hMutL(alpha)) heterodimeric MMR complex, our present findings further strengthen the link between BLM and processes involving correction of DNA mismatches, such as in the regulation of the fidelity of homologous recombination events.

The Bloom's syndrome helicase stimulates the activity of human topoisomerase IIIalpha.

Bloom's syndrome (BS) is a disorder associated with chromosomal instability and a predisposition to the development of cancer. The BS gene product, BLM, is a DNA helicase of the RecQ family that forms a complex in vitro and in vivo with topoisomerase IIIalpha. Here, we show that BLM stimulates the ability of topoisomerase IIIalpha to relax negatively supercoiled DNA. Moreover, DNA binding analyses indicate that BLM recruits topoisomerase IIIalpha to its DNA substrate. Consistent with this, a mutant form of BLM that retains helicase activity, but is unable to bind topoisomerase IIIalpha, fails to stimulate topoisomerase activity. These results indicate that a physical association between BLM and topoisomerase IIIalpha is a prerequisite for their functional biochemical interaction.

Direct association of Bloom's syndrome gene product with the human mismatch repair protein MLH1.

Bloom's syndrome (BS) is a rare genetic disorder characterised by genomic instability and cancer susceptibility. BLM, the gene mutated in BS, encodes a member of the RecQ family of DNA helicases. Here, we identify hMLH1, which is involved in mismatch repair (MMR) and recombination, as a protein that directly interacts with BLM both in vivo and in vitro, and that the two proteins co-localise to discrete nuclear foci. The interaction between BLM and hMLH1 appears to have been evolutionarily conserved, as Sgs1p, the Saccharomyces cerevisiae homologue of BLM, interacts with yeast Mlh1p. However, cell extracts derived from BS patients show no obvious defects in MMR compared to wild-type- and BLM-complemented BS cell extracts. We conclude that the hMLH1-BLM interaction is not essential for post-replicative MMR, but, more likely, is required for some aspect of genetic recombination.

Cleavage of the Bloom's syndrome gene product during apoptosis by caspase-3 results in an impaired interaction with topoisomerase IIIalpha.

In higher eukaryotes, the integration of signals triggered in response to certain types of stress can result in programmed cell death. Central to these events is the sequential activation of a cascade of proteinases known as caspases. The final activated effector caspases of this cascade digest a number of cellular proteins, in some cases increasing their enzymatic activity, in others destroying their function. Of the proteins shown to be targets for caspase-mediated proteolysis, a surprisingly large proportion are proteins involved in the signalling or repair of DNA damage. Here we investigate whether BLM, the product of the gene mutated in Bloom's syndrome, a human autosomal disease characterised by cancer predisposition and sunlight sensitivity, is cleaved during apoptosis. BLM interacts with topoisomerase IIIalpha and has been proposed to play an important role in maintaining genomic integrity through its roles in DNA repair and replication. We show that BLM is cleaved during apoptosis by caspase-3 and reveal that the main cleavage site is located at the junction between the N-terminal and central helicase domains of BLM. Proteolytic cleavage by caspase-3 produces a 120 kDa fragment, which contains the intact helicase domain and three smaller fragments, the relative amounts of which depend on time of incubation with caspase-3. The 120 kDa fragment retains the helicase activity of the intact BLM protein. However, its interaction with topoisomerase IIIalpha is severely impaired. Since the BLM-topoisomerase interaction is believed to be necessary for many of the replication and recombination functions of BLM, we suggest that caspase-3 cleavage of BLM could alter the localisation and/or function of BLM and that these changes may be important in the process of apoptosis.

The distribution and expression of the Bloom's syndrome gene product in normal and neoplastic human cells.

Bloom's syndrome (BS) is an autosomal recessive disorder associated with a predisposition to cancers of all types. Cells from BS sufferers display extreme genomic instability. The BS gene product, BLM, is a 159 kDa DNA helicase enzyme belonging to the RecQ family. Here, we have analysed the distribution of BLM in normal and tumour tissues from humans using a recently characterized, specific monoclonal antibody. BLM was found to be localized to nuclei in normal lymphoid tissues, but was largely absent from other normal tissues analysed with the exception of the proliferating compartment of certain tissues. In contrast, expression of BLM was observed in a variety of tumours of both lymphoid and epithelial origin. A strong correlation was observed between expression of BLM and the proliferative status of cells, as determined by staining for markers of cell proliferation (PCNA and Ki67). We conclude that BLM is a proliferation marker in normal and neoplastic cells in vivo, and, as a consequence, is expressed at a higher level in tumours than in normal quiescent tissues. © 2001 Cancer Research Campaign http://www.bjcancer.com

Covalent Modification of the Werner's Syndrome Gene Product with the Ubiquitin-related Protein, SUMO-1

Werner's syndrome is a potential model of accelerated human aging. The gene responsible for Werner's syndrome encodes a protein that has a helicase domain homologous to Escherichia coli RecQ. To identify binding partners that regulate the function in concert with Wrn, we screened for proteins using the yeast two-hybrid system with mouse Wrn as bait and found three. One was a novel protein, and the other two were mouse Ubc9 and SUMO-1. Ubc9 also interacted with the mouse homologue of the Bloom's syndrome gene product, another eukaryotic RecQ-type helicase, but not mouse DNA helicase Q1/RecQL (RecQL1). Deletion experiments indicated that both proteins interacted with the N-terminal segment of Wrn (amino acid 272-514). The interaction between Wrn and SUMO-1 was weaker than that between Wrn and Ubc9. Positive interaction was observed in the heterogeneous combination of Wrn and yeast Ubc9 (yUbc9), as well as yUbc9 and SUMO-1, in the two-hybrid system. The interaction between yUbc9 and SUMO-1 was abolished by deleting the C-terminal Gly residue of SUMO-1, which is reportedly required for the formation of Ubc9-SUMO-1 thioester linkage. The interaction of Wrn and SUMO-1 was also abolished by deleting the Gly residue, indicating that the interaction of Wrn and SUMO-1 is mediated by yUbc9 in the two-hybrid system. Finally, we confirmed by immunoblotting with an anti-SUMO-1 antibody that Wrn was covalently attached with SUMO-1.

The Bloom's syndrome gene product promotes branch migration of holliday junctions.

Bloom's syndrome (BS) is an autosomal recessive disorder associated with dwarfism, immunodeficiency, reduced fertility, and elevated levels of many types of cancer. BS cells show marked genomic instability; in particular, hyperrecombination between sister chromatids and homologous chromosomes. This instability is thought to result from defective processing of DNA replication intermediates. The gene mutated in BS, BLM, encodes a member of the RecQ family of DExH box DNA helicases, which also includes the Werner's syndrome gene product. We have investigated the mechanism by which BLM suppresses hyperrecombination. Here, we show that BLM selectively binds Holliday junctions in vitro and acts on recombination intermediates containing a Holliday junction to promote ATP-dependent branch migration. We present a model in which BLM disrupts potentially recombinogenic molecules that arise at sites of stalled replication forks. Our results have implications for the role of BLM as an anti-recombinase in the suppression of tumorigenesis.

The Bloom's Syndrome Gene Product Interacts with Topoisomerase III

Bloom's syndrome is a rare genetic disorder associated with loss of genomic integrity and a large increase in the incidence of many types of cancer at an early age. The Bloom's syndrome gene product, BLM, belongs to the RecQ family of DNA helicases, which also includes the human Werner's and Rothmund-Thomson syndrome gene products and the Sgs1 protein of Saccharomyces cerevisiae. This family shows strong evolutionary conservation of protein structure and function. Previous studies have shown that Sgs1p interacts both physically and genetically with topoisomerase III. Here, we have investigated whether this interaction has been conserved in human cells. We show that BLM and hTOPO IIIalpha, one of two human topoisomerase III homologues, co-localize in the nucleus of human cells and can be co-immunoprecipitated from human cell extracts. Moreover, the purified BLM and hTOPO IIIalpha proteins are able to bind specifically to each other in vitro, indicating that the interaction is direct. We have mapped two independent domains on BLM that are important for mediating the interaction with hTOPO IIIalpha. Furthermore, through characterizing a genetic interaction between BLM and TOP3 in S. cerevisiae, we have identified a functional role for the hTOPO IIIalpha interaction domains in BLM.