Unwinding protein complexes in ALTernative telomere maintenance

Telomeres are composed of specialized chromatin that includes DNA repair/recombination proteins, telomere DNA‐binding proteins and a number of three dimensional nucleic acid structures including G‐quartets and D‐loops. A number of studies suggest that the BLM and WRN recQ‐like helicases play important roles in recombination‐mediated mechanisms of telomere elongation or A lternative L engthening of T elomeres (ALT), processes that maintain/elongate telomeres in the absence of telomerase. BLM and WRN localize within ALT‐associated nuclear bodies in telomerase‐negative immortalized cell lines and interact with the telomere‐specific proteins POT1, TRF1 and TRF2. Helicase activity is modulated by these interactions. BLM functions in DNA double‐strand break repair processes such as non‐homologous end joining, homologous recombination‐mediated repair, resolution of stalled replication forks and synthesis‐dependent strand annealing, although its precise functions at the telomeres are speculative. WRN also functions in DNA replication, recombination and repair, and in addition to its helicase domain, includes an exonuclease domain not found in other recQ‐like helicases. The biochemical properties of BLM and WRN are, therefore, important in biological processes other than DNA replication, recombination and repair. In this review, we discuss some previous and recent findings of human rec‐Q‐like helicases and their role in telomere elongation during ALT processes. J. Cell. Biochem. 109: 7–15, 2010. © 2009 Wiley‐Liss, Inc.     

Telomere-binding protein TRF2 binds to and stimulates the Werner and Bloom syndrome helicases

Werner syndrome is a human premature aging disorder displaying cellular defects associated with telomere maintenance including genomic instability, premature senescence, and accelerated telomere erosion. The yeast homologue of the Werner protein (WRN), Sgs1, is required for recombination-mediated lengthening of telomeres in telomerase-deficient cells. In human cells, we report that WRN co-localizes and physically interacts with the critical telomere maintenance protein TRF2. This interaction is mediated by the RecQ conserved C-terminal region of WRN. In vitro, TRF2 demonstrates high affinity for WRN and for another RecQ family member, the Bloom syndrome protein (BLM). TRF2 interaction with either WRN or BLM results in a notable stimulation of their helicase activities. Furthermore, the WRN and BLM helicases, partnered with replication protein A, actively unwind long telomeric duplex regions that are pre-bound by TRF2. These results suggest that TRF2 functions with WRN, and possibly BLM, in a common pathway at telomeric ends.     

Telomerase-associated Protein 1, HSP90, and Topoisomerase II�� Associate Directly with the BLM Helicase in Immortalized Cells Using ALT and Modulate Its Helicase Activity Using Telomeric DNA Substrates

The BLM helicase associates with the telomere structural proteins TRF1 and TRF2 in immortalized cells using the alternative lengthening of telomere (ALT) pathways. This work focuses on identifying protein partners of BLM in cells using ALT. Mass spectrometry and immunoprecipitation techniques have identified three proteins that bind directly to BLM and TRF2 in ALT cells: telomerase-associated protein 1 (TEP1), heat shock protein 90 (HSP90), and topoisomerase IIα (TOPOIIα). BLM predominantly co-localizes with these proteins in foci actively synthesizing DNA during late S and G₂/M phases of the cell cycle when ALT is thought to occur. Immunoprecipitation studies also indicate that only HSP90 and TOPOIIα are components of a specific complex containing BLM, TRF1, and TRF2 but that this complex does not include TEP1. TEP1, TOPOIIα, and HSP90 interact directly with BLM in vitro and modulate its helicase activity on telomere-like DNA substrates but not on non-telomeric substrates. Initial studies suggest that knockdown of BLM in ALT cells reduces average telomere length but does not do so in cells using telomerase.Bloom syndrome (BS)⁴ is a genetic disease caused by mutation of both copies of the human BLM gene. It is characterized by sun sensitivity, small stature, immunodeficiency, male infertility, and an increased susceptibility to cancer of all sites and types. The high incidence of spontaneous chromosome breakage and other unique chromosomal anomalies in cells from BS patients indicate an increase in homologous recombination in somatic cells (1). Another notable feature of non-immortalized and immortalized cells from BS individuals is the presence of telomeric associations (TAs) between homologous chromosomes (2). Work from our group and others have suggested a role for BLM in recombination-mediated mechanisms of telomere elongation or ALT (alternative lengthening of telomeres), processes that maintain/elongate telomeres in the absence of telomerase (3–5). However, the exact mechanism by which BLM contributes to telomere stability is unknown.Several proteins interact with and regulate BLM helicase activity, including two telomere-specific proteins, TRF1 and TRF2 (6, 7). Although TRF2 stimulates BLM unwinding of telomeric and non-telomeric 3′-overhang substrates, TRF1 inhibits BLM unwinding of telomeric substrates. TRF2-mediated stimulation of BLM helicase activity on a telomeric substrate is observed when TRF2 is present in excess or with equimolar amount of TRF1 but not when TRF1 is present in molar excess. Both proteins associate with BLM specifically in ALT cells in vivo, suggesting their involvement in the ALT pathways. In addition to TRF1 and TRF2, the telomere single-strand DNA-binding protein POT1 strongly stimulates BLM helicase activity on long telomeric forked duplexes and D-loop structures (8). Other proteins also play an important role in telomere maintenance in telomerase-negative cells, including RAD50, NBS1, and MRE11, which co-localize with TRF1 and TRF2 in specialized ALT-associated promyelocytic leukemia (PML) nuclear bodies (APBs) (9–11). Thus, we hypothesize that BLM complex formation may be essential for the ALT mechanism, and its modification may occur dynamically during the specific nucleic acid transactions required to protect the telomere in cells using the ALT pathways.This study has identified previously unknown protein partners of BLM and TRF2 in ALT cells using double immunoprecipitation and mass spectrometry (MS). These include telomerase-associated protein 1 (TEP1), heat shock protein 90 (HSP90), and topoisomerase IIα (TOPOIIα). These proteins associate with BLM and TRF2 in cells using ALT but not in cells using telomerase and directly interact with BLM in vitro. This complex of proteins localizes to sites of new DNA synthesis in vivo in ALT cells, suggesting a role in telomere maintenance. We also identified HSP90 and TOPOIIα in another ALT-specific complex consisting of BLM, TRF1, and TRF2 but not TEP1. In vitro analyses demonstrate that HSP90 inhibits BLM helicase activity using both telomeric and non-telomeric substrates, whereas TEP1 and TOPOIIα initially slow the kinetics of BLM unwinding only using telomeric substrates. These findings suggest the presence of dynamic BLM-associated ALT complexes that include previously unidentified interacting proteins. The function of TEP1 in the BLM·TRF2 complex remains unclear, although its previously described interaction with the RNA subunit of telomerase (12) suggests an interesting hypothesis of cross-talk between mechanisms of telomere elongation.     

The roles of WRN and BLM RecQ helicases in the Alternative Lengthening of Telomeres

Approximately 10% of all cancers, but a higher proportion of sarcomas, use the recombination-based alternative lengthening of telomeres (ALT) to maintain telomeres. Two RecQ helicase genes, BLM and WRN, play important roles in homologous recombination repair and they have been implicated in telomeric recombination activity, but their precise roles in ALT are unclear. Using analysis of sequence variation present in human telomeres, we found that a WRN– ALT+ cell line lacks the class of complex telomere mutations attributed to inter-telomeric recombination in other ALT+ cell lines. This suggests that WRN facilitates inter-telomeric recombination when there are sequence differences between the donor and recipient molecules or that sister-telomere interactions are suppressed in the presence of WRN and this promotes inter-telomeric recombination. Depleting BLM in the WRN– ALT+ cell line increased the mutation frequency at telomeres and at the MS32 minisatellite, which is a marker of ALT. The absence of complex telomere mutations persisted in BLM-depleted clones, and there was a clear increase in sequence homogenization across the telomere and MS32 repeat arrays. These data indicate that BLM suppresses unequal sister chromatid interactions that result in excessive homogenization at MS32 and at telomeres in ALT+ cells.     

WRN Loss Induces Switching of Telomerase-Independent Mechanisms of Telomere Elongation

Telomere maintenance can occur in the presence of telomerase or in its absence, termed alternative lengthening of telomeres (ALT). ALT adds telomere repeats using recombination-based processes and DNA repair proteins that function in homologous recombination. Our previous work reported that the RecQ-like BLM helicase is required for ALT and that it unwinds telomeric substrates in vitro. WRN is also a RecQ-like helicase that shares many biochemical functions with BLM. WRN interacts with BLM, unwinds telomeric substrates, and co-localizes to ALT-associated PML bodies (APBs), suggesting that it may also be required for ALT processes. Using long-term siRNA knockdown of WRN in three ALT cell lines, we show that some, but not all, cell lines require WRN for telomere maintenance. VA-13 cells require WRN to prevent telomere loss and for the formation of APBs; Saos-2 cells do not. A third ALT cell line, U-2 OS, requires WRN for APB formation, however WRN loss results in p53-mediated apoptosis. In the absence of WRN and p53, U-2 OS cells undergo telomere loss for an intermediate number of population doublings (50–70), at which point they maintain telomere length even with the continued loss of WRN. WRN and the tumor suppressor BRCA1 co-localize to APBs in VA-13 and U-2 OS, but not in Saos-2 cells. WRN loss in U-2 OS is associated with a loss of BRCA1 from APBs. While the loss of WRN significantly increases telomere sister chromatid exchanges (T-SCE) in these three ALT cell lines, loss of both BRCA1 and WRN does not significantly alter T-SCE. This work demonstrates that ALT cell lines use different telomerase-independent maintenance mechanisms that variably require the WRN helicase and that some cells can switch from one mechanism to another that permits telomere elongation in the absence of WRN. Our data suggest that BRCA1 localization may define these mechanisms.     

Association of BLM and BRCA1 during Telomere Maintenance in ALT Cells

Fifteen percent of tumors utilize recombination-based alternative lengthening of telomeres (ALT) to maintain telomeres. The mechanisms underlying ALT are unclear but involve several proteins involved in homologous recombination including the BLM helicase, mutated in Bloom''s syndrome, and the BRCA1 tumor suppressor. Cells deficient in either BLM or BRCA1 have phenotypes consistent with telomere dysfunction. Although BLM associates with numerous DNA damage repair proteins including BRCA1 during DNA repair, the functional consequences of BLM-BRCA1 association in telomere maintenance are not completely understood. Our earlier work showed the involvement of BRCA1 in different mechanisms of ALT, and telomere shortening upon loss of BLM in ALT cells. In order to delineate their roles in telomere maintenance, we studied their association in telomere metabolism in cells using ALT. This work shows that BLM and BRCA1 co-localize with RAD50 at telomeres during S- and G2-phases of the cell cycle in immortalized human cells using ALT but not in cells using telomerase to maintain telomeres. Co-immunoprecipitation of BRCA1 and BLM is enhanced in ALT cells at G2. Furthermore, BRCA1 and BLM interact with RAD50 predominantly in S- and G2-phases, respectively. Biochemical assays demonstrate that full-length BRCA1 increases the unwinding rate of BLM three-fold in assays using a DNA substrate that models a forked structure composed of telomeric repeats. Our results suggest that BRCA1 participates in ALT through its interactions with RAD50 and BLM.     

The BLM helicase contributes to telomere maintenance through processing of late-replicating intermediate structures

Werner's syndrome (WS) and Bloom's syndrome (BS) are cancer predisposition disorders caused by loss of function of the RecQ helicases WRN or BLM, respectively. BS and WS are characterized by replication defects, hyperrecombination events and chromosomal aberrations, which are hallmarks of cancer. Inefficient replication of the G-rich telomeric strand contributes to chromosome aberrations in WS cells, demonstrating a link between WRN, telomeres and genomic stability. Herein, we provide evidence that BLM also contributes to chromosome-end maintenance. Telomere defects (TDs) are observed in BLM-deficient cells at an elevated frequency, which is similar to cells lacking a functional WRN helicase. Loss of both helicases exacerbates TDs and chromosome aberrations, indicating that BLM and WRN function independently in telomere maintenance. BLM localization, particularly its recruitment to telomeres, changes in response to replication dysfunction, such as in WRN-deficient cells or after aphidicolin treatment. Exposure to replication challenge causes an increase in decatenated deoxyribonucleic acid (DNA) structures and late-replicating intermediates (LRIs), which are visible as BLM-covered ultra-fine bridges (UFBs) in anaphase. A subset of UFBs originates from telomeric DNA and their frequency correlates with telomere replication defects. We propose that the BLM complex contributes to telomere maintenance through its activity in resolving LRIs.     

The Pif1 family helicase Pfh1 facilitates telomere replication and has an RPA-dependent role during telomere lengthening

Pif1 family helicases are evolutionary conserved 5′–3′ DNA helicases. Pfh1, the sole Schizosaccharomyces pombe Pif1 family DNA helicase, is essential for maintenance of both nuclear and mitochondrial DNAs. Here we show that its nuclear functions include roles in telomere replication and telomerase action. Pfh1 promoted semi-conservative replication through telomeric DNA, as replication forks moved more slowly through telomeres when Pfh1 levels were reduced. Unlike other organisms, S. pombe cells overexpressing Pfh1 displayed markedly longer telomeres. Because this lengthening occurred in the absence of homologous recombination but not in a replication protein A mutant (rad11-D223Y) that has defects in telomerase function, it is probably telomerase-mediated. The effects of Pfh1 on telomere replication and telomere length are likely direct as Pfh1 exhibited high telomere binding in cells expressing endogenous levels of Pfh1. These findings argue that Pfh1 is a positive regulator of telomere length and telomere replication.     

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 IIIα. Here, we show that BLM stimulates the ability of topoisomerase IIIα to relax negatively supercoiled DNA. Moreover, DNA binding analyses indicate that BLM recruits topoisomerase IIIα to its DNA substrate. Consistent with this, a mutant form of BLM that retains helicase activity, but is unable to bind topoisomerase IIIα, fails to stimulate topoisomerase activity. These results indicate that a physical association between BLM and topoisomerase IIIα is a prerequisite for their functional biochemical interaction.     

Mammalian BLM helicase is critical for integrating multiple pathways of meiotic recombination

Bloom’s syndrome (BS) is an autosomal recessive disorder characterized by growth retardation, cancer predisposition, and sterility. BS mutated (Blm), the gene mutated in BS patients, is one of five mammalian RecQ helicases. Although BLM has been shown to promote genome stability by assisting in the repair of DNA structures that arise during homologous recombination in somatic cells, less is known about its role in meiotic recombination primarily because of the embryonic lethality associated with Blm deletion. However, the localization of BLM protein on meiotic chromosomes together with evidence from yeast and other organisms implicates a role for BLM helicase in meiotic recombination events, prompting us to explore the meiotic phenotype of mice bearing a conditional mutant allele of Blm. In this study, we show that BLM deficiency does not affect entry into prophase I but causes severe defects in meiotic progression. This is exemplified by improper pairing and synapsis of homologous chromosomes and altered processing of recombination intermediates, resulting in increased chiasmata. Our data provide the first analysis of BLM function in mammalian meiosis and strongly argue that BLM is involved in proper pairing, synapsis, and segregation of homologous chromosomes; however, it is dispensable for the accumulation of recombination intermediates.     

SLX4 contributes to telomere preservation and regulated processing of telomeric joint molecule intermediates

SLX4 assembles a toolkit of endonucleases SLX1, MUS81 and XPF, which is recruited to telomeres via direct interaction of SLX4 with TRF2. Telomeres present an inherent obstacle for DNA replication and repair due to their high propensity to form branched DNA intermediates. Here we provide novel insight into the mechanism and regulation of the SLX4 complex in telomere preservation. SLX4 associates with telomeres throughout the cell cycle, peaking in late S phase and under genotoxic stress. Disruption of SLX4''s interaction with TRF2 or SLX1 and SLX1''s nuclease activity independently causes telomere fragility, suggesting a requirement of the SLX4 complex for nucleolytic resolution of branched intermediates during telomere replication. Indeed, the SLX1–SLX4 complex processes a variety of telomeric joint molecules in vitro. The nucleolytic activity of SLX1-SLX4 is negatively regulated by telomeric DNA-binding proteins TRF1 and TRF2 and is suppressed by the RecQ helicase BLM in vitro. In vivo, in the presence of functional BLM, telomeric circle formation and telomere sister chromatid exchange, both arising out of nucleolytic processing of telomeric homologous recombination intermediates, are suppressed. We propose that the SLX4-toolkit is a telomere accessory complex that, in conjunction with other telomere maintenance proteins, ensures unhindered, but regulated telomere maintenance.     

The Bloom's syndrome helicase (BLM) interacts physically and functionally with p12, the smallest subunit of human DNA polymerase delta

Bloom's syndrome (BS) is a cancer predisposition disorder caused by mutation of the BLM gene, encoding a member of the RecQ helicase family. Although the phenotype of BS cells is suggestive of a role for BLM in repair of stalled or damaged replication forks, thus far there has been no direct evidence that BLM associates with any of the three human replicative DNA polymerases. Here, we show that BLM interacts specifically in vitro and in vivo with p12, the smallest subunit of human POL δ (hPOL δ). The hPOL δ enzyme, as well as the isolated p12 subunit, stimulates the DNA helicase activity of BLM. Conversely, BLM stimulates hPOL δ strand displacement activity. Our results provide the first functional link between BLM and the replicative machinery in human cells, and suggest that BLM might be recruited to sites of disrupted replication through an interaction with hPOL δ. Finally, our data also define a novel role for the poorly characterized p12 subunit of hPOL δ.     

Yeast as a model system to study RecQ helicase function

Mutations in the highly conserved RecQ helicase, BLM, cause the rare cancer predisposition disorder, Bloom's syndrome. The orthologues of BLM in Saccharomyces cerevisiae and Schizosaccharomyces pombe are SGS1 and rqh1⁺, respectively. Studies in these yeast species have revealed a plethora of roles for the Sgs1 and Rqh1 proteins in repair of double strand breaks, restart of stalled replication forks, processing of aberrant intermediates that arise during meiotic recombination, and maintenance of telomeres. In this review, we focus on the known roles of Sgs1 and Rqh1 and how studies in yeast species have improved our knowledge of how BLM suppresses neoplastic transformation.     

洛美沙星对Bloom综合征解旋酶生物学特性影响的机理研究

Bloom综合征解旋酶(BLM)是RecQ家族DNA解旋酶中的一个重要成员,参与了DNA复制、修复、转录、重组以及端粒的维持等细胞代谢过程,在维持染色体的稳定性中具有重要作用.BLM解旋酶的突变可导致Bloom综合征.Bloom综合征是一种罕见隐性常染色体遗传疾病,患者遗传不稳定,并易患多种类型癌症.洛美沙星(LMX)可以抑制细胞内多种酶,并通过结合DNA干扰DNA代谢,从而治疗多种疾病,但是其具体的作用机理还未完全清楚.运用荧光偏振技术和自由磷检测技术,研究了LMX对BLM642～1290解旋酶DNA结合活性、解链活性和ATP酶活性的影响.运用荧光及紫外吸收光谱法研究了LMX与解旋酶结合的结合常数、结合位点数、作用力类型、结合距离等参数.结果表明,LMX与解旋酶之间能自发进行反应,两种分子有一个结合位点,通过静电引力和疏水作用力形成稳定的BLM-LMX复合物,且解旋酶的内源荧光被LMX静态猝灭,主要原因是非辐射能量转移.在这一过程中,LMX能抑制解旋酶的解链活性和ATP酶活性,而促进解旋酶的DNA结合活性.LMX对BLM解旋酶生物学活性影响的机理可能是LMX使解旋酶通过别构机制影响其ATP酶活性,并使酶的构象维持在较低解链活性的状态,通过抑制ATP催化水解与解链过程的偶联和阻止解旋酶的易位,从而抑制其解链.LMX能够促进解旋酶的DNA结合活性,可能是因为其C-6和C-7上的取代功能基团可以增加酶活力,以及增强药物、酶和DNA的结合,从而形成药物-酶-DNA复合物.这些结果为研究以DNA解旋酶为药物靶标的分子机理和理解喹诺酮类药物的作用机理奠定相关理论基础.     

Fanconi anemia and Bloom's syndrome crosstalk through FANCJ-BLM helicase interaction

Fanconi anemia (FA) and Bloom's syndrome (BS) are rare hereditary chromosomal instability disorders. FA displays bone marrow failure, acute myeloid leukemia, and head and neck cancers, whereas BS is characterized by growth retardation, immunodeficiency, and a wide spectrum of cancers. The BLM gene mutated in BS encodes a DNA helicase that functions in a protein complex to suppress sister-chromatid exchange. Of the 15 FA genetic complementation groups implicated in interstrand crosslink repair, FANCJ encodes a DNA helicase involved in recombinational repair and replication stress response. Based on evidence that BLM and FANCJ interact we suggest that crosstalk between BLM and FA pathways is more complex than previously thought. We propose testable models for how FANCJ and BLM coordinate to help cells deal with stalled replication forks or double-strand breaks (DSB). Understanding how BLM and FANCJ cooperate will help to elucidate an important pathway for maintaining genomic stability.     

The Bloom's syndrome protein (BLM) interacts with MLH1 but is not required for DNA mismatch repair

Bloom's syndrome (BS) is a rare autosomal recessive disorder characterized by pre- and postnatal growth deficiency, immunodeficiency, and a tremendous predisposition to a wide variety of cancers. Cells from BS individuals are characterized by a high incidence of chromosomal gaps and breaks, elevated sister chromatid exchange, quadriradial formations, and locus-specific mutations. BS is the consequence of mutations that lead to loss of function of BLM, a gene encoding a helicase with homology to the RecQ helicase family. To delineate the role of BLM in DNA replication, recombination, and repair we used a yeast two-hybrid screen to identify potential protein partners of the BLM helicase. The C terminus of BLM interacts directly with MLH1 in the yeast-two hybrid assay; far Western analysis and co-immunoprecipitations confirmed the interaction. Cell extracts deficient in BLM were competent for DNA mismatch repair. These data suggest that the BLM helicase and MLH1 function together in replication, recombination, or DNA repair events independent of single base mismatch repair.     

Possible anti-recombinogenic role of Bloom's syndrome helicase in double-strand break processing

Bloom’s syndrome (BS) which associates genetic instability and predisposition to cancer is caused by mutations in the BLM gene encoding a RecQ family 3′–5′ DNA helicase. It has been proposed that the generation of genetic instability in BS cells could result from an aberrant non-homologous DNA end joining (NHEJ), one of the two main DNA double-strand break (DSB) repair pathways in mammalian cells, the second major pathway being homologous recombination (HR). Using cell extracts, we report first that Ku70/80 and the catalytic subunit of the DNA-dependent protein kinase (DNA-PKcs), key factors of the end-joining machinery, and BLM are located in close proximity on DNA and that BLM binds to DNA only in the absence of ATP. In the presence of ATP, BLM is phosphorylated and dissociates from DNA in a strictly DNA-PKcs-dependent manner. We also show that BS cells display, in vivo, an accurate joining of DSBs, reflecting thus a functional NHEJ pathway. In sharp contrast, a 5-fold increase of the HR-mediated DNA DSB repair in BS cells was observed. These results support a model in which NHEJ activation mediates BLM dissociation from DNA, whereas, under conditions where HR is favored, e.g. at the replication fork, BLM exhibits an anti-recombinogenic role.     

Germline mutations of regulator of telomere elongation helicase 1, RTEL1, in Dyskeratosis congenita

Dyskeratosis congenita (DC) is an inherited bone marrow failure and cancer predisposition syndrome caused by aberrant telomere biology. The classic triad of dysplastic nails, abnormal skin pigmentation, and oral leukoplakia is diagnostic of DC, but substantial clinical heterogeneity exists; the clinically severe variant Hoyeraal Hreidarsson syndrome (HH) also includes cerebellar hypoplasia, severe immunodeficiency, enteropathy, and intrauterine growth retardation. Germline mutations in telomere biology genes account for approximately one-half of known DC families. Using exome sequencing, we identified mutations in RTEL1, a helicase with critical telomeric functions, in two families with HH. In the first family, two siblings with HH and very short telomeres inherited a premature stop codon from their mother who has short telomeres. The proband from the second family has HH and inherited a premature stop codon in RTEL1 from his father and a missense mutation from his mother, who also has short telomeres. In addition, inheritance of only the missense mutation led to very short telomeres in the proband’s brother. Targeted sequencing identified a different RTEL1 missense mutation in one additional DC proband who has bone marrow failure and short telomeres. Both missense mutations affect the helicase domain of RTEL1, and three in silico prediction algorithms suggest that they are likely deleterious. The nonsense mutations both cause truncation of the RTEL1 protein, resulting in loss of the PIP box; this may abrogate an important protein–protein interaction. These findings implicate a new telomere biology gene, RTEL1, in the etiology of DC.     

Characterization of the Caenorhabditis elegans HIM-6/BLM Helicase: Unwinding Recombination Intermediates

Mutations in three human RecQ genes are implicated in heritable human syndromes. Mutations in BLM, a RecQ gene, cause Bloom syndrome (BS), which is characterized by short stature, cancer predisposition, and sensitivity to sunlight. BLM is a RecQ DNA helicase that, with interacting proteins, is able to dissolve various DNA structures including double Holliday junctions. A BLM ortholog, him-6, has been identified in Caenorhabditis elegans, but little is known about its enzymatic activities or its in vivo roles. By purifying recombinant HIM-6 and performing biochemical assays, we determined that the HIM-6 has DNA-dependent ATPase activity HIM-6 and helicase activity that proceeds in the 3''-5'' direction and needs at least five 3'' overhanging nucleotides. HIM-6 is also able to unwind DNA structures including D-loops and Holliday junctions. Worms with him-6 mutations were defective in recovering the cell cycle arrest after HU treatment. These activities strongly support in vivo roles for HIM-6 in processing recombination intermediates.