Human Molecular Genetics, 2001, Vol. 10, No. 7 741-746
© 2001 Oxford University Press
DNA helicase deficiencies associated with cancer predisposition and premature ageing disorders
Imperial Cancer Research Fund Laboratories, University of Oxford, Weatherall Institute of Molecular Medicine, John Radcliffe Hospital, Oxford OX3 9DS, UK
Received 10 January 2001; Accepted 25 January 2001.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONRecQ FAMILY HELICASESHUMAN RecQ HELICASE-DEFICIENT...PROTEINS INTERACTING WITH HUMAN...WRN-INTERACTING PROTEINSBLM-INTERACTING PROTEINSREFERENCES |
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Deficiency in a helicase of the RecQ family is found in at least three human genetic disorders associated with cancer predisposition and/or premature ageing. The RecQ helicases encoded by the BLM, WRN and RECQ4 genes are defective in Bloom’s, Werner’s and Rothmund–Thomson syndromes, respectively. Cells derived from individuals with these disorders in each case show inherent genomic instability. Recent studies have demonstrated direct interactions between these RecQ helicases and human nuclear proteins required for several aspects of chromosome maintenance, including p53, BRCA1, topoisomerase III, replication protein A and DNA polymerase
. Here, we review this network of protein interactions, and the clues that they present regarding the potential roles of RecQ family members in DNA repair, replication and/or recombination pathways. |
INTRODUCTION |
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TOPABSTRACTINTRODUCTIONRecQ FAMILY HELICASESHUMAN RecQ HELICASE-DEFICIENT...PROTEINS INTERACTING WITH HUMAN...WRN-INTERACTING PROTEINSBLM-INTERACTING PROTEINSREFERENCES |
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DNA damage exists in many different forms and can arise as a result of defects in any aspect of DNA metabolism. To deal with DNA damage, all cells have developed a range of responses, which include pathways for DNA repair and arrest of the cell cycle (1,2). Mutations in DNA repair genes frequently lead to genome destabilization and a consequent increase in the frequency of mutations at other loci. Hence, germline mutations in genes involved in DNA repair could result in the appearance of a broad spectrum of disease phenotypes. In particular, the general maintenance of genome stability is crucial for the prevention of cancer because of a requirement to maintain the integrity of proto-oncogenes and tumour suppressor genes. This genome maintenance is achieved by mechanisms that eliminate, with high fidelity, DNA damage that occurs spontaneously, through the action of DNA reactive agents, or through intrinsic errors in DNA metabolism itself. One common feature of many processes in DNA metabolism is a requirement for the complementary strands of the DNA duplex to be separated through the action of a DNA helicase enzyme. DNA helicases utilize the energy derived from hydrolysis of ATP to perform essential roles in the processes of genetic recombination, transcription, DNA replication and DNA repair (3,4). Indeed, as much as 1% of eukaryotic genes may encode DNA and/or RNA helicase enzymes (5). In this review, we discuss recent findings linking deficiency in one family of DNA helicase enzymes, the RecQ family, to pathways essential for the prevention of tumorigenesis and premature ageing.
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RecQ FAMILY HELICASES |
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TOPABSTRACTINTRODUCTIONRecQ FAMILY HELICASESHUMAN RecQ HELICASE-DEFICIENT...PROTEINS INTERACTING WITH HUMAN...WRN-INTERACTING PROTEINSBLM-INTERACTING PROTEINSREFERENCES |
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The RecQ family is named after the RecQ protein of Escherichia coli, which is a component of the so-called RecF pathway of genetic recombination (6). Mutations of genes in this pathway lead to deficiency in conjugational recombination and sensitivity to UV light when the major pathway for recombination, defined by the RecBCD helicase, is impaired. However, in an otherwise wild-type background, recQ mutants show an elevated frequency of illegitimate recombination (7). The RecQ family includes representatives in prokaryotes and unicellular eukaryotes, as well as in vertebrates (Fig. 1) (8,9). RecQ family members share a highly conserved domain comprising approximately 450 amino acids, which includes seven sequence motifs found in many classes of DNA and RNA helicases. Amongst these motifs is an ATP binding sequence (the so-called Walker A-box) and a Dex H-box, which is a characteristic of the RecQ family. Outside this domain, there is only limited sequence similarity amongst the family members. Where studied, RecQ family members have been shown to be DNA helicases that translocate in the 3'
5' direction (10–17). WRN is unique in also being a 3'5' exonuclease dependent upon a functionally separable domain in the N-terminal region of the protein (Fig. 1) (18–20).
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Recent data indicate that mutations in genes encoding helicases are responsible for genomic instability disorders of man. The functional importance of RecQ family helicases in the maintenance of genome integrity is highlighted by the identification of five human RecQ family helicases, three of which are involved in autosomal recessive genomic instability disorders associated with cancer predisposition and/or premature ageing. These disorders, which are discussed in detail below are Bloom’s syndrome (BS), Werner’s syndrome (WS) and Rothmund–Thomson syndrome (RTS).
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HUMAN RecQ HELICASE-DEFICIENT DISORDERS |
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The BLM gene (21) is mutated in BS, a rare disorder associated with pleiotropic phenotypes including immunodeficiency, impaired fertility, proportional dwarfism, sun-induced facial erythema and a predisposition to cancer (mean age at cancer diagnosis of approximately 24 years) (22,23). This disorder is of particular interest because affected individuals are susceptible to the full range of cancers seen in the normal population. BS cells show a high frequency of genetic recombination events, particularly sister chromatid exchanges (SCEs) and an elevated rate of somatic mutation (9,22) (Table 1).
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Mutations in the WRN gene (24) give rise to WS, which is associated at a relatively early age with many, but not all, of the features of the normal ageing process (25). Hence, WS individuals show many age-related disorders that develop from puberty, including greying and thinning of the hair, cataracts, type II diabetes mellitus, osteoporosis and atherosclerosis. Moreover, WS individuals are also cancer-prone, although to a more limited extent than is seen in BS individuals, in particular displaying an elevated incidence of sarcomas (Table 1).
Recently, it has been shown that the RECQ4 gene (26) is mutated in some cases of RTS. In this disorder, affected individuals show growth deficiency, photosensitivity with poikilodermatous skin changes, cataracts, early greying and loss of hair, as well as some increase in cancer incidence (27). As with WS, the cancer predispositon in RTS individuals is of a limited range, mainly osteogenic sarcomas (Table 1).
One feature that links these three genetic disorders at the cellular level is inherent genomic instability (9). In the case of BS cells, this instability is manifested as a 10-fold elevated frequency of homologous recombination events, including reciprocal exchanges between sister-chromatids and homologous chromosomes (9,22,23). WS cells do not show elevated SCE frequencies, but they do display increased illegitimate recombination and a high frequency of large chromosomal deletions (25). The genomic instability of RTS cells has not been analysed in detail, but there are reports of an increased frequency of chromosome aberrations (28).
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PROTEINS INTERACTING WITH HUMAN RecQ FAMILY HELICASES |
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TOPABSTRACTINTRODUCTIONRecQ FAMILY HELICASESHUMAN RecQ HELICASE-DEFICIENT...PROTEINS INTERACTING WITH HUMAN...WRN-INTERACTING PROTEINSBLM-INTERACTING PROTEINSREFERENCES |
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The last year or so has seen the identification of numerous proteins that interact physically with human RecQ family members. Although in some cases the functional significance of these interactions remains to be identified, there is already sufficient evidence to indicate that RecQ helicases are important components of DNA repair, replication and recombination pathways (Table 1). Below, we review evidence that RecQ family helicases form complexes with other proteins involved in these processes.
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WRN-INTERACTING PROTEINS |
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WRN has been shown to form complexes with proteins involved both in cellular responses to DNA damage and in DNA replication (Table 2). The identification of a functional interaction between WRN and the p53 tumour suppressor protein serves to emphasize the role of the RecQ family in the maintenance of genomic stability (29,30). Mutations in p53, which has been dubbed the ‘guardian of the genome’, are seen in >50% of all sporadic cancers in humans. p53 functions in a highly dynamic and controlled manner; induction of p53 leads to cell cycle arrest in G1 and/or G2, allowing time for DNA repair to take place, but may additionally lead to apoptotic cell death (1,2). Moreover, the loss of p53 results in genomic instability. In WS cells, p53-mediated apoptosis is attenuated, while ectopic expression of WRN in these cells can rescue this deficiency (29). Overexpression of WRN results in elevated p53-dependent transcriptional activity and induction of p21Waf1 (29,30). The interaction between p53 and WRN takes place via their respective C-terminal domains. Interestingly, this is the region where the majority of missense mutations found in WS patients are located (25).
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Without detailed knowledge of the cellular functions of WRN, any discussion about the potential roles for the p53–WRN complex is necessarily speculative. However, there is increasing evidence to suggest that WRN acts at DNA replication origins or at sites of blocked replication forks (see discussion below and Table 2). Hence loss of the p53/WRN interaction could result in WRN being unable to recruit p53 to replication origins/forks in response to DNA damage. In normal cells, the p53–WRN complex may recognize abnormal DNA structures, such as stalled replication forks, leading either to a coordination of cell cycle and DNA repair events or to the induction of apoptosis. There are two possible routes for repair at this stage with a requirement for WRN. First, WRN could be involved directly in restoration of DNA replication by displacing (perhaps abnormal?) Okazaki fragments on the lagging strand and degrading the displaced DNA. Recently, it has been shown that the WRN exonuclease catalyses structure-dependent degradation of DNA, suggesting that WRN resolves abnormal DNA structures via both its helicase and its exonuclease activities (31). Consistent with this notion, a recent paper by Cooper et al. (32) supports a role for the WRN exonuclease activity during repair processes. It was shown the Ku70 and Ku86 DNA end-binding complex directly interacts with WRN, and stimulates its 3'
5' exonuclease activity. A second possible role for WRN in replication fork repair would be after removal of the damaged DNA strand at blocked forks. For example, it is possible that RAD51 (the human RecA homologue) could stabilize the replication fork at this stage, allowing the continuation of DNA synthesis without a need for re-initiation of replication (25). Alternatively, WRN could be involved in repair at blocked forks via homologous recombination; the role of WRN in this case could be to promote translocation of Holliday junctions, and prevent aberrant recombination events at sites of stalled replication forks by dissociating recombination intermediates (33). Indeed, there is now considerable evidence supporting a role for WRN in cellular response to DNA damage, and its presence at sites of DNA replication (Table 2).
The interaction of WRN with DNA polymerase (Table 2) provides a direct biochemical link between WRN and DNA synthesis. It was shown that WRN increases the rate of nucleotide incorporation by DNA polymerase in the absence of proliferating cell nuclear antigen (PCNA); however, WRN has no significant stimulatory effect on the DNA polymerase holoenzyme (polymerase –PCNA complex) (40). Therefore, Kamath-Loeb et al. (40) suggested that WRN is unlikely to function in normal processive DNA synthesis, which requires the polymerase –PCNA complex. Rather, they speculated that WRN may function in a replication restart pathway at sites where damaged DNA/unusual DNA secondary structures have blocked DNA replication and where the DNA replication machinery has detached from the DNA. It has also been shown that overexpressed WRN is able to recruit DNA polymerase to the nucleolus, suggesting that WRN could be involved in regulating the initiation and progression of DNA replication by recruiting polymerase to particular sites of DNA synthesis (41).
One potential way of regulating and coordinating the many roles of WRN could be by small ubiquitin-related modifier (SUMO) modification. Addition of SUMO to target proteins can change their localization or their interaction with other proteins. WRN has been shown to be covalently attached with SUMO-1 via the conjugating enzyme Ubc9 (42). Ubc9 has been shown to play a role in the degradation of certain proteins, including S and M phase cyclins (43). As discussed in more detail below in the section on BLM-interacting proteins, SUMO-1 modification is linked in some cases to regulation of intranuclear localization. SUMO modification is also involved in regulating p53 function, through interactions involving the Mdm2 protein and changes in the half-life of p53 (44). Potentially, SUMO modification of p53 and WRN could play a critical role in orchestrating the cross-talk between these proteins and hence regulate pathways for the maintenance of genome integrity.
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BLM-INTERACTING PROTEINS |
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TOPABSTRACTINTRODUCTIONRecQ FAMILY HELICASESHUMAN RecQ HELICASE-DEFICIENT...PROTEINS INTERACTING WITH HUMAN...WRN-INTERACTING PROTEINSBLM-INTERACTING PROTEINSREFERENCES |
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BRCA1 is the protein defective in some cases of hereditary breast cancer susceptibility. The recent identification of BLM as part of the BRCA1-associated genome surveillance complex (BASC), links BLM with a number of tumour suppressor and DNA damage repair proteins (45). The BASC complex includes MSH2, MHS6, MLH1, ATM, BLM, the RAD50–MRE11–NBS1 complex and DNA replication factor C. Many components of this complex have roles in recognition of DNA damage/unusual DNA structures, suggestive of this complex performing some kind of ‘sensor’ role. To examine the role of BLM within BASC, the subcellular localization of BLM and BRCA1 was analysed before and after exposure to DNA damaging agents. In untreated cells, BLM and BRCA1 colocalization was limited to a few bright nuclear foci. However, after treatment with hydroxyurea or ionizing radiation, colocalization was greatly enhanced in those cells that were in mid-to-late S phase or in G2. This could be indicative of specific requirement for BLM/BRCA1 in replication/repair of late replicating DNA. Consistent with a role for BLM (possibly within BASC) in recognizing abnormal DNA structures, we have shown recently that BLM is able to unwind a variety of unusual DNA structures, including G-quadruplex, synthetic X-junctions (models for the Holliday junction), bubbles and forked DNA (P. Mohaghegh et al., submitted for publication).
Recent data indicate that BLM binds to the 70 kDa subunit of the heterotrimeric, single-stranded DNA binding protein, replication protein A (RPA) (46). This interaction stimulates the helicase activity of BLM (46). RPA is involved in DNA replication, repair and recombination, and can be detected on meiotic prophase chromosomes where it appears to play a role in both homologous synapsis and recombination. BLM and RPA also colocalise in meiotic prophase nuclei of mouse spermatocytes (47). Potentially, BLM and RPA could work together to unwind various DNA structures to facilitate DNA recombination during meiosis, perhaps through resolving recombination intermediates.
A role for BLM in recombinational repair has been proposed recently by our group, as a result of the finding of a direct interaction between BLM and the RAD51 recombinase enzyme (48). RAD51 is the eukaryotic homologue of the E.coli RecA protein, which is vital for homologous recombination and recombinational repair of DNA double-strand breaks. BLM and RAD51 were found to interact directly in vitro and to co-immunoprecipitate from nuclear extracts. As discussed above, BLM localizes to nuclear foci in response to DNA damage and colocalizes with BRCA1. We have shown that BLM also colocalizes with a subset of RAD51 nuclear foci, which have been suggested to be sites of ongoing DNA repair. As with the BLM/BRCA1 colocalization, the number of colocalising BLM/RAD51 foci increases after treatment of cells with ionizing radiation. The precise functional role for the BLM/RAD51 interaction is not currently known. One possibility has been suggested by the recent work of van Brabant et al. (49). These authors showed that BLM preferentially binds to and melts DNA D-loops that can be formed by RAD51. D-loop structures model the initial intermediate formed during homologous recombination, and van Brabant et al. (49) suggested that BLM and RAD51 may have antagonistic roles in dealing with DNA structures arising at collapsed replication forks. A second possibility is that the BLM–RAD51 complex exists to coordinate different steps of homologous recombination. In this context, we have shown recently that BLM binds to the Holliday junction recombination intermediate and promotes ATP-dependent branch migration of these junctions (50).
Some, but not all, of the nuclear foci that include BASC, BLM and RAD51 colocalize with promyelocytic leukemia (PML) nuclear bodies. It was shown in three independent reports (51–53) that BLM localises to PML bodies in the nucleoplasm of normal cells. One report suggested, however, that BLM localizes to the nucleolus during S phase (53). In contrast, we have shown that BLM can be found in replication foci during S phase (54). It has been shown that BS cell lines bud out micronuclei during S phase (53), and micronuclei are proposed to be part of a p53-dependent process in response to the stalling of DNA replication forks. Interestingly, in PML–/– cell lines, BLM fails to accumulate in nuclear foci, and in these cell lines the level of SCEs is higher than in PML+/+ controls. These results suggest that PML is required for the localization of BLM to nuclear foci, pointing to PML being a regulator of BLM (52), and to a role for the PML body in regulating genome stability. The ability of PML to regulate BLM and other PML interacting proteins is potentially controlled by SUMO-1 modification of PML and other proteins, including possibly BLM (51).
The first direct biochemical evidence for a role for BLM in DNA replication was provided by studies with the BLM homologue in Xenopus (xBLM). Antibodies were raised against xBLM and used in a series of immunodepletion experiments. DNA replication was strongly inhibited (5- to 10-fold) in Xenopus egg extracts depleted of xBLM. This inhibition was rescued by the addition of recombinant xBLM (55). These results, showing a critical role for xBLM in DNA replication are apparently in contradiction to previous observations showing that yeast sgs1 and rqh1 mutants, as well as human BS cells lacking BLM, are viable and can perform near normal DNA replication (8,9).
During DNA replication, DNA helicases separate the complementary DNA strands; this process of unwinding produces torsional stress in the DNA, which is relieved by topoisomerases. It would not be surprising, therefore, if DNA helicases were to operate in close conjuction with topoisomerases. This has been shown for certain RecQ helicases. For example, E.coli RecQ helicase can unwind a covalently-closed double-stranded DNA substrate, and this activity stimulates E.coli topoisomerase III to fully catenate (interlink) two or more plasmid molecules (56). BLM also interacts with topoisomerase III, via both the N- and C-terminal domains of BLM (57). To date, only an interaction with one of the two human topoisomerase III isoforms, TOPO III
, has been shown (57). The finding of two binding sites on BLM for TOPO III suggests that these proteins might interact in a 1:2 ratio. Since topoisomerase III enzymes only make single-stranded DNA nicks, the incorporation of two topoisomerase molecules could allow double-strand breaks to take place. This could help to disrupt recombination intermediates between inappropriately paired DNA strands.
In summary, our knowledge of the biochemical properties of the RecQ helicases, in terms of enzymatic activities, substrate preferences and binding partners, has improved considerably in the last 2 years or so. Nevertheless, while there are tantalizing links between various RecQ family members and DNA replication and repair, there is still no clear view as to the precise role(s) that these enzymes play in any of these processes. Given the burgeoning interest in the human RecQ family members involved in the suppression of cancer and premature ageing, we expect the current level of ignorance to be relatively short-lived.
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ACKNOWLEDGEMENTS |
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We thank members of the ICRF Genome Integrity Group for useful discussions, Mrs J. Pepper for preparation of the manuscript and Dr C.J. Norbury for critical reading of the manuscript. Work in the authors’ laboratory is supported by the Imperial Cancer Research Fund.
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FOOTNOTES |
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+ To whom correspondence should be addressed. Tel: +44 1865 222417; Fax: +44 1865 222431; Email: hickson@icrf.icnet.uk
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