Possible involvement of RecQL4 in the repair of double-strand DNA breaks in Xenopus egg extracts

Positive and negative regulators of RAD51/DMC1 in homologous recombination and DNA replication

RAD51 recombinase plays a central role in homologous recombination (HR) by forming a nucleoprotein filament on single-stranded DNA (ssDNA) to catalyze homology search and strand exchange between the ssDNA and a homologous double-stranded DNA (dsDNA). The catalytic activity of RAD51 assembled on ssDNA is critical for the DNA-homology-mediated repair of DNA double-strand breaks in somatic and meiotic cells and restarting stalled replication forks during DNA replication. The RAD51-ssDNA complex also plays a structural role in protecting the regressed/reversed replication fork. Two types of regulators control RAD51 filament formation, stability, and dynamics, namely positive regulators, including mediators, and negative regulators, so-called remodelers. The appropriate balance of action by the two regulators assures genome stability. This review describes the roles of positive and negative RAD51 regulators in HR and DNA replication and its meiosis-specific homolog DMC1 in meiotic recombination. We also provide future study directions for a comprehensive understanding of RAD51/DMC1-mediated regulation in maintaining and inheriting genome integrity.

Biallelic variants in CRIPT cause a Rothmund-Thomson-like syndrome with increased cellular senescence

PURPOSE

Rothmund-Thomson syndrome (RTS) is characterized by poikiloderma, sparse hair, small stature, skeletal defects, cancer, and cataracts, resembling features of premature aging. RECQL4 and ANAPC1 are the 2 known disease genes associated with RTS in >70% of cases. We describe RTS-like features in 5 individuals with biallelic variants in CRIPT (OMIM 615789).

METHODS

Two newly identified and 4 published individuals with CRIPT variants were systematically compared with those with RTS using clinical data, computational analysis of photographs, histologic analysis of skin, and cellular studies on fibroblasts.

RESULTS

All CRIPT individuals fulfilled the diagnostic criteria for RTS and additionally had neurodevelopmental delay and seizures. Using computational gestalt analysis, CRIPT individuals showed greatest facial similarity with individuals with RTS. Skin biopsies revealed a high expression of senescence markers (p53/p16/p21) and the senescence-associated ß-galactosidase activity was elevated in CRIPT-deficient fibroblasts. RECQL4- and CRIPT-deficient fibroblasts showed an unremarkable mitotic progression and unremarkable number of mitotic errors and no or only mild sensitivity to genotoxic stress by ionizing radiation, mitomycin C, hydroxyurea, etoposide, and potassium bromate.

CONCLUSION

CRIPT causes an RTS-like syndrome associated with neurodevelopmental delay and epilepsy. At the cellular level, RECQL4- and CRIPT-deficient cells display increased senescence, suggesting shared molecular mechanisms leading to the clinical phenotypes.

DNA-PKcs-dependent phosphorylation of RECQL4 promotes NHEJ by stabilizing the NHEJ machinery at DNA double-strand breaks

Non-homologous end joining (NHEJ) is the major pathway that mediates the repair of DNA double-strand breaks (DSBs) generated by ionizing radiation (IR). Previously, the DNA helicase RECQL4 was implicated in promoting NHEJ, but its role in the pathway remains unresolved. In this study, we report that RECQL4 stabilizes the NHEJ machinery at DSBs to promote repair. Specifically, we find that RECQL4 interacts with the NHEJ core factor DNA-PK_cs and the interaction is increased following IR. RECQL4 promotes DNA end bridging mediated by DNA-PK_cs and Ku70/80 in vitro and the accumulation/retention of NHEJ factors at DSBs in vivo. Moreover, interaction between DNA-PK_cs and the other core NHEJ proteins following IR treatment is attenuated in the absence of RECQL4. These data indicate that RECQL4 promotes the stabilization of the NHEJ factors at DSBs to support formation of the NHEJ long-range synaptic complex. In addition, we observed that the kinase activity of DNA-PK_cs is required for accumulation of RECQL4 to DSBs and that DNA-PK_cs phosphorylates RECQL4 at six serine/threonine residues. Blocking phosphorylation at these sites reduced the recruitment of RECQL4 to DSBs, attenuated the interaction between RECQL4 and NHEJ factors, destabilized interactions between the NHEJ machinery, and resulted in decreased NHEJ. Collectively, these data illustrate reciprocal regulation between RECQL4 and DNA-PK_cs in NHEJ.

Role and Regulation of the RECQL4 Family during Genomic Integrity Maintenance

RECQL4 is a member of the evolutionarily conserved RecQ family of 3' to 5' DNA helicases. RECQL4 is critical for maintaining genomic stability through its functions in DNA repair, recombination, and replication. Unlike many DNA repair proteins, RECQL4 has unique functions in many of the central DNA repair pathways such as replication, telomere, double-strand break repair, base excision repair, mitochondrial maintenance, nucleotide excision repair, and crosslink repair. Consistent with these diverse roles, mutations in RECQL4 are associated with three distinct genetic diseases, which are characterized by developmental defects and/or cancer predisposition. In this review, we provide an overview of the roles and regulation of RECQL4 during maintenance of genome homeostasis.

Human RecQL4 as a Novel Molecular Target for Cancer Therapy

Human RecQ helicases play diverse roles in the maintenance of genomic stability. Inactivating mutations in 3 of the 5 human RecQ helicases are responsible for the pathogenesis of Werner syndrome (WS), Bloom syndrome (BS), Rothmund-Thomson syndrome (RTS), RAPADILINO, and Baller-Gerold syndrome (BGS). WS, BS, and RTS patients are at increased risk for developing many age-associated diseases including cancer. Mutations in RecQL1 and RecQL5 have not yet been associated with any human diseases so far. In terms of disease outcome, RecQL4 deserves special attention because mutations in RecQL4 result in 3 autosomal recessive syndromes (RTS type II, RAPADILINO, and BGS). RecQL4, like other human RecQ helicases, has been demonstrated to play a crucial role in the maintenance of genomic stability through participation in diverse DNA metabolic activities. Increased incidence of osteosarcoma in RecQL4-mutated RTS patients and elevated expression of RecQL4 in sporadic cancers including osteosarcoma suggest that loss or gain of RecQL4 expression is linked with cancer susceptibility. In this review, current and future perspectives are discussed on the potential use of RecQL4 as a novel cancer therapeutic target.

N-terminal region of RecQ4 inhibits non-homologous end joining and chromatin association of the Ku heterodimer in Xenopus egg extracts

RecQ4, a member of the RecQ helicase family, is required for the maintenance of genome integrity. RecQ4 has been shown to promote the following two DNA double-strand break (DSB) repair pathways: non-homologous end joining (NHEJ) and homologous recombination (HR). However, its molecular function has not been fully elucidated. In the present study, we aimed to investigate the role of RecQ4 in NHEJ using Xenopus egg extracts. The N-terminal 598 amino acid region of Xenopus RecQ4 (N598), which lacks a central helicase domain and a downstream C-terminal region, was added to the extracts and its effect on the joining of DNA ends was analyzed. We found that N598 inhibited the joining of linearized DNA ends in the extracts. In addition, N598 inhibited DSB-induced chromatin binding of Ku70, which is essential for NHEJ, while the DSB-induced chromatin binding of the HR-associated proteins, replication protein A (RPA) and Rad51, increased upon the addition of N598. These results suggest that RecQ4 possibly influences the choice of the DSB repair pathway by influencing the association of the Ku heterodimer with the DNA ends.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund–Thomson syndrome (RTS), Baller–Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

Human RecQ Helicases in DNA Double-Strand Break Repair

RecQ DNA helicases are a conserved protein family found in bacteria, fungus, plants, and animals. These helicases play important roles in multiple cellular functions, including DNA replication, transcription, DNA repair, and telomere maintenance. Humans have five RecQ helicases: RECQL1, Bloom syndrome protein (BLM), Werner syndrome helicase (WRN), RECQL4, and RECQL5. Defects in BLM and WRN cause autosomal disorders: Bloom syndrome (BS) and Werner syndrome (WS), respectively. Mutations in RECQL4 are associated with three genetic disorders, Rothmund-Thomson syndrome (RTS), Baller-Gerold syndrome (BGS), and RAPADILINO syndrome. Although no genetic disorders have been reported due to loss of RECQL1 or RECQL5, dysfunction of either gene is associated with tumorigenesis. Multiple genetically independent pathways have evolved that mediate the repair of DNA double-strand break (DSB), and RecQ helicases play pivotal roles in each of them. The importance of DSB repair is supported by the observations that defective DSB repair can cause chromosomal aberrations, genomic instability, senescence, or cell death, which ultimately can lead to premature aging, neurodegeneration, or tumorigenesis. In this review, we will introduce the human RecQ helicase family, describe in detail their roles in DSB repair, and provide relevance between the dysfunction of RecQ helicases and human diseases.

RECQ DNA Helicases and Osteosarcoma

The RECQ family of DNA helicases is a conserved group of enzymes that plays an important role in maintaining genomic stability. Humans possess five RECQ helicase genes, and mutations in three of them - BLM, WRN, and RECQL4 - are associated with the genetic disorders Bloom syndrome, Werner syndrome, and Rothmund-Thomson syndrome (RTS), respectively. These syndromes share overlapping clinical features, and importantly they are all associated with an increased risk of cancer. Patients with RTS have the highest specific risk of developing osteosarcoma compared to all other cancer predisposition syndromes; therefore, RTS serves as a relevant model to study the pathogenesis and molecular genetics of osteosarcoma. The "tumor suppressor" function of the RECQ helicases continues to be an area of active investigation. This chapter will focus primarily on the known cellular functions of RECQL4 and how these may relate to tumorigenesis, as well as ongoing efforts to understand RECQL4's functions in vivo using animal models. Understanding the RECQ pathways will provide insight into avenues for novel cancer therapies in the future.

Chromosome alignment maintenance requires the MAP RECQL4, mutated in the Rothmund-Thomson syndrome

RECQL4, which is mutated in the Rothmund–Thomson syndrome characterized by premature aging and cancer susceptibility, is a microtubule-associated protein required for mitotic chromosome alignment.

RecQ-like helicase 4 (RECQL4) is mutated in patients suffering from the Rothmund–Thomson syndrome, a genetic disease characterized by premature aging, skeletal malformations, and high cancer susceptibility. Known roles of RECQL4 in DNA replication and repair provide a possible explanation of chromosome instability observed in patient cells. Here, we demonstrate that RECQL4 is a microtubule-associated protein (MAP) localizing to the mitotic spindle. RECQL4 depletion in M-phase–arrested frog egg extracts does not affect spindle assembly per se, but interferes with maintaining chromosome alignment at the metaphase plate. Low doses of nocodazole depolymerize RECQL4-depleted spindles more easily, suggesting abnormal microtubule–kinetochore interaction. Surprisingly, inter-kinetochore distance of sister chromatids is larger in depleted extracts and patient fibroblasts. Consistent with a role to maintain stable chromosome alignment, RECQL4 down-regulation in HeLa cells causes chromosome misalignment and delays mitotic progression. Importantly, these chromosome alignment defects are independent from RECQL4’s reported roles in DNA replication and damage repair. Our data elucidate a novel function of RECQL4 in mitosis, and defects in mitotic chromosome alignment might be a contributing factor for the Rothmund–Thomson syndrome.

Human RecQL4 helicase plays multifaceted roles in the genomic stability of normal and cancer cells

Human RecQ helicases that share homology with E. coli RecQ helicase play critical roles in diverse biological activities such as DNA replication, transcription, recombination and repair. Mutations in three of the five human RecQ helicases (RecQ1, WRN, BLM, RecQL4 and RecQ5) result in autosomal recessive syndromes characterized by accelerated aging symptoms and cancer incidence. Mutational inactivation of Werner (WRN) and Bloom (BLM) genes results in Werner syndrome (WS) and Bloom syndrome (BS) respectively. However, mutations in RecQL4 result in three human disorders: (I) Rothmund-Thomson syndrome (RTS), (II) RAPADILINO and (III) Baller-Gerold syndrome (BGS). Cells from WS, BS and RTS are characterized by a unique chromosomal anomaly indicating that each of the RecQ helicases performs specialized function(s) in a non-redundant manner. Elucidating the biological functions of RecQ helicases will enable us to understand not only the aging process but also to determine the cause for age-associated human diseases. Recent biochemical and molecular studies have given new insights into the multifaceted roles of RecQL4 that range from genomic stability to carcinogenesis and beyond. This review summarizes some of the existing and emerging knowledge on diverse biological functions of RecQL4 and its significance as a potential molecular target for cancer therapy.

The role of RecQ helicases in non-homologous end-joining

DNA double-strand breaks are highly toxic DNA lesions that cause genomic instability, if not efficiently repaired. RecQ helicases are a family of highly conserved proteins that maintain genomic stability through their important roles in several DNA repair pathways, including DNA double-strand break repair. Double-strand breaks can be repaired by homologous recombination (HR) using sister chromatids as templates to facilitate precise DNA repair, or by an HR-independent mechanism known as non-homologous end-joining (NHEJ) (error-prone). NHEJ is a non-templated DNA repair process, in which DNA termini are directly ligated. Canonical NHEJ requires DNA-PKcs and Ku70/80, while alternative NHEJ pathways are DNA-PKcs and Ku70/80 independent. This review discusses the role of RecQ helicases in NHEJ, alternative (or back-up) NHEJ (B-NHEJ) and microhomology-mediated end-joining (MMEJ) in V(D)J recombination, class switch recombination and telomere maintenance.

RECQ helicase RECQL4 participates in non-homologous end joining and interacts with the Ku complex

RECQL4, a member of the RecQ helicase family, is a multifunctional participant in DNA metabolism. RECQL4 protein participates in several functions both in the nucleus and in the cytoplasm of the cell, and mutations in human RECQL4 are associated with three genetic disorders: Rothmund-Thomson, RAPADILINO and Baller-Gerold syndromes. We previously reported that RECQL4 is recruited to laser-induced DNA double-strand breaks (DSB). Here, we have characterized the functional roles of RECQL4 in the non-homologous end joining (NHEJ) pathway of DSB repair. In an in vitro NHEJ assay that depends on the activity of DNA-dependent protein kinase (DNA-PK), extracts from RECQL4 knockdown cells display reduced end-joining activity on DNA substrates with cohesive and non-cohesive ends. Depletion of RECQL4 also reduced the end joining activity on a GFP reporter plasmid in vivo. Knockdown of RECQL4 increased the sensitivity of cells to γ-irradiation and resulted in accumulation of 53BP1 foci after irradiation, indicating defects in the processing of DSB. We find that RECQL4 interacts with the Ku70/Ku80 heterodimer, part of the DNA-PK complex, via its N-terminal domain. Further, RECQL4 stimulates higher order DNA binding of Ku70/Ku80 to a blunt end DNA substrate. Taken together, these results implicate that RECQL4 participates in the NHEJ pathway of DSB repair via a functional interaction with the Ku70/Ku80 complex. This is the first study to provide both in vitro and in vivo evidence for a role of a RecQ helicase in NHEJ.

RECQ DNA helicases and osteosarcoma

The RECQ family of DNA helicases is a conserved group of enzymes that are important for maintaining genomic integrity. In humans, there are five RECQ helicase genes, and mutations in three of them-BLM, WRN, and RECQL4-are associated with the genetic disorders Bloom syndrome, Werner syndrome, and Rothmund-Thomson syndrome (RTS), respectively. Importantly all three diseases are cancer predisposition syndromes. Patients with RTS are highly and uniquely susceptible to developing osteosarcoma; thus, RTS provides a good model to study the pathogenesis of osteosarcoma. The "tumor suppressor" role of RECQL4 and the other RECQ helicases is an area of active investigation. This chapter reviews what is currently known about the cellular functions of RECQL4 and how these may relate to tumorigenesis, as well as ongoing efforts to understand RECQL4's functions in vivo using animal models. Understanding the RECQ pathways may provide insight into avenues for novel cancer therapies in the future.

The helicase and ATPase activities of RECQL4 are compromised by mutations reported in three human patients

RECQL4 is one of five members of the human RecQ helicase family, and is implicated in three syndromes displaying accelerating aging, developmental abnormalities and a predisposition to cancer. In this study, we purified three variants of RECQL4 carrying previously reported patient mutations. These three mutant proteins were analyzed for the known biochemical activities of RECQL4: DNA binding, unwinding of duplex DNA, ATP hydrolysis and annealing of simplex DNA. Further, the mutant proteins were evaluated for stability and recruitment to sites of laser-induced DNA damage. One mutant was helicase-dead, had marginal ATPase activity and may be structurally compromised, while the other two showed greatly reduced helicase and ATPase activities. The remaining biochemical activities and ability to recruit to damage sites were not significantly impaired for any of the mutants. Our findings demonstrate a consistent pattern of functional deficiency and provide further support for a helicase-dependent cellular function of RECQL4 in addition to its Nterminus-dependent role in initiation of replication, a function that may underlie the phenotype of RECQL4-linked disease.

Drosophila RecQ4 is directly involved in both DNA replication and the response to UV damage in S2 cells

The RecQ4 protein shows homology to both the S.cerevisiae DNA replication protein Sld2 and the DNA repair related RecQ helicases. Experimental data also suggest replication and repair functions for RecQ4, but the precise details of its involvement remain to be clarified.

Here we show that depletion of DmRecQ4 by dsRNA interference in S2 cells causes defects consistent with a replication function for the protein. The cells show reduced proliferation associated with an S phase block, reduced BrdU incorporation, and an increase in cells with a subG1 DNA content. At the molecular level we observe reduced chromatin association of DNA polymerase-alpha and PCNA. We also observe increased chromatin association of phosphorylated H2AvD - consistent with the presence of DNA damage and increased apoptosis.

Analysis of DmRecQ4 repair function suggests a direct role in NER, as the protein shows rapid but transient nuclear localisation after UV treatment. Re-localisation is not observed after etoposide or H₂O₂ treatment, indicating that the involvement of DmRecQ4 in repair is likely to be pathway specific.

Deletion analysis of DmRecQ4 suggests that the SLD2 domain was essential, but not sufficient, for replication function. In addition a DmRecQ4 N-terminal deletion could efficiently re-localise on UV treatment, suggesting that the determinants for this response are contained in the C terminus of the protein. Finally several deletions show differential rescue of dsRNA generated replication and proliferation phenotypes. These will be useful for a molecular analysis of the specific role of DmRecQ4 in different cellular pathways.

RECQL4 in genomic instability and aging

Helicases are ubiquitous proteins that unwind DNA and participate in DNA metabolism including replication, repair, transcription, and chromatin organization. The highly conserved RecQ helicase family proteins are important in these transactions and have been termed the guardians of the genome. Humans have five members of this family: WRN, BLM, RECQL4, RECQL1, and RECQL5. The first three of are associated with premature aging and cancer prone syndromes, but the latter two proteins have not yet been implicated in any human disease. Although WRN and BLM have been fairly well characterized, RECQL4 has only recently been intensively investigated. The sum of this work to date has shown that RECQL4 has helicase activity and localizes to telomeres and mitochondria. In addition, new protein partners are emerging, implicating RECQL4 in novel processes. Here, we describe these recent findings which place RECQL4 at the crossroads of genomic instability and aging processes.

RecQ helicases in DNA double strand break repair and telomere maintenance

Organisms are constantly exposed to various environmental insults which could adversely affect the stability of their genome. To protect their genomes against the harmful effect of these environmental insults, organisms have evolved highly diverse and efficient repair mechanisms. Defective DNA repair processes can lead to various kinds of chromosomal and developmental abnormalities. RecQ helicases are a family of evolutionarily conserved, DNA unwinding proteins which are actively engaged in various DNA metabolic processes, telomere maintenance and genome stability. Bacteria and lower eukaryotes, like yeast, have only one RecQ homolog, whereas higher eukaryotes including humans possess multiple RecQ helicases. These multiple RecQ helicases have redundant and/or non-redundant functions depending on the types of DNA damage and DNA repair pathways. Humans have five different RecQ helicases and defects in three of them cause autosomal recessive diseases leading to various kinds of cancer predisposition and/or aging phenotypes. Emerging evidence also suggests that the RecQ helicases have important roles in telomere maintenance. This review mainly focuses on recent knowledge about the roles of RecQ helicases in DNA double strand break repair and telomere maintenance which are important in preserving genome integrity.

RAPADILINO RECQL4 mutant protein lacks helicase and ATPase activity

The RecQ family of helicases has been shown to play an important role in maintaining genomic stability. In humans, this family has five members and mutations in three of these helicases, BLM, WRN and RECQL4, are associated with disease. Alterations in RECQL4 are associated with three diseases, Rothmund-Thomson syndrome, Baller-Gerold syndrome, and RAPADILINO syndrome. One of the more common mutations found in RECQL4 is the RAPADILINO mutation, c.1390+2delT which is a splice-site mutation leading to an in-frame skipping of exon 7 resulting in 44 amino acids being deleted from the protein (p.Ala420-Ala463del). In order to characterize the RAPADILINO RECQL4 mutant protein, it was expressed in bacteria and purified using an established protocol. Strand annealing, helicase, and ATPase assays were conducted to characterize the protein's activities relative to WT RECQL4. Here we show that strand annealing activity in the absence of ATP is unchanged from that of WT RECQL4. However, the RAPADILINO protein variant lacks helicase and ssDNA-stimulated ATPase activity. These observations help explain the underlying molecular etiology of the disease and our findings provide insight into the genotype and phenotype association among RECQL4 syndromes.

RECQL4 localizes to mitochondria and preserves mitochondrial DNA integrity

RECQL4 is associated with Rothmund-Thomson Syndrome (RTS), a rare autosomal recessive disorder characterized by premature aging, genomic instability, and cancer predisposition. RECQL4 is a member of the RecQ helicase family, and has many similarities to WRN protein, which is also implicated in premature aging. There is no information about whether any of the RecQ helicases play roles in mitochondrial biogenesis, which is strongly implicated in the aging process. Here, we used microscopy to visualize RECQL4 in mitochondria. Fractionation of human and mouse cells also showed that RECQL4 was present in mitochondria. Q-PCR amplification of mitochondrial DNA demonstrated that mtDNA damage accumulated in RECQL4-deficient cells. Microarray analysis suggested that mitochondrial bioenergetic pathways might be affected in RTS. Measurements of mitochondrial bioenergetics showed a reduction in the mitochondrial reserve capacity after lentiviral knockdown of RECQL4 in two different primary cell lines. Additionally, biochemical assays with RECQL4, mitochondrial transcription factor A, and mitochondrial DNA polymerase γ showed that the polymerase inhibited RECQL4's helicase activity. RECQL4 is the first 3'-5' RecQ helicase to be found in both human and mouse mitochondria, and the loss of RECQL4 alters mitochondrial integrity.

The helicase domain and C-terminus of human RecQL4 facilitate replication elongation on DNA templates damaged by ionizing radiation

The vertebrate RECQL4 (RECQ4) gene is thought to be the ortholog of budding yeast SLD2. However, RecQL4 contains within its C-terminus a RecQ-like helicase domain, which is absent in Sld2. We established human pre-B lymphocyte Nalm-6 cells, in which the endogenous RECQL4 gene was homozygously targeted such that the entire C-terminus would not be expressed. The RECQL4(ΔC/ΔC) cells behaved like the parental cells during unperturbed DNA replication or after treatment with agents that induce stalling of DNA replication forks, such as hydroxyurea (HU). However, after exposure to ionizing radiation (IR), the RECQL4(ΔC/ΔC) cells exhibited hypersensitivity, inability to complete S phase and prematurely terminated or paused DNA replication forks. Deletion of BLM, a gene that also encodes a RecQ helicase, had the opposite phenotype; an almost wild-type response to IR, but hypersensitivity to HU. Targeting both RECQL4 and BLM resulted in viable cells, which exhibited mostly additive phenotypes compared with those exhibited by the RECQL4(ΔC/ΔC) and the BLM(-/-) cells. We propose that RecQL4 facilitates DNA replication in cells that have been exposed to IR.

The N-terminal region of RECQL4 lacking the helicase domain is both essential and sufficient for the viability of vertebrate cells. Role of the N-terminal region of RECQL4 in cells

Rothmund-Thomson syndrome (RTS) is a rare genetic disorder characterized by premature aging, developmental abnormalities, and a predisposition to cancer. RTS is caused by mutations in the RECQL4 gene, which encodes one of the five human RecQ helicases. To identify the cellular functions of RECQL4, we generated a chicken DT40 cell line in which RECQL4 expression could be turned off by doxycycline (Dox). Upon exposure to Dox, cells stopped growing and underwent apoptosis. The cells could be rescued by expression of the N-terminal region of RECQL4 (amino acids 1-496), which lacks the helicase domain and has sequence similarity to yeast Sld2, which plays an essential function in the initiation of DNA replication in Saccharomyces cerevisiae. Smaller fragments of the N-terminal region of RECQL4 did not rescue the cells from lethality. RECQL4 gene knockout cells complemented with RECQL4 (1-496) showed relatively high sensitivity to DNA damaging agents that induce double strand breaks and cross-links, suggesting that the C-terminal region including the helicase domain of RECQL4 is involved in the repair of certain types of DNA lesions.

Human RecQL4 helicase plays critical roles in prostate carcinogenesis

Prostate cancer is the second leading cause of cancer-associated deaths among men in the western countries. Here, we report that human RecQL4 helicase, which is implicated in the pathogenesis of a subset of cancer-prone Rothmund-Thomson syndrome, is highly elevated in metastatic prostate cancer cell lines. Increased RecQL4 expression was also detected in human prostate tumor tissues as a function of tumor grade with the highest expression level in metastatic tumor samples, suggesting that RecQL4 may be a potential prognostic factor for advanced stage of prostate cancer. Transient and stable suppression of RecQL4 by small interfering RNA and short hairpin RNA vectors drastically reduced the growth and survival of metastatic prostate cancer cells, indicating that RecQL4 is a prosurvival factor for prostate cancer cells. RecQL4 suppression led to increased poly(ADP-ribose) polymerase (PARP) synthesis and RecQL4-suppressed prostate cancer cells underwent an extensive apoptotic death in a PARP-1-dependent manner. Most notably, RecQL4 knockdown in metastatic prostate cancer cells drastically reduced their cell invasiveness in vitro and tumorigenicity in vivo, showing that RecQL4 is essential for prostate cancer promotion. Observation of a direct interaction of retinoblastoma (Rb) and E2F1 proteins with RecQL4 promoter suggests that Rb-E2F1 pathway may regulate RecQL4 expression. Collectively, our study shows that RecQL4 is an essential factor for prostate carcinogenesis.

The involvement of human RECQL4 in DNA double-strand break repair

Rothmund-Thomson syndrome (RTS) is an autosomal recessive hereditary disorder associated with mutation in RECQL4 gene, a member of the human RecQ helicases. The disease is characterized by genomic instability, skeletal abnormalities and predisposition to malignant tumors, especially osteosarcomas. The precise role of RECQL4 in cellular pathways is largely unknown; however, recent evidence suggests its involvement in multiple DNA metabolic pathways. This study investigates the roles of RECQL4 in DNA double-strand break (DSB) repair. The results show that RECQL4-deficient fibroblasts are moderately sensitive to gamma-irradiation and accumulate more gammaH2AX and 53BP1 foci than control fibroblasts. This is suggestive of defects in efficient repair of DSB's in the RECQL4-deficient fibroblasts. Real time imaging of live cells using laser confocal microscopy shows that RECQL4 is recruited early to laser-induced DSBs and remains for a shorter duration than WRN and BLM, indicating its distinct role in repair of DSBs. Endogenous RECQL4 also colocalizes with gammaH2AX at the site of DSBs. The RECQL4 domain responsible for its DNA damage localization has been mapped to the unique N-terminus domain between amino acids 363-492, which shares no homology to recruitment domains of WRN and BLM to the DSBs. Further, the recruitment of RECQL4 to laser-induced DNA damage is independent of functional WRN, BLM or ATM proteins. These results suggest distinct cellular dynamics for RECQL4 protein at the site of laser-induced DSB and that it might play important roles in efficient repair of DSB's.

Conserved helicase domain of human RecQ4 is required for strand annealing-independent DNA unwinding

Humans have five members of the well conserved RecQ helicase family: RecQ1, Bloom syndrome protein (BLM), Werner syndrome protein (WRN), RecQ4, and RecQ5, which are all known for their roles in maintaining genome stability. BLM, WRN, and RecQ4 are associated with premature aging and cancer predisposition. Of the three, RecQ4's biological and cellular roles have been least thoroughly characterized. Here we tested the helicase activity of purified human RecQ4 on various substrates. Consistent with recent results, we detected ATP-dependent RecQ4 unwinding of forked duplexes. However, our results provide the first evidence that human RecQ4's unwinding is independent of strand annealing, and that it does not require the presence of excess ssDNA. Moreover, we demonstrate that a point mutation of the conserved lysine in the Walker A motif abolished helicase activity, implying that not the N-terminal portion, but the helicase domain is solely responsible for the enzyme's unwinding activity. In addition, we demonstrate a novel stimulation of RecQ4's helicase activity by replication protein A, similar to that of RecQ1, BLM, WRN, and RecQ5. Together, these data indicate that specific biochemical activities and protein partners of RecQ4 are conserved with those of the other RecQ helicases.

Rothmund-Thomson syndrome

Rothmund-Thomson syndrome (RTS) is a genodermatosis presenting with a characteristic facial rash (poikiloderma) associated with short stature, sparse scalp hair, sparse or absent eyelashes and/or eyebrows, juvenile cataracts, skeletal abnormalities, radial ray defects, premature aging and a predisposition to cancer. The prevalence is unknown but around 300 cases have been reported in the literature so far. The diagnostic hallmark is facial erythema, which spreads to the extremities but spares the trunk, and which manifests itself within the first year and then develops into poikiloderma. Two clinical subforms of RTS have been defined: RTSI characterised by poikiloderma, ectodermal dysplasia and juvenile cataracts, and RTSII characterised by poikiloderma, congenital bone defects and an increased risk of osteosarcoma in childhood and skin cancer later in life. The skeletal abnormalities may be overt (frontal bossing, saddle nose and congenital radial ray defects), and/or subtle (visible only by radiographic analysis). Gastrointestinal, respiratory and haematological signs have been reported in a few patients. RTS is transmitted in an autosomal recessive manner and is genetically heterogeneous: RTSII is caused by homozygous or compound heterozygous mutations in the RECQL4 helicase gene (detected in 60-65% of RTS patients), whereas the aetiology in RTSI remains unknown. Diagnosis is based on clinical findings (primarily on the age of onset, spreading and appearance of the poikiloderma) and molecular analysis for RECQL4 mutations. Missense mutations are rare, while frameshift, nonsense mutations and splice-site mutations prevail. A fully informative test requires transcript analysis not to overlook intronic deletions causing missplicing. The diagnosis of RTS should be considered in all patients with osteosarcoma, particularly if associated with skin changes. The differential diagnosis should include other causes of childhood poikiloderma (including dyskeratosis congenita, Kindler syndrome and Poikiloderma with Neutropaenia), other rare genodermatoses with prominent telangiectasias (including Bloom syndrome, Werner syndrome and Ataxia-telangiectasia) and the allelic disorders, RAPADILINO syndrome and Baller-Gerold syndrome, which also share some clinical features. A few mutations recur in all three RECQL4 diseases. Genetic counselling should be provided for RTS patients and their families, together with a recommendation for cancer surveillance for all patients with RTSII. Patients should be managed by a multidisciplinary team and offered long term follow-up. Treatment includes the use of pulsed dye laser photocoagulation to improve the telangiectatic component of the rash, surgical removal of the cataracts and standard treatment for individuals who develop cancer. Although some clinical signs suggest precocious aging, life expectancy is not impaired in RTS patients if they do not develop cancer. Outcomes in patients with osteosarcoma are similar in RTS and non-RTS patients, with a five-year survival rate of 60-70%. The sensitivity of RTS cells to genotoxic agents exploiting cells with a known RECQL4 status is being elucidated and is aimed at optimizing the chemotherapeutic regimen for osteosarcoma.

dRecQ4 is required for DNA synthesis and essential for cell proliferation in Drosophila

BACKGROUND

The family of RecQ DNA helicases plays an important role in the maintenance of genomic integrity. Mutations in three of the five known RecQ family members in humans, BLM, WRN and RecQ4, lead to disorders that are characterized by predisposition to cancer and premature aging.

METHODOLOGY/PRINCIPAL FINDINGS

To address the in vivo functions of Drosophila RecQ4 (dRecQ4), we generated mutant alleles of dRecQ4 using the targeted gene knock-out technique. Our data show that dRecQ4 mutants are homozygous lethal with defects in DNA replication, cell cycle progression and cell proliferation. Two sets of experiments suggest that dRecQ4 also plays a role in DNA double strand break repair. First, mutant animals exhibit sensitivity to gamma irradiation. Second, the efficiency of DsRed reconstitution via single strand annealing repair is significantly reduced in the dRecQ4 mutant animals. Rescue experiments further show that both the N-terminal domain and the helicase domain are essential to dRecQ4 function in vivo. The N-terminal domain is sufficient for the DNA repair function of dRecQ4.

CONCLUSIONS/SIGNIFICANCE

Together, our results show that dRecQ4 is an essential gene that plays an important role in not only DNA replication but also DNA repair and cell cycle progression in vivo.

DNA helicase activity in purified human RECQL4 protein

Human RECQL4 protein was expressed in insect cells using a baculovirus protein expression system and it was purified to near homogeneity. The protein sedimented at a position between catalase (230 kDa) and ferritin (440 kDa) in glycerol gradient centrifugation, suggesting that it forms homo-multimers. Activity to displace annealed 17-mer oligonucleotide in the presence of ATP was co-sedimented with hRECQL4 protein. In ion-exchange chromatography, both DNA helicase activity and single-stranded DNA-dependent ATPase activity were co-eluted with hRECQL4 protein. The requirements of ATP and Mg for the helicase activity were different from those for the ATPase activity. The data suggest that the helicase migrates on single-stranded DNA in a 3'-5' direction. These results suggest that the hRECQL4 protein exhibits DNA helicase activity.

Recurrent RECQL4 imbalance and increased gene expression levels are associated with structural chromosomal instability in sporadic osteosarcoma

Osteosarcoma (OS) is an aggressive bone tumor with complex abnormal karyotypes and a highly unstable genome, exhibiting both numerical- and structural-chromosomal instability (N- and S-CIN). Chromosomal rearrangements and genomic imbalances affecting 8q24 are frequent in OS. RECQL4 gene maps to this cytoband and encodes a putative helicase involved in the fidelity of DNA replication and repair. This protective genomic function of the protein is relevant because often patients with Rothmund-Thomson syndrome have constitutional mutations of RECQL4 and carry a very high risk of developing OS. To determine the relative level of expression of RECQL4 in OS, 18 sporadic tumors were studied by reverse transcription-polymerase chain reaction. All tumors overexpressed RECQL4 in comparison to control osteoblasts, and fluorescence in situ hybridization analysis of tumor DNA showed that expression levels were strongly copy number-dependent. Relative N- and S-CIN levels were determined by classifying copy number transitions within array comparative genomic hybridization profiles and by enumerating the frequency of break-apart fluorescence in situ hybridization within 8q24 using region-specific and control probes. Although there was no evidence that disruption of 8q24 in OS led to an elevated expression of RECQL4, there was a marked association between increased overall levels of S-CIN, determined by copy number transition frequency and higher levels of RECQL4.

p300-mediated acetylation of the Rothmund-Thomson-syndrome gene product RECQL4 regulates its subcellular localization

RECQL4 belongs to the conserved RecQ family of DNA helicases, members of which play important roles in the maintenance of genome stability in all organisms that have been examined. Although genetic alterations in the RECQL4 gene are reported to be associated with three autosomal recessive disorders (Rothmund-Thomson, RAPADILINO and Baller-Gerold syndromes), the molecular role of RECQL4 still remains poorly understood. Here, we show that RECQL4 specifically interacts with the histone acetyltransferase p300 (also known as p300 HAT), both in vivo and in vitro, and that p300 acetylates one or more of the lysine residues at positions 376, 380, 382, 385 and 386 of RECQL4. Furthermore, we report that these five lysine residues lie within a short motif of 30 amino acids that is essential for the nuclear localization of RECQL4. Remarkably, the acetylation of RECQL4 by p300 in vivo leads to a significant shift of a proportion of RECQL4 protein from the nucleus to the cytoplasm. This accumulation of the acetylated RECQL4 is a result of its inability to be imported into the nucleus. Our results provide the first evidence of a post-translational modification of the RECQL4 protein, and suggest that acetylation of RECQL4 by p300 regulates the trafficking of RECQL4 between the nucleus and the cytoplasm.

Repression of nascent strand elongation by deregulated Cdt1 during DNA replication in Xenopus egg extracts

Excess Cdt1 reportedly induces rereplication of chromatin in cultured cells and Xenopus egg extracts, suggesting that the regulation of Cdt1 activity by cell cycle-dependent proteolysis and expression of the Cdt1 inhibitor geminin is crucial for the inhibition of chromosomal overreplication between S phase and metaphase. We analyzed the consequences of excess Cdt1 for DNA replication and found that increased Cdt1 activity inhibited the elongation of nascent strands in Xenopus egg extracts. In Cdt1-supplemented extracts, overreplication was remarkably induced by the further addition of the Cdt1-binding domain of geminin (Gem79-130), which lacks licensing inhibitor activity. Further analyses indicated that fully active geminin, as well as Gem79-130, restored nascent strand elongation in Cdt1-supplemented extracts even after the Cdt1-induced stalling of replication fork elongation had been established. Our results demonstrate an unforeseen, negative role for Cdt1 in elongation and suggest that its function in the control of replication should be redefined. We propose a novel surveillance mechanism in which Cdt1 blocks nascent chain elongation after detecting illegitimate activation of the licensing system.

RecQ4 facilitates UV light-induced DNA damage repair through interaction with nucleotide excision repair factor xeroderma pigmentosum group A (XPA)

Mutations in the RECQL4 helicase gene have been linked to Rothmund-Thomson syndrome, which is characterized by genome instability, cancer susceptibility, and premature aging. To better define the cellular function of the RecQ4 protein, we investigated the subcellular localization of RecQ4 upon treatment of cells with different DNA-damaging agents including UV irradiation, 4-nitroquinoline 1-oxide, camptothecin, etoposide, hydroxyurea, and H(2)O(2). We found that RecQ4 formed discrete nuclear foci specifically in response to UV irradiation and 4-nitroquinoline 1-oxide. We demonstrated that functional RecQ4 was required for the efficient removal of UV lesions and could rescue UV sensitivity of RecQ4-deficient Rothmund-Thomson syndrome cells. Furthermore, UV treatment also resulted in the colocalization of the nuclear foci formed with RecQ4 and xeroderma pigmentosum group A in human cells. Consistently, RecQ4 could directly interact with xeroderma pigmentosum group A, and this interaction was stimulated by UV irradiation. By fractionating whole cell extracts into cytoplasmic, soluble nuclear, and chromatin-bound fractions, we observed that RecQ4 protein bound more tightly to chromatin upon UV irradiation. Taken together, our findings suggest a role of RecQ4 in the repair of UV-induced DNA damages in human cells.

Identification of new RECQL4 mutations in Caucasian Rothmund-Thomson patients and analysis of sensitivity to a wide range of genotoxic agents

Rothmund-Thomson syndrome (RTS), a rare recessive autosomal disorder, presents genome instability and clinical heterogeneity with growth deficiency, skin and bone defects, premature aging symptoms and cancer susceptibility. A subset of RTS patients presents mutations of the RECQL4 gene, member of the RecQ family of DNA helicases, including the RECQL2 (BLM) and RECQL3 (WRN) genes, defective in the cancer prone Bloom and Werner syndromes, respectively. Analysis of the RECQL4 gene in six clinically diagnosed RTS patients shows five patients, including two siblings, with eight mutations mainly located in the helicase domain, three patients presenting two mutations. The alterations include four missense mutations, one nonsense mutation and the same frameshift deletion, g.2881delG in exon 9 found in three patients. Seven RECQL4 polymorphisms, two being new, have also been identified. Primary RTS fibroblasts from these RTS patients show no sensitivity to a wide variety of genotoxic agents including ionizing or ultraviolet irradiation, nitrogen mustard, 4NQO, 8-MOP, Cis-Pt, MMC, H2O2, HU, or UV plus caffeine which could be related to the RECQL4 alterations identified here. This is in contrast with the DNA damage sensitive Bloom and Werner cells and highlights the complexity of the numerous RecQ protein functions implicated in the different cellular pathways required for maintaining genomic integrity.

Sensitivity of RECQL4-deficient fibroblasts from Rothmund-Thomson syndrome patients to genotoxic agents

RECQ helicase protein-like 4 (RECQL4) is a member of the human RECQ family of DNA helicases. Two-thirds of patients with Rothmund-Thomson syndrome (RTS) carry biallelic inactivating mutations in the RECQL4 gene. RTS is an autosomal recessive disorder characterized by poikiloderma, sparse hair, small stature, skeletal abnormalities, cataracts, and an increased risk of cancer. Mutations in two other RECQ helicases, BLM and WRN, are responsible for the cancer predisposition conditions Bloom and Werner syndromes, respectively. Previous studies have shown that BLM and WRN-deficient cells demonstrate increased sensitivity to hydroxyurea (HU), camptothecin (CPT), and 4-nitroquinoline 1-oxide (4NQO). Little is known about the sensitivity of RECQL4-deficient cells to these and other genotoxic agents. The purpose of this study was to determine if RTS cells display any distinct cellular phenotypes in response to DNA damaging agents or replication blocks that could provide insight into the molecular function of the RECQL4 protein. Our results show that primary fibroblasts from RTS patients carrying two deleterious RECQL4 mutations, compared to wild type (WT) fibroblasts, have increased sensitivity to HU, CPT, and doxorubicin (DOX), modest sensitivity to other DNA damaging agents including ultraviolet (UV) irradiation, ionizing radiation (IR), and cisplatin (CDDP), and relative resistance to 4NQO. The RECQ family of DNA helicases has been implicated in the regulation of DNA replication, recombination, and repair. Because HU, CPT, and DOX exert their effects primarily during S phase, these results support a greater role for the RECQL4 protein in DNA replication as opposed to repair of exogenous damage.