Telomere repeat DNA forms a large non-covalent complex with unique cohesive properties which is dissociated by Werner syndrome DNA helicase in the presence of replication protein A

Model of human aging: recent findings on Werner's and Hutchinson-Gilford progeria syndromes

The molecular mechanisms involved in human aging are complicated. Two progeria syndromes, Werner's syndrome (WS) and Hutchinson-Gilford progeria syndrome (HGPS), characterized by clinical features mimicking physiological aging at an early age, provide insights into the mechanisms of natural aging. Based on recent findings on WS and HGPS, we suggest a model of human aging. Human aging can be triggered by two main mechanisms, telomere shortening and DNA damage. In telomere-dependent aging, telomere shortening and dysfunction may lead to DNA damage responses which induce cellular senescence. In DNA damage-initiated aging, DNA damage accumulates, along with DNA repair deficiencies, resulting in genomic instability and accelerated cellular senescence. In addition, aging due to both mechanisms (DNA damage and telomere shortening) is strongly dependent on p53 status. These two mechanisms can also act cooperatively to increase the overall level ofgenomic instability, triggering the onset of human aging phenotypes.

[Mechanism of cell proliferation--cell cycle, oncogenes, and senescence]

Cell proliferation is regulated through a transition between the G0 phase and cell cycle. We isolated a mammalian temperature-sensitive mutant cell line defective in the function from the G0 phase to cell cycle. Senescent human somatic cells fail to enter into the cell cycle from the G0 phase with stimulation by any growth factor. Telomere shortening was found to be a cause of cellular senescence, and reexpression of telomerase immortalized human somatic cells. Immortalized human somatic cells showed normal phenotypes and were useful not only for basic research but also for clinical and applied fields. The importance of p53 and p21 activation/induction i now well accepted in the signal transduction process from telomere shortening to growth arrest, but the precise mechanism is largely unknown as yet. We found that the MAP kinase cascade and histone acetylase have an important role in the signaling process to express p21. Tumor tissues and cells were found to have strong telomerase activity, while most normal somatic human tissues showed very weak or no activity. Telomerase activity was shown to be a good marker for early tumor diagnosis because significant telomerase activity was detected in very early tumors or even in some precancerous tissues compared with adjacent normal tissues. Telomere/telomerase is a candidate target for cancer chemotherapeutics, and an agent that abrogated telomere functions was found to kill tumor cells effectively by inducing apoptosis whereas it showed no effect on the viability of normal cells.

Current advances in unraveling the function of the Werner syndrome protein

Werner syndrome (WS) is an autosomal recessive premature aging disease manifested by the mimicry of age-related phenotypes such as atherosclerosis, arteriosclerosis, cataracts, osteoporosis, soft tissue calcification, premature thinning, graying, and loss of hair, as well as a high incidence of some types of cancers. The gene product defective in WS, WRN, is a member of the RecQ family of DNA helicases that are widely distributed in nature and believed to play central roles in genomic stability of organisms ranging from prokaryotes to mammals. Interestingly, WRN is a bifunctional protein that is exceptional among RecQ helicases in that it also harbors an exonuclease activity. Furthermore, it preferentially operates on aberrant DNA structures believed to exist in vivo as intermediates in specific DNA transactions such as replication (forked DNA), recombination (Holliday junction, triplex and tetraplex DNA), and repair (partial duplex with single stranded bubble). In addition, WRN has been shown to physically and functionally interact with a variety of DNA-processing proteins, including those that are involved in resolving alternative DNA structures, repair DNA damage, and provide checkpoints for genomic stability. Despite significant research activity and considerable progress in understanding the biochemical and molecular genetic function of WRN, the in vivo molecular pathway(s) of WRN remain elusive. The following review focuses on the recent advances in the biochemistry of WRN and considers the putative in vivo functions of WRN in light of its many protein partners.

Genetic alterations in accelerated ageing syndromes

The molecular mechanisms leading to human senescence are still not known mostly because of the complexity of the process. Different research approaches are used to study ageing including studies of monogenic segmental progeroid syndromes. None of the known progerias represents true precocious ageing. Some of them, including Werner (WS), Bloom (BS), and Rothmund-Thomson syndromes (RTS) as well as combined xeroderma pigmentosa-Cockayne syndrome (XP-CS) are characterised by features resembling precocious ageing and the increased risk of malignant disease. Such phenotypes result from the mutations of the genes encoding proteins involved in the maintenance of genomic integrity, in most cases DNA helicases. Defective functioning of these proteins affects DNA repair, recombination, replication and transcription. Other segmental progeroid syndromes, such as Hutchinson-Gilford progeria (HGPS) and Cockayne syndrome are not associated with an increased risk of cancer. In this paper we present the clinical and molecular features of selected progeroid syndromes and describe the potential implications of these data for studies of ageing and cancer development.

Terminal telomere repeats are actually short in telomerase-negative immortal human cells

Telomerase-negative immortal human cells maintained telomere length by a mechanism called alternative lengthening of telomeres (ALT mechanism). These cells (ALT cells) have two prominent characteristics of long telomere repeats at each chromosome end revealed by Southern blotting (terminal restriction fragments: TRF) and the presence extrachromosomal telomere repeat (ECTR) DNA. We report here that the TRF length of ALT cells revealed by the conventional unidirectional (UD) current or pulse-field (PF) current electrophoresis appeared to be over estimated. The TRF length determined by the pulse inverse-field (PIF) current electrophoresis (2-9 kbp depending upon cell lines) was much smaller than that (ca. 23 kbp) by UD or PF current electrophoresis. These results were in consistent with very weak telomere staining in situ at chromosome ends in ALT cells. When a mixture of HinfI-digested genomic DNA of human diploid fibroblasts and synthetic telomere repeat DNA with similar size of ECTR DNA was electrophoresed using a UD current, the apparent TRF size shifted to larger molecular weight, while the size shift did not occur by PIF current electrophoresis. These results together with other data indicate that the unusually long TRF of ALT cells determined by using conventional electrophoresis is an artifact produced by a complex formed by short TRF and short ECTR DNA.

Telomere dysfunction in genome instability syndromes

Telomeres are nucleoprotein complexes located at the end of eukaryotic chromosomes. They have essential roles in preventing terminal fusions, protecting chromosome ends from degradation, and in chromosome positioning in the nucleus. These terminal structures consist of a tandemly repeated DNA sequence (TTAGGG in vertebrates) that varies in length from 5 to 15 kb in humans. Several proteins are attached to this telomeric DNA, some of which are also involved in different DNA damage response pathways, including Ku80, Mre11, NBS and BLM, among others. Mutations in the genes encoding these proteins cause a number of rare genetic syndromes characterized by chromosome and/or genetic instability and cancer predisposition. Deletions or mutations in any of these genes may also cause a telomere defect resulting in accelerated telomere shortening, lack of end-capping function, and/or end-to-end chromosome fusions. This telomere phenotype is also known to promote chromosomal instability and carcinogenesis. Therefore, it is essential to understand the interplay between telomere biology and genome stability. This review is focused in the dual role of chromosome fragility proteins in telomere maintenance.

WRN gene and other genetic factors affecting immortalization of human B-lymphoblastoid cell lines transformed by Epstein-Barr virus

The immortalization of human B-lymphoblastoid cell lines (LCL) transformed by Epstein-Barr virus (EBV) is accompanied by two major events: increase in telomerase activity and change in karyotype from normal diploid to aneuploidy. We investigated the effect of genetic factors on the incidence of immortalization by putting old and new data together to collect enough samples for statistical analysis. Among 50 LCL from normal individuals, 5 LCL (10.0%) were immortalized and the remaining 45 LCL were mortal. None of the 44 LCL (0%; P < 0.031 against normal individuals by chi square test) from patients having Werner syndrome (WS), a recessive genetic disorder showing premature aging, were immortalized. Among 11 LCL from a family with a tendency to have hereditary type 2 diabetes mellitus, 5 LCL (45.5%; P < 0.0040 against normal individuals, P < 0.00001 against WS patients) were immortalized. Duplicated measurements of the lifespan of 33 LCL showed a good coincidence (r=0.785) between the first and second estimations, indicating that each mortal LCL has a predetermined lifespan. These results strongly suggest that the normal WRN gene, the causative gene of WS, is essential for LCL to immortalize, and genetic factor(s) of a family having diabetes mellitus increases immortalization, implicating that host genetic factors affect immortalization of EBV and probably carcinogenesis by EBV.

Biochemical characterization of an exonuclease from Arabidopsis thaliana reveals similarities to the DNA exonuclease of the human Werner syndrome protein

The human Werner syndrome protein (hWRN-p) possessing DNA helicase and exonuclease activities is essential for genome stability. Plants have no homologue of this bifunctional protein, but surprisingly the Arabidopsis genome contains a small open reading frame (ORF) (AtWRNexo) with homology to the exonuclease domain of hWRN-p. Expression of this ORF in Escherichia coli revealed an exonuclease activity for AtWRN-exo-p with similarities but also some significant differences to hWRN-p. The protein digests recessed strands of DNA duplexes in the 3' --> 5' direction but hardly single-stranded DNA or blunt-ended duplexes. In contrast to the Werner exonuclease, AtWRNexo-p is also able to digest 3'-protruding strands. DNA with recessed 3'-PO4 and 3'-OH termini is degraded to a similar extent. AtWRNexo-p hydrolyzes the 3'-recessed strand termini of duplexes containing mismatched bases. AtWRNexo-p needs the divalent cation Mg2+ for activity, which can be replaced by Mn2+. Apurinic sites, cholesterol adducts, and oxidative DNA damage (such as 8-oxoadenine and 8-oxoguanine) inhibit or block the enzyme. Other DNA modifications, including uracil, hypoxanthine and ethenoadenine, did not inhibit AtWRNexo-p. A mutation of a conserved residue within the exonuclease domain (E135A) completely abolished the exonucleolytic activity. Our results indicate that a type of WRN-like exonuclease activity seems to be a common feature of the DNA metabolism of animals and plants.

RecQ helicases: caretakers of the genome

RecQ helicases are highly conserved from bacteria to man. Germline mutations in three of the five known family members in humans give rise to debilitating disorders that are characterized by, amongst other things, a predisposition to the development of cancer. One of these disorders--Bloom's syndrome--is uniquely associated with a predisposition to cancers of all types. So how do RecQ helicases protect against cancer? They seem to maintain genomic stability by functioning at the interface between DNA replication and DNA repair.

Initiation of eukaryotic DNA replication: regulation and mechanisms

The accurate and timely duplication of the genome is a major task for eukaryotic cells. This process requires the cooperation of multiple factors to ensure the stability of the genetic information of each cell. Mutations, rearrangements, or loss of chromosomes can be detrimental to a single cell as well as to the whole organism, causing failures, disease, or death. Because of the size of eukaryotic genomes, chromosomal duplication is accomplished in a multiparallel process. In human somatic cells between 10,000 and 100,000 parallel synthesis sites are present. This raises fundamental problems for eukaryotic cells to coordinate the start of DNA replication at each origin and to prevent replication of already duplicated DNA regions. Since these general phenomena were recognized in the middle of the 20th century the regulation and mechanisms of the initiation of eukaryotic DNA replication have been intensively investigated. These studies were carried out to find the essential factors involved in the process and to determine their functions during DNA replication. These studies gave rise to a model of the organization and the coordination of DNA replication within the eukaryotic cell. The elegant experiments carried out by Rao and Johnson (1970) (1), who fused cells in different phases of the cell cycle, showed that G1 cells are competent for replication of their chromosomes, but lack a specific diffusible factor required to activate their replicaton machinery and showed that G2 cells are incompetent for DNA replication. These findings suggested that eukaryotic cells exist in two states. In G1 phase, cells are competent to initiate DNA replication, which is subsequently triggered in S phase. After completion of S phase, cells in G2 are no longer able to initiate DNA replication and they require a transition through mitosis to reenable initiation of DNA replication to take place in the next S phase. The Xenopus cell-free replication system has proved a good model system in which to study DNA replication in vitro as well as the mechanism preventing rereplication within a single cell cycle (2). Studies using this system resulted in the development of a model postulating the existence of a replication licensing factor, which binds to chromatin before the G1-S transition and which is displaced during replication (2, 3). These results were supported by genetic and biochemical experiments in Saccharomyces cerevisiae (budding yeast) and Schizosaccharomyces pombe (fission yeast) (4, 5). The investigation of cell division cycle mutants and the budding yeast origin of replication resulted in the concept of a prereplicative and a postreplicative complex of initiation proteins (6-9). These three individual concepts have recently started to merge and it has become obvious that initiation in eukaryotes is generally governed by the same ubiquitous mechanisms.

The Werner syndrome helicase-nuclease--one protein, many mysteries

Werner syndrome (WS) is an autosomal recessive condition characterized by an early onset of age-related symptoms that include ocular cataracts, premature graying and loss of hair, arteriosclerosis and atherosclerosis, diabetes mellitus, osteoporosis, and a high incidence of some types of cancers. A major motivation for the study of WS is the expectation that elucidation of its underlying mechanisms will illuminate the basis for "normal" aging. In 1996, the gene responsible for the syndrome was positionally cloned. This advance launched an explosion of experiments aimed at unraveling the molecular mechanisms that lead to the WS phenotype. Soon thereafter, its protein product, WRN, was expressed, purified, and identified as a DNA helicase-exonuclease, a bifunctional enzyme that both unwinds DNA helices and cleaves nucleotides one at a time from the end of the DNA. WRN was shown to interact physically and functionally with several DNA-processing proteins, and WRN transgenic and null mutant mouse strains were generated and described. The substantial number of excellent reviews on WRN and WS that were published in the past 2 years (1-7) reflects the rapid pace of advances made in the field. Unlike those comprehensive articles, this review focuses on the biochemistry of the WRN protein and some aspects of its cell biology. Also considered are the putative functions of WRN in normal cells and the consequences of the loss of these functions in WS.

Werner Syndrome

Werner syndrome is a premature aging disease caused by the mutation in the WRN gene. The cloning and characterization of the WRN gene and its product allows investigators to study the disease and the human aging process at molecular level. This review summarizes the recent progresses on various aspects of the WRN research including functional analysis of the protein, interactive cloning, complexes formation, mouse models, and SNPs (single nucleotide polymorphisms). These in depth investigations have greatly advanced our understanding of the disease and elucidated future research direction for Werner syndrome and the human aging process.

The Saccharomyces cerevisiae WRN homolog Sgs1p participates in telomere maintenance in cells lacking telomerase

Werner syndrome (WS) is marked by early onset of features resembling aging, and is caused by loss of the RecQ family DNA helicase WRN. Precisely how loss of WRN leads to the phenotypes of WS is unknown. Cultured WS fibroblasts shorten their telomeres at an increased rate per population doubling and the premature senescence this loss induces can be bypassed by telomerase. Here we show that WRN co-localizes with telomeric factors in telomerase-independent immortalized human cells, and further that the budding yeast RecQ family helicase Sgs1p influences telomere metabolism in yeast cells lacking telomerase. Telomerase-deficient sgs1 mutants show increased rates of growth arrest in the G2/M phase of the cell cycle as telomeres shorten. In addition, telomerase-deficient sgs1 mutants have a defect in their ability to generate survivors of senescence that amplify telomeric TG1-3 repeats, and SGS1 functions in parallel with the recombination gene RAD51 to generate survivors. Our findings indicate that Sgs1p and WRN function in telomere maintenance, and suggest that telomere defects contribute to the pathogenesis of WS and perhaps other RecQ helicase diseases.

SGS1 is required for telomere elongation in the absence of telomerase

In S. cerevisiae, mutations in genes that encode telomerase components, such as the genes EST1, EST2, EST3, and TLC1, result in the loss of telomerase activity in vivo. Two telomerase-independent mechanisms can overcome the resulting senescence. Type I survival is characterized by amplification of the subtelomeric Y' elements with a short telomere repeat tract at the terminus. Type II survivors arise through the abrupt addition of long tracts of telomere repeats. Both mechanisms are dependent on RAD52 and on either RAD50 or RAD51. We show here that the telomere elongation pathway in yeast (type II) is dependent on SGS1, the yeast homolog of the gene products of Werner's (WRN) and Bloom's (BLM) syndromes. Survival in the absence of SGS1 and EST2 is dependent upon RAD52 and RAD51 but not RAD50. We propose that the RecQ family helicases are required for processing a DNA structure specific to eroding telomeres.