RTEL1 contributes to DNA replication and repair and telomere maintenance

Telomere maintenance and DNA repair are important processes that protect the genome against instability. mRtel1, an essential helicase, is a dominant factor setting telomere length in mice. In addition, mRtel1 is involved in DNA double-strand break repair. The role of mRtel1 in telomere maintenance and genome stability is poorly understood. Therefore we used mRtel1-deficient mouse embryonic stem cells to examine the function of mRtel1 in replication, DNA repair, recombination, and telomere maintenance. mRtel1-deficient mouse embryonic stem cells showed sensitivity to a range of DNA-damaging agents, highlighting its role in replication and genome maintenance. Deletion of mRtel1 increased the frequency of sister chromatid exchange events and suppressed gene replacement, demonstrating the involvement of the protein in homologous recombination. mRtel1 localized transiently at telomeres and is needed for efficient telomere replication. Of interest, in the absence of mRtel1, telomeres in embryonic stem cells appeared relatively stable in length, suggesting that mRtel1 is required to allow extension by telomerase. We propose that mRtel1 is a key protein for DNA replication, recombination, and repair and efficient elongation of telomeres by telomerase.     

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.     

Emerging roles of SIRT1 in vascular endothelial homeostasis

Sir2 is a NAD+-dependent deacetylase, which regulates life span in multiple model organisms in response to caloric restriction. Mammalian homologues of Sir2 comprise a family of seven proteins termed sirtuins (SIRT1-SIRT7), which have gained considerable attention for their impact on several important physiological processes associated with metabolism and stress resistance. In addition, recent studies point to SIRT1 as a key regulator of vascular endothelial homeostasis controlling angiogenesis, vascular tone and endothelial dysfunction. Here, we review the emerging role of SIRT1 as an important modulator of signaling networks critical for maintaining vascular endothelial homeostasis and discuss SIRT1 as a potential therapeutic target for cardiovascular diseases in the adult.     

SIRT1 contributes to telomere maintenance and augments global homologous recombination

Yeast Sir2 deacetylase is a component of the silent information regulator (SIR) complex encompassing Sir2/Sir3/Sir4. Sir2 is recruited to telomeres through Rap1, and this complex spreads into subtelomeric DNA via histone deacetylation. However, potential functions at telomeres for SIRT1, the mammalian orthologue of yeast Sir2, are less clear. We studied both loss of function (SIRT1 deficient) and gain of function (SIRT1(super)) mouse models. Our results indicate that SIRT1 is a positive regulator of telomere length in vivo and attenuates telomere shortening associated with aging, an effect dependent on telomerase activity. Using chromatin immunoprecipitation assays, we find that SIRT1 interacts with telomeric repeats in vivo. In addition, SIRT1 overexpression increases homologous recombination throughout the entire genome, including telomeres, centromeres, and chromosome arms. These findings link SIRT1 to telomere biology and global DNA repair and provide new mechanistic explanations for the known functions of SIRT1 in protection from DNA damage and some age-associated pathologies.     

Emerging roles of PPARdelta in metabolism

PPARdelta differs from the other two PPAR isotypes (alpha and gamma) by its more wide-spread tissue-specific expression pattern, its involvement in developmental processes and its profound impact on muscle and heart fat metabolism. Activation of PPARdelta modulates inflammatory responses of macrophages and is linked to altered lipoprotein metabolism, most importantly a significant raise of HDL cholesterol. PPARdelta activation in the liver decreases hepatic glucose output, thereby contributing to improved glucose tolerance and insulin sensitivity. Several studies have shown that PPARdelta polymorphisms are associated with plasma lipid levels, body mass index and the risk for diabetes and coronary heart disease. These findings support that high affinity PPARdelta agonists may be promising drugs of the future to treat the metabolic syndrome which is an expanding overweight-related health threat characterized by insulin resistance, hyperglycemia, dyslipidemia, hypertension, and accelerated atherosclerosis.     

DNA methyltransferases control telomere length and telomere recombination in mammalian cells

Here, we describe a role for mammalian DNA methyltransferases (DNMTs) in telomere length control. Mouse embryonic stem (ES) cells genetically deficient for DNMT1, or both DNMT3a and DNMT3b have dramatically elongated telomeres compared with wild-type controls. Mammalian telomere repeats (TTAGGG) lack the canonical CpG methylation site. However, we demonstrate that mouse subtelomeric regions are heavily methylated, and that this modification is decreased in DNMT-deficient cells. We show that other heterochromatic marks, such as histone 3 Lys 9 (H3K9) and histone 4 Lys 20 (H4K20) trimethylation, remain at both subtelomeric and telomeric regions in these cells. Lack of DNMTs also resulted in increased telomeric recombination as indicated by sister-chromatid exchanges involving telomeric sequences, and by the presence of 'alternative lengthening of telomeres' (ALT)-associated promyelocytic leukaemia (PML) bodies (APBs). This increased telomeric recombination may lead to telomere-length changes, although our results do not exclude a potential involvement of telomerase and telomere-binding proteins in the aberrant telomere elongation observed in DNMT-deficient cells. Together, these results demonstrate a previously unappreciated role for DNA methylation in maintaining telomere integrity.     

Role of mammalian Rad54 in telomere length maintenance

The homologous recombination (HR) DNA repair pathway participates in telomere length maintenance in yeast but its putative role at mammalian telomeres is unknown. Mammalian Rad54 is part of the HR machinery, and Rad54-deficient mice show a reduced HR capability. Here, we show that Rad54-deficient mice also show significantly shorter telomeres than wild-type controls, indicating that Rad54 activity plays an essential role in telomere length maintenance in mammals. Rad54 deficiency also resulted in an increased frequency of end-to-end chromosome fusions involving telomeres compared to the controls, suggesting a putative role of Rad54 in telomere capping. Finally, the study of mice doubly deficient for Rad54 and DNA-PKcs showed that telomere fusions due to DNA-PKcs deficiency were not rescued in the absence of Rad54, suggesting that they are not mediated by Rad54 activity.     

DNA repair, genome stability, and aging

Aging can be defined as progressive functional decline and increasing mortality over time. Here, we review evidence linking aging to nuclear DNA lesions: DNA damage accumulates with age, and DNA repair defects can cause phenotypes resembling premature aging. We discuss how cellular DNA damage responses may contribute to manifestations of aging. We review Sir2, a factor linking genomic stability, metabolism, and aging. We conclude with a general discussion of the role of mutant mice in aging research and avenues for future investigation.     

The MRN complex in double-strand break repair and telomere maintenance

Genomes are subject to constant threat by damaging agents that generate DNA double-strand breaks (DSBs). The ends of linear chromosomes need to be protected from DNA damage recognition and end-joining, and this is achieved through protein-DNA complexes known as telomeres. The Mre11-Rad50-Nbs1 (MRN) complex plays important roles in detection and signaling of DSBs, as well as the repair pathways of homologous recombination (HR) and non-homologous end-joining (NHEJ). In addition, MRN associates with telomeres and contributes to their maintenance. Here, we provide an overview of MRN functions at DSBs, and examine its roles in telomere maintenance and dysfunction.     

Mitochondrial DNA repair and aging

The mitochondrial electron transport chain plays an important role in energy production in aerobic organisms and is also a significant source of reactive oxygen species that damage DNA, RNA and proteins in the cell. Oxidative damage to the mitochondrial DNA is implicated in various degenerative diseases, cancer and aging. The importance of mitochondrial ROS in age-related degenerative diseases is further strengthened by studies using animal models, Caenorhabditis elegans, Drosophila and yeast. Research in the last several years shows that mitochondrial DNA is more susceptible to various carcinogens and ROS when compared to nuclear DNA. DNA damage in mammalian mitochondria is repaired by base excision repair (BER). Studies have shown that mitochondria contain all the enzymes required for BER. Mitochondrial DNA damage, if not repaired, leads to disruption of electron transport chain and production of more ROS. This vicious cycle of ROS production and mtDNA damage ultimately leads to energy depletion in the cell and apoptosis.     

Molecular characterization of the Schizosaccharomyces pombe nbs1+ gene involved in DNA repair and telomere maintenance

The human MRN complex is a multisubunit nuclease that is composed of Mre11, Rad50, and Nbs1 and is involved in homologous recombination and DNA damage checkpoints. Mutations of the MRN genes cause genetic disorders such as Nijmegen breakage syndrome. Here we identified a Schizosaccharomyces pombe nbs1(+) homologue by screening for mutants with mutations that caused methyl methanesulfonate (MMS) sensitivity and were synthetically lethal with the rad2Delta mutation. Nbs1 physically interacts with the C-terminal half of Rad32, the Schizosaccharomyces pombe Mre11 homologue, in a yeast two-hybrid assay. nbs1 mutants showed sensitivities to gamma-rays, UV, MMS, and hydroxyurea and displayed telomere shortening similar to the characteristics of rad32 and rad50 mutants. nbs1, rad32, and rad50 mutant cells were elongated and exhibited abnormal nuclear morphology. These findings indicate that S. pombe Nbs1 forms a complex with Rad32-Rad50 and is required for homologous recombination repair, telomere length regulation, and the maintenance of chromatin structure. Amino acid sequence features and some characteristics of the DNA repair function suggest that the S. pombe Rad32-Rad50-Nbs1 complex has functional similarity to the corresponding MRN complexes of higher eukaryotes. Therefore, S. pombe Nbs1 will provide an additional model system for studying the molecular function of the MRN complex associated with genetic diseases.     

DNA damage metabolism and aging

As a result of permanent exposure to low levels of various endogenous and exogenous genotoxic agents, large numbers of lesions are continuously induced in the DNA of cells of living organisms. Such lesions could lead to dysfunction of cells and tissues, and they might well be the underlying cause of the age-related reduction of homeostatic capacity and the increased incidence of cancer and other diseases of old age. The rate of damage induction as well as the persistence of the lesions depends on the activity, efficiency and reliability of a wide variety of molecular defense systems. However, a certain degree of imperfection seems to be a general characteristic of most of these defense systems and this could lead to a gradual accumulation of DNA alterations during aging. Even when the original lesions are quickly removed, they can still lead to secondary changes in the DNA, such as DNA-sequence changes and changes in gene expression. This process would be accelerated in case of the occurrence of an age-related decline in the efficiency of these molecular defense systems. This review deals with the present knowledge on the occurrence of 'spontaneous' DNA damage in aging organisms, its potential sources, the influence of preventive and processive cellular defense mechanisms and its consequences in terms of DNA-sequence changes, DNA conformational and configurational changes and changes in gene expression. In general, it can be concluded from the data discussed here that, in spite of a number of discrepancies and conflicting results, an age-related accumulation of DNA alterations occurs at all levels, e.g., chemical structure, DNA-sequence organization and gene expression.     

Human premature aging, DNA repair and RecQ helicases

Genomic instability leads to mutations, cellular dysfunction and aberrant phenotypes at the tissue and organism levels. A number of mechanisms have evolved to cope with endogenous or exogenous stress to prevent chromosomal instability and maintain cellular homeostasis. DNA helicases play important roles in the DNA damage response. The RecQ family of DNA helicases is of particular interest since several human RecQ helicases are defective in diseases associated with premature aging and cancer. In this review, we will provide an update on our understanding of the specific roles of human RecQ helicases in the maintenance of genomic stability through their catalytic activities and protein interactions in various pathways of cellular nucleic acid metabolism with an emphasis on DNA replication and repair. We will also discuss the clinical features of the premature aging disorders associated with RecQ helicase deficiencies and how they relate to the molecular defects.