Mapping genetic loci that determine leukocyte telomere length in a large sample of unselected female sibling pairs

Telomeres play a central role in cellular senescence and cancer pathobiology and are associated with age-related diseases such as atherosclerosis and dementia. Telomere length varies between individuals of the same age, is influenced by DNA-damaging factors such as oxidative stress, and is heritable. We performed a quantitative-trait linkage analysis using an approximate 10-cM genomewide map for mean leukocyte terminal-restriction fragment (TRF) lengths measured by Southern blotting, in 2,050 unselected women aged 18-80 years, comprising 1,025 complete dizygotic twin pairs. Heritability of mean batch-adjusted TRF was 36% (95% confidence interval [CI] 18%-48%), with a large common environmental effect of 49% (95% CI 40%-58%). Significant linkage was observed on chromosome 14 (LOD 3.9) at 14q23.2, and suggestive linkage at 10q26.13 (LOD 2.4) and 3p26.1 (LOD 2.7). This is the first report of loci, mapped in a sample of healthy individuals, that influence mean telomere variation in humans.     

Differences in telomere length between homologous chromosomes in humans

Telomeres are important structures for DNA replication and chromosome stability during cell growth. Telomere length has been correlated with the division potential of human cells and has been found to decrease with age in healthy individuals. Nevertheless, telomere lengths within the same cell are heterogeneous and certain chromosome arms typically have either short or long telomeres. Both the origin and the physiological consequences of this heterogeneity in telomere length remain unknown. In this study we used quantitative telomeric FISH combined with a method to identify the parental origin of chromosomes to show that significant differences in relative telomere intensities are frequently observed between chromosomal homologs in short-term stimulated cultures of peripheral blood lymphocytes. These differences appear to be stable for at least 4 months in vivo, but disappear after prolonged proliferation in vitro. The telomere length differences are also stable during in vitro growth of telomerase-negative fibroblast cells but can be abolished by exogenous telomerase expression in these cells. These findings suggest the existence of a mechanism maintaining differences in telomere length between chromosome homologs that is independent of telomere length itself.     

Inheritance of telomere length in a bird

Telomere dynamics are intensively studied in human ageing research and epidemiology, with many correlations reported between telomere length and age-related diseases, cancer and death. While telomere length is influenced by environmental factors there is also good evidence for a strong heritable component. In human, the mode of telomere length inheritance appears to be paternal and telomere length differs between sexes, with females having longer telomeres than males. Genetic factors, e.g. sex chromosomal inactivation, and non-genetic factors, e.g. antioxidant properties of oestrogen, have been suggested as possible explanations for these sex-specific telomere inheritance and telomere length differences. To test the influence of sex chromosomes on telomere length, we investigated inheritance and sex-specificity of telomere length in a bird species, the kakapo (Strigops habroptilus), in which females are the heterogametic sex (ZW) and males are the homogametic (ZZ) sex. We found that, contrary to findings in humans, telomere length was maternally inherited and also longer in males. These results argue against an effect of sex hormones on telomere length and suggest that factors associated with heterogamy may play a role in telomere inheritance and sex-specific differences in telomere length.     

Control of telomere length by a trimming mechanism that involves generation of t-circles

Telomere lengths are maintained in many cancer cells by the ribonucleoprotein enzyme telomerase but can be further elongated by increasing telomerase activity through the overexpression of telomerase components. We report here that increased telomerase activity results in increased telomere length that eventually reaches a plateau, accompanied by the generation of telomere length heterogeneity and the accumulation of extrachromosomal telomeric repeat DNA, principally in the form of telomeric circles (t-circles). Telomeric DNA was observed in promyelocytic leukemia bodies, but no intertelomeric copying or telomere exchange events were identified, and there was no increase in telomere dysfunction-induced foci. These data indicate that human cells possess a mechanism to negatively regulate telomere length by trimming telomeric DNA from the chromosome ends, most likely by t-loop resolution to form t-circles. Additionally, these results indicate that some phenotypic characteristics attributed to alternative lengthening of telomeres (ALT) result from increased mean telomere length, rather than from the ALT mechanism itself.     

Strain-specific telomere length revealed by single telomere length analysis in Caenorhabditis elegans

Terminal restriction fragment analysis is the only method currently available for measuring telomere length in Caenorhabditis elegans. Its limitations include low sensitivity and interference by the presence of interstitial telomeric sequences in the C.elegans genome. Here we report the adaptation of single telomere length analysis (STELA) to measure the length of telomeric repeats on the left arm of chromosome V in C.elegans. This highly sensitive PCR-based method allows telomere length measurement from as few as a single worm. The application of STELA to eight wild-type C.elegans strains revealed considerable strain-specific differences in telomere length. Within individual strains, short outlying telomeres were observed that were clearly distinct from the bulk telomere length distributions, suggesting that processes other than end-replication losses and telomerase-mediated lengthening may generate telomere length heterogeneity in C.elegans. The utility of this method was further demonstrated by the characterization of telomere shortening in mrt-2 mutants. We conclude that STELA appears to be a valuable tool for studying telomere biology in C.elegans.     

Sex-specific telomere length profiles and age-dependent erosion dynamics of individual chromosome arms in humans

During aging, telomeres are gradually shortened, eventually leading to cellular senescence. By T/C-FISH (telomere/centromere-FISH), we investigated human telomere length differences on single chromosome arms of 205 individuals in different age groups and sexes. For all chromosome arms, we found a linear correlation between telomere length and donor age. Generally, males had shorter telomeres and higher attrition rates. Every chromosome arm had its individual age-specific telomere length and erosion pattern, resulting in an unexpected heterogeneity in chromosome-specific regression lines. This differential erosion pattern, however, does not seem to be accidental, since we found a correlation between average telomere length of single chromosome arms in newborns and their annual attrition rate. Apart from the above-mentioned sex-specific discrepancies, chromosome arm-specific telomere lengths were strikingly similar in men and women. This implies a mechanism that arm specifically regulates the telomere length independent of gender, thus leading to interchromosomal telomere variations.     

Assessment of telomere length and factors that contribute to its stability.

Short strands of tandem hexameric repeats known as telomeres cap the ends of linear chromosomes. These repeats protect chromosomes from degradation and prevent chromosomal end-joining, a phenomenon that could occur due to the end-replication problem. Telomeres are maintained by the activity of the enzyme telomerase. The total number of telomeric repeats at the terminal end of a chromosome determines the telomere length, which in addition to its importance in chromosomal stabilization is a useful indicator of telomerase activity in normal and malignant tissues. Telomere length stability is one of the important factors that contribute to the proliferative capacity of many cancer cell types; therefore, the detection and estimation of telomere length is extremely important. Until relatively recently, telomere lengths were analyzed primarily using the standard Southern blot technique. However, the complexities of this technique have led to the search for more simple and rapid detection methods. Improvements such as the use of fluorescent probes and the ability to sort cells have greatly enhanced the ease and sensitivity of telomere length measurements. Recent advances, and the limitations of these techniques are evaluated. Drugs that assist in telomere shortening may contribute to tumor regression. Therefore, factors that contribute to telomere stability may influence the efficiency of the drugs that have potential in cancer therapy. These factors in relation to telomere length are also examined in this analysis.     

The role of telomere trimming in normal telomere length dynamics

Telomeres consist of repetitive DNA and associated proteins that protect chromosome ends from illicit DNA repair. It is well known that telomeric DNA is progressively eroded during cell division, until telomeres become too short and the cell stops dividing. There is a second mode of telomere shortening, however, which is a regulated form of telomere rapid deletion (TRD) termed telomere trimming that is reviewed here. Telomere trimming appears to involve resolution of recombination intermediate structures, which shortens the telomere by release of extrachromosomal telomeric DNA. This has been detected in human and in mouse cells and occurs both in somatic and germline cells, where it sets an upper limit on telomere length and contributes to a length equilibrium set-point in cells that have a telomere elongation mechanism. Telomere trimming thus represents an additional mechanism of telomere length control that contributes to normal telomere dynamics and cell proliferative potential.     

A genome-wide screen for essential yeast genes that affect telomere length maintenance

Telomeres are structures composed of repetitive DNA and proteins that protect the chromosomal ends in eukaryotic cells from fusion or degradation, thus contributing to genomic stability. Although telomere length varies between species, in all organisms studied telomere length appears to be controlled by a dynamic equilibrium between elongating mechanisms (mainly addition of repeats by the enzyme telomerase) and nucleases that shorten the telomeric sequences. Two previous studies have analyzed a collection of yeast deletion strains (deleted for nonessential genes) and found over 270 genes that affect telomere length (Telomere Length Maintenance or TLM genes). Here we complete the list of TLM by analyzing a collection of strains carrying hypomorphic alleles of most essential genes (DAmP collection). We identify 87 essential genes that affect telomere length in yeast. These genes interact with the nonessential TLM genes in a significant manner, and provide new insights on the mechanisms involved in telomere length maintenance. The newly identified genes span a variety of cellular processes, including protein degradation, pre-mRNA splicing and DNA replication.     

Mapping of a genetic locus that affects glycerol 3-phosphate transport in Bacillus subtilis.

Two types of fosfomycin-resistant mutants of Bacillus subtilis were isolated. Mutants of the first type (GlpT mutants) were resistant to at least 200 microgram of fosfomycin per ml and failed to take up exogenous glycerol 3-phosphate. Mutants of the second type were resistant to lower concentrations of fosfomycin and transported glycerol-3-phosphate as efficiently as wild-type bacteria. The glpT mutations, but not the mutations in the second type of fosfomycin-resistant mutants, map in the cysA-aroI region of the B. subtilis chromosome.     

Fine mapping of a major quantitative trait locus that regulates pod shattering in soybean

Legumes represent the second most important family of crop plants, accounting for ~27 % of the world’s crop production. While some legumes are grown as forages or vegetables, most crop legumes are grown for harvesting their nutritious seeds. The legume seeds are contained in the pod, which is composed of a single seed-bearing carpel that, when matures, splits open along two seams, a process called pod dehiscence or pod shattering. Pod shattering before or during harvest causes yield losses of grain legumes. Moreover, the dominant shattering trait of the wild progenitors is a limiting factor for efficient introgression of value-added traits into elite breeding lines. Knowledge of the genetic mechanisms underlying pod shattering will facilitate breeding of shattering-resistant varieties, expedite introgression of agronomically favorable traits from wild species to elite breeding lines, and enrich our understanding of the evolution of seed dispersal and crop domestication in diverse crop species. Here we report fine mapping of a major quantitative trait locus (designated as qPDH1) that regulates pod shattering in soybean (Glycine max). A combination of linkage and association mapping allowed us to delimit the qPDH1 locus within a 47-kb region on chromosome 16. The data reported here will facilitate positional cloning of the underlying gene and the development of breeder-friendly genetic markers for marker-assisted selection in soybean.     

Blood cell telomere length is a dynamic feature

There is a considerable heterogeneity in blood cell telomere length (TL) for individuals of similar age and recent studies have revealed that TL changes by time are dependent on TL at baseline. TL is partly inherited, but results from several studies indicate that e.g. life style and/or environmental factors can affect TL during life. Collectively, these studies imply that blood cell TL might fluctuate during a life time and that the actual TL at a defined time point is the result of potential regulatory mechanism(s) and environmental factors. We analyzed relative TL (RTL) in subsequent blood samples taken six months apart from 50 individuals and found significant associations between RTL changes and RTL at baseline. Individual RTL changes per month were more pronounced than the changes recorded in a previously studied population analyzed after 10 years' follow up. The data argues for an oscillating TL pattern which levels out at longer follow up times. In a separate group of five blood donors, a marked telomere loss was demonstrated within a six month period for one donor where after TL was stabilized. PCR determined RTL changes were verified by Southern blotting and STELA (single telomere elongation length analysis). The STELA demonstrated that for the donor with a marked telomere loss, the heterogeneity of the telomere distribution decreased considerably, with a noteworthy loss of the largest telomeres. In summary, the collected data support the concept that individual blood cell telomere length is a dynamic feature and this will be important to recognize in future studies of human telomere biology.     

The yeast telomere length regulator TEL2 encodes a protein that binds to telomeric DNA.

TEL2 is required for telomere length regulation and viability in Saccharomyces cerevisiae. To investigate the mechanism by which Tel2p regulates telomere length, the majority (65%) of the TEL2 ORF was fused to the 3'-end of the gene for maltose binding protein, expressed in bacteria and the purified protein used in DNA binding studies. Rap1p, the major yeast telomere binding protein, recognizes a 13 bp duplex site 5'-GGTGTGTGGGTGT-3' in yeast telomeric DNA with high affinity. Gel shift experiments revealed that the MBP-Tel2p fusion binds the double-stranded yeast telomeric Rap1p site in a sequence-specific manner. Analysis of mutated sites showed that MBP-Tel2p could bind 5'-GTGTGTGG-3' within this 13 bp site. Methylation interference analysis revealed that Tel2p contacts the 5'-terminal guanine in the major groove. MBP-Tel2p did not bind duplex telomeric DNA repeats from vertebrates, Tetrahymena or Oxytricha. These results suggest that Tel2p is a DNA binding protein that recognizes yeast telomeric DNA.     

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.     

Effects of telomere length on leukemogenesis

正Dear Editor,Most cancer cells maintain the length of their telomeres via telomerase(Günes and Rudolph, 2013; Kim et al., 1994).However, in some cancers, telomeres are maintained not by telomerase but by alternative lengthening of telomeres     

Flow cytometric measurement of telomere length

The regulation of telomere length may be involved in the cellular physiology of senescence, reproduction, cancer, immune response to infection, and possibly immune deficiency. The measurement of telomere length, critical to research in this area, has traditionally been performed by Southern blot analysis, which is cumbersome and time consuming. Several alternative methods have been described in recent years. Some, such as pulsed-field electrophoresis, slot blots, and centromere-to-telomere ratio measurements are essentially improvements to the Southern blot technique. However, other methods such as fluorescent in situ hybridization on metaphase chromosome spreads and flow cytometry-based fluorescent in situ hybridization represent a completely new technical approach to the problem. In this review, we compare methods, with particular emphasis placed on flow cytometric techniques for measuring telomere length in situ and identifying potential areas where improvements may still be made.     

Different mechanisms of telomere length regulation

Eukaryotic chromosomes are linear and have their, ends formed by DNA-protein structures, telomeres. At present more and more facts demonstrate the diversity of telomere functions. Telomeres protect the chromosome ends from degradation, fusion, recombination, and from the repair system that recognizes nicks in DNA strands. As shown recently, shortening of the telomeres is a cause of cell aging. In most organisms, telomeres are elongated by means of a special ribonucleoprotein complex; however, in some insects this takes place by either gene conversion or transposition of mobile elements. Evolutionary relations between different types of telomeres are discussed.     

Strength determines coalitional strategies in humans

Coalitions enhance survival and reproductive success in many social species, yet they generate contradictory impulses. Whereas a coalition increases the probability of successfully obtaining rewards for its members, it typically requires a division of rewards among members, thereby diminishing individual benefits. Non-human primate data indicate that coalition formation is more likely when an individual''s probability of success is low when competing alone. No comparable studies exist for humans. Here we show using a computerized competitive game that humans exhibit a systematic, intuitive strategy for coalition formation based on their own and others'' levels of perceived strength, a pattern that resembles coalition formation in chimpanzees, Pan troglodytes. Despite equal expected pay-offs for all strategies, subjects were more likely to form coalitions as their own level of perceived strength waned. Those chosen as coalition partners tended to be stronger individuals or arbitrarily designated ‘friends’. Results suggest a heuristic for human coalitionary decisions that rests on the perception of relative power rather than on the assessment of pay-offs.