Telomere directed fragmentation of mammalian chromosomes

Artificial chromosome technology and its potential application in plants

Plant genetic engineering and transgenic technology are powerful ways to study the function of genes and improve crop yield and quality in the past few years. However, only a few genes could be transformed by most available genetic engineering and transgenic technologies, so changes still need to be made to meet the demands for high throughput studies, such as investigating the whole genetic pathway of crop traits and avoiding undesirable genes simultaneously in the next generation. Plant artificial chromosome (PAC) technology provides a carrier which allows us to assemble multiple and specific genes to produce a variety of products by minichromosome. However, PAC technology also have limitations that may hinder its further development and application. In this review, we will introduce the current state of PACs technology from PACs formation, factors on PACs formation, problems and potential solutions of PACs and exogenous gene(s) integration.

Artificial generation of centromeres and kinetochores to understand their structure and function

The centromere is an essential genomic region that provides the surface to form the kinetochore, which binds to the spindle microtubes to mediate chromosome segregation during mitosis and meiosis. Centromeres of most organisms possess highly repetitive sequences, making it difficult to study these loci. However, an unusual centromere called a "neocentromere," which does not contain repetitive sequences, was discovered in a patient and can be generated experimentally. Recent advances in genome biology techniques allow us to analyze centromeric chromatin using neocentromeres. In addition to neocentromeres, artificial kinetochores have been generated on non-centromeric loci, using protein tethering systems. These are powerful tools to understand the mechanism of the centromere specification and kinetochore assembly. In this review, we introduce recent studies utilizing the neocentromeres and artificial kinetochores and discuss current problems in centromere biology.

Interstitial telomeric sequences in vertebrate chromosomes: Origin, function, instability and evolution

By definition, telomeric sequences are located at the very ends or terminal regions of chromosomes. However, several vertebrate species show blocks of (TTAGGG)n repeats present in non-terminal regions of chromosomes, the so-called interstitial telomeric sequences (ITSs), interstitial telomeric repeats or interstitial telomeric bands, which include those intrachromosomal telomeric-like repeats located near (pericentromeric ITSs) or within the centromere (centromeric ITSs) and those telomeric repeats located between the centromere and the telomere (i.e., truly interstitial telomeric sequences) of eukaryotic chromosomes. According with their sequence organization, localization and flanking sequences, ITSs can be classified into four types: 1) short ITSs, 2) subtelomeric ITSs, 3) fusion ITSs, and 4) heterochromatic ITSs. The first three types have been described mainly in the human genome, whereas heterochromatic ITSs have been found in several vertebrate species but not in humans. Several lines of evidence suggest that ITSs play a significant role in genome instability and evolution. This review aims to summarize our current knowledge about the origin, function, instability and evolution of these telomeric-like repeats in vertebrate chromosomes.

The epigenetic regulation of autonomous replicons

The discovery of autonomous replicating sequences (ARSs) in Saccharomyces cerevisiae in 1979 was considered a milestone in unraveling the regulation of replication in eukaryotic cells. However, shortly afterwards it became obvious that in Saccharomyces pombe and all other higher organisms ARSs were not sufficient to initiate independent replication. Understanding the mechanisms of replication is a major challenge in modern cell biology and is also a prerequisite to developing application-oriented autonomous replicons for gene therapeutic treatments. This review will focus on the development of non-viral episomal vectors, their use in gene therapeutic applications and our current knowledge about their epigenetic regulation.

A human artificial chromosome recapitulates the metabolism of native telomeres in mammalian cells

Telomeric and subtelomeric regions of human chromosomes largely consist of highly repetitive and redundant DNA sequences, resulting in a paucity of unique DNA sequences specific to individual telomeres. Accordingly, it is difficult to analyze telomere metabolism on a single-telomere basis. To circumvent this problem, we have exploited a human artificial chromosome (HAC#21) derived from human chromosome 21 (hChr21). HAC#21 was generated through truncation of the long arm of native hChr21 by the targeted telomere seeding technique. The newly established telomere of HAC#21 lacks canonical subtelomere structures but possesses unique sequences derived from the target vector backbone and the internal region of hChr21 used for telomere targeting, which enabled us to molecularly characterize the single HAC telomere. We established HeLa and NIH-3T3 sub-lines containing a single copy of HAC#21, where it was robustly maintained. The seeded telomere is associated with telomeric proteins over a length similar to that reported in native telomeres, and is faithfully replicated in mid-S phase in HeLa cells. We found that the seeded telomere on HAC#21 is transcribed from the newly juxtaposed site. The transcript, HAC-telRNA, shares several features with TERRA (telomeric repeat-containing RNA): it is a short-lived RNA polymerase II transcript, rarely contains a poly(A) tail, and associates with chromatin. Interestingly, HAC-telRNA undergoes splicing. These results suggest that transcription into TERRA is locally influenced by the subtelomeric context. Taken together, we have established human and mouse cell lines that will be useful for analyzing the behavior of a uniquely identifiable, functional telomere.

Epigenetic regulatory elements associate with specific histone modifications to prevent silencing of telomeric genes

In eukaryotic cells, transgene expression levels may be limited by an unfavourable chromatin structure at the integration site. Epigenetic regulators are DNA sequences which may protect transgenes from such position effect. We evaluated different epigenetic regulators for their ability to protect transgene expression at telomeres, which are commonly associated to low or inconsistent expression because of their repressive chromatin environment. Although to variable extents, matrix attachment regions (MARs), ubiquitous chromatin opening element (UCOE) and the chicken cHS4 insulator acted as barrier elements, protecting a telomeric-distal transgene from silencing. MARs also increased the probability of silent gene reactivation in time-course experiments. Additionally, all MARs improved the level of expression in non-silenced cells, unlike other elements. MARs were associated to histone marks usually linked to actively expressed genes, especially acetylation of histone H3 and H4, suggesting that they may prevent the spread of silencing chromatin by imposing acetylation marks on nearby nucleosomes. Alternatively, an UCOE was found to act by preventing deposition of repressive chromatin marks. We conclude that epigenetic DNA elements used to enhance and stabilize transgene expression all have specific epigenetic signature that might be at the basis of their mode of action.

Kinetics of DNA replication and telomerase reaction at a single-seeded telomere in human cells

In most cancer cells, telomerase is activated to elongate telomere DNA, thereby ensuring numerous rounds of cell divisions. It is thus important to understand how telomerase and the replication fork react with telomeres in human cells. However, the highly polymorphic and repetitive nature of the nucleotide sequences in human subtelomeric regions hampers the precise analysis of sequential events taking place at telomeres in S phase. Here, we have established HeLa cells harboring a single-seeded telomere abutted by a unique subtelomere DNA sequence, which has enabled us to specifically focus on the seeded telomere. We have also developed a modified chromatin immunoprecipitation (ChIP) method that uses restriction digestion instead of sonication to fragment chromatin DNA (RES-ChIP), and a method for immunoprecipitating 5-bromo-2'-deoxyuridine (BrdU)-labeled single-stranded DNA by incubating DNA with anti-BrdU antibody in the nondenaturing condition. We have shown that DNA replication of the seeded telomere takes place during a relatively narrow time window in S phase, and telomerase synthesizes telomere DNA after the replication. Moreover, we have demonstrated that the telomerase catalytic subunit TERT associates with telomeres before telomere DNA replication. These results provide a temporal and spatial framework for understanding DNA replication and telomerase reaction at human telomeres.

Construction of rice mini-chromosomes by telomere-mediated chromosomal truncation

Telomere truncation has been shown to be an efficient technology for the creation of mini-chromosomes that can be used as artificial chromosome platforms for genetic engineering. Artificial chromosome-based genetic engineering is considered to be superior to the existing techniques of randomized gene integration by Agrobacterium or biolistic-mediated genetic transformation. It organizes multiple transgenes as a unique genetic linkage block for subsequent manipulations in breeding. Telomere truncation technology relies on three components: the telomere sequence that mediates chromosomal truncation, a selection marker that allows the selection of transgenic events, and a site-specific recombination system that can be used to accept future genes into the mini-chromosome by gene targeting. These elements are usually pre-assembled before transformation, a process that is both time and labor consuming. We found in this research that the three elements could be mixed to transform plant cells in a biolistic transformation, and produced efficient chromosomal truncations and mini-chromosomes in rice. This system will allow rapid construction of mini-chromosomes with a flexible selection of resistant markers, site-specific recombination systems and other desirable elements. In addition, a rice telotrisomic line was used as the starting material for chromosomal truncations. Mini-chromosomes from the truncations of both the telocentric chromosome and other chromosomes were recovered. The mini-chromosomes remained stable during 2 years of subculture. The construction of mini-chromosomes in rice, an economically important crop, will provide a platform for future artificial chromosome-based genetic engineering of rice for stacking multiple genes.

Telomerase efficiently elongates highly transcribing telomeres in human cancer cells

RNA polymerase II transcribes the physical ends of linear eukaryotic chromosomes into a variety of long non-coding RNA molecules including telomeric repeat-containing RNA (TERRA). Since TERRA discovery, advances have been made in the characterization of TERRA biogenesis and regulation; on the contrary its associated functions remain elusive. Most of the biological roles so far proposed for TERRA are indeed based on in vitro experiments carried out using short TERRA-like RNA oligonucleotides. In particular, it has been suggested that TERRA inhibits telomerase activity. We have exploited two alternative cellular systems to test whether TERRA and/or telomere transcription influence telomerase-mediated telomere elongation in human cancer cells. In cells lacking the two DNA methyltransferases DNMT1 and DNMT3b, TERRA transcription and steady-state levels are greatly increased while telomerase is able to elongate telomeres normally. Similarly, telomerase can efficiently elongate transgenic inducible telomeres whose transcription has been experimentally augmented. Our data challenge the current hypothesis that TERRA functions as a general inhibitor of telomerase and suggest that telomere length homeostasis is maintained independently of TERRA and telomere transcription.

Transcription regulates telomere dynamics in human cancer cells

Telomeres are nucleoprotein structures capping the physical ends of linear eukaryotic chromosomes. Although largely heterochromatic, telomeres are transcribed into telomeric repeat-containing RNA (TERRA) molecules by RNA polymerase II. The functions associated with telomere transcription and TERRA remain ill defined. Here we show that the transcriptional activity of human telomeres directly regulates their movement during interphase. We find that chemical inhibition of global transcription dampens telomere motion, while global stimulation promotes it. Likewise, when DNA methyltransferase enzymes are deleted to augment telomere transcription, we observe increased telomere movement. Finally, using a cell line engineered with a unique transcriptionally inducible telomere, we show that transcription of one specific telomere stimulates only its own dynamics without overtly affecting its stability or its length. We reveal a new and unforeseen function for telomere transcription as a regulator of telomere motion, and speculate on the intriguing possibility that transcription-dependent telomere motion sustains the maintenance of functional and dysfunctional telomeres.

Telomere-mediated truncation of barley chromosomes

Engineered minichromosomes offer an enormous opportunity to plant biotechnology as they have the potential to simultaneously transfer and stably express multiple genes. Following a top-down approach, we truncated endogenous chromosomes in barley (Hordeum vulgare) by Agrobacterium-mediated transfer of T-DNA constructs containing telomere sequences. Blocks of Arabidopsis-like telomeric repeats were inserted into a binary vector suitable for stable transformation. After transfer of these constructs into immature embryos of diploid and tetraploid barley, chromosome truncation by T-DNA-induced de novo formation of telomeres could be confirmed by fluorescent in situ hybridisation, primer extension telomere repeat amplification and DNA gel blot analysis in regenerated plants. Telomere seeding connected to chromosome truncation was found in tetraploid plants only, indicating that genetic redundancy facilitates recovery of shortened chromosomes. Truncated chromosomes were transmissible in sexual reproduction, but were inherited at rates lower than expected according to Mendelian rules.

Induction of telomere-mediated chromosomal truncation and stability of truncated chromosomes in Arabidopsis thaliana

Minichromosomes possess functional centromeres and telomeres and thus should be stably inherited. They offer an enormous opportunity to plant biotechnology as they have the potential to simultaneously transfer and stably express multiple genes. Segregating independently of host chromosomes, they provide a platform for accelerating plant breeding. Following a top-down approach, we truncated endogenous chromosomes in Arabidopsis thaliana by Agrobacterium-mediated transfer of T-DNA constructs containing telomere sequences. Blocks of A. thaliana telomeric repeats were inserted into a binary vector suitable for stable transformation. After transfer of these constructs into the natural tetraploid A. thaliana accession Wa-1, chromosome truncation by T-DNA-induced de novo formation of telomeres could be confirmed by DNA gel blot analysis, PCR (polymerase chain reaction), and fluorescence in situ hybridisation. The addition of new telomere repeats in this process could start alternatively from within the T-DNA-derived telomere repeats or from adjacent sequences close to the right border of the T-DNA. Truncated chromosomes were transmissible in sexual reproduction, but were inherited at rates lower than expected according to Mendelian rules.

Parameters affecting telomere-mediated chromosomal truncation in Arabidopsis

Conversion of a double-strand break into a telomere is a dangerous, potentially lethal event. However, little is known about the mechanism and control of de novo telomere formation (DNTF). DNTF can be instigated by the insertion of a telomere repeat array (TRA) into the host genome, which seeds the formation of a new telomere, resulting in chromosome truncation. Such events are rare and concentrated at chromosome ends. Here, we introduce tetraploid Arabidopsis thaliana as a robust genetic model for DNTF. Transformation of a 2.6-kb TRA into tetraploid plants resulted in a DNTF efficiency of 56%, fivefold higher than in diploid plants and 50-fold higher than in human cells. DNTF events were recovered across the entire genome, indicating that genetic redundancy facilitates recovery of DNTF events. Although TRAs as short as 100 bp seeded new telomeres, these tracts were unstable unless they were extended above a 1-kb size threshold. Unexpectedly, DNTF efficiency increased in plants lacking telomerase, and DNTF rates were lower in plants null for Ku70 or Lig4, components of the nonhomologous end-joining repair pathway. We conclude that multiple competing pathways modulate DNTF, and that tetraploid Arabidopsis will be a powerful model for elucidating the molecular details of these processes.

Telomere truncation in plants

Telomeres are highly repetitive sequences at the ends of chromosomes that act as protection structure for chromosome stability. The integration of telomere sequences into the genome by genetic transformation can create chromosome instability because the integrated telomere sequences tend to form de novo telomeres at the site of integration. Thus, telomere repeats can be used to generate minichromosomes by telomere-mediated chromosome truncation in both plants and animals for chromosome studies as well as the applications in genetic engineering as engineered minichromosomes or artificial chromosomes. This protocol describes the procedure for telomere truncation of maize chromosomes by genetic transformation of telomere-containing constructs by both Agrobacterium- and biolistic-mediated transformations.

Increased sensitivity of subtelomeric regions to DNA double-strand breaks in a human cancer cell line

We previously reported that a single DNA double-strand break (DSB) near a telomere in mouse embryonic stem cells can result in chromosome instability. We have observed this same type of instability as a result of spontaneous telomere loss in human tumor cell lines, suggesting that a deficiency in the repair of DSBs near telomeres has a role in chromosome instability in human cancer. We have now investigated the frequency of the chromosome instability resulting from DSBs near telomeres in the EJ-30 human bladder carcinoma cell line to determine whether subtelomeric regions are sensitive to DSBs, as previously reported in yeast. These studies involved determining the frequency of large deletions, chromosome rearrangements, and chromosome instability resulting from I-SceI endonuclease-induced DSBs at interstitial and telomeric sites. As an internal control, we also analyzed the frequency of small deletions, which have been shown to be the most common type of mutation resulting from I-SceI-induced DSBs at interstitial sites. The results demonstrate that although the frequency of small deletions is similar at interstitial and telomeric DSBs, the frequency of large deletions and chromosome rearrangements is much greater at telomeric DSBs. DSB-induced chromosome rearrangements at telomeric sites also resulted in prolonged periods of chromosome instability. Telomeric regions in mammalian cells are therefore highly sensitive to DSBs, suggesting that spontaneous or ionizing radiation-induced DSBs at these locations may be responsible for many of the chromosome rearrangements that are associated with human cancer.

How shelterin protects mammalian telomeres

The genomes of prokaryotes and eukaryotic organelles are usually circular as are most plasmids and viral genomes. In contrast, the nuclear genomes of eukaryotes are organized on linear chromosomes, which require mechanisms to protect and replicate DNA ends. Eukaryotes navigate these problems with the advent of telomeres, protective nucleoprotein complexes at the ends of linear chromosomes, and telomerase, the enzyme that maintains the DNA in these structures. Mammalian telomeres contain a specific protein complex, shelterin, that functions to protect chromosome ends from all aspects of the DNA damage response and regulates telomere maintenance by telomerase. Recent experiments, discussed here, have revealed how shelterin represses the ATM and ATR kinase signaling pathways and hides chromosome ends from nonhomologous end joining and homology-directed repair.

Engineered telomere degradation models dyskeratosis congenita

Dyskeratosis congenita (DC) is an inherited bone marrow failure syndrome characterized by cutaneous symptoms, including hyperpigmentation and nail dystrophy. Some forms of DC are caused by mutations in telomerase, the enzyme that counteracts telomere shortening, suggesting a telomere-based disease mechanism. However, mice with extensively shortened telomeres due to telomerase deficiency do not develop the characteristics of DC, raising questions about the etiology of DC and/or mouse models for human telomere dysfunction. Here we describe mice engineered to undergo telomere degradation due to the absence of the shelterin component POT1b. When combined with reduced telomerase activity, POT1b deficiency elicits several characteristics of DC, including hyperpigmentation and fatal bone marrow failure at 4-5 mo of age. These results provide experimental support for the notion that DC is caused by telomere dysfunction, and demonstrate that key aspects of a human telomere-based disease can be modeled in the mouse.

Epigenetic and phenotypic consequences of a truncation disrupting the imprinted domain on distal mouse chromosome 7

The distal end of mouse chromosome 7 (Chr 7) contains a large cluster of imprinted genes. In this region two cis-acting imprinting centers, IC1 (H19 DMR) and IC2 (KvDMR1), define proximal and distal subdomains, respectively. To assess the functional independence of IC1 in the context of Chr 7, we developed a recombinase-mediated chromosome truncation strategy in embryonic stem cells and generated a terminal deletion allele, DelTel7, with a breakpoint in between the two subdomains. We obtained germ line transmission of the truncated Chr 7 and viable paternal heterozygotes, confirming the absence of developmentally required paternally expressed genes distal of Ins2. Conversely, maternal transmission of DelTel7 causes a midgestational lethality, consistent with loss of maternally expressed genes in the IC2 subdomain. Expression and DNA methylation analyses on DelTel7 heterozygotes demonstrate the independent imprinting of IC1 in absence of the entire IC2 subdomain. The evolutionarily conserved linkage between the subdomains is therefore not required for IC1 imprinting on Chr 7. Importantly, the developmental phenotype of maternal heterozygotes is rescued fully by a paternally inherited deletion of IC2. Thus, all the imprinted genes located in the region and required for normal development are silenced by an IC2-dependent mechanism on the paternal allele.

Endings in the middle: current knowledge of interstitial telomeric sequences

Interstitial telomeric sequences (ITSs) consist of tandem repeats of the canonical telomeric repeat and are common in mammals. They are localized at intrachromosomal sites, including those repeats located close to the centromeres and those found at interstitial sites, i.e., between the centromeres and the telomeres. ITSs might originate from ancestral intrachromosomal rearrangements (inversions and fusions), from differential crossing-over or from the repair of double-strand break during evolution. Three classes of ITSs have been described in the human genome, namely, short ITSs, long subtelomeric ITSs and fusion ITSs. The fourth class of ITSs, pericentromeric ITSs, has been found in other species. The function of ITSs can be inferred from the association of heritable diseases with ITS polymorphic variants, both in copy number and sequence. This is one of the most attractive aspects of ITS studies because it leads to new and useful markers for genetic linkage studies, forensic applications, and detection of genetic instability in tumors. Some ITSs also might be hotspots of chromosome breakage, rearrangement and amplification sites, based on the type of clastogens and the nature of ITSs. This study will contribute new knowledge with respect to ITSs' biology and mechanism, prevalence of diseases, risk evaluation and prevention of related diseases, thus facilitates the design of early detection markers for diseases caused by genomic instability.

Human artificial chromosome vectors meet stem cells: new prospects for gene delivery

The recent emergence of stem cell-based tissue engineering has now opened up new venues for gene therapy. The task now is to develop safe and effective vectors that can deliver therapeutic genes into specific stem cell lines and maintain long-term regulated expression of these genes. Human artificial chromosomes (HACs) possess several characteristics that require gene therapy vectors, including a stable episomal maintenance, and the capacity for large gene inserts. HACs can also carry genomic loci with regulatory elements, thus allowing for the expression of transgenes in a genetic environment similar to the chromosome. Currently, HACs are constructed by a two prone approaches. Using a top-down strategy, HACs can be generated from fragmenting endogenous chromosomes. By a bottom-up strategy, HACs can be created de novo from cloned chromosomal components using chromosome engineering. This review describes the current advances in developing HACs, with the main focus on their applications and potential value in gene delivery, such as HAC-mediated gene expression in embryonic, adult stem cells, and transgenic animals.

FV peptide induces apoptosis in HEp 2 and HeLa cells: an insight into the mechanism of induction

The present study is an attempt to evaluate the antiproliferative potential of peptide (7.6 kDa) from lionfish (Pterios volitans) venom on cultured HEp2 and HeLa cells. Different dose of purified peptide (1, 2 and 4 μg/ml) at different time points (12, 24 and 36 hrs) were tested for antiproliferative index of the peptide. Among them, 2 μg/ml at 24 hrs was found to effectively inhibit cancer cell growth in vitro and did not cause any adverse effect on normal human lymphocytes. Apoptosis was examined by propidium iodide staining, confirmed by the expression of caspase-8 and caspase-3, down regulation of Bcl-2 expression and DNA fragmentation in treated cells, when compared to untreated HEp2 and HeLa cells. Thus fish venom peptide was found to selectively induce apoptosis in cancer cell.

Telomere-mediated chromosomal truncation in maize

Direct repeats of Arabidopsis telomeric sequence were constructed to test telomere-mediated chromosomal truncation in maize. Two constructs with 2.6 kb of telomeric sequence were used to transform maize immature embryos by Agrobacterium-mediated transformation. One hundred seventy-six transgenic lines were recovered in which 231 transgene loci were revealed by a FISH analysis. To analyze chromosomal truncations that result in transgenes located near chromosomal termini, Southern hybridization analyses were performed. A pattern of smear in truncated lines was seen as compared with discrete bands for internal integrations, because telomeres in different cells are elongated differently by telomerase. When multiple restriction enzymes were used to map the transgene positions, the size of the smears shifted in accordance with the locations of restriction sites on the construct. This result demonstrated that the transgene was present at the end of the chromosome immediately before the integrated telomere sequence. Direct evidence for chromosomal truncation came from the results of FISH karyotyping, which revealed broken chromosomes with transgene signals at the ends. These results demonstrate that telomere-mediated chromosomal truncation operates in plant species. This technology will be useful for chromosomal engineering in maize as well as other plant species.

Human artificial chromosomes: potential applications and clinical considerations

Human artificial chromosomes demonstrate promise as a novel class of nonintegrative gene therapy vectors. The authors outline current developments in human artificial chromosome technology and examine their potential for clinical application.

Engineering chromosomes for delivery of therapeutic genes

The ability to create fully functional human chromosome vectors represents a potentially exciting gene-delivery system for the correction of human genetic disorders with several advantages over viral delivery systems. However, for the full potential of chromosome-based gene-delivery vectors to be realized, several key obstacles must be overcome. Methods must be developed to insert therapeutic genes reliably and efficiently and to enable the stable transfer of the resulting chromosomal vectors to different therapeutic cell types. Research to achieve these outcomes continues to encounter major challenges; however recent developments have reiterated the potential of chromosome-based vectors for therapeutic gene delivery. Here we review the different strategies under development and discuss the advantages and problems associated with each.

Artificial and engineered chromosomes: developments and prospects for gene therapy

At the gene therapy session of the ICCXV Chromosome Conference (2004), recent advances in the construction of engineered chromosomes and de novo human artificial chromosomes were presented. The long-term aims of these studies are to develop vectors as tools for studying genome and chromosome function and for delivering genes into cells for therapeutic applications. There are two primary advantages of chromosome-based vector systems over most conventional vectors for gene delivery. First, the transferred DNA can be stably maintained without the risks associated with insertion, and second, large DNA segments encompassing genes and their regulatory elements can be introduced, leading to more reliable transgene expression. There is clearly a need for safe and effective gene transfer vectors to correct genetic defects. Among the topics discussed at the gene therapy session and the main focus of this review are requirements for de novo human artificial chromosome formation, assembly of chromatin on de novo human artificial chromosomes, advances in vector construction, and chromosome transfer to cells and animals.

Artificial and engineered chromosomes: non-integrating vectors for gene therapy

Non-integrating gene-delivery platforms demonstrate promise as potentially ideal gene-therapy vector systems. Although several approaches are under development, there is little consensus as to what constitutes a true 'artificial' versus an 'engineered' human chromosome. Recent progress must be evaluated in light of significant technical challenges that remain before such vectors achieve clinical utility. Here, we examine the principal classes of non-integrating vectors, ranging from episomes to engineered mini-chromosomes to true human artificial chromosomes. We compare their potential as practical gene-transfer platforms and summarize recent advances towards eventual applications in gene therapy. Although chromosome-engineering technology has advanced considerably within recent years, difficulties in establishing composition of matter and effective vector delivery currently prevent artificial or engineered chromosomes being accepted as viable gene-delivery platforms.

Efficient assembly of de novo human artificial chromosomes from large genomic loci

Background

Human Artificial Chromosomes (HACs) are potentially useful vectors for gene transfer studies and for functional annotation of the genome because of their suitability for cloning, manipulating and transferring large segments of the genome. However, development of HACs for the transfer of large genomic loci into mammalian cells has been limited by difficulties in manipulating high-molecular weight DNA, as well as by the low overall frequencies of de novo HAC formation. Indeed, to date, only a small number of large (>100 kb) genomic loci have been reported to be successfully packaged into de novo HACs.

Results

We have developed novel methodologies to enable efficient assembly of HAC vectors containing any genomic locus of interest. We report here the creation of a novel, bimolecular system based on bacterial artificial chromosomes (BACs) for the construction of HACs incorporating any defined genomic region. We have utilized this vector system to rapidly design, construct and validate multiple de novo HACs containing large (100–200 kb) genomic loci including therapeutically significant genes for human growth hormone (HGH), polycystic kidney disease (PKD1) and ß-globin. We report significant differences in the ability of different genomic loci to support de novo HAC formation, suggesting possible effects of cis-acting genomic elements. Finally, as a proof of principle, we have observed sustained ß-globin gene expression from HACs incorporating the entire 200 kb ß-globin genomic locus for over 90 days in the absence of selection.

Conclusion

Taken together, these results are significant for the development of HAC vector technology, as they enable high-throughput assembly and functional validation of HACs containing any large genomic locus. We have evaluated the impact of different genomic loci on the frequency of HAC formation and identified segments of genomic DNA that appear to facilitate de novo HAC formation. These genomic loci may be useful for identifying discrete functional elements that may be incorporated into future generations of HAC vectors.

Rapid creation of BAC-based human artificial chromosome vectors by transposition with synthetic alpha-satellite arrays

Efficient construction of BAC-based human artificial chromosomes (HACs) requires optimization of each key functional unit as well as development of techniques for the rapid and reliable manipulation of high-molecular weight BAC vectors. Here, we have created synthetic chromosome 17-derived alpha-satellite arrays, based on the 16-monomer repeat length typical of natural D17Z1 arrays, in which the consensus CENP-B box elements are either completely absent (0/16 monomers) or increased in density (16/16 monomers) compared to D17Z1 alpha-satellite (5/16 monomers). Using these vectors, we show that the presence of CENP-B box elements is a requirement for efficient de novo centromere formation and that increasing the density of CENP-B box elements may enhance the efficiency of de novo centromere formation. Furthermore, we have developed a novel, high-throughput methodology that permits the rapid conversion of any genomic BAC target into a HAC vector by transposon-mediated modification with synthetic alpha-satellite arrays and other key functional units. Taken together, these approaches offer the potential to significantly advance the utility of BAC-based HACs for functional annotation of the genome and for applications in gene transfer.

Determination of gaps by contig alignment with telomere-mediated chromosomal fragmentation in Candida albicans

We have adopted a method of telomere-mediated chromosome fragmentation in order to demonstrate the alignment of contigs and determination of gaps. We established the order and orientation of four contigs of Candida albicans chromosome 5 and determined the sizes of three gaps between these contigs. We confirmed this proposed alignment of contigs, as well as gap sizes, by sequencing one gap and analyzing three mega deletions of approximately 41 kbp, 58 kbp, and 77 kbp, which covered two other gaps. These gaps could be also conveniently sequenced, which is an important step in establishing a complete sequence. The combined length of contigs and gaps covered approximately 422 kbp, which is one third of chromosome 5. Telomere-mediated chromosome fragmentation, used here for the first time to align the contigs of C. albicans and determine the gaps, proved to be a reliable method. The method could be helpful in sequencing projects of other diploid organisms, in particular those in which centromeres have not been identified. In addition, our approach can be used to assign any contig to a chromosome, or to induce the loss of a specific chromosome.

Regulation of telomerase by telomeric proteins

Telomeres are essential for genome stability in all eukaryotes. Changes in telomere functions and the associated chromosomal abnormalities have been implicated in human aging and cancer. Telomeres are composed of repetitive sequences that can be maintained by telomerase, a complex containing a reverse transcriptase (hTERT in humans and Est2 in budding yeast), a template RNA (hTERC in humans and Tlc1 in yeast), and accessory factors (the Est1 proteins and dyskerin in humans and Est1, Est3, and Sm proteins in budding yeast). Telomerase is regulated in cis by proteins that bind to telomeric DNA. This regulation can take place at the telomere terminus, involving single-stranded DNA-binding proteins (POT1 in humans and Cdc13 in budding yeast), which have been proposed to contribute to the recruitment of telomerase and may also regulate the extent or frequency of elongation. In addition, proteins that bind along the length of the telomere (TRF1/TIN2/tankyrase in humans and Rap1/Rif1/Rif2 in budding yeast) are part of a negative feedback loop that regulates telomere length. Here we discuss the details of telomerase and its regulation by the telomere.

Modulation of telomere length dynamics by the subtelomeric region of tetrahymena telomeres

Tetrahymena telomeres usually consist of approximately 250 base pairs of T(2)G(4) repeats, but they can grow to reach a new length set point of up to 900 base pairs when kept in log culture at 30 degrees C. We have examined the growth profile of individual macronuclear telomeres and have found that the rate and extent of telomere growth are affected by the subtelomeric region. When the sequence of the rDNA subtelomeric region was altered, we observed a decrease in telomere growth regardless of whether the GC content was increased or decreased. In both cases, the ordered structure of the subtelomeric chromatin was disrupted, but the effect on the telomeric complex was relatively minor. Examination of the telomeres from non-rDNA chromosomes showed that each telomere exhibited a unique and characteristic growth profile. The subtelomeric regions from individual chromosome ends did not share common sequence elements, and they each had a different chromatin structure. Thus, telomere growth is likely to be regulated by the organization of the subtelomeric chromatin rather than by a specific DNA element. Our findings suggest that at each telomere the telomeric complex and subtelomeric chromatin cooperate to form a unique higher order chromatin structure that controls telomere length.

Telomere length homeostasis is achieved via a switch between telomerase- extendible and -nonextendible states

Telomerase counteracts telomere erosion that stems from incomplete chromosome end replication and nucleolytic processing. A precise understanding of telomere length homeostasis has been hampered by the lack of assays that delineate the nonuniform telomere extension events of single chromosome molecules. Here, we measure telomere elongation at nucleotide resolution in Saccharomyces cerevisiae. The number of nucleotides added to a telomere in a single cell cycle varies between a few to more than 100 nucleotides and is independent of telomere length. Telomerase does not act on every telomere in each cell cycle, however. Instead, it exhibits an increasing preference for telomeres as their lengths decline. Deletion of the telomeric proteins Rif1 or Rif2 gives rise to longer telomeres by increasing the frequency of elongation events. Thus, by taking a molecular snapshot of a single round of telomere replication, we demonstrate that telomere length homeostasis is achieved via a switch between telomerase-extendible and -nonextendible states.

Stable inheritance of telomere chromatin structure and function in the absence of telomeric repeats

It is generally believed that telomeric repeats are a necessary and sufficient cis-element for telomere function. Here we show that telomere structure and meiotic function are stably inherited in fission yeast circular chromosomes that have lost all telomeric repeats. We found that the telomeric repeat binding protein, Taz1, and the heterochromatin protein, Swi6, remain associated with subtelomeres in the absence of telomeric repeats. We also found that the fusion point of circular chromosomes that lack telomeric repeats associates with SPB (the yeast counterpart of the centrosome) in the premeiotic horsetail stage, similarly to wild-type telomeres. However, a taz1+ deletion/reintroduction experiment revealed that the maintenance of Taz1 binding and premeiotic function is achieved via different strategies. Taz1 is recruited to subtelomeres by an autonomous element present in subtelomeric DNA, thus in a genetic mechanism. In contrast, the premeiotic subtelomere-SPB association is maintained in an epigenetic manner. These results shed light on the previously unrecognized role played by the subtelomere and underscore the robust nature of the functional telomere complex that is maintained by both genetic and epigenetic mechanisms. Furthermore, we suggest that the establishment and the maintenance of the functional telomere complex are mechanistically distinguishable.

Telomere instability in a human tumor cell line expressing a dominant-negative WRN protein

Werner Syndrome (WS) is an autosomal recessive disease characterized by premature aging and chromosome instability. The protein involved in WS, WRN, is a RecQ-type helicase that also has exonuclease activity. WRN has been demonstrated to bind to a variety of other proteins, including RPA, DNA-PKcs, and TRF2, suggesting that WRN is involved in DNA replication, repair, recombination, and telomere maintenance. In culture, WS cells show premature senescence, which can be overcome by transfection with an expression vector containing the gene for the catalytic subunit of telomerase. However, telomerase expression does not eliminate chromosome instability in WS cells, which led to the proposal that telomere loss is not the cause of the high rate of chromosome rearrangements in WS cells. In the present study, we have investigated how a WRN protein containing a dominant-negative mutation (K577M-WRN) influences the stability of telomeres in a human tumor cell line expressing telomerase. The results demonstrate an increased rate of telomere loss and chromosome fusion in cells expressing K577M-WRN. Expression of K577M-WRN results in reduced levels of telomerase activity, however, the absence of detectable changes in average telomere length demonstrates that WRN-associated telomere loss results from stochastic events involving complete telomere loss or loss of telomere capping function. Thus, telomere loss can contribute to chromosome instability in cells deficient in WRN regardless of the expression of telomerase activity.

POT1 as a terminal transducer of TRF1 telomere length control

Human telomere maintenance is essential for the protection of chromosome ends, and changes in telomere length have been implicated in ageing and cancer. Human telomere length is regulated by the TTAGGG-repeat-binding protein TRF1 and its interacting partners tankyrase 1, TIN2 and PINX1 (refs 5-9). As the TRF1 complex binds to the duplex DNA of the telomere, it is unclear how it can affect telomerase, which acts on the single-stranded 3' telomeric overhang. Here we show that the TRF1 complex interacts with a single-stranded telomeric DNA-binding protein--protection of telomeres 1 (POT1)--and that human POT1 controls telomerase-mediated telomere elongation. The presence of POT1 on telomeres was diminished when the amount of single-stranded DNA was reduced. Furthermore, POT1 binding was regulated by the TRF1 complex in response to telomere length. A mutant form of POT1 lacking the DNA-binding domain abrogated TRF1-mediated control of telomere length, and induced rapid and extensive telomere elongation. We propose that the interaction between the TRF1 complex and POT1 affects the loading of POT1 on the single-stranded telomeric DNA, thus transmitting information about telomere length to the telomere terminus, where telomerase is regulated.

The number of vertebrate repeats can be regulated at yeast telomeres by Rap1-independent mechanisms

The number of telomeric DNA repeats at chromosome ends is maintained around a mean value by a dynamic balance between elongation and shortening. In particular, proteins binding along the duplex part of telomeric DNA set the number of repeats by progressively limiting telomere growth. The paradigm of this counting mechanism is the Rap1 protein in Saccharomyces cerevisiae. We demonstrate here that a Rap1-independent mechanism regulates the number of yeast telomeric repeats (TG(1-3)) and of vertebrate repeats (T(2)AG(3)) when TEL1, a yeast ortholog of the human gene encoding the ATM kinase, is inactivated. In addition, we show that a T(2)AG(3)-only telomere can be formed and maintained in humanized yeast cells carrying a template mutation of the gene encoding the telomerase RNA, which leads to the synthesis of vertebrate instead of yeast repeats. Genetic and biochemical evidences indicate that this telomere is regulated in a Rap1-independent manner, both in TEL1 and in tel1Delta humanized yeast cells. Altogether, these findings shed light on multiple repeat-counting mechanisms, which may share critical features between lower and higher eukaryotes.

Cloning of human centromeres by transformation-associated recombination in yeast and generation of functional human artificial chromosomes

Human centromeres remain poorly characterized regions of the human genome despite their importance for the maintenance of chromosomes. In part this is due to the difficulty of cloning of highly repetitive DNA fragments and distinguishing chromosome-specific clones in a genomic library. In this work we report the highly selective isolation of human centromeric DNA using transformation-associated recombination (TAR) cloning. A TAR vector with alphoid DNA monomers as targeting sequences was used to isolate large centromeric regions of human chromosomes 2, 5, 8, 11, 15, 19, 21 and 22 from human cells as well as monochromosomal hybrid cells. The alphoid DNA array was also isolated from the 12 Mb human mini-chromosome DeltaYq74 that contained the minimum amount of alphoid DNA required for proper chromosome segregation. Preliminary results of the structural analyses of different centromeres are reported in this paper. The ability of the cloned human centromeric regions to support human artificial chromosome (HAC) formation was assessed by transfection into human HT1080 cells. Centromeric clones from DeltaYq74 did not support the formation of HACs, indicating that the requirements for the existence of a functional centromere on an endogenous chromosome and those for forming a de novo centromere may be distinct. A construct with an alphoid DNA array from chromosome 22 with no detectable CENP-B motifs formed mitotically stable HACs in the absence of drug selection without detectable acquisition of host DNAs. In summary, our results demonstrated that TAR cloning is a useful tool for investigating human centromere organization and the structural requirements for formation of HAC vectors that might have a potential for therapeutic applications.

DNA amplification by breakage/fusion/bridge cycles initiated by spontaneous telomere loss in a human cancer cell line

The development of genomic instability is an important step in generating the multiple genetic changes required for cancer. One consequence of genomic instability is the overexpression of oncogenes due to gene amplification. One mechanism for gene amplification is the breakage/fusion/bridge (B/F/B) cycle that involves the repeated fusion and breakage of chromosomes following the loss of a telomere. B/F/B cycles have been associated with low-copy gene amplification in human cancer cells, and have been proposed to be an initiating event in high-copy gene amplification. We have found that spontaneous telomere loss on a marker chromosome 16 in a human tumor cell line results in sister chromatid fusion and prolonged periods of chromosome instability. The high rate of anaphase bridges involving chromosome 16 demonstrates that this instability results from B/F/B cycles. The amplification of subtelomeric DNA on the marker chromosome provides conclusive evidence that B/F/B cycles initiated by spontaneous telomere loss are a mechanism for gene amplification in human cancer cells.

Construction of a swine artificial chromosome: a novel vector for transgenesis in the pig

A de novo SAC was constructed by making use of YAC technology and a humanized yeast strain. The construct (about 310 kb) contained pig centromeric DNA and the Neo gene. The construct was introduced into a pig cell line by yeast-mammalian cell fusion and G418 resistant clones were obtained. One clone was characterized by FISH and shown to contain an episomally located microchromosome containing YAC, Neo and pig centromere sequences. FISH analysis over time showed that the SAC was mitotically stable for at least 34 generations in the absence of selection. The size of the SAC was determined by confocal microscopy of the SAC and shown to be approximately 7 Mb, which is about 25-fold greater than the size of the original YAC. From its behavior in pulsed field gel electrophoresis, FISH analysis of stretched DNA fibers, and its appearance under scanning confocal microscopy, it was concluded that the SAC is a circularized and multimerized derivative of the original YAC. Possible applications as vectors for animal transgenesis are discussed.

Telomere Erosion-Induced Mitotic Catastrophe in Continuously Grown Chinese Hamster Don Cells

We have previously reported that telomere erosion is the earliest chromatin modification in cells entering the apoptotic pathway. The purpose of this investigation was to determine whether loss of telomeric DNA was involved in inducing mitotic catastrophe and death in Chinese hamster Don cells. Don, a male Chinese hamster-derived cell line which requires daily subculturing to remain diploid, was grown without subculturing for 1-4 days at 37 degrees C and analyzed cytologically. Our results indicated that (1) the frequency of metaphase chromosomes with structural anomalies was significantly higher in 3-day continuously grown cells than in 1-day control cells (8.2% vs 5.7%; P < 0.01), (2) the mitotic index was considerably lower in 3-day continuously grown cells (0.13%) than in control cells (3.64%), (3) cells grown for 3 days continuously showed a higher incidence (7.6%) of endoreduplicated metaphase chromosomes than did control cells (4.9%), (4) 4-day continuously grown Don cells showed significantly smaller amounts of telomeric DNA in interphase nuclei than did control cells, and (5) apoptotic cells were more frequent in 4-day cell cultures (40.6%) than in control cells (4.3%). These results support our earlier observations and contribute additional support for our hypothesis that telomere reduction is the cause of mitotic catastrophe and that cell death in continuously grown Don cells occurs because of the loss of telomeric DNA.

Chromosome instability as a result of double-strand breaks near telomeres in mouse embryonic stem cells

Telomeres are essential for protecting the ends of chromosomes and preventing chromosome fusion. Telomere loss has been proposed to play an important role in the chromosomal rearrangements associated with tumorigenesis. To determine the relationship between telomere loss and chromosome instability in mammalian cells, we investigated the events resulting from the introduction of a double-strand break near a telomere with I-SceI endonuclease in mouse embryonic stem cells. The inactivation of a selectable marker gene adjacent to a telomere as a result of the I-SceI-induced double-strand break involved either the addition of a telomere at the site of the break or the formation of inverted repeats and large tandem duplications on the end of the chromosome. Nucleotide sequence analysis demonstrated large deletions and little or no complementarity at the recombination sites involved in the formation of the inverted repeats. The formation of inverted repeats was followed by a period of chromosome instability, characterized by amplification of the subtelomeric region, translocation of chromosomal fragments onto the end of the chromosome, and the formation of dicentric chromosomes. Despite this heterogeneity, the rearranged chromosomes eventually acquired telomeres and were stable in most of the cells in the population at the time of analysis. Our observations are consistent with a model in which broken chromosomes that do not regain a telomere undergo sister chromatid fusion involving nonhomologous end joining. Sister chromatid fusion is followed by chromosome instability resulting from breakage-fusion-bridge cycles involving the sister chromatids and rearrangements with other chromosomes. This process results in highly rearranged chromosomes that eventually become stable through the addition of a telomere onto the broken end. We have observed similar events after spontaneous telomere loss in a human tumor cell line, suggesting that chromosome instability resulting from telomere loss plays a role in chromosomal rearrangements associated with tumor cell progression.

Novel substituted methylenedioxy lignan suppresses proliferation of cancer cells by inhibiting telomerase and activation of c-myc and caspases leading to apoptosis

Conventional solvent fractionation and bioactivity based target assays were used to identify a new anti-cancer molecule from Phyllanthus urinaria, a herbal medicinal plant used in South India. At each step of the purification process the different fractions that were isolated were tested for specific anti-proliferative activity by assays measuring the inhibition of [(3)H]thymidine incorporation, and trypan blue drug exclusion. The ethyl acetate fraction that contained the bioactivity was further purified and resolved by HPLC on a preparative column. The purity of each of the fractions and their bioactivity were checked. Fraction 3 demonstrated a single spot on TLC and showed maximum anti-proliferative activity. This fraction was further purified and the structure was defined as 7'-hydroxy-3',4',5,9,9'-pentamethoxy-3,4-methylene dioxy lignan using NMR and mass spectrometry analysis. The pure compound and the crude ethyl acetate fraction which showed anti-proliferative activities were examined for ability to target specific markers of apoptosis like bcl2, c-myc and caspases and for effects on telomerase. Four specific cancer cell lines HEp2, EL-1 monocytes, HeLa and MCP7 were used in this study. The results indicate that 7'-hydroxy-3',4',5,9,9'-pentamethoxy-3,4-methylene dioxy lignan was capable of inhibiting telomerase activity and also could inhibit bcl2 and activate caspase 3 and caspase 8 whose significance in the induction of apoptosis is well known. We believe that this compound could serve as a valuable chemotherapeutic drug after further evaluations.

Advances in human artificial chromosome technology

Human artificial chromosome (HAC) technology has developed rapidly over the past four years. Recent reports show that HACs are useful gene transfer vectors in expression studies and important tools for determining human chromosome function. HACs have been used to complement gene deficiencies in human cultured cells by transfer of large genomic loci also containing the regulatory elements for appropriate expression. And, they now offer the possibility to express large human transgenes in animals, especially in mouse models of human genetic diseases.

Efficiency of de novo centromere formation in human artificial chromosomes

In a comparative study, we show that human artificial chromosome (HAC) vectors based on alpha-satellite (alphoid) DNA from chromosome 17 but not the Y chromosome regularly form HACs in HT1080 human cells. We constructed four structurally similar HAC vectors, two with chromosome 17 or Y alphoid DNA (17alpha, Yalpha) and two with 17alpha or Yalpha and the hypoxanthine guanine phosphoribosyltransferase locus (HPRT1). The 17alpha HAC vectors generated artificial minichromosomes in 32-79% of the HT1080 clones screened, compared with only approximately 4% for the Yalpha HAC vectors, indicating that Yalpha is inefficient at forming a de novo centromere. The 17alpha HAC vectors produced megabase-sized, circular HACs containing multiple copies of alphoid fragments (60-250 kb) interspersed with either vector or HPRT1 DNA. The 17alpha-HPRT1 HACs were less stable than those with 17alpha only, and these results may influence the design of new HAC gene transfer vectors.

Activation of p53 protein by telomeric (TTAGGG)n repeats

Genome instability is a primary factor leading to the activation of the p53 tumor suppressor protein. Telomeric repeat (TR) sequences are also responsible for genome integrity. By capping the termini of the chromosomes, TRs prevent them undergoing nucleolytic degradation, ligation or chromosome fusion. Interestingly, telomere shortening was suggested to activate p53, which in turn may cause primary cells to senesce. In order to elucidate the nature of a possible cross talk between the two, we introduced into cells TRs of defined length and investigated their effect on p53 activation and subsequent cellular response. We found that the introduction of a TR into cells leads to stabilization of the p53 protein. This stabilization was specific to TRs and was not observed in response to exposure of cells to plasmids containing non-TR sequences. p53 stabilization requires the presence of an intact p53 oligomerization domain. TR-activated p53 exhibited enhanced transcriptional activity. Eventually, TRs induced p53-dependent growth suppression, measured as a reduction in colony formation.

FISH analysis of telomeric repeat sequences and their involvement in chromosomal aberrations induced by radiomimetic compounds in hamster cells

The behaviour of telomeric repeat sequences in Chinese hamster CHO and CHE cell lines treated with the radiomimetic drugs bleomycin (BLM) and streptonigrin (STN) and the effect of these drugs on telomerase activity was investigated. Fluorescence in situ hybridisation revealed that 18% of the scored aberrations induced by BLM and 14% of those induced by STN in CHO cells exhibited telomeric repeat signals. In CHE cells, 29% of the total aberrations induced by BLM and 45% of those induced by STN involved telomeric repeat sequences. Acentric fragments labelled along their entire length and translocations of telomeric repeat sequences were also found in both cell lines. These results suggest that telomeric repeat sequences are preferentially involved in chromosome breakage, fragility and recombination induced by radiomimetic agents. In addition, some of the damaged CHE cells exhibited one or more chromosomes with additional zones of hybridisation, indicating the possible amplification of (TTAGGG)(n) repeats by telomerase. However, the fact that none of the radiomimetic compounds tested produced any effect on telomerase activity suggests that this enzyme is not related to the assumed amplification events induced by BLM and STN in CHE cells.

Transient expression of wild-type or biologically inactive telomerase allows the formation of artificial telomeres in mortal human cells

Telomere seeding, the formation of artificial telomeres, has been routinely successful in immortalized but not normal human cells. We compared seeding efficiencies in preimmortal and immortal SV40-transformed cells using plasmid telomeres with T(2)AG(3) tracts of 1600 and 3200 bp. Seeding occurred only in immortal cells, indicating that transformed preimmortal cells behave like normal cells vis à vis formation of new telomeres and that T-antigen inhibition of cellular checkpoints is insufficient to allow seeding. Telomerase is active in immortal but not preimmortal cells, which do not express the reverse transcriptase hTERT. Upon transient expression of hTERT, seeds with 1600 bp of T(2)AG(3) formed telomeres in preimmortal cells. Comparable seeding efficiencies were obtained with wild-type hTERT or the HA-tagged protein that is catalytically active but unable to maintain endogenous telomeres. No seeding occurred with catalytically inactive hTERT. Given that telomerase expression was transient and that longer seeds did not form telomeres in the absence of the enzyme, seeding may not be elicited merely by elongation of telomeric sequences. We propose that modification of the telomeric terminus by telomerase may contribute to telomere seeding by leading to formation of a structure that impedes rejoining of this terminus with chromosomal sequences.

A telomere-mediated chromosome fragmentation approach to assess mitotic stability and ploidy alterations of Leishmania chromosomes

We have used a telomere-associated chromosome fragmentation strategy to induce internal chromosome-specific breakage of Leishmania chromosomes. The integration of telomeric repeats from the kinetoplastid Trypanosoma brucei into defined positions of the Leishmania genome by homologous recombination can induce chromosome breakage accompanied by the deletion of the chromosomal part that is distal to the site of the break. The cloned telomeric DNA at the end of the truncated chromosomes is functional and it can seed the formation of new telomeric repeats. We found that genome ploidy is often altered upon telomere-mediated chromosome fragmentation events resulting in large chromosomal deletions. In most cases diploidy is either preserved, or partial trisomic cells are observed, but interestingly we report here the generation of partial haploid mutants in this diploid organism. Partial haploid Leishmania mutants should facilitate studies on the function of chromosome-assigned genes. We also present several lines of evidence for the presence of sequences involved in chromosome mitotic stability and segregation during cell cycle in this parasitic protozoan. Telomere-directed chromosome fragmentation studies in Leishmania may constitute a useful tool to assay for centromere function.