Telomere | EPFL Graph Search

A telomere (ˈtɛləmɪər,_ˈtiːlə-; ) is a region of repetitive nucleotide sequences associated with specialized proteins at the ends of linear chromosomes. Telomeres are a widespread genetic feature most commonly found in eukaryotes. In most, if not all species possessing them, they protect the terminal regions of chromosomal DNA from progressive degradation and ensure the integrity of linear chromosomes by preventing DNA repair systems from mistaking the very ends of the DNA strand for a double-strand break. Discovery In 1938, when the young North American geneticist Hermann J. Müller used to work with flies of the species Drosophila melanogaster, exposed to X rays at the Edinburgh Animal Genetics Institute (United Kingdom), he did not foresee the implications that his findings would have in the molecular biology and genetics in the following 70 years. He had just observed that the ends of the irradiated chromosomes, different from the other genome, did not present alterat

Suv-HMTases and their impact on tumor formation and telomere regulation

Fabian Jordi

Epigenetic alterations during tumor formation and progression have become an increasingly important aspect of cancer biology. Loss of global methylation at histone H4 lysine 20, mediated by the Suv4-20 HMTases, and loss of methylation at histone H3 lysine 9 in Suv39 HMTase deficient mice were shown to promote tumor formation. A key function of Suv-HMTases is the generation and maintenance of constitutive heterochromatin at telomeres which plays a central role during tumorigenesis and stem cell activity. Transgenic mice were generated to address the role of Suv39h1 and Suv4-20h1 in tumorigenesis and telomere biology. Overexpression of Suv39h1 in pMEFs results in reduced transformation frequency and preliminary results suggest reduced papilloma formation during skin carcinogenesis. Suv39h1 overexpression in keratinocytes reduces the proliferative potential of skin stem cells in vitro. Suv39h1 overexpression in pMEFs leads to increased chromatin compaction at telomeric and pericentric repeats resulting in telomere shortening. In line with the effect of Suv39h1 overexpression Suv4-20h1 transgenic mice show reduced retarded papilloma formation during skin carcinogenesis. All those findings taken together confirm a role for Suv-HMTases in epigenetic control of telomeric heterochromatin and proliferation, identifying Suv39h1 and Suv4-20h1 as potential tumor suppressors in vivo

2008

Identification and characterization of proteins that protect human telomeres from fragility

Anna Christina Näger

Difficulties to replicate telomeres - the ends of our chromosomes - can cause telomere shortening andgenome instability. These difficulties are due to the repetitive DNA sequence and distinct structures at telomeresthat challenge the semi-conservative DNA replication machinery. Among the proteins that help toovercome the obstacles to telomere replication are factors known to be mutated in genetic diseases (forinstance WRN, BLM, RTEL1, CTC1), but we suspect that there are still factors and pathways unknown.DNA replication defects at telomeres manifest as smeary or multiple telomeric fluorescence in situ hybridization(FISH) signals on metaphase chromosomes - commonly called telomere fragility. To identify proteinsthat protect human telomeres from fragility, we depleted 71 proteins (including 8 controls) from HeLa cellsusing siRNA pools and analysed their metaphase chromosomes for fragile telomeres. The majority of theseproteins had been identified by QTIP-iPOND - a proteomic analysis of chromatin near telomere replicationforks. Among the 71 proteins that were depleted in the siRNA screen, we identified 29 novel proteins thatprevent telomere fragility and are therefore crucial to maintain genome stability. The telomere fragilityscreen validated QTIP-iPOND as a technique to identify telomere replication proteins. The hits identified inour telomere fragility screen may for example have functions in regulating telomeric chromatin compositionand compaction, regulating transcription of the telomeric long non-coding RNA TERRA and its associationwith telomeric chromatin, dealing with replication stress and stalled replication forks, modulating DNAdamage signalling and repair, modulating sister chromatid cohesion, and preventing oxidative stress.One of the 29 novel proteins identified in the telomere fragility screen is chromobox 8 (CBX8) whose functionat telomeres was investigated in further detail in this study. CBX8 is a component of the canonical polycombrepressive complex 1 (PRC1), an epigenetic regulator of gene expression. However, CBX8 also hasfunctions that are independent of the PRC1 complex. We show that human CBX8 prevents telomere fragilitycaused by RNA-DNA hybrids which could stall replication forks and induce homologous recombination.CBX8 depletion or knockout did however not affect telomeric RNA-DNA hybrid levels. We hypothesize thatCBX8 may be involved in repair activities at stalled replication forks that arise when the replisome encounterspersistent RNA-DNA hybrids. Moreover, we found that CBX8 localizes to telomeric repeat-binding factor2 (TRF2)-deficient telomeres in a manner that is dependent on RNA-DNA hybrids and ATM signalling.We demonstrate that H3K27me3 (trimethylated histone 3 lysine 27) marks, which have previously beensuggested to help recruitment of CBX proteins to chromatin via the CBX chromodomain, are dispensable forCBX8 recruitment to TRF2-deficient telomeres. Furthermore, we found that CBX8 promotes non-homologousend joining (NHEJ)-mediated telomere fusions of dysfunctional telomeres. We therefore hypothesizethat CBX8 is involved in repair activities that influence the DNA repair pathway choice at TRF2-deficient telomeres.

EPFL2022

Telomeric Repeat Containing RNAs and the Response to Telomere Deprotection in Saccharomyces Cerevisiae

Andrea Bernardo Panza

Telomeres are the DNA/RNA/protein structures at the end of linear eukaryotic chromosomes, which protect them against the DNA damage repair machinery, preventing chromosome end-to-end fusions and aberrant recombination. Moreover, telomeres compensate for the erosion that occurs at every cycle of DNA replication maintaining telomere length through the action of the telomerase reverse transcriptase. Even if telomeres have many features in common with heterochromatic DNA, they have been shown to be transcribed into telomeric repeat-containing RNA (TERRA) both in vertebrates and plants. In the first part of this work we demonstrate that TERRA expression is conserved in the yeast Saccharomyces cerevisiae. Moreover, TERRA is transcribed by RNA polymerase II, it is polyadenylated and its levels in wild type cells are maintained low through the action of the 5’ to 3’ RNA exonuclease Rati. Indeed cells harboring the thermosensitive mutant allele ratl-1 accumulate high levels of TERRA even at semipermissive temperature. Moreover, ratl-1 mutant cells display a telomere shortening phenotype that is rescued by RNase H overexpression, indicating that DNA/RNA hybrids, which can accumulate in this mutant, inhibit the normal telomere maintenance. In the second part of this work we tried to define TERRA function(s) in response to telomere alterations in yeast cells. We observed that TERRA increases during senescence and survivor formation in est2A mutant cells, which undergo through telomere shortening due to telomerase deletion. The observed increase in TERRA does not require the recombination machinery. Moreover, though TERRA levels are not altered by global DNA damage, they are raised in cdcl3-l, stnl-13 and ku70A mutants, which display telomere resection and DNA damage checkpoint activation at restrictive temperature, suggesting a specific role for TERRA in response to telomere alterations. Since the TERRA increase in telomere deprotection-mutants is suppressed by the deletion of either the 5’ to 3’ exonuclease EXOl or one of the checkpoint factors MEC1, TEL1 and RAD9, we suggest that TERRA is part of a specific checkpoint response, which is induced upon telomere resection. Finally, since est2A cells harboring the ratl-1 mutation display a prolonged senescence phenotype, we suspect a role for TERRA in telomere protection and in particular in preventing telomeric recombination.

EPFL2012