UMR3244 – Dynamique de l’information génétique

Publications de l’équipe

Année de publication : 2007

Dennis Kappei, J Arturo Londoño-Vallejo (2007 Dec 7)

Telomere length inheritance and aging.

Mechanisms of ageing and development : 17-26 En savoir plus
Résumé

Telomere shortening accompanies human aging, and premature aging syndromes are often associated with short telomeres. These two observations are central to the hypothesis that telomere length directly influences longevity. If true, genetically determined mechanisms of telomere length homeostasis should significantly contribute to variations of longevity in the human population. On the other hand, telomere shortening is also observed in the course of many aging-associated disorders but determining whether it is a cause or a consequence is not an easy task. Here, we review the most relevant experimental and descriptive data relating telomere length, as a quantitative trait, to aging and longevity.

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Natalia S Zhdanova, Julia M Minina, Tatjana V Karamisheva, Irena Draskovic, Nikolai B Rubtsov, Jose-Arturo Londoño-Vallejo (2007 Sep 28)

The very long telomeres in Sorex granarius (Soricidae, Eulipothyphla) contain ribosomal DNA.

Chromosome research : an international journal on the molecular, supramolecular and evolutionary aspects of chromosome biology : 881-90 En savoir plus
Résumé

Two closely related shrew species, Sorex granarius and Sorex araneus, in which Robertsonian rearrangements have played a primary role in karyotype evolution, present very distinct telomere length patterns. S. granarius displays hyperlong telomeres specifically associated with the short arms of acrocentrics, whereas telomere lengths in S. araneus are rather short and homogenous. Using a combined approach of chromosome and fibre FISH, modified Q-FISH, 3D-FISH, Ag-NOR staining and TRF analysis, we carried out a comparative analysis of telomeric repeats and rDNA distribution on chromosome ends of Sorex granarius. Our results show that rDNA sequences forming active nuclear organizing regions are interspersed with the long telomere tracts of all short arms of acrocentrics. These observations suggest that the major rearrangements that gave rise to today’s karyotype in S. granarius were accompanied by a profound reorganization of chromosome ends, which comprised extensive amplification of telomeric and rDNA repeats on the short arms of acrocentrics and finally contributed to the stabilization of telomeres. This is the first time that such telomeric structures have been observed in any mammalian species.

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Anne-Sophie Gabet, Rosita Accardi, Angelica Bellopede, Susanne Popp, Petra Boukamp, Bakary S Sylla, J Arturo Londoño-Vallejo, Massimo Tommasino (2007 Sep 28)

Impairment of the telomere/telomerase system and genomic instability are associated with keratinocyte immortalization induced by the skin human papillomavirus type 38.

FASEB journal : official publication of the Federation of American Societies for Experimental Biology : 622-32 En savoir plus
Résumé

The skin human papillomavirus (HPV) types belonging to the genus beta of the HPV phylogenetic tree appear to be associated with nonmelanoma skin cancer. We previously showed that the beta HPV type 38 E6 and E7 oncoproteins are able to inactivate the tumor suppressors p53 and retinoblastoma. Here, both viral proteins were expressed in primary human skin keratinocytes in order to study their effects on the telomere/telomerase system. We show that immortalization of skin keratinocytes induced by HPV38 E6/E7 is associated with hTERT gene overexpression. This event is, in part, explained by the accumulation of the p53-related protein, DeltaNp73. Despite elevated levels of hTERT mRNA, the telomerase activity detected in HPV38 E6/E7 keratinocytes was lower than that observed in HPV16 E6/E7 keratinocytes. The low telomerase activation in highly proliferative HPV38 E6/E7 keratinocytes resulted in the presence of extremely short and unstable telomeres. In addition, we observed anaphase bridges, mitotic multipolarity, and dramatic genomic aberrations. Interestingly, the ectopic expression of hTERT prevents both telomere erosion and genomic instability. Thus, we showed that in HPV38 E6/E7 keratinocytes characterized by unscheduled proliferation, suboptimal activation of telomerase and subsequent extensive telomere shortening result in genomic instability facilitating cellular immortalization.

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Hong-Yan Du, Rachel Idol, Sara Robledo, Jennifer Ivanovich, Ping An, Arturo Londono-Vallejo, David B Wilson, Philip J Mason, Monica Bessler (2007 Sep 19)

Telomerase reverse transcriptase haploinsufficiency and telomere length in individuals with 5p- syndrome.

Aging cell : 689-97 En savoir plus
Résumé

Telomerase, which maintains the ends of chromosomes, consists of two core components, the telomerase reverse transcriptase (TERT) and the telomerase RNA (TERC). Haploinsufficiency for TERC or TERT leads to progressive telomere shortening and autosomal dominant dyskeratosis congenita (DC). The clinical manifestations of autosomal dominant DC are thought to occur when telomeres become critically short, but the rate of telomere shortening in this condition is unknown. Here, we investigated the consequences of de novo TERT gene deletions in a large cohort of individuals with 5p- syndrome. The study group included 41 individuals in which the chromosome deletion resulted in loss of one copy of the TERT gene at 5p15.33. Telomere length in peripheral blood cells from these individuals, although within the normal range, was on average shorter than in normal controls. The shortening was more significant in older individuals suggesting an accelerated age-dependent shortening. In contrast, individuals with autosomal dominant DC due to an inherited TERC gene deletion had very short telomeres, and the telomeres were equally short regardless of the age. Although some individuals with 5p- syndrome showed clinical features that were reminiscent of autosomal dominant DC, these features did not correlate with telomere length, suggesting that these were not caused by critically short telomeres. We conclude that a TERT gene deletion leads to slightly shorter telomeres within one generation. However, our results suggest that several generations of TERT haploinsufficiency are needed to produce the very short telomeres seen in patients with DC.

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J Arturo Londoño-Vallejo (2007 Aug 31)

Telomere instability and cancer.

Biochimie : 73-82 En savoir plus
Résumé

Telomeres are required to preserve genome integrity, chromosome stability, nuclear architecture and chromosome pairing during meiosis. Given that telomerase activity is limiting or absent in most somatic tissues, shortening of telomeres during development and aging is the rule. In vitro, telomere length operates as a mechanism to prevent uncontrolled cell growth and therefore defines the proliferation potential of a cell. In vitro, in somatic cells that have lost proliferation control, shortening of telomeres becomes the main source of genome instability leading to genetic or epigenetic changes that may allow cells to become immortal and to acquire tumor phenotypes. In vivo, mice models have indisputably shown both the protective and the promoting role of very short telomeres in cancer development. In humans, although telomere shortening and other types of telomere dysfunction probably contribute to the genome instability often detected in tumors, the specific contributions of such instability to the development of cancer remain largely undetermined.

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Eric Gilson, Arturo Londoño-Vallejo (2007 Aug 30)

Telomere length profiles in humans: all ends are not equal.

Cell cycle (Georgetown, Tex.) : 2486-94 En savoir plus
Résumé

Telomere length is an important parameter of telomere function since it determines number of aspects controlling chromosome stability and cell division. Since telomeres shorten with age in humans and premature aging syndromes are often associated with the presence of short telomeres, it has been proposed that telomere length is also an important parameter for organismal aging. How mean telomere lengths are determined in humans remains puzzling, but it is clear that genetic and epigenetic factors appear to be of great importance. Experimental evidence obtained from many different organisms has provided the basis for a widely accepted counting mechanism based on a negative feedback loop for telomerase activity at the level of individual telomeres. In addition, recent studies in both normal and pathological contexts point to the existence of chromosome-specific mechanisms of telomere length regulation determining a telomere length profile, which is inherited and maintained throughout life. In this review, we recapitulate the available data, propose a synthetic view of telomere length control mechanisms in humans and suggest new approaches to test current hypotheses.

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Valérie Borde (2007 Aug 4)

The multiple roles of the Mre11 complex for meiotic recombination.

Chromosome research : an international journal on the molecular, supramolecular and evolutionary aspects of chromosome biology : 551-63 En savoir plus
Résumé

During the first meiotic prophase, numerous DNA double-strand breaks (DSB) are formed in the genome in order to initiate recombination between homologous chromosomes. The conserved Mre11 complex, formed of Mre11, Rad50 and Nbs1 (Xrs2 in Saccharomyces cerevisiae) proteins, plays a crucial role in mitotic cells for sensing and repairing DSB. In meiosis the Mre11 complex is also required for meiotic recombination. Depending on the organisms, the Mre11 complex is required for the formation of the DSB catalysed by the transesterase Spo11 protein. It then plays a unique function in removing covalently attached Spo11 from the 5′ extremity of the breaks through its nuclease activity, to allow further break resection. Finally, the Mre11 complex also plays a role during meiosis in bridging DNA molecules together and in sensing Spo11 DSB and activating the DNA damage checkpoint. In this article the different biochemical functions of the Mre11 complex required during meiosis are reviewed, as well as the consequences of Mre11 complex inactivation for meiosis in several organisms. Finally, I describe the meiotic phenotypes of several animal models that have been developed to model hypomorphic mutations of the Mre11 complex, involved in humans in some genetic instability disorders.

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Franck Toledo, Olivier Bluteau, Iva Simeonova (2007 Jul 17)

[The activation of p53 in tumors: a promising strategy against cancer].

Médecine sciences : M/S : 565-7 En savoir plus
Résumé

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Franck Toledo, Geoffrey M Wahl (2007 May 15)

MDM2 and MDM4: p53 regulators as targets in anticancer therapy.

The international journal of biochemistry & cell biology : 1476-82 En savoir plus
Résumé

The gene TP53, encoding transcription factor p53, is mutated or deleted in half of human cancers, demonstrating the crucial role of p53 in tumor suppression. Importantly, p53 inactivation in cancers can also result from the amplification/overexpression of its specific inhibitors MDM2 and MDM4 (also known as MDMX). The presence of wild-type p53 in those tumors with MDM2 or MDM4 overexpression stimulates the search for new therapeutic agents to selectively reactivate it. This short survey highlights recent insights into MDM2 and MDM4 regulatory functions and their implications for the design of future p53-based anticancer strategies. We now know that MDM2 and MDM4 inhibit p53 in distinct and complementary ways: MDM4 regulates p53 activity, while MDM2 mainly regulates p53 stability. Upon DNA damage, MDM2-dependent degradation of itself and MDM4 contribute significantly to p53 stabilization and activation. These and other data imply that the combined use of MDM2 and MDM4 antagonists in cancer cells expressing wild-type p53 should activate p53 more significantly than agents that only antagonize MDM2, resulting in more effective anti-tumor activity.

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Année de publication : 2006

Nicolas Robine, Norio Uematsu, Franck Amiot, Xavier Gidrol, Emmanuel Barillot, Alain Nicolas, Valérie Borde (2006 Dec 26)

Genome-wide redistribution of meiotic double-strand breaks in Saccharomyces cerevisiae.

Molecular and cellular biology : 1868-80 En savoir plus
Résumé

Meiotic recombination is initiated by the formation of programmed DNA double-strand breaks (DSBs) catalyzed by the Spo11 protein. DSBs are not randomly distributed along chromosomes. To better understand factors that control the distribution of DSBs in budding yeast, we have examined the genome-wide binding and cleavage properties of the Gal4 DNA binding domain (Gal4BD)-Spo11 fusion protein. We found that Gal4BD-Spo11 cleaves only a subset of its binding sites, indicating that the association of Spo11 with chromatin is not sufficient for DSB formation. In centromere-associated regions, the centromere itself prevents DSB cleavage by tethered Gal4BD-Spo11 since its displacement restores targeted DSB formation. In addition, we observed that new DSBs introduced by Gal4BD-Spo11 inhibit surrounding DSB formation over long distances (up to 60 kb), keeping constant the number of DSBs per chromosomal region. Together, these results demonstrate that the targeting of Spo11 to new chromosomal locations leads to both local stimulation and genome-wide redistribution of recombination initiation and that some chromosomal regions are inherently cold regardless of the presence of Spo11.

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Franck Toledo, Crystal J Lee, Kurt A Krummel, Luo-Wei Rodewald, Chung-Wen Liu, Geoffrey M Wahl (2006 Dec 13)

Mouse mutants reveal that putative protein interaction sites in the p53 proline-rich domain are dispensable for tumor suppression.

Molecular and cellular biology : 1425-32 En savoir plus
Résumé

The stability and activity of tumor suppressor p53 are tightly regulated and partially depend on the p53 proline-rich domain (PRD). We recently analyzed mice expressing p53 with a deletion of the PRD (p53(DeltaP)). p53(DeltaP), a weak transactivator hypersensitive to Mdm2-mediated degradation, is unable to suppress oncogene-induced tumors. This phenotype could result from the loss of two motifs: Pin1 sites proposed to influence p53 stabilization and PXXP motifs proposed to mediate protein interactions. We investigated the importance of these motifs by generating mice encoding point mutations in the PRD. p53(TTAA) contains mutations suppressing all putative Pin1 sites in the PRD, while p53(AXXA) lacks PXXP motifs but retains one intact Pin1 site. Both mutant proteins accumulated in response to DNA damage, although the accumulation of p53(TTAA) was partially impaired. Importantly, p53(TTAA) and p53(AXXA) are efficient transactivators and potent suppressors of oncogene-induced tumors. Thus, Pin1 sites in the PRD may modulate p53 stability but do not significantly affect function. In addition, PXXP motifs are not essential, but structure dictated by the presence of prolines, PXXXXP motifs that may mediate protein interactions, and/or the length of this region appears to be functionally significant. These results may explain why the sequence of the p53 PRD is so variable in evolution.

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Franck Toledo, Geoffrey M Wahl (2006 Nov 28)

Regulating the p53 pathway: in vitro hypotheses, in vivo veritas.

Nature reviews. Cancer : 909-23 En savoir plus
Résumé

Mutations in TP53, the gene that encodes the tumour suppressor p53, are found in 50% of human cancers, and increased levels of its negative regulators MDM2 and MDM4 (also known as MDMX) downregulate p53 function in many of the rest. Understanding p53 regulation remains a crucial goal to design broadly applicable anticancer strategies based on this pathway. This Review of in vitro studies, human tumour data and recent mouse models shows that p53 post-translational modifications have modulatory roles, and MDM2 and MDM4 have more profound roles for regulating p53. Importantly, MDM4 emerges as an independent target for drug development, as its inactivation is crucial for full p53 activation.

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Maria Antonietta Cerone, J Arturo Londoño-Vallejo, Chantal Autexier (2006 Aug 8)

Telomerase inhibition enhances the response to anticancer drug treatment in human breast cancer cells.

Molecular cancer therapeutics : 1669-75 En savoir plus
Résumé

Breast cancer is the most common malignancy among women. Current therapies for breast tumors are based on the use of chemotherapeutic drugs that are quite toxic for the patients and often result in resistance. Telomerase is up-regulated in 95% of breast carcinomas but not in adjacent normal tissues. Therefore, it represents a very promising target for anticancer therapies. Unfortunately, the antiproliferative effects of telomerase inhibition require extensive telomere shortening before they are fully present. Combining telomerase inhibition with common chemotherapeutic drugs can be used to reduce this lag phase and induce tumor cell death more effectively. Few studies have analyzed the effects of telomerase inhibition in combination with anticancer drugs in breast cancer cells. In this study, we inhibited telomerase activity in two breast cancer cell lines using a dominant-negative human telomerase reverse transcriptase and analyzed cell viability after treatment with different anticancer compounds. We found that dominant-negative human telomerase reverse transcriptase efficiently inhibits telomerase activity and causes telomere shortening over time. Moreover, cells in which telomerase was suppressed were more sensitive to anticancer agents independently of their mechanism of action and this sensitization was dependent on the presence of shorter telomeres. Altogether, our data show that blocking telomere length maintenance in combination with anticancer drugs can be used as an effective way to induce death of breast cancer cells.

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J Graakjaer, J A Londono-Vallejo, K Christensen, S Kølvraa (2006 Jun 29)

The pattern of chromosome-specific variations in telomere length in humans shows signs of heritability and is maintained through life.

Annals of the New York Academy of Sciences : 311-6 En savoir plus
Résumé

This paper characterizes the distribution of telomere length on individual chromosome arms in humans. By fluorescent in situ hybridization (FISH), followed by computer-assisted analysis of digital images, it is shown that the distribution of telomere length on individual chromosome arms is not random, but that humans have a common telomere profile. This profile exists in lymphocytes, amniocytes and fibroblasts, and seems to be conserved during life. A closer look at the overall pattern of the profile shows that the length of the telomeres in general follows the total chromosome length. In addition to the common profile, it is found that each person has specific characteristics, which are also conserved throughout life. Studying both twins and families we have obtained indications that these individual characteristics are at least partly inherited. Altogether, our results suggest that the length of individual telomeres might occasionally play a role in the heritability of life span.

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M A Cerone, J A Londoño-Vallejo, C Autexier (2006 Jun 13)

Mutated telomeres sensitize tumor cells to anticancer drugs independently of telomere shortening and mechanisms of telomere maintenance.

Oncogene : 7411-20 En savoir plus
Résumé

Telomerase is a ribonucleoprotein complex that maintains the stability of chromosome ends and regulates replicative potential. Telomerase is upregulated in over 85% of human tumors, but not in adjacent normal tissues and represents a promising target for anticancer therapy. Most telomerase-based therapies rely on the inhibition of telomerase activity and require extensive telomere shortening before inducing any antiproliferative effect. Disturbances of telomere structure rather than length may be more effective in inducing cell death. Telomerase RNA subunits (hTRs) with mutations in the template region reconstitute active holoenzymes that incorporate mutated telomeric sequences. Here, we analysed the feasibility of an anticancer approach based on the combination of telomere destabilization and conventional chemotherapeutic drugs. We show that a mutant template hTR dictates the synthesis of mutated telomeric repeats in telomerase-positive cancer cells, without significantly affecting their viability and proliferative ability. Nevertheless, the mutant hTR increased sensitivity to anticancer drugs in cells with different initial telomere lengths and mechanisms of telomere maintenance and without requiring overall telomere shortening. This report is the first to show that interfering with telomere structure maintenance in a telomerase-dependent manner may be used to increase the susceptibility of tumor cells to anticancer drugs and may lead to the development of a general therapy for the treatment of human cancers.

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