Repression of an alternative mechanism for lengthening of telomeres in somatic cell hybrids

Perrem, Kilian; Bryan, Tracy M.; Englezou, Anna; Hackl, Torben; Moy, Elsa L.; Reddel, Roger R.

doi:10.1038/sj.onc.1202752

Cited by 98 publications

(92 citation statements)

References 37 publications

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“…Inhibition of telomerase will put tumors that are initially telomerase positive under strong selection pressure to activate ALT. Because repression of ALT results in senescence and cell death (Nakabayashi et al 1997;Perrem et al 1999), ALT, like telomerase, may be an attractive drug target. The use of both classes of drugs to inhibit telomere maintenance in tumor cells may help prevent the emergence of drug resistance.…”

Section: Discussionmentioning

confidence: 99%

Telomere-based crisis: functional differences between telomerase activation and ALT in tumor progression

Chang¹,

Khoo²,

Naylor³

et al. 2003

Genes Dev.

167

107

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Telomerase activation is a common feature of most advanced human cancers and is postulated to restore genomic stability to a level permissive for cell viability and tumor progression. Here, we used genetically defined transformed mouse embryonic fibroblast (MEF) cultures derived from late generation mTerc −/− Ink4a/Arf −/− mice to explore more directly how telomere-based crisis relates to the evolution of cancer cell genomes and to tumor biology. An exhaustive serial analysis of cytogenetic profiles over extensive passage in culture revealed that the emergence of chromosomal fusions (including dicentrics) coincided with onset of deletions and complex nonreciprocal translocations (NRTs), whereas mTerc-transduced cultures maintained intact chromosomes and stable genomes. Despite a high degree of telomere dysfunction and genomic instability, transformed late passage mTerc −/− Ink4a/Arf −/− cultures retained the capacity to form subcutaneous tumors in immunocompromised mice. However, even moderate levels of telomere dysfunction completely abrogated the capacity of these cells to form lung metastases after tail-vein injection, whereas mTerc reconstitution alone conferred robust metastatic activity in these cells. Finally, serial subcutaneous tumor formation using late passage transformed mTerc −/− Ink4a/Arf −/− cultures revealed clear evidence of telomerase-independent alternative lengthening of telomeres (ALT). Significantly, despite a marked increase in telomere reserve, cells derived from the ALT+ subcutaneous tumors were unable to generate lung metastases, indicating in vivo functional differences in these principal mechanisms of telomere maintenance. Together, these results are consistent with the model that although telomere dysfunction provokes chromosomal aberrations that initiate carcinogenesis, telomerase-mediated telomere maintenance enables such initiated cells to efficiently achieve a fully malignant endpoint, including metastasis.[Keywords: Telomere; telomerase; metastasis; ALT (alternative lengthening of telomeres); chromosomal instability; SKY] Supplemental material is available at http://www.genesdev.org. Tumor formation represents the phenotypic endpoint of a mutational process that endows cells with a critical threshold of cancer-relevant genetic lesions. From a cytogenetic standpoint, epithelial neoplasms or carcinomas fall into one of two major classes-the first possessing microsatellite instability with retention of grossly normal karyotypes, and the second presenting with markedly altered chromosomal counts and structure. These two classes have been operationally defined as "microsatellite instability neoplasias" and "chromosomal instability neoplasias" (MIN and CIN), respectively (for review, see Lengauer et al. 1998). The molecular mechanism underlying MIN is clearly related to defective DNA mismatch repair. The basis for CIN is less well understood, although defects in mitotic checkpoint, nonhomologous end-joining, and DNA replication, as well as increased oxidative stress and telomere dysfu...

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Section: Discussionmentioning

confidence: 99%

Telomere-based crisis: functional differences between telomerase activation and ALT in tumor progression

Chang¹,

Khoo²,

Naylor³

et al. 2003

Genes Dev.

167

107

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“…Although ALT is characterized by the presence of elongated and heterogeneous telomeres in the absence of TA, [11][12][13] it has also been shown that a small proportion of telomerase positive tumors may exhibit an ALT TRF pattern. 12 Perrem et al 39 demonstrated that the ALT telomere phenotype was abrogated in cell hybrids between ALT and telomerase-positive cells. Thus, this would indicate that our 3 telomerase-positive tumors contained subpopulations of cells using telomerase or ALT for maintaining telomere length.…”

Section: Discussionmentioning

confidence: 99%

Telomerase activity and hTERT mRNA expression can be heterogeneous and does not correlate with telomere length in soft tissue sarcomas

Yan

Benhattar

Coindre

et al. 2002

Intl Journal of Cancer

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In a previous study, we showed that telomerase activity (TA) and human telomerase reverse transcriptase (hTERT) mRNA expression were undetectable in benign mesenchymal lesions and low-grade soft tissue sarcomas (STSs), but detectable in about 50% of intermediate-/high-grade STSs. We wondered if this lack of TA or hTERT mRNA expression could be related to the tumor sample examined and if there was a relationship between the former 2 parameters and telomere length. Two separate tumor samples from 37 STSs were examined for telomerase activity, using the telomerase repeat amplification protocol (TRAP) assay and for hTERT mRNA expression, using RT-PCR. Considerable attention has been drawn recently to the regulation of telomere length and telomerase activity in the process of cellular senescence and immortalization. 1,2 Human telomeres are specialized terminal chromosomal structures composed of TTAGGG repeats. Shortening of telomeres has been suggested to represent the mitotic clock of cell cycle; telomere shortening is observed in most dividing somatic cells, leading to cell senescence when reaching a critical level. 1,2 Telomerase, a ribonucleoprotein enzyme that is capable of synthesizing telomeric repeats, plays a pivotal role in the process of telomere length maintenance and cell immortalization. Telomerase activity (TA) has been shown to be expressed in a majority of human carcinomas 1,2 but only few data exist regarding TA in sarcomas. [3][4][5][6][7][8][9] In a previous study, 3 we showed that TA is detectable in only about 50% of intermediate-/high-grade soft tissue sarcomas (STS) and undetectable in benign lesions and low-grade STS. These results are in keeping with those of Schneider Stock et al. detecting TA in 38% (23/60) of their STS 4 and 69% of liposarcomas. 5 A rapid literature survey shows, however, that TA in STS may vary greatly, ranging from 17% 7 to 100% 9 of the cases examined. Some authors also found a positive correlation between TA and tumor aggressiveness in sarcomas 4 and, more specifically, in liposarcomas 5,6,9 whereas others did not. 3 Telomere length has rarely been examined in STS. 5,10 Examining a series of 30 sarcomas, Schneider-Stock et al. 10 showed that telomere TRF length was heterogeneous in this tumor category. Unfortunately, they didn't examine their tumors for TA and, thus, it was not known if telomere length correlated with TA or not. In a further study focusing on liposarcomas only, the same group found a positive correlation between the presence of TA, tumor aggressiveness, high tumor grade and high proliferative activity but not between TA and TRF length. This lack of correlation between TA and TRF length may have several causes including the activation of telomerase-independent mechanism(s) of telomere lengthening referred to as alternative lengthening of telomeres (ALT) 11-13 that seems to occur by means of telomeretelomere recombination. 13,14 In our first study on STS, we observed some degree of heterogeneity in TA expression when examining several, otherwise morphol...

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“…However, some of these genes may act through other pathways because the mortal phenotype can be restored in somatic cell hybrids despite the presence of telomerase activity (as detected by an in vitro assay, which does not necessarily re¯ect continuing telomerase activity at the telomere) . That ALT results from recessive mutation(s) was demonstrated by the observation that fusion of an ALT+ immortal cell line with normal cells resulted in senescent hybrids that had lost the ALT telomere phenotype (Perrem et al, 1999). Further, some telomerase-positive immortalized cells appear to contain repressors of ALT activity: ALT was repressed in immortal hybrid cells formed by fusing an ALT+ cell line from immortalization complementation group A with either of two telomerase-positive cell lines from the same complementation group (Perrem et al, 1999.…”

Section: Alt Genetics and Repressionmentioning

confidence: 94%

“…That ALT results from recessive mutation(s) was demonstrated by the observation that fusion of an ALT+ immortal cell line with normal cells resulted in senescent hybrids that had lost the ALT telomere phenotype (Perrem et al, 1999). Further, some telomerase-positive immortalized cells appear to contain repressors of ALT activity: ALT was repressed in immortal hybrid cells formed by fusing an ALT+ cell line from immortalization complementation group A with either of two telomerase-positive cell lines from the same complementation group (Perrem et al, 1999. In contrast, fusion of ALT+ and telomerase-positive cell lines from immortalization complementation group D resulted in immortal hybrids , and by PD 77 a few weeks later immortalized cells had overgrown the culture; telomerase activity was not detectable at any time (Yeager et al, 1999).…”

Section: Alt Genetics and Repressionmentioning

confidence: 99%

Alternative lengthening of telomeres in mammalian cells

et al. 2002

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Some immortalized mammalian cell lines and tumors maintain or increase the overall length of their telomeres in the absence of telomerase activity by one or more mechanisms referred to as alternative lengthening of telomeres (ALT). Characteristics of human ALT cells include great heterogeneity of telomere size (ranging from undetectable to abnormally long) within individual cells, and ALT-associated PML bodies (APBs) that contain extrachromosomal telomeric DNA, telomere-speci®c binding proteins, and proteins involved in DNA recombination and replication. Activation of ALT during immortalization involves recessive mutations in genes that are as yet unidenti®ed. Repressors of ALT activity are present in normal cells and some telomerase-positive cells. Telomere length dynamics in ALT cells suggest a recombinational mechanism. Inter-telomeric copying occurs, consistent with a mechanism in which singlestranded DNA at one telomere terminus invades another telomere and uses it as a copy template resulting in net increase in telomeric sequence. It is possible that t-loops, linear and/or circular extrachromosomal telomeric DNA, and the proteins found in APBs, may be involved in the mechanism. ALT and telomerase activity can co-exist within cultured cells, and within tumors. The existence of ALT adds some complexity to proposed uses of telomererelated parameters in cancer diagnosis and prognosis, and poses challenges for the design of anticancer therapeutics designed to inhibit telomere maintenance.

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Repression of an alternative mechanism for lengthening of telomeres in somatic cell hybrids

Cited by 98 publications

References 37 publications

Telomere-based crisis: functional differences between telomerase activation and ALT in tumor progression

Telomere-based crisis: functional differences between telomerase activation and ALT in tumor progression

Telomerase activity and hTERT mRNA expression can be heterogeneous and does not correlate with telomere length in soft tissue sarcomas

Alternative lengthening of telomeres in mammalian cells

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