Abstract
Chromosome aberrations are the hallmark of cancer cells. Although a few specific chromosome aberrations are frequently detected in some types of cancer, the majority of karyotypic abnormalities tend to differ between different histological types and between individuals with the same type of cancer. Recent work indicates that telomeres may be directly involved in shaping the karyotypes of tumor cells. In particular, the heterogeneity of telomere lengths within cells may have a direct influence on the frequency with which chromosomes engage in telomeric fusions and in subsequent breakage-fusion-bridge cycles. Since telomere length distribution among chromosome arms is a polymorphic trait, difference in distributions between individuals may account, at least in part, for the karyotypic differences found among tumors of the same type. Conversely, if single telomere lengths happen to be inherited, the segregation of particularly short telomeres in families may increase the incidence of specific chromosome aberrations during tumor evolution, and perhaps contribute, along with other factors, to cancer predisposition.

Keywords: Telomere length heterogeneity; Crisis; Chromosome instability; CIN.

1. Introduction
Telomeres are complex nucleoprotein structures at the ends of linear chromosomes that are essential to preserve genome integrity [1]. A functional telomere requires both a minimum DNA length and the presence of capping proteins, several of which have been identified and characterized [2]. These proteins, most of which are expressed constitutively, are necessary for protection, stability, replication and/or elongation of telomeres [2]. In the absence of telomerase, telomeres shorten progressively due to the inability of conventional DNA polymerases to completely duplicate linear molecules (the "end-replication problem" [3, 4]). In most telomerase-negative cells, the initial telomere length correlates with their replicative potential and cells enter a senescent state when telomeres reach a critical threshold [5, 6]. Thus, telomere length serves as a counting mechanism to control cell proliferation [7], although it is possible that the signal triggering the senescence block originates from a structural alteration rather than from the actual length of telomeres [8-10].

2. Telomere lengths are highly heterogeneous within human somatic cells
When analyzed by Southern blot hybridization, telomere restriction fragments appear highly heterogeneous in length. This length heterogeneity arises mainly from the fact that the DNA sample was extracted from thousands of cells with different replication histories, which results in marked mean telomere length differences between cells. There are also intracellular sources of length heterogeneity such as the variations in the position of the last restriction site available at every chromosome end, and the differences in the telomere repeat content between chromosome extremities.
The heterogeneity of telomere lengths within cells is best appreciated through in situ hybridization techniques [11, 12], the most sophisticated of which allow the quantification of fluorescent signals (Q-FISH) [13-15]. The intensity of the signals may then be translated into kilobases or used as arbitrary values, relative or not to other telomeres in the cell [16, 17].
Several studies using Q-FISH approaches have attempted to identify typical telomere length patterns linked to particular chromosomes. It has been proposed, for example, that the length of an individual telomere in a cell is correlated either to the length of the chromosome arm [18, 19] or to the size of the whole chromosome [20]. Another study carried out in several individuals and including different types of cells from the same individuals found that chromosome specific telomeres followed similar patterns in all individuals, with particular telomeres always being identified as the longest (4q) or the shortest (17p) in all cells and all individuals [21]. However, since all these studies have made measurements in diploid metaphases, the conclusions were based on results obtained by averaging the telomere intensity detected on two telomeres located on homologous chromosome arms. This method has the major disadvantage of omitting potentially important information due to length variation between alleles.

3. Telomere lengths are allele specific
Two different approaches that determine the parental origin of chromosomes have been developed to study telomere length in single chromosome extremities.
The first approach exploits the extensive polymorphism of human subtelomeric regions [22]. Variations in these regions can affect very large segments of DNA (up to several hundreds of kilobases) and can be revealed by in situ hybridization on metaphase chromosomes using labeled subtelomeric cosmids [23]. Homologous chromosomes are distinguished in each metaphase and fluorescent signals corresponding to allele-specific telomeres are measured using a specific peptide nucleic acid (PNA) probe [17]. Using this approach on PHA-stimulated peripheral blood cells from healthy donors, it was shown that significant relative length differences between telomeres located at allelic positions exist and that these differences are stable with time [17]. On the other hand, when studying telomerase negative diploid cells growing in vitro, no significant differences in the shortening kinetics of individual telomeres were detected, each allele retaining its relative length up to the senescence point. This is consistent with the idea that the end replication problem or other factors that may affect single telomeres stochastically in telomerase negative cells (such as recombination events [24] or deletions following oxidative damage [25]) do not contribute, at least in vitro, to the heterogeneity of relative telomere lengths among individual chromosome arms within a cell [17].
Recently, Baird and colleagues developed a PCR-based approach, called STELA (for Single TElomere Length Analysis), to study allele specific telomere lengths [26]. Telomere extremities in purified DNA are modified with linkers and a PCR reaction using allele-specific primers allows the amplification of fragments that precisely reflect the telomere lengths carried by that allele. Using this approach, they showed that the telomere lengths on Xp/Yp can be highly heterogeneous and allele specific [26]. STELA, which has been so far only reported for the Xp/Yp telomere, provides a very precise estimation of actual telomere lengths and may be, at least theoretically, applied to very small DNA samples. However, its extension to all human telomeres will require sequence information, not yet available, for each allele as well as information on allelic frequencies.

4. The length of individual telomeres is genetically determined
Average telomere length in humans appears to be under strong genetic control [27]. The possibility of identifying the parental origin of at least a subset of human chromosome pairs allows a more careful analysis of factors contributing to telomere length differences between chromosomes.
As mentioned above, these factors may relate to the global structure of the chromosome such as the length of arms, the presence of large centromeric heterochromatin, or perhaps the influence of large epigenetic chromosome modifications, as suggested in the case of chromosome X [28]. These factors may also depend on the local telomeric structure, most presumably through the presence of juxtatelomeric associated sequences that may either influence the formation of particular structures (such as the T-loop [29]) or carry non canonical binding sites for telomeric factors [30]. Finally, particular telomeres may be preferentially elongated by telomerase [31, 32] or be stochastically affected by DNA damaging agents (such as free radicals) leading to dramatic loss of telomeric sequences [33].
We have used the Q-FISH/subtelomeric FISH approach to measure the relative length of a total of 216 pairs of allelic telomeres (located on the p and q arms of chromosomes 1, 3, 5, 6, 7, 8, 9, 11, 15, 16 and 19) in PHA-stimulated PBLs from more than forty healthy unrelated individuals (unpublished data). Since they have different parental origin, intra-individual alleles are considered genetically unrelated (i.e. the probability of being Identical By Descent -IBD- is essentially zero). A correlation analysis shows that there exists a low, but statistically significant, positive correlation between the lengths of allelic telomeres (Fig. 1A), suggesting common factors that influence the length of telomeres located on homologous chromosomes. However, this chromosome-dependent effect explains little of the overall telomere length variation within cells and appears to be weak relative to other factors contributing to allelic differences.

Figure 1. The relative lengths of individual telomeres are determined in the zygote.
Correlation analyses of relative telomere lengths, as measured by Q-FISH, were carried out by comparing either allelic positions in unrelated individuals (IBD=0) or genetically related chromosome extremities in twins, both DZ (IBD=0.5-075) and MZ (IBD=1) (A). The intra-individual correlation between allelic telomeres is slightly positive, indicating a weak chromosome effect on telomere length. In twins, the inter-individual correlation of genetically related extremities increases with the probability of being IBD. MZ twins, who were more than 80 years old at the time of blood sampling, show the highest correlation, indicating a strict maintenance of relative telomere lengths throughout life. Two different hypotheses may be proposed: Differences among individual telomere lengths may be present in the zygote (B, left) either as a result of random variations among chromosome extremities or because allele specific lengths are carried by the germ cells. After the splitting of the zygote, these differences are maintained during development and the extra-uterine life. Alternatively, telomere lengths contributed by germ cells may be highly homogeneous after fecundation (C, left) but allele-specific lengths are rapidly defined during development. In this case, allele specific subtelomeric sequences likely influence telomere maintenance at the local level.

5. The heterogeneity of telomere lengths influences the karyotype evolution of transformed cells in vitro
A potential implication of the single telomere length polymorphism is the possibility that the senescence signal originates on different chromosome extremities, depending on the individual. In fact, experimental evidence indicates that the trigger may require the accumulation of several very short telomeres suggesting a threshold effect or the possibility that, when there is only a small number of short telomeres, they can be repaired or at least temporarily hidden from the DNA-damage sensing systems [35]. However, when cells become "transformed" by mutations or by viral oncogenes, the mechanisms limiting cell replication are disabled and proliferation continues, bringing telomere length below its stability threshold [11, 36]. A great deal of experimental data suggests that excessive telomere shortening leads to chromosome breakage-fusion-bridge (BFB) cycles and, eventually, to generalized genome instability and cell death (crisis) (Figure 2) [37-44]. Only cells that have acquired a telomere maintenance mechanism can escape from this crisis [45-51]. Whether telomere-driven genomic instability directly contributes to the acquisition of the immortal phenotype remains to be demonstrated. It is possible, however, that some of the resulting rearrangements favor the acquisition/establishment of phenotypes contributing to the unlimited replicative potential characteristic of post-crisis cells [44, 52, 53].

Figure 2. Impact of telomere length heterogeneity in the karyotype evolution of pre-immortal cells.
Telomeres shorten due to cell proliferation. If mechanisms triggering mitotic senescence in the presence of critically short telomeres are disabled, the cell enters a period of genome instability driven by telomeric fusions and BFB cycles. Chromosome arms carrying the shortest telomeres in the cell will be the first to become unstable. The instability will progressively affect other chromosome arms until the cell reaches full crisis and die or telomerase is reactivated. The extent and quality of karyotype abnormalities observed in the rescued "immortal" cells will depend on both the initial distribution of telomere lengths and the timing of telomerase reactivation.

Shortest	Early	Late	Longest	Early	Late
telomeres			telomeres
18q	++++	++++	13q	-	-
19q	+++	++++	8p	+	++
19p	+++	++++	16q	-	-
20q	++++	++++	22p	-	-
6q	-	+++	18p	-	++
4p	++	+++	5Ap*	-	-
22q	-	+	21p	-	-
15p	-	+++	Xp	-	-
20p	-	+	13p	+	+
1Bq	+	+++	8q	-	-
8p	+	++	6q	-	+++
21q	-	+++	2p	-	-
16p	-	+	5qB	-	-
5Aq	-	++	1qA	-	-

Table 1. Implication in post-crisis karyotypic aberrations of chromosome arms previously carrying either long or short telomeres in SV40 ER transformed human kidney epithelial cells.
Single telomere lengths were measured by Q-FISH on metaphase chromosomes and chromosome arms were ranked accordingly [59]. Ranking data from 18 metaphases were combined to determine the location of the shortest and longest telomeres in this cell line. 8 post-crisis HA1 cell lines (all of them telomerase-positive) were obtained in independent experiments and analyzed by spectral karyotyping (unpublished). Early and late refer to the delay of emergence of post-crisis cells, which could also reflect the timing of telomerase reactivation. Four cell lines were examined in each group. A plus sign indicates that a chromosome alteration involving that chromosome arm was detected at least once in one of the cell lines. Some chromosome arms appear as carrying both short and long telomeres (8p and 6q). Presumably, they belong to different homologous chromosomes. Sometimes, homologous chromosomes can be differentiated (A and B) on the basis of subtelomeric polymorphisms [23]. In some cases, these polymorphisms can also be used to distinguish which of two homologues is involved in chromosome aberrations [59].

6. Shortening of telomeres is likely involved in the chromosome instability of human cancers
Genetic instability is the hallmark of most human cancers. Like post-crisis cells, cells derived from tumors typically harbor numerous karyotypic abnormalities such as aneuploidy and non reciprocal chromosome translocations. Chromosome instability (CIN) is frequently observed in human cancers and, in some cases, distinctive, recurrent, chromosome rearrangements are associated with a particular type of cancer (mostly hematological malignancies) [60]. However, many solid tumors present complex, nearly random, patterns of chromosome translocations, that appear unrelated even in tumors belonging to the same histological type or subtype [61-63]. The molecular bases for the CIN phenotype remain elusive and its role in conferring any growth advantage or aggressive behavior on tumor cells remains conjectural (for a recent mathematical model on the contribution of CIN to tumorigenesis, see [64]).
Numerous studies, mostly based on mouse models, clearly implicate dysfunctional telomeres in the early phases of cancer development [53, 65-67]. Recently, studies carried out on human specimens have shed new light on the role of telomeres in the genesis of CIN in cancer. In situ quantitative FISH telomere analyses (Q-FISH) of certain types of tumors have shown that marked telomere attrition does take place in vivo at early stages of cancer development [68-71] and that this telomere attrition correlates with genome instability [72]. Furthermore, the examination of chromosomal breakpoint profiles found in some solid tumors suggested that most of the genome instability observed during the initial stages of tumor development could be explained by telomere dysfunction due to shortening. Indeed, the observed rearrangements of chromosomes in low grade tumors were most likely caused by initial BFB cycles, leading to chromosome breakage near the extremities, whereas rearrangements in high grade tumors pointed to chromosome breaks randomly distributed in the genome [73, 74], probably the result of repeated BFBs.
These observations are consistent with the hypothesis that shortening of telomeres in transformed cells in vivo leads to the BFBs events that initiate CIN at the early stages of cancer development [75, 76].

7. Potential implications of telomere length polymorphism in CIN
As just noted, telomere dysfunction caused by exaggerated shortening seems to play an important role in the genome instabilities that accompany the development of tumors in man [73, 74]. Moreover, since telomeres shorten physiologically with age, this shortening has been suggested to contribute to the mechanisms that lead to increased cancer frequencies in aged people [65, 77, 78]. The data just discussed suggest a more precise molecular link between telomere shortening and chromosome instabilities in cancer. During tumor development, the telomere length distribution characteristic of each individual could have, a direct impact on the way chromosome instability contributes. Extrapolated to in vivo tumors, the in-vitro results suggest that telomere-initiated BFBs may follow similar patterns in different tissues from the same individual, at least during the earliest phases of cell transformation. Later, however, reiterated BFBs will unavoidably involve randomly generated non-telomeric extremities, potentially leading to karyotype heterogeneity within tumors. In agreement with this is the observation that BFBs are more frequent in tumors bearing numerous nonspecific chromosome aberrations, as well as marked intratumor heterogeneity, than in tumors with tumor-specific aberrations and low variability [76]. Interestingly, chromosome aberrations frequently detected in tumors may also be found as chromosome aberrations in PBLs [79, 80]. This might be best explained by the observations that telomere lengths of individual chromosome arms are conserved among tissues in the same individual [21] and that individuals with carcinomas have PBLs with shorter telomeres than controls [81].

8. Future prospects
PBLs can be used as surrogates for determining the distribution of telomere lengths in pre-cancerous cells and to compare this distribution to the incidence of karyotype alterations found in the tumor from the same patient. The distribution of telomere lengths among chromosome arms being different among individuals [17, 34], it is predicted that the associated risk for any specific chromosome arm to become unstable due to excessive telomere shortening should also vary among individuals with the same type of tumor. If confirmed, such variable risk could explain, at least partially, the marked genetic heterogeneity found in solid tumors. Finally, if telomere lengths on specific chromosome arms are inherited from the parents, the segregation of short telomeres in the progeny may predispose such chromosomes (but only upon cell transformation) to telomere dysfunction and chromosome rearrangements. Depending on the chromosome involved, this predisposition to undergo BFBs may contribute to the global risk of cancer development that runs in some families.

Acknowledgements
Work in the author's laboratory is supported by the "Association pour la Recherche contre le Cancer" (grant ARC4779). I thank Héra Der-Sarkissian for the karyotype analyses of post-crisis cells. Thanks to Jesper Graakjaer, Leigh Pascoe and Silvia Bacchetti for insightful discussions and to S. B., Frank Graham and Roger R. Reddel for comments and suggestions on the manuscript.

References

[1] E. H. Blackburn, Switching and signaling at the telomere, Cell 106 (2001) 661-673.

[2] E. H. Blackburn, Telomere states and cell fates, Nature 408 (2000) 53-56.

[3] A. M. Olovnikov, [Principle of marginotomy in template synthesis of polynucleotides], Dokl. Akad. Nauk. SSSR. 201 (1971) 1496-1499.

[4] J. D. Watson, Origin of concatemeric T7 DNA, Nat. New Biol. 239 (1972) 197-201.

[5] R. C. Allsopp, H. Vaziri, C. Patterson, S. Goldstein, E. V. Younglai, A. B. Futcher, C. W. Greider, C. B. Harley, Telomere length predicts replicative capacity of human fibroblasts, Proc. Natl. Acad. Sci. U. S. A. 89 (1992) 10114-10118.

[6] R. C. Allsopp, E. Chang, M. Kashefi-Aazam, E. I. Rogaev, M. A. Piatyszek, J. W. Shay, C. B. Harley, Telomere shortening is associated with cell division in vitro and in vivo, Exp. Cell Res. 220 (1995) 194-200.

[7] W. E. Wright, J. W. Shay, Cellular senescence as a tumor-protection mechanism: the essential role of counting, Curr. Opin. Genet. Dev. 11 (2001) 98-103.

[8] K. Masutomi, E. Y. Yu, S. Khurts, I. Ben-Porath, J. L. Currier, G. B. Metz, M. W. Brooks, S. Kaneko, S. Murakami, J. A. DeCaprio, R. A. Weinberg, S. A. Stewart, W. C. Hahn, Telomerase maintains telomere structure in normal human cells, Cell 114 (2003) 241-253.

[9] S. A. Stewart, I. Ben-Porath, V. J. Carey, B. F. O'Connor, W. C. Hahn, R. A. Weinberg, Erosion of the telomeric single-strand overhang at replicative senescence, Nat. Genet. 33 (2003) 492-496.

[10] J. Karlseder, A. Smogorzewska, T. de Lange, Senescence induced by altered telomere state, not telomere loss, Science 295 (2002) 2446-2449.

[11] S. Henderson, R. Allsopp, D. Spector, S. S. Wang, C. Harley, In situ analysis of changes in telomere size during replicative aging and cell transformation, J. Cell Biol. 134 (1996) 1-12.

[12] P. M. Lansdorp, N. P. Verwoerd, F. M. van de Rijke, V. Dragowska, M. T. Little, R. W. Dirks, A. K. Raap, H. J. Tanke, Heterogeneity in telomere length of human chromosomes, Hum. Mol. Genet. 5 (1996) 685-691.

[13] M. Hultdin, E. Gronlund, K. Norrback, E. Eriksson-Lindstrom, T. Just, G. Roos, Telomere analysis by fluorescence in situ hybridization and flow cytometry, Nucleic Acids Res. 26 (1998) 3651-3656.

[14] S. Bekaert, S. Koll, O. Thas, P. Van Oostveldt, Comparing telomere length of sister chromatids in human lymphocytes using three-dimensional confocal microscopy, Cytometry 48 (2002) 34-44.

[15] S. S. Poon, U. M. Martens, R. K. Ward, P. M. Lansdorp, Telomere length measurements using digital fluorescence microscopy, Cytometry. 36 (1999) 267-278.

[16] J. M. Zijlmans, U. M. Martens, S. S. Poon, A. K. Raap, H. J. Tanke, R. K. Ward, P. M. Lansdorp, Telomeres in the mouse have large inter-chromosomal variations in the number of T2AG3 repeats, Proc. Natl. Acad. Sci. U. S. A. 94 (1997) 7423-7428.

[17] J. A. Londoño-Vallejo, H. DerSarkissian, L. Cazes, G. Thomas, Differences in telomere length between homologous chromosomes in humans, Nucleic Acids Res. 29 (2001) 3164-3171.

[18] P. Slijepcevic, Telomere length regulation--a view from the individual chromosome perspective, Exp. Cell Res. 244 (1998) 268-274.

[19] P. Slijepcevic, Telomere length and telomere-centromere relationships?, Mutat. Res. 404 (1998) 215-220.

[20] J. Graakjaer, C. Bischoff, L. Korsholm, S. Holstebroe, W. Vach, V. A. Bohr, K. Christensen, S. Kolvraa, The pattern of chromosome-specific variations in telomere length in humans is determined by inherited, telomere-near factors and is maintained throughout life, Mech. Ageing Dev. 124 (2003) 629-640.

[21] U. M. Martens, J. M. Zijlmans, S. S. Poon, W. Dragowska, J. Yui, E. A. Chavez, R. K. Ward, P. M. Lansdorp, Short telomeres on human chromosome 17p, Nat. Genet. 18 (1998) 76-80.

[22] J. Flint, G. P. Bates, K. Clark, A. Dorman, D. Willingham, B. A. Roe, G. Micklem, D. R. Higgs, E. J. Louis, Sequence comparison of human and yeast telomeres identifies structurally distinct subtelomeric domains, Hum. Mol. Genet. 6 (1997) 1305-1313.

[23] H. Der-Sarkissian, G. Vergnaud, Y. M. Borde, G. Thomas, J. A. Londono-Vallejo, Segmental polymorphisms in the proterminal regions of a subset of human chromosomes, Genome Res. 12 (2002) 1673-1678.

[24] R. R. Reddel, Alternative lengthening of telomeres, telomerase, and cancer, Cancer Lett. 194 (2003) 155-162.

[25] T. von Zglinicki, Role of oxidative stress in telomere length regulation and replicative senescence, Ann. N. Y. Acad. Sci. 908 (2000) 99-110.

[26] D. M. Baird, J. Rowson, D. Wynford-Thomas, D. Kipling, Extensive allelic variation and ultrashort telomeres in senescent human cells, Nat. Genet. 33 (2003) 203-207.

[27] P. E. Slagboom, S. Droog, D. I. Boomsma, Genetic determination of telomere size in humans: a twin study of three age groups, Am. J. Hum. Genet. 55 (1994) 876-882.

[28] J. Surralles, M. P. Hande, R. Marcos, P. M. Lansdorp, Accelerated telomere shortening in the human inactive X chromosome, Am. J. Hum. Genet. 65 (1999) 1617-1622.

[29] T. de Lange, Protection of mammalian telomeres, Oncogene 21 (2002) 532-540.

[30] D. M. Baird, J. Coleman, Z. H. Rosser, N. J. Royle, High levels of sequence polymorphism and linkage disequilibrium at the telomere of 12q: implications for telomere biology and human evolution, Am. J. Hum. Genet. 66 (2000) 235-250.

[31] S. Marcand, V. Brevet, E. Gilson, Progressive cis-inhibition of telomerase upon telomere elongation, Embo J. 18 (1999) 3509-3519.

[32] Y. Liu, H. Kha, M. Ungrin, M. O. Robinson, L. Harrington, Preferential maintenance of critically short telomeres in mammalian cells heterozygous for mTert, Proc. Natl. Acad. Sci. U. S. A. 99 (2002) 3597-3602.

[33] T. von Zglinicki, R. Pilger, N. Sitte, Accumulation of single-strand breaks is the major cause of telomere shortening in human fibroblasts, Free Radic. Biol. Med. 28 (2000) 64-74.

[34] J. Graakjaer, L. Pascoe, H. Der-Sarkissian, G. Thomas, S. Kolvraa, K. Christensen, J. Londoño-Vallejo, The relative lengths of individula telomeres are defined in the zygote and strictly maintained during life, Aging cell (2004) Online publication, 15 Apr, doi: 10.1111/j.1474-9728.2004.00092.x.

[35] U. M. Martens, E. A. Chavez, S. S. Poon, C. Schmoor, P. M. Lansdorp, Accumulation of short telomeres in human fibroblasts prior to replicative senescence, Exp. Cell Res. 256 (2000) 291-299.

[36] C. M. Counter, A. A. Avilion, C. E. LeFeuvre, N. G. Stewart, C. W. Greider, C. B. Harley, S. Bacchetti, Telomere shortening associated with chromosome instability is arrested in immortal cells which express telomerase activity, Embo J. 11 (1992) 1921-1929.

[37] C. Ducray, J. P. Pommier, L. Martins, F. D. Boussin, L. Sabatier, Telomere dynamics, end-to-end fusions and telomerase activation during the human fibroblast immortalization process, Oncogene 18 (1999) 4211-4223.

[38] J. A. Hackett, C. W. Greider, Balancing instability: dual roles for telomerase and telomere dysfunction in tumorigenesis, Oncogene 21 (2002) 619-626.

[39] J. A. Hackett, D. M. Feldser, C. W. Greider, Telomere dysfunction increases mutation rate and genomic instability, Cell 106 (2001) 275-286.

[40] M. A. Blasco, H. W. Lee, M. P. Hande, E. Samper, P. M. Lansdorp, R. A. De Pinho, C. W. Greider, Telomere shortening and tumor formation by mouse cells lacking telomerase RNA, Cell 91 (1997) 25-34.

[41] F. Ishikawa, Telomere crisis, the driving force in cancer cell evolution, Biochem. Biophys. Res. Commun. 230 (1997) 1-6.

[42] E. L. Duncan, R. R. Reddel, Genetic changes associated with immortalization. A review, Biochemistry (Mosc.) 62 (1997) 1263-1274.

[43] T. L. Halvorsen, G. Leibowitz, F. Levine, Telomerase activity is sufficient To allow transformed cells To escape from crisis, Mol. Cell Biol. 19 (1999) 1864-1870.

[44] R. S. Maser, R. A. DePinho, Connecting chromosomes, crisis, and cancer, Science 297 (2002) 565-569.

[45] T. M. Bryan, A. Englezou, J. Gupta, S. Bacchetti, R. R. Reddel, Telomere elongation in immortal human cells without detectable telomerase activity, Embo J. 14 (1995) 4240-4248.

[46] C. M. Counter, W. C. Hahn, W. Wei, S. D. Caddle, R. L. Beijersbergen, P. M. Lansdorp, J. M. Sedivy, R. A. Weinberg, Dissociation among in vitro telomerase activity, telomere maintenance, and cellular immortalization, Proc. Natl. Acad. Sci. U. S. A. 95 (1998) 14723-14728.

[47] C. M. Counter, F. M. Botelho, P. Wang, C. B. Harley, S. Bacchetti, Stabilization of short telomeres and telomerase activity accompany immortalization of Epstein-Barr virus-transformed human B lymphocytes, J. Virol. 68 (1994) 3410-3414.

[48] M. C. Montalto, J. S. Phillips, F. A. Ray, Telomerase activation in human fibroblasts during escape from crisis, J. Cell Physiol. 180 (1999) 46-52.

[49] W. Guo, M. K. Kang, H. J. Kim, N. H. Park, Immortalization of human oral keratinocytes is associated with elevation of telomerase activity and shortening of telomere length, Oncol. Rep. 5 (1998) 799-804.

[50] T. M. Bryan, R. R. Reddel, Telomere dynamics and telomerase activity in in vitro immortalised human cells, Eur. J. Cancer 33 (1997) 767-773.

[51] J. D. Coursen, W. P. Bennett, L. Gollahon, J. W. Shay, C. C. Harris, Genomic instability and telomerase activity in human bronchial epithelial cells during immortalization by human papillomavirus-16 E6 and E7 genes, Exp. Cell Res. 235 (1997) 245-253.

[52] S. E. Artandi, R. A. DePinho, A critical role for telomeres in suppressing and facilitating carcinogenesis, Curr. Opin. Genet. Dev. 10 (2000) 39-46.

[53] R. C. O'Hagan, S. Chang, R. S. Maser, R. Mohan, S. E. Artandi, L. Chin, R. A. DePinho, Telomere dysfunction provokes regional amplification and deletion in cancer genomes, Cancer Cell 2 (2002) 149-155.

[54] J. N. O'Sullivan, M. P. Bronner, T. A. Brentnall, J. C. Finley, W. T. Shen, S. Emerson, M. J. Emond, K. A. Gollahon, A. H. Moskovitz, D. A. Crispin, J. D. Potter, P. S. Rabinovitch, Chromosomal instability in ulcerative colitis is related to telomere shortening, Nat. Genet. 32 (2002) 280-284.

[55] C. N. Sprung, G. Afshar, E. A. Chavez, P. Lansdorp, L. Sabatier, J. P. Murnane, Telomere instability in a human cancer cell line, Mutat. Res. 429 (1999) 209-223.

[56] W. Deng, S. W. Tsao, X. Y. Guan, J. N. Lucas, A. L. Cheung, Role of short telomeres in inducing preferential chromosomal aberrations in human ovarian surface epithelial cells: A combined telomere quantitative fluorescence in situ hybridization and whole-chromosome painting study, Genes Chromosomes Cancer 37 (2003) 92-97.

[57] C. Desmaze, J. C. Soria, M. A. Freulet-Marriere, N. Mathieu, L. Sabatier, Telomere-driven genomic instability in cancer cells, Cancer Lett. 194 (2003) 173-182.

[58] M. T. Hemann, M. A. Strong, L. Y. Hao, C. W. Greider, The shortest telomere, not average telomere length, is critical for cell viability and chromosome stability, Cell 107 (2001) 67-77.

[59] H. Der-Sarkissian, S. Bacchetti, L. Cazes, J. A. Londono-Vallejo, The shortest telomeres drive karyotype evolution in transformed cells, Oncogene 23 (2004) 1221-1228.

[60] C. Lengauer, K. W. Kinzler, B. Vogelstein, Genetic instabilities in human cancers, Nature 396 (1998) 643-649.

[61] F. Mertens, B. Johansson, M. Hoglund, F. Mitelman, Chromosomal imbalance maps of malignant solid tumors: a cytogenetic survey of 3185 neoplasms, Cancer Res. 57 (1997) 2765-2780.

[62] M. Hoglund, D. Gisselsson, N. Mandahl, B. Johansson, F. Mertens, F. Mitelman, T. Sall, Multivariate analyses of genomic imbalances in solid tumors reveal distinct and converging pathways of karyotypic evolution, Genes Chromosomes Cancer 31 (2001) 156-171.

[63] B. Johansson, F. Mertens, F. Mitelman, Primary vs. secondary neoplasia-associated chromosomal abnormalities--balanced rearrangements vs. genomic imbalances?, Genes Chromosomes Cancer 16 (1996) 155-163.

[64] N. L. Komarova, D. Wodarz, The optimal rate of chromosome loss for the inactivation of tumor suppressor genes in cancer, Proc. Natl. Acad. Sci. U. S. A. 101 (2004) 7017-7021.

[65] K. L. Rudolph, S. Chang, H. W. Lee, M. Blasco, G. J. Gottlieb, C. Greider, R. A. DePinho, Longevity, stress response, and cancer in aging telomerase-deficient mice, Cell 96 (1999) 701-712.

[66] S. E. Artandi, S. Chang, S. L. Lee, S. Alson, G. J. Gottlieb, L. Chin, R. A. DePinho, Telomere dysfunction promotes non-reciprocal translocations and epithelial cancers in mice, Nature 406 (2000) 641-645.

[67] K. L. Rudolph, M. Millard, M. W. Bosenberg, R. A. DePinho, Telomere dysfunction and evolution of intestinal carcinoma in mice and humans, Nat. Genet. 28 (2001) 155-159.

[68] A. K. Meeker, J. L. Hicks, E. Gabrielson, W. M. Strauss, A. M. De Marzo, P. Argani, Telomere shortening occurs in subsets of normal breast epithelium as well as in situ and invasive carcinoma, Am. J. Pathol. 164 (2004) 925-935.

[69] A. K. Meeker, J. L. Hicks, E. A. Platz, G. E. March, C. J. Bennett, M. J. Delannoy, A. M. De Marzo, Telomere shortening is an early somatic DNA alteration in human prostate tumorigenesis, Cancer Res. 62 (2002) 6405-6409.

[70] N. T. van Heek, A. K. Meeker, S. E. Kern, C. J. Yeo, K. D. Lillemoe, J. L. Cameron, G. J. Offerhaus, J. L. Hicks, R. E. Wilentz, M. G. Goggins, A. M. De Marzo, R. H. Hruban, A. Maitra, Telomere shortening is nearly universal in pancreatic intraepithelial neoplasia, Am. J. Pathol. 161 (2002) 1541-1547.

[71] A. K. Meeker, W. R. Gage, J. L. Hicks, I. Simon, J. R. Coffman, E. A. Platz, G. E. March, A. M. De Marzo, Telomere length assessment in human archival tissues: combined telomere fluorescence in situ hybridization and immunostaining, Am. J. Pathol. 160 (2002) 1259-1268.

[72] D. Gisselsson, T. Jonson, C. Yu, C. Martins, N. Mandahl, J. Wiegant, Y. Jin, F. Mertens, C. Jin, Centrosomal abnormalities, multipolar mitoses, and chromosomal instability in head and neck tumours with dysfunctional telomeres, Br. J. Cancer 87 (2002) 202-207.

[73] D. Gisselsson, T. Jonson, A. Petersen, B. Strombeck, P. Dal Cin, M. Hoglund, F. Mitelman, F. Mertens, N. Mandahl, Telomere dysfunction triggers extensive DNA fragmentation and evolution of complex chromosome abnormalities in human malignant tumors, Proc. Natl. Acad. Sci. U. S. A. 98 (2001) 12683-12688.

[74] D. Gisselsson, Chromosome instability in cancer: how, when, and why?, Adv. Cancer Res. 87 (2003) 1-29.

[75] C. Lengauer, How do tumors make ends meet?, Proc. Natl. Acad. Sci. U. S. A. 98 (2001) 12331-12333.

[76] D. Gisselsson, L. Pettersson, M. Hoglund, M. Heidenblad, L. Gorunova, J. Wiegant, F. Mertens, P. Dal Cin, F. Mitelman, N. Mandahl, Chromosomal breakage-fusion-bridge events cause genetic intratumor heterogeneity, Proc. Natl. Acad. Sci. U. S. A. 97 (2000) 5357-5362.

[77] V. N. Anisimov, The relationship between aging and carcinogenesis: a critical appraisal, Crit. Rev. Oncol. Hematol. 45 (2003) 277-304.

[78] R. A. DePinho, The age of cancer, Nature 408 (2000) 248-254.

[79] X. Wu, B. J. Dave, H. Jiang, S. Pathak, M. R. Spitz, Lung carcinoma patients with a family history of cancer and lymphocyte primary chromosome 9 aberrations, Cancer 79 (1997) 1527-1532.

[80] Y. Zhu, M. R. Spitz, S. Strom, G. E. Tomlinson, C. I. Amos, J. D. Minna, X. Wu, A case-control analysis of lymphocytic chromosome 9 aberrations in lung cancer, Int. J. Cancer. 102 (2002) 536-540.

[81] X. Wu, C. I. Amos, Y. Zhu, H. Zhao, B. H. Grossman, J. W. Shay, S. Luo, W. K. Hong, M. R. Spitz, Telomere dysfunction: a potential cancer predisposition factor, J. Natl. Cancer Inst. 95 (2003) 1211-1218.

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