Alternative lengthening of telomeres is characterized by high rates of telomeric exchange.

Telomere maintenance activity is a hallmark of cancer. In some telomerase-negative tumors, telomeres become lengthened by alternative lengthening of telomeres (ALT), a recombination-mediated DNA replication process in which telomeres use other telomeric DNA as a copy template. Using chromosome orientation fluorescence in situ hybridization, we found that postreplicative exchange events involving a telomere and another TTAGGG-repeat tract occur at remarkably high frequencies in ALT cells (range 28-280/100 metaphases) and rarely or never in non-ALT cells, including cell lines with very long telomeres. Like the ALT phenotype itself, the telomeric exchanges were not suppressed when telomerase was activated in ALT cells. These exchanges are telomere specific because there was no correlation with sister chromatid exchange rates at interstitial locations, and they were not observed in non-ALT Bloom syndrome cells with very high sister chromatid exchange rates.

The shortest telomeres drive karyotype evolution in transformed cells.

Maintenance of telomeres is essential for chromosome stability. In the absence of telomerase, telomeres shorten with cell division until they approach a stability threshold, at which point cells enter senescence. When senescence-signaling pathways are inactive, further telomere shortening leads to chromosome instability characterized by telomeric fusions and breakage-fusion-bridge (BFB) cycles. Since the distribution of telomere lengths among chromosome extremities is heterogeneous, we wondered about the impact of such variability on the stability of particular chromosome arms. We correlated the initial length of individual telomeres in telomerase-negative-transformed cells with the stability of the corresponding chromosome arms during the precrisis period. We show that arms carrying the shortest telomeres are the first to become unstable and this instability affects the chromosome homologues with shorter telomeres almost exclusively. The analysis of several postcrisis cell populations, which had stabilized their telomeres by re-expressing telomerase, showed that the karyotypic outcome is strongly influenced by the initial telomere length heterogeneity. The timing of telomerase re-expression also seems to play a role in limiting the extent of karyotypic changes, probably by reducing the frequency of telomeric fusions and hence BFB. Since the distribution of telomere lengths within somatic cells is proper to every individual, our results predict that the risk for a particular chromosome arm of becoming unstable early in tumorigenesis will differ between individuals and contribute directly to the heterogeneity of chromosome aberrations found in tumors.

The relative lengths of individual telomeres are defined in the zygote and strictly maintained during life.

Previous studies have indicated that average telomere length is partly inherited (Slagboom et al., 1994; Rufer et al., 1999) and that there is an inherited telomere pattern in each cell (Graakjaer et al., 2003); (Londoño-Vallejo et al., 2001). In this study, we quantify the importance of the initially inherited telomere lengths within cells, in relation to other factors that influence telomere length during life. We have estimated the inheritance by measuring telomere length in monozygotic (MZ) twins using Q-FISH with a telomere specific peptide nucleic acid (PNA)-probe. Homologous chromosomes were identified using subtelomeric polymorphic markers. We found that identical homologous telomeres from two aged MZ twins show significantly less differences in relative telomere length than when comparing the two homologues within one individual. This result means that towards the end of life, individual telomeres retain the characteristic relative length they had at the outset of life and that any length alteration during the lifespan impacts equally on genetically identical homologues. As the result applies across independent individuals, we conclude that, at least in lymphocytes, epigenetic/environmental effects on relative telomere length are relatively minor during life.

Allele-specific relative telomere lengths are inherited.

Previous studies have indicated that single relative telomere lengths are defined in the zygote. In order to explore the possibility that single telomere lengths segregate in families, we compared relative telomere lengths obtained from allelic chromosome extremities transmitted from parent to child, representing a total of 31 independent meiotic events. We find a significant positive correlation of 0.65 (P=0.0004) between these telomere lengths, whereas the correlation between the non-transmitted parental homologue and the transmitted homologue in the child is not statistically significant (r=0.16; P=0.195). This study indicates that, even though there is a telomerase-mediated maintenance/elongation of telomeres in germ cells, allele-specific relative telomere lengths are preserved in the next generation.

Segmental polymorphisms in the proterminal regions of a subset of human chromosomes.

The subtelomeric domains of chromosomes are probably the most rapidly evolving structures of the human genome. The highly variable distribution of large duplicated subtelomeric segments has indicated that frequent exchanges between nonhomologous chromosomes may have been taking place during recent genome evolution. We have studied the extent and variability of such duplications using in situ hybridization techniques and a set of well-defined subtelomeric cosmid probes that identify discrete regions within the subtelomeric domain. In addition to reciprocal translocation and illegitimate recombination events that could explain the observed mosaic pattern of subtelomeric regions, it is likely that homology-based recombination mechanisms have also contributed to the spread of distal subtelomeric sequences among particular groups of nonhomologous chromosome arms. The frequency and distribution of large-scale subtelomeric polymorphisms may have direct implications for the design of chromosome-specific probes that are aimed at the identification of cryptic subtelomeric deletions. Furthermore, our results indicate that the relevance of some of the telomere closures proposed within the present Human Genome Sequence draft are restricted to specific allelic variants of unknown frequencies.