Normal telomere erosion rates at the single cell level in Werner syndrome fibroblast cells

The aim of this study was to investigate whether the accelerated replicative senescence seen in Werner syndrome (WS) fibroblasts is due to accelerated telomere loss per cell division. Using single telomere length analysis (STELA) we show that the mean rate of telomere shortening in WS bulk cultures ranges between that of normal fibroblasts [99 bp/population doubling (PD)] and four times that of normal (355 bp/PD). The telomere erosion rate in the fastest eroding strain slows in the later stages of culture to that observed in normal fibroblasts, and appears to be correlated with a reduction in the heterogeneity of the telomere-length distributions. Telomere erosion rates in clones of WS cells are much reduced compared with bulk cultures, as are the variances of the telomere-length distributions. The overall lack of length heterogeneity and the normal erosion rates of the clonal populations are consistent with simple end-replication losses as the major contributor to telomere erosion in WS cells. We propose that telomere dynamics at the single cell level in WS fibroblasts are not significantly different from those in normal fibroblasts, and suggest that the accelerated replicative decline seen in WS fibroblasts does not result from accelerated telomere erosion

Telomere instability in the male germline

Telomeres play a key role in upholding the integrity of the genome, and telomerase expression in spermatogonial stem cells is responsible for the maintenance of telomere length in the human male germline. We have previously described extensive allelic variation in somatic cell telomere length that is set in the zygote, the ultimate source of which may be the germline. This implies that despite telomerase activity, substantial telomere length variation can be generated and tolerated in the germline; in order to investigate this further, we have examined the nature of telomere length variation in the human male germline. Here, we describe an analysis of both genome-wide telomere length and single molecule analysis of specific chromosome ends in human sperm. We observed individual specific differences in genome-wide telomere length. This variation may result from genetic differences within the components that determine the telomere length setting of each individual. Superimposed on the genome wide telomere length setting was a stochastic component of variation that generates germ-cells containing severely truncated telomeres. If not re-lengthened during early embryogenesis, such telomeres may limit the replicative capacity of cells derived from the zygote and have the potential to create fusagenic chromosomes, unbalanced translocations and terminal micro-deletions. These data may have implications for the genetic determination of ageing, genetic disease and fertility

Extensive allelic variation and ultrashort telomeres in senescent human cells

By imposing a limit on the proliferative lifespan of most somatic cells, telomere erosion represents an innate mechanism for tumor suppression1 and may contribute to age-related disease2. A detailed understanding of the pathways that link shortened telomeres to replicative senescence has been severely hindered by the inability of current methods to analyze telomere dynamics in detail. Here we describe single telomere length analysis (STELA), a PCR-based approach that accurately measures the full spectrum of telomere lengths from individual chromosomes. STELA analysis of human XpYp telomeres in fibroblasts identifies several features of telomere biology. We observe bimodal distributions of telomeres in normal fibroblasts; these distributions result from inter-allelic differences of up to 6.5 kb, indicating that unexpectedly large-scale differences in zygotic telomere length are maintained throughout development. Most telomeres shorten in a gradual fashion consistent with simple losses through end replication, and the rates of erosion are independent of allele size. Superimposed on this are occasional, more substantial changes in length, which may be the consequence of additional mutational mechanisms. Notably, some alleles show almost complete loss of TTAGGG repeats at senescence

Structural stability and chromosome-specific telomere length is governed by cis-acting determinants in humans

Single telomere length analysis (STELA) of the XpYp telomere has revealed extensive allelic variation and ultra-short telomeres in senescent cells. Superimposed on end-replication losses are additional mutational events that result in large-scale changes in telomere length. In order to establish if the dynamics of the XpYp telomere are typical of human telomeres, here we describe an analysis using STELA of the telomeres of 2p, 11q, 12q, 17p and XpYp. The dynamics of telomere loss (erosion rates and stochastic length changes) was conserved among 2p, 11q, 12q and XpYp within the same cell strains and was dependent on the replicative kinetics of the cells in culture. However, of the telomeres analysed, the telomere of 17p was more stable with a striking paucity of large-scale length changes, and exhibited the shortest recorded allelic distribution (300 bp) in senescent cells and displayed a general, but not absolute, trend towards being the shortest telomere. Ectopic over-expression of hTERT homogenized both allelic and chromosome-specific telomeric distributions. However, telomerase-expressing cancer cells displayed both allelic variation and chromosome-specific telomere length, with 17p displaying the shortest allelic telomere length. Although other telomeres in the genome may share the properties of 17p, these data suggest that physiological levels of telomerase allow differential telomere length regulation and indicate the presence of cis-acting factors that govern both telomeric stability and chromosome-specific telomere length in the presence of telomerase

Telomere dynamics during replicative senescence are not directly modulated by conditions of oxidative stress in IMR90 fibroblast cells

The replicative lifespan of many cell types is determined by the length of telomeres in the initiating cell population. In 20% oxygen, IMR90 cells have a shorter replicative lifespan compared to that achieved in conditions that lower the levels of oxidative stress. We sought to address the role of telomere dynamics in determining the replicative lifespan of IMR90 cells. We analysed clonal populations cultured in parallel in 3 and 20% oxygen. We observed that, at senescence, telomere length was shorter in 3% oxygen and this was proportional to the lifespan extension. We observed no detectable difference in the rate of telomere erosion in the two culture conditions, however as the cells approached senescence the growth rate of the cultures slowed with a commensurate increase in the rate of telomere erosion. We conclude that, in 20% oxygen senescence of IMR90 is telomere-independent, but telomere-dependent in 3% oxygen

Mre11 modulates the fidelity of fusion between short telomeres in human cells

The loss of telomere function can result in the fusion of telomeres with other telomeric loci, or non-telomeric double-stranded DNA breaks. Sequence analysis of fusion events between short dysfunctional telomeres in human cells has revealed that fusion is characterized by a distinct molecular signature consisting of extensive deletions and micro-homology at the fusion points. This signature is consistent with alternative error-prone end-joining processes. We have examined the role that Mre11 may play in the fusion of short telomeres in human cells; to do this, we have analysed telomere fusion events in cells derived from ataxia-telangiectasia-like disorder (ATLD) patients that exhibit hypomorphic mutations in MRE11. The telomere dynamics of ATLD fibroblasts were indistinguishable from wild-type fibroblasts and they were proficient in the fusion of short telomeres. However, we observed a high frequency of insertion of DNA sequences at the fusion points that created localized sequence duplications. These data indicate that Mre11 plays a role in the fusion of short dysfunctional telomeres in human cells and are consistent with the hypothesis that as part of the MRN complex it serves to stabilize the joining complex, thereby controlling the fidelity of the fusion reaction