High incidence of rapid telomere loss in telomerase-deficient Caenorhabditis elegans

Telomerase is essential to maintain telomere length in most eukaryotes. Other functions for telomerase have been proposed but molecular mechanisms remain unclear. We studied Caenorhabditis elegans with a mutation in the trt-1 telomerase reverse transcriptase gene. Mutant animals showed a progressive decrease in brood size and typically failed to reproduce after five generations. Using PCR analysis to measure the length of individual telomere repeat tracks on the left arm of chromosome V we observed that trt-1 mutants lost ∼125bp of telomeric DNA per generation. Chromosome fusions involving complex recombination reactions were observed in late generations. Strikingly, trt-1 mutant animals displayed a high frequency of telomeres with many fewer repeats than average. Such outlying short telomeres were not observed in mrt-2 mutants displaying progressive telomere loss very similar to trt-1 mutants. We speculate that, apart from maintaining the average telomere length, telomerase is required to prevent or repair sporadic telomere truncations that are unrelated to the typical ‘end-replication’ problems.

Role of Telomerase in Hematopoietic Stem Cells

Studies in hematopoietic stem cell (HSC) biology are often focused on "self-renewal" and differentiation. Implicit in the word self-renewal is that the two daughter cells generated by a self-renewal division are identical to the parental cell. Strictly speaking, this is not possible because DNA is continuously damaged and repaired by DNA-repair mechanisms that are not 100% efficient. It is important to note that the efficiency of DNA repair varies greatly among different stem cell types. For example, embryonic stem cells are quite resistant to DNA damage and maintain the length of telomere repeats on serial passage, whereas HSCs are quite sensitive to DNA damage and less able to maintain telomere length. Most likely, differences between stem cell types in DNA repair and telomere maintenance pathways coevolved with cell mass, turnover, reproductive strategy, and life span. This idea has given rise to the notion that many aspects of normal aging could primarily reflect limitations in DNA repair and telomere-maintenance pathways in the (stem) cells of the soma. In humans, levels of telomerase in HSCs are under extremely tight control, as is illustrated by the marrow failure in patients with (mild) telomerase deficiencies. Here, the role of telomerase in human HSC biology is reviewed, and it is proposed that telomerase has an important role in the repair of G-rich DNA.

Major cutbacks at chromosome ends

To distinguish a telomere from a double-strand break, a minimum number of telomere repeats must 'cap' each chromosome end. The length of each repeat array will reflect a unique history of addition and losses. Telomere losses are predicted to occur slowly but surely with every replication cycle (referred to as 'typical' telomere loss) in addition to intermittently and, potentially, rapidly ('sporadic'). Recent studies have shown that sporadic telomere losses can result from failure to properly repair (oxidative) damage to telomeric DNA, from failure to properly process higher-order structures of G-rich DNA and from homologous recombination reactions. Differences in telomere-erosion pathways between normal and malignant cells provide novel targets for the prevention and therapy of disease.

Dissociation of telomerase activity and telomere length maintenance in primitive human hematopoietic cells

Primitive human hematopoietic cells have low endogenous telomerase activity, yet telomeres are not maintained. In contrast, ectopic telomerase expression in fibroblasts and other cells leads to telomere length maintenance or elongation. It is unclear whether this disparity can be attributed to telomerase level or stems from fundamentally different telomere biology. Here, we show that telomerase overexpression does not prevent proliferation-associated telomere shortening in human hematopoietic cells, pointing to the existence of cell type-specific differences in telomere dynamics. Furthermore, we observed eventual stabilization of telomere length without detectable changes in telomerase activity during establishment of two leukemic cell lines from normal cord blood cells, indicating that additional cooperating events are required for telomere maintenance in immortalized human hematopoietic cells.

The telomerase reverse transcriptase regulates chromatin state and DNA damage responses

Constitutive expression of telomerase prevents senescence and crisis by maintaining telomere homeostasis. However, recent evidence suggests that telomerase is dynamically regulated in normal cells and also contributes to transformation independently of net telomere elongation. Here, we show that suppression of the telomerase catalytic subunit [human telomerase reverse transcriptase (hTERT)] expression abrogates the cellular response to DNA double strand breaks. Loss of hTERT does not alter short-term telomere integrity but instead affects the overall configuration of chromatin. Cells lacking hTERT exhibit increased radiosensitivity, diminished capacity for DNA repair, and fragmented chromosomes, demonstrating that loss of hTERT impairs the DNA damage response.

Identification and functional characterization of 2 variant alleles of the telomerase RNA template gene (TERC) in a patient with dyskeratosis congenita

Heterozygous mutations of the human telomerase RNA template gene (TERC) have been described in patients with acquired aplastic anemia and the autosomal dominant form of dyskeratosis congenita (DKC). Patients with mutations in both TERC alleles have not yet been reported. Here, we report a patient with DKC who inherited 2 distinct TERC sequence variants from her parents; a deletion (216_229del) in one and a point mutation (37A>G) in the other allele of the TERC gene. Her marrow was hypocellular and showed an abnormal clone [46, XX t(7;21)(q34;q22)]. The telomere lengths in leukocytes of the patient and her relatives were shorter than those of the age-matched controls and were progressively shorter in subsequent generations of family members with the 216_229del allele. Telomerase enzymatic levels in lymphocytes from the patient were approximately half of those measured in healthy controls. The 216_229del mutation failed to reconstitute telomerase activity in transfected cells, but, when coexpressed with the 37A>G variant, telomerase activity was only modestly suppressed. These clinical and laboratory findings support the concept that telomerase levels in human hematopoietic stem cells are tightly controlled as even moderately reduced levels result in accelerated telomere shortening and eventual marrow failure.

Mutations in TERT, the gene for telomerase reverse transcriptase, in aplastic anemia

BACKGROUND

Mutations in TERC, the gene for the RNA component of telomerase, cause short telomeres in congenital aplastic anemia and in some cases of apparently acquired hematopoietic failure. We investigated whether mutations in genes for other components of telomerase also occur in aplastic anemia.

METHODS

We screened blood or marrow cells from 124 patients with apparently acquired aplastic anemia and 282 control subjects for sequence variations in the TERT, DKC1, NHP2, and NOP10 genes; an additional 81 patients and 246 controls were examined for genetic variations in TERT. Telomere lengths and the telomerase activity of peripheral-blood leukocytes were evaluated in patients carrying genetic variants. Identified mutations were transfected into telomerase-deficient cell lines to examine their effects and their mechanism of action on telomerase function.

RESULTS

Five heterozygous, nonsynonymous mutations (which cause an amino acid change in the corresponding protein) were identified in TERT, the gene for the telomerase reverse transcriptase catalytic enzyme, among seven unrelated patients. Leukocytes from these patients had short telomeres and low telomerase enzymatic activity. In three of these patients, the mutation was also detected in buccal mucosa cells. Family members carrying the mutations also had short telomeres and reduced telomerase activity but no evident hematologic abnormality. The results of coexpression of wild-type TERT and TERT with aplastic anemia-associated mutations in a telomerase-deficient cell line suggested that haploinsufficiency was the mechanism of telomere shortening due to TERT mutations.

CONCLUSIONS

Heterozygous mutations in the TERT gene impair telomerase activity by haploinsufficiency and may be risk factors for marrow failure.

Functional characterization of telomerase RNA variants found in patients with hematologic disorders

Human telomerase uses a specific cellular RNA, called hTERC, as the template to synthesize telomere repeats at chromosome ends. Approximately 10% to 15% of patients with aplastic anemia or other bone marrow failure syndromes are carriers of hTERC sequence variants whose functional significance, in most cases, is unknown. We screened 10 reported and 2 newly discovered hTERC variants from such patients and found that 10 of these negatively affected telomerase enzymatic function when they were used to reconstitute telomerase enzymatic function in human cells. Most functional deficits were due to perturbations of hTERC secondary structure and correlated well with the degrees of telomere shortening and reduced telomerase activity observed in peripheral blood lymphocytes of the representative patients. We also found no evidence of dominant-negative activity in any of the mutants. Therefore, loss of telomerase activity and of telomere maintenance resulting from inherited hTERC mutations may limit marrow stem cell renewal and predispose some patients to bone marrow failure.

Telomere loss, senescence, and genetic instability in CD4+ T lymphocytes overexpressing hTERT

Little is known about the long-term consequences of overexpression of the human telomerase reverse transcriptase (hTERT) gene in T lymphocytes. To address this issue, we transduced polyclonal as well as clonally derived populations of naive and memory CD44 T cells from 2 healthy donors (aged 24 and 34 years) with retroviral vectors encoding green fluorescence protein (GFP) and hTERT (GFP-hTERT) or GFP alone. After transduction, cells were sorted on the basis of GFP expression and cultured in vitro until senescence. T cells transduced with hTERT exhibited high stable telomerase activity throughout the culture period. Relative to GFP controls, minor changes in overall gene expression were observed yet the proliferative lifespan of the hTERT-transduced populations was significantly increased and the rate of telomere loss was lower. Nevertheless, hTERT-transduced cells showed progressive telomere loss and had shorter telomeres at senescence than controls (2.3 +/- 0.3 kilobase [kb] versus 3.4 +/- 0.1 kb). Furthermore, a population of cells with 4N DNA consisting of binucleated cells with connected nuclei emerged in the hTERT-transduced cells prior to senescence. We conclude that overexpression of hTERT in CD4+ T cells provides a proliferative advantage independent of the average telomere length but does not prevent eventual genetic instability and replicative senescence.

CD27 expression promotes long-term survival of functional effector-memory CD8+ cytotoxic T lymphocytes in HIV-infected patients

Human immunodeficiency virus (HIV)-specific CD8(+) T cells persist in high frequencies in HIV-infected patients despite impaired CD4(+) T helper response to the virus, but, unlike other differentiated effector cytotoxic T lymphocytes, most continue to express the tumor necrosis factor receptor family member CD27. Because the ligand for CD27 (CD70) is also overexpressed in HIV-infected hosts, we examined the nature of expression and potential functional consequences of CD27 expression on HIV-specific CD8(+) T cells. Analysis of CD27(+) and CD27(-) T cells derived from the same HIV-specific clone revealed that retention of CD27 did not interfere with acquisition of effector functions, and that after T cell receptor stimulation, CD27(+) cells that concurrently were triggered via CD27 exhibited more resistance to apoptosis, interleukin 2 production, and proliferation than CD27(-) T cells. After transfer back into an HIV-infected patient, autologous HIV-specific CD27(-) T cells rapidly disappeared, but CD27(+) T cells derived from the same clone persisted at high frequency. Our findings suggest that the CD27-CD70 interaction in HIV infection may provide CD27(+) CD8(+) T cells with a survival advantage and compensate for limiting or absent CD4(+) T help to maintain the CD8 response.

Prediction of survival in follicular lymphoma based on molecular features of tumor-infiltrating immune cells

BACKGROUND

Patients with follicular lymphoma may survive for periods of less than 1 year to more than 20 years after diagnosis. We used gene-expression profiles of tumor-biopsy specimens obtained at diagnosis to develop a molecular predictor of the length of survival.

METHODS

Gene-expression profiling was performed on 191 biopsy specimens obtained from patients with untreated follicular lymphoma. Supervised methods were used to discover expression patterns associated with the length of survival in a training set of 95 specimens. A molecular predictor of survival was constructed from these genes and validated in an independent test set of 96 specimens.

RESULTS

Individual genes that predicted the length of survival were grouped into gene-expression signatures on the basis of their expression in the training set, and two such signatures were used to construct a survival predictor. The two signatures allowed patients with specimens in the test set to be divided into four quartiles with widely disparate median lengths of survival (13.6, 11.1, 10.8, and 3.9 years), independently of clinical prognostic variables. Flow cytometry showed that these signatures reflected gene expression by nonmalignant tumor-infiltrating immune cells.

CONCLUSIONS

The length of survival among patients with follicular lymphoma correlates with the molecular features of nonmalignant immune cells present in the tumor at diagnosis.

Estimating human hematopoietic stem cell kinetics using granulocyte telomere lengths

OBJECTIVE

To study in vivo behavior of hematopoietic stem cells (HSC).

MATERIALS AND METHODS

Behavior of HSC is difficult to study because one cannot observe and track cells within the marrow microenvironment. Therefore, information must be obtained from indirect means, such as competitive repopulation assays or surrogate studies, such as observations of telomere shortening in granulocytes. In this article, we use granulocyte telomere length data and a novel approach, stochastic simulation, to derive replication rates of HSC. The approach is first applied to cats and then to humans.

RESULTS

Human HSC replicate infrequently, on average once per 45 weeks (range: once per 23 to once per 67 weeks).

CONCLUSIONS

This rate is substantially slower than the average replication rates estimated for murine (once per 2.5 weeks) and feline (once per 8.3-10 weeks) HSC in vivo.

Telomeres, telomerase, and hematopoietic stem cell biology

Telomeres are composed of the tandem DNA repeats and associated proteins that cap the end of linear chromosomes. They provide stability to the chromosome and protect against DNA loss associated with cellular replication. Telomeres are maintained by the reverse transcriptase, telomerase. The regulation of telomere length and telomerase activity is a complex and dynamic process that is tightly linked to cell cycle regulation. Hematopoietic stem cells have an impressive but finite proliferative potential and demonstrate telomeric shortening during replicative aging despite expression of low levels of telomerase. Recently, the important role of telomeres in human illness has been highlighted by studies of the rare genetic disorder dyskeratosis congenita. Here we review the role of telomeres and telomerase in the function and regulation of the hematopoietic stem cell compartment and their importance in hematologic disease.

Regulation of murine telomere length by Rtel: an essential gene encoding a helicase-like protein

Little is known about the genes that regulate telomere length diversity between mammalian species. A candidate gene locus was previously mapped to a region on distal mouse Chr 2q. Within this region, we identified a gene similar to the dog-1 DNA helicase-like gene in C. elegans. We cloned this Regulator of telomere length (Rtel) gene and inactivated its expression in mice. Rtel(-/-) mice died between days 10 and 11.5 of gestation with defects in the nervous system, heart, vasculature, and extraembryonic tissues. Rtel(-/-) embryonic stem cells showed telomere loss and displayed many chromosome breaks and fusions upon differentiation in vitro. Crosses of Rtel(+/-) mice with Mus spretus showed that Rtel from the Mus musculus parent is required for telomere elongation of M. spretus chromosomes in F1 cells. We conclude that Rtel is an essential gene that regulates telomere length and prevents genetic instability.

Telomere length in subpopulations of human hematopoietic cells

In order to test the hypothesis that the telomere length in human hematopoietic cells correlates with their proliferative potential, we analyzed the telomere length in highly purified subpopulations of bone marrow cells. Cells were sorted on the basis of CD34 and CD38 cell surface markers, and two samples were additionally sorted on the basis of Hoechst 33342 dye efflux allowing isolation of side population (SP) cells. The telomere length in limiting numbers of sorted cells was analyzed using a newly developed fluorescence in situ hybridization (flow-FISH) method in which hybridization of telomere probe in cells of interest is measured relative to control cells in the same tube. In all seven bone marrow samples analyzed, the telomere length in CD34(+)CD38(-) cells was longer than in CD34(+)CD38(+) cells from the same donor (p < 0.02). Results with sorted SP cells were less clear: the telomere fluorescence in these cells was very heterogeneous, and a reproducible difference in telomere length relative to CD34(+)CD38(-) cells could not be observed. We conclude that the telomere length in subpopulations of hematopoietic cells does appear to be correlated with the known proliferative potential of such cells and that further characterization of cells on the basis of telomere length is warranted for enrichment of very rare precursors of hematopoietic and other tissues.

Strain-specific telomere length revealed by single telomere length analysis in Caenorhabditis elegans

Terminal restriction fragment analysis is the only method currently available for measuring telomere length in Caenorhabditis elegans. Its limitations include low sensitivity and interference by the presence of interstitial telomeric sequences in the C.elegans genome. Here we report the adaptation of single telomere length analysis (STELA) to measure the length of telomeric repeats on the left arm of chromosome V in C.elegans. This highly sensitive PCR-based method allows telomere length measurement from as few as a single worm. The application of STELA to eight wild-type C.elegans strains revealed considerable strain-specific differences in telomere length. Within individual strains, short outlying telomeres were observed that were clearly distinct from the bulk telomere length distributions, suggesting that processes other than end-replication losses and telomerase-mediated lengthening may generate telomere length heterogeneity in C.elegans. The utility of this method was further demonstrated by the characterization of telomere shortening in mrt-2 mutants. We conclude that STELA appears to be a valuable tool for studying telomere biology in C.elegans.

Effect of TERT and ATM on gene expression profiles in human fibroblasts

Telomeres protect chromosomes from degradation, end-to-end fusion, and illegitimate recombination. Loss of telomeres may lead to cell death or senescence or may cause genomic instability, leading to tumor formation. Expression of human telomerase reverse transcriptase (TERT) in human fibroblast cells elongates their telomeres and extends their lifespan. Ataxia telangiectasia mutated (ATM) deficiency in A-T human fibroblasts results in accelerated telomere shortening, abnormal cell-cycle response to DNA damage, and early senescence. Gene expression profiling was performed by serial analysis of gene expression (SAGE) on BJ normal human skin fibroblasts, A-T cells, and BJ and A-T cells transduced with TERT cDNA and expressing telomerase activity. In the four SAGE libraries, 36,921 unique SAGE tags were detected. Pairwise comparisons between the libraries showed differential expression levels of 1%-8% of the tags. Transcripts affected by both TERT and ATM were identified according to expression patterns, making them good candidates for further studies of pathways affected by both TERT and ATM. These include MT2A, P4HB, LGALS1, CFL1, LDHA, S100A10, EIF3S8, RANBP9, and SEC63. These genes are involved in apoptosis or processes related to cell growth, and most have been found to be deregulated in cancer. Our results have provided further insight into the roles of TERT and ATM by identifying genes likely to be involved in their function. Supplementary material for this article can be found on the Genes, Chromosomes and Cancer website at http://www.interscience.wiley.com/jpages/1045-2257/suppmat/index.html.

Late presentation of dyskeratosis congenita as apparently acquired aplastic anaemia due to mutations in telomerase RNA

Aplastic anaemia in adults is usually acquired, but rarely constitutional types of bone marrow failure can occur late in life. We assessed two families with onset of pancytopenia in adults and detected two novel point mutations in the telomerase RNA gene (TERC) in each family. This gene is abnormal in some kindreds with dyskeratosis congenita. Individuals in our families with mutated TERC did not have physical signs of dyskeratosis congenita, and their blood counts were nearly normal, but all had severely shortened telomeres, reduced haemopoietic function, and raised serum erythropoietin and thrombopoietin. Bone marrow failure of variable severity due to dyskeratosis congenita, historically characterised by associated physical anomalies and early pancytopenia, may be present in otherwise phenotypically normal adults, and can masquerade as acquired aplastic anaemia.

Telomerase is limiting the growth of acute myeloid leukemia cells

Telomeres play an important role in the proliferation and senescence of normal and malignant cells. To test the role of telomerase in acute myeloid leukemia (AML), we expressed the telomerase reverse transcriptase (hTERT) gene, a dominant-negative hTERT (DN-hTERT) (D868A, D869A) gene, or a gene encoding green fluorescence protein (GFP) in the leukemia cell line K562 and in primary AML cells from different patients, using retroviral vectors. Cells transduced with hTERT exhibited elevated levels of telomerase activity compared to GFP controls, whereas cells expressing DN-hTERT had decreased telomerase activity. K562 populations transduced with DN-hTERT showed reduced clonogenicity, telomere dysfunction and increased numbers of apoptotic cells compared to GFP- or hTERT-transduced cells. Two of four clones transduced with DN-hTERT died after 30 and 53 population doublings, respectively. Transduced AML cells were tested in primary colony-forming unit (CFU) and suspension culture assays. Relative to hTERT- and GFP-transduced controls, AML cells transfected with DN-hTERT produced fewer CFU and showed lower engraftment after transplantation into sublethally irradiated beta(2)-m(-/-) nonobese diabetic/severe combined immunodeficient mice. We conclude that telomerase is limiting the growth of the leukemic cell line K562 and primary AML progenitor cells. Our data warrant further studies of the therapeutic use of telomerase inhibitors in AML.

Telomere length measurements in leukocyte subsets by automated multicolor flow-FISH

BACKGROUND

Telomeres are essential protein-DNA structures at the end of chromosomes which are implicated in genome stability and cell replication. The average length of telomere repeats can be measured by in situ hybridization and flow cytometry [flow-FISH]. Such telomere length values reflect telomere shortening (resulting from cell divisions, oxidative damage and other causes) and telomere elongation (mainly resulting from telomerase activity) of the chromosome-specific telomere length inherited in the gametes. Here we report improvements in flow-FISH methodology that enable measurements of telomere length in subsets of human nucleated blood cells.

METHODS AND RESULTS

In order to measure the telomere length in granulocytes, naive T cells, memory T cells, B cells and natural killer (NK)/NKT cells within a blood sample, we combined flow-FISH with antibody-staining (Multicolor flow-FISH). Most steps in the staining protocol were automated using a 96-well microdispenser device. The minimum detectable difference in telomere length and the reproducibility of the method are in the range of 0.2-0.5 kb and measurements can be made with as few as a thousand cells.

CONCLUSIONS

Automated multicolor flow-FISH will greatly facilitate studies of telomere length regulation in subsets of nucleated blood cells, especially when only few cells are available and when differences in telomere length are small.

Mutations of the human telomerase RNA gene (TERC) in aplastic anemia and myelodysplastic syndrome

Mutations in the human telomerase RNA (TERC) occur in autosomal dominant dyskeratosis congenita (DKC). Because of the possibility that TERC mutations might underlie seemingly acquired forms of bone marrow failure, we examined blood samples from a large number of patients with aplastic anemia (AA), paroxysmal nocturnal hemoglobinuria (PNH), and myelodysplasia (MDS). Only 3 of 210 cases showed heterozygous TERC mutations: both nucleotide 305 (n305) (G>A) and n322 (G>A) were within the conserved region (CR) 4-CR5 domain; n450 (G>A) was localized to the boxH/ACA domain. However, only one patient (with a mutation at n305 [G>A]) had clinical characteristics suggesting DKC; her blood cells contained short telomeres and her sister also suffered from bone marrow failure. Another 21 patients with short telomeres did not show TERC mutations. Our results suggest that cryptic DKC, at least secondary to mutations in the TERC gene, is an improbable diagnosis in patients with otherwise typical AA, PNH, and MDS.

Telomerase levels control the lifespan of human T lymphocytes

The loss of telomeric DNA with each cell division contributes to the limited replicative lifespan of human T lymphocytes. Although telomerase is transiently expressed in T lymphocytes upon activation, it is insufficient to confer immortality. We have previously shown that immortalization of human CD8+ T lymphocytes can be achieved by ectopic expression of the human telomerase reverse transcriptase (hTERT) gene, which encodes for the catalytic component of the telomerase complex. To study the role of endogenous hTERT in the lifespan of human T cells, we blocked endogenous hTERT expression by ectopic expression of dominant-negative (DN) hTERT. Cells expressing DN-hTERT had a decreased lifespan and showed cytogenetic abnormalities, including chromosome ends without detectable telomeric DNA as well as chromosome fusions. These results indicate that while endogenous hTERT cannot prevent overall telomere shortening, it has a major influence on the longevity of human T cells. Furthermore, we show that up-regulation of hTERT in T cells upon activation decreases over time in culture. Long-term-cultured T cells also show a decreased expression of c-myc upon activation, resulting in less c-myc-induced transcription of hTERT. Moreover, memory T cells, which have expanded in vivo upon antigen encounter, expressed a lower level of hTERT upon activation than naive cells from the same donor. The observed inverse correlation between telomerase levels and replicative history suggests that telomerase levels in T cells are limiting and increasingly insufficient to sustain their proliferation.

Telomere shortening in hematopoietic stem cell transplantation: a potential mechanism for late graft failure?

Telomeres serve to maintain the structural integrity of chromosomes, yet each somatic cell division is associated with a decrease in telomere length. The cumulative decrease in telomere length can impose an upper limit for the number of cell divisions that can occur before a cell senesces. When studied in vitro with fibroblasts, this limit is referred to as the Hayflick limit and usually occurs after 40 to 80 cell doublings. In theory, a similar replicative potential in a hematopoietic stem cell could support hematopoiesis in a person for more than 100 years. However, stem cells differentiate, and the telomere length differs among chromosomes within a single cell, among cell types, and among age-matched individuals. This variation in telomere length raises the possibility that long-term hematopoiesis by transplanted stem cells could, depending on the telomere length of the engrafted stem cell and the proliferative demand to which it is subjected, reach a Hayflick limit during the life span of the patient. Although significant shortening of telomeres is reported to occur within the first year posttransplantation, as yet no evidence has indicated that this shortening is associated with marrow function. In this review, we summarize reports on telomere shortening in stem cell transplantation recipients and report 2 cases in which graft failure is associated with significant telomere shortening.