Haploinsufficiency of telomerase reverse transcriptase leads to anticipation in autosomal dominant dyskeratosis congenita

Genetic anticipation is associated with telomere shortening in hereditary breast cancer

There is increasing evidence suggesting that short telomeres and subsequent genomic instability contribute to malignant transformation. Telomere shortening has been described as a mechanism to explain genetic anticipation in dyskeratosis congenita and Li-Fraumeni syndrome. Since genetic anticipation has been observed in familial breast cancer, we aimed to study telomere length in familial breast cancer patients and hypothesized that genetic defects causing this disease would affect telomere maintenance resulting in shortened telomeres. Here, we first investigated age anticipation in mother-daughter pairs with breast cancer in 623 breast cancer families, classified as BRCA1, BRCA2, and BRCAX. Moreover, we analyzed telomere length in DNA from peripheral blood leukocytes by quantitative PCR in a set of 198 hereditary breast cancer patients, and compared them with 267 control samples and 71 sporadic breast cancer patients. Changes in telomere length in mother-daughter pairs from breast cancer families and controls were also evaluated to address differences through generations. We demonstrated that short telomeres characterize hereditary but not sporadic breast cancer. We have defined a group of BRCAX families with short telomeres, suggesting that telomere maintenance genes might be susceptibility genes for breast cancer. Significantly, we described that progressive telomere shortening is associated with earlier onset of breast cancer in successive generations of affected families. Our results provide evidence that telomere shortening is associated with earlier age of cancer onset in successive generations, suggesting that it might be a mechanism of genetic anticipation in hereditary breast cancer.

The fact that accelerated telomere shortening accompanies different premature aging syndromes and seems to be associated with increased risk of cancer development prompted us to analyze the role of telomere length in hereditary breast cancer. In this study we found that telomeres of peripheral blood cells from familial breast cancer patients were significantly shorter than those from the control population. Women carrying a mutation in BRCA1 or BRCA2 genes, and a subset of BRCAX families, were characterized by short telomeres. We also demonstrated that genetic anticipation, the successive earlier age of onset of cancer, in these families was associated with a decrease of the telomere length in affected daughters compared to their mothers. This is the first study suggesting that telomere shortening may contribute to anticipation in breast cancer families and that analysis of telomere length in hereditary breast cancer may affect the design of surveillance programs for affected families.

Dyskeratosis congenita as a disorder of telomere maintenance

Since 1998, there have been great advances in our understanding of the pathogenesis of dyskeratosis congenita (DC), a rare inherited bone marrow failure and cancer predisposition syndrome with prominent mucocutaneous abnormalities and features of premature aging. DC is now characterized molecularly by the presence of short age-adjusted telomeres. Mutations in seven genes have been unequivocally associated with DC, each with a role in telomere length maintenance. These observations, combined with knowledge that progressive telomere shortening can impose a proliferative barrier on dividing cells and contribute to chromosome instability, have led to the understanding that extreme telomere shortening drives the clinical features of DC. However, some of the genes implicated in DC encode proteins that are also components of H/ACA-ribonucleoprotein enzymes, which are responsible for the post-translational modification of ribosomal and spliceosomal RNAs, raising the question whether alterations in these activities play a role in the pathogenesis of DC. In addition, recent reports suggest that some cases of DC may not be characterized by short age-adjusted telomeres. This review will highlight our current knowledge of the telomere length defects in DC and the factors involved in its development.

TIN2 protein dyskeratosis congenita missense mutants are defective in association with telomerase

Dyskeratosis congenita (DC) is a progressive and heterogeneous congenital disorder that affects multiple systems and is characterized by bone marrow failure and a triad of abnormal skin pigmentation, nail dystrophy, and oral leukoplakia. One common feature for all DC patients is abnormally short telomeres and defects in telomere biology. Most of the known DC mutations have been found to affect core components of the telomerase holoenzyme. Recently, multiple mutations in the gene encoding the telomeric protein TIN2 have been identified in DC patients with intact telomerase genes, but the molecular mechanisms underlying TIN2 mutation-mediated DC remain unknown. Here, we demonstrate that ectopic expression of TIN2 with DC missense mutations in human cells led to accelerated telomere shortening, similar to the telomere phenotypes found in DC patients. However, this telomere shortening was not accompanied by changes in total telomerase activity, localization of TIN2, or telomere end protection status. Interestingly, we found TIN2 to participate in the TPP1-dependent recruitment of telomerase activity. Furthermore, DC mutations in TIN2 led to its decreased ability to associate with TERC and telomerase activity. Taken together, our data suggest that TIN2 mutations in DC may compromise the telomere recruitment of telomerase, leading to telomere shortening and the associated pathogenesis.

Telomere replication: poised but puzzling

Faithful replication of chromosomes is essential for maintaining genome stability. Telomeres, the chromosomal termini, pose quite a challenge to replication machinery due to the complexity in their structures and sequences. Efficient and complete replication of chromosomes is critical to prevent aberrant telomeres as well as to avoid unnecessary loss of telomere DNA. Compelling evidence supports the emerging picture of synergistic actions between DNA replication proteins and telomere protective components in telomere synthesis. This review discusses the actions of various replication and telomere-specific binding proteins that ensure accurate telomere replication and their roles in telomere maintenance and protection.

Lung transplantation for pulmonary fibrosis in dyskeratosis congenita: Case Report and systematic literature review

Background

Dyskeratosis congenita (DC) is a progressive, multi-system, inherited disorder of telomere biology with high risks of morbidity and mortality from bone marrow failure, hematologic malignancy, solid tumors and pulmonary fibrosis. Hematopoietic stem cell transplantation (HSCT) can cure the bone marrow failure, but it does not eliminate the risks of other complications, for which life-long surveillance is required. Pulmonary fibrosis is a progressive and lethal complication of DC.

Case presentation

In this report, we describe a patient with DC who developed pulmonary fibrosis seven years after HSCT for severe aplastic anemia, and was successfully treated with bilateral lung transplantation. We also performed a systematic literature review to understand the burden of pulmonary disease in patients with DC who did or did not receive an HSCT. Including our patient, we identified 49 DC patients with pulmonary disease (12 after HSCT and 37 without HSCT), and 509 with no reported pulmonary complications.

Conclusion

Our current case and literature review indicate that pulmonary morbidity is one of the major contributors to poor quality of life and reduced long-term survival in DC. We suggest that lung transplantation be considered for patients with DC who develop pulmonary fibrosis with no concurrent evidence of multi-organ failure.

Progerin and telomere dysfunction collaborate to trigger cellular senescence in normal human fibroblasts

Hutchinson-Gilford progeria syndrome (HGPS), a devastating premature aging disease, is caused by a point mutation in the lamin A gene (LMNA). This mutation constitutively activates a cryptic splice donor site, resulting in a mutant lamin A protein known as progerin. Recent studies have demonstrated that progerin is also produced at low levels in normal human cells and tissues. However, the cause-and-effect relationship between normal aging and progerin production in normal individuals has not yet been determined. In this study, we have shown in normal human fibroblasts that progressive telomere damage during cellular senescence plays a causative role in activating progerin production. Progressive telomere damage was also found to lead to extensive changes in alternative splicing in multiple other genes. Interestingly, elevated progerin production was not seen during cellular senescence that does not entail telomere shortening. Taken together, our results suggest a synergistic relationship between telomere dysfunction and progerin production during the induction of cell senescence, providing mechanistic insight into how progerin may participate in the normal aging process.

Partial pneumonectomy of telomerase null mice carrying shortened telomeres initiates cell growth arrest resulting in a limited compensatory growth response

Telomerase mutations and significantly shortened chromosomal telomeres have recently been implicated in human lung pathologies. Natural telomere shortening is an inevitable consequence of aging, which is also a risk factor for development of lung disease. However, the impact of shortened telomeres and telomerase dysfunction on the ability of lung cells to respond to significant challenge is still largely unknown. We have previously shown that lungs of late generation, telomerase null B6.Cg-Terc(tm1Rdp) mice feature alveolar simplification and chronic stress signaling at baseline, a phenocopy of aged lung. To determine the role telomerase plays when the lung is challenged, B6.Cg-Terc(tm1Rdp) mice carrying shortened telomeres and wild-type controls were subjected to partial pneumonectomy. We found that telomerase activity was strongly induced in alveolar epithelial type 2 cells (AEC2) of the remaining lung immediately following surgery. Eighty-six percent of wild-type animals survived the procedure and exhibited a burst of early compensatory growth marked by upregulation of proliferation, stress response, and DNA repair pathways in AEC2. In B6.Cg-Terc(tm1Rdp) mice carrying shortened telomeres, response to pneumonectomy was characterized by decreased survival, diminished compensatory lung growth, attenuated distal lung progenitor cell response, persistent DNA damage, and cell growth arrest. Overall, survival correlated strongly with telomere length. We conclude that functional telomerase and properly maintained telomeres play key roles in both long-term survival and the early phase of compensatory lung growth following partial pneumonectomy.

Phenotypes in mTERT⁺/⁻ and mTERT⁻/⁻ mice are due to short telomeres, not telomere-independent functions of telomerase reverse transcriptase

Telomerase is essential for telomere length maintenance. Mutations in either of the two core components of telomerase, telomerase RNA (TR) or the catalytic protein component telomerase reverse transcriptase (TERT), cause the genetic disorders dyskeratosis congenita, pulmonary fibrosis, and other degenerative diseases. Overexpression of the TERT protein has been reported to have telomere length-independent roles, including regulation of the Wnt signaling pathway. To examine the phenotypes of TERT haploinsufficiency and determine whether loss of function of TERT has effects other than those associated with telomere shortening, we characterized both mTERT⁺/⁻ and mTERT⁻/⁻ mice on the CAST/EiJ genetic background. Phenotypic analysis showed a loss of tissue renewal capacity with progressive breeding of heterozygous mice that was indistinguishable from that of mTR-deficient mice. mTERT⁻/⁻ mice, from heterozygous mTERT⁺/⁻ mouse crosses, were born at the expected Mendelian ratio (26.5%; n = 1,080 pups), indicating no embryonic lethality of this genotype. We looked for, and failed to find, hallmarks of Wnt deficiency in various adult and embryonic tissues, including those of the lungs, kidneys, brain, and skeleton. Finally, mTERT⁻/⁻ cells showed wild-type levels of Wnt signaling in vitro. Thus, while TERT overexpression in some settings may activate the Wnt pathway, loss of function in a physiological setting has no apparent effects on Wnt signaling. Our results indicate that both TERT and TR are haploinsufficient and that their deficiency leads to telomere shortening, which limits tissue renewal. Our studies imply that hypomorphic loss-of-function alleles of hTERT and hTR should cause a similar disease spectrum in humans.

Familial myelodysplastic syndromes: a review of the literature

Familial cases of myelodysplastic syndromes are rare, but are immensely valuable for the investigation of the molecular pathogenesis of myelodysplasia in general. The best-characterized familial myelodysplastic syndrome is that of familial platelet disorder with propensity to myeloid malignancy, caused by heterozygous germline RUNX1 mutations. Recently, there has been an increase in the number of reported cases, allowing for better understanding of the incidence, clinical features, and pathogenesis of this disorder. These recent cases have highlighted the clinical variability of the disorder and confirmed that many patients lack a bleeding and/or thrombocytopenia history. Additionally, several cases of T-acute lymphoblastic leukemia have now been reported, confirming a risk of lymphoid leukemia in patients with inherited RUNX1 mutations. Furthermore, an increased awareness of clinicians has helped detect a number of additional families affected by inherited myelodysplastic syndromes, resulting in the identification of novel causative mechanisms of disease, such as RUNX1 deficiency resulting from constitutional microdeletions of 21q22 and myelodysplasia-associated with telomerase deficiency. Awareness of predisposition to myelodysplastic syndromes and acute myeloid leukemia in families may be of critical importance in the management of younger patients with myelodysplasia in whom allogeneic hematopoietic stem cell transplantation is considered. Such families should be investigated for inherited deficiencies of RUNX1 and/or telomerase to prevent the use of an affected sibling as a donor for transplantation. Here we provide an update on familial platelet disorder in addition to a review of other known familial myelodysplastic syndromes.

Telomere shortening and loss of self-renewal in dyskeratosis congenita induced pluripotent stem cells

The differentiation of patient-derived induced pluripotent stem cells (iPSCs) to committed fates such as neurons, muscle and liver is a powerful approach for understanding key parameters of human development and disease. Whether undifferentiated iPSCs themselves can be used to probe disease mechanisms is uncertain. Dyskeratosis congenita is characterized by defective maintenance of blood, pulmonary tissue and epidermal tissues and is caused by mutations in genes controlling telomere homeostasis. Short telomeres, a hallmark of dyskeratosis congenita, impair tissue stem cell function in mouse models, indicating that a tissue stem cell defect may underlie the pathophysiology of dyskeratosis congenita. Here we show that even in the undifferentiated state, iPSCs from dyskeratosis congenita patients harbour the precise biochemical defects characteristic of each form of the disease and that the magnitude of the telomere maintenance defect in iPSCs correlates with clinical severity. In iPSCs from patients with heterozygous mutations in TERT, the telomerase reverse transcriptase, a 50% reduction in telomerase levels blunts the natural telomere elongation that accompanies reprogramming. In contrast, mutation of dyskerin (DKC1) in X-linked dyskeratosis congenita severely impairs telomerase activity by blocking telomerase assembly and disrupts telomere elongation during reprogramming. In iPSCs from a form of dyskeratosis congenita caused by mutations in TCAB1 (also known as WRAP53), telomerase catalytic activity is unperturbed, yet the ability of telomerase to lengthen telomeres is abrogated, because telomerase mislocalizes from Cajal bodies to nucleoli within the iPSCs. Extended culture of DKC1-mutant iPSCs leads to progressive telomere shortening and eventual loss of self-renewal, indicating that a similar process occurs in tissue stem cells in dyskeratosis congenita patients. These findings in iPSCs from dyskeratosis congenita patients reveal that undifferentiated iPSCs accurately recapitulate features of a human stem cell disease and may serve as a cell-culture-based system for the development of targeted therapeutics.

Sex differences in sand lizard telomere inheritance: paternal epigenetic effects increases telomere heritability and offspring survival

Background

To date, the only estimate of the heritability of telomere length in wild populations comes from humans. Thus, there is a need for analysis of natural populations with respect to how telomeres evolve.

Methodology/Principal Findings

Here, we show that telomere length is heritable in free-ranging sand lizards, Lacerta agilis. More importantly, heritability estimates analysed within, and contrasted between, the sexes are markedly different; son-sire heritability is much higher relative to daughter-dam heritability. We assess the effect of paternal age on Telomere Length (TL) and show that in this species, paternal age at conception is the best predictor of TL in sons. Neither paternal age per se at blood sampling for telomere screening, nor corresponding age in sons impact TL in sons. Processes maintaining telomere length are also associated with negative fitness effects, most notably by increasing the risk of cancer and show variation across different categories of individuals (e.g. males vs. females). We therefore tested whether TL influences offspring survival in their first year of life. Indeed such effects were present and independent of sex-biased offspring mortality and offspring malformations.

Conclusions/Significance

TL show differences in sex-specific heritability with implications for differences between the sexes with respect to ongoing telomere selection. Paternal age influences the length of telomeres in sons and longer telomeres enhance offspring survival.

Dyskeratosis congenita and the DNA damage response

Dyskeratosis congenita (DC) is a heterogeneous bone marrow failure disorder with known mutations in components of telomerase and telomere shelterin. Recent work in a mouse model with a dyskerin mutation has implicated an increased DNA damage response as part of the cellular pathology, while mouse models with Terc and Tert mutations displayed a normal response. To clarify how these contradictory results might apply to DC pathology in humans, we studied the cellular phenotype in primary cells from DC patients of several genetic subtypes, focussing on T lymphocytes to remain close to the haematopoietic system. We observed novel cell cycle abnormalities in conjunction with impaired growth and an increase in apoptosis. Using flow cytometry and confocal microscopy we examined induction of the DNA damage proteins γ-H2AX and 53BP1 and the cell cycle protein TP53 (p53). We found an increase in damage foci at telomeres in lymphocytes and an increase in the basal level of DNA damage in fibroblasts, but crucially no increased response to DNA damaging agents in either cell type. As the response to induced DNA damage was normal and levels of global DNA damage were inconsistent between cell types, DNA damage may contribute differently to the pathology in different tissues.

Ancestral mutation in telomerase causes defects in repeat addition processivity and manifests as familial pulmonary fibrosis

The telomerase reverse transcriptase synthesizes new telomeres onto chromosome ends by copying from a short template within its integral RNA component. During telomere synthesis, telomerase adds multiple short DNA repeats successively, a property known as repeat addition processivity. However, the consequences of defects in processivity on telomere length maintenance are not fully known. Germline mutations in telomerase cause haploinsufficiency in syndromes of telomere shortening, which most commonly manifest in the age-related disease idiopathic pulmonary fibrosis. We identified two pulmonary fibrosis families that share two non-synonymous substitutions in the catalytic domain of the telomerase reverse transcriptase gene hTERT: V791I and V867M. The two variants fell on the same hTERT allele and were associated with telomere shortening. Genealogy suggested that the pedigrees shared a single ancestor from the nineteenth century, and genetic studies confirmed the two families had a common founder. Functional studies indicated that, although the double mutant did not dramatically affect first repeat addition, hTERT V791I-V867M showed severe defects in telomere repeat addition processivity in vitro. Our data identify an ancestral mutation in telomerase with a novel loss-of-function mechanism. They indicate that telomere repeat addition processivity is a critical determinant of telomere length and telomere-mediated disease.

Syndrome complex of bone marrow failure and pulmonary fibrosis predicts germline defects in telomerase

Mutations in the essential telomerase components hTERT and hTR cause dyskeratosis congenita, a bone marrow failure syndrome characterized by mucocutaneous features. Some (~ 3%) sporadic aplastic anemia (AA) and idiopathic pulmonary fibrosis cases also carry mutations in hTERT and hTR. Even though it can affect clinical outcome, because the mutation frequency is rare, genetic testing is not standard. We examined whether the cooccurrence of bone marrow failure and pulmonary fibrosis in the same individual or family enriches for the presence of a telomerase mutation. Ten consecutive individuals with a total of 36 family members who fulfilled these criteria carried a germline mutant telomerase gene (100%). The mean age of onset for individuals with AA was significantly younger than that for those with pulmonary fibrosis (14 vs 51; P < .0001). Families displayed autosomal dominant inheritance and there was an evolving pattern of genetic anticipation, with the older generation primarily affected by pulmonary fibrosis and successive generations by bone marrow failure. The cooccurrence of AA and pulmonary fibrosis in a single patient or family is highly predictive for the presence of a germline telomerase defect. This diagnosis affects the choice of bone marrow transplantation preparative regimen and can prevent morbidity.

Decreased dyskerin levels as a mechanism of telomere shortening in X-linked dyskeratosis congenita

Dyskeratosis congenita (DC) is a premature ageing syndrome characterised by short telomeres. An X-linked form of DC is caused by mutations in DKC1 which encodes dyskerin, a telomerase component that is essential for telomerase RNA stability. However, mutations in DKC1 are identifiable in only half of X-linked DC families. A four generation family with pulmonary fibrosis and features of DC was identified. Affected males showed the classic mucocutaneous features of DC and died prematurely from pulmonary fibrosis. Although there were no coding sequence or splicing variants, genome wide linkage analysis of 16 individuals across four generations identified significant linkage at the DKC1 locus, and was accompanied by reduced dyskerin protein levels in affected males. Decreased dyskerin levels were associated with compromised telomerase RNA levels and very short telomeres. These data identify decreased dyskerin levels as a novel mechanism of DC, and indicate that intact dyskerin levels, in the absence of coding mutations, are critical for telomerase RNA stability and for in vivo telomere maintenance.

Short telomeres compromise β-cell signaling and survival

The genetic factors that underlie the increasing incidence of diabetes with age are poorly understood. We examined whether telomere length, which is inherited and known to shorten with age, plays a role in the age-dependent increased incidence of diabetes. We show that in mice with short telomeres, insulin secretion is impaired and leads to glucose intolerance despite the presence of an intact β-cell mass. In ex vivo studies, short telomeres induced cell-autonomous defects in β-cells including reduced mitochondrial membrane hyperpolarization and Ca²⁺ influx which limited insulin release. To examine the mechanism, we looked for evidence of apoptosis but found no baseline increase in β-cells with short telomeres. However, there was evidence of all the hallmarks of senescence including slower proliferation of β-cells and accumulation of p16^INK4a. Specifically, we identified gene expression changes in pathways which are essential for Ca²⁺-mediated exocytosis. We also show that telomere length is additive to the damaging effect of endoplasmic reticulum stress which occurs in the late stages of type 2 diabetes. This additive effect manifests as more severe hyperglycemia in Akita mice with short telomeres which had a profound loss of β-cell mass and increased β-cell apoptosis. Our data indicate that short telomeres can affect β-cell metabolism even in the presence of intact β-cell number, thus identifying a novel mechanism of telomere-mediated disease. They implicate telomere length as a determinant of β-cell function and diabetes pathogenesis.

The role of telomeres in the ageing of human skin

Skin is a self-renewing tissue that is required to go through extensive proliferation throughout the lifespan of an organism. Telomere shortening acts as a mitotic clock that prevents aberrant proliferation such as cancer. A consequence of this protection is cellular senescence and ageing. The telomerase enzyme complex maintains telomere length in germline cells and in cancer cells. Telomerase is also active in certain somatic cells such as those in the epidermis but is almost undetectable in the dermis. Increasing evidence indicates that telomerase plays a significant role in maintenance of skin function and proliferation. Mutations in telomerase component genes in the disease dyskeratosis congenita result in numerous epidermal abnormalities. Studies also indicate that telomerase activity in epidermal stem cells might have roles that go beyond telomere elongation. Telomeres in skin cells may be particularly susceptible to accelerated shortening because of both proliferation and DNA-damaging agents such as reactive oxygen species. Skin might present an accessible tissue for manipulation of telomerase activity and telomere length with the potential of ameliorating skin diseases associated with ageing.

An evolutionary review of human telomere biology: the thrifty telomere hypothesis and notes on potential adaptive paternal effects

Telomeres, repetitive DNA sequences found at the ends of linear chromosomes, play a role in regulating cellular proliferation, and shorten with increasing age in proliferating human tissues. The rate of age-related shortening of telomeres is highest early in life and decreases with age. Shortened telomeres are thought to limit the proliferation of cells and are associated with increased morbidity and mortality. Although natural selection is widely assumed to operate against long telomeres because they entail increased cancer risk, the evidence for this is mixed. Instead, here it is proposed that telomere length is primarily limited by energetic constraints. Cell proliferation is energetically expensive, so shorter telomeres should lead to a thrifty phenotype. Shorter telomeres are proposed to restrain adaptive immunity as an energy saving mechanism. Such a limited immune system, however, might also result in chronic infections, inflammatory stress, premature aging, and death--a more "disposable soma." With an increased reproductive lifespan, the fitness costs of premature aging are higher and longer telomeres will be favored by selection. Telomeres exhibit a paternal effect whereby the offspring of older fathers have longer telomeres due to increased telomere lengths of sperm with age. This paternal effect is proposed to be an adaptive signal of the expected age of male reproduction in the environment offspring are born into. The offspring of lineages of older fathers will tend to have longer, and thereby less thrifty, telomeres, better preparing them for an environment with higher expected ages at reproduction.

Subclinical lung disease, macrocytosis, and premature graying in kindreds with telomerase (TERT) mutations

BACKGROUND

Mutations in the human gene encoding the protein component of telomerase (TERT) are the most common genetic defect in patients with familial idiopathic pulmonary fibrosis (IPF). The subclinical phenotypes of asymptomatic members of these families have not been evaluated with respect to TERT mutation status or telomere length.

METHODS

We measured a variety of pulmonary, blood, skin, and bone parameters for 20 subjects with heterozygous TERT mutations (carriers) and 20 family members who had not inherited a TERT mutation (noncarriers) to identify the spectrum of phenotypes associated with mutations in this gene. The two groups were matched for sex, age, and cigarette smoking. Three TERT mutation carriers had IPF (IPF carriers). The rest of the carriers were apparently healthy (asymptomatic carriers) and were compared with the noncarriers.

RESULTS

Asymptomatic carriers exhibited significantly lower diffusing capacity of lung for carbon monoxide (Dlco), impaired recruitment of Dlco with exercise, radiographic signs of lung fibrosis, and increased fractional lung tissue volume quantified by high-resolution chest CT scan than noncarriers. RBC and platelet counts were significantly lower, and the mean corpuscular volume and mean corpuscular hemoglobin concentration were significantly higher in carriers than in noncarriers. Carriers reported significantly earlier graying of hair than noncarriers. TERT mutation status is more accurately predicted by short telomere lengths than any of these measured phenotypes.

CONCLUSIONS

TERT mutation carriers exhibit early preclinical signs of lung fibrosis, bone marrow dysfunction, and premature graying. These clinical features and short telomere lengths characterize patients with germline TERT mutations.

Behaviour of telomere and telomerase during aging and regeneration in zebrafish

Telomere length and telomerase activity are important factors in the pathobiology of human diseases. Age-related diseases and premature aging syndromes are characterized by short telomeres, which can compromise cell viability, whereas tumour cells can prevent telomere loss by aberrantly upregulating telomerase. The zebrafish (Danio rerio) offers multiple experimental manipulation advantages over other vertebrate models and, therefore, it has been recently considered as a potential model for aging, cancer, and regeneration studies. However, it has only partially been exploited to shed light on these fundamental biological processes. The aim of this study was, therefore, to investigate telomere length and telomerase expression and activity in different strains of zebrafish obtained from different stock centres to determine whether they undergo any changes during aging and regeneration. We found that although both telomerase expression and telomere length increased from embryo to adulthood stages, they drastically declined in aged fish despite telomerase activity was detected in different tissues of old fish. In addition, we observed a weaker upregulation of telomerase expression in regenerating fins of old fish, which well correlates with their impaired regeneration capacity. Strikingly, telomeres were elongated or maintained during the fin regeneration process at all ages and after repeated amputations, likely to support high cell proliferation rates. We conclude that the expression of telomerase and telomere length are closely related during the entire life cycle of the fish and that these two parameters can be used as biomarkers of aging in zebrafish. Our results also reveal a direct relationship between the expression of telomerase, telomere length and the efficiency of tissue regeneration.

Disruption of telomerase trafficking by TCAB1 mutation causes dyskeratosis congenita

Dyskeratosis congenita (DC) is a genetic disorder of defective tissue maintenance and cancer predisposition caused by short telomeres and impaired stem cell function. Telomerase mutations are thought to precipitate DC by reducing either the catalytic activity or the overall levels of the telomerase complex. However, the underlying genetic mutations and the mechanisms of telomere shortening remain unknown for as many as 50% of DC patients, who lack mutations in genes controlling telomere homeostasis. Here, we show that disruption of telomerase trafficking accounts for unknown cases of DC. We identify DC patients with missense mutations in TCAB1, a telomerase holoenzyme protein that facilitates trafficking of telomerase to Cajal bodies. Compound heterozygous mutations in TCAB1 disrupt telomerase localization to Cajal bodies, resulting in misdirection of telomerase RNA to nucleoli, which prevents telomerase from elongating telomeres. Our findings establish telomerase mislocalization as a novel cause of DC, and suggest that telomerase trafficking defects may contribute more broadly to the pathogenesis of telomere-related disease.

Dyskeratosis congenita

Dyskeratosis congenita (DC) is a multisystem inherited syndrome exhibiting marked clinical and genetic heterogeneity. In its classic form, it is characterized by mucocutaneous abnormalities, BM failure, and a predisposition to cancer. BM failure is the principal cause of premature mortality. Studies over the last 15 years have led to significant advances, with 8 DC genes (DKC1, TERC, TERT, NOP10, NHP2, TIN2, C16orf57, and TCAB1) having been characterized. Seven of these are important in telomere maintenance either because they encode components of the telomerase enzyme complex (DKC1, TERC, TERT, NOP10, NHP2, and TCAB1) or the shelterin complex (TINF2). DC is therefore principally a disease of defective telomere maintenance and patients usually have very short telomeres. The genetic advances have led to the unification of DC with several other disorders, including the severe multisystem disorders Hoyeraal-Hreidarsson and Revesz syndromes, as well as a subset of patients with aplastic anemia, myelodysplasia, leukemia, and idiopathic pulmonary fibrosis. This wide spectrum of diseases ranging from classic DC to aplastic anemia can be regarded as disorders of defective telomere maintenance-"the telomereopathies." These advances have increased our understanding of normal hematopoiesis and highlighted the important role of telomerase and telomeres in human biology. They are also facilitating the diagnosis (especially when presentation is atypical) and management of DC.

Emberger syndrome-primary lymphedema with myelodysplasia: report of seven new cases

Four reports have been published on an association between acute myeloid leukaemia (AML) and primary lymphedema, with or without congenital deafness. We report seven new cases, including one extended family, confirming this entity as a genetic syndrome. The lymphedema typically presents in one or both lower limbs, before the hematological abnormalities, with onset between infancy and puberty and frequently affecting the genitalia. The AML is often preceded by pancytopenia or myelodysplasia with a high incidence of monosomy 7 in the bone marrow (five propositi and two relatives). Associated anomalies included hypotelorism, epicanthic folds, long tapering fingers and/or neck webbing (four patients), recurrent cellulitis in the affected limb (four patients), generalized warts (two patients), and congenital, high frequency sensorineural deafness (one patient). Children with lower limb and genital lymphedema should be screened for hematological abnormalities and immunodeficiency.

Too short stature, too many stigmata

Dyskeratosis congenita (DC) is a rare disease characterised by bone marrow failure and skin manifestations. Patients with DC may exhibit short stature that is not usually related to growth hormone (GH) deficiency. Replacement treatment with GH should be done cautiously as it can predispose to haematological malignancy. We present a 10-year-old boy with DC and GH deficiency.

A sequence-dependent exonuclease activity from Tetrahymena thermophila

Background

Telomere function requires a highly conserved G rich 3'- overhang. This structure is formed by 5'-resection of the C-rich telomere strand. However, while many nucleases have been suggested to play a role in processing, it is not yet clear which nucleases carry out this 5'-resection.

Results

We used biochemical purification to identify a sequence-dependent exonuclease activity in Tetrahymena thermophila cell extracts. The nuclease activity showed specificity for 5'-ends containing AA or AC sequences, unlike Exo1, which showed sequence-independent cleavage. The Tetrahymena nuclease was active on both phosphorylated and unphosphorylated substrates whereas Exo1 requires a 5'-phosphate for cleavage.

Conclusions

The specificities of the enzyme indicate that this novel Tetrahymena exonuclease is distinct from Exo1 and has properties required for 3'-overhang formations at telomeres.

Workshop on idiopathic pulmonary fibrosis in older adults

Idiopathic pulmonary fibrosis (IPF), a heterogeneous disease with respect to clinical presentation and rates of progression, disproportionately affects older adults. The diagnosis of IPF is descriptive, based on clinical, radiologic, and histopathologic examination, and definitive diagnosis is hampered by poor interobserver agreement and lack of a consensus definition. There are no effective treatments. Cellular, molecular, genetic, and environmental risk factors have been identified for IPF, but the initiating event and the characteristics of preclinical stages are not known. IPF is predominantly a disease of older adults, and the processes underlying normal aging might significantly influence the development of IPF. Yet, the biology of aging and the principles of medical care for this population have been typically ignored in basic, translational, or clinical IPF research. In August 2009, the Association of Specialty Professors, in collaboration with the American College of Chest Physicians, the American Geriatrics Society, the National Institute on Aging, and the National Heart, Lung, and Blood Institute, held a workshop, summarized herein, to review what is known, to identify research gaps at the interface of aging and IPF, and to suggest priority areas for future research. Efforts to answer the questions identified will require the integration of geriatrics, gerontology, and pulmonary research, but these efforts have great potential to improve care for patients with IPF.

Recent progress in dyskeratosis congenita

Dyskeratosis congenita (DC) is an inherited disease associated with nail dystrophy, abnormal skin pigmentation, oral leukoplakia, bone marrow failure and a predisposition to cancer. DC is a disease of defective telomere maintenance and patients with DC have very short telomeres. To date, mutations in six genes of telomerase and telomere components have been identified in patients with DC. Recently, mutations in telomerase and telomere components were also identified in patients with aplastic anemia, pulmonary fibrosis, and liver diseases who did not have mucocutaneous manifestations. These findings imply that defective telomere maintenance may cause not only classical DC but also a broad spectrum of diseases previously thought to be idiopathic, and have led to a new concept of diseases, termed "syndromes of telomere shortening". An understanding of the role of telomeres in these diseases is indispensable for diagnosis, genetic counseling and clinical management.

Dyskeratosis congenita

Dyskeratosis congenita (DC) was originally defined as a rare inherited bone marrow failure (BMF) syndrome associated with distinct mucocutaneous features. Today DC is defined by its pathogenetic mechanism and mutations in components of the telomere maintenance machinery resulting in excessively short telomeres in highly proliferating tissues. With this new definition the disease spectrum has broadened and ranges from intrauterine growth retardation, cerebellar hypoplasia, and death in early childhood to asymptomatic mutation carriers whose descendants are predisposed to malignancy, BMF, or pulmonary disease. The degree of telomere dysfunction is the major determinant of disease onset and manifestations.

Mutations in C16orf57 and normal-length telomeres unify a subset of patients with dyskeratosis congenita, poikiloderma with neutropenia and Rothmund-Thomson syndrome

Dyskeratosis congenita (DC) is an inherited poikiloderma which in addition to the skin abnormalities is typically associated with nail dystrophy, leucoplakia, bone marrow failure, cancer predisposition and other features. Approximately 50% of DC patients remain genetically uncharacterized. All the DC genes identified to date are important in telomere maintenance. To determine the genetic basis of the remaining cases of DC, we undertook linkage analysis in 20 families and identified a common candidate gene region on chromosome 16 in a subset of these. This region included the C16orf57 gene recently identified to be mutated in poikiloderma with neutropenia (PN), an inherited poikiloderma displaying significant clinical overlap with DC. Analysis of the C16orf57 gene in our uncharacterized DC patients revealed homozygous mutations in 6 of 132 families. In addition, three of six families previously classified as Rothmund–Thomson syndrome (RTS—a poikiloderma that is sometimes confused with PN) were also found to have homozygous C16orf57 mutations. Given the role of the previous DC genes in telomere maintenance, telomere length was analysed in these patients and found to be comparable to age-matched controls. These findings suggest that mutations in C16orf57 unify a distinct set of families which clinically can be categorized as DC, PN or RTS. This study also highlights the multi-system nature (wider than just poikiloderma and neutropenia) of the clinical features of affected individuals (and therefore house-keeping function of C16orf57), a possible role for C16orf57 in apoptosis, as well as a distinct difference from previously characterized DC patients because telomere length was normal.

Telomerase inhibition targets clonogenic multiple myeloma cells through telomere length-dependent and independent mechanisms

Background

Plasma cells constitute the majority of tumor cells in multiple myeloma (MM) but lack the potential for sustained clonogenic growth. In contrast, clonotypic B cells can engraft and recapitulate disease in immunodeficient mice suggesting they serve as the MM cancer stem cell (CSC). These tumor initiating B cells also share functional features with normal stem cells such as drug resistance and self-renewal potential. Therefore, the cellular processes that regulate normal stem cells may serve as therapeutic targets in MM. Telomerase activity is required for the maintenance of normal adult stem cells, and we examined the activity of the telomerase inhibitor imetelstat against MM CSC. Moreover, we carried out both long and short-term inhibition studies to examine telomere length-dependent and independent activities.

Methodology/Principal Findings

Human MM CSC were isolated from cell lines and primary clinical specimens and treated with imetelstat, a specific inhibitor of the reverse transcriptase activity of telomerase. Two weeks of exposure to imetelstat resulted in a significant reduction in telomere length and the inhibition of clonogenic MM growth both in vitro and in vivo. In addition to these relatively long-term effects, 72 hours of imetelstat treatment inhibited clonogenic growth that was associated with MM CSC differentiation based on expression of the plasma cell antigen CD138 and the stem cell marker aldehyde dehydrogenase. Short-term treatment of MM CSC also decreased the expression of genes typically expressed by stem cells (OCT3/4, SOX2, NANOG, and BMI1) as revealed by quantitative real-time PCR.

Conclusions

Telomerase activity regulates the clonogenic growth of MM CSC. Moreover, reductions in MM growth following both long and short-term telomerase inhibition suggest that it impacts CSC through telomere length-dependent and independent mechanisms.

InTERTpreting telomerase structure and function

The Nobel Prize in Physiology or Medicine was recently awarded to Elizabeth Blackburn, Carol Greider and Jack Szostak for their pioneering studies on chromosome termini (telomeres) and their discovery of telomerase, the enzyme that synthesizes telomeres. Telomerase is a unique cellular reverse transcriptase that contains an integral RNA subunit, the telomerase RNA and a catalytic protein subunit, the telomerase reverse transcriptase (TERT), as well as several species-specific accessory proteins. Telomerase is essential for genome stability and is associated with a broad spectrum of human diseases including various forms of cancer, bone marrow failure and pulmonary fibrosis. A better understanding of telomerase structure and function will shed important insights into how this enzyme contributes to human disease. To this end, a series of high-resolution structural studies have provided critical information on TERT architecture and may ultimately elucidate novel targets for therapeutic intervention. In this review, we discuss the current knowledge of TERT structure and function, revealed through the detailed analysis of TERT from model organisms. To emphasize the physiological importance of telomeres and telomerase, we also present a general discussion of the human diseases associated with telomerase dysfunction.

Telomere lengths, pulmonary fibrosis and telomerase (TERT) mutations

Background

Telomerase is an enzyme that catalyzes the addition of nucleotides on the ends of chromosomes. Rare loss of function mutations in the gene that encodes the protein component of telomerase (TERT) have been described in patients with idiopathic pulmonary fibrosis (IPF). Here we examine the telomere lengths and pulmonary fibrosis phenotype seen in multiple kindreds with heterozygous TERT mutations.

Methods and Findings

We have identified 134 individuals with heterozygous TERT mutations from 21 unrelated families. Available medical records, surgical lung biopsies and radiographs were evaluated retrospectively. Genomic DNA isolated from circulating leukocytes has been used to measure telomere lengths with a quantitative PCR assay. We find that telomere lengths of TERT mutation carriers decrease in an age-dependent manner and show progressive shortening with successive generations of mutation inheritance. Family members without TERT mutations have a shorter mean telomere length than normal, demonstrating epigenetic inheritance of shortened telomere lengths in the absence of an inherited TERT mutation. Pulmonary fibrosis is an age-dependent phenotype not seen in mutation carriers less than 40 years of age but found in 60% of men 60 years or older; its development is associated with environmental exposures including cigarette smoking. A radiographic CT pattern of usual interstitial pneumonia (UIP), which is consistent with a diagnosis of IPF, is seen in 74% of cases and a pathologic pattern of UIP is seen in 86% of surgical lung biopsies. Pulmonary fibrosis associated with TERT mutations is progressive and lethal with a mean survival of 3 years after diagnosis. Overall, TERT mutation carriers demonstrate reduced life expectancy, with a mean age of death of 58 and 67 years for males and females, respectively.

Conclusions

A subset of pulmonary fibrosis, like dyskeratosis congenita, bone marrow failure, and liver disease, represents a “telomeropathy” caused by germline mutations in telomerase and characterized by short telomere lengths. Family members within kindreds who do not inherit the TERT mutation have shorter telomere lengths than controls, demonstrating epigenetic inheritance of a shortened parental telomere length set-point.

Variation at theTERTlocus and predisposition for cancer

Telomerase and the control of telomere length are intimately linked to the process of tumourigenesis in humans. Here I review the evidence that variation at the 5p15.33 locus, which contains theTERTgene (encoding the catalytic subunit of telomerase), might play a role in the determination of cancer risk. Mutations in the coding regions ofTERTcan affect telomerase activity and telomere length, and create severe clinical phenotypes, including bone marrow failure syndromes and a substantive increase in cancer frequency. Variants within theTERTgene have been associated with increased risk of haematological malignancies, including myelodysplastic syndrome and acute myeloid leukaemia as well as chronic lymphocytic leukaemia. Furthermore, there is good evidence from a number of independent genome-wide association studies to implicate variants at the 5p15.33 locus in cancer risk at several different sites: lung cancer, basal cell carcinoma and pancreatic cancer show strong associations, while bladder, prostate and cervical cancer and glioma also show risk alleles in this region. Thus, multiple independent lines of evidence have implicated variation in theTERTgene as a risk factor for cancer. The mechanistic basis of these risk variants is yet to be established; however, the basic biology suggests that telomere length control is a tantalising candidate mechanism underlying cancer risk.

Pathophysiology and management of inherited bone marrow failure syndromes

The inherited marrow failure syndromes are a diverse set of genetic disorders characterized by hematopoietic aplasia and cancer predisposition. The clinical phenotypes are highly variable and much broader than previously recognized. The medical management of the inherited marrow failure syndromes differs from that of acquired aplastic anemia or malignancies arising in the general population. Diagnostic workup, molecular pathogenesis, and clinical treatment are reviewed.

Defining the pathogenic role of telomerase mutations in myelodysplastic syndrome and acute myeloid leukemia

The primary pathology in many cases of myelodysplasia (MDS) and acute myeloid leukemia (AML) remains unknown. In some cases, two or more affected members have been identified in the same family. To date, mutations in two genes have been directly implicated: the hematopoietic transcription factors RUNX1 (runt-related transcription factor 1) and CEBPA (CCATT-box enhancer binding protein alpha). However, there are also other familial cases of MDS/AML where the genetic basis remains unknown. Both MDS, and to a lesser extent AML, have been observed in cases of the bone marrow failure syndrome dyskeratosis congenita, in which telomerase mutations have been identified. Recently, an increased incidence of telomerase reverse transcriptase mutations has been reported in a series of de novo AML. We have now identified novel mutations in the telomerase RNA (TERC) or telomerase reverse transcriptase component (TERT) within 4 of 20 families presenting with familial MDS/AML. Functional analysis has demonstrated that all mutations adversely impact on telomerase activity in vitro, and affected individuals have short telomeres. These families, in conjunction with a review of previously published cases, help to further define the pathological role of telomerase mutations in MDS/AML and have implications for the biology, treatment and screening regimen of de novo cases.

[Molecular diagnosis and therapeutic measures in patients with dyskeratosis congenita]

Dyskeratosis congenita is a rare genetically heterogeneous disorder characterized by bone marrow failure and premature ageing. Current knowledge on clinical manifestations, molecular pathomechanisms, diagnostic criteria and therapeutic possibilities of patients with dyskeratosis congenita are described. Mutation analysis of the gene encoding for dyskerin revealed the c.IVS2-5C>G splice site mutation. The importance of early diagnosis in order to prevent severe invasive infections and non-infectious complications is emphasized. Family screening is important to identify carriers as prenatal genetic diagnosis conveys great benefits for family planning.

Telomere biology and telomere diseases: implications for practice and research

The recent recognition of genetic defects in telomeres and telomere repair in multiple human diseases has practical implications for hematologists and oncologists and their patients; consequences for future clinical research in hematology and other subspecialties; and even importance in the interpretation of animal experiments involving cell propagation. Telomere diseases include constitutional marrow failure as dyskeratosis congenita, some apparently acquired aplastic anemia, myelodysplasia and acute myeloid leukemia; pulmonary fibrosis; and hepatic nodular regenerative hyperplasia and cirrhosis. Accelerated telomere attrition is a likely pathophysiology of cancer arising from chronic inflammation. Telomerase can be modulated by sex hormones, which may explain the activity of androgens in marrow failure. Measurement of telomere length of peripheral blood leukocytes is a simple screening clinical assay. Detection of a mutation in a patient has implications for therapy, prognosis, monitoring, and genetic counseling. For research in hematology and oncology, telomere biology could be assessed as a risk for secondary malignancies and in graft-versus-host disease, for progression in a variety of blood cancers, and as potentially modifiable by hormone replacement strategies.

Investigation of human telomerase holoenzyme assembly, activity, and processivity using disease-linked subunit variants

After the initial discovery of human telomerase deficiency in the X-linked form of the bone marrow failure syndrome dyskeratosis congenita, mutations in genes encoding telomerase subunits have been identified in patients with a wide spectrum of disorders. Structure/function studies of disease-linked variants of human telomerase RNA (hTR) or telomerase reverse transcriptase (TERT) have exploited in vitro reconstitution of the enzyme catalytic core and/or a PCR-amplified activity assay readout that would not reflect alterations of cellular RNP assembly efficiency, telomeric primer recognition, and/or repeat addition processivity. Here we used telomerase reconstitution in vivo and direct telomeric-repeat primer extension activity assays to compare the ribonucleoprotein (RNP) assembly and activity properties of disease-linked subunit variants in holoenzyme context. Analysis of a large panel of hTR variants revealed numerous biochemical mechanisms for telomerase loss of function, including reduced association of hTR with TERT, reduced RNP catalytic activity, or loss in fidelity of telomeric repeat synthesis. An absolute correlation exists between hTR loss of function and hematopoietic deficiency, but there is no readily apparent telomerase deficiency imposed by an hTR variant linked to pulmonary fibrosis. Some disease-linked TERT variants have altered properties of holoenzyme assembly or repeat addition processivity, but other TERT variants linked to either pulmonary fibrosis or hematopoietic deficiency retained normal hTR interaction and RNP catalytic activity. Combined with additional hTR structure/function studies, our results establish a new resolution of insight into hTR structural requirements for hTR-TERT interaction and for the catalytic cycle of human telomerase holoenzyme.

Xenotropic murine leukemia virus-related virus is susceptible to AZT

The xenotropic murine leukemia virus-related virus (XMRV) is a human retrovirus, recently isolated from tissues of prostate cancer patients with impaired RNase L activity. In this study, we evaluated 10 licensed anti-HIV-1 compounds for their activity against XMRV, including protease inhibitors (PI), nucleoside reverse transcriptase (RT) inhibitors (NRTI), non-nucleoside RT inhibitors (NNRTI) and an integrase inhibitor. No PI affected XMRV production; even high concentrations of Ritonavir failed to inhibit the maturation of XMRV Gag polyproteins. Among the NRTI, NNRTI and integrase inhibitors used in this study, only AZT blocked XMRV infection and replication through inhibition of viral reverse transcription. This sensitivity of XMRV to AZT may be explained by the modest homology in the motif D sequences of HIV-1 and XMRV reverse transcriptases. If XMRV becomes established as an etiological agent for prostate cancer or other diseases, AZT may be useful for preventing or treating XMRV infections in humans.

Abnormal expression of telomerase/apoptosis limits type II alveolar epithelial cell replication in the early remodeling of usual interstitial pneumonia/idiopathic pulmonary fibrosis

Idiopathic pulmonary fibrosis is a distinctive, usually fatal, type of chronic fibrosing interstitial pneumonia of unknown cause that increases in prevalence with advanced age, characterized by failure of alveolar re-epithelization and progressive scar formation. Recently, limitation of the replicative capacity of tissues determined by telomerase/apoptosis balance has been implicated in pathogenesis of age-related diseases. In this study, we validated the importance of the expression of type 2 alveolar epithelial cells telomerase protein and studied the relationships between telomerase and apoptosis in early remodeling of usual interstitial pneumonia. We determined type 2 alveolar epithelial cells density, telomerase expression, and apoptosis in surgical lung biopsies from 24 patients with usual interstitial pneumonia, and in normal lung tissues from 18 subjects. We used immunohistochemistry, deoxynucleotidyl transferase method of end labeling, electron microscopy, and histomorphometry to evaluate the amount of type 2 alveolar epithelial cells staining for surfactant-A, telomerase, and in situ detection of apoptotic cells. Unaffected areas of usual interstitial pneumonia and normal lung tissue had similar densities of type 2 alveolar epithelial cells, but a significant minor subpopulation of type 2 alveolar epithelial cells was telomerase positive and a large population was telomerase negative. A significant inverse association was found between low type 2 alveolar epithelial cell telomerase expression and high apoptosis in unaffected areas of usual interstitial pneumonia. Although type 2 alveolar epithelial cell telomerase expression was higher than apoptosis in NLT group, no significant association was found between them. Electron microscopy confirmed epithelial apoptosis, alveolar collapse, and initial fibroplasia. We conclude that abnormal type 2 alveolar epithelial cells telomerase/apoptosis balance may reduce alveolar epithelial regenerative capacity, thus contributing to the early remodeling response in usual interstitial pneumonia.

Short telomeres are sufficient to cause the degenerative defects associated with aging

Telomerase function is critical for telomere maintenance. Mutations in telomerase components lead to telomere shortening and progressive bone marrow failure in the premature aging syndrome dyskeratosis congenita. Short telomeres are also acquired with aging, yet the role that they play in mediating age-related disease is not fully known. We generated wild-type mice that have short telomeres. In these mice, we identified hematopoietic and immune defects that resembled those present in dyskeratosis congenita patients. When mice with short telomeres were interbred, telomere length was only incrementally restored, and even several generations later, wild-type mice with short telomeres still displayed degenerative defects. Our findings implicate telomere length as a unique heritable trait that, when short, is sufficient to mediate the degenerative defects of aging, even when telomerase is wild-type.

A spectrum of severe familial liver disorders associate with telomerase mutations

Background

Telomerase is an enzyme specialized in maintaining telomere lengths in highly proliferative cells. Loss-of-function mutations cause critical telomere shortening and are associated with the bone marrow failure syndromes dyskeratosis congenita and aplastic anemia and with idiopathic pulmonary fibrosis. Here, we sought to determine the spectrum of clinical manifestations associated with telomerase loss-of-function mutations.

Methodology/Principal Findings

Sixty-nine individuals from five unrelated families with a variety of hematologic, hepatic, and autoimmune disorders were screened for telomerase complex gene mutations; leukocyte telomere length was measured by flow fluorescence in situ hybridization in mutation carriers and some non-carriers; the effects of the identified mutations on telomerase activity were determined; and genetic and clinical data were correlated. In six generations of a large family, a loss-of-function mutation in the telomerase enzyme gene TERT associated with severe telomere shortening and a range of hematologic manifestations, from macrocytosis to acute myeloid leukemia, with severe liver diseases marked by fibrosis and inflammation, and one case of idiopathic pulmonary fibrosis but not with autoimmune disorders. Additionally, we identified four unrelated families in which loss-of-function TERC or TERT gene mutations tracked with marrow failure, pulmonary fibrosis, and a spectrum of liver disorders.

Conclusions/Significance

These results indicate that heterozygous telomerase loss-of-function mutations associate with but are not determinant of a large spectrum of hematologic and liver abnormalities, with the latter sometimes occurring in the absence of marrow failure. Our findings, along with the link between pulmonary fibrosis and telomerase mutations, also suggest a common pathogenic mechanism for fibrotic diseases in which defective telomere repair plays important role.

Telomeres and telomerase in cancer

Myriad genetic and epigenetic alterations are required to drive normal cells toward malignant transformation. These somatic events commandeer many signaling pathways that cooperate to endow aspiring cancer cells with a full range of biological capabilities needed to grow, disseminate and ultimately kill its host. Cancer genomes are highly rearranged and are characterized by complex translocations and regional copy number alterations that target loci harboring cancer-relevant genes. Efforts to uncover the underlying mechanisms driving genome instability in cancer have revealed a prominent role for telomeres. Telomeres are nucleoprotein structures that protect the ends of eukaryotic chromosomes and are particularly vulnerable due to progressive shortening during each round of DNA replication and, thus, a lifetime of tissue renewal places the organism at risk for increasing chromosomal instability. Indeed, telomere erosion has been documented in aging tissues and hyperproliferative disease states-conditions strongly associated with increased cancer risk. Telomere dysfunction can produce the opposing pathophysiological states of degenerative aging or cancer with the specific outcome dictated by the integrity of DNA damage checkpoint responses. In most advanced cancers, telomerase is reactivated and serves to maintain telomere length and emerging data have also documented the capacity of telomerase to directly regulate cancer-promoting pathways. This review covers the role of telomeres and telomerase in the biology of normal tissue stem/progenitor cells and in the development of cancer.

Comparing effects of mTR and mTERT deletion on gene expression and DNA damage response: a critical examination of telomere length maintenance-independent roles of telomerase

Telomerase, the essential enzyme that maintains telomere length, contains two core components, TERT and TR. Early studies in yeast and mouse showed that loss of telomerase leads to phenotypes only after several generations, due to telomere shortening. However, recent studies have suggested additional roles for telomerase components in transcription and the response to DNA damage. To examine these potential telomere length maintenance-independent roles of telomerase components, we examined first generation mTR(-/-) and mTERT(-/-) mice with long telomeres. We used gene expression profiling and found no genes that were differentially expressed in mTR(-/-) G1 mice and mTERT(-/-) G1 mice compared with wild-type mice. We also compared the response to DNA damage in mTR(-/-)G1 and mTERT(-/-) G1 mouse embryonic fibroblasts, and found no increase in the response to DNA damage in the absence of either telomerase component compared to wild-type. We conclude that, under physiologic conditions, neither mTR nor mTERT acts as a transcription factor or plays a role in the DNA damage response.

Telomerase reverse transcriptase-dependent telomere equilibration mitigates tissue dysfunction in mTert heterozygotes

Autosomal dominant mutations in telomere-associated factors elicit a disease known as dyskeratosis congenita (DKC), and patients suffer proliferative abnormalities associated with telomere erosion. Mice that are heterozygous for telomerase genes (Tert or Terc, hereafter referred to as mTert and mTerc) are useful models of telomerase haploinsufficiency, but do not strictly mimic DKC. In strains with long telomeres (>60 kbp), animals that are heterozygous for mTert undergo telomere erosion for nine generations and remain phenotypically normal. In an mTerc heterozygous strain with short telomeres (<15 kbp), early mortality arises after five to six generations, but dyskeratosis occurs only upon the further loss of mPot1b. We show that prolonged mTert heterozygosity (for greater than ten generations) did not elicit disease, even upon heterozygote interbreeding, and that telomeres reset to wild-type lengths. This lengthening did not occur in nullizygotes, and short telomeres inherited from mTert null parents were rescued only in heterozygous progeny. In the bone marrow, nullizygotes remained competent for radioprotection for three generations. Thus, gradual telomere erosion in the presence of telomerase may enable subsequent telomere extension, similar to that described in budding yeast. We speculate whether such adaptation occurs in normal human cells (or whether it could be induced in DKC-derived cells), and whether it might mitigate the impact of telomerase inhibition upon stem cells during cancer therapy.