The enigma of excessively long telomeres in cancer: lessons learned from rare human POT1 variants

CoPixie, a novel algorithm for single-particle track colocalization, enables efficient quantification of telomerase dynamics at telomeres

Single-particle imaging and tracking can be combined with colocalization analysis to study the dynamic interactions between macromolecules in living cells. Indeed, single-particle tracking has been extensively used to study protein-DNA interactions and dynamics. Still, unbiased identification and quantification of binding events at specific genomic loci remains challenging. Herein, we describe CoPixie, a new software that identifies colocalization events between a theoretically unlimited number of imaging channels, including single-particle movies. CoPixie is an object-based colocalization algorithm that relies on both pixel and trajectory overlap to determine colocalization between molecules. We employed CoPixie with live-cell single-molecule imaging of telomerase and telomeres, to test the model that cancer-associated POT1 mutations facilitate telomere accessibility. We show that POT1 mutants Y223C, D224N or K90E increase telomere accessibility for telomerase interaction. However, unlike the POT1-D224N mutant, the POT1-Y223C and POT1-K90E mutations also increase the duration of long-lasting telomerase interactions at telomeres. Our data reveal that telomere elongation in cells expressing cancer-associated POT1 mutants arises from the dual impact of these mutations on telomere accessibility and telomerase retention at telomeres. CoPixie can be used to explore a variety of questions involving macromolecular interactions in living cells, including between proteins and nucleic acids, from multicolor single-particle tracks.

POT1 tumour predisposition: a broader spectrum of associated malignancies and proposal for additional screening program

Protection of Telomeres Protein 1 (POT1) protein is an essential subunit of the shelterin telomere binding complex, regulating telomere length. Some POT1 gene pathogenic variants (PV) lead to telomere elongation, genomic instability and higher risk of cancer. POT1 tumour predisposition syndrome (POT1-TPD) has autosomal dominant inheritance and unknown penetrance. It is associated with increased risk of cutaneous melanoma, chronic lymphocytic leukaemia, angiosarcoma and gliomas. In this work, we aim to describe a broader cancer phenotype related to POT1-TPD, in three families (two with a four generation pedigree, one with a five generation pedigree). The three index cases were referred to our oncogenetic centre for genetic counselling due to their personal history of cancer. Two underwent clinical exome sequencing of 4,867 genes associated with Mendelian genetic diseases, and another underwent gene panel sequencing including POT1, which identified three different POT1 PV: NC_000007.14(NM_015450.2):c.349C>T; NC_000007.14(NM_015450.2):c.233T>C and NC_000007.14(NM_015450.2):c.818G>A; already described in the literature. Referenced relatives, did a target genetic test (according to the POT1 PV identified in the family). In total, 37 individuals were tested (51.4% females), median age of 46 (22-81) years, with POT1 PV detected in 22. POT1-TPD was observed, but also a higher incidence of other cancers (other sarcomas, papillary thyroid cancer, early onset prostate cancer and leukaemia). These findings contribute to an increase in our knowledge about POT1 PV, and it can play a role in the definition of future POT1 PV screening criteria, POT1 carrier surveillance protocols (possibly considering screening for all types of sarcomas) and in genetic counselling.

Replicative senescence is ATM driven, reversible, and accelerated by hyperactivation of ATM at normoxia

Programmed telomere shortening limits tumorigenesis through the induction of replicative senescence. Here we address three long-standing questions concerning senescence. First, we show that the ATM kinase is solely responsible for the induction of replicative senescence. Senescence was delayed by ATM inhibition (ATMi) or overexpression of TRF2, the shelterin subunit dedicated to ATM repression. In contrast, there was no evidence for ATR signaling contributing to replicative senescence even when ATMi was combined with ATR inhibition. Second, we show ATMi can induce apparently normal cell divisions in a subset of senescent cells, indicating that senescence can be reversed. Third, we show that the extended replicative life span at low (physiological) oxygen is due to diminished ATM activity. At low oxygen, cells show a decreased ATM response to dysfunctional telomeres and genome-wide DSBs compared to 20% oxygen. As this effect could be reversed by NAC, the attenuated response of ATM to critically short telomeres and the resulting extended life span at low oxygen is likely due to ROS-induced formation of cysteine disulfide-bridges that crosslink ATM dimers into a form that is not activated by DSBs. These findings show how primary human cells detect shortened telomeres and reveal the molecular mechanism underlying the telomere tumor suppressor pathway.

Can telomeric changes orchestrate the development of autoinflammatory skin diseases?

Telomeres, the safeguarding caps at the tips of chromosomes, are pivotal in the aging process of cells and have been linked to skin ailments and inflammatory conditions. Telomeres undergo a gradual reduction in length and factors such as oxidative stress hasten this diminishing process. Skin diseases including inflammatory conditions can be correlated with the shortening of telomeres and the persistent activation of DNA damage response in skin tissues. Telomere dysfunction could disrupt the balance of the skin, impairs wound healing, and may contribute to abnormal cytokine production. Skin aging and processes related to telomeres may function as one of the triggers for skin diseases. The presence of proinflammatory cytokines and dysfunctional telomeres in conditions such as Dyskeratosis Congenita implies a possible connection between the shortening of telomeres and the onset of chronic inflammatory skin disorders. In autoinflammatory skin diseases, chronic inflammation hinders wound healing thus aggravating the progression of the disease. The NF-ĸB pathway might contribute to the initiation or progression of chronic disorders by influencing mechanisms associated with telomere biology. The intricate connections between telomeres, telomerase, telomere-associated proteins, and skin diseases are still a complex puzzle to be solved. Here, we provide an overview of the impact of telomeres on both health and disease with a specific emphasis on their role in skin, inflammation and autoinflammatory skin disorders.

Does Effective Population Size Govern Evolutionary Differences in Telomere Length?

Lengths of telomeres vary by an order of magnitude across mammalian species. Similarly, age- and sex-standardized telomere lengths differ by up to 1 kb (14%) across human populations. How to explain these differences? Telomeres play a central role in senescence and aging, and genes that affect telomere length are likely under weak selection (i.e. telomere length is a trait that is subject to nearly neutral evolution). Importantly, natural selection is more effective in large populations than in small populations. Here, we propose that observed differences in telomere length across species and populations are largely due to differences in effective population sizes. In this perspective, we present preliminary evolutionary genetic evidence supporting this hypothesis and highlight the need for more data.

Telomere length and cancer risk: finding Goldilocks

Telomeres are the nucleoprotein complex at chromosome ends essential in genomic stability. Baseline telomere length (TL) is determined by rare and common germline genetic variants but shortens with age and is susceptible to certain environmental exposures. Cellular senescence or apoptosis are normally triggered when telomeres reach a critically short length, but cancer cells overcome these protective mechanisms and continue to divide despite chromosomal instability. Rare germline variants in telomere maintenance genes cause exceedingly short telomeres for age (< 1st percentile) and the telomere biology disorders, which are associated with elevated risks of bone marrow failure, myelodysplastic syndrome, acute myeloid leukemia, and squamous cell carcinoma of the head/neck and anogenital regions. Long telomeres due to rare germline variants in the same or different telomere maintenance genes are associated with elevated risks of other cancers, such as chronic lymphocytic leukemia or sarcoma. Early epidemiology studies of TL in the general population lacked reproducibility but new methods, including creation of a TL polygenic score using common variants, have found longer telomeres associated with excess risks of renal cell carcinoma, glioma, lung cancer, and others. It has become clear that when it comes to TL and cancer etiology, not too short, not too long, but "just right" telomeres are important in minimizing cancer risk.

Targeting 'histone mark': Advanced approaches in epigenetic regulation of telomere dynamics in cancer

Telomere integrity is required for the maintenance of genome stability and prevention of oncogenic transformation of cells. Recent evidence suggests the presence of epigenetic modifications as an important regulator of mammalian telomeres. Telomeric and subtelomeric regions are rich in epigenetic marks that regulate telomere length majorly through DNA methylation and post-translational histone modifications. Specific histone modifying enzymes play an integral role in establishing telomeric histone codes necessary for the maintenance of structural integrity. Alterations of crucial histone moieties and histone modifiers cause deregulations in the telomeric chromatin leading to carcinogenic manifestations. This review delves into the significance of histone modifications and their influence on telomere dynamics concerning cancer. Additionally, it highlights the existing research gaps that hold the potential to drive the development of therapeutic interventions targeting the telomere epigenome.

Germline POT1 Variants: A Critical Perspective on POT1 Tumor Predisposition Syndrome

The Protection of Telomere 1 (POT1) gene was identified as a melanoma predisposition candidate nearly 10 years ago. Thereafter, various cancers have been proposed as associated with germline POT1 variants in the context of the so-called POT1 Predisposition Tumor Syndrome (POT1-TPD). While the key role, and related risks, of the alterations in POT1 in melanoma are established, the correlation between germline POT1 variants and the susceptibility to other cancers partially lacks evidence, due also to the rarity of POT1-TPD. Issues range from the absence of functional or segregation studies to biased datasets or the need for a revised classification of variants. Furthermore, a proposal of a surveillance protocol related to the cancers associated with POT1 pathogenic variants requires reliable data to avoid an excessive, possibly unjustified, burden for POT1 variant carriers. We propose a critical perspective regarding data published over the last 10 years that correlate POT1 variants to various types of cancer, other than cutaneous melanoma, to offer food for thought for the specialists who manage cancer predisposition syndromes and to stimulate a debate on the grey areas that have been exposed.

Characterization of POT1 tumor predisposition syndrome: Tumor prevalence in a clinically diverse hereditary cancer cohort

PURPOSE

Germline variants in POT1 have been implicated in predisposition to melanoma, sarcoma, and glioma in limited studies. Here, we determine the prevalence of cancer types in individuals with POT1 pathogenic variants (PVs) undergoing multigene panel testing (MGPT) for a broad variety of cancer indications.

METHODS

We performed a retrospective review of data provided on clinical documents from individuals with POT1 PVs identified via MGPT over a 5-year period. Tumor prevalence in POT1 PV heterozygotes was compared with MGPT-negative wild-type (WT) controls using χ2 test.

RESULTS

POT1 PVs were identified in 227 individuals. POT1 PV and WT (n = 13,315) cohorts had a similar proportion of reported tumors (69.6% and 69.2%, respectively); however, POT1 PV heterozygotes were more likely to be diagnosed with multiple tumors (18.9% vs 8.7%; P < .001). Compared with POT1 WT, we identified a significant increase in melanoma (odds ratio 7.03; 95% CI 4.7-10.5; P < .001) and sarcoma (odds ratio 6.6; 95% CI 3.1-13.9; P < .001).

CONCLUSION

This analysis of the largest POT1 PV cohort to date validates the inclusion of POT1 in hereditary cancer MGPT and has the potential to impact clinical management recommendations, particularly for patients and families at risk for melanoma and sarcoma.

TINF2 is a major susceptibility gene in Danish patients with multiple primary melanoma

TINF2 encodes the TINF2 protein, which is a subunit in the shelterin complex critical for telomere regulation. Three recent studies have associated six truncating germline variants in TINF2 that have previously been associated with a cancer predisposition syndrome (CPS) caused by elongation of the telomeres. This has added TINF2 to the long telomere syndrome genes, together with other telomere maintenance genes such as ACD, POT1, TERF2IP, and TERT. We report a clinical study of 102 Danish patients with multiple primary melanoma (MPM) in which a germline truncating variant in TINF2 (p.(Arg265Ter)) was identified in four unrelated participants. The telomere lengths of three variant carriers were >90% percentile. In a routine diagnostic setting, the variant was identified in two more families, including an additional MPM patient and monozygotic twins with thyroid cancer and other cancer types. A total of 10 individuals from six independent families were confirmed carriers, all with cancer history, predominantly melanoma. Our findings suggest a major role of TINF2 in Danish patients with MPM. In addition to melanoma, other cancers in the six families include thyroid, renal, breast, and sarcoma, supporting a CPS in which melanoma, thyroid cancer, and sarcoma predominate. Further studies are needed to establish the full spectrum of associated cancer types and characterize lifetime cancer risk in carriers.

Telomere biology and ribosome biogenesis: structural and functional interconnections

Telomeres are nucleoprotein structures that play a pivotal role in the protection and maintenance of eukaryotic chromosomes. Telomeres and the enzyme telomerase, which replenishes telomeric DNA lost during replication, are important factors necessary to ensure continued cell proliferation. Cell proliferation is also dependent on proper and efficient protein synthesis, which is carried out by ribosomes. Mutations in genes involved in either ribosome biogenesis or telomere biology result in cellular abnormalities and can cause human genetic diseases, defined as ribosomopathies and telomeropathies, respectively. Interestingly, recent discoveries indicate that many of the ribosome assembly and rRNA maturation factors have additional noncanonical functions in telomere biology. Similarly, several key proteins and enzymes involved in telomere biology, including telomerase, have unexpected roles in rRNA transcription and maturation. These observations point to an intriguing cross-talk mechanism potentially explaining the multiple pleiotropic symptoms of mutations in many causal genes identified in various telomeropathy and ribosomopathy diseases. In this review, we provide a brief summary of eukaryotic telomere and rDNA loci structures, highlight several universal features of rRNA and telomerase biogenesis, evaluate intriguing interconnections between telomere biology and ribosome assembly, and conclude with an assessment of overlapping features of human diseases of telomeropathies and ribosomopathies.

A pan-sarcoma landscape of telomeric content shows that alterations in RAD51B and GID4 are associated with higher telomeric content

Tumor cells need to activate a telomere maintenance mechanism, enabling limitless replication. The bulk of evidence supports that sarcomas predominantly use alternative lengthening of telomeres (ALT) mechanism, commonly associated with alterations in ATRX and DAXX. In our dataset, only 12.3% of sarcomas harbored alterations in these genes. Thus, we checked for the presence of other genomic determinants of high telomeric content in sarcomas. Our dataset consisted of 13555 sarcoma samples, sequenced as a part of routine clinical care on the FoundationOne®Heme platform. We observed a median telomeric content of 622.3 telomeric reads per GC-matched million reads (TRPM) across all samples. In agreement with previous studies, telomeric content was significantly higher in ATRX altered and POT1 altered sarcomas. We further observed that sarcomas with alterations in RAD51B or GID4 were enriched in samples with high telomeric content, specifically within uterus leiomyosarcoma for RAD51B and soft tissue sarcoma (not otherwise specified, nos) for GID4, Furthermore, RAD51B and POT1 alterations were mutually exclusive with ATRX and DAXX alterations, suggestive of functional redundancy. Our results propose a role played by RAD51B and GID4 in telomere elongation in sarcomas and open research opportunities for agents aimed at targeting this critical pathway in tumorigenesis.

Exploring Genetic Interactions with Telomere Protection Gene pot1 in Fission Yeast

The regulation of telomere length has a significant impact on cancer risk and aging in humans. Circular chromosomes are found in humans and are often unstable during mitosis, resulting in genome instability. Some types of cancer have a high frequency of a circular chromosome. Fission yeast is a good model for studying the formation and stability of circular chromosomes as deletion of pot1 (encoding a telomere protection protein) results in rapid telomere degradation and chromosome fusion. Pot1 binds to single-stranded telomere DNA and is conserved from fission yeast to humans. Loss of pot1 leads to viable strains in which all three fission yeast chromosomes become circular. In this review, I will introduce pot1 genetic interactions as these inform on processes such as the degradation of uncapped telomeres, chromosome fusion, and maintenance of circular chromosomes. Therefore, exploring genes that genetically interact with pot1 contributes to finding new genes and/or new functions of genes related to the maintenance of telomeres and/or circular chromosomes.

POT1 mutations are frequent and associated with Ki-67 index in canine diffuse large B-cell lymphoma

Diffuse large B-cell lymphoma (DLBCL) represents one of the most frequent and deadliest neoplasia in dogs worldwide and is characterized by a remarkable degree of clinical heterogeneity, with poor chances to anticipate the outcome. Even if in the last years some recurrently mutated genes have been identified, the genetic origin of canine DLBCL (cDLBCL) is not yet completely understood. The aim of the present study was to assess the prevalence of POT1 mutations in cDLBCL and to elucidate the role of such gene in the pathogenesis of this tumor. Mutations in POT1 were retrieved in 34% of cases, in line with previous reports, but no significant associations with any clinico-pathological variable were identified. Likewise, POT1 mutations are not predictive of worse prognosis. Interestingly, Ki-67 index was significantly higher in dogs harboring POT1 mutations compared to wild-type ones. These results suggest that POT1 mutations may exert their pathogenic role in cDLBCL by promoting cellular proliferation.

Quantification of 8-oxoG in Plant Telomeres

Chemical modifications in DNA impact gene regulation and chromatin structure. DNA oxidation, for example, alters gene expression, DNA synthesis and cell cycle progression. Modification of telomeric DNA by oxidation is emerging as a marker of genotoxic damage and is associated with reduced genome integrity and changes in telomere length and telomerase activity. 8-oxoguanine (8-oxoG) is the most studied and common outcome of oxidative damage in DNA. The G-rich nature of telomeric DNA is proposed to make it a hotspot for oxidation, but because telomeres make up only a tiny fraction of the genome, it has been difficult to directly test this hypothesis by studying dynamic DNA modifications specific to this region in vivo. Here, we present a new, robust method to differentially enrich telomeric DNA in solution, coupled with downstream methods for determination of chemical modification. Specifically, we measure 8-oxoG in Arabidopsis thaliana telomeres under normal and oxidative stress conditions. We show that telomere length is unchanged in response to oxidative stress in three different wild-type accessions. Furthermore, we report that while telomeric DNA comprises only 0.02–0.07% of the total genome, telomeres contribute between 0.2 and 15% of the total 8-oxoG. That is, plant telomeres accumulate 8-oxoG at levels approximately 100-fold higher than the rest of the genome under standard growth conditions. Moreover, they are the primary targets of further damage upon oxidative stress. Interestingly, the accumulation of 8-oxoG in the chromosome body seems to be inversely proportional to telomere length. These findings support the hypothesis that telomeres are hotspots of 8-oxoG and may function as sentinels of oxidative stress in plants.

Plasticity, pleiotropy and fitness trade-offs in Arabidopsis genotypes with different telomere lengths

Telomere length has been implicated in the organismal response to stress, but the underlying mechanisms are unknown. Here we examine the impact of telomere length changes on the responses to three contrasting abiotic environments in Arabidopsis, and measure 32 fitness, developmental, physiological and leaf-level anatomical traits. We report that telomere length in wild-type and short-telomere mutants is resistant to abiotic stress, while the elongated telomeres in ku70 mutants are more plastic. We detected significant pleiotropic effects of telomere length on flowering time and key leaf physiological and anatomical traits. Furthermore, our data reveal a significant genotype by environment (G × E) interaction for reproductive fitness, with the benefits and costs to performance depending on the growth conditions. These results imply that life-history trade-offs between flowering time and reproductive fitness are impacted by telomere length variation. We postulate that telomere length in plants is subject to natural selection imposed by different environments.

Association of Mitochondrial DNA Copy Number and Telomere Length with Prevalent and Incident Cancer and Cancer Mortality in Women: A Prospective Swedish Population-Based Study

Changes in mitochondrial DNA copy number (mtDNA-CN) and telomere length have, separately, been proposed as risk factors for various cancer types. However, those results are conflicting. Here, mtDNA-CN and relative telomere length were measured in 3225 middle-aged women included in a large population-based prospective cohort. The baseline mtDNA-CN in patients with prevalent breast cancer was significantly higher (12.39 copies/µL) than cancer-free individuals. During an average of 15.2 years of follow-up, 520 patients were diagnosed with cancer. Lower mtDNA-CN was associated with decreased risk of genital organ cancer (hazard ratio (HR), 0.84), and shorter telomere length was associated with increased risk of urinary system cancer (HR, 1.79). Furthermore, mtDNA-CN was inversely associated with all-cause (HR, 1.20) and cancer-specific mortality (HR, 1.21) when considering all cancer types. Surprisingly, shorter telomere length was associated with decreased risk of cancer-specific mortality when considering all cancer types (HR, 0.85). Finally, lower mtDNA-CN and shorter telomere length were associated with increased risk of both all-cause and cancer-specific mortality in genital organ cancer patients. In this study population, we found that mtDNA-CN and telomere length were significantly associated with prevalent and incident cancer and cancer mortality. However, these associations were cancer type specific and need further investigation.

Multifunctionality of the Telomere-Capping Shelterin Complex Explained by Variations in Its Protein Composition

Protecting telomere from the DNA damage response is essential to avoid the entry into cellular senescence and organismal aging. The progressive telomere DNA shortening in dividing somatic cells, programmed during development, leads to critically short telomeres that trigger replicative senescence and thereby contribute to aging. In several organisms, including mammals, telomeres are protected by a protein complex named Shelterin that counteract at various levels the DNA damage response at chromosome ends through the specific function of each of its subunits. The changes in Shelterin structure and function during development and aging is thus an intense area of research. Here, we review our knowledge on the existence of several Shelterin subcomplexes and the functional independence between them. This leads us to discuss the possibility that the multifunctionality of the Shelterin complex is determined by the formation of different subcomplexes whose composition may change during aging.

The Power of Stress: The Telo-Hormesis Hypothesis

Adaptative response to stress is a strategy conserved across evolution to promote survival. In this context, the groundbreaking findings of Miroslav Radman on the adaptative value of changing mutation rates opened new avenues in our understanding of stress response. Inspired by this work, we explore here the putative beneficial effects of changing the ends of eukaryotic chromosomes, the telomeres, in response to stress. We first summarize basic principles in telomere biology and then describe how various types of stress can alter telomere structure and functions. Finally, we discuss the hypothesis of stress-induced telomere signaling with hormetic effects.

Cancer-associated POT1 mutations lead to telomere elongation without induction of a DNA damage response

Mutations in the shelterin protein POT1 are associated with chronic lymphocytic leukemia (CLL), Hodgkin lymphoma, angiosarcoma, melanoma, and other cancers. These cancer‐associated POT1 (caPOT1) mutations are generally heterozygous, missense, or nonsense mutations occurring throughout the POT1 reading frame. Cancers with caPOT1 mutations have elongated telomeres and show increased genomic instability, but which of the two phenotypes promotes tumorigenesis is unclear. We tested the effects of CAS9‐engineered caPOT1 mutations in human embryonic and hematopoietic stem cells (hESCs and HSCs, respectively). HSCs with caPOT1 mutations did not show overt telomere damage. In vitro and in vivo competition experiments showed the caPOT1 mutations did not confer a selective disadvantage. Since DNA damage signaling is known to affect the fitness of HSCs, the data argue that caPOT1 mutations do not cause significant telomere damage. Furthermore, hESC lines with caPOT1 mutations showed no detectable telomere damage response while showing consistent telomere elongation. Thus, caPOT1 mutations are likely selected for during cancer progression because of their ability to elongate telomeres and extend the proliferative capacity of the incipient cancer cells.

Colorectal Cancer Stem Cell States Uncovered by Simultaneous Single-Cell Analysis of Transcriptome and Telomeres

Cancer stem cells (CSCs) presumably contribute to tumor progression and drug resistance, yet their definitive features have remained elusive. Here, simultaneous measurement of telomere length and transcriptome in the same cells enables systematic assessment of CSCs in primary colorectal cancer (CRC). The in-depth transcriptome profiled by SMART-seq2 is independently validated by high-throughput scRNA-seq using 10 × Genomics. It is found that rare CSCs exist in dormant state and display plasticity toward cancer epithelial cells (EPCs) that essentially are presumptive tumor-initiating cells (TICs), while both retaining the prominent signaling pathways including WNT, TGF-β, and HIPPO/YAP. Moreover, CSCs exhibit chromosome copy number variation (CNV) pattern resembling cancer EPCs but distinct from normal stem cells, suggesting the phylogenetic relationship between CSCs and cancer EPCs. Notably, CSCs maintain shorter telomeres and possess minimal telomerase activity consistent with their nonproliferative nature, unlike cancer EPCs. Additionally, the specific signature of CSCs particularly NOTUM, SMOC2, BAMBI, PHLDA1, and TNFRSF19 correlates with the prognosis of CRC. These findings characterize the heterogeneity of CSCs and their linkage to cancer EPCs/TICs, some of which are conventionally regarded as CSCs.

POT1 stability and binding measured by fluorescence thermal shift assays

The protein POT1 (Protection of Telomeres 1) is an integral part of the shelterin complex that protects the ends of human chromosomes from degradation or end fusions. It is the only component of shelterin that binds single-stranded DNA. We describe here the application of two separate fluorescent thermal shift assays (FTSA) that provide quantitative biophysical characterization of POT1 stability and its interactions. The first assay uses Sypro Orange™ and monitors the thermal stability of POT1 and its binding under a variety of conditions. This assay is useful for the quality control of POT1 preparations, for biophysical characterization of its DNA binding and, potentially, as an efficient screening tool for binding of small molecule drug candidates. The second assay uses a FRET-labeled human telomeric G-quadruplex structure that reveals the effects of POT1 binding on thermal stability from the DNA frame of reference. These complementary assays provide efficient biophysical approaches for the quantitative characterization of multiple aspects of POT1 structure and function. The results from these assays provide thermodynamics details of POT1 folding, the sequence selectivity of its DNA binding and the thermodynamic profile for its binding to its preferred DNA binding sequence. Most significantly, results from these assays elucidate two mechanisms for the inhibition of POT1 -DNA interactions. The first is by competitive inhibition at the POT1 DNA binding site. The second is indirect and is by stabilization of G-quadruplex formation within the normal POT1 single-stranded DNA sequence to prevent POT1 binding.

Identification and characterization of novel ACD variants: modulation of TPP1 protein level offsets the impact of germline loss-of-function variants on telomere length

Telomere biology disorders, largely characterized by telomere lengths below the first centile for age, are caused by variants in genes associated with telomere replication, structure, or function. One of these genes, ACD, which encodes the shelterin protein TPP1, is associated with both autosomal dominantly and autosomal recessively inherited telomere biology disorders. TPP1 recruits telomerase to telomeres and stimulates telomerase processivity. Several studies probing the effect of various synthetic or patient-derived variants have mapped specific residues and regions of TPP1 that are important for interaction with TERT, the catalytic component of telomerase. However, these studies have come to differing conclusions regarding ACD haploinsufficiency. Here, we report a proband with compound heterozygous novel variants in ACD (NM_001082486.1)-c.505_507delGAG, p.(Glu169del); and c.619delG, p.(Asp207Thrfs*22)-and a second proband with a heterozygous chromosomal deletion encompassing ACD: arr[hg19] 16q22.1(67,628,846-67,813,408)x1. Clinical data, including symptoms and telomere length within the pedigrees, suggested that loss of one ACD allele was insufficient to induce telomere shortening or confer clinical features. Further analyses of lymphoblastoid cell lines showed decreased nascent ACD RNA and steady-state mRNA, but normal TPP1 protein levels, in cells containing heterozygous ACD c.619delG, p.(Asp207Thrfs*22), or the ACD-encompassing chromosomal deletion compared to controls. Based on our results, we conclude that cells are able to compensate for loss of one ACD allele by activating a mechanism to maintain TPP1 protein levels, thus maintaining normal telomere length.

TINF2 is a haploinsufficient tumor suppressor that limits telomere length

Telomere shortening is a presumed tumor suppressor pathway that imposes a proliferative barrier (the Hayflick limit) during tumorigenesis. This model predicts that excessively long somatic telomeres predispose to cancer. Here, we describe cancer-prone families with two unique TINF2 mutations that truncate TIN2, a shelterin subunit that controls telomere length. Patient lymphocyte telomeres were unusually long. We show that the truncated TIN2 proteins do not localize to telomeres, suggesting that the mutations create loss-of-function alleles. Heterozygous knock-in of the mutations or deletion of one copy of TINF2 resulted in excessive telomere elongation in clonal lines, indicating that TINF2 is haploinsufficient for telomere length control. In contrast, telomere protection and genome stability were maintained in all heterozygous clones. The data establish that the TINF2 truncations predispose to a tumor syndrome. We conclude that TINF2 acts as a haploinsufficient tumor suppressor that limits telomere length to ensure a timely Hayflick limit.

A single-molecule view of telomerase regulation at telomeres

Telomerase plays a key role in the immortalization of cancer cells by maintaining telomeres length. Using single-molecule imaging of telomerase RNA molecules in cancer cells, we recently reported novel insights into the role of Cajal bodies in telomerase biogenesis and the regulation of telomerase recruitment to telomeres.

Role of Telomeres and Telomeric Proteins in Human Malignancies and Their Therapeutic Potential

Telomeres are the ends of linear chromosomes comprised of repetitive nucleotide sequences in humans. Telomeres preserve chromosomal stability and genomic integrity. Telomere length shortens with every cell division in somatic cells, eventually resulting in replicative senescence once telomere length becomes critically short. Telomere shortening can be overcome by telomerase enzyme activity that is undetectable in somatic cells, while being active in germline cells, stem cells, and immune cells. Telomeres are bound by a shelterin complex that regulates telomere lengthening as well as protects them from being identified as DNA damage sites. Telomeres are transcribed by RNA polymerase II, and generate a long noncoding RNA called telomeric repeat-containing RNA (TERRA), which plays a key role in regulating subtelomeric gene expression. Replicative immortality and genome instability are hallmarks of cancer and to attain them cancer cells exploit telomere maintenance and telomere protection mechanisms. Thus, understanding the role of telomeres and their associated proteins in cancer initiation, progression and treatment is very important. The present review highlights the critical role of various telomeric components with recently established functions in cancer. Further, current strategies to target various telomeric components including human telomerase reverse transcriptase (hTERT) as a therapeutic approach in human malignancies are discussed.