Telomere length and telomerase complex mutations in pediatric acute myeloid leukemia

A telomere-related gene risk model for predicting prognosis and treatment response in acute myeloid leukemia

Acute myeloid leukemia (AML) is a prevalent hematological malignancy among adults. Recent studies suggest that the length of telomeres could significantly affect both the risk of developing AML and the overall survival (OS). Despite the limited focus on the prognostic value of telomere-related genes (TRGs) in AML, our study aims at addressing this gap by compiling a list of TRGs from TelNet, as well as collecting clinical information and TRGs expression data through the Gene Expression Omnibus (GEO) database. The GSE37642 dataset, sourced from GEO and based on the GPL96 platform, was divided into training and validation sets at a 6:4 ratio. Additionally, the GSE71014 dataset (based on the GPL10558 platform), GSE12417 dataset (based on the GPL96 and GPL570 platforms), and another portion of the GSE37642 dataset (based on the GPL570 platform) were designated as external testing sets. Univariate Cox regression analysis identified 96 TRGs significantly associated with OS. Subsequent Lasso-Cox stepwise regression analysis pinpointed eight TRGs (MCPH1, SLC25A6, STK19, PSAT1, KCTD15, DNMT3B, PSMD5, and TAF2) exhibiting robust predictive potential for patient survival. Both univariate and multivariate survival analyses unveiled TRG risk scores and age as independent prognostic variables. To refine the accuracy of survival prognosis, we developed both a nomogram integrating clinical parameters and a predictive risk score model based on TRGs. In subsequent investigations, associations were emphasized not solely regarding the TRG risk score and immune infiltration patterns but also concerning the response to immune-checkpoint inhibitor (ICI) therapy. In summary, the establishment of a telomere-associated genetic risk model offers a valuable tool for prognosticating AML outcomes, thereby facilitating informed treatment decisions.

Telomerase-targeted therapies in myeloid malignancies

Human telomeres are tandem arrays that are predominantly composed of 5'-TTAGGG-3' nucleotide sequences at the terminal ends of chromosomes. These sequences serve 2 primary functions: they preserve genomic integrity by protecting the ends of chromosomes, preventing inappropriate degradation by DNA repair mechanisms, and they prevent loss of genetic information during cellular division. When telomeres shorten to reach a critical length, termed the Hayflick limit, cell senescence or death is triggered. Telomerase is a key enzyme involved in synthesizing and maintaining the length of telomeres within rapidly dividing cells and is upregulated across nearly all malignant cells. Accordingly, targeting telomerase to inhibit uncontrolled cell growth has been an area of great interest for decades. In this review, we summarize telomere and telomerase biology because it relates to both physiologic and malignant cells. We discuss the development of telomere- and telomerase-targeted therapeutic candidates within the realm of myeloid malignancies. We overview all mechanisms of targeting telomerase that are currently in development, with a particular focus on imetelstat, an oligonucleotide with direct telomerase inhibitory properties that has advanced the furthest in clinical development and has demonstrated promising data in multiple myeloid malignancies.

Role of telomere length in human carcinogenesis (Review)

Cancer is considered the most important clinical, social and economic issue regarding cause‑specific disability‑adjusted life years among all human pathologies. Exogenous, endogenous and individual factors, including genetic predisposition, participate in cancer triggering. Telomeres are specific DNA structures positioned at the end of chromosomes and consist of repetitive nucleotide sequences, which, together with shelterin proteins, facilitate the maintenance of chromosome stability, while protecting them from genomic erosion. Even though the connection between telomere status and carcinogenesis has been identified, the absence of a universal or even a cancer‑specific trend renders consent even more complex. It is indicative that both short and long telomere lengths have been associated with a high risk of cancer incidence. When evaluating risk associations between cancer and telomere length, a disparity appears to emerge. Even though shorter telomeres have been adopted as a marker of poorer health status and an older biological age, longer telomeres due to increased cell growth potential are associated with the acquirement of cancer‑initiating somatic mutations. Therefore, the present review aimed to comprehensively present the multifaceted pattern of telomere length and cancer incidence association.

Telomere biology: from disorders to hematological diseases

Variations in the length of telomeres and pathogenic variants involved in telomere length maintenance have been correlated with several human diseases. Recent breakthroughs in telomere biology knowledge have contributed to the identification of illnesses named "telomeropathies" and revealed an association between telomere length and disease outcome. This review emphasizes the biology and physiology aspects of telomeres and describes prototype diseases in which telomeres are implicated in their pathophysiology. We also provide information on the role of telomeres in hematological diseases ranging from bone marrow failure syndromes to acute and chronic leukemias.

Imetelstat Induces Leukemia Stem Cell Death in Pediatric Acute Myeloid Leukemia Patient-Derived Xenografts

Acute myeloid leukemia (AML) in children remains deadly, despite the use of maximally intensive therapy. Because leukemia stem cells (LSCs) significantly contribute to chemoresistance and relapse, therapies that specifically target the LSCs are likely to be more beneficial in improving outcome. LSCs are known to have high telomerase activity and telomerase activity is negatively correlated with survival in pediatric AML. We evaluated the preclinical efficacy of imetelstat, an oligonucleotide inhibitor of telomerase activity in patient-derived xenograft (PDX) lines of pediatric AML. Imetelstat treatment significantly increased apoptosis/death of the LSC population in a dose-dependent manner in six pediatric AML PDX lines ex vivo, while it had limited activity on the stem cell population in normal bone marrow specimens. These results were validated in vivo in two distinct PDX models wherein imetelstat as single agent or in combination with chemotherapy greatly reduced the LSC percentage and prolonged median survival. Imetelstat combination with DNA hypomethylating agent azacitidine was also beneficial in extending survival. Secondary transplantation experiments showed delayed engraftment and improved survival of mice receiving imetelstat-treated cells, confirming the diminished LSC population. Thus, our data suggest that imetelstat represents an effective therapeutic strategy for pediatric AML.

Telomerase-based therapies in haematological malignancies

Telomeres are specialized genetic structures present at the end of all eukaryotic linear chromosomes. They progressively get shortened after each cell division due to end replication problems. Telomere shortening (TS) and chromosomal instability cause apoptosis and massive cell death. Following oncogene activation and inactivation of tumour suppressor genes, cells acquire mechanisms such as telomerase expression and alternative lengthening of telomeres to maintain telomere length (TL) and prevent initiation of cellular senescence or apoptosis. Significant TS, telomerase activation and alteration in expression of telomere-associated proteins are frequent features of different haematological malignancies that reflect on the progression, response to therapy and recurrence of these diseases. Telomerase is a ribonucleoprotein enzyme that has a pivotal role in maintaining the TL. However, telomerase activity in most somatic cells is insufficient to prevent TS. In 85-90% of tumour cells, the critically short telomeric length is maintained by telomerase activation. Thus, overexpression of telomerase in most tumour cells is a potential target for cancer therapy. In this review, alteration of telomeres, telomerase and telomere-associated proteins in different haematological malignancies and related telomerase-based therapies are discussed.

TERT genetic variability and telomere length as factors affecting survival and risk in acute myeloid leukaemia

Acute myeloid leukaemia (AML) is a neoplasm of immature myeloid cells characterized by various cytogenetic alterations. The present study showed that in addition to the FLT3-ITD and NPM1 mutation status, telomere length (TL) and telomerase reverse transcriptase (TERT) gene polymorphisms may affect risk and overall survival (OS) in AML. TL was longer in healthy controls than in AML patients and positively correlated with age in the patients, but not in healthy subjects. TL was found to be independently affected by the presence of the FLT3-ITD mutation. As for the TERT gene polymorphism, AML patients with the TERT rs2853669 CC genotype were characterized by significantly shorter OS than patients carrying the T allele. Another observation in our study is the difference in TL and OS in patients belonging to various risk stratification groups related to the FLT3-ITD and NPM1 mutation status. Patients with adverse risk classification (mutation in FLT3-ITD and lack of mutation in NPM1) presented with the shortest telomeres and significantly worse OS. In conclusion, OS of AML patients appears to be affected by TERT gene variability and TL in addition to other well-established factors such as age, WBC count, or FLT3-ITD and NPM1 mutation status.

Evaluation of pretreatment telomere length as a prognostic marker in intermediate-risk acute myeloid leukemia

INTRODUCTION

The current framework for risk stratification is still insufficient for highly heterogeneous intermediate-risk acute myeloid leukemia (IRC-AML), which lacks specific genomic abnormalities.

METHODS

In order to incorporate novel biomarkers to refine current risk stratification strategies for patients with this subtype, we investigated pretreatment telomere length (TL), which is essential for maintaining genomic stability, in 204 adults with de novo AML (non-acute promyelocytic leukemia).

RESULTS

We found that TL measured at diagnosis did not decrease with advancing age in 204 patients with AML (R²  = 0.001, P = .695). A multivariate analysis demonstrated that short TL was independently associated with an inferior relapse-free survival (hazard ratio [HR] 3.08, 95% confidence interval [CI] 1.48-6.41, P = .003); event-free survival (HR 2.14, 95% CI 1.12-4.08, P = .021); and overall survival (HR 2.26, 95% CI 1.09-4.67, P = .028) in IRC-AML patients. In addition, IRC-AML patients with short TL also exhibited an increased cumulative incidence of hematologic relapse (HR 2.32, 95% CI 1.08-5.26, P = .032).

CONCLUSION

Short TL is an independent prognostic factor for poor prognosis in patients with IRC-AML and may represent a novel mechanism that links genomic stability and disease progression.

The Dual Role of ROS in Hematological Malignancies: Stem Cell Protection and Cancer Cell Metastasis

BACKGROUND AND OBJECTIVE

Reactive oxygen species (ROS) play crucial role in hematopoiesis, regulation of differentiation, self-renewal, and the balance between quiescence and proliferation of hematopoietic stem cells (HSCs). The HSCs are a small population of undifferentiated cells that reside in the bone marrow (BM) and can undergo self-renewal by giving rise to mature cells.

METHODS

Relevant literature was identified through a PubMed search (2000-2019) of English-language papers using the following terms: reactive oxygen species, hematopoietic stem cell, leukemic stem cell, leukemia and chemotherapy.

RESULTS

HSCs are very sensitive to high levels of ROS and increased production of ROS have been attributed to HSC aging. HSC aging induced by both cell intrinsic and extrinsic factors is linked to impaired HSC self-renewal and regeneration. In addition, the elevated ROS levels might even trigger differentiation of Leukemic stem cells (LSCs) and ROS may be involved in the initiation and progression of hematological malignancies, such as leukemia.

CONCLUSION

Targeting genes involved in ROS in LSCs and HSCs are increasingly being used as a critical target for therapeutic interventions. Appropriate concentration of ROS may be an optimal therapeutic target for treatment of leukemia during chemotherapy, but still more studies are required to better understanding of the of ROS role in blood disorders.

Chromosomal Instability in Acute Myeloid Leukemia

Chromosomal instability (CIN), the increasing rate in which cells acquire new chromosomal alterations, is one of the hallmarks of cancer. Many studies highlighted CIN as an important mechanism in the origin, progression, and relapse of acute myeloid leukemia (AML). The ambivalent feature of CIN as a cancer-promoting or cancer-suppressing mechanism might explain the prognostic variability. The latter, however, is described in very few studies. This review highlights the important CIN mechanisms in AML, showing that CIN signatures can occur largely in all the three major AML types (de novo AML, secondary-AML, and therapy-related-AML). CIN features in AML could also be age-related and reflect the heterogeneity of the disease. Although most of these abnormalities show an adverse prognostic value, they also offer a strong new perspective on personalized therapy approaches, which goes beyond assessing CIN in vitro in patient tumor samples to predict prognosis. Current and emerging AML therapies are exploring CIN to improve AML treatment, which includes blocking CIN or increasing CIN beyond the limit threshold to induce cell death. We argue that the characterization of CIN features, not included yet in the routine diagnostic of AML patients, might provide a better stratification of patients and be extended to a more personalized therapeutic approach.

DNA Repair Expression Profiling to Identify High-Risk Cytogenetically Normal Acute Myeloid Leukemia and Define New Therapeutic Targets

Cytogenetically normal acute myeloid leukemias (CN-AML) represent about 50% of total adult AML. Despite the well-known prognosis role of gene mutations such as NPM1 mutations of FLT3 internal tandem duplication (FLT3-ITD), clinical outcomes remain heterogeneous in this subset of AML. Given the role of genomic instability in leukemogenesis, expression analysis of DNA repair genes might be relevant to sharpen prognosis evaluation in CN-AML. A publicly available gene expression profile dataset from two independent cohorts of patients with CN-AML were analyzed (GSE12417). We investigated the prognostic value of 175 genes involved in DNA repair. Among these genes, 23 were associated with a prognostic value. The prognostic information provided by these genes was summed in a DNA repair score, allowing to define a group of patients (n = 87; 53.7%) with poor median overall survival (OS) of 233 days (95% CI: 184-260). These results were confirmed in two validation cohorts. In multivariate Cox analysis, the DNA repair score, NPM1, and FLT3-ITD mutational status remained independent prognosis factors in CN-AML. Combining these parameters allowed the identification of three risk groups with different clinical outcomes in both training and validation cohorts. Combined with NPM1 and FLT3 mutational status, our GE-based DNA repair score might be used as a biomarker to predict outcomes for patients with CN-AML. DNA repair score has the potential to identify CN-AML patients whose tumor cells are dependent on specific DNA repair pathways to design new therapeutic avenues.

Short-term treatment with imetelstat sensitizes hematopoietic malignant cells to a genotoxic agent via suppression of the telomerase-mediated DNA repair process

Imetelstat is a specific and competitive inhibitor of telomerase enzymatic activity. We demonstrated that imetelstat could interfere with the DNA repair process and enhance the effect of DNA damaging agents using hematological tumor cell lines. Short-term administration of imetelstat enhanced growth suppression by anticancer agents and radiation. It also upregulated γH2AX expression induced by irradiation. Immunofluorescence staining showed that both human telomerase reverse transcriptase (hTERT) and γH2AX were upregulated and co-localized in the nucleus of peripheral blood mononuclear cells after irradiation, suggesting that hTERT was involved in the DNA-DSB repair process. Imetelstat enhanced growth inhibitory effect of cytotoxic agents in short-term culture without shortening of telomeres, indicating that this effect was attributed by telomere length independent mechanism. Our results suggest that the combination of short-term treatment with imetelstat and cytotoxic agent is a promising strategy to treat a wide variety of hematopoietic malignancies.

Small-Molecule PAPD5 Inhibitors Restore Telomerase Activity in Patient Stem Cells

Genetic lesions that reduce telomerase activity inhibit stem cell replication and cause a range of incurable diseases, including dyskeratosis congenita (DC) and pulmonary fibrosis (PF). Modalities to restore telomerase in stem cells throughout the body remain unclear. Here, we describe small-molecule PAPD5 inhibitors that demonstrate telomere restoration in vitro, in stem cell models, and in vivo. PAPD5 is a non-canonical polymerase that oligoadenylates and destabilizes telomerase RNA component (TERC). We identified BCH001, a specific PAPD5 inhibitor that restored telomerase activity and telomere length in DC patient induced pluripotent stem cells. When human blood stem cells engineered to carry DC-causing PARN mutations were xenotransplanted into immunodeficient mice, oral treatment with a repurposed PAPD5 inhibitor, the dihydroquinolizinone RG7834, rescued TERC 3' end maturation and telomere length. These findings pave the way for developing systemic telomere therapeutics to counteract stem cell exhaustion in DC, PF, and possibly other aging-related diseases.

Telomerase reverse transcriptase gene amplification in hematological malignancies

Background

Telomere is a complex DNA–protein structure located at the end of all eukaryotic chromosomes. The major role of human telomerase is to catalyze the addition of telomeric repeat sequences TTAGGG onto chromosome ends for stabilization of telomere length in attaining cellular immortality and may therefore be a critical step in carcinogenesis. Expression of significant levels of telomerase can dramatically increase proliferative life span and promote cellular immortality, thereby contributing to the malignant phenotype. The purpose of this study is to investigate telomerase reverse transcriptase (TERT) gene amplification in hematological neoplasms, e.g., multiple myeloma (MM), B-non-Hodgkin lymphoma (B-NHL), and acute myeloid leukemia (AML), using FISH technique and to evaluate its potential use as a prognostic marker.

Results

TERT amplification was detected in all groups of the participant patients (15 MM, 15 B-NHL, and 15 AML patients), with higher incidence in AML patients (53.3%). A significant association between the pattern of presentation and telomerase amplification was detected in 88.9% of the relapsed patients who demonstrated amplification of TERT. TERT amplification shows a significant association with p53 deletion and a highly significant association with poor prognosis.

Conclusions

TERT gene amplification is significantly associated with hematological malignancies and may play a critical role in carcinogenesis; thus, elucidation of their regulatory mechanism is highly demanding. Higher amplification was found in relapsed cases than de novo cases which highlight its potential implication in clinical analysis and disease monitoring. Moreover, our results suggest the future use of TERT gene as a potential prognostic marker that may aid in treatment decision and chemotherapy.

Telomere Maintenance in Pediatric Cancer

Telomere length has been proposed as a biomarker of biological age and a risk factor for age-related diseases and cancer. Substantial progress has been made in recent decades in understanding the complex molecular relationships in this research field. However, the majority of telomere studies have been conducted in adults. The data on telomere dynamics in pediatric cancers is limited, and interpretation can be challenging, especially in cases where results are contrasting to those in adult entities. This review describes recent advances in the molecular characterization of structure and function of telomeres, regulation of telomerase activity in cancer pathogenesis in general, and highlights the key advances that have expanded our views on telomere biology in pediatric cancer, with special emphasis on the central role of telomere maintenance in neuroblastoma. Furthermore, open questions in the field of telomere maintenance research are discussed in the context of recently published literature.

Beyond JAK-2: potential targets for myeloproliferative neoplasm therapy

INTRODUCTION

The outlook for patients with myeloproliferative neoplasms, particularly myelofibrosis, has improved in recent years, with greater understanding of the pathogenesis and the subsequent development of a plethora of new agents. Areas covered: This article will discuss some of the advances in the field in recent years and explore in greater detail some of the most advanced emerging agents as well as those with greatest potential. An extensive literature review has been performed to identify recent clinical trials and any relevant pre-clinical work. Expert commentary: Important discoveries regarding molecular pathogenesis have led to advances in diagnostic algorithms, prognosis and ultimately also treatment strategies. However, the therapeutic armamentarium for MPN is still largely inadequate to cope with significant challenges including normalization of life span, reduction of cardiovascular complications, prevention of hematological progression and improved quality of life. Sadly, no currently available drugs have shown clear evidence of disease-modifying activity and results of early phase I and II clinical trials have been quite disappointing to date, with toxicities sometimes limiting and a lack of meaningful biological surrogate end points.

Immortalization of human AE pre-leukemia cells by hTERT allows leukemic transformation

Human CD34+ hematopoietic stem and progenitor cells (HSPC) expressing fusion protein AML1-ETO (AE), generated by the t(8;21)(q22;q22) rearrangement, manifest enhanced self-renewal and dysregulated differentiation without leukemic transformation, representing a pre-leukemia stage. Enabling replicative immortalization via telomerase reactivation is a crucial step in cancer development. However, AE expression alone is not sufficient to maintain high telomerase activity to immortalize human HSPC cells, which may hamper transformation. Here, we investigated the cooperativity of telomerase reverse transcriptase (hTERT), the catalytic subunit of telomerase, and AE in disease progression. Enforced expression of hTERT immortalized human AE pre-leukemia cells in a telomere-lengthening independent manner, and improved the pre-leukemia stem cell function by enhancing cell proliferation and survival. AE-hTERT cells retained cytokine dependency and multi-lineage differentiation potential similar to parental AE clones. Over the short-term, AE-hTERT cells did not show features of stepwise transformation, with no leukemogenecity evident upon initial injection into immunodeficient mice. Strikingly, after extended culture, we observed full transformation of one AE-hTERT clone, which recapitulated the disease evolution process in patients and emphasizes the importance of acquiring cooperating mutations in t(8;21) AML leukemogenesis. In summary, achieving unlimited proliferative potential via hTERT activation, and thereby allowing for acquisition of additional mutations, is a critical link for transition from pre-leukemia to overt disease in human cells. AE-hTERT cells represent a tractable model to study cooperating genetic lesions important for t(8;21) AML disease progression.

Shorter Remission Telomere Length Predicts Delayed Neutrophil Recovery After Acute Myeloid Leukemia Therapy: A Report From the Children's Oncology Group

Purpose Suboptimal outcomes for children with acute myeloid leukemia (AML) necessitate maximally intensive therapy. Consequently, serious adverse events, such as prolonged periods of profound myelosuppression, contribute to AML treatment-related mortality. Telomeres, the repetitive DNA-protein structures at chromosome ends, influence cellular replicative capacity in that critically short telomeres can induce cell senescence or apoptosis. Our objective was to evaluate the impact of telomere length on duration of post-therapy neutropenia in a pediatric AML cohort. Patients and Methods Patients were diagnosed with de novo AML, enrolled in Children's Oncology Group study AAML0531, and included those with (n = 53) and without (n = 62) significantly delayed neutrophil recovery after chemotherapy. We used quantitative polymerase chain reaction to measure telomere content (TC), a validated proxy for telomere length, from remission bone marrow samples obtained after the second induction chemotherapy course. Results Less TC was significantly associated with prolonged neutropenia after the fourth ( P < .001) and fifth chemotherapy courses ( P = .002). Cox regression adjusting for age at diagnosis confirmed that TC remained independently predictive of time to recovery of absolute neutrophil count for both the fourth and fifth courses ( P = .002 and .009, respectively). DNA from patients was analyzed for germline mutations in four telomere maintenance genes associated with telomere biology disorders. Sequence analysis revealed no enrichment of rare or novel variants in the delayed recovery group. Conclusion Our results suggest that TC at end of AML induction is associated with hematopoietic reconstitution capacity independently of age and may identify those at highest risk for markedly delayed bone marrow recovery after AML therapy.

Maintenance of telomere length in AML

The importance of telomere length to human health, aging, and cancer continues to be underappreciated. This review examines some basics of telomere biology and relates how telomere function, telomerase activity, and mutations in TERC or TERT are involved in bone marrow failure, leukemias, and other cancers. Given the challenge to obtain accurate data on telomerase activity and telomere length in specific cell types, the situation in acute myeloid leukemia (AML) remains puzzling. In most cancers, telomerase levels are increased after cells have encountered a "telomere crisis," which is typically associated with poor prognosis. Cells emerging from "telomere crisis" have defective DNA damage responses, resulting, for example, from loss of p53. Such cells often express elevated telomerase levels as a result of point mutations in the TERT promoter or amplification of the TERT gene. While telomeres in AML blasts are typically shorter than expected for normal leukocytes, most AML cells do not show evidence of having gone through a "telomere crisis." In chronic myeloid leukemia (CML), the difference between the telomere length in nonmalignant T cells and malignant blasts from the same patient was found to correlate with the remaining duration of the chronic phase. This observation supports that a mitotic clock is ticking in CML stem cells and that disease progression in CML heralds the onset of a "telomere crisis." The presence of very short telomeres in tumor cells was found to predict disease progression in chronic lymphocytic leukemia, myeloma, and various solid tumors. In view of these findings longitudinal studies of telomere length in AML appear worthwhile.

Telomerase in hematologic malignancies

PURPOSE OF REVIEW

The activation of telomere maintenance pathways has long been regarded as a key hallmark of cancer and this has propelled the development of novel inhibitors of telomerase. In this review, we detail the background biology on telomere maintenance in health and disease, then concentrate on the recent preclinical and clinical development behind targeting telomerase in blood cancers.

RECENT FINDINGS

Preclinical and clinical studies have shown that imetelstat, a competitive inhibitor of telomerase, has activity in certain hematologic malignancies, in particular the myeloproliferative neoplasms and acute myeloid leukemia.

SUMMARY

Telomerase inhibition has shown remarkable efficacy in myeloid malignancies, and current and future preclinical and clinical studies are necessary to comprehensively investigate its underlying mechanism of action. Future work should identify the potential genetic susceptibilities to telomerase inhibition therapy, and evaluate rational combinations of telomerase inhibitors with chemotherapy and other novel agents. Robust preclinical evaluation is essential to best translate these new agents successfully into our clinical treatment algorithm for myeloid and other blood cancers.

Biological and clinical implications of telomere dysfunction in myeloid malignancies

Telomeres at the ends of linear chromosomes protect the genome. Telomeres shorten with each round of cell division, placing a finite limit on cell growth. Telomere attrition is associated with cell senescence and apoptosis. Telomerase, a specialized ribonucleoprotein complex, maintains telomeres homeostasis through repeat addition of telomere sequences to the 3' telomeric overhang. Telomere biology is closely related to cancer and normal aging. Upregulation of telomerase or activation of the alternative pathway of telomere lengthening is a hallmark of cancer cells, making telomerase an attractive target for cancer therapeutics. In this review, we will discuss telomere biology and the prognostic implications of telomere length in acute myeloid leukemia, and review exciting new investigational approaches using telomerase inhibitors in acute myeloid leukemia and other myeloid malignancies.

Telomere length is an independent prognostic marker in MDS but not in de novo AML

Telomere dysfunction is implicated in the generation of large-scale genomic rearrangements that drive progression to malignancy. In this study we used high-resolution single telomere length analysis (STELA) to examine the potential role of telomere dysfunction in 80 myelodysplastic syndrome (MDS) and 95 de novo acute myeloid leukaemia (AML) patients. Despite the MDS cohort being older, they had significantly longer telomeres than the AML cohort (P < 0·0001) where telomere length was also significantly shorter in younger AML patients (age <60 years) (P = 0·02) and in FLT3 internal tandem duplication-mutated AML patients (P = 0·03). Using a previously determined telomere length threshold for telomere dysfunction (3·81 kb) did not provide prognostic resolution in AML [Hazard ratio (HR) = 0·68, P = 0·2]. In contrast, the same length threshold was highly prognostic for overall survival in the MDS cohort (HR = 5·0, P < 0·0001). Furthermore, this telomere length threshold was an independent parameter in multivariate analysis when adjusted for age, gender, cytogenetic risk group, number of cytopenias and International Prognostic Scoring System (IPSS) score (HR = 2·27, P < 0·0001). Therefore, telomere length should be assessed in a larger prospective study to confirm its prognostic role in MDS with a view to integrating this variable into a revised IPSS.

Telomerase and telomere biology in hematological diseases: A new therapeutic target

Telomeres are structures confined at the ends of eukaryotic chromosomes. With each cell division, telomeric repeats are lost because DNA polymerases are incapable to fully duplicate the very ends of linear chromosomes. Loss of repeats causes cell senescence, and apoptosis. Telomerase neutralizes loss of telomeric sequences by adding telomere repeats at the 3' telomeric overhang. Telomere biology is frequently associated with human cancer and dysfunctional telomeres have been proved to participate to genetic instability. This review covers the information on telomerase expression and genetic alterations in the most relevant types of hematological diseases. Telomere erosion hampers the capability of hematopoietic stem cells to effectively replicate, clinically resulting in bone marrow failure. Furthermore, telomerase mutations are genetic risk factors for the occurrence of some hematologic cancers. New discoveries in telomere structure and telomerase functions have led to an increasing interest in targeting telomeres and telomerase in anti-cancer therapy.

Genomic instability is a principle pathologic feature of FLT3 ITD kinase activity in acute myeloid leukemia leading to clonal evolution and disease progression

Acute Myeloid Leukemia with FLT3 ITD mutations are associated with a poor prognosis characterized by a higher relapse rate, shorter relapse free survival, and decreased likelihood of response to therapy at relapse. FLT3 ITD signaling drives cell proliferation and survival. FLT3 ITD AML disease progression is associated with cytogenetic evolution and acquired tyrosine kinase inhibitor (TKI) resistance suggesting a potential role of genomic instability. There is growing evidence demonstrating a relationship between FLT3 signaling and increased DNA damage, specifically through increased reactive oxygen species (ROS) resulting in double-strand breaks (DSB), as well as impaired DNA repair, involving deficiencies in the non-homologous end joining (NHEJ), alternative non-homologous end joining (ALT NHEJ) and homologous recombination (HR) pathways. The role of genomic instability in the pathogenesis of FLT3 ITD AML warrants further examination as it offers potential therapeutic targets.

Telomerase Activation in Hematological Malignancies

Telomerase expression and telomere maintenance are critical for cell proliferation and survival, and they play important roles in development and cancer, including hematological malignancies. Transcriptional regulation of the rate-limiting subunit of human telomerase reverse transcriptase gen (hTERT) is a complex process, and unveiling the mechanisms behind its reactivation is an important step for the development of diagnostic and therapeutic applications. Here, we review the main mechanisms of telomerase activation and the associated hematologic malignancies.

Short telomere length and its correlation with gene mutations in myelodysplastic syndrome

Background

Telomere erosion can lead to genomic instability and cancer progression. It has been suggested that the shortest telomere, not the average telomere length (TL), is critical for cell viability. Some studies have shown shorter TL in myelodysplastic syndrome (MDS) patients but the critically short telomeres, the variability of TL within individual patient has not been evaluated. Thus, we aimed to investigate the TL of MDS patients and assessed the association of TL with recurrent genetic mutations in MDS.

Methods

We measured the TL of bone marrow nucleated cells for diagnostic samples at a single-cell level by quantitative fluorescence in situ hybridization (Q-FISH) for 58 MDS patients and analyzed the minimum, median, average, standard deviation, average of the 0th to 10th percentile TL within a patient, and the proportion of cells with TL that is shorter than the lowest 10th percentile of the normal control (NC). The correlations of TL to clinical parameters, cytogenetic results, and genetic mutations were assessed.

Results

MDS patients showed eroded telomeres and narrow distribution compared to the NC (P < 0.001, P = 0.018, respectively). Patients with mutation showed significantly lesser cells with short TL, below the lowest 10th percentile of the NC (P = 0.017), but no differences in TL were found according to mutations/cytogenetic abnormalities except for CSF3R mutation. However, those patients with a high percentage (≥80 %) of cells with short TL showed poorer overall survival (P = 0.021), and this was an independent prognostic factor, along with TP53, U2AF1 mutation, and high BM blast count (P = 0.044, 0.001, 0.004, 0.012, respectively).

Conclusions

The shortest TL, which determines the fate of the cell, was significantly shorter, and higher burden of cells with short TL were found in MDS, which correlated with poor survival, suggesting the need to measure TL in single cells by Q-FISH.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-016-0287-9) contains supplementary material, which is available to authorized users.

Impact of telomere length on survival in classic and variant hairy cell leukemia

Telomeres, which protect the ends of chromosomes, are shortened in several hematologic malignancies, often with adverse prognostic implications, but their effect on prognosis of classic and variant hairy cell leukemia (HCL and HCLv) has not been reported. HCL/HCLv genomic DNA from 46 patients was studied by PCR to determine the ratio of telomere to single copy gene number (T/S). T/S was unrelated to diagnosis of HCL or HCLv (p=0.27), but shorter T/S was associated with unmutated immunoglobulin rearrangements (p=0.033) and age above the median at diagnosis (p=0.017). Low T/S was associated with shorter overall survival from diagnosis (OS), particularly T/S <0.655 (p=0.0064, adjusted p=0.019). Shorter OS was also associated with presence of unmutated (p<0.0001) or IGHV4-34+ (p<0.0001) rearrangements, or increasing age (p=0.0002). Multivariable analysis with Cox modeling showed that short T/S along with either unmutated or IGHV4-34+ rearrangements remained associated with reduced OS (p=0.0071, p=0.0024, respectively) after age adjustment. While T/S is relatively long in HCL and the disease usually indolent with excellent survival, shortened telomeres in HCL/HCLv are associated with decreased survival. Shortened T/S could represent a risk factor needing further investigation/intervention to determine if non-chemotherapy treatment options, in addition to or instead of chemotherapy, might be particularly useful.

Association between TERT promoter polymorphisms and acute myeloid leukemia risk and prognosis

Telomerase reverse transcriptase gene (TERT) promoter mutations are identified in many malignancies but not in hematological malignancies. Here we analyzed TERT and protection of telomeres 1 gene (POT1) mutations, and four different TERT SNVs in 226 acute myeloid leukemia (AML) patients and 806 healthy individuals in a case referent design, where also overall survival was assessed. A significant association for increased risk of AML was found for TERT SNVs, rs2853669 (OR = 2.45, p = 0.00015) and rs2736100 (OR = 1.5, p = 0.03). The overall survival for patients with CC genotype of rs2853669 was significantly shorter compared to those with TT or TC genotypes (p = 0.036 and 0.029 respectively). The influence of TERT rs2853669 CC on survival was confirmed in multivariable Cox regression analysis as an independent risk biomarker in addition to high risk group, higher age and treatment. No hot spot TERT promoter mutations at -228C > T or -250C > T or POT1 mutations could be identified in this AML cohort. We show that rs2853669 CC may be a risk factor for the development of AML that may also be used as a prognostic marker to identify high risk normal karyotype-AML (NK-AML) patients, for treatment guidance.

Telomerase inhibition effectively targets mouse and human AML stem cells and delays relapse following chemotherapy

Acute myeloid leukemia (AML) is an aggressive and lethal blood cancer maintained by rare populations of leukemia stem cells (LSCs). Selective targeting of LSCs is a promising approach for treating AML and preventing relapse following chemotherapy, and developing such therapeutic modalities is a key priority. Here, we show that targeting telomerase activity eradicates AML LSCs. Genetic deletion of the telomerase subunit Terc in a retroviral mouse AML model induces cell-cycle arrest and apoptosis of LSCs, and depletion of telomerase-deficient LSCs is partially rescued by p53 knockdown. Murine Terc(-/-) LSCs express a specific gene expression signature that can be identified in human AML patient cohorts and is positively correlated with patient survival following chemotherapy. In xenografts of primary human AML, genetic or pharmacological inhibition of telomerase targets LSCs, impairs leukemia progression, and delays relapse following chemotherapy. Altogether, these results establish telomerase inhibition as an effective strategy for eliminating AML LSCs.

Telomerase inhibition effectively targets mouse and human AML stem cells and delays relapse following chemotherapy

Acute myeloid leukemia (AML) is an aggressive and lethal blood cancer maintained by rare populations of leukemia stem cells (LSCs). Selective targeting of LSCs is a promising approach for treating AML and preventing relapse following chemotherapy, and developing such therapeutic modalities is a key priority. Here, we show that targeting telomerase activity eradicates AML LSCs. Genetic deletion of the telomerase subunit Terc in a retroviral mouse AML model induces cell-cycle arrest and apoptosis of LSCs, and depletion of telomerase-deficient LSCs is partially rescued by p53 knockdown. Murine Terc(-/-) LSCs express a specific gene expression signature that can be identified in human AML patient cohorts and is positively correlated with patient survival following chemotherapy. In xenografts of primary human AML, genetic or pharmacological inhibition of telomerase targets LSCs, impairs leukemia progression, and delays relapse following chemotherapy. Altogether, these results establish telomerase inhibition as an effective strategy for eliminating AML LSCs.

Telomerase reverse transcriptase (TERT) A1062T mutation as a prognostic factor in Egyptian patients with acute myeloid leukemia (AML)

This study aimed to evaluate the incidence and clinical and prognostic impact of TERT A1062T mutation in AML patients treated at Mansoura Oncology Center. Screening for TERT A1062T mutation in exon 15 of the TERT gene was performed on diagnostic DNA samples from 153 AML patients and 197 healthy subjects as a control group by using sequence-specific primers. TERT A1062T mutation was detected in 18 cases out of 153 patients (11.8 %) and in one out of 197 control group subjects (0.51 %). The achievement of complete remission was significantly higher in AML group with wild type as compared to that in the mutant one (53.3 vs 16.7 %, respectively). In addition, the relapse rate was significantly higher in the mutant patients as compared to those with wild type (62.5 vs 28.2 %, respectively). The AML patients with TERT (A1062T) mutation had shorter overall survival than patients with wild type (P = 0.001). In a multivariable analysis, TERT (A1062T) mutational status is independently worse predictor factor (P = 0.007) when controlling for cytogenetic status (P = <0.001), performance status (P = <0.001) and bone marrow blast cells (P = 0.001). In conclusion, TERT A1062T mutation is an independent negative prognostic factor in AML patients. Therefore, molecular testing for TERT A1062T mutation in patients with AML is recommended in order to delineate their prognostic status.