Targeting Telomeres and Telomerase: Studies in Aging and Disease Utilizing CRISPR/Cas9 Technology

Comprehensive analyses of the cancer-associated fibroblast subtypes and their score system for prediction of outcomes and immunosuppressive microenvironment in prostate cancer

BACKGROUND

Cancer-associated fibroblasts (CAFs) drive cancer progression and treatment failure on one hand, while their tumor-restraining functions are also observed on the other. Recent single cell RNA sequencing (scRNA-seq) analyses demonstrates heterogeneity of CAFs and defines molecular subtypes of CAFs, which help explain their different functions. However, it remains unclear whether these CAF subtypes have the same or different biological/clinical implications in prostate cancer (PCa) or other malignancies.

METHODS

PCa cells were incubated with supernatant from normal fibroblasts and CAFs to assess their effects on cell behaviors. Sequencing, genomic, and clinical data were collected from TCGA, MSKCC, CPGEA and GEO databases. CAF molecular subtypes and total CAF scores were constructed and grouped into low and high groups based on CAF-specific gene expression. Progression free interval (PFI), clinicopathological features, telomere length, immune cell infiltration, drug treatment and somatic mutations were compared among CAF molecular subtypes and low/high score groups.

RESULTS

The PCa CAF-derived supernatant promoted PCa cell proliferation and invasion. Based on differentially expressed genes identified by scRNA-seq analyses, we classified CAFs into 6 molecular subtypes in PCa tumors, and each subtype was then categorized into score-high and low groups according to the subtype-specific gene expression level. Such score models in 6 CAF subtypes all predicted PFI. Telomeres were significantly shorter in high-score tumors. The total CAF score from 6 CAF subtypes was also associated with PFI in PCa patients inversely, which was consistent with results from cellular experiments. Immunosuppressive microenvironment occurred more frequently in tumors with a high CAF score, which was characterized by increased CTLA4 expression and indicated better responses to CTLA4 inhibitors. Moreover, this model can also serve as a useful PFI predictor in pan-cancers.

CONCLUSION

By combining scRNA-seq and bulk RNA-seq data analyses, we develop a CAF subtype score system as a prognostic factor for PCa and other cancer types. This model system also helps distinguish different immune-suppressive mechanisms in PCa, suggesting its implications in predicting response to immunotherapy. Thus, the present findings should contribute to personalized PCa intervention.

Recent advances in optical biosensing and imaging of telomerase activity and relevant signal amplification strategies

Telomerase as a new valuable biomarker for early diagnosis and prognosis evaluation of cancer has attracted much interest in the field of biosensors, cell imaging, and drug screening. In this review, we mainly focus on different optical techniques and various signal amplification strategies for telomerase activity determination. Fluorometric, colorimetry, chemiluminescence, surface-enhanced Raman scattering (SERS), and dual-mode techniques for telomerase sensing and imaging are summarized. Signal amplification strategies include two categories: one is nucleic acid-based amplification, such as rolling circle amplification (RCA), the hybridization chain reaction (HCR), and catalytic hairpin assembly (CHA); the other is nanomaterial-assisted amplification, including metal nanoclusters, quantum dots, transition metal compounds, graphene oxide, and DNA nanomaterials. Challenges and prospects are also discussed to provide new insights for future development of multifunctional strategies and techniques for in situ and in vivo analysis of biomarkers for accurate cancer diagnosis.

Combining old and new concepts in targeting telomerase for cancer therapy: transient, immediate, complete and combinatory attack (TICCA)

Telomerase can overcome replicative senescence by elongation of telomeres but is also a specific element in most cancer cells. It is expressed more vastly than any other tumor marker. Telomerase as a tumor target inducing replicative immortality can be overcome by only one other mechanism: alternative lengthening of telomeres (ALT). This limits the probability to develop resistance to treatments. Moreover, telomerase inhibition offers some degree of specificity with a low risk of toxicity in normal cells. Nevertheless, only one telomerase antagonist reached late preclinical studies. The underlying causes, the pitfalls of telomerase-based therapies, and future chances based on recent technical advancements are summarized in this review. Based on new findings and approaches, we propose a concept how long-term survival in telomerase-based cancer therapies can be significantly improved: the TICCA (Transient Immediate Complete and Combinatory Attack) strategy.

Differential DNA Methylation of THOR and hTAPAS in the Regulation of hTERT and the Diagnosis of Cancer

Simple Summary

Because of its high prevalence of >45% in 9 out of 11 (82%) cancer types screened, THOR hypermethylation has been suggested to be a frequent telomerase-activating mechanism in hTERT-expressing tumor types, e.g., in cancers of the prostate, breast, blood, colon, lung, bladder, and brain. In this prime example, we present detailed DNA methylation profiles in urothelial cancer that reveal the exact positions of the most differentially methylated CpG dinucleotides within the THOR region in order to design an efficient Methylation-Specific PCR (MSPCR) approach for diagnostic and prognostic purposes. Furthermore, our data suggest an epigenetic mechanism regulating hTERT expression through the methylation status of THOR and lncRNA hTAPAS.

Abstract

Background: Although DNA methylation in the gene promoters usually represses gene expression, the TERT hypermethylated oncological region (THOR) located 5′ of the hTERT gene is hypermethylated when hTERT is expressed in diverse cancer types, including urothelial cancer (UC). Methods: Comprehensive MeDIP and DNA methylation array analyses complemented by the technically independent method of bisulfite genomic sequencing were applied on pathologically reviewed and classified urothelial carcinoma specimens and healthy urothelial tissue samples to reveal the methylation status of THOR in detail. Results: The detailed DNA methylation profiles reveal the exact positions of differentially methylated CpG dinucleotides within THOR in urothelial cancer and provide evidence ofa diverging role of methylation of these CpGs in the regulation of hTERT. In particular, our data suggest a regulating mechanism in which THOR methylation acts on hTERT expression through epigenetic silencing of the lncRNA hTERT antisense promoter-associated (hTAPAS), which represses hTERT. Conclusions: These findings precisely define the most differentially methylated CpGs of THOR in early urothelial cancer, enabling optimal design of Methylation-Specific PCR (MSPCR) primers to reliably probe these methylation differences for diagnostic and prognostic purposes. In addition, this strategy presents a prime example that is also applicable to many other malignancies. Finally, the first evidence for the underlying epigenetic mechanism regulating hTERT expression through the methylation status of THOR is provided.

CRISPR/Cas: A New Tool in the Research of Telomeres and Telomerase as Well as a Novel Form of Cancer Therapy

Due to their close connection with senescence, aging, and disease, telomeres and telomerase provide a unique and vital research route for boosting longevity and health span. Despite significant advances during the last three decades, earlier studies into these two biological players were impeded by the difficulty of achieving real-time changes inside living cells. As a result of the clustered regularly interspaced short palindromic repeats (CRISPR)-associated system’s (Cas) method, targeted genetic studies are now underway to change telomerase, the genes that govern it as well as telomeres. This review will discuss studies that have utilized CRISPR-related technologies to target and modify genes relevant to telomeres and telomerase as well as to develop targeted anti-cancer therapies. These studies greatly improve our knowledge and understanding of cellular and molecular mechanisms that underlie cancer development and aging.

Transcriptional Activation of Heterochromatin by Recruitment of dCas9 Activators

The transition from silenced heterochromatin to a biologically active state and vice versa is a fundamental part of the implementation of cell type-specific gene expression programs. To reveal structure-function relationships and dissect the underlying mechanisms, experiments that ectopically induce transcription are highly informative. In particular, the approach to perturb chromatin states by recruiting fusions of the catalytically inactive dCas9 protein in a sequence-specific manner to a locus of interest has been used in numerous applications. Here, we describe how this approach can be applied to activate pericentric heterochromatin (PCH) in mouse cells as a prototypic silenced state by providing protocols for the following workflow: (a) Recruitment of dCas9 fusion constructs with the strong transcriptional activator VPR to PCH. (b) Analysis of the resulting changes in chromatin compaction, epigenetic marks, and active transcription by fluorescence microscopy-based readouts. (c) Automated analysis of the resulting images with a set of scripts in the R programming language. Furthermore, we discuss how parameters for chromatin decondensation and active transcription are extracted from these experiments and can be combined with other readouts to gain insights into PCH activation.

Seeding drug discovery: Telomeric tankyrase as a pharmacological target for the pathophysiology of high-altitude hypoxia

Cellular exposure to extreme environments leads to the expression of multiple proteins that participate in pathophysiological manifestations. Hypobaric hypoxia at high altitude (HA) generates reactive oxygen species (ROS) that can damage telomeres. Tankyrase (TNKS) belongs to multiple telomeric protein complexes and is actively involved in DNA damage repair. Although published research on TNKS indicates its possible role in cancer and other hypoxic diseases, its role in HA sicknesses remains elusive. Understanding the roles of telomeres, telomerase, and TNKS could ameliorate physiological issues experienced at HA. In addition, telomeric TNKS could be a potential biomarker in hypoxia-induced sicknesses or acclimatization. Thus, a new research avenue on TNKS linked to HA sickness might lead to the discovery of drugs for hypobaric hypoxia.

Folic Acid Inhibits Aging-Induced Telomere Attrition and Apoptosis in Astrocytes In Vivo and In Vitro

Folic acid (FA) has been reported to inhibit astrocyte apoptosis and improve aging-induced disorders; however, its role in telomere attrition remains unclear. In present study, 4-month-old senescence-accelerated mouse prone 8 (SAMP8) mice were assigned to four treatment groups for the in vivo experiment: FA-deficient diet (FA-D) group, FA-normal diet (FA-N) group, low FA-supplemented diet (FA-L) group, and high FA-supplemented diet (FA-H) group. These mice were euthanized when 10 months old. There was also a young SAMP8 (4 months old) control group (Con-Y) fed with FA-normal diet. In in vitro study, primary cultures of astrocytes from hippocampus and cerebral cortex were incubated for five generations with various concentrations of FA (0-40 μM) and were assigned to five groups: FA 0 μM (generation 5), FA 10 μM (generation 5), FA 20 μM (generation 5), FA 40 μM (generation 5), and FA 10 μM (generation 1). The results showed that FA supplementation inhibited aging-induced astrocytosis, astrocyte apoptosis, neurodegeneration, and prevented telomere attrition in hippocampus and cortex of SAMP8 mice. FA supplementation also decreased apoptosis and telomere attrition, and increased telomerase activity, in primary cultures of astrocytes. These results showed that it may be one of the mechanisms that FA inhibiting aging-induced apoptosis of astrocyte by alleviating telomere attrition.

Alternative Splicing of Human Telomerase Reverse Transcriptase (hTERT) and Its Implications in Physiological and Pathological Processes

Alternative splicing (AS) of human telomerase catalytic subunit (hTERT, human telomerase reverse transcriptase) pre-mRNA strongly regulates telomerase activity. Several proteins can regulate AS in a cell type-specific manner and determine the functions of cells. In addition to being involved in telomerase activity regulation, AS provides cells with different splice variants that may have alternative biological activities. The modulation of telomerase activity through the induction of hTERT AS is involved in the development of different cancer types and embryos, and the differentiation of stem cells. Regulatory T cells may suppress the proliferation of target human and murine T and B lymphocytes and NK cells in a contact-independent manner involving activation of TERT AS. This review focuses on the mechanism of regulation of hTERT pre-mRNA AS and the involvement of splice variants in physiological and pathological processes.

The role of telomerase and viruses interaction in cancer development, and telomerase-dependent therapeutic approaches

Human telomerase reverse transcriptase (hTERT) is an enzyme that is critically involved in elongating and maintaining telomeres length to control cell life span and replicative potential. Telomerase activity is continuously expressed in human germ-line cells and most cancer cells, whereas it is suppressed in most somatic cells. In normal cells, by reducing telomerase activity and progressively shortening the telomeres, the cells progress to the senescence or apoptosis process. However, in cancer cells, telomere lengths remain constant due to telomerase's reactivation, and cells continue to proliferate and inhibit apoptosis, and ultimately lead to cancer development and human death due to metastasis. Studies demonstrated that several DNA and RNA oncoviruses could interact with telomerase by integrating their genome sequence within the host cell telomeres specifically. Through the activation of the hTERT promoter and lengthening the telomere, these cells contributes to cancer development. Since oncoviruses can activate telomerase and increase hTERT expression, there are several therapeutic strategies based on targeting the telomerase of cancer cells like telomerase-targeted peptide vaccines, hTERT-targeting dendritic cells (DCs), hTERT-targeting gene therapy, and hTERT-targeting CRISPR/Cas9 system that can overcome tumor-mediated toleration mechanisms and specifically apoptosis in cancer cells. This study reviews available data on the molecular structure of telomerase and the role of oncoviruses and telomerase interaction in cancer development and telomerase-dependent therapeutic approaches to conquest the cancer cells.

TERT promoter mutations and GABP transcription factors in carcinogenesis: More foes than friends

The transcriptional de-repression of the telomerase reverse transcriptase (TERT) gene and subsequent activation of telomerase is a prerequisite step in malignant transformation and progression. Recently, the gain-of-function mutation of the TERT promoter was identified in many types of human malignancies, and the mutated promoter acquires de novo ETS binding motifs through which the TERT transcription is activated. The ETS family transcription factors GABPA and GABPB1 have been shown to act as master drivers for the mutant TERT promoter activity. Indeed, GABPA or GABPB1 depletion leads to the down-regulation of TERT expression in the mutant TERT promoter-bearing cancer cells, and is thus proposed as targets for cancer therapy. Surprisingly, however, despite its key role in activating the mutant TERT promoter and telomerase, GABPA may itself function as a potent tumor suppressor in several malignancies. In this review, we address the collaboration between GABPA and mutant TERT promoter in cancer development, discuss selection trade-offs among different activities of GABPA in cancer evolution, and underscore the suppressive function of GABPA in cancer progression and implications in precision oncology.

Therapeutic Targets in Telomerase and Telomere Biology of Cancers

Telomeres play an important role to conserve genomic integrity by protecting the ends of chromosomes in normal cells. Since, their progressive shortening during successive cell division which lead to chromosomal instability. Notably, telomere length is perpetuated by telomerase in large majority of cancers, thereby ensure indefinite cell proliferation-a hallmark of cancer-and this unique feature has provided telomerase as the preferred target for drug development in cancer therapeutics. Cancer cells have acquired the potential to have telomere length maintenance by telomerase activation- up-regulation of hTERT gene expression in tumor cells is synchronized by multiple genetic and epigenetic modification mechanisms viz hTERT structural variants, hTERT promoter mutation and epigenetic modifications through hTERT promoter methylation which have been implicated in various cancers initiation and progression. In view of these facts, strategies have been made to target the underlining molecular mechanisms involved in telomerase reactivation as well as of telomere structure with special reference to distortion of sheltrin proteins. This review is focussed on extensive understanding of telomere and telomerase biology. which will provide indispensable informations for enhancing the efficiency of rational anticancer drug design. However, there is also an urgent need for better understanding of cell signalling pathways for alternative lengthening of telomere which is present in telomerase negative cancer for therapeutic targets.

CRISPR/Cas9-Mediated TERT Disruption in Cancer Cells

Mammalian telomere lengths are primarily regulated by telomerase, a ribonucleoprotein consisting of a reverse transcriptase (TERT) and an RNA subunit (TERC). TERC is constitutively expressed in all cells, whereas TERT expression is temporally and spatially regulated, such that in most adult somatic cells, TERT is inactivated and telomerase activity is undetectable. Most tumor cells activate TERT as a mechanism for preventing progressive telomere attrition to achieve proliferative immortality. Therefore, inactivating TERT has been considered to be a promising means of cancer therapy. Here we applied the CRISPR/Cas9 gene editing system to target the TERT gene in cancer cells. We report that disruption of TERT severely compromises cancer cell survival in vitro and in vivo. Haploinsufficiency of TERT in tumor cells is sufficient to result in telomere attrition and growth retardation in vitro. In vivo, TERT haploinsufficient tumor cells failed to form xenograft after transplantation to nude mice. Our work demonstrates that gene editing-mediated TERT knockout is a potential therapeutic option for treating cancer.

Telomere length and associated factors in older adults with hypertension

Telomeres and telomerase play important roles in the occurrence and development of hypertension. This review was performed to clarify the factors that influence telomere length and telomerase activity in older patients and elucidate the association of these factors with hypertension. A PubMed search and critical review of studies assessing the risk factors underlying the association of hypertension with telomere length and telomerase activity was performed. Telomere length and telomerase activity were found to be associated with hypertension. The factors that influence telomere length and telomerase activity in older patients with hypertension include genetics, demographics, social and environmental factors, chronic disease, psychological factors, and antihypertensive drug treatment. A better understanding of the molecular mechanisms underlying the association of hypertension with telomere length and telomerase activity may help to reduce the incidence of hypertension.