Special Issue "Telomerase Activity in Human Cells"

A special issue of Genes (ISSN 2073-4425). This special issue belongs to the section "Human Genomics and Genetic Diseases".

Deadline for manuscript submissions: closed (30 April 2016).

Special Issue Editor

Dr. Gabriele Saretzki
E-Mail Website
Guest Editor
Newcastle Institute for Ageing and Institute for Cell and Molecular Biosciences, Campus for Ageing and Vitality, Newcastle University, Newcastle upon Tyne NE4 5PL, UK
Interests: telomerase in ageing and cancer; TERT in mitochondria; oxidative stress; mitochondria; senescence and ageing
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

The activity of the reverse transcriptase telomerase is a canonical function to maintain telomeres, the ends of linear chromosomes. Telomeres shorten in the absence of telomerase, causing senescence and ageing. In contrast to other organisms, telomerase activity is downregulated early in development in many somatic human tissues. However, some cell types, such as lymphocytes, adult stem cells, and endothelial cells retain, or can upregulate, telomerase activity. Importantly, this activity is strongly controlled by physiological conditions. In contrast, telomerase activity is continuously expressed at a high level in the majority of cancer cells, contributing to their indefinite proliferation potential. Although telomerase activity has been vigorously investigated over the last few decades, many questions still remain open regarding the mechanisms of physiological regulation in normal cells, as well as its up-regulation during tumourigenesis. The complex regulation at the levels of transcription, splicing, and posttranscriptional activation certainly contribute to that. Recently, interventions into its activation to counteract telomere shortening in healthy tissues, as well as its inhibition as tumour therapy, have been suggested and trials have been started with no final breakthrough yet. Thus, we still need to better understand the biology and regulation of telomerase activity in order to interfere with it successfully.

Dr. Gabriele Saretzki
Guest Editor

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Keywords

  • telomerase
  • reverse transcriptase
  • activity
  • regulation
  • cancer
  • telomere
  • senescence
  • ageing

Published Papers (14 papers)

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Research

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Article
Lack of TERT Promoter Mutations in Human B-Cell Non-Hodgkin Lymphoma
Genes 2016, 7(11), 93; https://doi.org/10.3390/genes7110093 - 25 Oct 2016
Cited by 7 | Viewed by 2871
Abstract
Non-Hodgkin lymphomas (NHL) are a heterogeneous group of immune cell neoplasms that comprise molecularly distinct lymphoma subtypes. Recent work has identified high frequency promoter point mutations in the telomerase reverse transcriptase (TERT) gene of different cancer types, including melanoma, glioma, liver [...] Read more.
Non-Hodgkin lymphomas (NHL) are a heterogeneous group of immune cell neoplasms that comprise molecularly distinct lymphoma subtypes. Recent work has identified high frequency promoter point mutations in the telomerase reverse transcriptase (TERT) gene of different cancer types, including melanoma, glioma, liver and bladder cancer. TERT promoter mutations appear to correlate with increased TERT expression and telomerase activity in these cancers. In contrast, breast, pancreatic, and prostate cancer rarely demonstrate mutations in this region of the gene. TERT promoter mutation prevalence in NHL has not been thoroughly tested thus far. We screened 105 B-cell lymphoid malignancies encompassing nine NHL subtypes and acute lymphoblastic leukemia, for TERT promoter mutations. Our results suggest that TERT promoter mutations are rare or absent in most NHL. Thus, the classical TERT promoter mutations may not play a major oncogenic role in TERT expression and telomerase activation in NHL. Full article
(This article belongs to the Special Issue Telomerase Activity in Human Cells)
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Article
Telomere Transcripts Target Telomerase in Human Cancer Cells
Genes 2016, 7(8), 46; https://doi.org/10.3390/genes7080046 - 16 Aug 2016
Cited by 13 | Viewed by 3193
Abstract
Long non-coding transcripts from telomeres, called telomeric repeat-containing RNA (TERRA), were identified as blocking telomerase activity (TA), a telomere maintenance mechanism (TMM), in tumors. We expressed recombinant TERRA transcripts in tumor cell lines with TA and with alternative lengthening of telomeres (ALT) to [...] Read more.
Long non-coding transcripts from telomeres, called telomeric repeat-containing RNA (TERRA), were identified as blocking telomerase activity (TA), a telomere maintenance mechanism (TMM), in tumors. We expressed recombinant TERRA transcripts in tumor cell lines with TA and with alternative lengthening of telomeres (ALT) to study effects on TMM and cell growth. Adeno- and lentivirus constructs (AV and LV) were established for transient and stable expression of approximately 130 units of telomere hexanucleotide repeats under control of cytomegalovirus (CMV) and human RNase P RNA H1 (hH1) promoters with and without polyadenylation, respectively. Six human tumor cell lines either using telomerase or ALT were infected and analyzed for TA levels. Pre-infection cells using telomerase had 1%–3% of the TERRA expression levels of ALT cells. AV and LV expression of recombinant TERRA in telomerase positive cells showed a 1.3–2.6 fold increase in TERRA levels, and a decrease in TA of 25%–58%. Dominant-negative or small hairpin RNA (shRNA) viral expression against human telomerase reverse transcriptase (hTERT) results in senescence, not induced by TERRA expression. Population doubling time, cell viability and TL (telomere length) were not impacted by ectopic TERRA expression. Clonal growth was reduced by TERRA expression in TA but not ALT cell lines. ALT cells were not affected by treatments applied. Established cell models and tools may be used to better understand the role of TERRA in the cell, especially for targeting telomerase. Full article
(This article belongs to the Special Issue Telomerase Activity in Human Cells)
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Article
Telomerase Activity is Downregulated Early During Human Brain Development
Genes 2016, 7(6), 27; https://doi.org/10.3390/genes7060027 - 16 Jun 2016
Cited by 19 | Viewed by 3088
Abstract
Changes in hTERT splice variant expression have been proposed to facilitate the decrease of telomerase activity during fetal development in various human tissues. Here, we analyzed the expression of telomerase RNA (hTR), wild type and α-spliced hTERT in developing human fetal brain (post [...] Read more.
Changes in hTERT splice variant expression have been proposed to facilitate the decrease of telomerase activity during fetal development in various human tissues. Here, we analyzed the expression of telomerase RNA (hTR), wild type and α-spliced hTERT in developing human fetal brain (post conception weeks, pcw, 6–19) and in young and old cortices using qPCR and correlated it to telomerase activity measured by TRAP assay. Decrease of telomerase activity occurred early during brain development and correlated strongest to decreased hTR expression. The expression of α-spliced hTERT increased between pcw 10 and 19, while that of wild type hTERT remained unchanged. Lack of expression differences between young and old cortices suggests that most changes seem to occur early during human brain development. Using in vitro differentiation of neural precursor stem cells (NPSCs) derived at pcw 6 we found a decrease in telomerase activity but no major expression changes in telomerase associated genes. Thus, they do not seem to model the mechanisms for the decrease in telomerase activity in fetal brains. Our results suggest that decreased hTR levels, as well as transient increase in α-spliced hTERT, might both contribute to downregulation of telomerase activity during early human brain development between 6 and 17 pcw. Full article
(This article belongs to the Special Issue Telomerase Activity in Human Cells)
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Review

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Review
Telomerase Regulation from Beginning to the End
Genes 2016, 7(9), 64; https://doi.org/10.3390/genes7090064 - 14 Sep 2016
Cited by 49 | Viewed by 4060
Abstract
The vast body of literature regarding human telomere maintenance is a true testament to the importance of understanding telomere regulation in both normal and diseased states. In this review, our goal was simple: tell the telomerase story from the biogenesis of its parts [...] Read more.
The vast body of literature regarding human telomere maintenance is a true testament to the importance of understanding telomere regulation in both normal and diseased states. In this review, our goal was simple: tell the telomerase story from the biogenesis of its parts to its maturity as a complex and function at its site of action, emphasizing new developments and how they contribute to the foundational knowledge of telomerase and telomere biology. Full article
(This article belongs to the Special Issue Telomerase Activity in Human Cells)
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Review
Telomerase Activation in Hematological Malignancies
Genes 2016, 7(9), 61; https://doi.org/10.3390/genes7090061 - 07 Sep 2016
Cited by 16 | Viewed by 2492
Abstract
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 [...] Read more.
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. Full article
(This article belongs to the Special Issue Telomerase Activity in Human Cells)
Review
The Telomere/Telomerase System in Chronic Inflammatory Diseases. Cause or Effect?
Genes 2016, 7(9), 60; https://doi.org/10.3390/genes7090060 - 03 Sep 2016
Cited by 64 | Viewed by 5723
Abstract
Telomeres are specialized nucleoprotein structures located at the end of linear chromosomes and telomerase is the enzyme responsible for telomere elongation. Telomerase activity is a key component of many cancer cells responsible for rapid cell division but it has also been found by [...] Read more.
Telomeres are specialized nucleoprotein structures located at the end of linear chromosomes and telomerase is the enzyme responsible for telomere elongation. Telomerase activity is a key component of many cancer cells responsible for rapid cell division but it has also been found by many laboratories around the world that telomere/telomerase biology is dysfunctional in many other chronic conditions as well. These conditions are characterized by chronic inflammation, a situation mostly overlooked by physicians regarding patient treatment. Among others, these conditions include diabetes, renal failure, chronic obstructive pulmonary disease, etc. Since researchers have in many cases identified the association between telomerase and inflammation but there are still many missing links regarding this correlation, the latest findings about this phenomenon will be discussed by reviewing the literature. Our focus will be describing telomere/telomerase status in chronic diseases under the prism of inflammation, reporting molecular findings where available and proposing possible future approaches. Full article
(This article belongs to the Special Issue Telomerase Activity in Human Cells)
Review
Telomeres and Telomerase in Cardiovascular Diseases
Genes 2016, 7(9), 58; https://doi.org/10.3390/genes7090058 - 01 Sep 2016
Cited by 79 | Viewed by 7196
Abstract
Telomeres are tandem repeat DNA sequences present at the ends of each eukaryotic chromosome to stabilize the genome structure integrity. Telomere lengths progressively shorten with each cell division. Inflammation and oxidative stress, which are implicated as major mechanisms underlying cardiovascular diseases, increase the [...] Read more.
Telomeres are tandem repeat DNA sequences present at the ends of each eukaryotic chromosome to stabilize the genome structure integrity. Telomere lengths progressively shorten with each cell division. Inflammation and oxidative stress, which are implicated as major mechanisms underlying cardiovascular diseases, increase the rate of telomere shortening and lead to cellular senescence. In clinical studies, cardiovascular risk factors such as smoking, obesity, sedentary lifestyle, and hypertension have been associated with short leukocyte telomere length. In addition, low telomerase activity and short leukocyte telomere length have been observed in atherosclerotic plaque and associated with plaque instability, thus stroke or acute myocardial infarction. The aging myocardium with telomere shortening and accumulation of senescent cells limits the tissue regenerative capacity, contributing to systolic or diastolic heart failure. In addition, patients with ion-channel defects might have genetic imbalance caused by oxidative stress-related accelerated telomere shortening, which may subsequently cause sudden cardiac death. Telomere length can serve as a marker for the biological status of previous cell divisions and DNA damage with inflammation and oxidative stress. It can be integrated into current risk prediction and stratification models for cardiovascular diseases and can be used in precise personalized treatments. In this review, we summarize the current understanding of telomeres and telomerase in the aging process and their association with cardiovascular diseases. In addition, we discuss therapeutic interventions targeting the telomere system in cardiovascular disease treatments. Full article
(This article belongs to the Special Issue Telomerase Activity in Human Cells)
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Review
Transcription Regulation of the Human Telomerase Reverse Transcriptase (hTERT) Gene
Genes 2016, 7(8), 50; https://doi.org/10.3390/genes7080050 - 18 Aug 2016
Cited by 82 | Viewed by 6331
Abstract
Embryonic stem cells and induced pluripotent stem cells have the ability to maintain their telomere length via expression of an enzymatic complex called telomerase. Similarly, more than 85%–90% of cancer cells are found to upregulate the expression of telomerase, conferring them with the [...] Read more.
Embryonic stem cells and induced pluripotent stem cells have the ability to maintain their telomere length via expression of an enzymatic complex called telomerase. Similarly, more than 85%–90% of cancer cells are found to upregulate the expression of telomerase, conferring them with the potential to proliferate indefinitely. Telomerase Reverse Transcriptase (TERT), the catalytic subunit of telomerase holoenzyme, is the rate-limiting factor in reconstituting telomerase activity in vivo. To date, the expression and function of the human Telomerase Reverse Transcriptase (hTERT) gene are known to be regulated at various molecular levels (including genetic, mRNA, protein and subcellular localization) by a number of diverse factors. Among these means of regulation, transcription modulation is the most important, as evident in its tight regulation in cancer cell survival as well as pluripotent stem cell maintenance and differentiation. Here, we discuss how hTERT gene transcription is regulated, mainly focusing on the contribution of trans-acting factors such as transcription factors and epigenetic modifiers, as well as genetic alterations in hTERT proximal promoter. Full article
(This article belongs to the Special Issue Telomerase Activity in Human Cells)
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Review
Telomerase: The Devil Inside
Genes 2016, 7(8), 43; https://doi.org/10.3390/genes7080043 - 29 Jul 2016
Cited by 20 | Viewed by 4409
Abstract
High telomerase activity is detected in nearly all human cancers but most human cells are devoid of telomerase activity. There is well-documented evidence that reactivation of telomerase occurs during cellular transformation. In humans, tumors can rely in reactivation of telomerase or originate in [...] Read more.
High telomerase activity is detected in nearly all human cancers but most human cells are devoid of telomerase activity. There is well-documented evidence that reactivation of telomerase occurs during cellular transformation. In humans, tumors can rely in reactivation of telomerase or originate in a telomerase positive stem/progenitor cell, or rely in alternative lengthening of telomeres, a telomerase-independent telomere-length maintenance mechanism. In this review, we will focus on the telomerase positive tumors. In this context, the recent findings that telomerase reverse transcriptase (TERT) promoter mutations represent the most common non-coding mutations in human cancer have flared up the long-standing discussion whether cancer originates from telomerase positive stem cells or telomerase reactivation is a final step in cellular transformation. Here, we will discuss the pros and cons of both concepts in the context of telomere length-dependent and telomere length-independent functions of telomerase. Together, these observations may provoke a re-evaluation of telomere and telomerase based therapies, both in telomerase inhibition for cancer therapy and telomerase activation for tissue regeneration and anti-ageing strategies. Full article
(This article belongs to the Special Issue Telomerase Activity in Human Cells)
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Review
Therapeutic Targeting of Telomerase
Genes 2016, 7(7), 39; https://doi.org/10.3390/genes7070039 - 21 Jul 2016
Cited by 77 | Viewed by 5063
Abstract
Telomere length and cell function can be preserved by the human reverse transcriptase telomerase (hTERT), which synthesizes the new telomeric DNA from a RNA template, but is normally restricted to cells needing a high proliferative capacity, such as stem cells. Consequently, telomerase-based therapies [...] Read more.
Telomere length and cell function can be preserved by the human reverse transcriptase telomerase (hTERT), which synthesizes the new telomeric DNA from a RNA template, but is normally restricted to cells needing a high proliferative capacity, such as stem cells. Consequently, telomerase-based therapies to elongate short telomeres are developed, some of which have successfully reached the stage I in clinical trials. Telomerase is also permissive for tumorigenesis and 90% of all malignant tumors use telomerase to obtain immortality. Thus, reversal of telomerase upregulation in tumor cells is a potential strategy to treat cancer. Natural and small-molecule telomerase inhibitors, immunotherapeutic approaches, oligonucleotide inhibitors, and telomerase-directed gene therapy are useful treatment strategies. Telomerase is more widely expressed than any other tumor marker. The low expression in normal tissues, together with the longer telomeres in normal stem cells versus cancer cells, provides some degree of specificity with low risk of toxicity. However, long term telomerase inhibition may elicit negative effects in highly-proliferative cells which need telomerase for survival, and it may interfere with telomere-independent physiological functions. Moreover, only a few hTERT molecules are required to overcome senescence in cancer cells, and telomerase inhibition requires proliferating cells over a sufficient number of population doublings to induce tumor suppressive senescence. These limitations may explain the moderate success rates in many clinical studies. Despite extensive studies, only one vaccine and one telomerase antagonist are routinely used in clinical work. For complete eradication of all subpopulations of cancer cells a simultaneous targeting of several mechanisms will likely be needed. Possible technical improvements have been proposed including the development of more specific inhibitors, methods to increase the efficacy of vaccination methods, and personalized approaches. Telomerase activation and cell rejuvenation is successfully used in regenerative medicine for tissue engineering and reconstructive surgery. However, there are also a number of pitfalls in the treatment with telomerase activating procedures for the whole organism and for longer periods of time. Extended cell lifespan may accumulate rare genetic and epigenetic aberrations that can contribute to malignant transformation. Therefore, novel vector systems have been developed for a ‘mild’ integration of telomerase into the host genome and loss of the vector in rapidly-proliferating cells. It is currently unclear if this technique can also be used in human beings to treat chronic diseases, such as atherosclerosis. Full article
(This article belongs to the Special Issue Telomerase Activity in Human Cells)
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Review
Cancer-Specific Telomerase Reverse Transcriptase (TERT) Promoter Mutations: Biological and Clinical Implications
Genes 2016, 7(7), 38; https://doi.org/10.3390/genes7070038 - 18 Jul 2016
Cited by 65 | Viewed by 5950
Abstract
The accumulated evidence has pointed to a key role of telomerase in carcinogenesis. As a RNA-dependent DNA polymerase, telomerase synthesizes telomeric DNA at the end of linear chromosomes, and attenuates or prevents telomere erosion associated with cell divisions. By lengthening telomeres, telomerase extends [...] Read more.
The accumulated evidence has pointed to a key role of telomerase in carcinogenesis. As a RNA-dependent DNA polymerase, telomerase synthesizes telomeric DNA at the end of linear chromosomes, and attenuates or prevents telomere erosion associated with cell divisions. By lengthening telomeres, telomerase extends cellular life-span or even induces immortalization. Consistent with its functional activity, telomerase is silent in most human normal somatic cells while active only in germ-line, stem and other highly proliferative cells. In contrast, telomerase activation widely occurs in human cancer and the enzymatic activity is detectable in up to 90% of malignancies. Recently, hotspot point mutations in the regulatory region of the telomerase reverse transcriptase (TERT) gene, encoding the core catalytic component of telomerase, was identified as a novel mechanism to activate telomerase in cancer. This review discusses the cancer-specific TERT promoter mutations and potential biological and clinical significances. Full article
(This article belongs to the Special Issue Telomerase Activity in Human Cells)
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Review
Human Specific Regulation of the Telomerase Reverse Transcriptase Gene
Genes 2016, 7(7), 30; https://doi.org/10.3390/genes7070030 - 28 Jun 2016
Cited by 19 | Viewed by 3547
Abstract
Telomerase, regulated primarily by the transcription of its catalytic subunit telomerase reverse transcriptase (TERT), is critical for controlling cell proliferation and tissue homeostasis by maintaining telomere length. Although there is a high conservation between human and mouse TERT genes, the regulation of their [...] Read more.
Telomerase, regulated primarily by the transcription of its catalytic subunit telomerase reverse transcriptase (TERT), is critical for controlling cell proliferation and tissue homeostasis by maintaining telomere length. Although there is a high conservation between human and mouse TERT genes, the regulation of their transcription is significantly different in these two species. Whereas mTERT expression is widely detected in adult mice, hTERT is expressed at extremely low levels in most adult human tissues and cells. As a result, mice do not exhibit telomere-mediated replicative aging, but telomere shortening is a critical factor of human aging and its stabilization is essential for cancer development in humans. The chromatin environment and epigenetic modifications of the hTERT locus, the binding of transcriptional factors to its promoter, and recruitment of nucleosome modifying complexes all play essential roles in restricting its transcription in different cell types. In this review, we will discuss recent progress in understanding the molecular mechanisms of TERT regulation in human and mouse tissues and cells, and during cancer development. Full article
(This article belongs to the Special Issue Telomerase Activity in Human Cells)
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Review
Role of Telomerase in the Cardiovascular System
Genes 2016, 7(6), 29; https://doi.org/10.3390/genes7060029 - 17 Jun 2016
Cited by 19 | Viewed by 3192
Abstract
Aging is one major risk factor for the incidence of cardiovascular diseases and the development of atherosclerosis. One important enzyme known to be involved in aging processes is Telomerase Reverse Transcriptase (TERT). After the discovery of the enzyme in humans, TERT had initially [...] Read more.
Aging is one major risk factor for the incidence of cardiovascular diseases and the development of atherosclerosis. One important enzyme known to be involved in aging processes is Telomerase Reverse Transcriptase (TERT). After the discovery of the enzyme in humans, TERT had initially only been attributed to germ line cells, stem cells and cancer cells. However, over the last few years it has become clear that TERT is also active in cells of the cardiovascular system including cardiac myocytes, endothelial cells, smooth muscle cells and fibroblasts. Interference with the activity of this enzyme greatly contributes to cardiovascular diseases. This review will summarize the findings on the role of TERT in cardiovascular cells. Moreover, recent findings concerning TERT in different mouse models with respect to cardiovascular diseases will be described. Finally, the extranuclear functions of TERT will be covered within this review. Full article
(This article belongs to the Special Issue Telomerase Activity in Human Cells)
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Review
Telomere and Telomerase Therapeutics in Cancer
Genes 2016, 7(6), 22; https://doi.org/10.3390/genes7060022 - 26 May 2016
Cited by 58 | Viewed by 3849
Abstract
Telomerase is a reverse transcriptase capable of utilizing an integrated RNA component as a template to add protective tandem telomeric single strand DNA repeats, TTAGGG, to the ends of chromosomes. Telomere dysfunction and telomerase reactivation are observed in approximately 90% of human cancers; [...] Read more.
Telomerase is a reverse transcriptase capable of utilizing an integrated RNA component as a template to add protective tandem telomeric single strand DNA repeats, TTAGGG, to the ends of chromosomes. Telomere dysfunction and telomerase reactivation are observed in approximately 90% of human cancers; hence, telomerase activation plays a unique role as a nearly universal step on the path to malignancy. In the past two decades, multiple telomerase targeting therapeutic strategies have been pursued, including direct telomerase inhibition, telomerase interference, hTERT or hTERC promoter driven therapy, telomere-based approaches, and telomerase vaccines. Many of these strategies have entered clinical development, and some have now advanced to phase III clinical trials. In the coming years, one or more of these new telomerase-targeting drugs may be expected to enter the pharmacopeia of standard care. Here, we briefly review the molecular functions of telomerase in cancer and provide an update about the preclinical and clinical development of telomerase targeting therapeutics. Full article
(This article belongs to the Special Issue Telomerase Activity in Human Cells)
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