Rearrangement of upstream sequences of the hTERT gene during cellular immortalization

Telomere dysfunction in Tert knockout mice delays BrafV600E -induced melanoma development

Telomerase activation is a crucial step in melanomagenesis, often occurring because of ultraviolet radiation (UVR)-induced mutations at the telomerase gene (TERT) promoter and rendering TERT transcription in response to the activated Raf-MAP kinase pathway by BRAFV600E mutation. Due to the excessively long telomeres in mice, this process does not occur during melanomagenesis in mouse models. To investigate the impact of telomere dysfunction on melanomagenesis, BrafV600E was induced in generations 1 and 4 (G1 and G4) of Tert-/- mice. Our findings revealed that, regardless of UVR exposure, melanoma development was delayed in G4 mice, which had shorter telomeres compared to G1 and wild-type C57BL/6J (G0) mice. Moreover, many G4 tumors displayed an accumulation of excessive DNA damage, as evidenced by increased γH2A.X staining. Tumors from UVR-exposed mice exhibited elevated p53 protein expression. Cultured tumor cells isolated from G4 mice displayed abundant chromosomal fusions and rearrangements, indicative of telomere dysfunction in these cells. Additionally, tumor cells derived from UVB-exposed mice exhibited constitutively elevated expression of mutant p53 proteins, suggesting that p53 was a target of UVB-induced mutagenesis. Taken together, our findings suggest that telomere dysfunction hampers melanomagenesis, and targeting telomere crisis-mediated genomic instability may hold promise for the prevention and treatment of melanoma.

Targeting oncogenic TERT promoter variants by allele-specific epigenome editing

BACKGROUND

Activation of dominant oncogenes by small or structural genomic alterations is a common driver mechanism in many cancers. Silencing of such dominantly activated oncogenic alleles, thus, is a promising strategy to treat cancer. Recently, allele-specific epigenome editing (ASEE) has been described as a means to reduce transcription of genes in an allele-specific manner. In cancer, specificity to an oncogenic allele can be reached by either targeting directly a pathogenic single-nucleotide variant or a polymorphic single-nucleotide variant linked to the oncogenic allele. To investigate the potential of ASEE in cancer, we here explored this approach by targeting variants at the TERT promoter region. The TERT promoter region has been described as one of the most frequently mutated non-coding cancer drivers.

RESULTS

Sequencing of the TERT promoter in cancer cell lines showed 53% (41/77) to contain at least one heterozygous sequence variant allowing allele distinction. We chose the hepatoblastoma cell line Hep-G2 and the lung cancer cell line A-549 for this proof-of-principle study, as they contained two different kinds of variants, namely the activating mutation C228T in the TERT core promoter and the common SNP rs2853669 in the THOR region, respectively. These variants were targeted in an allele-specific manner using sgRNA-guided dCas9-DNMT3A-3L complexes. In both cell lines, we successfully introduced DNA methylation specifically to the on-target allele of the TERT promoter with limited background methylation on the off-target allele or an off-target locus (VEGFA), respectively. We observed a maximum CpG methylation gain of 39% and 76% on the target allele when targeting the activating mutation and the common SNP, respectively. The epigenome editing translated into reduced TERT RNA expression in Hep-G2.

CONCLUSIONS

We applied an ASEE-mediated approach to silence TERT allele specifically. Our results show that the concept of dominant oncogene inactivation by allele-specific epigenome editing can be successfully translated into cancer models. This new strategy may have important advantages in comparison with existing therapeutic approaches, e.g., targeting telomerase, especially with regard to reducing adverse side effects.

Reactivation of telomerase reverse transcriptase expression in cancer: the role of TERT promoter mutations

Telomerase activity and telomere elongation are essential conditions for the unlimited proliferation of neoplastic cells. Point mutations in the core promoter region of the telomerase reverse transcriptase (TERT) gene have been found to occur at high frequencies in several tumour types and considered a primary cause of telomerase reactivation in cancer cells. These mutations promote TERT gene expression by multiple mechanisms, including the generation of novel binding sites for nuclear transcription factors, displacement of negative regulators from DNA G-quadruplexes, recruitment of epigenetic activators and disruption of long-range interactions between TERT locus and telomeres. Furthermore, TERT promoter mutations cooperate with TPP1 promoter nucleotide changes to lengthen telomeres and with mutated BRAF and FGFR3 oncoproteins to enhance oncogenic signalling in cancer cells. TERT promoter mutations have been recognized as an early marker of tumour development or a major indicator of poor outcome and reduced patients survival in several cancer types. In this review, we summarize recent findings on the role of TERT promoter mutations, telomerase expression and telomeres elongation in cancer development, their clinical significance and therapeutic opportunities.

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.

Methylation of Subtelomeric Chromatin Modifies the Expression of the lncRNA TERRA, Disturbing Telomere Homeostasis

The long noncoding RNA (lncRNA) telomeric repeat-containing RNA (TERRA) has been associated with telomeric homeostasis, telomerase recruitment, and the process of chromosome healing; nevertheless, the impact of this association has not been investigated during the carcinogenic process. Determining whether changes in TERRA expression are a cause or a consequence of cell transformation is a complex task because studies are usually carried out using either cancerous cells or tumor samples. To determine the role of this lncRNA in cellular aging and chromosome healing, we evaluated telomeric integrity and TERRA expression during the establishment of a clone of untransformed myeloid cells. We found that reduced expression of TERRA disturbed the telomeric homeostasis of certain loci, but the expression of the lncRNA was affected only when the methylation of subtelomeric bivalent chromatin domains was compromised. We conclude that the disruption in TERRA homeostasis is a consequence of cellular transformation and that changes in its expression profile can lead to telomeric and genomic instability.

Telomeric Repeat-Containing RNA (TERRA): A Review of the Literature and First Assessment in Cutaneous T-Cell Lymphomas

Telomeric Repeat-containing RNA (TERRA) are long non-coding RNAs transcribed from telomeric DNA sequences from multiple chromosome ends. Major research efforts have been made to understand TERRA roles and functions in several physiological and pathological processes. We summarize herein available data regarding TERRA’s roles in human cells and we report the first investigation in cutaneous T-cells lymphomas (CTCL) using real-time PCR. Among the TERRA analysed, our data suggest a particular role for TERRA 16p downregulation and TERRA 11q upregulation in CTCL lymphomagenesis.

Telomeres and Cancer: Resolving the Paradox

Decades of study on cell cycle regulation have provided great insight into human cellular life span barriers, as well as their dysregulation during tumorigenesis. Telomeres, the extremities of linear chromosomes, perform an essential role in implementing these proliferative boundaries and preventing the propagation of potentially cancerous cells. The tumor-suppressive function of telomeres relies on their ability to initiate DNA damage signaling pathways and downstream cellular events, ranging from cell cycle perturbation to inflammation and cell death. While the tumor-suppressor role of telomeres is undoubtable, recent advances have pointed to telomeres as a major source of many of the genomic aberrations found in both early- and late-stage cancers, including the most recently discovered mutational phenomenon of chromothripsis. Telomere shortening appears as a double-edged sword that can function in opposing directions in carcinogenesis. This review focuses on the current knowledge of the dual role of telomeres in cancer and suggests a new perspective to reconcile the paradox of telomeres and their implications in cancer etiology.

Mechanisms of TERT Reactivation and Its Interaction with BRAFV600E

The telomerase reverse transcriptase (TERT) gene, which is repressed in most differentiated human cells, can be reactivated by somatic TERT alterations and epigenetic modulations. Moreover, the recruitment, accessibility, and binding of transcription factors also affect the regulation of TERT expression. Reactivated TERT contributes to the development and progression of cancer through telomere lengthening-dependent and independent ways. In particular, because of recent advances in high-throughput sequencing technologies, studies on genomic alterations in various cancers that cause increased TERT transcriptional activity have been actively conducted. TERT reactivation has been reported to be associated with poor prognosis in several cancers, and TERT promoter mutations are among the most potent prognostic markers in thyroid cancer. In particular, when a TERT promoter mutation coexists with the BRAFV600E mutation, these mutations exert synergistic effects on a poor prognosis. Efforts have been made to uncover the mechanisms of these synergistic interactions. In this review, we discuss the role of TERT reactivation in tumorigenesis, the mechanisms of TERT reactivation across all human cancers and in thyroid cancer, and the mechanisms of interactions between BRAFV600E and TERT promoter mutations.

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

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

TERT promoter mutations and telomeres during tumorigenesis

Telomerase regulation and telomere shortening act as a strong tumor suppressor mechanism in human somatic cells. Point mutations in the promoter of telomerase reverse transcriptase (TERT) are the most frequent non-coding mutation in cancer. These TERT promoter mutations (TPMs) create de novo ETS factor binding sites upstream of the start codon of the gene, which can be bound by different ETS factors. TPMs can occur early during tumorigenesis and are thought to be among the first mutations in melanoma, glioblastoma and hepatocellular carcinoma. Despite their association with increased TERT levels, TPMs do not prohibit telomere shortening and TPM-harboring cancers present with short telomeres. Their short telomere length combined with their high prevalence and specificity for cancer makes TPMs an attractive target for future therapeutic exploitation of telomerase inhibition and telomere deprotection-induced cell death.

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.

Mechanisms underlying the activation of TERT transcription and telomerase activity in human cancer: old actors and new players

Long-lived species Homo sapiens have evolved robust protection mechanisms against cancer by repressing telomerase and maintaining short telomeres, thereby delaying the onset of the majority of cancer types until post-reproductive age. Indeed, telomerase is silent in most differentiated human cells, predominantly due to the transcriptional repression of its catalytic component telomerase reverse transcriptase (TERT) gene. The lack of telomerase/TERT expression leads to progressive telomere erosion in dividing human cells, whereas critically shortened telomere length induces a permanent growth arrest stage named replicative senescence. TERT/telomerase activation has been experimentally shown to be essential to cellular immortalization and malignant transformation by stabilizing telomere length and erasing the senescence barrier. Consistently, TERT expression/telomerase activity is detectable in up to 90% of human primary cancers. Compelling evidence has also accumulated that TERT contributes to cancer development and progression via multiple activities beyond its canonical telomere-lengthening function. Given these key roles of telomerase and TERT in oncogenesis, great efforts have been made to decipher mechanisms underlying telomerase activation and TERT induction. In the last two decades since the TERT gene and promoter were cloned, the derepression of the TERT gene has been shown to be achieved typically at a transcriptional level through dysregulation of oncogenic factors or signaling, post-transcriptional/translational regulation and genomic amplification. However, advances in high-throughput next-generation sequencing technologies have prompted a revolution in cancer genomics, which leads to the recent discovery that genomic alterations take center stage in activating the TERT gene. In this review article, we summarize critical mechanisms activating TERT transcription, with special emphases on the contribution of TERT promoter mutations and structural alterations at the TERT locus, and briefly discuss the underlying implications of these genomic events-driven TERT hyperactivity in cancer initiation/progression and potential clinical applications as well.

Telomere Maintenance Mechanisms in Cancer

Tumour cells can adopt telomere maintenance mechanisms (TMMs) to avoid telomere shortening, an inevitable process due to successive cell divisions. In most tumour cells, telomere length (TL) is maintained by reactivation of telomerase, while a small part acquires immortality through the telomerase-independent alternative lengthening of telomeres (ALT) mechanism. In the last years, a great amount of data was generated, and different TMMs were reported and explained in detail, benefiting from genome-scale studies of major importance. In this review, we address seven different TMMs in tumour cells: mutations of the TERT promoter (TERTp), amplification of the genes TERT and TERC, polymorphic variants of the TERT gene and of its promoter, rearrangements of the TERT gene, epigenetic changes, ALT, and non-defined TMM (NDTMM). We gathered information from over fifty thousand patients reported in 288 papers in the last years. This wide data collection enabled us to portray, by organ/system and histotypes, the prevalence of TERTp mutations, TERT and TERC amplifications, and ALT in human tumours. Based on this information, we discuss the putative future clinical impact of the aforementioned mechanisms on the malignant transformation process in different setups, and provide insights for screening, prognosis, and patient management stratification.

TERT-mediated and ATRX-mediated Telomere Maintenance and Neuroblastoma

Neuroblastomas (NB) are one of the most common extracranial solid tumors in children, and they frequently display high heterogeneity in the disease course. With ongoing research, more information regarding the genetic etiology and molecular mechanisms underlying these contrasting phenotypes is being uncovered. The proto-oncogene MYCN is amplified in approximately 20% of NB cases and is considered a indicator of poor prognosis and an indicator of high-risk NB. The poor prognosis of high risk NB is incompletely explained by MYCN amplification. Recently, massive parallel sequencing studies reported several relatively common gene alterations, such as ATRX mutation and TERT rearrangement that are involved in telomere maintenance through telomerase activity and alternative lengthening of telomeres. Thus, these are important for understanding the etiology and molecular pathogenesis of NB, and hence, for identifying diagnostic and treatment markers. Development of telomerase inhibitors and identification of alternative lengthening of telomeres related targets will contribute to the individualized treatment for high-risk NB. In this mini-review, we will discuss the research progress of TERT-mediated and ATRX-mediated telomere maintenance and NB, especially high-risk tumors.

Detection of Aberrant TERT Promoter Methylation by Combined Bisulfite Restriction Enzyme Analysis for Cancer Diagnosis

Aberrant CpG dinucleotide methylation in a specific region of the telomerase reverse transcriptase (TERT) promoter is associated with increased TERT mRNA levels and malignancy in several cancer types. However, routine screening of this region to aid cancer diagnosis can be challenging because i) several established methylation assays may inaccurately report on hypermethylation of this particular region, ii) interpreting the results of methylation assays can sometimes be difficult for clinical laboratories, and iii) use of high-throughput methylation assays for a few patient samples can be cost prohibitive. Herein, we describe the use of combined bisulfite restriction enzyme analysis (COBRA) as a diagnostic tool for detecting the hypermethylated TERT promoter using in vitro methylated and unmethylated genomic DNA as well as genomic DNA from four melanomas and two benign melanocytic lesions. We compare COBRA with MassARRAY, a more commonly used high-throughput approach, in screening for promoter hypermethylation in 28 formalin-fixed, paraffin-embedded neuroblastoma samples. COBRA sensitively and specifically detected samples with hypermethylated TERT promoter and was as effective as MassARRAY at differentiating high-risk from benign or low-risk tumors. This study demonstrates the utility of this low-cost, technically straightforward, and easily interpretable assay for cancer diagnosis in tumors of an ambiguous nature.

Regulation of the Human Telomerase Gene TERT by Telomere Position Effect-Over Long Distances (TPE-OLD): Implications for Aging and Cancer

Telomerase is expressed in early human development and then becomes silenced in most normal tissues. Because ~90% of primary human tumors express telomerase and generally maintain very short telomeres, telomerase is carefully regulated, particularly in large, long-lived mammals. In the current report, we provide substantial evidence for a new regulatory control mechanism of the rate limiting catalytic protein component of telomerase (hTERT) that is determined by the length of telomeres. We document that normal, young human cells with long telomeres have a repressed hTERT epigenetic status (chromatin and DNA methylation), but the epigenetic status is altered when telomeres become short. The change in epigenetic status correlates with altered expression of TERT and genes near to TERT, indicating a change in chromatin. Furthermore, we identified a chromosome 5p telomere loop to a region near TERT in human cells with long telomeres that is disengaged with increased cell divisions as telomeres progressively shorten. Finally, we provide support for a role of the TRF2 protein, and possibly TERRA, in the telomere looping maintenance mechanism through interactions with interstitial TTAGGG repeats. This provides new insights into how the changes in genome structure during replicative aging result in an increased susceptibility to age-related diseases and cancer prior to the initiation of a DNA damage signal.

Repression of telomerase gene promoter requires human-specific genomic context and is mediated by multiple HDAC1-containing corepressor complexes

The human telomerase reverse transcriptase (hTERT) gene is repressed in most somatic cells, whereas the expression of the mouse mTert gene is widely detected. To understand the mechanisms of this human-specific repression, we constructed bacterial artificial chromosome (BAC) reporters using human and mouse genomic DNAs encompassing the TERT genes and neighboring loci. Upon chromosomal integration, the hTERT, but not the mTert, reporter was stringently repressed in telomerase-negative human cells in a histone deacetylase (HDAC)-dependent manner, replicating the expression of their respective endogenous genes. In chimeric BACs, the mTert promoter became strongly repressed in the human genomic context, but the hTERT promoter was highly active in the mouse genomic context. Furthermore, an unrelated herpes simplex virus-thymidine kinase (HSV-TK) promoter was strongly repressed in the human, but not in the mouse, genomic context. These results demonstrated that the repression of hTERT gene was dictated by distal elements and its chromatin environment. This repression depended on class I HDACs and involved multiple corepressor complexes, including HDAC1/2-containing Sin3B, nucleosome remodeling and histone deacetylase (NuRD), and corepressor of RE1 silencing transcription factor (CoREST) complexes. Together, our data indicate that the lack of telomerase expression in most human somatic cells results from its repressive genomic environment, providing new insight into the mechanism of long-recognized differential telomerase regulation in mammalian species.-Cheng, D., Zhao, Y., Wang, S., Zhang, F., Russo, M., McMahon, S. B., Zhu, J. Repression of telomerase gene promoter requires human-specific genomic context and is mediated by multiple HDAC1-containing corepressor complexes.

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.

Human Specific Regulation of the Telomerase Reverse Transcriptase Gene

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.

TERT rearrangements are frequent in neuroblastoma and identify aggressive tumors

Whole-genome sequencing detected structural rearrangements of TERT in 17 of 75 high-stage neuroblastomas, with five cases resulting from chromothripsis. Rearrangements were associated with increased TERT expression and targeted regions immediately up- and downstream of TERT, positioning a super-enhancer close to the breakpoints in seven cases. TERT rearrangements (23%), ATRX deletions (11%) and MYCN amplifications (37%) identify three almost non-overlapping groups of high-stage neuroblastoma, each associated with very poor prognosis.

Telomerase activation by genomic rearrangements in high-risk neuroblastoma

Neuroblastoma is a malignant paediatric tumour of the sympathetic nervous system. Roughly half of these tumours regress spontaneously or are cured by limited therapy. By contrast, high-risk neuroblastomas have an unfavourable clinical course despite intensive multimodal treatment, and their molecular basis has remained largely elusive. Here we have performed whole-genome sequencing of 56 neuroblastomas (high-risk, n = 39; low-risk, n = 17) and discovered recurrent genomic rearrangements affecting a chromosomal region at 5p15.33 proximal of the telomerase reverse transcriptase gene (TERT). These rearrangements occurred only in high-risk neuroblastomas (12/39, 31%) in a mutually exclusive fashion with MYCN amplifications and ATRX mutations, which are known genetic events in this tumour type. In an extended case series (n = 217), TERT rearrangements defined a subgroup of high-risk tumours with particularly poor outcome. Despite a large structural diversity of these rearrangements, they all induced massive transcriptional upregulation of TERT. In the remaining high-risk tumours, TERT expression was also elevated in MYCN-amplified tumours, whereas alternative lengthening of telomeres was present in neuroblastomas without TERT or MYCN alterations, suggesting that telomere lengthening represents a central mechanism defining this subtype. The 5p15.33 rearrangements juxtapose the TERT coding sequence to strong enhancer elements, resulting in massive chromatin remodelling and DNA methylation of the affected region. Supporting a functional role of TERT, neuroblastoma cell lines bearing rearrangements or amplified MYCN exhibited both upregulated TERT expression and enzymatic telomerase activity. In summary, our findings show that remodelling of the genomic context abrogates transcriptional silencing of TERT in high-risk neuroblastoma and places telomerase activation in the centre of transformation in a large fraction of these tumours.

Therapeutic targeting of replicative immortality

One of the hallmarks of malignant cell populations is the ability to undergo continuous proliferation. This property allows clonal lineages to acquire sequential aberrations that can fuel increasingly autonomous growth, invasiveness, and therapeutic resistance. Innate cellular mechanisms have evolved to regulate replicative potential as a hedge against malignant progression. When activated in the absence of normal terminal differentiation cues, these mechanisms can result in a state of persistent cytostasis. This state, termed “senescence,” can be triggered by intrinsic cellular processes such as telomere dysfunction and oncogene expression, and by exogenous factors such as DNA damaging agents or oxidative environments. Despite differences in upstream signaling, senescence often involves convergent interdependent activation of tumor suppressors p53 and p16/pRB, but can be induced, albeit with reduced sensitivity, when these suppressors are compromised. Doses of conventional genotoxic drugs required to achieve cancer cell senescence are often much lower than doses required to achieve outright cell death. Additional therapies, such as those targeting cyclin dependent kinases or components of the PI3K signaling pathway, may induce senescence specifically in cancer cells by circumventing defects in tumor suppressor pathways or exploiting cancer cells’ heightened requirements for telomerase. Such treatments sufficient to induce cancer cell senescence could provide increased patient survival with fewer and less severe side effects than conventional cytotoxic regimens. This positive aspect is countered by important caveats regarding senescence reversibility, genomic instability, and paracrine effects that may increase heterogeneity and adaptive resistance of surviving cancer cells. Nevertheless, agents that effectively disrupt replicative immortality will likely be valuable components of new combinatorial approaches to cancer therapy.

Genomics of chromophobe renal cell carcinoma: implications from a rare tumor for pan-cancer studies

Chromophobe Renal Cell Carcinoma (ChRCC) is a rare subtype of the renal cell carcinomas, a heterogenous group of cancers arising from the nephron. Recently, The Cancer Genome Atlas (TCGA) profiled this understudied disease using multiple data platforms, including whole exome sequencing, whole genome sequencing (WGS), and mitochondrial DNA (mtDNA) sequencing. The insights gained from this study would have implications for other types of kidney cancer as well as for cancer biology in general. Global molecular patterns in ChRCC provided clues as to this cancer's cell of origin, which is distinct from that of the other renal cell carcinomas, illustrating an approach that might be applied towards elucidating the cell of origin of other cancer types. MtDNA sequencing revealed loss-of-function mutations in NADH dehydrogenase subunits, highlighting the role of deregulated metabolism in this and other cancers. Analysis of WGS data led to the discovery of recurrent genomic rearrangements involving TERT promoter region, which were associated with very high expression levels of TERT, pointing to a potential mechanism for TERT deregulation that might be found in other cancers. WGS data, generated by large scale efforts such as TCGA and the International Cancer Genomics Consortium (ICGC), could be more extensively mined across various cancer types, to uncover structural variants, mtDNA mutations, themes of tumor metabolic properties, as well as noncoding point mutations. TCGA's data on ChRCC should continue to serve as a resource for future pan-cancer as well as kidney cancer studies, and highlight the value of investigations into rare tumor types to globally inform principals of cancer biology.

A Combinatory Strategy for Detection of Live CTCs Using Microfiltration and a New Telomerase-Selective Adenovirus

Circulating tumor cells (CTC) have become an important biomarker for early cancer diagnosis, prognosis, and treatment monitoring. Recently, a replication-competent recombinant adenovirus driven by a human telomerase gene (hTERT) promoter was shown to detect live CTCs in blood samples of patients with cancer. Here, we report a new class of adenoviruses containing regulatory elements that repress the hTERT gene in normal cells. Compared with the virus with only the hTERT core promoter, the new viruses showed better selectivity for replication in cancer cells than in normal cells. In particular, Ad5GTSe, containing three extra copies of a repressor element, displayed a superior tropism for cancer cells among leukocytes and was thus selected for CTC detection in blood samples. To further improve the efficiency and specificity of CTC identification, we tested a combinatory strategy of microfiltration enrichment using flexible micro spring arrays and Ad5GTSe imaging. Our experiments showed that this method efficiently detected both cancer cells spiked into healthy blood and potential CTCs in blood samples of patients with breast and pancreatic cancer, demonstrating its potential as a highly sensitive and reliable system for detection and capture of CTCs of different tumor types.

The somatic genomic landscape of chromophobe renal cell carcinoma

We describe the landscape of somatic genomic alterations of 66 chromophobe renal cell carcinomas (ChRCCs) on the basis of multidimensional and comprehensive characterization, including mtDNA and whole-genome sequencing. The result is consistent that ChRCC originates from the distal nephron compared with other kidney cancers with more proximal origins. Combined mtDNA and gene expression analysis implicates changes in mitochondrial function as a component of the disease biology, while suggesting alternative roles for mtDNA mutations in cancers relying on oxidative phosphorylation. Genomic rearrangements lead to recurrent structural breakpoints within TERT promoter region, which correlates with highly elevated TERT expression and manifestation of kataegis, representing a mechanism of TERT upregulation in cancer distinct from previously observed amplifications and point mutations.

Dual roles of c-Myc in the regulation of hTERT gene

Human telomerase gene hTERT is important for cancer and aging. hTERT promoter is regulated by multiple transcription factors (TFs) and its activity is dependent on the chromatin environment. However, it remains unsolved how the interplay between TFs and chromatin environment controls hTERT transcription. In this study, we employed the recombinase-mediated BAC targeting and BAC recombineering techniques to dissect the functions of two proximal E-box sites at -165 and +44 nt in regulating the hTERT promoter in the native genomic contexts. Our data showed that mutations of these sites abolished promoter binding by c-Myc/Max, USF1 and USF2, decreased hTERT promoter activity, and prevented its activation by overexpressed c-Myc. Upon inhibition of histone deacetylases, mutant and wildtype promoters were induced to the same level, indicating that the E-boxes functioned to de-repress the hTERT promoter and allowed its transcription in a repressive chromatin environment. Unexpectedly, knockdown of endogenous c-Myc/Max proteins activated hTERT promoter. This activation did not require the proximal E-boxes but was accompanied by increased promoter accessibility, as indicated by augmented active histone marks and binding of multiple TFs at the promoter. Our studies demonstrated that c-Myc/Max functioned in maintaining chromatin-dependent repression of the hTERT gene in addition to activating its promoter.

Recurrent pre-existing and acquired DNA copy number alterations, including focal TERT gains, in neuroblastoma central nervous system metastases

Stage 4 neuroblastomas have a high rate of local and metastatic relapse and associated disease mortality. The central nervous system (CNS) is currently one of the most common isolated relapse sites, yet the genomic alterations that contribute to these metastases are unknown. This study sought to identify recurrent DNA copy number alterations (CNAs) and target genes relating to neuroblastoma CNS metastases by studying 19 pre-CNS primary tumors and 27 CNS metastases, including 12 matched pairs. SNP microarray analyses revealed that MYCN amplified (MYCNA) tumors had recurrent CNAs different from non-MYCNA cohorts. Several CNAs known to be prevalent among primary neuroblastomas occurred more frequently in CNS metastases, including 4p-, 7q+, 12q+, and 19q- in non-MYCNA metastases, and 9p- and 14q- irrespective of MYCNA status. In addition, novel CNS metastases-related CNAs included 18q22.1 gains in non-MYCNA pre-CNS primaries and 5p15.33 gains and 15q26.1→tel losses in non-MYCNA CNS metastases. Based on minimal common regions, gene expression, and biological properties, TERT (5p), NR2F2 (15q), ALDH1A3 (15q), CDKN2A (9p), and possibly CDH7 and CDH19 (18q) were candidate genes associated with the CNS metastatic process. Notably, the 5p15 minimal common region contained only TERT, and non-MYCNA CNS metastases with focal 5p15 gains had increased TERT expression, similar to MYCNA tumors. These findings suggest that a specific genomic lesion (18q22.1 gain) predisposes to CNS metastases and that distinct lesions are recurrently acquired during metastatic progression. Among the acquired lesions, increased TERT copy number and expression appears likely to function in lieu of MYCNA to promote CNS metastasis.

Medical genetics and epigenetics of telomerase

Telomerase is a specialized reverse transcriptase that extends and maintains the terminal ends of chromosomes, or telomeres. Since its discovery in 1985 by Nobel Laureates Elizabeth Blackburn and Carol Greider, thousands of articles have emerged detailing its significance in telomere function and cell survival. This review provides a current assessment on the importance of telomerase regulation and relates it in terms of medical genetics. In this review, we discuss the recent findings on telomerase regulation, focusing on epigenetics and non-coding RNAs regulation of telomerase, such as microRNAs and the recently discovered telomeric-repeat containing RNA transcripts. Human genetic disorders that develop due to mutations in telomerase subunits, the role of single nucleotide polymorphisms in genes encoding telomerase components and diseases as a result of telomerase regulation going awry are also discussed. Continual investigation of the complex regulation of telomerase will further our insight into the use of controlling telomerase activity in medicine.

Robust activation of the human but not mouse telomerase gene during the induction of pluripotency

Pluripotent stem cells (PSCs) express telomerase and have unlimited proliferative potential. To study telomerase activation during reprogramming, 3 classes of embryonic stem cell (ESC)-like clones were isolated from mouse fibroblasts containing a transgenic hTERT reporter. Class I expressed few pluripotency markers, whereas class II contained many, but not Oct4, Nanog, and Sox2. Neither class of cells differentiated efficiently. Class III cells, the fully reprogrammed induced PSCs (iPSCs), expressed all pluripotency markers, formed teratomas indistinguishable from those of mESCs, and underwent efficient osteogenic differentiation in vitro. Interestingly, whereas the endogenous mTERT gene expression was only moderately increased during reprogramming, the hTERT promoter was strongly activated in class II cells and was further elevated in class III cells. Treatment of class II cells with chemical inhibitors of MEKs and glycogen synthase kinase 3 resulted in their further reprogramming into class III cells, accompanied by a strong activation of hTERT promoter. In reprogrammed human cells, the endogenous telomerase level, although variable among different clones, was dramatically elevated. Only in cells with the highest telomerase were telomeres restored to the lengths in hESCs. Our data, for the first time, demonstrated that the hTERT promoter was strongly activated in discrete steps, revealing a critical difference in human and mouse cell reprogramming. Because telomere elongation is crucial for self-renewal of hPSCs and replicative aging of their differentiated progeny, these findings have important implications in the generation and applications of iPSCs.

Chromatin and epigenetic regulation of the telomerase reverse transcriptase gene

Telomerase expression and telomere maintenance are critical for long-term cell proliferation and survival, and they play important roles in development, aging, and cancer. Cumulating evidence has indicated that regulation of the rate-limiting subunit of human telomerase reverse transcriptase gene (hTERT) is a complex process in normal cells and many cancer cells. In addition to a number of transcriptional activators and repressors, the chromatin environment and epigenetic status of the endogenous hTERT locus are also pivotal for its regulation in normal human somatic cells and in tumorigenesis.