Accelerating drug development for neuroblastoma: Summary of the Second Neuroblastoma Drug Development Strategy forum from Innovative Therapies for Children with Cancer and International Society of Paediatric Oncology Europe Neuroblastoma

Only one class of targeted agents (anti-GD2 antibodies) has been incorporated into front-line therapy for neuroblastoma since the 1980s. The Neuroblastoma New Drug Development Strategy (NDDS) initiative commenced in 2012 to accelerate the development of new drugs for neuroblastoma. Advances have occurred, with eight of nine high-priority targets being evaluated in paediatric trials including anaplastic lymphoma kinase inhibitors being investigated in front-line, but significant challenges remain. This article reports the conclusions of the second NDDS forum, which expanded across the Atlantic to further develop the initiative. Pre-clinical and clinical data for 40 genetic targets and mechanisms of action were prioritised and drugs were identified for early-phase trials. Strategies to develop drugs targeting TERT, telomere maintenance, ATRX, alternative lengthening of telomeres (ALT), BRIP1 and RRM2 as well as direct targeting of MYCN are high priority and should be championed for drug discovery. Promising pre-clinical data suggest that targeting of ALT by ATM or PARP inhibition may be potential strategies. Drugs targeting CDK2/9, CDK7, ATR and telomere maintenance should enter paediatric clinical development rapidly. Optimising the response to anti-GD2 by combinations with chemotherapy, targeted agents and other immunological targets are crucial. Delivering this strategy in the face of small patient cohorts, genomically defined subpopulations and a large number of permutations of combination trials, demands even greater international collaboration. In conclusion, the NDDS provides an internationally agreed, biologically driven selection of prioritised genetic targets and drugs. Improvements in the strategy for conducting trials in neuroblastoma will accelerate bringing these new drugs more rapidly to front-line therapy.

Abstract 973: Telomere stress potentiates host STING-dependent anti-tumor immunity

Telomerase is an attractive target for anti-tumor therapy as it is almost universally expressed in cancer cells but not in normal cells. Here we show that treatment with a telomerase-mediated telomere-targeting drug, 6-thio-2'-deoxyguanosine (6-thio-dG), leads to tumor regression in innate and adaptive immune-dependent manners in syngeneic and humanized mouse colon cancer models. 6-thio-dG treatment causes telomere-associated DNA damages that are sensed by dendritic cells (DCs) and activates the host cytosolic DNA sensing STING/IFN-I pathway, resulting in enhanced cross-priming capacity of DCs and tumor-specific CD8 T-cell activation. Moreover, 6-thio-dG overcomes resistances to checkpoint blockade in advanced mouse cancer models. Together, our results unveil how 6-thio-dG induced telomere stress increases innate sensing and adaptive anti-tumor immunity and provide a strong rationale for sequentially combining telomerase directed telomere-targeted therapy with immunotherapy.

Citation Format: Anli Zhang, Ilgen Mender, Zhenhua Ren, Chuanhui Han, Yafang Deng, Silvia Siteni, Huiyu Li, Jerry W. Shay, Yang-Xin Fu. Telomere stress potentiates host STING-dependent anti-tumor immunity [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 973.

Immortalized normal human lung epithelial cell models for studying lung cancer biology

Primary cultures of human lung epithelial cells are ideal representatives of normal lung epithelial cells, and while there are certain novel approaches for the long-term culture of lung epithelial cells, the cells eventually undergo irreversible growth arrest, limiting their experimental utility, particularly the ability to widely distribute these cultures and their clonal derivatives to the broader research community. Therefore, the establishment of immortalized normal human lung epithelial cell strains has garnered considerable attention. The number and type of oncogenic changes necessary for the tumorigenic transformation of normal cells could be determined using "normal" cell lines immortalized with the simian virus 40 (SV40) large T antigen (LT). A primary report suggested that LT, human telomerase reverse transcriptase (hTERT), and oncogenic RAS transformed normal lung epithelial cells into tumorigenic cells. Since LT inactivates the tumor suppressors p53 and RB, at least four alterations would be necessary. However, the SV40 small T antigen (ST), a different oncoprotein, was also introduced simultaneously with LT in the above-mentioned study. Furthermore, the possible uncharacterized functions of LT remained largely obscure. Therefore, no definitive conclusion could be arrived in these studies. Subsequent studies used methods that did not involve the use of oncoproteins and revealed that at least five genetic changes were necessary for full tumorigenic transformation. hTERT-immortalized normal human lung epithelial cell lines established without using viral oncoproteins were also used for investigating several aspects of lung cancer, such as epithelial to mesenchymal transition and the cancer stem cell theory. The use of immortalized normal lung epithelial cell models has improved our understanding of lung cancer pathogenesis and these models can serve as valuable research tools.

Dysfunctional telomeres trigger cellular senescence mediated by cyclic GMP-AMP synthase

Defective DNA damage response (DDR) signaling is a common mechanism that initiates and maintains the cellular senescence phenotype. Dysfunctional telomeres activate DDR signaling, genomic instability, and cellular senescence, but the links among these events remains unclear. Here, using an array of biochemical and imaging techniques, including a highly regulatable CRISPR/Cas9 strategy to induce DNA double strand breaks specifically in the telomeres, ChIP, telomere immunofluorescence, fluorescence in situ hybridization (FISH), micronuclei imaging, and the telomere shortest length assay (TeSLA), we show that chromosome mis-segregation due to imperfect DDR signaling in response to dysfunctional telomeres creates a preponderance of chromatin fragments in the cytosol, which leads to a premature senescence phenotype. We found that this phenomenon is caused not by telomere shortening, but by cyclic GMP-AMP synthase (cGAS) recognizing cytosolic chromatin fragments and then activating the stimulator of interferon genes (STING) cytosolic DNA-sensing pathway and downstream interferon signaling. Significantly, genetic and pharmacological manipulation of cGAS not only attenuated immune signaling, but also prevented premature cellular senescence in response to dysfunctional telomeres. The findings of our study uncover a cellular intrinsic mechanism involving the cGAS-mediated cytosolic self-DNA-sensing pathway that initiates premature senescence independently of telomere shortening.

Chemical intervention of influenza virus mRNA nuclear export

Influenza A viruses are human pathogens with limited therapeutic options. Therefore, it is crucial to devise strategies for the identification of new classes of antiviral medications. The influenza A virus genome is constituted of 8 RNA segments. Two of these viral RNAs are transcribed into mRNAs that are alternatively spliced. The M1 mRNA encodes the M1 protein but is also alternatively spliced to yield the M2 mRNA during infection. M1 to M2 mRNA splicing occurs at nuclear speckles, and M1 and M2 mRNAs are exported to the cytoplasm for translation. M1 and M2 proteins are critical for viral trafficking, assembly, and budding. Here we show that gene knockout of the cellular protein NS1-BP, a constituent of the M mRNA speckle-export pathway and a binding partner of the virulence factor NS1 protein, inhibits M mRNA nuclear export without altering bulk cellular mRNA export, providing an avenue to preferentially target influenza virus. We performed a high-content, image-based chemical screen using single-molecule RNA-FISH to label viral M mRNAs followed by multistep quantitative approaches to assess cellular mRNA and cell toxicity. We identified inhibitors of viral mRNA biogenesis and nuclear export that exhibited no significant activity towards bulk cellular mRNA at non-cytotoxic concentrations. Among the hits is a small molecule that preferentially inhibits nuclear export of a subset of viral and cellular mRNAs without altering bulk cellular mRNA export. These findings underscore specific nuclear export requirements for viral mRNAs and phenocopy down-regulation of the mRNA export factor UAP56. This RNA export inhibitor impaired replication of diverse influenza A virus strains at non-toxic concentrations. Thus, this screening strategy yielded compounds that alone or in combination may serve as leads to new ways of treating influenza virus infection and are novel tools for studying viral RNA trafficking in the nucleus.

SLC43A3 Is a Biomarker of Sensitivity to the Telomeric DNA Damage Mediator 6-Thio-2'-Deoxyguanosine

Cell membrane transporters facilitate the passage of nucleobases and nucleosides for nucleotide synthesis and metabolism, and are important for the delivery of nucleoside analogues used in anticancer drug therapy. Here, we investigated if cell membrane transporters are involved in the cellular uptake of the nucleoside analogue DNA damage mediator 6-thio-2'-deoxyguanosine (6-thio-dG). A large panel of non-small cell lung cancer (NSCLC) cell lines (73 of 77) were sensitive to 6-thio-dG; only four NSCLC lines were resistant to 6-thio-dG. When analyzed by microarray and RNA sequencing, the resistant NSCLC cell lines clustered together, providing a molecular signature for patients that may not respond to 6-thio-dG. Significant downregulation of solute carrier family 43 A3 (SLC43A3), an equilibrative nucleobase transporter, was identified as a candidate in this molecular resistance signature. High levels of SLC43A3 mRNA predicted sensitivity to 6-thio-dG and therefore SLC43A3 could serve as a promising biomarker for 6-thio-dG sensitivity in patients with NSCLC. SIGNIFICANCE: These findings identify a biomarker of resistance to the telomeric DNA damage mediator 6-thio-2'-deoxyguanosine.

TRA-1-60-positive/CD45low cells found in the peripheral blood of prostate cancer patients with metastatic disease - A proof-of-concept study

Purpose

Over 90% of all cancer related deaths are due to metastasis. However, current diagnostic tools can't reliably discriminate between invasive and localized cancers.

Patients and methods

In this proof-of-concept study, we employed the embryonic stem cell marker TRA-1-60 (TRA+) to identify TRA + cells within the blood of prostate cancer patients and searched for TRA + cells in men with metastatic and localized cancers. We isolated whole peripheral blood mononuclear cells from 26 metastatic prostate cancer patients, from 13 patients with localized prostate cancer and from 17 healthy controls. Cells were stained for DAPI, CD45 and TRA + by immunofluorescence and imaged by epi-fluorescence microscopy. Imaged-based software was used both to identify TRA + cells, and to analyze CD45 levels in TRA+ and negative cells.

Results

We found high numbers of TRA + cells within the blood of metastatic cancer patients, whereas healthy individuals or men with localized prostate cancer showed none or very low numbers of TRA + cells. Further analysis of the CD45 levels of TRA + cells revealed a small population of TRA + cells with almost undetectable CD45 levels that were found frequently in metastatic prostate cancer patients. By excluding CD45 positive cells from the TRA + cell pool, we were able to refine the assay to be highly specific in identifying men with metastatic disease. In fact, the difference of CD45 levels between TRA+ and negative cells was a robust measure to distinguish between men with localized and metastatic prostate cancers in this small patient cohort.

Conclusions

The data suggest that metastatic prostate cancer patient have significant numbers of TRA+/CD45^low cells which might represent a potential tool for diagnostic assessment in the future.

Lung cancer progression using fast switching multiple ion beam radiation and countermeasure prevention

Most of the research in understanding space radiation-induced cancer progression and risk assessment has been performed using mono-energetic single-ion beams. However, the space radiation environment consists of a wide variety of ion species with a various range of energies. Using the fast beam switching technology developed at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory (BNL), ion species can be switched rapidly allowing investigators to use multiple ions with different energies to simulate more closely the radiation environment found in space. Here, we exposed a lung cancer susceptible mouse model (K-ras^LA-1) to three sequential ion beams: Proton (H) (120 MeV/n) 20 cGy, Helium (He) (250 MeV/n) 5.0 cGy, and Silicon (Si) (300 MeV/n) 5.0 cGy with a dose rate of 0.5 cGy/min. Using three ion beams we performed whole body irradiation with a total dose of 30 cGy in two different orders: 3B-1 (H→He→Si) and 3B-2 (Si→He→H) and used 30 cGy H single-ion beam as a reference. In this study we show that whole-body irradiation with H→He→Si increases the incidence of premalignant lesions and systemic oxidative stress in mice 100 days post-irradiation more than (Si→He→H) and H only irradiation. Additionally, we observed an increase in adenomas with atypia and adenocarcinomas in H→He→Si irradiated mice but not in (Si→He→H) or H (30 cGy) only irradiated mice. When we used the H→He→Si irradiation sequence but skipped a day before exposing the mice to Si, we did not observe the increased incidence of cancer initiation and progression. We also found that a non-toxic anti-inflammatory, anti-oxidative radioprotector (CDDO-EA) reduced H→He→Si induced oxidative stress and cancer initiation almost back to baseline. Thus, exposure to H→He→Si elicits significant changes in lung cancer initiation that can be mitigated using CDDO-EA.

Proliferation of adult human bronchial epithelial cells without a telomere maintenance mechanism for over 200 population doublings

To date, there is no direct evidence of telomerase activity in adult lung epithelial cells, but typical culture conditions only support cell proliferation for 30-40 population doublings (PD), a point at which telomeres remain relatively long. Here we report that in in vitro low stress culture conditions consisting of a fibroblast feeder layer, rho-associated coiled coil protein kinase inhibitor (ROCKi), and low oxygen (2%), normal human bronchial epithelial basal progenitor cells (HBECs) divide for over 200 PD without engaging a telomere maintenance mechanism (almost four times the "Hayflick limit"). HBECs exhibit critically short telomeres at 200 PD and the population of cells start to undergo replicative senescence. Subcloning these late passage cells to clonal density, to mimic lung injury in vivo, selects for rare subsets of HBECs that activate low levels of telomerase activity to maintain short telomeres. CRISPR/Cas9 knockout of human telomerase reverse transcriptase or treatment with the telomerase-mediated telomere targeting agent 6-thio-2'deoxyguanosine abrogates colony growth in these late passage cultures (>200 PD) but not in early passage cultures (<200 PD). To our knowledge, this is the first study to report such long-term growth of HBECs without a telomere maintenance mechanism. This report also provides direct evidence of telomerase activation in HBECs near senescence when telomeres are critically short. This novel cell culture system provides an experimental model to understand how telomerase is regulated in normal adult tissues.

Decellularized mice colons as models to study the contribution of the extracellular matrix to cell behavior and colon cancer progression

Current 3D culture models to study colorectal cancer lack architectural support and signaling proteins provided by the tissue extracellular matrix (ECM) which may influence cell behavior and cancer progression. Therefore, the ability to study cancer cells in the context of a matrix that is physiologically more relevant and to understand how the ECM affects cancer progression has been understudied. To address this, we developed an ex-vivo 3D system, provided by intact wild type (WT) and colon cancer susceptible decellularized mouse colons (DMC), to support the growth of human cancer cells. DMC are free of viable cells but still contain extracellular matrix proteins including subsets of collagens. Stiffness, an important mechanical property, is also maintained in DMCs. Importantly, we observed that the DMC is permissive for cell proliferation and differentiation of a human colon cancer cell line (HT-29). Notably, the ability of cells in the WT DMC to differentiate was also greater when compared to Matrigel™, an extracellular matrix extract from a mouse tumor cell line. Additionally, we observed in invasion assays that DMC obtained from polyps from a colon cancer susceptible mouse model facilitated increased cell migration/invasion of colorectal cancer cells and immortalized non-tumor colonic epithelial cells compared to DMC from WT mice. Finally, using mass spectrometry, we identified extracellular matrix proteins that are more abundant in DMC from a colorectal cancer mouse model compared to age and sex-matched WT mice. We propose that these abundantly expressed proteins in the tumor microenvironment are potentially involved in colorectal cancer progression. STATEMENT OF SIGNIFICANCE: Decellularized matrices, when properly produced, are attractive biomaterials for tissue regeneration and replacement. We show here that the mouse decellularized matrices can also be repurposed to elucidate how the extracellular matrix influences human cell behavior and cancer progression. To do this we produce decellularized matrices, from mice colonic tissue, that have preserved tissue mechanical and structural properties. We demonstrate that the matrix better supports the differentiation of HT-29 cells, a colonic cancer cell line, compared to Matrigel™. Additionally, we show that the extracellular matrix contributes to colon cancer progression via invasion assays using extracellular matrix extracts. Finally, we use mass spectrometry to identify ECM proteins that are more abundant in colonic polyps compared to adjacent tissue regions. This model system may have therapeutic implications for colorectal cancer patients.

MAP9 Loss Triggers Chromosomal Instability, Initiates Colorectal Tumorigenesis, and Is Associated with Poor Survival of Patients with Colorectal Cancer

PURPOSE

Chromosomal instability (CIN) is a common phenomenon in colorectal cancer, but its role and underlying cause remain unknown. We have identified that mitotic regulator microtubule-associated protein 9 (MAP9) is a critical regulator of CIN in colorectal cancer. We thus studied the effect of MAP9 loss on colorectal cancer in Map9-knockout mice and in cell lines.

EXPERIMENTAL DESIGN

We generated colon epithelial-specific Map9-knockout mice and evaluated colorectal cancer development. Effect of Map9 knockout on colorectal cancer progression was determined in chemical or Apc^{Min /+} -induced colorectal cancer. Molecular mechanism of MAP9 was determined using spectral karyotyping, microtubule assays, and whole-genome sequencing (WGS). Clinical significance of MAP9 was examined in 141 patients with CRC.

RESULTS

Spontaneous colonic tumors (9.1%) were developed in colon epithelium-specific Map9-knockout mice at 17 months, but none was observed in wild-type littermates. Map9 deletion accelerated colorectal cancer formation both in Apc^{Min /+} mice and azoxymethane-treated mice, and reduced survival in Apc^{Min /+} mice. Mechanistically, MAP9 stabilized microtubules and mediated mitotic spindle assembly. MAP9 also maintained the spindle pole integrity and protected K-fiber from depolymerization at spindle poles. MAP9 loss induced severe mitosis failure, chromosome segregation errors, and aneuploidy, leading to transformation of normal colon epithelial cells. WGS confirmed enhanced CIN in intestinal tumors from Map9 knockout Apc^{Min /+} mice. In patients with colorectal cancer, MAP9 was frequently silenced and its downregulation was associated with poor survival.

CONCLUSIONS

MAP9 is a microtubule stabilizer that contributes to spindle stability and inhibits colorectal tumorigenesis, supporting the role of MAP9 as a tumor suppressor for preventing CIN in colorectal cancer.

Quantitative mitochondrial DNA copy number determination using droplet digital PCR with single-cell resolution

Mitochondria are involved in a number of diverse cellular functions, including energy production, metabolic regulation, apoptosis, calcium homeostasis, cell proliferation, and motility, as well as free radical generation. Mitochondrial DNA (mtDNA) is present at hundreds to thousands of copies per cell in a tissue-specific manner. mtDNA copy number also varies during aging and disease progression and therefore might be considered as a biomarker that mirrors alterations within the human body. Here, we present a new quantitative, highly sensitive droplet digital PCR (ddPCR) method, droplet digital mitochondrial DNA measurement (ddMDM), to measure mtDNA copy number not only from cell populations but also from single cells. Our developed assay can generate data in as little as 3 h, is optimized for 96-well plates, and also allows the direct use of cell lysates without the need for DNA purification or nuclear reference genes. We show that ddMDM is able to detect differences between samples whose mtDNA copy number was close enough as to be indistinguishable by other commonly used mtDNA quantitation methods. By utilizing ddMDM, we show quantitative changes in mtDNA content per cell across a wide variety of physiological contexts including cancer progression, cell cycle progression, human T cell activation, and human aging.

In perspective: An update on telomere targeting in cancer

Engaging a telomere maintenance mechanism during DNA replication is essential for almost all advanced cancers. The conversion from normal and premalignant somatic cells to advanced malignant cells often results (85%-90%) from the reactivation of the functional ribonucleoprotein holoenzyme complex, referred to as telomerase. Modulation of the human telomerase reverse transcriptase (hTERT) appears to be rate limiting to produce functional telomerase and engage a telomere maintenance mechanism. The remaining 10% to 15% of cancers overcome progressively shortened telomeres by activating an alternative lengthening of telomeres (ALT) maintenance mechanism, through a DNA recombination pathway. Exploration into the specific mechanisms of telomere maintenance in cancer have led to the development of drugs such as Imetelstat (GRN163L), BIBR1532, 6-thio-dG, VE-822, and NVP-BEZ235 being investigated as therapeutic approaches for treating telomerase and ALT tumors. The successful use of 6-thio-dG (a nucleoside preferentially recognized by telomerase) that targets and uncaps telomeres in telomerase positive but not normal telomerase silent cells has recently shown impressive effects on multiple types of cancer. For example, 6-thio-dG overcomes therapy-resistant cancers in a fast-acting mechanism potentially providing an alternative or additional route of treatment for patients with cancer. In this perspective, we provide a synopsis of the current landscape of telomeres and telomerase processing in cancer development and how this new knowledge may improve outcomes for patients with cancer.

Infection with genotoxin-producing Salmonella enterica synergises with loss of the tumour suppressor APC in promoting genomic instability via the PI3K pathway in colonic epithelial cells

Several commensal and pathogenic Gram‐negative bacteria produce DNA‐damaging toxins that are considered bona fide carcinogenic agents. The microbiota of colorectal cancer (CRC) patients is enriched in genotoxin‐producing bacteria, but their role in the pathogenesis of CRC is poorly understood. The adenomatous polyposis coli (APC) gene is mutated in familial adenomatous polyposis and in the majority of sporadic CRCs. We investigated whether the loss of APC alters the response of colonic epithelial cells to infection by Salmonella enterica, the only genotoxin‐producing bacterium associated with cancer in humans. Using 2D and organotypic 3D cultures, we found that APC deficiency was associated with sustained activation of the DNA damage response, reduced capacity to repair different types of damage, including DNA breaks and oxidative damage, and failure to induce cell cycle arrest. The reduced DNA repair capacity and inability to activate adequate checkpoint responses was associated with increased genomic instability in APC‐deficient cells exposed to the genotoxic bacterium. Inhibition of the checkpoint response was dependent on activation of the phosphatidylinositol 3‐kinase pathway. These findings highlight the synergistic effect of the loss of APC and infection with genotoxin‐producing bacteria in promoting a microenvironment conducive to malignant transformation.

MYC promotes tryptophan uptake and metabolism by the kynurenine pathway in colon cancer

Tumors display increased uptake and processing of nutrients to fulfill the demands of rapidly proliferating cancer cells. Seminal studies have shown that the proto-oncogene MYC promotes metabolic reprogramming by altering glutamine uptake and metabolism in cancer cells. How MYC regulates the metabolism of other amino acids in cancer is not fully understood. Using high-performance liquid chromatography (HPLC)-tandem mass spectrometry (LC-MS/MS), we found that MYC increased intracellular levels of tryptophan and tryptophan metabolites in the kynurenine pathway. MYC induced the expression of the tryptophan transporters SLC7A5 and SLC1A5 and the enzyme arylformamidase (AFMID), involved in the conversion of tryptophan into kynurenine. SLC7A5, SLC1A5, and AFMID were elevated in colon cancer cells and tissues, and kynurenine was significantly greater in tumor samples than in the respective adjacent normal tissue from patients with colon cancer. Compared with normal human colonic epithelial cells, colon cancer cells were more sensitive to the depletion of tryptophan. Blocking enzymes in the kynurenine pathway caused preferential death of established colon cancer cells and transformed colonic organoids. We found that only kynurenine and no other tryptophan metabolite promotes the nuclear translocation of the transcription factor aryl hydrocarbon receptor (AHR). Blocking the interaction between AHR and kynurenine with CH223191 reduced the proliferation of colon cancer cells. Therefore, we propose that limiting cellular kynurenine or its downstream targets could present a new strategy to reduce the proliferation of MYC-dependent cancer cells.

Transient introduction of human telomerase mRNA improves hallmarks of progeria cells

Hutchinson-Gilford progeria syndrome (HGPS) is characterized by accelerated senescence due to a de novo mutation in the LMNA gene. The mutation produces an abnormal lamin A protein called progerin that lacks the splice site necessary to remove a farnesylated domain. Subsequently, progerin accumulates in the nuclear envelope, disrupting nuclear architecture, chromatin organization, and gene expression. These alterations are often associated with rapid telomere erosion and cellular aging. Here, we further characterize the cellular and molecular abnormalities in HGPS cells and report a significant reversal of some of these abnormalities by introduction of in vitro transcribed and purified human telomerase (hTERT) mRNA. There is intra-individual heterogeneity of expression of telomere-associated proteins DNA PKcs/Ku70/Ku80, with low-expressing cells having shorter telomeres. In addition, the loss of the heterochromatin marker H3K9me3 in progeria is associated with accelerated telomere erosion. In HGPS cell lines characterized by short telomeres, transient transfections with hTERT mRNA increase telomere length, increase expression of telomere-associated proteins, increase proliferative capacity and cellular lifespan, and reverse manifestations of cellular senescence as assessed by β-galactosidase expression and secretion of inflammatory cytokines. Unexpectedly, mRNA hTERT also improves nuclear morphology. In combination with the farnesyltransferase inhibitor (FTI) lonafarnib, hTERT mRNA promotes HGPS cell proliferation. Our findings demonstrate transient expression of human telomerase in combination with FTIs could represent an improved therapeutic approach for HGPS.

Telomeres and telomerase: three decades of progress

Many recent advances have emerged in the telomere and telomerase fields. This Timeline article highlights the key advances that have expanded our views on the mechanistic underpinnings of telomeres and telomerase and their roles in ageing and disease. Three decades ago, the classic view was that telomeres protected the natural ends of linear chromosomes and that telomerase was a specific telomere-terminal transferase necessary for the replication of chromosome ends in single-celled organisms. While this concept is still correct, many diverse fields associated with telomeres and telomerase have substantially matured. These areas include the discovery of most of the key molecular components of telomerase, implications for limits to cellular replication, identification and characterization of human genetic disorders that result in premature telomere shortening, the concept that inhibiting telomerase might be a successful therapeutic strategy and roles for telomeres in regulating gene expression. We discuss progress in these areas and conclude with challenges and unanswered questions in the field.

Catalysis-dependent inactivation of human telomerase and its reactivation by intracellular telomerase-activating factors (iTAFs)

Human telomerase maintains genome stability by adding telomeric repeats to the ends of linear chromosomes. Although previous studies have revealed profound insights into telomerase functions, the low cellular abundance of functional telomerase and the difficulties in quantifying its activity leave its thermodynamic and kinetic properties only partially characterized. Employing a stable cell line overexpressing both the human telomerase RNA component and the N-terminally biotinylated human telomerase reverse transcriptase and using a newly developed method to count individual extension products, we demonstrate here that human telomerase holoenzymes contain fast- and slow-acting catalytic sites. Surprisingly, both active sites became inactive after two consecutive rounds of catalysis, named single-run catalysis. The fast active sites turned off ∼40-fold quicker than the slow ones and exhibited higher affinities to DNA substrates. In a dimeric enzyme, the two active sites work in tandem, with the faster site functioning before the slower one, and in the monomeric enzyme, the active sites also perform single-run catalysis. Interestingly, inactive enzymes could be reactivated by intracellular telomerase-activating factors (iTAFs) from multiple cell types. We conclude that the single-run catalysis and the iTAF-triggered reactivation serve as an unprecedented control circuit for dynamic regulation of telomerase. They endow native telomerase holoenzymes with the ability to match their total number of active sites to the number of telomeres they extend. We propose that the exquisite kinetic control of telomerase activity may play important roles in both cell division and cell aging.

Clustered telomeres in phase-separated nuclear condensates engage mitotic DNA synthesis through BLM and RAD52

Alternative lengthening of telomeres (ALT) is a telomerase-independent telomere maintenance mechanism that occurs in a subset of cancers. One of the hallmarks of ALT cancer is the excessively clustered telomeres in promyelocytic leukemia (PML) bodies, represented as large bright telomere foci. Here, we present a model system that generates telomere clustering in nuclear polySUMO (small ubiquitin-like modification)/polySIM (SUMO-interacting motif) condensates, analogous to PML bodies, and thus artificially engineered ALT-associated PML body (APB)-like condensates in vivo. We observed that the ALT-like phenotypes (i.e., a small fraction of heterogeneous telomere lengths and formation of C circles) are rapidly induced by introducing the APB-like condensates together with BLM through its helicase domain, accompanied by ssDNA generation and RPA accumulation at telomeres. Moreover, these events lead to mitotic DNA synthesis (MiDAS) at telomeres mediated by RAD52 through its highly conserved N-terminal domain. We propose that the clustering of large amounts of telomeres in human cancers promotes ALT that is mediated by MiDAS, analogous to Saccharomyces cerevisiae type II ALT survivors.

Design and Synthesis of TASIN Analogues Specifically Targeting Colorectal Cancer Cell Lines with Mutant Adenomatous Polyposis Coli (APC)

Despite advances in targeted anticancer therapies, there are still no small-molecule-based therapies available that specifically target colorectal cancer (CRC) development and progression, the second leading cause of cancer deaths. We previously disclosed the discovery of truncating adenomatous polyposis coli (APC)-selective inhibitor 1 (TASIN-1), a small molecule that specifically targets colorectal cancer cells lines with truncating mutations in the adenomatous polyposis coli (APC) tumor suppressor gene through inhibition of cholesterol biosynthesis. Here, we report a medicinal chemistry evaluation of a collection of TASIN analogues and activity against colon cancer cell lines and an isogenic cell line pair reporting on the status of APC-dependent selectivity. A number of potent and selective analogues were identified, including compounds with good metabolic stability and pharmacokinetic properties. The compounds reported herein represent a first-in-class genotype-selective series that specifically target apc mutations present in the majority of CRC patients and serve as a translational platform toward a targeted therapy for colon cancer.

Telomere length and telomerase activity in T cells are biomarkers of high-performing centenarians

It is generally recognized that the function of the immune system declines with increased age and one of the major immune changes is impaired T-cell responses upon antigen presentation/stimulation. Some "high-performing" centenarians (100+ years old) are remarkably successful in escaping, or largely postponing, major age-related diseases. However, the majority of centenarians ("low-performing") have experienced these pathologies and are forced to reside in long-term nursing facilities. Previous studies have pooled all centenarians examining heterogeneous populations of resting/unstimulated peripheral blood mononuclear cells (PBMCs). T cells represent around 60% of PBMC and are in a quiescence state when unstimulated. However, upon stimulation, T cells rapidly divide and exhibit dramatic changes in gene expression. We have compared stimulated T-cell responses and identified a set of transcripts expressed in vitro that are dramatically different in high- vs. low-performing centenarians. We have also identified several other measurements that are different between high- and low-performing centenarians: (a) The amount of proliferation following in vitro stimulation is dramatically greater in high-performing centenarians compared to 67- to 83-year-old controls and low-performing centenarians; (b) telomere length is greater in the high-performing centenarians; and (c) telomerase activity following stimulation is greater in the high-performing centenarians. In addition, we have validated a number of genes whose expression is directly related to telomere length and these are potential fundamental biomarkers of aging that may influence the risk and progression of multiple aging conditions.

Reflections on telomere dynamics and ageing-related diseases in humans

Epidemiological studies have principally relied on measurements of telomere length (TL) in leucocytes, which reflects TL in other somatic cells. Leucocyte TL (LTL) displays vast variation across individuals—a phenomenon already observed in newborns. It is highly heritable, longer in females than males and in individuals of African ancestry than European ancestry. LTL is also longer in offspring conceived by older men. The traditional view regards LTL as a passive biomarker of human ageing. However, new evidence suggests that a dynamic interplay between selective evolutionary forces and TL might result in trade-offs for specific health outcomes. From a biological perspective, an active role of TL in ageing-related human diseases could occur because short telomeres increase the risk of a category of diseases related to restricted cell proliferation and tissue degeneration, including cardiovascular disease, whereas long telomeres increase the risk of another category of diseases related to increased proliferative growth, including major cancers. To understand the role of telomere biology in ageing-related diseases, it is essential to expand telomere research to newborns and children and seek further insight into the underlying causes of the variation in TL due to ancestry and geographical location.

This article is part of the theme issue ‘Understanding diversity in telomere dynamics’.

Comparison of telomere length measurement methods

The strengths and limitations of the major methods developed to measure telomere lengths (TLs) in cells and tissues are presented in this review. These include Q-PCR (Quantitative Polymerase Chain Reaction), TRF (Terminal Restriction Fragment) analysis, a variety of Q-FISH (Quantitative Fluorescence In Situ Hybridization) methods, STELA (Single TElomere Length Analysis) and TeSLA (Telomere Shortest Length Assay). For each method, we will cover information about validation studies, including reproducibility in independent laboratories, accuracy, reliability and sensitivity for measuring not only the average but also the shortest telomeres. There is substantial evidence that it is the shortest telomeres that trigger DNA damage responses leading to replicative senescence in mammals. However, the most commonly used TL measurement methods generally provide information on average or relative TL, but it is the shortest telomeres that leads to telomere dysfunction (identified by TIF, Telomere dysfunction Induced Foci) and limit cell proliferation in the absence of a telomere maintenance mechanism, such as telomerase. As the length of the shortest telomeres is a key biomarker determining cell fate and the onset of senescence, a new technique (TeSLA) that provides quantitative information about all the shortest telomeres will be highlighted.This article is part of the theme issue 'Understanding diversity in telomere dynamics'.

Telomere length-dependent transcription and epigenetic modifications in promoters remote from telomere ends

Telomere-binding proteins constituting the shelterin complex have been studied primarily for telomeric functions. However, mounting evidence shows non-telomeric binding and gene regulation by shelterin factors. This raises a key question—do telomeres impact binding of shelterin proteins at distal non-telomeric sites? Here we show that binding of the telomere-repeat-binding-factor-2 (TRF2) at promoters ~60 Mb from telomeres depends on telomere length in human cells. Promoter TRF2 occupancy was depleted in cells with elongated telomeres resulting in altered TRF2-mediated transcription of distal genes. In addition, histone modifications—activation (H3K4me1 and H3K4me3) as well as silencing marks (H3K27me3)—at distal promoters were telomere length-dependent. These demonstrate that transcription, and the epigenetic state, of telomere-distal promoters can be influenced by telomere length. Molecular links between telomeres and the extra-telomeric genome, emerging from findings here, might have important implications in telomere-related physiology, particularly ageing and cancer.

Telomeres (special DNA-protein assemblies that protect chromosome ends) affect ageing and diseases such as cancer. Although this has been recognized for many years, biological processes that connect telomeres to ageing, cancer and other cellular functions remain to be fully understood. Certain proteins, believed to be only telomere-associated, engage DNA outside telomeres. This raises an interesting question. Does telomere length influence how telomere-binding proteins associate with DNA at regions distal from telomeres. If so, how does this impact function? Motivated by these questions, in the present studies we tested if extra-telomeric binding of the well-known telomere-repeat-binding-actor-2 (TRF2) depends on telomere length. Our results show that the level of DNA-bound TRF2 at telomere-distal sites changes as telomeres shorten or elongate. Consequently, TRF2-mediated gene regulation affects many genes. Notably, histone modifications that dictate chromatin compaction and access to regulatory factors, at sites distant from telomere ends also depended on telomere length. Together, this links the state of telomeres to gene regulation and epigenetics directly in ways not previously appreciated that might impact a more complete understanding of molecular processes underlying ageing and cancer.