Clonal inactivation of TERT impairs stem cell competition.

Telomerase is intimately associated with stem cells and cancer, because it catalytically elongates telomeres-nucleoprotein caps that protect chromosome ends1. Overexpression of telomerase reverse transcriptase (TERT) enhances the proliferation of cells in a telomere-independent manner2-8, but so far, loss-of-function studies have provided no evidence that TERT has a direct role in stem cell function. In many tissues, homeostasis is shaped by stem cell competition, a process in which stem cells compete on the basis of inherent fitness. Here we show that conditional deletion of Tert in the spermatogonial stem cell (SSC)-containing population in mice markedly impairs competitive clone formation. Using lineage tracing from the Tert locus, we find that TERT-expressing SSCs yield long-lived clones, but that clonal inactivation of TERT promotes stem cell differentiation and a genome-wide reduction in open chromatin. This role for TERT in competitive clone formation occurs independently of both its reverse transcriptase activity and the canonical telomerase complex. Inactivation of TERT causes reduced activity of the MYC oncogene, and transgenic expression of MYC in the TERT-deleted pool of SSCs efficiently rescues clone formation. Together, these data reveal a catalytic-activity-independent requirement for TERT in enhancing stem cell competition, uncover a genetic connection between TERT and MYC and suggest that a selective advantage for stem cells with high levels of TERT contributes to telomere elongation in the male germline during homeostasis and ageing.

Digital telomere measurement by long-read sequencing distinguishes healthy aging from disease.

Telomere length is an important biomarker of organismal aging and cellular replicative potential, but existing measurement methods are limited in resolution and accuracy. Here, we deploy digital telomere measurement (DTM) by nanopore sequencing to understand how distributions of human telomere length change with age and disease. We measure telomere attrition and de novo elongation with up to 30 bp resolution in genetically defined populations of human cells, in blood cells from healthy donors and in blood cells from patients with genetic defects in telomere maintenance. We find that human aging is accompanied by a progressive loss of long telomeres and an accumulation of shorter telomeres. In patients with defects in telomere maintenance, the accumulation of short telomeres is more pronounced and correlates with phenotypic severity. We apply machine learning to train a binary classification model that distinguishes healthy individuals from those with telomere biology disorders. This sequencing and bioinformatic pipeline will advance our understanding of telomere maintenance mechanisms and the use of telomere length as a clinical biomarker of aging and disease.

Digital telomere measurement by long-read sequencing distinguishes healthy aging from disease.

Telomere length is an important biomarker of organismal aging and cellular replicative potential, but existing measurement methods are limited in resolution and accuracy. Here, we deploy digital telomere measurement by nanopore sequencing to understand how distributions of human telomere length change with age and disease. We measure telomere attrition and de novo elongation with unprecedented resolution in genetically defined populations of human cells, in blood cells from healthy donors and in blood cells from patients with genetic defects in telomere maintenance. We find that human aging is accompanied by a progressive loss of long telomeres and an accumulation of shorter telomeres. In patients with defects in telomere maintenance, the accumulation of short telomeres is more pronounced and correlates with phenotypic severity. We apply machine learning to train a binary classification model that distinguishes healthy individuals from those with telomere biology disorders. This sequencing and bioinformatic pipeline will advance our understanding of telomere maintenance mechanisms and the use of telomere length as a clinical biomarker of aging and disease.

TRF2 rescues telomere attrition and prolongs cell survival in Duchenne muscular dystrophy cardiomyocytes derived from human iPSCs.

Duchenne muscular dystrophy (DMD) is a severe muscle wasting disease caused by the lack of dystrophin. Heart failure, driven by cardiomyocyte death, fibrosis, and the development of dilated cardiomyopathy, is the leading cause of death in DMD patients. Current treatments decrease the mechanical load on the heart but do not address the root cause of dilated cardiomyopathy: cardiomyocyte death. Previously, we showed that telomere shortening is a hallmark of DMD cardiomyocytes. Here, we test whether prevention of telomere attrition is possible in cardiomyocytes differentiated from patient-derived induced pluripotent stem cells (iPSC-CMs) and if preventing telomere shortening impacts cardiomyocyte function. We observe reduced cell size, nuclear size, and sarcomere density in DMD iPSC-CMs compared with healthy isogenic controls. We find that expression of just one telomere-binding protein, telomeric repeat-binding factor 2 (TRF2), a core component of the shelterin complex, prevents telomere attrition and rescues deficiencies in cell size as well as sarcomere density. We employ a bioengineered platform to micropattern cardiomyocytes for calcium imaging and perform Southern blots of telomere restriction fragments, the gold standard for telomere length assessments. Importantly, preservation of telomere lengths in DMD cardiomyocytes improves their viability. These data provide evidence that preventing telomere attrition ameliorates deficits in cell morphology, activation of the DNA damage response, and premature cell death, suggesting that TRF2 is a key player in DMD-associated cardiac failure.

TGS1 impacts snRNA 3'-end processing, ameliorates survival motor neuron-dependent neurological phenotypes in vivo and prevents neurodegeneration.

Trimethylguanosine synthase 1 (TGS1) is a highly conserved enzyme that converts the 5'-monomethylguanosine cap of small nuclear RNAs (snRNAs) to a trimethylguanosine cap. Here, we show that loss of TGS1 in Caenorhabditis elegans, Drosophila melanogaster and Danio rerio results in neurological phenotypes similar to those caused by survival motor neuron (SMN) deficiency. Importantly, expression of human TGS1 ameliorates the SMN-dependent neurological phenotypes in both flies and worms, revealing that TGS1 can partly counteract the effects of SMN deficiency. TGS1 loss in HeLa cells leads to the accumulation of immature U2 and U4atac snRNAs with long 3' tails that are often uridylated. snRNAs with defective 3' terminations also accumulate in Drosophila Tgs1 mutants. Consistent with defective snRNA maturation, TGS1 and SMN mutant cells also exhibit partially overlapping transcriptome alterations that include aberrantly spliced and readthrough transcripts. Together, these results identify a neuroprotective function for TGS1 and reinforce the view that defective snRNA maturation affects neuronal viability and function.

Targeted replacement of full-length CFTR in human airway stem cells by CRISPR-Cas9 for pan-mutation correction in the endogenous locus.

Cystic fibrosis (CF) is a monogenic disease caused by impaired production and/or function of the CF transmembrane conductance regulator (CFTR) protein. Although we have previously shown correction of the most common pathogenic mutation, there are many other pathogenic mutations throughout the CF gene. An autologous airway stem cell therapy in which the CFTR cDNA is precisely inserted into the CFTR locus may enable the development of a durable cure for almost all CF patients, irrespective of the causal mutation. Here, we use CRISPR-Cas9 and two adeno-associated viruses (AAVs) carrying the two halves of the CFTR cDNA to sequentially insert the full CFTR cDNA along with a truncated CD19 (tCD19) enrichment tag in upper airway basal stem cells (UABCs) and human bronchial epithelial cells (HBECs). The modified cells were enriched to obtain 60%-80% tCD19+ UABCs and HBECs from 11 different CF donors with a variety of mutations. Differentiated epithelial monolayers cultured at air-liquid interface showed restored CFTR function that was >70% of the CFTR function in non-CF controls. Thus, our study enables the development of a therapy for almost all CF patients, including patients who cannot be treated using recently approved modulator therapies.

Systemic delivery of a targeted synthetic immunostimulant transforms the immune landscape for effective tumor regression.

Promoting immune activation within the tumor microenvironment (TME) is a promising therapeutic strategy to reverse tumor immunosuppression and elicit anti-tumor immunity. To enable tumor-localized immunotherapy following intravenous administration, we chemically conjugated a polyspecific integrin-binding peptide (PIP) to an immunostimulant (Toll-like receptor 9 [TLR9] agonist: CpG) to generate a tumor-targeted immunomodulatory agent, referred to as PIP-CpG. We demonstrate that systemic delivery of PIP-CpG induces tumor regression and enhances therapeutic efficacy compared with untargeted CpG in aggressive murine breast and pancreatic cancer models. Furthermore, PIP-CpG transforms the immune-suppressive TME dominated by myeloid-derived suppressor cells into a lymphocyte-rich TME infiltrated with activated CD8+ T cells, CD4+ T cells, and B cells. Finally, we show that T cells are required for therapeutic efficacy and that PIP-CpG treatment generates tumor-specific CD8+ T cells. These data demonstrate that conjugation to a synthetic tumor-targeted peptide can improve the efficacy of systemically administered immunostimulants and lead to durable anti-tumor immune responses.

The S-adenosylmethionine analog sinefungin inhibits the trimethylguanosine synthase TGS1 to promote telomerase activity and telomere lengthening.

Mutations in many genes that control the expression, the function, or the stability of telomerase cause telomere biology disorders (TBDs), such as dyskeratosis congenita, pulmonary fibrosis, and aplastic anemia. Mutations in a subset of the genes associated with TBDs cause reductions of the telomerase RNA moiety hTR, thus limiting telomerase activity. We have recently found that loss of the trimethylguanosine synthase TGS1 increases both hTR abundance and telomerase activity and leads to telomere elongation. Here, we show that treatment with the S-adenosylmethionine analog sinefungin inhibits TGS1 activity, increases the hTR levels, and promotes telomere lengthening in different cell types. Our results hold promise for restoring telomere length in stem and progenitor cells from TBD patients with reduced hTR levels.

Acinar cell clonal expansion in pancreas homeostasis and carcinogenesis.

Pancreatic ductal adenocarcinoma (PDAC) is one of the leading causes of cancer deaths worldwide1. Studies in human tissues and in mouse models have suggested that for many cancers, stem cells sustain early mutations driving tumour development2,3. For the pancreas, however, mechanisms underlying cellular renewal and initiation of PDAC remain unresolved. Here, using lineage tracing from the endogenous telomerase reverse transcriptase (Tert) locus, we identify a rare TERT-positive subpopulation of pancreatic acinar cells dispersed throughout the exocrine compartment. During homeostasis, these TERThigh acinar cells renew the pancreas by forming expanding clones of acinar cells, whereas randomly marked acinar cells do not form these clones. Specific expression of mutant Kras in TERThigh acinar cells accelerates acinar clone formation and causes transdifferentiation to ductal pre-invasive pancreatic intraepithelial neoplasms by upregulating Ras-MAPK signalling and activating the downstream kinase ERK (phospho-ERK). In resected human pancreatic neoplasms, we find that foci of phospho-ERK-positive acinar cells are common and frequently contain activating KRAS mutations, suggesting that these acinar regions represent an early cancer precursor lesion. These data support a model in which rare TERThigh acinar cells may sustain KRAS mutations, driving acinar cell expansion and creating a field of aberrant cells initiating pancreatic tumorigenesis.

Analysis of RNA conformation in endogenously assembled RNPs by icSHAPE.

The majority of the mammalian genome is transcribed into non-coding RNAs, many of which co-evolve with RNA-binding proteins (RBPs) to function as biochemically defined and tractable ribonucleoproteins (RNPs). Here, we applied icSHAPE- a robust and versatile RNA structural probing pipeline- to endogenous RNPs purified from nuclei, providing base-resolution structural rationale for RNP activity and subcellular localization. Combining with genetic and biochemical reconstitutions, structural and functional alternations can be directly attributed to a given RBP without ambiguity. For complete details on the use and execution of this protocol, please refer to Chen et al. (2018).

A novel DDB2 mutation causes defective recognition of UV-induced DNA damages and prevalent equine squamous cell carcinoma.

Squamous cell carcinoma (SCC) occurs frequently in the human Xeroderma Pigmentosum (XP) syndrome and is characterized by deficient UV-damage repair. SCC is the most common equine ocular cancer and the only associated genetic risk factor is a UV-damage repair protein. Specifically, a missense mutation in horse DDB2 (T338M) was strongly associated with both limbal SCC and third eyelid SCC in three breeds of horses (Halflinger, Belgian, and Rocky Mountain Horses) and was hypothesized to impair binding to UV-damaged DNA. Here, we investigate DDB2-T338M mutant's capacity to recognize UV lesions in vitro and in vivo, together with human XP mutants DDB2-R273H and -K244E. We show that the recombinant DDB2-T338M assembles with DDB1, but fails to show any detectable binding to DNA substrates with or without UV lesions, due to a potential structural disruption of the rigid DNA recognition ß-loop. Consistently, we demonstrate that the cellular DDB2-T338M is defective in its recruitment to focally radiated DNA damages, and in its access to chromatin. Thus, we provide direct functional evidence indicating the DDB2-T338M recapitulates molecular defects of human XP mutants, and is the causal loss-of-function allele that gives rise to equine ocular SCCs. Our findings shed new light on the mechanism of DNA recognition by UV-DDB and on the initiation of ocular malignancy.

Regulation of human telomerase in homeostasis and disease.

Telomerase is a ribonucleoprotein complex, the catalytic core of which includes the telomerase reverse transcriptase (TERT) and the non-coding human telomerase RNA (hTR), which serves as a template for the addition of telomeric repeats to chromosome ends. Telomerase expression is restricted in humans to certain cell types, and telomerase levels are tightly controlled in normal conditions. Increased levels of telomerase are found in the vast majority of human cancers, and we have recently begun to understand the mechanisms by which cancer cells increase telomerase activity. Conversely, germline mutations in telomerase-relevant genes that decrease telomerase function cause a range of genetic disorders, including dyskeratosis congenita, idiopathic pulmonary fibrosis and bone marrow failure. In this Review, we discuss the transcriptional regulation of human TERT, hTR processing, assembly of the telomerase complex, the cellular localization of telomerase and its recruitment to telomeres, and the regulation of telomerase activity. We also discuss the disease relevance of each of these steps of telomerase biogenesis.

Loss of Human TGS1 Hypermethylase Promotes Increased Telomerase RNA and Telomere Elongation.

Biogenesis of the human telomerase RNA (hTR) involves a complex series of posttranscriptional modifications, including hypermethylation of the 5' mono-methylguanosine cap to a tri-methylguanosine cap (TMG). How the TMG cap affects hTR maturation is unknown. Here, we show that depletion of trimethylguanosine synthase 1 (TGS1), the enzyme responsible for cap hypermethylation, increases levels of hTR and telomerase. Diminished trimethylation increases hTR association with the cap-binding complex (CBC) and with Sm chaperone proteins. Loss of TGS1 causes an increase in accumulation of mature hTR in both the nucleus and the cytoplasm compared with controls. In TGS1 mutant cells, increased hTR assembles with telomerase reverse transcriptase (TERT) protein to yield elevated active telomerase complexes and increased telomerase activity, resulting in telomere elongation in cultured human cells. Our results show that TGS1-mediated hypermethylation of the hTR cap inhibits hTR accumulation, restrains levels of assembled telomerase, and limits telomere elongation.

HiChIRP reveals RNA-associated chromosome conformation.

Modular domains of long non-coding RNAs can serve as scaffolds to bring distant regions of the linear genome into spatial proximity. Here, we present HiChIRP, a method leveraging bio-orthogonal chemistry and optimized chromosome conformation capture conditions, which enables interrogation of chromatin architecture focused around a specific RNA of interest down to approximately ten copies per cell. HiChIRP of three nuclear RNAs reveals insights into promoter interactions (7SK), telomere biology (telomerase RNA component) and inflammatory gene regulation (lincRNA-EPS).

Disruption of Telomerase RNA Maturation Kinetics Precipitates Disease.

Mutations in RNA-processing enzymes are increasingly linked to human disease. Telomerase RNA and related noncoding RNAs require 3' end-processing steps, including oligoadenylation. Germline mutations in poly(A)ribonuclease (PARN) cause accumulation of extended human telomerase RNA (hTR) species and precipitate dyskeratosis congenita and pulmonary fibrosis. Here, we develop nascent RNAend-seq to measure processing rates of RNA precursors. We find that mature hTR derives from extended precursors but that in PARN-mutant cells hTR maturation kinetically stalls and unprocessed precursors are degraded. Loss of poly(A)polymerase PAPD5 in PARN-mutant cells accelerates hTR maturation and restores hTR processing, indicating that oligoadenylation and deadenylation set rates of hTR maturation. The H/ACA domain mediates hTR maturation by precisely defining the 3' end, recruiting poly(A)polymerase activity, and conferring sensitivity to PARN regulation. These data reveal a feedforward circuit in which post-transcriptional oligoadenylation controls RNA maturation kinetics. Similar alterations in RNA processing rates may contribute to mechanisms of RNA-based human disease.

An Activity Switch in Human Telomerase Based on RNA Conformation and Shaped by TCAB1.

Ribonucleoprotein enzymes require dynamic conformations of their RNA constituents for regulated catalysis. Human telomerase employs a non-coding RNA (hTR) with a bipartite arrangement of domains-a template-containing core and a distal three-way junction (CR4/5) that stimulates catalysis through unknown means. Here, we show that telomerase activity unexpectedly depends upon the holoenzyme protein TCAB1, which in turn controls conformation of CR4/5. Cells lacking TCAB1 exhibit a marked reduction in telomerase catalysis without affecting enzyme assembly. Instead, TCAB1 inactivation causes unfolding of CR4/5 helices that are required for catalysis and for association with the telomerase reverse-transcriptase (TERT). CR4/5 mutations derived from patients with telomere biology disorders provoke defects in catalysis and TERT binding similar to TCAB1 inactivation. These findings reveal a conformational "activity switch" in human telomerase RNA controlling catalysis and TERT engagement. The identification of two discrete catalytic states for telomerase suggests an intramolecular means for controlling telomerase in cancers and progenitor cells.

Distributed hepatocytes expressing telomerase repopulate the liver in homeostasis and injury.

Hepatocytes are replenished gradually during homeostasis and robustly after liver injury1, 2. In adults, new hepatocytes originate from the existing hepatocyte pool3-8, but the cellular source of renewing hepatocytes remains unclear. Telomerase is expressed in many stem cell populations, and mutations in telomerase pathway genes have been linked to liver diseases9-11. Here we identify a subset of hepatocytes that expresses high levels of telomerase and show that this hepatocyte subset repopulates the liver during homeostasis and injury. Using lineage tracing from the telomerase reverse transcriptase (Tert) locus in mice, we demonstrate that rare hepatocytes with high telomerase expression (TERTHigh hepatocytes) are distributed throughout the liver lobule. During homeostasis, these cells regenerate hepatocytes in all lobular zones, and both self-renew and differentiate to yield expanding hepatocyte clones that eventually dominate the liver. In response to injury, the repopulating activity of TERTHigh hepatocytes is accelerated and their progeny cross zonal boundaries. RNA sequencing shows that metabolic genes are downregulated in TERTHigh hepatocytes, indicating that metabolic activity and repopulating activity may be segregated within the hepatocyte lineage. Genetic ablation of TERTHigh hepatocytes combined with chemical injury causes a marked increase in stellate cell activation and fibrosis. These results provide support for a 'distributed model' of hepatocyte renewal in which a subset of hepatocytes dispersed throughout the lobule clonally expands to maintain liver mass.

Purification of GFRα1+ and GFRα1- Spermatogonial Stem Cells Reveals a Niche-Dependent Mechanism for Fate Determination.

Undifferentiated spermatogonia comprise a pool of stem cells and progenitor cells that show heterogeneous expression of markers, including the cell surface receptor GFRα1. Technical challenges in isolation of GFRα1+ versus GFRα1- undifferentiated spermatogonia have precluded the comparative molecular characterization of these subpopulations and their functional evaluation as stem cells. Here, we develop a method to purify these subpopulations by fluorescence-activated cell sorting and show that GFRα1+ and GFRα1- undifferentiated spermatogonia both demonstrate elevated transplantation activity, while differing principally in receptor tyrosine kinase signaling and cell cycle. We identify the cell surface molecule melanocyte cell adhesion molecule (MCAM) as differentially expressed in these populations and show that antibodies to MCAM allow isolation of highly enriched populations of GFRα1+ and GFRα1- spermatogonia from adult, wild-type mice. In germ cell culture, GFRα1- cells upregulate MCAM expression in response to glial cell line-derived neurotrophic factor (GDNF)/fibroblast growth factor (FGF) stimulation. In transplanted hosts, GFRα1- spermatogonia yield GFRα1+ spermatogonia and restore spermatogenesis, albeit at lower rates than their GFRα1+ counterparts. Together, these data provide support for a model of a stem cell pool in which the GFRα1+ and GFRα1- cells are closely related but show key cell-intrinsic differences and can interconvert between the two states based, in part, on access to niche factors.

Approaching TERRA Firma: Genomic Functions of Telomeric Noncoding RNA.

Functions of the telomeric repeat-containing RNA (TERRA), the long noncoding RNA (lncRNA) transcribed from telomeres, have eluded researchers. In this issue of Cell, Graf el al. and Chu et al. uncover new regulatory roles for TERRA at the telomere and at distant genomic sites.

Control of Cellular Aging, Tissue Function, and Cancer by p53 Downstream of Telomeres.

Telomeres, the nucleoprotein complex at the ends of eukaryotic chromosomes, perform an essential cellular role in part by preventing the chromosomal end from initiating a DNA-damage response. This function of telomeres can be compromised as telomeres erode either as a consequence of cell division in culture or as a normal part of cellular ageing in proliferative tissues. Telomere dysfunction in this context leads to DNA-damage signaling and activation of the tumor-suppressor protein p53, which then can prompt either cellular senescence or apoptosis. By culling cells with dysfunctional telomeres, p53 plays a critical role in protecting tissues against the effects of critically short telomeres. However, as telomere dysfunction worsens, p53 likely exacerbates short telomere-driven tissue failure diseases, including pulmonary fibrosis, aplastic anemia, and liver cirrhosis. In cells lacking p53, unchecked telomere shortening drives chromosomal end-to-end fusions and cycles of chromosome fusion-bridge-breakage. Incipient cancer cells confronting these telomere barriers must disable p53 signaling to avoid senescence and eventually up-regulate telomerase to achieve cellular immortality. The recent findings of highly recurrent activating mutations in the promoter for the telomerase reverse transcriptase (TERT) gene in diverse human cancers, together with the widespread mutations in p53 in cancer, provide support for the idea that circumvention of a telomere-p53 checkpoint is essential for malignant progression in human cancer.