N-terminal residues of human dyskerin are required for interactions with telomerase RNA that prevent RNA degradation

Telomeres, telomerase, and cancer: mechanisms, biomarkers, and therapeutics

Telomeres and telomerase play crucial roles in the initiation and progression of cancer. As biomarkers, they aid in distinguishing benign from malignant tissues. Despite the promising therapeutic potential of targeting telomeres and telomerase for therapy, translating this concept from the laboratory to the clinic remains challenging. Many candidate drugs remain in the experimental stage, with only a few advancing to clinical trials. This review explores the relationship between telomeres, telomerase, and cancer, synthesizing their roles as biomarkers and reviewing the outcomes of completed trials. We propose that changes in telomere length and telomerase activity can be used to stratify cancer stages. Furthermore, we suggest that differential expression of telomere and telomerase components at the subcellular level holds promise as a biomarker. From a therapeutic standpoint, combining telomerase-targeted therapies with drugs that mitigate the adverse effects of telomerase inhibition may offer a viable strategy.

RIOK2 transcriptionally regulates TRiC and dyskerin complexes to prevent telomere shortening

Telomere shortening is a prominent hallmark of aging and is emerging as a characteristic feature of Myelodysplastic Syndromes (MDS) and Idiopathic Pulmonary Fibrosis (IPF). Optimal telomerase activity prevents progressive shortening of telomeres that triggers DNA damage responses. However, the upstream regulation of telomerase holoenzyme components remains poorly defined. Here, we identify RIOK2, a master regulator of human blood cell development, as a critical transcription factor for telomere maintenance. Mechanistically, loss of RIOK2 or its DNA-binding/transactivation properties downregulates mRNA expression of both TRiC and dyskerin complex subunits that impairs telomerase activity, thereby causing telomere shortening. We further show that RIOK2 expression is diminished in aged individuals and IPF patients, and it strongly correlates with shortened telomeres in MDS patient-derived bone marrow cells. Importantly, ectopic expression of RIOK2 alleviates telomere shortening in IPF patient-derived primary lung fibroblasts. Hence, increasing RIOK2 levels prevents telomere shortening, thus offering therapeutic strategies for telomere biology disorders.

The C-terminal extension of dyskerin is a dyskeratosis congenita mutational hotspot that modulates interaction with telomerase RNA and subcellular localization

Dyskerin is a component of the human telomerase complex and is involved in stabilizing the human telomerase RNA (hTR). Many mutations in the DKC1 gene encoding dyskerin are found in X-linked dyskeratosis congenita (X-DC), a premature aging disorder and other related diseases. The C-terminal extension (CTE) of dyskerin contributes to its interaction with the molecular chaperone SHQ1 during the early stage of telomerase biogenesis. Disease mutations in this region were proposed to disrupt dyskerin-SHQ1 interaction and destabilize dyskerin, reducing hTR levels indirectly. However, biochemical evidence supporting this hypothesis is still lacking. In addition, the effects of many CTE disease mutations on hTR have not been examined. In this study, we tested eight dyskerin CTE variants and showed that they failed to maintain hTR levels. These mutants showed slightly reduced but not abolished interaction with SHQ1, and caused defective binding to hTR. Deletion of the CTE further reduced binding to hTR, and perturbed localization of dyskerin to the Cajal bodies and the nucleolus, and the interaction with TCAB1 as well as GAR1. Our findings suggest impaired dyskerin-hTR interaction in cells as a previously overlooked mechanism through which dyskerin CTE mutations cause X-DC and related telomere syndromes.

2.7 Å cryo-EM structure of human telomerase H/ACA ribonucleoprotein

Telomerase is a ribonucleoprotein (RNP) enzyme that extends telomeric repeats at eukaryotic chromosome ends to counterbalance telomere loss caused by incomplete genome replication. Human telomerase is comprised of two distinct functional lobes tethered by telomerase RNA (hTR): a catalytic core, responsible for DNA extension; and a Hinge and ACA (H/ACA) box RNP, responsible for telomerase biogenesis. H/ACA RNPs also have a general role in pseudouridylation of spliceosomal and ribosomal RNAs, which is critical for the biogenesis of the spliceosome and ribosome. Much of our structural understanding of eukaryotic H/ACA RNPs comes from structures of the human telomerase H/ACA RNP. Here we report a 2.7 Å cryo-electron microscopy structure of the telomerase H/ACA RNP. The significant improvement in resolution over previous 3.3 Å to 8.2 Å structures allows us to uncover new molecular interactions within the H/ACA RNP. Many disease mutations are mapped to these interaction sites. The structure also reveals unprecedented insights into a region critical for pseudouridylation in canonical H/ACA RNPs. Together, our work advances understanding of telomerase-related disease mutations and the mechanism of pseudouridylation by eukaryotic H/ACA RNPs.

Control of protein synthesis through mRNA pseudouridylation by dyskerin

Posttranscriptional modifications of mRNA have emerged as regulators of gene expression. Although pseudouridylation is the most abundant, its biological role remains poorly understood. Here, we demonstrate that the pseudouridine synthase dyskerin associates with RNA polymerase II, binds to thousands of mRNAs, and is responsible for their pseudouridylation, an action that occurs in chromatin and does not appear to require a guide RNA with full complementarity. In cells lacking dyskerin, mRNA pseudouridylation is reduced, while at the same time, de novo protein synthesis is enhanced, indicating that this modification interferes with translation. Accordingly, mRNAs with fewer pseudouridines due to knockdown of dyskerin are translated more efficiently. Moreover, mRNA pseudouridylation is severely reduced in patients with dyskeratosis congenita caused by inherited mutations in the gene encoding dyskerin (i.e., DKC1). Our findings demonstrate that pseudouridylation by dyskerin modulates mRNA translatability, with important implications for both normal development and disease.

Functional genomics for curation of variants in telomere biology disorder associated genes: A systematic review

PURPOSE

Patients with an underlying telomere biology disorder (TBD) have variable clinical presentations, and they can be challenging to diagnose clinically. A genomic diagnosis for patients presenting with TBD is vital for optimal treatment. Unfortunately, many variants identified during diagnostic testing are variants of uncertain significance. This complicates management decisions, delays treatment, and risks nonuptake of potentially curative therapies. Improved application of functional genomic evidence may reduce variants of uncertain significance classifications.

METHODS

We systematically searched the literature for published functional assays interrogating TBD gene variants. When possible, established likely benign/benign and likely pathogenic/pathogenic variants were used to estimate the assay sensitivity, specificity, positive predictive value, negative predictive value, and odds of pathogenicity.

RESULTS

In total, 3131 articles were screened and 151 met inclusion criteria. Sufficient data to enable a PS3/BS3 recommendation were available for TERT variants only. We recommend that PS3 and BS3 can be applied at a moderate and supportive level, respectively. PS3/BS3 application was limited by a lack of assay standardization and limited inclusion of benign variants.

CONCLUSION

Further assay standardization and assessment of benign variants are required for optimal use of the PS3/BS3 criterion for TBD gene variant classification.

Telomere biology disorders: time for moving towards the clinic?

Telomere biology disorders (TBDs) are a group of rare diseases caused by mutations that impair telomere maintenance. Mutations that cause reduced levels of TERC/hTR, the telomerase RNA component, are found in most TBD patients and include loss-of-function mutations in hTR itself, in hTR-binding proteins [NOP10, NHP2, NAF1, ZCCHC8, and dyskerin (DKC1)], and in proteins required for hTR processing (PARN). These patients show diverse clinical presentations that most commonly include bone marrow failure (BMF)/aplastic anemia (AA), pulmonary fibrosis, and liver cirrhosis. There are no curative therapies for TBD patients. An understanding of hTR biogenesis, maturation, and degradation has identified pathways and pharmacological agents targeting the poly(A) polymerase PAPD5, which adds 3'-oligoadenosine tails to hTR to promote hTR degradation, and TGS1, which modifies the 5'-cap structure of hTR to enhance degradation, as possible therapeutic approaches. Critical next steps will be clinical trials to establish the effectiveness and potential side effects of these compounds in TBD patients.

Domain specific mutations in dyskerin disrupt 3' end processing of scaRNA13

Mutations in DKC1 (encoding dyskerin) cause telomere diseases including dyskeratosis congenita (DC) by decreasing steady-state levels of TERC, the non-coding RNA component of telomerase. How DKC1 mutations variably impact numerous other snoRNAs remains unclear, which is a barrier to understanding disease mechanisms in DC beyond impaired telomere maintenance. Here, using DC patient iPSCs, we show that mutations in the dyskerin N-terminal extension domain (NTE) dysregulate scaRNA13. In iPSCs carrying the del37L NTE mutation or engineered to carry NTE mutations via CRISPR/Cas9, but not in those with C-terminal mutations, we found scaRNA13 transcripts with aberrant 3' extensions, as seen when the exoribonuclease PARN is mutated in DC. Biogenesis of scaRNA13 was rescued by repair of the del37L DKC1 mutation by genome-editing, or genetic or pharmacological inactivation of the polymerase PAPD5, which counteracts PARN. Inspection of the human telomerase cryo-EM structure revealed that in addition to mediating intermolecular dyskerin interactions, the NTE interacts with terminal residues of the associated snoRNA, indicating a role for this domain in 3' end definition. Our results provide mechanistic insights into the interplay of dyskerin and the PARN/PAPD5 axis in the biogenesis and accumulation of snoRNAs beyond TERC, broadening our understanding of ncRNA dysregulation in human diseases.

Dyskerin Downregulation Can Induce ER Stress and Promote Autophagy via AKT-mTOR Signaling Deregulation

Dyskerin is an evolutionarily conserved nucleolar protein implicated in a wide range of fundamental biological roles, including telomere maintenance and ribosome biogenesis. Germline mutations of DKC1, the human gene encoding dyskerin, cause the hereditary disorders known as X-linked dyskeratosis congenita (X-DC). Moreover, dyskerin is upregulated in several cancers. Due to the pleiotropic functions of dyskerin, the X-DC clinical features overlap with those of both telomeropathies and ribosomopathies. In this paper, we evaluate the telomerase-independent effects of dyskerin depletion on cellular physiology by using inducible DCK1 knockdown. This system allows the downregulation of DKC1 expression within a short timeframe. We report that, in these cellular systems, dyskerin depletion induces the accumulation of unfolded/misfolded proteins in the endoplasmic reticulum, which in turn induces the activation of the PERK branch of the unfolded protein response. We also demonstrate that the PERK-eIF2a-ATF4-CHOP signaling pathway, activated by dyskerin downregulation, triggers a functional autophagic flux through the inhibition of the PI3K/AKT/mTOR pathway. By revealing a novel unpredicted connection between the loss of dyskerin, autophagy and UPR, our results establish a firm link between the lowering of dyskerin levels and the activation of the ER stress response, that plays a key role in the pathogenesis of several diseases.

Telomere Shortening and Its Association with Cell Dysfunction in Lung Diseases

Telomeres are localized at the end of chromosomes to provide genome stability; however, the telomere length tends to be shortened with each cell division inducing a progressive telomere shortening (TS). In addition to age, other factors, such as exposure to pollutants, diet, stress, and disruptions in the shelterin protein complex or genes associated with telomerase induce TS. This phenomenon favors cellular senescence and genotoxic stress, which increases the risk of the development and progression of lung diseases such as idiopathic pulmonary fibrosis, chronic obstructive pulmonary disease, SARS-CoV-2 infection, and lung cancer. In an infectious environment, immune cells that exhibit TS are associated with severe lymphopenia and death, whereas in a noninfectious context, naïve T cells that exhibit TS are related to cancer progression and enhanced inflammatory processes. In this review, we discuss how TS modifies the function of the immune system cells, making them inefficient in maintaining homeostasis in the lung. Finally, we discuss the advances in drug and gene therapy for lung diseases where TS could be used as a target for future treatments.

Dyskerin: an essential pseudouridine synthase with multifaceted roles in ribosome biogenesis, splicing, and telomere maintenance

Dyskerin and its homologs are ancient and conserved enzymes that catalyze the most common post-transcriptional modification found in cells, pseudouridylation. The resulting pseudouridines provide stability to RNA molecules and regulate ribosome biogenesis and splicing events. Dyskerin does not act independently-it is the core component of a protein heterotetramer, which associates with RNAs that contain the H/ACA motif. The variety of H/ACA RNAs that guide the function of this ribonucleoprotein (RNP) complex highlights the diversity of cellular processes in which dyskerin participates. When associated with small nucleolar (sno) RNAs, it regulates ribosomal (r) RNAs and ribosome biogenesis. By interacting with small Cajal body (sca) RNAs, it targets small nuclear (sn) RNAs to regulate pre-mRNA splicing. As a component of the telomerase holoenzyme, dyskerin binds to the telomerase RNA to modulate telomere maintenance. In a disease context, dyskerin malfunction can result in multiple detrimental phenotypes. Mutations in DKC1, the gene that encodes dyskerin, cause the premature aging syndrome X-linked dyskeratosis congenita (X-DC), a still incurable disorder that typically leads to bone marrow failure. In this review, we present the classical and most recent findings on this essential protein, discussing the evolutionary, structural, and functional aspects of dyskerin and the H/ACA RNP. The latest research underscores the role that dyskerin plays in the regulation of gene expression, translation efficiency, and telomere maintenance, along with the impacts that defective dyskerin has on aging, cell proliferation, haematopoietic potential, and cancer.

Dual Inhibition of DKC1 and MEK1/2 Synergistically Restrains the Growth of Colorectal Cancer Cells

Colorectal cancer, one of the most commonly diagnosed cancers worldwide, is often accompanied by uncontrolled proliferation of tumor cells. Dyskerin pseudouridine synthase 1 (DKC1), screened using the genome-wide RNAi strategy, is a previously unidentified key regulator that promotes colorectal cancer cell proliferation. Enforced expression of DKC1, but not its catalytically inactive mutant D125A, accelerates cell growth in vitro and in vivo. DKC1 knockdown or its inhibitor pyrazofurin attenuates cell proliferation. Proteomics, RNA immunoprecipitation (RIP)-seq, and RNA decay analyses reveal that DKC1 binds to and stabilizes the mRNA of several ribosomal proteins (RPs), including RPL10A, RPL22L1, RPL34, and RPS3. DKC1 depletion significantly accelerates mRNA decay of these RPs, which mediates the oncogenic function of DKC1. Interestingly, these DKC1-regulated RPs also interact with HRAS and suppress the RAS/RAF/MEK/ERK pathway. Pyrazofurin and trametinib combination synergistically restrains colorectal cancer cell growth in vitro and in vivo. Furthermore, DKC1 is markedly upregulated in colorectal cancer tissues compared to adjacent normal tissues. Colorectal cancer patients with higher DKC1 expression has consistently poorer overall survival and progression-free survival outcomes. Taken together, these data suggest that DKC1 is an essential gene and candidate therapeutic target for colorectal cancer.

SUMOylation- and GAR1-Dependent Regulation of Dyskerin Nuclear and Subnuclear Localization

The nuclear and subnuclear compartmentalization of the telomerase-associated protein and H/ACA ribonucleoprotein component dyskerin is an important although incompletely understood aspect of H/ACA ribonucleoprotein function. Four SUMOylation sites were previously identified in the C-terminal nuclear/nucleolar localization signal (N/NoLS) of dyskerin. We found that a cytoplasmic localized C-terminal truncation variant of dyskerin lacking most of the C-terminal N/NoLS represents an under-SUMOylated variant of dyskerin compared to wild-type dyskerin. We demonstrate that mimicking constitutive SUMOylation of dyskerin using a SUMO3 fusion construct can drive nuclear accumulation of this variant and that the SUMO site K467 in this N/NoLS is particularly important for the subnuclear localization of dyskerin to the nucleolus in a mature H/ACA complex assembly- and SUMO-dependent manner. We also characterize a novel SUMO-interacting motif in the mature H/ACA complex component GAR1 that mediates the interaction between dyskerin and GAR1. Mislocalization of dyskerin, either in the cytoplasm or excluded from the nucleolus, disrupts dyskerin function and leads to reduced interaction of dyskerin with the telomerase RNA. These data indicate a role for dyskerin C-terminal N/NoLS SUMOylation in regulating the nuclear and subnuclear localization of dyskerin, which is essential for dyskerin function as both a telomerase-associated protein and as an H/ACA ribonucleoprotein.

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.

Regulation of human telomerase RNA biogenesis and localization

Maintenance of telomeres is essential for genome integrity and replicative capacity in eukaryotic cells. Telomerase, the ribonucleoprotein complex that catalyses telomere synthesis is minimally composed of a reverse transcriptase and an RNA component. The sequence and structural domains of human telomerase RNA (hTR) have been extensively characterized, while the regulation of hTR transcription, maturation, and localization, is not fully understood. Here, we provide an up-to-date review of hTR, with an emphasis on current breakthroughs uncovering the mechanisms of hTR maturation and localization.

Molecular mechanisms of telomere biology disorders

Genetic mutations that affect telomerase function or telomere maintenance result in a variety of diseases collectively called telomeropathies. This wide spectrum of disorders, which include dyskeratosis congenita, pulmonary fibrosis, and aplastic anemia, is characterized by severely short telomeres, often resulting in hematopoietic stem cell failure in the most severe cases. Recent work has focused on understanding the molecular basis of these diseases. Mutations in the catalytic TERT and TR subunits of telomerase compromise activity, while others, such as those found in the telomeric protein TPP1, reduce the recruitment of telomerase to the telomere. Mutant telomerase-associated proteins TCAB1 and dyskerin and the telomerase RNA maturation component poly(A)-specific ribonuclease affect the maturation and stability of telomerase. In contrast, disease-associated mutations in either CTC1 or RTEL1 are more broadly associated with telomere replication defects. Yet even with the recent surge in studies decoding the mechanisms underlying these diseases, a significant proportion of dyskeratosis congenita mutations remain uncharacterized or poorly understood. Here we review the current understanding of the molecular basis of telomeropathies and highlight experimental data that illustrate how genetic mutations drive telomere shortening and dysfunction in these patients. This review connects insights from both clinical and molecular studies to create a comprehensive view of the underlying mechanisms that drive these diseases. Through this, we emphasize recent advances in therapeutics and pinpoint disease-associated variants that remain poorly defined in their mechanism of action. Finally, we suggest future avenues of research that will deepen our understanding of telomere biology and telomere-related disease.

The secrets of telomerase: Retrospective analysis and future prospects

Telomerase plays a significant role to maintain and regulate the telomere length, cellular immortality and senescence by the addition of guanine-rich repetitive sequences. Chronic inflammation or oxidative stress-induced infection downregulates TERT gene modifying telomerase activity thus contributing to the early steps of gastric carcinogenesis process. Furthermore, telomere-telomerase system performs fundamental role in the pathogenesis and progression of diabetes mellitus as well as in its vascular intricacy. The cessation of cell proliferation in cultured cells by inhibiting the telomerase activity of transformed cells renders the rationale for culling of telomerase as a target therapy for the treatment of metabolic disorders and various types of cancers. In this article, we have briefly described the role of immune system and malignant cells in the expression of telomerase with critical analysis on the gaps and potential for future studies. The key findings regarding the secrets of the telomerase summarized in this article will help in future treatment modalities for the prevention of various types of cancers and metabolic disorders notably diabetes mellitus.

Functional diversity of small nucleolar RNAs

Small nucleolar RNAs (snoRNAs) are short non-protein-coding RNAs with a long-recognized role in tuning ribosomal and spliceosomal function by guiding ribose methylation and pseudouridylation at targeted nucleotide residues of ribosomal and small nuclear RNAs, respectively. SnoRNAs are increasingly being implicated in regulation of new types of post-transcriptional processes, for example rRNA acetylation, modulation of splicing patterns, control of mRNA abundance and translational efficiency, or they themselves are processed to shorter stable RNA species that seem to be the principal or alternative bioactive isoform. Intriguingly, some display unusual cellular localization under exogenous stimuli, or tissue-specific distribution. Here, we discuss the new and unforeseen roles attributed to snoRNAs, focusing on the presumed mechanisms of action. Furthermore, we review the experimental approaches to study snoRNA function, including high resolution RNA:protein and RNA:RNA interaction mapping, techniques for analyzing modifications on targeted RNAs, and cellular and animal models used in snoRNA biology research.

Dyskerin Mutations Present in Dyskeratosis Congenita Patients Increase Oxidative Stress and DNA Damage Signalling in Dictyostelium Discoideum

Dyskerin is a protein involved in the formation of small nucleolar and small Cajal body ribonucleoproteins. These complexes participate in RNA pseudouridylation and are also components of the telomerase complex required for telomere elongation. Dyskerin mutations cause a rare disease, X-linked dyskeratosis congenita, with no curative treatment. The social amoeba Dictyostelium discoideum contains a gene coding for a dyskerin homologous protein. In this article D. discoideum mutant strains that have mutations corresponding to mutations found in dyskeratosis congenita patients are described. The phenotype of the mutant strains has been studied and no alterations were observed in pseudouridylation activity and telomere structure. Mutant strains showed increased proliferation on liquid culture but reduced growth feeding on bacteria. The results obtained indicated the existence of increased DNA damage response and reactive oxygen species, as also reported in human Dyskeratosis congenita cells and some other disease models. These data, together with the haploid character of D. discoideum vegetative cells, that resemble the genomic structure of the human dyskerin gene, located in the X chromosome, support the conclusion that D. discoideum can be a good model system for the study of this disease.