snoRNPs Regulate Telomerase Activity in Neuroblastoma and Are Associated with Poor Prognosis

Suppressed miR-128-3p combined with TERT overexpression predicts dismal outcomes for neuroblastoma

BACKGROUND

Molecular and clinical diversity of neuroblastomas is notorious. The activating TERT rearrangements have been associated with dismal prognosis. Suppression of miR-128-3p may complement and enhance the adverse effects of TERT overexpression.

OBJECTIVE

The study aimed at evaluation of prognostic significance of the miR-128-3p/TERT expression in patients with primary neuroblastoma.

METHODS

RNA samples isolated from fresh-frozen tumor specimens (n= 103) were reverse transcribed for evaluation of miR-128-3p and TERT expression by qPCR. The normalized expression levels were tested for correlations with the event-free survival (EFS). ROC-analysis was used to establish threshold expression levels (TLs) for the possible best prediction of the outcomes. The median follow-up was 57 months.

RESULTS

Both TERT overexpression and miR-128-3p downregulation were independently associated with superior rates of adverse events (p= 0.027, TL =-2.32 log10 and p= 0.080, TL =-1.33 log10, respectively). The MYCN single-copy patients were stratified into groups based on the character of alterations in expression of the studied transcripts. Five-year EFS in the groups of patients with elevated TERT/normal miR-128-3p expression and normal TERT/reduced miR-128-3p expression were 0.74 ± 0.08 and 0.60 ± 0.16, respectively. The patients with elevated TERT/reduced miR-128-3p expression had the worst outcomes, with 5-year EFS of 0.40 ± 0.16 compared with 0.91 ± 0.06 for the patients with unaltered levels of both transcripts (p< 0.001). Cumulative incidence of relapse/progression for the groups constituted 0.23 ± 0.08, 0.40 ± 0.16, 0.60 ± 0.16 and 0.09 ± 0.06, respectively. Moreover, the loss of miR-128-3p was qualified as independent adverse predictor which outperformed the conventional clinical and genetic risk factors in the multivariate Cox regression model of EFS.

CONCLUSIONS

Combined expression levels of miR-128-3p and TERT represent a novel prognostic biomarker for neuroblastoma.

Analysis of Telomere Maintenance Related Genes Reveals NOP10 as a New Metastatic-Risk Marker in Pheochromocytoma/Paraganglioma

Simple Summary

Telomere maintenance involving TERT and ATRX genes has been recently described in metastatic pheochromocytoma and paraganglioma, reinforcing the importance of immortalization mechanisms in the progression of these tumors. Thus, the aim of this study was to analyze additional telomere-related genes to uncover potential new markers capable of identifying metastatic-risk patients more accurately. After analyzing 29 telomere-related genes, we were able to validate the predictive value of TERT and ATRX in mPPGL progression. In addition, we were able to identify NOP10 as a novel prognostic risk marker of mPPGLs, which also facilitates telomerase-dependent telomere length maintenance in these tumors. Interestingly, NOP10 overexpression assessment by IHC could be easily included within the current battery of markers for stratifying PPGL patients to fine-tune their clinical diagnoses.

Abstract

One of the main problems we face with PPGL is the lack of molecular markers capable of predicting the development of metastases in patients. Telomere-related genes, such as TERT and ATRX, have been recently described in PPGL, supporting the association between the activation of immortalization mechanisms and disease progression. However, the contribution of other genes involving telomere preservation machinery has not been previously investigated. In this work, we aimed to analyze the prognostic value of a comprehensive set of genes involved in telomere maintenance. For this study, we collected 165 PPGL samples (97 non-metastatic/63 metastatic), genetically characterized, in which the expression of 29 genes of interest was studied by NGS. Three of the 29 genes studied, TERT, ATRX and NOP10, showed differential expression between metastatic and non-metastatic cases, and alterations in these genes were associated with a shorter time to progression, independent of SDHB-status. We studied telomere length by Q-FISH in patient samples and in an in vitro model. NOP10 overexpressing tumors displayed an intermediate-length telomere phenotype without ALT, and in vitro results suggest that NOP10 has a role in telomerase-dependent telomere maintenance. We also propose the implementation of NOP10 IHC to better stratify PPGL patients.

Small nucleolar RNA and its potential role in breast cancer - A comprehensive review

Small Nucleolar RNAs (snoRNAs) are known for their canonical functions, including ribosome biogenesis and RNA modification. snoRNAs act as endogenous sponges that regulate miRNA expression. Thus, precise snoRNA expression is critical for fine-tuning miRNA expression. snoRNAs processed into miRNA-like sequences play a crucial role in regulating the expression of protein-coding genes similar to that of miRNAs. Recent studies have linked snoRNA deregulation to breast cancer (BC). Inappropriate snoRNA expression contributes to BC pathology by facilitating breast cells to acquire cancer hallmarks. Since snoRNAs show significant differential expression in normal and cancer conditions, measuring snoRNA levels could be useful for BC prognosis and diagnosis. The present article provides a comprehensive overview of the role of snoRNAs in breast cancer pathology. More specifically, we have discussed the regulation, biological function, signaling pathways, and clinical utility of abnormally expressed snoRNAs in BC. Besides, we have also discussed the role of snoRNA host genes in breast tumorigenesis and emerging and future research directions in the field of snoRNA and cancer.

Nucleolar protein 10 (NOP10) predicts poor prognosis in invasive breast cancer

Purpose

Nucleolar protein 10 (NOP10) is required for ribosome biogenesis and telomere maintenance and plays a key role in carcinogenesis. This study aims to evaluate the clinical and prognostic significance of NOP10 in breast cancer (BC).

Methods

NOP10 expression was assessed at mRNA level employing the Molecular Taxonomy of Breast Cancer International Consortium (METABRIC) (n = 1980) and Cancer Genome Atlas (TCGA) BC cohorts (n = 854). Protein expression was evaluated on tissue microarray of a large BC cohort (n = 1081) using immunohistochemistry. The correlation between NOP10 expression, clinicopathological parameters and patient outcome was assessed.

Results

NOP10 expression was detected in the nucleus and nucleolus of the tumour cells. At the transcriptomic and proteomic levels, NOP10 was significantly associated with aggressive BC features including high tumour grade, high nucleolar score and poor Nottingham Prognostic Index. High NOP10 protein expression was an independent predictor of poor outcome in the whole cohort and in triple-negative BC (TNBC) class (p = 0.002 & p = 0.014, respectively). In chemotherapy- treated patients, high NOP10 protein expression was significantly associated with shorter survival (p = 0.03) and was predictive of higher risk of death (p = 0.028) and development of distant metastasis (p = 0.02) independent of tumour size, nodal stage and tumour grade.

Conclusion

High NOP10 expression is a poor prognostic biomarker in BC and its expression can help in predicting chemotherapy resistance. Functional assessments are necessary to decipher the underlying mechanisms and to reveal its potential therapeutic values in various BC subtypes especially in the aggressive TNBC class.

Electronic supplementary material

The online version of this article (doi:10.1007/s10549-020-05999-3) contains supplementary material, which is available to authorized users.

The nucleolar-related protein Dyskerin pseudouridine synthase 1 (DKC1) predicts poor prognosis in breast cancer

BACKGROUND

Hypertrophy of the nucleolus is a distinctive cytological feature of malignant cells and corresponds to aggressive behaviour. This study aimed to identify the key gene associated with nucleolar prominence (NP) in breast cancer (BC) and determine its prognostic significance.

METHODS

From The Cancer Genome Atlas (TCGA) cohort, digital whole slide images identified cancers having NP served as label and an information theory algorithm was applied to find which mRNA gene best explained NP. Dyskerin Pseudouridine Synthase 1 (DKC1) was identified. DKC1 expression was assessed using mRNA data of Molecular Taxonomy of Breast Cancer International Consortium (METABRIC, n = 1980) and TCGA (n = 855). DKC1 protein expression was assessed using immunohistochemistry in Nottingham BC cohort (n = 943).

RESULTS

Nuclear and nucleolar expressions of DKC1 protein were significantly associated with higher tumour grade (p < 0.0001), high nucleolar score (p < 0.001) and poor Nottingham Prognostic Index (p < 0.0001). High DKC1 expression was associated with shorter BC-specific survival (BCSS). In multivariate analysis, DKC1 mRNA and protein expressions were independent risk factors for BCSS (p < 0.01).

CONCLUSION

DKC1 expression is strongly correlated with NP and its overexpression in BC is associated with unfavourable clinicopathological characteristics and poor outcome. This has been a detailed example in the correlation of phenotype with genotype.

Allosteric and Orthosteric Activators of mGluR8 Differentially Affect the Chemotherapeutic-Induced Human Neuroblastoma SH-SY5Y Cell Damage: The Impact of Cell Differentiation State

The participation of group III metabotropic glutamate receptors (mGluRs) in cancer growth and progression is still an understudied issue. Based on our recent data on high expression of mGluR8 in human neuroblastoma SH-SY5Y cells, in this study, we evaluated the effect of an mGluR8-specific positive allosteric modulator (PAM: AZ12216052) and orthosteric agonist [(S)-3,4-DCPG ((S)-3,4-dicarboxyphenylglycine)] on chemotherapeutic (doxorubicin, irinotecan or cisplatin)-evoked cell damage in undifferentiated (UN-) and retinoic acid-differentiated (RA-) SH-SY5Y cells. The data showed that AZ12216052 as well as a group III mGluR antagonist (UBP1112) but not (S)-3,4-DCPG partially inhibited the cell damage evoked by doxorubicin, irinotecan or cisplatin in UN-SH-SY5Y cells. In RA-SH-SY5Y, we observed only a modest protective effect of mGluR8 PAM. In contrast, both types of mGluR8 activators significantly enhanced toxic effects of doxorubicin and irinotecan in RA-SH-SY5Y cells. These data suggest that in undifferentiated neuroblastoma malignant cells, some mGluR8 modulators can decrease cytotoxic effects of chemotherapeutics which exclude them from the group of putative anticancer agents. On the other hand, in SH-SY5Y cells differentiated to a more mature neuron-like phenotype, that is non-malignant cells, the mGluR8 activators can aggravate the chemotherapeutic neurotoxicity which is a well-known undesired effect of these drugs. Our pharmacological data add new observations to the unexplored field of research on the role of mGluR8 in cancer, pointing to complexity of response which could be mediated by particular types of mGluR8 ligands at least in neuroblastoma cells.

TERT-mediated and ATRX-mediated Telomere Maintenance and Neuroblastoma

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

A functional connection between dyskerin and energy metabolism

The human DKC1 gene encodes dyskerin, an evolutionarily conserved nuclear protein whose overexpression represents a common trait of many types of aggressive sporadic cancers. As a crucial component of the nuclear H/ACA snoRNP complexes, dyskerin is involved in a variety of essential processes, including telomere maintenance, splicing efficiency, ribosome biogenesis, snoRNAs stabilization and stress response. Although multiple minor dyskerin splicing isoforms have been identified, their functions remain to be defined. Considering that low-abundance splice variants could contribute to the wide functional repertoire attributed to dyskerin, possibly having more specialized tasks or playing significant roles in changing cell status, we investigated in more detail the biological roles of a truncated dyskerin isoform that lacks the C-terminal nuclear localization signal and shows a prevalent cytoplasmic localization. Here we show that this dyskerin variant can boost energy metabolism and improve respiration, ultimately conferring a ROS adaptive response and a growth advantage to cells. These results reveal an unexpected involvement of DKC1 in energy metabolism, highlighting a previously underscored role in the regulation of metabolic cell homeostasis.

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Human dyskerin is an evolutionary conserved component of nuclear H/ACA snoRNPs.

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The functional role of a truncated dyskerin isoform (Iso3) is analyzed.

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Iso3 overexpression boosts energy metabolism and induces a ROS adaptive response.

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Iso3 connects dyskerin with mitochondrial functionality and redox homeostasis.

Acute telomerase components depletion triggers oxidative stress as an early event previous to telomeric shortening

Loss of function of dyskerin (DKC1), NOP10 and TIN2 are responsible for different inheritance patterns of Dyskeratosis congenita (DC; ORPHA1775). They are key components of telomerase (DKC1 and NOP10) and shelterin (TIN2), and play an important role in telomere homeostasis. They participate in several fundamental cellular processes by contributing to Dyskeratosis congenita through mechanisms that are not fully understood. Presence of oxidative stress was postulated to result from telomerase ablation. However, the resulting disturbed redox status can promote telomere attrition by generating a vicious circle, which promotes cellular senescence. This fact prompted us to study if acute loss of DKC1, NOP10 and TINF2 can promote redox disequilibrium as an early event when telomere shortening has not yet taken place. We generated siRNA-mediated (DKC1, NOP10 and TINF2) cell lines by RNA interference, which was confirmed by mRNA and protein expression analyses. No telomere shortening occurred in any silenced cell line. Depletion of H/ACA ribonucleoproteins DKC1 and NOP10 diminished telomerase activity via TERC down-regulation, and produced alterations in pseudouridylation and ribosomal biogenesis. An increase in the GSSG/GSH ratio, carbonylated proteins and oxidized peroxiredoxin-6 was observed, in addition to MnSOD and TRX1 overexpression in the siRNA DC cells. Likewise, high PARylation levels and high PARP1 protein expression were detected. In contrast, the silenced TINF2 cells did not alter any evaluated oxidative stress marker. Altogether these findings lead us to conclude that loss of DKC1 and NOP10 functions induces oxidative stress in a telomere shortening independent manner.

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Transient silencing of DKC1 and NOP10 genes produce oxidative stress.

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Cells depleted of DKC1 and NOP10 are susceptible to DNA damage.

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Acute DKC1 and NOP10 depletion disrupts RNA maturation.

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Oxidative stress is an early event previous to telomere shortening.

A new role for human dyskerin in vesicular trafficking

Dyskerin is an essential, conserved, multifunctional protein found in the nucleolus, whose loss of function causes the rare genetic diseases X‐linked dyskeratosis congenita and Hoyeraal‐Hreidarsson syndrome. To further investigate the wide range of dyskerin's biological roles, we set up stable cell lines able to trigger inducible protein knockdown and allow a detailed analysis of the cascade of events occurring within a short time frame. We report that dyskerin depletion quickly induces cytoskeleton remodeling and significant alterations in endocytic Ras‐related protein Rab‐5A/Rab11 trafficking. These effects arise in different cell lines well before the onset of telomere shortening, which is widely considered the main cause of dyskerin‐related diseases. Given that vesicular trafficking affects many homeostatic and differentiative processes, these findings add novel insights into the molecular mechanisms underlining the pleiotropic manifestation of the dyskerin loss‐of‐function phenotype.

Neuroblastoma patient-derived xenograft cells cultured in stem-cell promoting medium retain tumorigenic and metastatic capacities but differentiate in serum

Cultured cancer cells serve as important models for preclinical testing of anti-cancer compounds. However, the optimal conditions for retaining original tumor features during in vitro culturing of cancer cells have not been investigated in detail. Here we show that serum-free conditions are critical for maintaining an immature phenotype of neuroblastoma cells isolated from orthotopic patient-derived xenografts (PDXs). PDX cells could be grown either as spheres or adherent on laminin in serum-free conditions with retained patient-specific genomic aberrations as well as tumorigenic and metastatic capabilities. However, addition of serum led to morphological changes, neuronal differentiation and reduced cell proliferation. The epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF) were central for PDX cell proliferation and MYCN expression, and also hindered the serum-induced differentiation. Although serum induced a robust expression of neurotrophin receptors, stimulation with their cognate ligands did not induce further sympathetic differentiation, which likely reflects a block in PDX cell differentiation capacity coupled to their tumor genotype. Finally, PDX cells cultured as spheres or adherent on laminin responded similarly to various cytotoxic drugs, suggesting that both conditions are suitable in vitro screening models for neuroblastoma-targeting compounds.

Dysregulation of H/ACA ribonucleoprotein components in chronic lymphocytic leukemia

Telomeres are protective repeats of TTAGGG sequences located at the end of human chromosomes. They are essential to maintain chromosomal integrity and genome stability. Telomerase is a ribonucleoprotein complex containing an internal RNA template (hTR) and a catalytic subunit (hTERT). The human hTR gene consists of three major domains; among them the H/ACA domain is essential for telomere biogenesis. H/ACA ribonucleoprotein (RNP) complex is composed of four evolutionary conserved proteins, including dyskerin (encoded by DKC1 gene), NOP10, NHP2 and GAR1. In this study, we have evaluated the expression profile of the H/ACA RNP complex genes: DKC1, NOP10, NHP2 and GAR1, as well as hTERT and hTR mRNA levels, in patients with chronic lymphocytic leukemia (CLL). Results were correlated with the number and type of genetic alteration detected by conventional cytogenetics and FISH (fluorescence in situ hybridization), IGHV (immunoglobulin heavy chain variable region) mutational status, telomere length (TL) and clinico-pathological characteristics of patients. Our results showed significant decreased expression of GAR1, NOP10, DKC1 and hTR, as well as increased mRNA levels of hTERT in patients compared to controls (p≤0.04). A positive correlation between the expression of GAR1-NHP2, GAR1-NOP10, and NOP10-NHP2 (p<0.0001), were observed. The analysis taking into account prognostic factors showed a significant increased expression of hTERT gene in unmutated-IGHV cases compared to mutated-CLL patients (p = 0.0185). The comparisons among FISH groups exhibited increased expression of DKC1 in cases with two or more alterations with respect to no abnormalities, trisomy 12 and del13q14, and of NHP2 and NOP10 compared to those with del13q14 (p = 0.03). The analysis according to TL showed a significant increased expression of hTERT (p = 0.0074) and DKC1 (p = 0.0036) in patients with short telomeres compared to those with long TL. No association between gene expression and clinical parameters was found. Our results suggest a role for these telomere associated genes in genomic instability and telomere dysfunction in CLL.

Transcript signatures that predict outcome and identify targetable pathways in MYCN-amplified neuroblastoma

BACKGROUND

In the pediatric cancer neuroblastoma (NB), patients are stratified into low, intermediate or high-risk subsets based in part on MYCN amplification status. While MYCN amplification in general predicts unfavorable outcome, no clinical or genomic factors have been identified that predict outcome within these cohorts of high-risk patients. In particular, it is currently not possible at diagnosis to determine which high-risk neuroblastoma patients will ultimately fail upfront therapy.

EXPERIMENTAL DESIGN

We analyzed the prognostic potential of most published gene expression signatures for NB and developed a new prognostic signature to predict outcome for patients with MYCN amplification. Network and pathway analyses identified candidate therapeutic targets for this MYCN-amplified patient subset with poor outcome.

RESULTS

Most signatures have a high capacity to predict outcome of unselected NB patients. However, the majority of published signatures, as well as most randomly generated signatures, are highly confounded by MYCN amplification, and fail to predict outcome in subpopulations of high-risk patients with MYCN-amplified NB. We identify a MYCN module signature that predicts patient outcome for those with MYCN-amplified tumors, that also predicts potential tractable therapeutic signaling pathways and targets including the DNA repair enzyme Poly [ADP-ribose] polymerase 1 (PARP1).

CONCLUSION

Many prognostic signatures for NB are confounded by MYCN amplification and fail to predict outcome for the subset of high-risk patients with MYCN amplification. We report a MYCN module signature that is associated with distinct patient outcomes, and predicts candidate therapeutic targets in DNA repair pathways, including PARP1 in MYCN-amplified NB.

New prospects for targeting telomerase beyond the telomere

Telomerase activity is responsible for the maintenance of chromosome end structures (telomeres) and cancer cell immortality in most human malignancies, making telomerase an attractive therapeutic target. The rationale for targeting components of the telomerase holoenzyme has been strengthened by accumulating evidence indicating that these molecules have extra-telomeric functions in tumour cell survival and proliferation. This Review discusses current knowledge of the biogenesis, structure and multiple functions of telomerase-associated molecules intertwined with recent advances in drug discovery approaches. We also describe the fertile ground available for the pursuit of next-generation small-molecule inhibitors of telomerase.

Intratumoral diversity of telomere length in individual neuroblastoma tumors

The purpose of the work was to investigate telomere length (TL) and mechanisms involved in TL maintenance in individual neuroblastoma (NB) tumors. Primary NB tumors from 102 patients, ninety Italian and twelve Spanish, diagnosed from 2000 to 2008 were studied. TL was investigated by quantitative fluorescence in situ hybridization (IQ-FISH) that allows to analyze individual cells in paraffin-embedded tissues. Fluorescence intensity of chromosome 2 centromere was used as internal control to normalize TL values to ploidy. Human telomerase reverse transcriptase (hTERT) expression was detected by immunofluorescence in 99/102 NB specimens.

The main findings are the following: 1) two intratumoral subpopulations of cancer cells displaying telomeres of different length were identified in 32/102 tumors belonging to all stages. 2) hTERT expression was detected in 99/102 tumors, of which 31 displayed high expression and 68 low expression. Alternative lengthening of telomeres (ALT)-mechanism was present in 60/102 tumors, 20 of which showed high hTERT expression. Neither ALT-mechanism nor hTERT expression correlated with heterogeneous TL. 3) High hTERT expression and ALT positivity were associated with significantly reduced Overall Survival. 4) High hTERT expression predicted relapse irrespective of patient age. Intratumoral diversity in TL represents a novel feature in NB.

In conclusion, diversity of TL in individual NB tumors was strongly associated with disease progression and death, suggesting that these findings are of translational relevance. The combination of high hTERT expression and ALT positivity may represent a novel biomarker of poor prognosis that deserves further investigation.

MYC-Driven Neuroblastomas Are Addicted to a Telomerase-Independent Function of Dyskerin

The RNA-binding protein dyskerin, encoded by the DKC1 gene, functions as a core component of the telomerase holoenzyme as well as ribonuclear protein complexes involved in RNA processing and ribosome biogenesis. The diverse roles of dyskerin across many facets of RNA biology implicate its potential contribution to malignancy. In this study, we examined the expression and function of dyskerin in neuroblastoma. We show that DKC1 mRNA levels were elevated relative to normal cells across a panel of 15 neuroblastoma cell lines, where both N-Myc and c-Myc directly targeted the DKC1 promoter. Upregulation of MYCN was shown to dramatically increase DKC1 expression. In two independent neuroblastoma patient cohorts, high DKC1 expression correlated strongly with poor event-free and overall survival (P < 0.0001), independently of established prognostic factors. RNAi-mediated depletion of dyskerin inhibited neuroblastoma cell proliferation, including cells immortalized via the telomerase-independent ALT mechanism. Furthermore, dyskerin attenuation impaired anchorage-independent proliferation and tumor growth. Overexpression of the telomerase RNA component, hTR, demonstrated that this proliferative impairment was not a consequence of telomerase suppression. Instead, ribosomal stress, evidenced by depletion of small nucleolar RNAs and nuclear dispersal of ribosomal proteins, was the likely cause of the proliferative impairment in dyskerin-depleted cells. Accordingly, dyskerin suppression caused p53-dependent G1 cell-cycle arrest in p53 wild-type cells, and a p53-independent pathway impaired proliferation in cells with p53 dysfunction. Together, our findings highlight dyskerin as a new therapeutic target in neuroblastoma with crucial telomerase-independent functions and broader implications for the spectrum of malignancies driven by MYC family oncogenes. Cancer Res; 76(12); 3604-17. ©2016 AACR.

Differential requirements for H/ACA ribonucleoprotein components in cell proliferation and response to DNA damage

H/ACA ribonucleoproteins (RNPs) are comprised of four conserved proteins, dyskerin, NHP2, NOP10, and GAR1, and a function-specifying, noncoding H/ACA RNA. H/ACA RNPs contribute to telomerase assembly and stabilization, and posttranscriptional processing of nascent ribosomal RNA and spliceosomal RNA. However, very little is known about the coordinated action of the four proteins in other biologic processes. As described herein, we observed a differential requirement for the proteins in cell proliferation and identified a possible reliance for these factors in regulation of specific DNA damage biomarkers. In particular, GAR1 expression was upregulated following exposure to all forms of genotoxic stress tested. In contrast, levels of the other proteins were either reduced or unaffected. Only GAR1 showed an altered subcellular localization with a shift from the nucleolus to the nucleoplasm after ultraviolet-C irradiation and doxorubicin treatments. Transient siRNA-mediated depletion of GAR1 and dyskerin arrested cell proliferation, whereas loss of either NHP2 or NOP10 had no effect. Finally, loss of dyskerin, GAR1, NHP2, and NOP10, respectively, limited the accumulation of DNA damage biomarkers. However, the individual responses were dependent upon the specific type of damage incurred. In general, loss of GAR1 had the most suppressive effect on the biomarkers tested. Since the specific responses to genotoxic stress, the contribution of each protein to cell proliferation, and the activation of DNA damage biomarkers were not equivalent, this suggests the possibility that at least some of the proteins, most notably GAR1, may potentially function independently of their respective roles within H/ACA RNP complexes.

Individual patient risk stratification of high-risk neuroblastomas using a two-gene score suited for clinical use

Several gene expression-based prognostic signatures have been described in neuroblastoma, but none have successfully been applied in the clinic. Here we have developed a clinically applicable prognostic gene signature, both with regards to number of genes and analysis platform. Importantly, it does not require comparison between patients and is applicable amongst high-risk patients. The signature is based on a two-gene score (R-score) with prognostic power in high-stage tumours (stage 4 and/or MYCN-amplified diagnosed after 18 months of age). QPCR-based and array-based analyses of matched cDNAs confirmed cross platform (array-qPCR) transferability. We also defined a fixed cut-off value identifying prognostically differing subsets of high-risk patients on an individual patient basis. This gene expression signature independently contributes to the current neuroblastoma classification system, and if prospectively validated could provide further stratification of high-risk patients, and potential upfront identification of a group of patients that are in need of new/additional treatment regimens.

Acute dyskerin depletion triggers cellular senescence and renders osteosarcoma cells resistant to genotoxic stress-induced apoptosis

Dyskerin is a conserved, nucleolar RNA-binding protein implicated in an increasing array of fundamental cellular processes. Germline mutation in the dyskerin gene (DKC1) is the cause of X-linked dyskeratosis congenita (DC). Conversely, wild-type dyskerin is overexpressed in sporadic cancers, and high-levels may be associated with poor prognosis. It was previously reported that acute loss of dyskerin function via siRNA-mediated depletion slowed the proliferation of transformed cell lines. However, the mechanisms remained unclear. Using human U2OS osteosarcoma cells, we show that siRNA-mediated dyskerin depletion induced cellular senescence as evidenced by proliferative arrest, senescence-associated heterochromatinization and a senescence-associated molecular profile. Senescence can render cells resistant to apoptosis. Conversely, chromatin relaxation can reverse the repressive effects of senescence-associated heterochromatinization on apoptosis. To this end, genotoxic stress-induced apoptosis was suppressed in dyskerin-depleted cells. In contrast, agents that induce chromatin relaxation, including histone deacetylase inhibitors and the DNA intercalator chloroquine, sensitized dyskerin-depleted cells to apoptosis. Dyskerin is a core component of the telomerase complex and plays an important role in telomere homeostasis. Defective telomere maintenance resulting in premature senescence is thought to primarily underlie the pathogenesis of X-linked DC. Since U2OS cells are telomerase-negative, this leads us to conclude that loss of dyskerin function can also induce cellular senescence via mechanisms independent of telomere shortening.