The PARN, TOE1, and USB1 RNA deadenylases and their roles in non-coding RNA regulation

The levels of non-coding RNAs (ncRNAs) are regulated by transcription, RNA processing, and RNA degradation pathways. One mechanism for the degradation of ncRNAs involves the addition of oligo(A) tails by non-canonical poly(A) polymerases, which then recruit processive sequence-independent 3' to 5' exonucleases for RNA degradation. This pathway of decay is also regulated by three 3' to 5' exoribonucleases, USB1, PARN, and TOE1, which remove oligo(A) tails and thereby can protect ncRNAs from decay in a manner analogous to the deubiquitination of proteins. Loss-of-function mutations in these genes lead to premature degradation of some ncRNAs and lead to specific human diseases such as Poikiloderma with Neutropenia (PN) for USB1, Dyskeratosis Congenita (DC) for PARN and Pontocerebellar Hypoplasia type 7 (PCH7) for TOE1. Herein, we review the biochemical properties of USB1, PARN, and TOE1, how they modulate ncRNA levels, and their roles in human diseases.

Interstitial lung diseases associated with mutations of poly(A)-specific ribonuclease: A multicentre retrospective study

BACKGROUND AND OBJECTIVE

Poly(A)-specific ribonuclease (PARN) mutations have been associated with familial pulmonary fibrosis. This study aims to describe the phenotype of patients with interstitial lung disease (ILD) and heterozygous PARN mutations.

METHODS

We performed a retrospective, observational, non-interventional study of patients with an ILD diagnosis and a pathogenic heterozygous PARN mutation followed up in a centre of the OrphaLung network.

RESULTS

We included 31 patients (29 from 16 kindreds and two sporadic patients). The median age at ILD diagnosis was 59 years (range 54 to 63). In total, 23 (74%) patients had a smoking history and/or fibrogenic exposure. The pulmonary phenotypes were heterogenous, but the most frequent diagnosis was idiopathic pulmonary fibrosis (n = 12, 39%). Haematological abnormalities were identified in three patients and liver disease in two. In total, 21 patients received a specific treatment for ILD: steroids (n = 13), antifibrotic agents (n = 11), immunosuppressants (n = 5) and N-acetyl cysteine (n = 2). The median forced vital capacity decline for the whole sample was 256 ml/year (range -363 to -148). After a median follow-up of 32 months (range 18 to 66), 10 patients had died and six had undergone lung transplantation. The median transplantation-free survival was 54 months (95% CI 29 to ∞). Extra-pulmonary features were less frequent with PARN mutation than telomerase reverse transcriptase (TERT) or telomerase RNA component (TERC) mutation.

CONCLUSION

IPF is common among individuals with PARN mutation, but other ILD subtypes may be observed.

Poly (A)-specific ribonuclease (PARN): More than just "mRNA stock clearing"

In eukaryotic cells, the balance between the synthesis and the degradation decides the steady-state levels of messenger RNAs (mRNA). The removal of adenosine residues from the poly(A) tail, called deadenylation, is the first and the most crucial step in the process of mRNA degradation. Poly (A)-specific ribonuclease (PARN) is one such enzyme that catalyses the process of deadenylation. Although PARN has been primarily known as the regulator of the mRNA stability, recent evidence clearly suggests several other functions of PARN, including a role in embryogenesis, oocyte maturation, cell-cycle progression, telomere biology, non-coding RNA maturation and ribosome biogenesis. Also, deregulated PARN activity is shown to be a hallmark of specific disease conditions. Pathogenic variants in the PARN gene have been observed in various cancers and inherited bone marrow failure syndromes. The focus in this review is to highlight the emerging functions of PARN, particularly in the context of human diseases.

Multiple bilateral hip fractures in a patient with dyskeratosis congenita caused by a novel mutation in the PARN gene

We report a case of a young male patient with clinical signs of dyskeratosis congenita who presented with multiple bilateral low-traumatic hip fractures. Whole exome sequencing (WES) showed a previously unreported mutation in the poly(A)-specific ribonuclease (PARN) gene. Zoledronic acid 5 mg over 3 years was effective at preventing further fractures. A male patient was referred to our clinic at age 24 due to multiple bilateral hip fractures. At the time of admission, the patient's height was 160 cm and weight 40 kg; bone mineral density (BMD) at the lumbar spine was normal (L1-L4 0.0 Z-score). The patient was found to have abnormal skin pigmentation, hyperkeratosis of palms and soles, nail dystrophy, and signs of bone marrow failure (BMF). Bone fragility first presented at 5 years old with a wrist fracture, followed by multiple bilateral low-traumatic hip fractures without falls from 14 to 24 years. WES showed a previously unreported mutation (NM_002582.3: c.1652delA; p.His551fs) in the poly(A)-specific ribonuclease (PARN) gene. Flow fish telomere measurement result was 5.9 (reference range 8.0-12.6), which is consistent with the DC diagnosis. Permanent fixation with internal metal rods and zoledronic acid 5 mg over 3 years was effective at preventing further fractures over 4 years of follow-up. Additionally, BMF did not progress over 4 years of observation. DC associated with PARN gene mutations might predispose to low-traumatic multiple hip fractures in adolescents and young adults. Treatment with zoledronic acid in this case was effective and safe at preventing further fractures.

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.

Distinct Poly(A) nucleases have differential impact on sut-2 dependent tauopathy phenotypes

Aging drives pathological accumulation of proteins such as tau, causing neurodegenerative dementia disorders like Alzheimer's disease. Previously we showed loss of function mutations in the gene encoding the poly(A) RNA binding protein SUT-2/MSUT2 suppress tau-mediated neurotoxicity in C. elegans neurons, cultured human cells, and mouse brain, while loss of PABPN1 had the opposite effect (Wheeler et al., 2019). Here we found that blocking poly(A) tail extension with cordycepin exacerbates tauopathy in cultured human cells, which is rescued by MSUT2 knockdown. To further investigate the molecular mechanisms of poly(A) RNA-mediated tauopathy suppression, we examined whether genes encoding poly(A) nucleases also modulated tauopathy in a C. elegans tauopathy model. We found that loss of function mutations in C. elegans ccr-4 and panl-2 genes enhanced tauopathy phenotypes in tau transgenic C. elegans while loss of parn-2 partially suppressed tauopathy. In addition, loss of parn-1 blocked tauopathy suppression by loss of parn-2. Epistasis analysis showed that sut-2 loss of function suppressed the tauopathy enhancement caused by loss of ccr-4 and SUT-2 overexpression exacerbated tauopathy even in the presence of parn-2 loss of function in tau transgenic C. elegans. Thus sut-2 modulation of tauopathy is epistatic to ccr-4 and parn-2. We found that human deadenylases do not colocalize with human MSUT2 in nuclear speckles; however, expression levels of TOE1, the homolog of parn-2, correlated with that of MSUT2 in post-mortem Alzheimer's disease patient brains. Alzheimer's disease patients with low TOE1 levels exhibited significantly increased pathological tau deposition and loss of NeuN staining. Taken together, this work suggests suppressing tauopathy cannot be accomplished by simply extending poly(A) tails, but rather a more complex relationship exists between tau, sut-2/MSUT2 function, and control of poly(A) RNA metabolism, and that parn-2/TOE1 may be altered in tauopathy in a similar way.

Small Molecules Restore Telomeres in Patient Stem Cells

Genetic defects in telomere maintenance result in stem cell exhaustion and a spectrum of telomere biology diseases. Systemic treatments beyond organ transplantation are lacking for these diseases. Nagpal and colleagues identified small molecules that restore telomere maintenance in patient-derived stem cells, offering a promising therapy for telomere biology diseases.

Small-Molecule PAPD5 Inhibitors Restore Telomerase Activity in Patient Stem Cells

Genetic lesions that reduce telomerase activity inhibit stem cell replication and cause a range of incurable diseases, including dyskeratosis congenita (DC) and pulmonary fibrosis (PF). Modalities to restore telomerase in stem cells throughout the body remain unclear. Here, we describe small-molecule PAPD5 inhibitors that demonstrate telomere restoration in vitro, in stem cell models, and in vivo. PAPD5 is a non-canonical polymerase that oligoadenylates and destabilizes telomerase RNA component (TERC). We identified BCH001, a specific PAPD5 inhibitor that restored telomerase activity and telomere length in DC patient induced pluripotent stem cells. When human blood stem cells engineered to carry DC-causing PARN mutations were xenotransplanted into immunodeficient mice, oral treatment with a repurposed PAPD5 inhibitor, the dihydroquinolizinone RG7834, rescued TERC 3' end maturation and telomere length. These findings pave the way for developing systemic telomere therapeutics to counteract stem cell exhaustion in DC, PF, and possibly other aging-related diseases.

The Recovery from Sulfur Starvation Is Independent from the mRNA Degradation Initiation Enzyme PARN in Arabidopsis

When plants are exposed to sulfur limitation, they upregulate the sulfate assimilation pathway at the expense of growth-promoting measures. Upon cessation of the stress, however, protective measures are deactivated, and growth is restored. In accordance with these findings, transcripts of sulfur-deficiency marker genes are rapidly degraded when starved plants are resupplied with sulfur. Yet it remains unclear which enzymes are responsible for the degradation of transcripts during the recovery from starvation. In eukaryotes, mRNA decay is often initiated by the cleavage of poly(A) tails via deadenylases. As mutations in the poly(A) ribonuclease PARN have been linked to altered abiotic stress responses in Arabidopsis thaliana, we investigated the role of PARN in the recovery from sulfur starvation. Despite the presence of putative PARN-recruiting AU-rich elements in sulfur-responsive transcripts, sulfur-depleted PARN hypomorphic mutants were able to reset their transcriptome to pre-starvation conditions just as readily as wildtype plants. Currently, the subcellular localization of PARN is disputed, with studies reporting both nuclear and cytosolic localization. We detected PARN in cytoplasmic speckles and reconciled the diverging views in literature by identifying two PARN splice variants whose predicted localization is in agreement with those observations.

PARN and TOE1 Constitute a 3' End Maturation Module for Nuclear Non-coding RNAs

Poly(A)-specific ribonuclease (PARN) and target of EGR1 protein 1 (TOE1) are nuclear granule-associated deadenylases, whose mutations are linked to multiple human diseases. Here, we applied mTAIL-seq and RNA sequencing (RNA-seq) to systematically identify the substrates of PARN and TOE1 and elucidate their molecular functions. We found that PARN and TOE1 do not modulate the length of mRNA poly(A) tails. Rather, they promote the maturation of nuclear small non-coding RNAs (ncRNAs). PARN and TOE1 act redundantly on some ncRNAs, most prominently small Cajal body-specific RNAs (scaRNAs). scaRNAs are strongly downregulated when PARN and TOE1 are compromised together, leading to defects in small nuclear RNA (snRNA) pseudouridylation. They also function redundantly in the biogenesis of telomerase RNA component (TERC), which shares sequence motifs found in H/ACA box scaRNAs. Our findings extend the knowledge of nuclear ncRNA biogenesis, and they provide insights into the pathology of PARN/TOE1-associated genetic disorders whose therapeutic treatments are currently unavailable.

From incomplete penetrance with normal telomere length to severe disease and telomere shortening in a family with monoallelic and biallelic PARN pathogenic variants

PARN encodes poly(A)-specific ribonuclease. Biallelic and monoallelic PARN variants are associated with Hoyeraal-Hreidarsson syndrome/dyskeratosis congenita and idiopathic pulmonary fibrosis (IPF), respectively. The molecular features associated with incomplete penetrance of PARN-associated IPF have not been described. We report a family with a rare missense, p.Y91C, and a novel insertion, p.(I274*), PARN variant. We found PARN p.Y91C had reduced deadenylase activity and the p.(I274*) transcript was depleted. Detailed analysis of the consequences of these variants revealed that, while PARN protein was lowest in the severely affected biallelic child who had the shortest telomeres, it was also reduced in his mother with the p.(I274*) variant but telomeres at the 50th percentile. Increased adenylation of telomerase RNA, human telomerase RNA, and certain small nucleolar RNAs, and impaired ribosomal RNA maturation were observed in cells derived from the severely affected biallelic carrier, but not in the other, less affected biallelic carrier, who had less severely shortened telomeres, nor in the monoallelic carriers who were unaffected and had telomeres ranging from the 1st to the 50th percentiles. We identified hsa-miR-202-5p as a potential negative regulator of PARN. We propose one or more genetic modifiers influence the impact of PARN variants on its targets and this underlies incomplete penetrance of PARN-associated disease.

Lack of effective translational regulation of PLD expression and exosome biogenesis in triple-negative breast cancer cells

Triple-negative breast cancer (TNBC) is an aggressive subtype of breast cancer that is difficult to treat since cells lack the three receptors (ES, PR, or HER) that the most effective treatments target. We have used a well-established TNBC cell line (MDA-MB-231) from which we found evidence in support for a phospholipase D (PLD)-mediated tumor growth and metastasis: high levels of expression of PLD, as well as the absence of inhibitory miRs (such as miR-203) and 3'-mRNA PARN deadenylase activity in these cells. Such findings are not present in a luminal B cell line, MCF-7, and we propose a new miR•PARN•PLD node that is not uniform across breast cancer molecular subtypes and as such TNBC could be pharmacologically targeted differentially. We review the participation of PLD and phosphatidic acid (PA), its enzymatic product, as new "players" in breast cancer biology, with the aspects of regulation of the tumor microenvironment, macrophage polarization, regulation of PLD transcripts by specific miRs and deadenylases, and PLD-regulated exosome biogenesis. A new signaling miR•PARN•PLD node could serve as new biomarkers for TNBC abnormal signaling and metastatic disease staging, potentially before metastases are able to be visualized using conventional imaging.

PABP Cooperates with the CCR4-NOT Complex to Promote mRNA Deadenylation and Block Precocious Decay

Multiple deadenylases are known in vertebrates, the PAN2-PAN3 (PAN2/3) and CCR4-NOT (CNOT) complexes, and PARN, yet their differential functions remain ambiguous. Moreover, the role of poly(A) binding protein (PABP) is obscure, limiting our understanding of the deadenylation mechanism. Here, we show that CNOT serves as a predominant nonspecific deadenylase for cytoplasmic poly(A)⁺ RNAs, and PABP promotes deadenylation while preventing premature uridylation and decay. PAN2/3 selectively trims long tails (>∼150 nt) with minimal effect on transcriptome, whereas PARN does not affect mRNA deadenylation. CAF1 and CCR4, catalytic subunits of CNOT, display distinct activities: CAF1 trims naked poly(A) segments and is blocked by PABPC, whereas CCR4 is activated by PABPC to shorten PABPC-protected sequences. Concerted actions of CAF1 and CCR4 delineate the ∼27 nt periodic PABPC footprints along shortening tail. Our study unveils distinct functions of deadenylases and PABPC, re-drawing the view on mRNA deadenylation and regulation.

Poly(A)-specific ribonuclease sculpts the 3' ends of microRNAs

The 3' ends of metazoan microRNAs (miRNAs) are initially defined by the RNase III enzymes during maturation, but subsequently experience extensive modifications by several enzymatic activities. For example, terminal nucleotidyltransferases (TENTs) elongate miRNAs by adding one or a few nucleotides to their 3' ends, which occasionally leads to differential regulation of miRNA stability or function. However, the catalytic entities that shorten miRNAs and the molecular consequences of such shortening are less well understood, especially in vertebrates. Here, we report that poly(A)-specific ribonuclease (PARN) sculpts the 3' ends of miRNAs in human cells. By generating PARN knockout cells and characterizing their miRNAome, we demonstrate that PARN digests the 3' extensions of miRNAs that are derived from the genome or attached by TENTs, thereby effectively reducing the length of miRNAs. Surprisingly, PARN-mediated shortening has little impact on miRNA stability, suggesting that this process likely operates to finalize miRNA maturation, rather than to initiate miRNA decay. PARN-mediated shortening is pervasive across most miRNAs and appears to be a conserved mechanism contributing to the 3' end formation of vertebrate miRNAs. Our findings add miRNAs to the expanding list of noncoding RNAs whose 3' end formation depends on PARN.

The RNase PARN Controls the Levels of Specific miRNAs that Contribute to p53 Regulation

PARN loss-of-function mutations cause a severe form of the hereditary disease dyskeratosis congenita (DC). PARN deficiency affects the stability of non-coding RNAs such as human telomerase RNA (hTR), but these effects do not explain the severe disease in patients. We demonstrate that PARN deficiency affects the levels of numerous miRNAs in human cells. PARN regulates miRNA levels by stabilizing either mature or precursor miRNAs by removing oligo(A) tails added by the poly(A) polymerase PAPD5, which if remaining recruit the exonuclease DIS3L or DIS3L2 to degrade the miRNA. PARN knockdown destabilizes multiple miRNAs that repress p53 translation, which leads to an increase in p53 accumulation in a Dicer-dependent manner, thus explaining why PARN-defective patients show p53 accumulation. This work also reveals that DIS3L and DIS3L2 are critical 3' to 5' exonucleases that regulate miRNA stability, with the addition and removal of 3' end extensions controlling miRNA levels in the cell.

Fidelity in RNA-based recognition of transposable elements

Genomes are under constant threat of invasion by transposable elements and other genomic parasites. How can host genomes recognize these elements and target them for degradation? This requires a system that is highly adaptable, and at the same time highly specific. Current data suggest that perturbation of transcription patterns by transposon insertions could be detected by the RNAi surveillance pathway. Multiple transposon insertions might generate sufficient amounts of primal small RNAs to initiate generation of secondary small RNAs and silencing. At the same time primal small RNAs need to be constantly degraded to reduce the level of noise small RNAs below the threshold required for initiation of silencing. Failure in RNA degradation results in loss of fidelity of small RNA pathways and silencing of ectopic targets.

This article is part of the theme issue ‘5′ and 3′ modifications controlling RNA degradation’.

Nucleolin phosphorylation regulates PARN deadenylase activity during cellular stress response

Nucleolin (NCL) is an abundant stress-responsive, RNA-binding phosphoprotein that controls gene expression by regulating either mRNA stability and/or translation. NCL binds to the AU-rich element (ARE) in the 3'UTR of target mRNAs, mediates miRNA functions in the nearby target sequences, and regulates mRNA deadenylation. However, the mechanism by which NCL phosphorylation affects these functions and the identity of the deadenylase involved, remain largely unexplored. Earlier we demonstrated that NCL phosphorylation is vital for cell cycle progression and proliferation, whereas phosphorylation-deficient NCL at six consensus CK2 sites confers dominant-negative effect on proliferation by increasing p53 expression, possibly mimicking cellular DNA damage conditions. In this study, we show that NCL phosphorylation at those CK2 consensus sites in the N-terminus is necessary to induce deadenylation upon oncogenic stimuli and UV stress. NCL-WT, but not hypophosphorylated NCL-6/S*A, activates poly (A)-specific ribonuclease (PARN) deadenylase activity. We further demonstrate that NCL interacts directly with PARN, and under non-stress conditions also forms (a) complex (es) with factors that regulate deadenylation, such as p53 and the ARE-binding protein HuR. Upon UV stress, the interaction of hypophosphorylated NCL-6/S*A with these proteins is favored. As an RNA-binding protein, NCL interacts with PARN deadenylase substrates such as TP53 and BCL2 mRNAs, playing a role in their downregulation under non-stress conditions. For the first time, we show that NCL phosphorylation offers specificity to its protein-protein, protein-RNA interactions, resulting in the PARN deadenylase regulation, and hence gene expression, during cellular stress responses.

Expanding the repertoire of deadenylases

Deadenylases belong to an expanding family of exoribonucleases involved mainly in mRNA stability and turnover, with the exception of PARN which has additional roles in the biogenesis of several important non-coding RNAs, including miRNAs and piRNAs. Recently, PARN in C. elegans and its homolog PNLDC1 in B. mori were reported as the elusive trimmers mediating piRNA biogenesis. In addition, characterization of mammalian PNLDC1 in comparison to PARN, showed that is specifically expressed in embryonic stem and germ cells, as well as during early embryo development. Moreover, its expression is correlated with epigenetic events mediated by the de novo DNMT3b methyltransferase and knockdown in stem cells upregulates important genes that regulate multipotency. The recent data suggest that at least some new deadenylases may have expanded roles in cell metabolism as regulators of gene expression, through mRNA deadenylation, ncRNAs biogenesis and ncRNA-mediated mRNA targeting, linking essential mechanisms that regulate epigenetic control and transition events during differentiation. The possible roles of mammalian PNLDC1 along those dynamic networks are discussed in the light of new extremely important findings.

PARN Modulates Y RNA Stability and Its 3'-End Formation

Loss-of-function mutations in 3'-to-5' exoribonucleases have been implicated in hereditary human diseases. For example, PARN mutations cause a severe form of dyskeratosis congenita (DC), wherein PARN deficiency leads to human telomerase RNA instability. Since the DC phenotype in PARN patients is even more severe than that of loss-of-function alleles in telomerase components, we hypothesized that PARN would also be required for the stability of other RNAs. Here, we show that PARN depletion reduces the levels of abundant human Y RNAs, which might contribute to the severe phenotype of DC observed in patients. Depletion of PAPD5 or the cytoplasmic exonuclease DIS3L rescues the effect of PARN depletion on Y RNA levels, suggesting that PARN stabilizes Y RNAs by removing oligoadenylated tails added by PAPD5, which would otherwise recruit DIS3L for Y RNA degradation. Through deep sequencing of 3' ends, we provide evidence that PARN can also deadenylate the U6 and RMRP RNAs without affecting their levels. Moreover, we observed widespread posttranscriptional oligoadenylation, uridylation, and guanylation of U6 and Y RNA 3' ends, suggesting that in mammalian cells, the formation of a 3' end for noncoding RNAs can be a complex process governed by the activities of various 3'-end polymerases and exonucleases.

FXR1a-associated microRNP: A driver of specialized non-canonical translation in quiescent conditions

Eukaryotic protein synthesis is a multifaceted process that requires coordination of a set of translation factors in a particular cellular state. During normal growth and proliferation, cells generally make their proteome via conventional translation that utilizes canonical translation factors. When faced with environmental stress such as growth factor deprivation, or in response to biological cues such as developmental signals, cells can reduce canonical translation. In this situation, cells adapt alternative modes of translation to make specific proteins necessary for required biological functions under these distinct conditions. To date, a number of alternative translation mechanisms have been reported, which include non-canonical, cap dependent translation and cap independent translation such as IRES mediated translation. Here, we discuss one of the alternative modes of translation mediated by a specialized microRNA complex, FXR1a-microRNP that promotes non-canonical, cap dependent translation in quiescent conditions, where canonical translation is reduced due to low mTOR activity.

Hoyeraal-Hreidarsson Syndrome due to PARN Mutations: Fourteen Years of Follow-Up

BACKGROUND

Hoyeraal-Hreidarsson syndrome is a dyskeratosis congenita-related telomere biology disorder that presents in infancy with intrauterine growth retardation, immunodeficiency, and cerebellar hypoplasia in addition to the triad of nail dysplasia, skin pigmentation, and oral leukoplakia. Individuals with Hoyeraal-Hreidarsson syndrome often develop bone marrow failure in early childhood. Germline mutations in DKC1, TERT, TINF2, RTEL1, ACD, or PARN cause about 60% of individuals with Hoyeraal-Hreidarsson syndrome.

PATIENT DESCRIPTION

We describe 14 years of follow-up of an individual with Hoyeraal-Hreidarsson syndrome who initially presented as an infant with intrauterine growth retardation, microcephaly, and central nervous system calcifications. He was diagnosed with Hoyeraal-Hreidarsson syndrome at age 6 years and had a complicated medical history including severe developmental delay, cerebellar hypoplasia, esophageal and urethral stenosis, hip avascular necrosis, immunodeficiency, and bone marrow failure evolving to myelodysplastic syndrome requiring hematopoietic cell transplantation at age 14 years. He had progressive skin pigmentation, oral leukoplakia, and nail dysplasia leading to anonychia. Whole exome sequencing identified novel biallelic variants in PARN.

CONCLUSIONS

This patient illustrates that the constellation of intrauterine growth retardation, central nervous system calcifications, and cerebellar hypoplasia, esophageal or urethral stenosis, and cytopenias, in the absence of congenital infection, may be due to Hoyeraal-Hreidarsson syndrome. Early diagnosis of Hoyeraal-Hreidarsson syndrome is important to optimize medical management and provide genetic counseling.

Molecular recognition of mRNA 5' cap by 3' poly(A)-specific ribonuclease (PARN) differs from interactions known for other cap-binding proteins

The mRNA 5' cap structure plays a pivotal role in coordination of eukaryotic translation and mRNA degradation. Poly(A)-specific ribonuclease (PARN) is a dimeric exoribonuclease that efficiently degrades mRNA 3' poly(A) tails while also simultaneously interacting with the mRNA 5' cap. The cap binding amplifies the processivity of PARN action. We used surface plasmon resonance kinetic analysis, quantitative equilibrium fluorescence titrations and circular dichroism to study the cap binding properties of PARN. The molecular mechanism of 5' cap recognition by PARN has been demonstrated to differ from interactions seen for other known cap-binding proteins in that: i) the auxiliary biological function of 5' cap binding by the 3' degrading enzyme is accomplished by negative cooperativity of PARN dimer subunits; ii) non-coulombic interactions are major factors in the complex formation; and iii) PARN has versatile activity toward alternative forms of the cap. These characteristics contribute to stabilization of the PARN-cap complex needed for the deadenylation processivity. Our studies provide a consistent biophysical basis for elucidation of the processive mechanism of PARN-mediated 3' mRNA deadenylation and provide a new framework to interpret the role of the 5' cap in mRNA degradation.

The molecular genetics of the telomere biology disorders

The importance of telomere function for human health is exemplified by a collection of Mendelian disorders referred to as the telomere biology disorders (TBDs), telomeropathies, or syndromes of telomere shortening. Collectively, the TBDs cover a spectrum of conditions from multisystem disease presenting in infancy to isolated disease presentations in adulthood, most notably idiopathic pulmonary fibrosis. Eleven genes have been found mutated in the TBDs to date, each of which is linked to some aspect of telomere maintenance. This review summarizes the molecular defects that result from mutations in these genes, highlighting recent advances, including the addition of PARN to the TBD gene family and the discovery of heterozygous mutations in RTEL1 as a cause of familial pulmonary fibrosis.

Discovery, synthesis and biochemical profiling of purine-2,6-dione derivatives as inhibitors of the human poly(A)-selective ribonuclease Caf1

Eukaryotic mRNA contains a 3′ poly(A) tail, which plays important roles in the regulation of mRNA stability and translation. Well-characterized enzymes involved in the shortening of the poly(A) tail include the multi-subunit Ccr4-Not deadenylase, which contains the Caf1 (Pop2) and Ccr4 catalytic components, and poly(A)-specific ribonuclease (PARN). Two Mg²⁺ ions present in the active sites of these ribonucleases are required for RNA cleavage. Here, we report the discovery, synthesis and biochemical profiling of purine-2,6-dione derivatives as (sub)micromolar inhibitors of Caf1.

Posttranscriptional regulation of gene expression-adding another layer of complexity to the DNA damage response

In response to DNA damage, cells activate a complex, kinase-based signaling network to arrest the cell cycle and allow time for DNA repair, or, if the extend of damage is beyond repair capacity, induce apoptosis. This signaling network, which is collectively referred to as the DNA damage response (DDR), is primarily thought to consist of two components—a rapid phosphorylation-driven signaling cascade that results in immediate inhibition of Cdk/cyclin complexes and a delayed transcriptional response that promotes a prolonged cell cycle arrest through the induction of Cdk inhibitors, such as p21. In recent years a third layer of complexity has emerged that involves potent posttranscriptional regulatory mechanisms that control the cellular response to DNA damage. Although much has been written on the relevance of the DDR in cancer and on the post-transcriptional role of microRNAs (miRs) in cancer, the post-transcriptional regulation of the DDR by non-coding RNAs and RNA-binding proteins (RBPs) still remains elusive in large parts. Here, we review the recent developments in this exciting new area of research in the cellular response to genotoxic stress. We put specific emphasis on the role of RBPs and the control of their function through DNA damage-activated protein kinases.