High Readthrough Research Articles

WITHDRAWN: Detection of N3-methylcytidine in RNA at single base resolution using Protoscript II to read through and generate high mutation rates

N3-methylcytidine (m3C), an RNA modification involving methylation of cytidine at the Watson-Crick face, has been identified in human tRNA, mitochondrial tRNA (mt-tRNA), and mRNA. In tRNA and mt-tRNA, it has been shown to play important roles in tRNA structure and translation. m3C remains understudied in mRNA and other low-abundance RNAs due to its markedly lower levels, the lack of high-resolution techniques to reproducibly map its location, and under-characterization of its effector proteins on these RNA types. Here we describe a new approach which utilizes the high readthrough and mutation rates of Protoscript II (PSII) at m3C sites in combination with AlkB demethylation to detect m3C at single-base resolution. We first validate the method on synthetic probes. Then, using this method, we successfully identify the known m3C sites on tRNA and mt-tRNA and show that none of the m3C sites identified in cytosolic tRNA are likely METTL8 substrates. In contrast, the two m3C sites detected in mt-tRNA are installed by METTL8 since they are sensitive to METTL8 knockdown (KD) and over expression (OE). These results demonstrate that the method can be used to investigate changes in the m3C level in many sites using an unbiased approach. Overall, this study introduces a convenient high-resolution method for m3C mapping with high readthrough rates, paving the way for future mechanistic studies on the biological function of m3C and its effector proteins.

Balancing Nonsense Mutation Readthrough and Toxicity of Designer Aminoglycosides for Treatment of Genetic Diseases.

New derivatives of aminoglycosides with a side chain 1,2-aminoalcohol at the 5" position of ring III were designed, synthesized, and biologically evaluated. The novel lead structure (compound 6), exhibiting substantially enhanced selectivity toward eukaryotic versus prokaryotic ribosome, high readthrough activity, and considerably lower toxicity than the previous lead compounds, was discovered. Balanced readthrough activity and toxicity of 6 were demonstrated in three different nonsense DNA-constructs underlying the genetic diseases, cystic fibrosis and Usher syndrome, and in two different cell lines, baby hamster kidney and human embryonic kidney cells. Molecular dynamics simulations within the A site of the 80S yeast ribosome demonstrated a remarkable kinetic stability of 6, which potentially determines its high readthrough activity.

Identification of 2,6-Diaminopurine As a Candidate for Hemophilia a Ribosomal Readthrough Therapy

Nonsense mutations introduce premature termination codons (PTCs) and are responsible for 11% hereditary genetic disorders including hemophilias. Drug-induced ribosomal readthrough over PTCs is an emerging strategy to treat the genetic disorders caused by nonsense mutations. Ribosomal readthrough therapy aims to allow translation beyond the PTC, creating a full-length protein and possible phenotypic rescue. Ataluren (PTC124), which induces readthrough by binding to the ribosomal 60S subunit, was recently granted conditional approval in the European Union for the treatment of Duchenne muscular dystrophy caused by nonsense mutations. Most nonsense mutations in FVIII and FIX lead to severe diseases. In theory, hemophilias are ideally suited for drug-induced readthrough therapy, because even a small increase in FVIII or FIX activity to over 1% could significantly improve bleeding symptoms in patients with nonsense mutations. Thus, ribosomal readthrough therapy has the potential to provide a small-molecule, personalized treatments based on patient's mutation profile. We previously investigated different readthrough drugs on different FVIII nonsense mutations and found that geneticin, but not gentamicin and Ataluren, is an effective readthrough agent for a number of such mutations. Recently, 2,6-diaminopurine (DAP), a natural compound found in extracts of the edible mushroom Lepista inversa, has shown high-efficiency correction of UGA nonsense mutations and low toxicity. DAP is thought to act as an inhibitor of a tRNA-specific 2'-O-methyltransferase and promotes the interaction of the near-cognate tRNATrp with the UGA stop codon. We introduced 22 UGA mutations, including the most common recurring mutations from hemophilia A (HA) patients into the B domain deleted FVIII-Gassia luciferase fusion (BDD-Gluc) protein. Cells stably expressing these mutant fusion proteins were treated with DAP and geneticin. DAP induced higher secreted luciferase activity, FVIII antigen and cofactor activity levels in cell culture media than geneticin treatments. A majority of DAP-treated mutants produced FVIII co-factor activities and antigens that are over 2% of wild-type level, with several mutants reaching 10-15% levels. Using an immunoprecipitation-immunoblotting assay, we detected full-length BDD-Gluc fusion proteins in culture media and cell lysates of both DAP- and geneticin-treated cells expressing nonsense mutants. Two major factors affect the outcome of readthrough therapies are the readthrough efficiency of drugs and the nature of replacement of amino acid (AA) residues. Studies had shown that a cytosine at +4 position is a favorable nucleotide sequence context. Readthrough efficiency of F8 mRNA is influenced, but not determined by nucleotide sequence contexts of the PTC. It was reported that only tryptophan was replaced with DAP-induced readthrough while three potential AAs can be replaced with geneticin treatment. We systematically substituted PTCs with potential replacement AAs and found that activities of readthrough products are determined by the compatibility of replacement AA residues with FVIII secretion and activity. Our results suggest that DAP, with its high readthrough efficiency and low toxicity, is a promising drug candidate for HA patients carrying susceptible UGA mutations. This cell-based study identifies candidate patient mutations and provides a rationale for a future clinical study of DAP or one of its derivatives as a therapeutic for these patients.

Transcriptome-wide investigation of stop codon readthrough in Saccharomyces cerevisiae.

Translation of mRNA into a polypeptide is terminated when the release factor eRF1 recognizes a UAA, UAG, or UGA stop codon in the ribosomal A site and stimulates nascent peptide release. However, stop codon readthrough can occur when a near-cognate tRNA outcompetes eRF1 in decoding the stop codon, resulting in the continuation of the elongation phase of protein synthesis. At the end of a conventional mRNA coding region, readthrough allows translation into the mRNA 3’-UTR. Previous studies with reporter systems have shown that the efficiency of termination or readthrough is modulated by cis-acting elements other than stop codon identity, including two nucleotides 5’ of the stop codon, six nucleotides 3’ of the stop codon in the ribosomal mRNA channel, and stem-loop structures in the mRNA 3’-UTR. It is unknown whether these elements are important at a genome-wide level and whether other mRNA features proximal to the stop codon significantly affect termination and readthrough efficiencies in vivo. Accordingly, we carried out ribosome profiling analyses of yeast cells expressing wild-type or temperature-sensitive eRF1 and developed bioinformatics strategies to calculate readthrough efficiency, and to identify mRNA and peptide features which influence that efficiency. We found that the stop codon (nt +1 to +3), the nucleotide after it (nt +4), the codon in the P site (nt -3 to -1), and 3’-UTR length are the most influential features in the control of readthrough efficiency, while nts +5 to +9 had milder effects. Additionally, we found low readthrough genes to have shorter 3’-UTRs compared to high readthrough genes in cells with thermally inactivated eRF1, while this trend was reversed in wild-type cells. Together, our results demonstrated the general roles of known regulatory elements in genome-wide regulation and identified several new mRNA or peptide features affecting the efficiency of translation termination and readthrough.

Targeting Nonsense: Optimization of 1,2,4-Oxadiazole TRIDs to Rescue CFTR Expression and Functionality in Cystic Fibrosis Cell Model Systems.

Cystic fibrosis (CF) patients develop a severe form of the disease when the cystic fibrosis transmembrane conductance regulator (CFTR) gene is affected by nonsense mutations. Nonsense mutations are responsible for the presence of a premature termination codon (PTC) in the mRNA, creating a lack of functional protein. In this context, translational readthrough-inducing drugs (TRIDs) represent a promising approach to correct the basic defect caused by PTCs. By using computational optimization and biological screening, we identified three new small molecules showing high readthrough activity. The activity of these compounds has been verified by evaluating CFTR expression and functionality after treatment with the selected molecules in cells expressing nonsense–CFTR–mRNA. Additionally, the channel functionality was measured by the halide sensitive yellow fluorescent protein (YFP) quenching assay. All three of the new TRIDs displayed high readthrough activity and low toxicity and can be considered for further evaluation as a therapeutic approach toward the second major cause of CF.

Metabolic stress promotes stop-codon readthrough and phenotypic heterogeneity

Accurate protein synthesis is a tightly controlled biological process with multiple quality control steps safeguarded by aminoacyl-transfer RNA (tRNA) synthetases and the ribosome. Reduced translational accuracy leads to various physiological changes in both prokaryotes and eukaryotes. Termination of translation is signaled by stop codons and catalyzed by release factors. Occasionally, stop codons can be suppressed by near-cognate aminoacyl-tRNAs, resulting in protein variants with extended C termini. We have recently shown that stop-codon readthrough is heterogeneous among single bacterial cells. However, little is known about how environmental factors affect the level and heterogeneity of stop-codon readthrough. In this study, we have combined dual-fluorescence reporters, mass spectrometry, mathematical modeling, and single-cell approaches to demonstrate that a metabolic stress caused by excess carbon substantially increases both the level and heterogeneity of stop-codon readthrough. Excess carbon leads to accumulation of acid metabolites, which lower the pH and the activity of release factors to promote readthrough. Furthermore, our time-lapse microscopy experiments show that single cells with high readthrough levels are more adapted to severe acid stress conditions and are more sensitive to an aminoglycoside antibiotic. Our work thus reveals a metabolic stress that promotes translational heterogeneity and phenotypic diversity.

Noncoding transcription influences the replication initiation program through chromatin regulation.

In eukaryotic organisms, replication initiation follows a temporal program. Among the parameters that regulate this program in Saccharomyces cerevisiae, chromatin structure has been at the center of attention without considering the contribution of transcription. Here, we revisit the replication initiation program in the light of widespread genomic noncoding transcription. We find that noncoding RNA transcription termination in the vicinity of autonomously replicating sequences (ARSs) shields replication initiation from transcriptional readthrough. Consistently, high natural nascent transcription correlates with low ARS efficiency and late replication timing. High readthrough transcription is also linked to increased nucleosome occupancy and high levels of H3K36me3. Moreover, forcing ARS readthrough transcription promotes these chromatin features. Finally, replication initiation defects induced by increased transcriptional readthrough are partially rescued in the absence of H3K36 methylation. Altogether, these observations indicate that natural noncoding transcription into ARSs influences replication initiation through chromatin regulation.

Rescuing the CFTR protein function: Introducing 1,3,4-oxadiazoles as translational readthrough inducing drugs

Nonsense mutations in the CFTR gene prematurely terminate translation of the CFTR mRNA leading to the production of a truncated protein that lacks normal function causing a more severe form of the cystic fibrosis (CF) disease. About 10% of patients affected by CF show a nonsense mutation. A potential treatment of this alteration is to promote translational readthrough of premature termination codons (PTCs) by Translational Readthrough Inducing Drugs (TRIDs) such as PTC124. In this context we aimed to compare the activity of PTC124 with analogues differing in the heteroatoms position in the central heterocyclic core. By a validated protocol consisting of computational screening, synthesis and biological tests we identified a new small molecule (NV2445) with 1,3,4-oxadiazole core showing a high readthrough activity. Moreover, we evaluated the CFTR functionality after NV2445 treatment in CF model systems and in cells expressing a nonsense-CFTR-mRNA. Finally, we studied the supramolecular interactions between TRIDs and CFTR-mRNA to assess the biological target/mechanism and compared the predicted ADME properties of NV2445 and PTC124.

Rules of UGA-N decoding by near-cognate tRNAs and analysis of readthrough on short uORFs in yeast

The molecular mechanism of stop codon recognition by the release factor eRF1 in complex with eRF3 has been described in great detail; however, our understanding of what determines the difference in termination efficiencies among various stop codon tetranucleotides and how near-cognate (nc) tRNAs recode stop codons during programmed readthrough in Saccharomyces cerevisiae is still poor. Here, we show that UGA-C as the only tetranucleotide of all four possible combinations dramatically exacerbated the readthrough phenotype of the stop codon recognition-deficient mutants in eRF1. Since the same is true also for UAA-C and UAG-C, we propose that the exceptionally high readthrough levels that all three stop codons display when followed by cytosine are partially caused by the compromised sampling ability of eRF1, which specifically senses cytosine at the +4 position. The difference in termination efficiencies among the remaining three UGA-N tetranucleotides is then given by their varying preferences for nc-tRNAs. In particular, UGA-A allows increased incorporation of Trp-tRNA whereas UGA-G and UGA-C favor Cys-tRNA. Our findings thus expand the repertoire of general decoding rules by showing that the +4 base determines the preferred selection of nc-tRNAs and, in the case of cytosine, it also genetically interacts with eRF1. Finally, using an example of the GCN4 translational control governed by four short uORFs, we also show how the evolution of this mechanism dealt with undesirable readthrough on those uORFs that serve as the key translation reinitiation promoting features of the GCN4 regulation, as both of these otherwise counteracting activities, readthrough versus reinitiation, are mediated by eIF3.

Translational readthrough-promoting drugs enhance pseudoknot-mediated suppression of the stop codon at the Moloney murine leukemia virus gag–pol junction.

Translational readthrough-promoting drugs enhance the incorporation of amino acids at stop codons and can thus bypass premature termination during protein synthesis. The polymerase (Pol) proteins of Moloney murine leukemia virus (MoMLV) are synthesized as a large Gag–Pol fusion protein, formed by the readthrough of a stop codon at the end of the gag ORF. The downstream pol ORF lacks its own start codon, and Pol protein synthesis is wholly dependent on translation of the upstream gag gene and the readthrough event for expression. Here, we explored the effects of readthrough-promoting drugs – aminoglycoside antibiotics and the small molecule ataluren – on the efficiency of readthrough of the stop codon in the context of the MoMLV genome. We showed that these compounds increased readthrough of the stop codon at the MoMLV gag–pol junction in vivo above the already high basal level and that the resulting elevated gag–pol readthrough had deleterious effects on virus replication. We also showed that readthrough efficiency could be driven to even higher levels in vitro, and that the combination of the small molecules and the RNA structure at the MoMLV stop codon could achieve extremely high readthrough efficiencies.

Transcription and histone methylation changes correlate with imprint acquisition in male germ cells

Genomic imprinting in mammals is controlled by DNA methylation imprints that are acquired in the gametes, at essential sequence elements called 'imprinting control regions' (ICRs). What signals paternal imprint acquisition in male germ cells remains unknown. To address this question, we explored histone methylation at ICRs in mouse primordial germ cells (PGCs). By 13.5 days post coitum (d.p.c.), H3 lysine-9 and H4 lysine-20 trimethylation are depleted from ICRs in male (and female) PGCs, indicating that these modifications do not signal subsequent imprint acquisition, which initiates at ∼15.5 d.p.c. Furthermore, during male PGC development, H3 lysine-4 trimethylation becomes biallelically enriched at 'maternal' ICRs, which are protected against DNA methylation, and whose promoters are active in the male germ cells. Remarkably, high transcriptional read-through is detected at the paternal ICRs H19-DMR and Ig-DMR at the time of imprint establishment, from one of the strands predominantly. Combined, our data evoke a model in which differential histone modification states linked to transcriptional events may signal the specificity of imprint acquisition during spermatogenesis.

PTC124-Mediated Translational Readthrough of a Nonsense Mutation Causing Usher Syndrome Type 1C

We investigated the therapeutic potential of the premature termination codon (PTC) readthrough-inducing drug PTC124 in treating the retinal phenotype of Usher syndrome, caused by a nonsense mutation in the USH1C gene. Applications in cell culture, organotypic retina cultures, and mice in vivo revealed significant readthrough and the recovery of protein function. In comparison with other readthrough drugs, namely the clinically approved readthrough-inducing aminoglycoside gentamicin, PTC124 exhibits significant better retinal biocompatibility. Its high readthrough efficiency in combination with excellent biocompatibility makes PTC124 a promising therapeutic agent for PTCs in USH1C, as well as other ocular and nonocular genetic diseases.

Overlapping enhancer/promoter and transcriptional termination signals in the lentiviral long terminal repeat

Oncoretrovirus, but not lentivirus, displays a high transcriptional readthrough activity in the 3' long terminal repeat (LTR) (Zaiss et al. J. Virol. 76, 7209–7219, 2002). However, the U3-deleted, self-inactivating (SIN) lentiviral LTR also exhibits high transcriptional readthrough activity. Since the canonical "core" polyadenylation signal (AAUAAA) of the lentivirus is located in the R-U5 region, the above finding suggests that additional RNA termination signals must be present in the U3 region. Insertion of alternative termination signals including panhuman T cell leukemia virus type I polyadenylation signal, a 3' end small intron, and a tertiary tRNA motif into the lentiviral SIN LTR did not restore the transcriptional termination function. Functional dissection of the U3 region revealed that 70–80% of the termination signals reside in the transcriptional control region within 124 nt overlapping NFκB, Sp1 and TATA binding sites. Serial deletion analysis of the transcriptional control region indicates that the lentiviral enhancer/promoter elements are essential to the RNA termination function. These results characterize the mechanism of lentiviral transcriptional readthrough, which addresses important fundamental and practical issue of RNA readthrough influencing lentiviral gene function and vector safety.

823. Dissecting Overlapping Functions of Transcriptional Enhancer and Polyadenylation in the Lentiviral U3

The transcriptional readthrough activity of the 3' long terminal repeat (LTR) of lentiviral vectors is critical to the safety profile of this vector system in future clinical applications. We have previously reported that the 3' polyadenylation (polyA) readthrough activity of the murine leukemia virus long terminal repeat (LTR) is much higher than that of the human immunodeficiency virus type 1 (HIV-1) LTR; however, deletion of the U3 or HIV-1 LTR to generate a self-inactivating (SIN) lentiviral vector results in a high transcriptional readthrough activity (J. Virol. 76: 7209-7219, 2002). This strongly suggests that other transcriptional termination signals (TS) besides the polyA signal (AAUAAA) in the R region are present in the U3 region. By serially inserting back different lengths of the U3 sequence, we demonstrated that the readthrough activity of the HIV-1 3' LTR was inversely correlated with the length of the U3, suggesting that the termination signals span across the entire U3 region. However, the transcriptional control region, co ntaining 124 nt of the U3 sequence, preserves |[sim]| 70-80% of the wild type TS function. Furthermore, deletion of the Sp1 and NF-kB binding sites abolished most of the TS function. Knocking down NF-kB expression by siRNA down-regulated the TS function. The introduction of foreign TS elements including human T cell leukemia virus type I (HTLV-I) polyA signal, human growth hormone gene intron, a tRNA tertiary structure, or a tetracycline responsive element (TRE) with a TRE-binding factor did not restore the TS function. Therefore, the lentiviral U3 contains an overlapping transcriptional enhancer and termination function which is difficult to be separated. Further improving the transcriptional termination but not the enhancer/promoter function of the 3' LTR will be critical to safety improvement of this vector system.

Computational identification and sequence analysis of stop codon readthrough genes in Oryza sativa

Using an approach based on the Readthrough Candidate Extraction System (RCES), we extracted 111 candidates from 9620 gene sequences of rice. The results of homology search and sequence analysis demonstrated that these candidates included actual readthrough genes that would be important for further investigating the mechanism of translation termination regulated by readthrough event, and could also give some useful clues for functional genome annotation. Between the candidates and non-candidates of gene sequences in rice, there exist significant base biases at the positions surrounding the stop codons. These positions, especially both −1 and +4, are referred to as part of an extended stop signal. In candidates, G at position −1, and G or C at position +4 are much more favored than that in non-candidates. Both stop sequence patterns, GUAGC and GUGAG, might drive high readthrough efficiency in rice. Secondary structure analysis revealed that the −1 and +1 amino acids around the first stop codon of candidates have a strong bias toward arginine, particularly the +1 position (20.7%), which indicated that the amino acids at the readthrough region being frequently located in the hydrophilic region of β-turn might be a determinant for efficient translation termination or not.

Effect of B. subtilis TRNA(Trp) on readthrough rate at an opal UGA codon.

Bacillus subtilis has been thought to have a high readthrough rate at the UGA stop codon because no opal suppressor tRNA has been isolated so far [Lovett et al. (1991) J. Bacteriol. 173, 1810-1812]. To examine whether a tRNATrp which we have characterized [Matsugi et al. (1992) Nucleic Acids Res. 20, 3514] has the ability to read the UGA codon, in vitro translation was performed with a synthetic mRNA containing a test codon, UGA, UAG, UAA, or UGG, in a reading frame. Addition of Trp-tRNATrp to the system significantly increased the readthrough rate only in the case of UGA. This suggests that this tRNATrp has a dual recognition pattern in B. subtilis, i.e., for the canonical tryptophan codon and for readthrough at the UGA stop codon.

Mutations of the beta subunit of RNA polymerase alter both transcription pausing and transcription termination in the trp operon leader region in vitro.

RNA polymerase was purified from rifampicin-resistant mutants of Escherichia coli which exhibit altered transcription termination at the trp operon attenuator in vivo. These mutant polymerases were used to investigate transcription pausing at the trp leader pause site and transcription termination at the trp attenuator. The mutant polymerases examined in vitro mimic their in vivo termination responses; i.e. RNA polymerase isolated from a mutant which displays high transcriptional read-through of the trp operon in vivo allows greater transcriptional read-through in vitro, while RNA polymerase prepared from a mutant which has reduced read-through in vivo exhibits greater termination of transcription in vitro. The observed differences are not due to the presence of--or response to--alternate secondary structures in the trp leader transcript since deletion of the DNA segment corresponding to some of these alternate structures does not affect termination efficiency. The mutant polymerases also have comparable effects on the kinetics of transcription pausing at the trp leader pause site; the termination-deficient polymerase exhibits diminished pausing while the termination-proficient polymerase displays enhanced pausing. We suggest that this correlation reflects polymerase recognition of similar features of RNA secondary structures in the pause and termination events. In addition, since single mutational changes in RNA polymerase affect two activities, pausing and termination, it is likely that a single site or region of the polymerase is involved in both events.