Pharmacological read-through of nonsense ARSB mutations as a potential therapeutic approach for mucopolysaccharidosis VI

Ataluren prevented bone loss induced by ovariectomy and aging in mice through the BMP-SMAD signaling pathway

Both estrogen deficiency and aging may lead to osteoporosis. Developing novel drugs for treating osteoporosis is a popular research direction. We screened several potential therapeutic agents through a new deep learning-based efficacy prediction system (DLEPS) using transcriptional profiles for osteoporosis. DLEPS screening led to a potential novel drug examinee, ataluren, for treating osteoporosis. Ataluren significantly reversed bone loss in ovariectomized mice. Next, ataluren significantly increased human bone marrow-derived mesenchymal stem cell (hBMMSC) osteogenic differentiation without cytotoxicity, indicated by the high expression index of osteogenic differentiation genes (OCN , BGLAP, ALP, COL1A, BMP2, RUNX2). Mechanistically, ataluren exerted its function through the BMP-SMAD pathway. Furthermore, it activated SMAD phosphorylation but osteogenic differentiation was attenuated by BMP2-SMAD inhibitors or small interfering RNA of BMP2. Finally, ataluren significantly reversed bone loss in aged mice. In summary, our findings suggest that the DLEPS-screened ataluren may be a therapeutic agent against osteoporosis by aiding hBMMSC osteogenic differentiation.

Mucopolysaccharidosis Type VI, an Updated Overview of the Disease

Mucopolysaccharidosis type VI, or Maroteaux-Lamy syndrome, is a rare, autosomal recessive genetic disease, mainly affecting the pediatric age group. The disease is due to pathogenic variants of the ARSB gene, coding for the lysosomal hydrolase N-acetylgalactosamine 4-sulfatase (arylsulfatase B, ASB). The enzyme deficit causes a pathological accumulation of the undegraded glycosaminoglycans dermatan-sulphate and chondroitin-sulphate, natural substrates of ASB activity. Intracellular and extracellular deposits progressively take to a pathological scenario, often severe, involving most organ-systems and generally starting from the osteoarticular apparatus. Neurocognitive and behavioral abilities, commonly described as maintained, have been actually investigated by few studies. The disease, first described in 1963, has a reported prevalence between 0.36 and 1.3 per 100,000 live births across the continents. With this paper, we wish to contribute an updated overview of the disease from the clinical, diagnostic, and therapeutic sides. The numerous in vitro and in vivo preclinical studies conducted in the last 10-15 years to dissect the disease pathogenesis, the efficacy of the available therapeutic treatment (enzyme replacement therapy), as well as new therapies under study are here described. This review also highlights the need to identify new disease biomarkers, potentially speeding up the diagnostic process and the monitoring of therapeutic efficacy.

Evidence in disease and non-disease contexts that nonsense mutations cause altered splicing via motif disruption

Transcripts containing premature termination codons (PTCs) can be subject to nonsense-associated alternative splicing (NAS). Two models have been evoked to explain this, scanning and splice motif disruption. The latter postulates that exonic cis motifs, such as exonic splice enhancers (ESEs), are disrupted by nonsense mutations. We employ genome-wide transcriptomic and k-mer enrichment methods to scrutinize this model. First, we show that ESEs are prone to disruptive nonsense mutations owing to their purine richness and paucity of TGA, TAA and TAG. The motif model correctly predicts that NAS rates should be low (we estimate 5–30%) and approximately in line with estimates for the rate at which random point mutations disrupt splicing (8–20%). Further, we find that, as expected, NAS-associated PTCs are predictable from nucleotide-based machine learning approaches to predict splice disruption and, at least for pathogenic variants, are enriched in ESEs. Finally, we find that both in and out of frame mutations to TAA, TGA or TAG are associated with exon skipping. While a higher relative frequency of such skip-inducing mutations in-frame than out of frame lends some credence to the scanning model, these results reinforce the importance of considering splice motif modulation to understand the etiology of PTC-associated disease.

Ataluren-Promising Therapeutic Premature Termination Codon Readthrough Frontrunner

Around 12% of hereditary disease-causing mutations are in-frame nonsense mutations. The expression of genes containing nonsense mutations potentially leads to the production of truncated proteins with residual or virtually no function. However, the translation of transcripts containing premature stop codons resulting in full-length protein expression can be achieved using readthrough agents. Among them, only ataluren was approved in several countries to treat nonsense mutation Duchenne muscular dystrophy (DMD) patients. This review summarizes ataluren’s journey from its identification, via first in vitro activity experiments, to clinical trials in DMD, cystic fibrosis, and aniridia. Additionally, data on its pharmacokinetics and mechanism of action are presented. The range of diseases with underlying nonsense mutations is described for which ataluren therapy seems to be promising. What is more, experiments in which ataluren did not show its readthrough activity are also included, and reasons for their failures are discussed.

Diagnosis of Mucopolysaccharidoses

The mucopolysaccharidoses (MPSs) include 11 different conditions caused by specific enzyme deficiencies in the degradation pathway of glycosaminoglycans (GAGs). Although most MPS types present increased levels of GAGs in tissues, including blood and urine, diagnosis is challenging as specific enzyme assays are needed for the correct diagnosis. Enzyme assays are usually performed in blood, with some samples (as leukocytes) providing a final diagnosis, while others (such as dried blood spots) still being considered as screening methods. The identification of variants in the specific genes that encode each MPS-related enzyme is helpful for diagnosis confirmation (when needed), carrier detection, genetic counseling, prenatal diagnosis (preferably in combination with enzyme assays) and phenotype prediction. Although the usual diagnostic flow in high-risk patients starts with the measurement of urinary GAGs, it continues with specific enzyme assays and is completed with mutation identification; there is a growing trend to have genotype-based investigations performed at the beginning of the investigation. In such cases, confirmation of pathogenicity of the variants identified should be confirmed by measurement of enzyme activity and/or identification and/or quantification of GAG species. As there is a growing number of countries performing newborn screening for MPS diseases, the investigation of a low enzyme activity by the measurement of GAG species concentration and identification of gene mutations in the same DBS sample is recommended before the suspicion of MPS is taken to the family. With specific therapies already available for most MPS patients, and with clinical trials in progress for many conditions, the specific diagnosis of MPS as early as possible is becoming increasingly necessary. In this review, we describe traditional and the most up to date diagnostic methods for mucopolysaccharidoses.

Identification of Novel ARSB Genes Necessary for p-Benzoquinone Biosynthesis in the Larval Oral Secretion Participating in External Immune Defense in the Red Palm Weevil

External secretions, composed of a variety of chemical components, are among the most important traits that endow insects with the ability to defend themselves against predators, parasites, or other adversities, especially pathogens. Thus, these exudates play a crucial role in external immunity. Red palm weevil larvae are prolific in this regard, producing large quantities of p-benzoquinone, which is present in their oral secretion. Benzoquinone with antimicrobial activity has been proven to be an active ingredient and key factor for external immunity in a previous study. To obtain a better understanding of the genetic and molecular basis of external immune secretions, we identify genes necessary for p-benzoquinone synthesis. Three novel ARSB genes, namely, RfARSB-0311, RfARSB-11581, and RfARSB-14322, are screened, isolated, and molecularly characterized on the basis of transcriptome data. To determine whether these genes are highly and specifically expressed in the secretory gland, we perform tissue/organ-specific expression profile analysis. The functions of these genes are further determined by examining the antimicrobial activity of the secretions and quantification of p-benzoquinone after RNAi. All the results reveal that the ARSB gene family can regulate the secretory volume of p-benzoquinone by participating in the biosynthesis of quinones, thus altering the host’s external immune inhibitory efficiency.

Current and Emerging Therapies for Mucopolysaccharidoses

Mucopolysaccharidoses (MPSs) are caused by deficiencies of specific lysosomal enzymes that affect the degradation of mucopolysaccharides or glycosaminoglycans (GAGs). Enzyme replacement therapies are available for an increasing number of MPSs since more than 15 years. Together with hematopoietic stem cell transplantation, these enzyme therapies are currently the gold standard of causal treatment in MPS. Both treatments can improve symptoms and prognosis, but they do not cure these severe conditions. The limitations of intravenous enzyme replacement and cell therapy can be summarized as the development of immune reactions against the therapeutic molecules/cells and failure to restore enduring and sufficient drug exposures in all relevant tissues. Thus innovative approaches include small molecules and encapsulated cells that do not induce immune reactions, gene therapy approaches that aim for sustained enzyme expression, and new enzymes that are able to penetrate barriers to drug distribution like the blood-brain barrier. This chapter provides an update on the state of development of these new therapies and highlights current challenges.

miR-154-5p Functions as an Important Regulator of Angiotensin II-Mediated Heart Remodeling

Chronic hypertension, valvular heart disease, and heart infarction cause cardiac remodeling and potentially lead to a series of pathological and structural changes in the left ventricular myocardium and a progressive decrease in heart function. Angiotensin II (AngII) plays a key role in the onset and development of cardiac remodeling. Many microRNAs (miRNAs), including miR-154-5p, may be involved in the development of cardiac remolding, but the underlying molecular mechanisms remain unclear. We aimed to characterize the function of miR-154-5p and reveal its mechanisms in cardiac remodeling induced by AngII. First, angiotensin II led to concurrent increases in miR-154-5p expression and cardiac remodeling in adult C57BL/6J mice. Second, overexpression of miR-154-5p to a level similar to that induced by AngII was sufficient to trigger cardiomyocyte hypertrophy and apoptosis, which is associated with profound activation of oxidative stress and inflammation. Treatment with a miR-154-5p inhibitor noticeably reversed these changes. Third, miR-154-5p directly inhibited arylsulfatase B (Arsb) expression by interacting with its 3′-UTR and promoted cardiomyocyte hypertrophy and apoptosis. Lastly, the angiotensin type 1 receptor blocker telmisartan attenuated AngII-induced cardiac hypertrophy, apoptosis, and fibrosis by blocking the increase in miR-154-5p expression. Moreover, upon miR-154-5p overexpression in isolated cardiomyocytes, the protective effect of telmisartan was partially abolished. Based on these results, increased cardiac miR-154-5p expression is both necessary and sufficient for AngII-induced cardiomyocyte hypertrophy and apoptosis, suggesting that the upregulation of miR-154-5p may be a crucial pathological factor and a potential therapeutic target for cardiac remodeling.

Treatment of brain disease in the mucopolysaccharidoses

The mucopolysaccharidosis (MPS) disorders are a group of lysosomal storage diseases caused by lysosomal enzyme deficits that lead to glycosaminoglycan accumulation, affecting various tissues throughout the body based on the specific enzyme deficiency. These disorders are characterized by their progressive nature and a variety of somatic manifestations and neurological symptoms. There are established treatments for some MPS disorders, but these mostly alleviate somatic and non-neurological symptoms and do not cure the disease. Patients with MPS I, II, III, and VII can present with neurological manifestations such as neurocognitive decline and behavioral problems. Treatment of these neurological manifestations remains challenging due to the blood-brain barrier (BBB) that limits delivery of therapeutic agents to the central nervous system (CNS). New therapies that circumvent this barrier and target brain disease in MPS are currently under development. They primarily focus on facilitating penetration of drugs through the BBB, delivery of recombinant enzyme to the brain by gene therapy, or direct CNS administration. This review summarizes existing and potential future treatment approaches that target brain disease in MPS. The information in this review is based on current literature and presentations and discussions during a closed meeting by an international group of experts with extensive experience in managing and treating MPS.

Small molecules as therapeutic agents for inborn errors of metabolism

Most inborn errors of metabolism (IEM) remain without effective treatment mainly due to the incapacity of conventional therapeutic approaches to target the neurological symptomatology and to ameliorate the multisystemic involvement frequently observed in these patients. However, in recent years, the therapeutic use of small molecules has emerged as a promising approach for treating this heterogeneous group of disorders. In this review, we focus on the use of therapeutically active small molecules to treat IEM, including readthrough agents, pharmacological chaperones, proteostasis regulators, substrate inhibitors, and autophagy inducers. The small molecules reviewed herein act at different cellular levels, and this knowledge provides new tools to set up innovative treatment approaches for particular IEM. We review the molecular mechanism underlying therapeutic properties of small molecules, methodologies used to screen for these compounds, and their applicability in preclinical and clinical practice.

Targeting Nonsense Mutations in Diseases with Translational Read-Through-Inducing Drugs (TRIDs)

In recent years, remarkable advances in the ability to diagnose genetic disorders have been made. The identification of disease-causing genes allows the development of gene-specific therapies with the ultimate goal to develop personalized medicines for each patient according to their own specific genetic defect. In-depth genotyping of many different genes has revealed that ~12% of inherited genetic disorders are caused by in-frame nonsense mutations. Nonsense (non-coding) mutations are caused by point mutations, which generate premature termination codons (PTCs) that cause premature translational termination of the mRNA, and subsequently inhibit normal full-length protein expression. Recently, a gene-based therapeutic approach for genetic diseases caused by nonsense mutations has emerged, namely the so-called translational read-through (TR) therapy. Read-through therapy is based on the discovery that small molecules, known as TR-inducing drugs (TRIDs), allow the translation machinery to suppress a nonsense codon, elongate the nascent peptide chain, and consequently result in the synthesis of full-length protein. Several TRIDs are currently under investigation and research has been performed on several genetic disorders caused by nonsense mutations over the years. These findings have raised hope for the usage of TR therapy as a gene-based pharmacogenetic therapy for nonsense mutations in various genes responsible for a variety of genetic diseases.

Nonsense Suppression as an Approach to Treat Lysosomal Storage Diseases

In-frame premature termination codons (PTCs) (also referred to as nonsense mutations) comprise ~10% of all disease-associated gene lesions. PTCs reduce gene expression in two ways. First, PTCs prematurely terminate translation of an mRNA, leading to the production of a truncated polypeptide that often lacks normal function and/or is unstable. Second, PTCs trigger degradation of an mRNA by activating nonsense-mediated mRNA decay (NMD), a cellular pathway that recognizes and degrades mRNAs containing a PTC. Thus, translation termination and NMD are putative therapeutic targets for the development of treatments for genetic diseases caused by PTCs. Over the past decade, significant progress has been made in the identification of compounds with the ability to suppress translation termination of PTCs (also referred to as readthrough). More recently, NMD inhibitors have also been explored as a way to enhance the efficiency of PTC suppression. Due to their relatively low threshold for correction, lysosomal storage diseases are a particularly relevant group of diseases to investigate the feasibility of nonsense suppression as a therapeutic approach. In this review, the current status of PTC suppression and NMD inhibition as potential treatments for lysosomal storage diseases will be discussed.

Novel mutations of the arylsulphatase B (ARSB) gene in Indian patients with mucopolysaccharidosis type VI

Background & objectives:

Mucopolysaccharidosis type VI (MPS VI) is a rare, autosomal recessive lysosomal storage disorder caused by deficient enzymatic activity of N-acetyl galactosamine-4-sulphatase resulting from mutations in the arylsulphatase B (ARSB) gene. The ARSB gene is located on chromosome 5q11-q13 and is composed of eight exons. More than hundred ARSB mutations have been reported so far, but the mutation spectrum of MPS VI in India is still unknown. Hence, the aim of the present study was to identify the mutational spectrum in patients with MPS VI in India and to study the genotype-phenotype association and functional outcomes of these mutations.

Methods:

Molecular characterization of the ARSB gene by Sanger sequencing was done for 15 patients (aged 15 months to 11 yr) who were enzymatically confirmed to have MPS VI. Age of onset, clinical progression and enzyme activity levels in each patient were studied to look for genotype-phenotype association. Haplotype analysis performed for unrelated patients with the recurring mutation W450C, was suggestive of a founder effect. Sequence and structural analyses of the ARSB protein using standard software were carried out to determine the impact of detected mutations on the function of the ARSB protein.

Results:

A total of 12 mutations were identified, of which nine were novel mutations namely, p.D53N, p.L98R, p.Y103SfsX9, p.W353X, p.H393R, p.F166fsX18, p.I220fsX5, p.W450L, and p.W450C, and three were known mutations (p.D54N, p.A237D and p.S320R). The nine novel sequence variants were confirmed not to be polymorphic variants by performing sequencing in 50 unaffected individuals from the same ethnic population.

Interpretation & conclusions:

Nine novel mutations were identified in MPS VI cases from India in the present study. The study also provides some insights into the genotype-phenotype association in MPS VI.

Carbohydrate-Processing Enzymes of the Lysosome: Diseases Caused by Misfolded Mutants and Sugar Mimetics as Correcting Pharmacological Chaperones

Lysosomal storage diseases are hereditary disorders caused by mutations on genes encoding for one of the more than fifty lysosomal enzymes involved in the highly ordered degradation cascades of glycans, glycoconjugates, and other complex biomolecules in the lysosome. Several of these metabolic disorders are associated with the absence or the lack of activity of carbohydrate-processing enzymes in this cell compartment. In a recently introduced therapy concept, for susceptible mutants, small substrate-related molecules (so-called pharmacological chaperones), such as reversible inhibitors of these enzymes, may serve as templates for the correct folding and transport of the respective protein mutant, thus improving its concentration and, consequently, its enzymatic activity in the lysosome. Carbohydrate-processing enzymes in the lysosome, related lysosomal diseases, and the scope and limitations of reported reversible inhibitors as pharmacological chaperones are discussed with a view to possibly extending and improving research efforts in this area of orphan diseases.

Molecular diagnosis of 65 families with mucopolysaccharidosis type II (Hunter syndrome) characterized by 16 novel mutations in the IDS gene: Genetic, pathological, and structural studies on iduronate-2-sulfatase

Mucopolysaccharidosis type II (MPS II: also called as Hunter syndrome) is an X-linked recessive lysosomal storage disorder characterized by the accumulation of extracellular glycosaminoglycans due to the deficiency of the enzyme iduronate-2-sulfatase (IDS). Previous observations suggested that MPS II can be classified into two distinct disease subtypes: (1) severe type of MPS II involves a decline in the cognitive ability of a patient and (2) attenuated type of MPS II exhibits no such intellectual phenotype. To determine whether such disease subtypes of MPS II could be explained by genetic diagnosis, we analyzed mutations in the IDS gene of 65 patients suffering from MPS II among the Japanese population who were diagnosed with both the accumulation of urinary glycosaminoglycans and a decrease in their IDS enzyme activity between 2004 and 2014. Among the specimens examined, we identified the following mutations: 33 missense, 8 nonsense, 7 frameshift, 4 intronic changes affecting splicing, 8 recombinations involving IDS-IDS2, and 7 other mutations including 4 large deletions. Consistent with the previous data, the results of our study showed that most of the attenuated phenotype was derived from the missense mutations of the IDS gene, whereas mutations associated with a large structural alteration including recombination, splicing, frameshift, and nonsense mutations were linked to the severe phenotype of MPS II. Furthermore, we conducted a homology modeling study of IDS P120R and N534I mutant as representatives of the causative mutation of the severe and attenuated type of MPS II, respectively. We found that the substitution of P120R of the IDS enzyme was predicted to deform the α-helix generated by I119-F123, leading to the major structural alteration of the wild-type IDS enzyme. In sharp contrast, the effect of the structural alteration of N534I was marginal; thus, this mutation was pathogenically predicted to be associated with the attenuated type of MPS II. These results suggest that a combination of the genomic diagnosis of the IDS gene and the structural prediction of the IDS enzyme could enable the prediction of a phenotype more effectively.

Emerging drugs for the treatment of mucopolysaccharidoses

INTRODUCTION

Despite being reported for the first time almost one century ago, only in the last few decades effective have treatments become available for the mucopolysaccharidoses (MPSs), a group of 11 inherited metabolic diseases that affect lysosomal function. These diseases are progressive, usually severe, and, in a significant number of cases, involve cognitive impairment.

AREAS COVERED

This review will not cover established treatments such as bone marrow/hematopoietic stem cell transplantation and classic intravenous enzyme replacement therapy (ERT), whose long-term outcomes have already been published (MPS I, MPS II, and MPS VI), but it instead focuses on emerging therapies for MPSs. That includes intravenous ERT for MPS IVA and VII, intrathecal ERT, ERT with fusion proteins, substrate reduction therapy, gene therapy, and other novel approaches.

EXPERT OPINION

The available treatments have resulted in improvements for several disease manifestations, but they still do not represent a cure for these diseases; thus, it is important to develop alternative methods to approach the unmet needs (i.e. bone disease, heart valve disease, corneal opacity, and central nervous system (CNS) involvement). The work in progress with novel approaches makes us confident that in 2017, when MPS will commemorate 100 years of its first report, we will be much closer to an effective cure for these challenging conditions.

Progress toward improved therapies for inborn errors of metabolism

Because of their prevalence, severity and lack of effective treatments, inborn errors of metabolism need novel and more effective therapeutic approaches. The opportunity for an early treatment coming from expanded newborn screening has made this need even more urgent. To meet this demand, a growing number of novel treatments are entering in the phase of clinical development. Strategies to overcome the detrimental consequences of the enzyme deficiencies responsible for inborn errors of metabolism have been focused on multiple fronts at the levels of the gene, RNA, protein and whole cell. These strategies have been accomplished using a wide spectrum of approaches ranging from small molecules to enzyme replacement therapy, cell and gene therapy. The applications of new technologies in the field of inborn errors of metabolism, such as genome editing, RNA interference and cell reprogramming, along with progress in pre-existing strategies, such as gene therapy or cell transplantation, have tremendous potential for clinical translation.

Effect of Readthrough Treatment in Fibroblasts of Patients Affected by Lysosomal Diseases Caused by Premature Termination Codons

Aminoglycoside antibiotics, such as gentamicin, may induce premature termination codon (PTC) readthrough and elude the nonsense-mediated mRNA decay mechanism. Because PTCs are frequently involved in lysosomal diseases, readthrough compounds may be useful as potential therapeutic agents. The aim of our study was to identify patients responsive to gentamicin treatment in order to be used as positive controls to further screen for other PTC readthrough compounds. With this aim, fibroblasts from 11 patients affected by 6 different lysosomal diseases carrying PTCs were treated with gentamicin. Treatment response was evaluated by measuring enzymatic activity, abnormal metabolite accumulation, mRNA expression, protein localization, and cell viability. The potential effect of readthrough was also analyzed by in silico predictions. Results showed that fibroblasts from 5/11 patients exhibited an up to 3-fold increase of enzymatic activity after gentamicin treatment. Accordingly, cell lines tested showed enhanced well-localized protein and/or increased mRNA expression levels and/or reduced metabolite accumulation. Interestingly, these cell lines also showed increased enzymatic activity after PTC124 treatment, which is a PTC readthrough-promoting compound. In conclusion, our results provide a proof-of-concept that PTCs can be effectively suppressed by readthrough drugs, with different efficiencies depending on the genetic context. The screening of new compounds with readthrough activity is a strategy that can be used to develop efficient therapies for diseases caused by PTC mutations.

Evaluation of Aminoglycoside and Non-Aminoglycoside Compounds for Stop-Codon Readthrough Therapy in Four Lysosomal Storage Diseases

Nonsense mutations are quite prevalent in inherited diseases. Readthrough drugs could provide a therapeutic option for any disease caused by this type of mutation. Geneticin (G418) and gentamicin were among the first to be described. Novel compounds have been generated, but only a few have shown improved results. PTC124 is the only compound to have reached clinical trials. Here we first investigated the readthrough effects of gentamicin on fibroblasts from one patient with Sanfilippo B, one with Sanfilippo C, and one with Maroteaux-Lamy. We found that ARSB activity (Maroteaux-Lamy case) resulted in an increase of 2–3 folds and that the amount of this enzyme within the lysosomes was also increased, after treatment. Since the other two cases (Sanfilippo B and Sanfilippo C) did not respond to gentamicin, the treatments were extended with the use of geneticin and five non-aminoglycoside (PTC124, RTC13, RTC14, BZ6 and BZ16) readthrough compounds (RTCs). No recovery was observed at the enzyme activity level. However, mRNA recovery was observed in both cases, nearly a two-fold increase for Sanfilippo B fibroblasts with G418 and around 1.5 fold increase for Sanfilippo C cells with RTC14 and PTC124. Afterwards, some of the products were assessed through in vitro analyses for seven mutations in genes responsible for those diseases and, also, for Niemann-Pick A/B. Using the coupled transcription/translation system (TNT), the best results were obtained for SMPD1 mutations with G418, reaching a 35% recovery at 0.25 μg/ml, for the p.W168X mutation. The use of COS cells transfected with mutant cDNAs gave positive results for most of the mutations with some of the drugs, although to a different extent. The higher enzyme activity recovery, of around two-fold increase, was found for gentamicin on the ARSB p.W146X mutation. Our results are promising and consistent with those of other groups. Further studies of novel compounds are necessary to find those with more consistent efficacy and fewer toxic effects.

Translational read-through of the RP2 Arg120stop mutation in patient iPSC-derived retinal pigment epithelium cells

Mutations in the RP2 gene lead to a severe form of X-linked retinitis pigmentosa. RP2 patients frequently present with nonsense mutations and no treatments are currently available to restore RP2 function. In this study, we reprogrammed fibroblasts from an RP2 patient carrying the nonsense mutation c.519C>T (p.R120X) into induced pluripotent stem cells (iPSC), and differentiated these cells into retinal pigment epithelial cells (RPE) to study the mechanisms of disease and test potential therapies. RP2 protein was undetectable in the RP2 R120X patient cells, suggesting a disease mechanism caused by complete lack of RP2 protein. The RP2 patient fibroblasts and iPSC-derived RPE cells showed phenotypic defects in IFT20 localization, Golgi cohesion and Gβ1 trafficking. These phenotypes were corrected by over-expressing GFP-tagged RP2. Using the translational read-through inducing drugs (TRIDs) G418 and PTC124 (Ataluren), we were able to restore up to 20% of endogenous, full-length RP2 protein in R120X cells. This level of restored RP2 was sufficient to reverse the cellular phenotypic defects observed in both the R120X patient fibroblasts and iPSC-RPE cells. This is the first proof-of-concept study to demonstrate successful read-through and restoration of RP2 function for the R120X nonsense mutation. The ability of the restored RP2 protein level to reverse the observed cellular phenotypes in cells lacking RP2 indicates that translational read-through could be clinically beneficial for patients.

Ataluren treatment of patients with nonsense mutation dystrophinopathy

INTRODUCTION

Dystrophinopathy is a rare, severe muscle disorder, and nonsense mutations are found in 13% of cases. Ataluren was developed to enable ribosomal readthrough of premature stop codons in nonsense mutation (nm) genetic disorders.

METHODS

Randomized, double-blind, placebo-controlled study; males ≥ 5 years with nm-dystrophinopathy received study drug orally 3 times daily, ataluren 10, 10, 20 mg/kg (N=57); ataluren 20, 20, 40 mg/kg (N=60); or placebo (N=57) for 48 weeks. The primary endpoint was change in 6-Minute Walk Distance (6MWD) at Week 48.

RESULTS

Ataluren was generally well tolerated. The primary endpoint favored ataluren 10, 10, 20 mg/kg versus placebo; the week 48 6MWD Δ=31.3 meters, post hoc P=0.056. Secondary endpoints (timed function tests) showed meaningful differences between ataluren 10, 10, 20 mg/kg, and placebo.

CONCLUSIONS

As the first investigational new drug targeting the underlying cause of nm-dystrophinopathy, ataluren offers promise as a treatment for this orphan genetic disorder with high unmet medical need.

Nonsense-mediated decay in genetic disease: friend or foe?

Eukaryotic cells utilize various RNA quality control mechanisms to ensure high fidelity of gene expression, thus protecting against the accumulation of nonfunctional RNA and the subsequent production of abnormal peptides. Messenger RNAs (mRNAs) are largely responsible for protein production, and mRNA quality control is particularly important for protecting the cell against the downstream effects of genetic mutations. Nonsense-mediated decay (NMD) is an evolutionarily conserved mRNA quality control system in all eukaryotes that degrades transcripts containing premature termination codons (PTCs). By degrading these aberrant transcripts, NMD acts to prevent the production of truncated proteins that could otherwise harm the cell through various insults, such as dominant negative effects or the ER stress response. Although NMD functions to protect the cell against the deleterious effects of aberrant mRNA, there is a growing body of evidence that mutation-, codon-, gene-, cell-, and tissue-specific differences in NMD efficiency can alter the underlying pathology of genetic disease. In addition, the protective role that NMD plays in genetic disease can undermine current therapeutic strategies aimed at increasing the production of full-length functional protein from genes harboring nonsense mutations. Here, we review the normal function of this RNA surveillance pathway and how it is regulated, provide current evidence for the role that it plays in modulating genetic disease phenotypes, and how NMD can be used as a therapeutic target.

Therapeutics based on stop codon readthrough

Nonsense suppression therapy encompasses approaches aimed at suppressing translation termination at in-frame premature termination codons (PTCs, also known as nonsense mutations) to restore deficient protein function. In this review, we examine the current status of PTC suppression as a therapy for genetic diseases caused by nonsense mutations. We discuss what is currently known about the mechanism of PTC suppression as well as therapeutic approaches under development to suppress PTCs. The approaches considered include readthrough drugs, suppressor tRNAs, PTC pseudouridylation, and inhibition of nonsense-mediated mRNA decay. We also discuss the barriers that currently limit the clinical application of nonsense suppression therapy and suggest how some of these difficulties may be overcome. Finally, we consider how PTC suppression may play a role in the clinical treatment of genetic diseases caused by nonsense mutations.

Comparison of read-through effects of aminoglycosides and PTC124 on rescuing nonsense mutations of HERG gene associated with long QT syndrome

Aminoglycosides promote the readthrough of premature stop codons introduced by nonsense mutations to produce full-length proteins in genetic disease models. The read-through effects of different aminoglycosides and PTC124 on HERG gene have yet to be adequately elucidated. The wild-type (WT) or mutant genes were transiently transfected in HEK293 cells. The read-through effect was examined by adding drugs into culture medium for 24 h. Western blot analysis and patch clamping were performed to evaluate the expression and function of the genes. The mRNA levels were determined using qPCR. The results showed that G418 and PTC124 significantly increased the protein expression of R1014X mutant in a dose-dependent manner and produced a full-length protein. The maximal protein levels after G418, gentamicin or PTC124 treatment were 39.1±2.4, 18.6±0.3 or 10.3±1.0%, respectively, of the WT level. Tobramycin did not exhibit a read-through effect. The mRNA levels, however, did not differ between WT and mutant gene. The tail current densities of R1014X channels at 40 mV were 22.57±2.26 pA/pF for G418, 16.21±1.49 pA/pF for gentamicin and 9.62±0.73 pA/pF for PTC124. The leftward shift of the activation curve was corrected only by G418 and gentamicin. The read-through effects of W927X, R863X and E698X revealed that as the mutation site approached the N-terminal, the rescue efficiency was decreased. The above results suggest that aminoglycosides and PTC124 induced different effects on rescue nonsense mutations of the HERG gene. The mutation site was a significant factor in determining the pharmacological rescue efficiency.

Ataluren as an agent for therapeutic nonsense suppression

The interplay of translation and mRNA turnover has helped unveil how the regulation of gene expression is a continuum in which events that occur during the birth of a transcript in the nucleus can have profound effects on subsequent steps in the cytoplasm. Exemplifying this continuum is nonsense-mediated mRNA decay (NMD), the process wherein a premature stop codon affects both translation and mRNA decay. Studies of NMD helped lead us to the therapeutic concept of treating a subset of patients suffering from multiple genetic disorders due to nonsense mutations with a single small-molecule drug that modulates the translation termination process at a premature nonsense codon. Here we review both translation termination and NMD, and our subsequent efforts over the past 15 years that led to the identification, characterization, and clinical testing of ataluren, a new therapeutic with the potential to treat a broad range of genetic disorders due to nonsense mutations.