Reversal of cystic fibrosis phenotype in a cultured Delta508 cystic fibrosis transmembrane conductance regulator cell line by oligonucleotide insertion

One Size Does Not Fit All: The Past, Present and Future of Cystic Fibrosis Causal Therapies

Cystic fibrosis (CF) is the most common monogenic disorder, caused by mutations in the CF transmembrane conductance regulator (CFTR) gene. Over the last 30 years, tremendous progress has been made in understanding the molecular basis of CF and the development of treatments that target the underlying defects in CF. Currently, a highly effective CFTR modulator treatment (Kalydeco™/Trikafta™) is available for 90% of people with CF. In this review, we will give an extensive overview of past and ongoing efforts in the development of therapies targeting the molecular defects in CF. We will discuss strategies targeting the CFTR protein (i.e., CFTR modulators such as correctors and potentiators), its cellular environment (i.e., proteostasis modulation, stabilization at the plasma membrane), the CFTR mRNA (i.e., amplifiers, nonsense mediated mRNA decay suppressors, translational readthrough inducing drugs) or the CFTR gene (gene therapies). Finally, we will focus on how these efforts can be applied to the 15% of people with CF for whom no causal therapy is available yet.

Gene-specific nonsense-mediated mRNA decay targeting for cystic fibrosis therapy

Low CFTR mRNA expression due to nonsense-mediated mRNA decay (NMD) is a major hurdle in developing a therapy for cystic fibrosis (CF) caused by the W1282X mutation in the CFTR gene. CFTR-W1282X truncated protein retains partial function, so increasing its levels by inhibiting NMD of its mRNA will likely be beneficial. Because NMD regulates the normal expression of many genes, gene-specific stabilization of CFTR-W1282X mRNA expression is more desirable than general NMD inhibition. Synthetic antisense oligonucleotides (ASOs) designed to prevent binding of exon junction complexes (EJC) downstream of premature termination codons (PTCs) attenuate NMD in a gene-specific manner. We describe cocktails of three ASOs that specifically increase the expression of CFTR-W1282X mRNA and CFTR protein upon delivery into human bronchial epithelial cells. This treatment increases the CFTR-mediated chloride current. These results set the stage for clinical development of an allele-specific therapy for CF caused by the W1282X mutation.

The W1282X nonsense mutation in the CFTR gene causes cystic fibrosis by reducing its mRNA and functional protein levels. Here the authors developed antisense-oligonucleotide cocktails that restore CFTR protein function by gene-specific stabilization of CFTR mRNA.

Exon-skipping antisense oligonucleotides for cystic fibrosis therapy

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene cause cystic fibrosis (CF), and the CFTR-W1282X nonsense mutation causes a severe form of CF. Although Trikafta and other CFTR-modulation therapies benefit most CF patients, targeted therapy for patients with the W1282X mutation is lacking. The CFTR-W1282X protein has residual activity but is expressed at a very low level due to nonsense-mediated messenger RNA (mRNA) decay (NMD). NMD-suppression therapy and read-through therapy are actively being researched for CFTR nonsense mutants. NMD suppression could increase the mutant CFTR mRNA, and read-through therapies may increase the levels of full-length CFTR protein. However, these approaches have limitations and potential side effects: because the NMD machinery also regulates the expression of many normal mRNAs, broad inhibition of the pathway is not desirable, and read-through drugs are inefficient partly because the mutant mRNA template is subject to NMD. To bypass these issues, we pursued an exon-skipping antisense oligonucleotide (ASO) strategy to achieve gene-specific NMD evasion. A cocktail of two splice-site-targeting ASOs induced the expression of CFTR mRNA without the premature-termination-codon-containing exon 23 (CFTR-Δex23), which is an in-frame exon. Treatment of human bronchial epithelial cells with this cocktail of ASOs that target the splice sites flanking exon 23 results in efficient skipping of exon 23 and an increase in CFTR-Δex23 protein. The splice-switching ASO cocktail increases the CFTR-mediated chloride current in human bronchial epithelial cells. Our results set the stage for developing an allele-specific therapy for CF caused by the W1282X mutation.

Antisense technology: an overview and prospectus

Antisense technology is now beginning to deliver on its promise to treat diseases by targeting RNA. Nine single-stranded antisense oligonucleotide (ASO) drugs representing four chemical classes, two mechanisms of action and four routes of administration have been approved for commercial use, including the first RNA-targeted drug to be a major commercial success, nusinersen. Although all the approved drugs are for use in patients with rare diseases, many of the ASOs in late- and middle-stage clinical development are intended to treat patients with very common diseases. ASOs in development are showing substantial improvements in potency and performance based on advances in medicinal chemistry, understanding of molecular mechanisms and targeted delivery. Moreover, the ASOs in development include additional mechanisms of action and routes of administration such as aerosol and oral formulations. Here, we describe the key technological advances that have enabled this progress and discuss recent clinical trials that illustrate the impact of these advances on the performance of ASOs in a wide range of therapeutic applications. We also consider strategic issues such as target selection and provide perspectives on the future of the field.

Cystic fibrosis - Ten promising therapeutic approaches in the current era of care

INTRODUCTION

Cystic fibrosis (CF) is a genetic disease affecting multiple organ systems. Research and innovations in novel therapeutic agents and health care delivery have resulted in dramatic improvements in quality of life and survival for people with CF. Despite this, significant disease burden persists for many and this is compounded by disparities in treatment access and care which globally necessitates further work to improve outcomes. Because of the advent of numerous therapies which include gene-targeted modulators in parallel with specialized care delivery models, innovative efforts continue.

AREAS COVERED

In this review, we discuss the available data on investigational agents in clinical development and currently available treatments for CF. We also evaluate approaches to care delivery, consider treatment gaps, and propose future directions for advancement.

EXPERT OPINION

Since the discovery of the CF gene, CFTR modulators have provided a hallmark of success, even though it was thought not previously possible. This has led to reinvigorated efforts and innovations in treatment approaches and care delivery. Numerous challenges remain because of genetic and phenotypic heterogeneity, access issues, and therapeutic costs, but the collaborative approach between stakeholders for continued innovation fuels optimism. Abbreviations: CF cystic fibrosis; CFF Cystic Fibrosis Foundation (USA); CFTR cystic fibrosis transmembrane regulator; CRISPR clustered regularly interspaced short palindromic repeats; COX cyclo oxygenase; FDA US Food and Drug Administration; FEV1% forced expiratory volume in one second % predicted; F508del deletion of phenylalanine (F) in the 508th position (most common mutation); G551D substitution of the amino acid glycine by aspartate at position 551 in the nucleotide binding domain-1 of the CFTR gene; LMIC low- and middle-income country; LTB4 leukotriene B4; MDT multi-disciplinary care team; NO nitric oxide; NSAIDs non-steroidal anti-inflammatory drugs; SLPI secretory leukocyte protease inhibitor.

Gene Therapy for Cystic Fibrosis: Progress and Challenges of Genome Editing

Since the early days of its conceptualization and application, human gene transfer held the promise of a permanent solution to genetic diseases including cystic fibrosis (CF). This field went through alternated periods of enthusiasm and distrust. The development of refined technologies allowing site specific modification with programmable nucleases highly revived the gene therapy field. CRISPR nucleases and derived technologies tremendously facilitate genome manipulation offering diversified strategies to reverse mutations. Here we discuss the advancement of gene therapy, from therapeutic nucleic acids to genome editing techniques, designed to reverse genetic defects in CF. We provide a roadmap through technologies and strategies tailored to correct different types of mutations in the cystic fibrosis transmembrane regulator (CFTR) gene, and their applications for the development of experimental models valuable for the advancement of CF therapies.

Ethylcellulose nanoparticles as a new "in vitro" transfection tool for antisense oligonucleotide delivery

Oil-in-water nano-emulsions have been obtained in the HEPES 20 mM buffer solution / [Alkylamidoammonium:Kolliphor EL = 1:1] / [6 wt% ethylcellulose in ethyl acetate] system over a wide oil-to-surfactant range and above 35 wt% aqueous component at 25 °C. The nano-emulsion with an oil-to-surfactant ratio of 70/30 and 95 wt% aqueous component was used for nanoparticles preparation. These nanoparticles (mean diameter around 90 nm and zeta potential of +22 mV) were non-toxic to HeLa cells up to a concentration of 3 mM of cationic species. Successful complexation with an antisense phosphorothioate oligonucleotide targeting Renilla luciferase mRNA was achieved at cationic/anionic charge ratios above 16, as confirmed by zeta potential measurements and an electrophoretic mobility shift assay, provided that no Fetal Bovine Serum is present in the cell culture medium. Importantly, Renilla luciferase gene inhibition shows an optimum efficiency (40%) for the cationic/anionic ratio 28, which makes these complexes promising for "in vitro" cell transfection.

Evaluation of eluforsen, a novel RNA oligonucleotide for restoration of CFTR function in in vitro and murine models of p.Phe508del cystic fibrosis

Cystic fibrosis (CF) is caused by mutations in the gene encoding the epithelial chloride channel CF transmembrane conductance regulator (CFTR) protein. The most common mutation is a deletion of three nucleotides leading to the loss of phenylalanine at position 508 (p.Phe508del) in the protein. This study evaluates eluforsen, a novel, single-stranded, 33-nucleotide antisense oligonucleotide designed to restore CFTR function, in in vitro and in vivo models of p.Phe508del CF. The aims of the study were to demonstrate cellular uptake of eluforsen, and its efficacy in functional restoration of p.Phe508del-CFTR both in vitro and in vivo. In vitro, the effect of eluforsen was investigated in human CF pancreatic adenocarcinoma cells and human bronchial epithelial cells. Two mouse models were used to evaluate eluforsen in vivo. In vitro, eluforsen improved chloride efflux in CF pancreatic adenocarcinoma cell cultures and increased short-circuit current in primary human bronchial epithelial cells, both indicating restoration of CFTR function. In vivo, eluforsen was taken up by airway epithelium following oro-tracheal administration in mice, resulting in systemic exposure of eluforsen. In female F508del-CFTR mice, eluforsen significantly increased CFTR-mediated saliva secretion (used as a measure of CFTR function, equivalent to the sweat test in humans). Similarly, intranasal administration of eluforsen significantly improved nasal potential difference (NPD), and therefore CFTR conductance, in two CF mouse models. These findings indicate that eluforsen improved CFTR function in cell and animal models of p.Phe508del-CFTR-mediated CF and supported further development of eluforsen in human clinical trials, where eluforsen has also been shown to improve CFTR activity as measured by NPD.

Emerging Therapeutic Approaches for Cystic Fibrosis. From Gene Editing to Personalized Medicine

An improved understanding of the cystic fibrosis (CF) transmembrane conductance regulator (CFTR) protein structure and the consequences of CFTR gene mutations have allowed the development of novel therapies targeting specific defects underlying CF. Some strategies are mutation specific and have already reached clinical development; some strategies include a read-through of the specific premature termination codons (read-through therapies, nonsense mediated decay pathway inhibitors for Class I mutations); correction of CFTR folding and trafficking to the apical plasma membrane (correctors for Class II mutations); and an increase in the function of CFTR channel (potentiators therapy for Class III mutations and any mutant with a residual function located at the membrane). Other therapies that are in preclinical development are not mutation specific and include gene therapy to edit the genome and stem cell therapy to repair the airway tissue. These strategies that are directed at the basic CF defects are now revolutionizing the treatment for patients and should positively impact their survival rates.

The potential of antisense oligonucleotide therapies for inherited childhood lung diseases

Antisense oligonucleotides are an emerging therapeutic option to treat diseases with known genetic origin. In the age of personalised medicines, antisense oligonucleotides can sometimes be designed to target and bypass or overcome a patient's genetic mutation, in particular those lesions that compromise normal pre-mRNA processing. Antisense oligonucleotides can alter gene expression through a variety of mechanisms as determined by the chemistry and antisense oligomer design. Through targeting the pre-mRNA, antisense oligonucleotides can alter splicing and induce a specific spliceoform or disrupt the reading frame, target an RNA transcript for degradation through RNaseH activation, block ribosome initiation of protein translation or disrupt miRNA function. The recent accelerated approval of eteplirsen (renamed Exondys 51™) by the Food and Drug Administration, for the treatment of Duchenne muscular dystrophy, and nusinersen, for the treatment of spinal muscular atrophy, herald a new and exciting era in splice-switching antisense oligonucleotide applications to treat inherited diseases. This review considers the potential of antisense oligonucleotides to treat inherited lung diseases of childhood with a focus on cystic fibrosis and disorders of surfactant protein metabolism.

In vitro repair of a defective EGFP transcript and translation into a functional protein

Twin ribozymes mediate the exchange of a short patch of RNA against an exogenous oligonucleotide within a suitable RNA substrate. Thus, twin ribozymes are promising tools for RNA repair, i.e. for the treatment of genetic disorders at the mRNA level. A number of twin ribozyme-mediated RNA fragment exchange reactions have been successfully demonstrated using short model substrates. Herein we show for the first time a twin ribozyme-mediated in vitro repair of a full-length transcript and translation into a functional protein. The system is based on the repair of a designed mutant EGFP mRNA containing the four-base deletion ΔACTC (190-193). Upon twin ribozyme-mediated replacement of a patch of 15 nucleotides with an externally added repair oligonucleotide (19 mer) the wild type sequence of the EGFP transcript could be restored with 32% yield. This is the first time that such a high twin ribozyme-mediated repair yield, so far observed only for short model substrates, has been obtained for a full-length mRNA. Translation of the repaired EGFP-ΔACTC mRNA produces functional EGFP, as detected by the restored fluorescence.

Nucleic Acid Therapies for Cystic Fibrosis

Nucleic acid therapeutics are an established class of drugs that enable specific targeting of a gene of interest. This diverse family of drugs includes antisense oligonucleotides, siRNAs, and mRNA replacement therapies, which can elicit both gene repression and activation, primarily at the RNA level. Recent advances in medicinal chemistry have increased drug potency and enhanced delivery and distribution to a broad array of tissue and cell types. A key advantage of nucleic acid therapeutics is in their application to monogenic diseases. Cystic fibrosis (CF) is one such disease that affects ∼70,000 people globally. This severe disease is an excellent candidate for nucleic acid therapies, as it is due to a genetic defect in a single epithelial chloride channel. Although CF affects many tissues, the primary cause of patient mortality is lung disease. Here we review the various nucleic acid therapeutic modalities and their mechanisms of action, the opportunities and challenges associated with application of nucleic acid drugs to the lung pathology of CF, and the current state and prospects for nucleic acid drugs for the treatment of CF.

Evolving Mechanistic Views and Emerging Therapeutic Strategies for Cystic Fibrosis-Related Diabetes

Diabetes is a common and important complication of cystic fibrosis, an autosomal recessive genetic disease due to mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. Cystic fibrosis-related diabetes (CFRD) is associated with profound detrimental effects on the disease course and mortality and is expected to increase in prevalence as the survival of patients with cystic fibrosis continues to improve. Despite progress in the functional characterization of CFTR molecular defects, the mechanistic basis of CFRD is not well understood, in part because of the relative inaccessibility of the pancreatic tissue and the limited availability of representative animal models. This review presents a concise overview of the current understanding of CFRD pathogenesis and provides a cutting-edge update on novel findings from human and animal studies. Potential contributions from paracrine mechanisms and β-cell compensatory mechanisms are highlighted, as well as functional β-cell and α-cell defects, incretin defects, exocrine pancreatic insufficiency, and loss of islet cell mass. State-of-the-art and emerging treatment options are explored, including advances in insulin administration, CFTR modulators, cell replacement, gene replacement, and gene editing therapies.

New treatments targeting the basic defects in cystic fibrosis

Cystic fibrosis (CF) is a monogenic autosomal recessive disorder affecting around 75,000 individuals worldwide. It is a multi-system disease but the main morbidity and mortality is caused by chronic lung disease. Due to newborn screening, a multidisciplinary approach to care and intensive symptomatic treatment, the prognosis has dramatically improved over the last decades and there are currently more adults than children in many countries. However, CF is still a very severe disease with a current median age of life expectancy in the fourth decade of life. The disease is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene which encodes the CFTR protein, a protein kinase A-activated ATP-gated anion channel that regulates the transport of electrolytes such as chloride and bicarbonate. More than 2000 mutations have been reported, although not all of these have functional consequences. An enormous research effort and progress has been made in understanding the consequences of these mutations on the CFTR protein structure and function, and this has led to the approval of two new drug therapies that are able to bind to defective CFTR proteins and partially restore their function. They are mutation-specific therapies and available at present for specific mutations only. They are the first personalized medicine for CF with a possible disease-modifying effect. A pipeline of other compounds is under development with different mechanisms of action. It is foreseeable that new combinations of compounds will further improve the correction of CFTR function. Other strategies including premature stop codon read-through drugs, antisense oligonucleotides that correct the basic defect at the mRNA level or gene editing to restore the defective gene as well as gene therapy approaches are all in the pipeline. All these strategies are needed to develop disease-modifying therapies for all patients with CF.

New horizons for cystic fibrosis treatment

Cystic fibrosis is an inherited multi-system disease associated with chronic lung infection, malabsorption, salt loss syndromes, male infertility and leading to numerous comorbidities. The landscape in cystic fibrosis care has changed markedly with currently more adult patients than children in many countries. Over 2000 different mutations in the CFTR gene have been reported and the majority are extremely rare. Understanding how CFTR mutations translate to disturbed synthesis or function of the CFTR protein has opened the way to 'personalized' treatments to correct the basic defect. The first 2 drugs have reached the clinic: a CFTR potentiator to augment CFTR channel function, and the combination of this potentiator with a corrector to increase CFTR expression at the cell membrane. To obtain robust correction of CFTR expression at the cell membrane, combinations of correctors with additive efficacy are under investigation. Other mutation type-specific treatments under clinical investigation are premature stop codon-read through drugs and antisense oligonucleotides that correct the basic defect at the mRNA level. Restoring the defective gene by gene editing can already be achieved ex vivo. Mutation agnostic treatments are explored as well: stabilizing CFTR expression at the cell membrane, circumventing the CFTR channel by blocking or activating other ion channels, and gene therapy. Combinations of these therapies can be anticipated. The pipeline of corrective strategies under clinical investigation is increasing continuously and a rising number of pharmaceutical companies are entering the field.

Genetic medicines for CF: Hype versus reality

Since identification of the CFTR gene over 25 years ago, gene therapy for cystic fibrosis (CF) has been actively developed. More recently gene therapy has been joined by other forms of "genetic medicines" including mRNA delivery, as well as genome editing and mRNA repair-based strategies. Proof-of-concept that gene therapy can stabilize the progression of CF lung disease has recently been established in a Phase IIb trial. An early phase study to assess the safety and explore efficacy of CFTR mRNA repair is ongoing, while mRNA delivery and genome editing-based strategies are currently at the pre-clinical phase of development. This review has been written jointly by some of those involved in the various CF "genetic medicine" fields and will summarize the current state-of-the-art, as well as discuss future developments. Where applicable, it highlights common problems faced by each of the strategies, and also tries to highlight where a specific strategy may have an advantage on the pathway to clinical translation. We hope that this review will contribute to the ongoing discussion about the hype versus reality of genetic medicine-based treatment approaches in CF. Pediatr Pulmonol. 2016;51:S5-S17. © 2016 Wiley Periodicals, Inc.

The notorious R.N.A. in the spotlight - drug or target for the treatment of disease

mRNA is an attractive drug target for therapeutic interventions. In this review we highlight the current state, clinical trials, and developments in antisense therapy, including the classical approaches like RNaseH-dependent oligomers, splice-switching oligomers, aptamers, and therapeutic RNA interference. Furthermore, we provide an overview on emerging concepts for using RNA in therapeutic settings including protein replacement by in-vitro-transcribed mRNAs, mRNA as vaccines and anti-allergic drugs. Finally, we give a brief outlook on early-stage RNA repair approaches that apply endogenous or engineered proteins in combination with short RNAs or chemically stabilized oligomers for the re-programming of point mutations, RNA modifications, and frame shift mutations directly on the endogenous mRNA.

Impact of gene editing on the study of cystic fibrosis

Cystic fibrosis (CF) is a chronic and progressive autosomal recessive disorder of secretory epithelial cells, which causes obstructions in the lung airways and pancreatic ducts of 70,000 people worldwide (for recent review see Cutting Nat Rev Genet 16(1):45-56, 2015). The finding that mutations in the CFTR gene cause CF (Kerem et al. Science 245(4922):1073-1080, 1989; Riordan et al. Science 245(4922):1066-1073, 1989; Rommens et al. Science 245(4922):1059-1065, 1989), was hailed as the very happy middle of a story whose end is a cure for a fatal disease (Koshland Science 245(4922):1029, 1989). However, despite two licensed drugs (Ramsey et al. N Engl J Med 365(18):1663-1672, 2011; Wainwright et al. N Engl J Med 373(3):220-231, 2015), and a formal demonstration that repeated administration of CFTR cDNA to patients is safe and effects a modest but significant stabilisation of disease (Alton et al. Lancet Respir Med 3(9):684-691, 2015), we are still a long way from a cure, with many patients taking over 100 tablets per day, and a mean age at death of 28 years. The aim of this review is to discuss the impact on the study of CF of gene-editing techniques as they have developed over the last 30 years, up to and including the possibility of editing as a therapeutic approach.

New and emerging targeted therapies for cystic fibrosis

Cystic fibrosis (CF) is a monogenic autosomal recessive disorder that affects about 70,000 people worldwide. The clinical manifestations of the disease are caused by defects in the cystic fibrosis transmembrane conductance regulator (CFTR) protein. The discovery of the CFTR gene in 1989 has led to a sophisticated understanding of how thousands of mutations in the CFTR gene affect the structure and function of the CFTR protein. Much progress has been made over the past decade with the development of orally bioavailable small molecule drugs that target defective CFTR proteins caused by specific mutations. Furthermore, there is considerable optimism about the prospect of gene replacement or editing therapies to correct all mutations in cystic fibrosis. The recent approvals of ivacaftor and lumacaftor represent the genesis of a new era of precision medicine in the treatment of this condition. These drugs are having a positive impact on the lives of people with cystic fibrosis and are potentially disease modifying. This review provides an update on advances in our understanding of the structure and function of the CFTR, with a focus on state of the art targeted drugs that are in development.

Strategies in early clinical development for the treatment of basic defects of cystic fibrosis

INTRODUCTION

Twenty-six years after the identification of the gene responsible for cystic fibrosis (CF), controversies still surround the pathogenesis of the disease that continues to burden and shorten lives. Therefore, finding effective therapeutic strategies that target the basic defect of CF is crucially needed.

AREAS COVERED

This review offers a comprehensive survey of fundamental therapies in early stages of development for the treatment of CF. The first part describes recent strategies targeting the basic defect either at the gene or at the transcript level. The second part summarizes a panel of novel strategies targeting protein repair. The third part reports strategies targeting non-CFTR channels.

EXPERT OPINION

Recent major breakthroughs in CF therapy have been made, raising hope to find a cure for CF. Apart from Vertex corrector and potentiator molecules (lumacaftor, ivacaftor, VX-661) and from ataluren, used to correct nonsense mutations, most compounds being currently tested are in very early (I-II) phases of development and definitive clinical results are keenly expected. Among the broad list of molecules and strategies being tested, the QR-010 compound and inhibitors of phosphodiesterase type 5 (sildenafil, vardenafil) could reveal a strong potentiality as therapeutic candidates to cure CF.

Searching for a cure for cystic fibrosis. A 25-year quest in a nutshell

After 25 years of intensive search, there is not yet a cure for cystic fibrosis (CF). However, the quest has led to major breakthroughs in understanding the basic disease defect and defining strategies to correct it. The first cystic fibrosis transmembrane conductance regulator (CFTR) modulators have been introduced in clinic. Some show an impressive clinical benefit, like the potentiator ivacaftor for the 4% of patients with a class III defect. Others offer at present only a limited benefit, like the combination corrector lumacaftor plus potentiator ivacaftor for subjects homozygous for F508del. These findings prove that the basic defect in CF can be modified and hold the promise that one day CF will no longer be a life-shortening disease.

CONCLUSION

This review updates the clinician on recent achievements as well as on the CF research pipeline.

WHAT IS KNOWN

Cystic fibrosis (CF) is a common and life-shortening disease that currently cannot be cured. However, for each of the six CF mutation classes, disease-modifying drugs are under way.

WHAT IS NEW

This review is a concise update for the clinician on new drugs that reached the CF clinical pipeline. The research strategies in CF have become a paradigm for clinical trials in other inherited diseases.

Cystic fibrosis: toward personalized therapies

Cystic fibrosis (CF), the most common, life-threatening monogenetic disease in Caucasians, is caused by mutations in the CFTR gene, encoding a cAMP- and cGMP-regulated epithelial chloride channel. Symptomatic therapies treating end-organ manifestations have increased the life expectancy of CF patients toward a mean of 40 years. The recent development of CFTR-targeted drugs that emerged from high-throughput screening and are capable of correcting the basic defect promises to transform the therapeutic landscape from a trial-and-error prescription to personalized medicine. This stratified approach is tailored to a specific functional class of mutations in CFTR, but can be refined further to an individual level by exploiting recent advances in ex vivo drug testing methods. These tests range from CFTR functional measurements in rectal biopsies donated by a CF patient to the use of patient-derived intestinal or pulmonary organoids. Such organoids may serve as an inexhaustible source of epithelial cells that can be stored in biobanks and allow medium- to high-throughput screening of CFTR activators, correctors and potentiators on the basis of a simple microscopic assay monitoring organoid swelling. Thus the recent breakthrough in stem cell biology allowing the culturing of mini-organs from individual patients is not only relevant for future stem cell therapy, but may also allow the preclinical testing of new drugs or combinations that are optimally suited for an individual patient.

From protein synthesis to genetic insertion

In 1946, (14)C-cyanide made its appearance as an offshoot of the Atomic Energy Program. Our colleague Robert Loftfield built it into (14)C-alanine by the Strecker synthesis, and a lusty program directed toward uncovering the unknown mechanism of protein synthesis grew out of this beginning. The necessity for an undiscovered series of steps and enzymes was soon evident. A cell free system was developed, and a succession of components necessary for this new pathway tumbled out. ATP dependence, amino acid activation, the ribosome as the site of polypeptide formation, discovery of tRNA as the translation molecule linking the gene and protein sequence, and GTP as the essential energy ingredient in peptide chain extension all appeared from our laboratory within the next decade. A little later the AP(4)N family, whose functions remain imperfectly defined, of intracellular molecules was discovered. Isolation of specific species of RNA became a high priority, and we sequenced a small segment of the 3' end of the Rous sarcoma virus, just inside the poly(A) tail, at the same time the Gilbert group at Harvard was sequencing the 5' end. The sequence identity and polarity of the two ends suggested a circular intermediate in replication and predicted correctly that a synthetic antisense oligonucleotide targeted against this sequence might be a specific inhibitor of replication. More recently, we have evolved a technique that appears to achieve a trinucleotide insertion into tissue culture cells bearing a specific Delta508 mRNA triplet deletion, resulting in phenotypic reversion in the tissue culture.