A highly specific SpCas9 variant is identified by in vivo screening in yeast

Despite the utility of CRISPR-Cas9 nucleases for genome editing, the potential for off-target activity limits their application, especially for therapeutic purposes. We developed a yeast-based assay to identify optimized Streptococcus pyogenes Cas9 (SpCas9) variants that enables simultaneous evaluation of on- and off-target activity. We screened a library of SpCas9 variants carrying random mutations in the REC3 domain and identified mutations that increased editing accuracy while maintaining editing efficiency. We combined four beneficial mutations to generate evoCas9, a variant that has fidelity exceeding both wild-type (79-fold improvement) and rationally designed Cas9 variants (fourfold average improvement), while maintaining near wild-type on-target editing efficiency (90% median residual activity). Evaluating evoCas9 on endogenous genomic loci, we demonstrated a substantially improved specificity and observed no off-target sites for four of the eight single guide RNAs (sgRNAs) tested. Finally, we showed that following long-term expression (40 d), evoCas9 strongly limited the nonspecific cleavage of a difficult-to-discriminate off-target site and fully abrogated the cleavage of two additional off-target sites.

VSV-G-Enveloped Vesicles for Traceless Delivery of CRISPR-Cas9

The method of delivery of CRISPR-Cas9 into target cells is a strong determinant of efficacy and specificity in genome editing. Even though high efficiency of Cas9 delivery is necessary for optimal editing, its long-term and high levels of expression correlate with increased off-target activity. We developed vesicles (VEsiCas) carrying CRISPR-SpCas9 ribonucleoprotein complexes (RNPs) that are efficiently delivered into target cells through the fusogenic glycoprotein of the vesicular stomatitis virus (VSV-G). A crucial step for VEsiCas production is the synthesis of the single guide RNA (sgRNA) mediated by the T7 RNA polymerase in the cytoplasm of producing cells as opposed to canonical U6-driven Pol III nuclear transcription. In VEsiCas, the absence of DNA encoding SpCas9 and sgRNA allows rapid clearance of the nuclease components in target cells, which correlates with reduced genome-wide off-target cleavages. Compared with SpCas9 RNPs electroporation, which is currently the method of choice to obtain transient SpCas9 activity, VEsiCas deliver the nuclease with higher efficiency and lower toxicity. We show that a wide variety of cells can be edited through VEsiCas, including a variety of transformed cells, induced pluripotent stem cells (iPSCs), and cardiomyocytes, in vivo. VEsiCas is a traceless CRISPR-Cas9 delivery tool for efficient and safe genome editing that represents a further advancement toward the therapeutic use of the CRISPR-Cas9 technology.

Allele specific repair of splicing mutations in cystic fibrosis through AsCas12a genome editing

Cystic fibrosis (CF) is an autosomal recessive disease caused by mutations in the CFTR gene. The 3272-26A>G and 3849+10kbC>T CFTR mutations alter the correct splicing of the CFTR gene, generating new acceptor and donor splice sites respectively. Here we develop a genome editing approach to permanently correct these genetic defects, using a single crRNA and the Acidaminococcus sp. BV3L6, AsCas12a. This genetic repair strategy is highly precise, showing very strong discrimination between the wild-type and mutant sequence and a complete absence of detectable off-targets. The efficacy of this gene correction strategy is verified in intestinal organoids and airway epithelial cells derived from CF patients carrying the 3272-26A>G or 3849+10kbC>T mutations, showing efficient repair and complete functional recovery of the CFTR channel. These results demonstrate that allele-specific genome editing with AsCas12a can correct aberrant CFTR splicing mutations, paving the way for a permanent splicing correction in genetic diseases.

Tobacco Etch Virus protease: A shortcut across biotechnologies

About thirty years ago, studies on the RNA genome of Tobacco Etch Virus revealed the presence of an efficient and specific protease, called Tobacco Etch Virus protease (TEVp), that was part of the Nuclear Inclusion a (NIa) enzyme. TEVp is an efficient and specific protease of 27kDa that has become a valuable biotechnological tool. Nowadays TEVp is a unique endopeptidase largely exploited in biotechnology from industrial applications to in vitro and in vivo cellular studies. A number of TEVp mutants with different rate of cleavage, stability and specificity have been reported. Similarly, a panel of different target cleavage sites, derived from the canonical ENLYFQ-G/S site, has been established. In this review we describe these aspects of TEVp and some of its multiple applications. A particular focus is on the use and molecular biology of TEVp in living cells and organisms.

Mechanism-based traps enable protease and hydrolase substrate discovery

Hydrolase enzymes, including proteases, are encoded by 2-3% of the genes in the human genome and 14% of these enzymes are active drug targets1. However, the activities and substrate specificities of many proteases-especially those embedded in membranes-and other hydrolases remain unknown. Here we report a strategy for creating mechanism-based, light-activated protease and hydrolase substrate traps in complex mixtures and live mammalian cells. The traps capture substrates of hydrolases, which normally use a serine or cysteine nucleophile. Replacing the catalytic nucleophile with genetically encoded 2,3-diaminopropionic acid allows the first step reaction to form an acyl-enzyme intermediate in which a substrate fragment is covalently linked to the enzyme through a stable amide bond2; this enables stringent purification and identification of substrates. We identify new substrates for proteases, including an intramembrane mammalian rhomboid protease RHBDL4 (refs. 3,4). We demonstrate that RHBDL4 can shed luminal fragments of endoplasmic reticulum-resident type I transmembrane proteins to the extracellular space, as well as promoting non-canonical secretion of endogenous soluble endoplasmic reticulum-resident chaperones. We also discover that the putative serine hydrolase retinoblastoma binding protein 9 (ref. 5) is an aminopeptidase with a preference for removing aromatic amino acids in human cells. Our results exemplify a powerful paradigm for identifying the substrates and activities of hydrolase enzymes.

Pharmacological inactivation of the prion protein by targeting a folding intermediate

Recent computational advancements in the simulation of biochemical processes allow investigating the mechanisms involved in protein regulation with realistic physics-based models, at an atomistic level of resolution. These techniques allowed us to design a drug discovery approach, named Pharmacological Protein Inactivation by Folding Intermediate Targeting (PPI-FIT), based on the rationale of negatively regulating protein levels by targeting folding intermediates. Here, PPI-FIT was tested for the first time on the cellular prion protein (PrP), a cell surface glycoprotein playing a key role in fatal and transmissible neurodegenerative pathologies known as prion diseases. We predicted the all-atom structure of an intermediate appearing along the folding pathway of PrP and identified four different small molecule ligands for this conformer, all capable of selectively lowering the load of the protein by promoting its degradation. Our data support the notion that the level of target proteins could be modulated by acting on their folding pathways, implying a previously unappreciated role for folding intermediates in the biological regulation of protein expression.

Spagnolli, Massignan, Astolfi et al. design a new drug discovery approach, termed Pharmacological Protein Inactivation by Folding Intermediate Targeting, in which folding intermediates of disease-causing proteins are targeted. They test it on the cellular prion protein, identifying ligands stabilizing a folding intermediate and consequently promoting its degradation by the cellular quality control machinery.

Refactored genetic codes enable bidirectional genetic isolation

The near-universal genetic code defines the correspondence between codons in genes and amino acids in proteins. We refactored the structure of the genetic code in Escherichia coli and created orthogonal genetic codes that restrict the escape of synthetic genetic information into natural life. We developed orthogonal and mutually orthogonal horizontal gene transfer systems, which permit the transfer of genetic information between organisms that use the same genetic code but restrict the transfer of genetic information between organisms that use different genetic codes. Moreover, we showed that locking refactored codes into synthetic organisms completely blocks invasion by mobile genetic elements, including viruses, which carry their own translation factors and successfully invade organisms with canonical and compressed genetic codes.

CD4 and BST-2/tetherin proteins retro-translocate from endoplasmic reticulum to cytosol as partially folded and multimeric molecules

CD4 and BST-2/Tetherin are cellular membrane proteins targeted to degradation by the HIV-1 protein Vpu. In both cases proteasomal degradation following recruitment into the ERAD pathway has been described. CD4 is a type I transmembrane glycoprotein, with four extracellular immunoglobulin-like domains containing three intrachain disulfide bridges. BST-2/Tetherin is an atypical type II transmembrane glycoprotein with an N-terminal transmembrane domain and a C-terminal glycophosphatidylinositol anchor, which dimerizes through three interchain bridges. We investigated spontaneous and Vpu-induced retro-translocation of CD4 and BST-2/Tetherin using our novel biotinylation technique in living cells to determine ER-to-cytosol retro-translocation of proteins. We found that CD4 retro-translocates with oxidized intrachain disulfide bridges, and only upon proteasomal inhibition does it accumulate in the cytosol as already reduced and deglycosylated molecules. Similarly, BST-2/Tetherin is first exposed to the cytosol as a dimeric oxidized complex and then becomes deglycosylated and reduced to monomers. These results raise questions on the required features of the putative retro-translocon, suggesting alternative retro-translocation mechanisms for membrane proteins in which complete cysteine reduction and unfolding are not always strictly required before ER to cytosol dislocation.

Inherited determinants of early recurrent somatic mutations in prostate cancer

Prostate cancer is a highly heritable molecularly and clinically heterogeneous disease. To discover germline events involved in prostate cancer predisposition, we develop a computational approach to nominate heritable facilitators of somatic genomic events in the context of the androgen receptor signaling. Here, we use a ranking score and benign prostate transcriptomes to identify a non-coding polymorphic regulatory element at 7p14.3 that associates with DNA repair and hormone-regulated transcript levels and with an early recurrent prostate cancer-specific somatic mutation in the Speckle-Type POZ protein (SPOP) gene. The locus shows allele-specific activity that is concomitantly modulated by androgen receptor and by CCAAT/enhancer-binding protein (C/EBP) beta (CEBPB). Deletion of this locus via CRISPR-Cas9 leads to deregulation of the genes predicted to interact with the 7p14.3 locus by Hi-C chromosome conformation capture data. This study suggests that a polymorphism at 7p14.3 may predispose to SPOP mutant prostate cancer subclass through a hormone-dependent DNA damage response.

Prostate cancer is a heterogeneous disease, and many cases show somatic mutations of SPOP. Here, the authors show that a non-coding polymorphic regulatory element at 7p14.3 may predispose to SPOP mutant prostate cancer subclass through a hormone dependent DNA damage response.

Calcium cytotoxicity sensitizes prostate cancer cells to standard-of-care treatments for locally advanced tumors

Therapy resistance is a major roadblock in oncology. Exacerbation of molecular dysfunctions typical of cancer cells have proven effective in twisting oncogenic mechanisms to lethal conditions, thus offering new therapeutic avenues for cancer treatment. Here, we demonstrate that selective agonists of Transient Receptor Potential cation channel subfamily M member 8 (TRPM8), a cation channel characteristic of the prostate epithelium frequently overexpressed in advanced stage III/IV prostate cancers (PCa), sensitize therapy refractory models of PCa to radio, chemo or hormonal treatment. Overall, our study demonstrates that pharmacological-induced Ca²⁺ cytotoxicity is an actionable strategy to sensitize cancer cells to standard therapies.

Thermally Driven Membrane Phase Transitions Enable Content Reshuffling in Primitive Cells

Self-assembling single-chain amphiphiles available in the prebiotic environment likely played a fundamental role in the advent of primitive cell cycles. However, the instability of prebiotic fatty acid-based membranes to temperature and pH seems to suggest that primitive cells could only host prebiotically relevant processes in a narrow range of nonfluctuating environmental conditions. Here we propose that membrane phase transitions, driven by environmental fluctuations, enabled the generation of daughter protocells with reshuffled content. A reversible membrane-to-oil phase transition accounts for the dissolution of fatty acid-based vesicles at high temperatures and the concomitant release of protocellular content. At low temperatures, fatty acid bilayers reassemble and encapsulate reshuffled material in a new cohort of protocells. Notably, we find that our disassembly/reassembly cycle drives the emergence of functional RNA-containing primitive cells from parent nonfunctional compartments. Thus, by exploiting the intrinsic instability of prebiotic fatty acid vesicles, our results point at an environmentally driven tunable prebiotic process, which supports the release and reshuffling of oligonucleotides and membrane components, potentially leading to a new generation of protocells with superior traits. In the absence of protocellular transport machinery, the environmentally driven disassembly/assembly cycle proposed herein would have plausibly supported protocellular content reshuffling transmitted to primitive cell progeny, hinting at a potential mechanism important to initiate Darwinian evolution of early life forms.

The VCP/p97 and YOD1 Proteins Have Different Substrate-dependent Activities in Endoplasmic Reticulum-associated Degradation (ERAD)

Endoplasmic reticulum-associated degradation (ERAD) is an essential quality control mechanism of the folding state of proteins in the secretory pathway that targets unfolded/misfolded polypeptides for proteasomal degradation. The cytosolic p97/valosin-containing protein is an essential ATPase for degradation of ERAD substrates. It has been considered necessary during retro-translocation to extract proteins from the endoplasmic reticulum that are otherwise supposed to accumulate in the endoplasmic reticulum lumen. The activity of the p97-associated deubiquitinylase YOD1 is also required for substrate disposal. We used the in vivo biotinylation retro-translocation assay in mammalian cells under conditions of impaired p97 or YOD1 activity to directly discriminate their requirements and diverse functions in ERAD. Using different ERAD substrates, we found that both proteins participate in two distinct retro-translocation steps. For CD4 and MHC-Iα, which are induced to degradation by the HIV-1 protein Vpu and by the CMV immunoevasins US2 and US11, respectively, p97 and YOD1 have a retro-translocation-triggering role. In contrast, for three other spontaneous ERAD model substrates (NS1, NHK-α1AT, and BST-2/Tetherin), p97 and YOD1 are required in the downstream events of substrate deglycosylation and proteasomal degradation.

CRISPR-Csy4-Mediated Editing of Rotavirus Double-Stranded RNA Genome

CRISPR-nucleases have been widely applied for editing cellular and viral genomes, but nuclease-mediated genome editing of double-stranded RNA (dsRNA) viruses has not yet been reported. Here, by engineering CRISPR-Csy4 nuclease to localize to rotavirus viral factories, we achieve the nuclease-mediated genome editing of rotavirus, an important human and livestock pathogen with a multisegmented dsRNA genome. Rotavirus replication intermediates cleaved by Csy4 is edited through the formation of precise deletions in the targeted genome segments in a single replication cycle. Using CRISPR-Csy4-mediated editing of rotavirus genome, we label the products of rotavirus secondary transcription made by newly assembled viral particles during rotavirus replication, demonstrating that this step largely contributes to the overall production of viral proteins. We anticipate that the nuclease-mediated cleavage of dsRNA virus genomes will promote an advanced level of understanding of viral replication and host-pathogen interactions, also offering opportunities to develop therapeutics.

New tags for recombinant protein detection and O-glycosylation reporters

Monoclonal antibodies (mAbs), because of their unique specificity, are irreplaceable tools for scientific research. Precise mapping of the antigenic determinants allows the development of epitope tagging approaches to be used with recombinant proteins for several purposes. Here we describe a new family of tags derived from the epitope recognized by a single highly specific mAb (anti-roTag mAb), which was obtained from a pool of mAbs reacting with the rotavirus nonstructural protein 5 (NSP5). The variable regions of the anti-roTag mAb were identified and their binding capacity verified upon expression as a single-chain/miniAb. The minimal epitope, termed roTag, was identified as a 10 amino acid sequence (SISSSIFKNE). The affinity of the anti-roTag/roTag interaction was found to be comparable to that of the anti-SV5/SV5 tag interaction. roTag was successfully used for detection of several recombinant cytosolic, secretory and membrane proteins. Two additional variants of roTag of 10 and 13 amino acids containing O-glycosylation susceptible sites (termed OG-tag and roTagO) were constructed and characterised. These tags were useful to detect proteins passing through the Golgi apparatus, the site of O-glycosylation.

Continuous synthesis of E. coli genome sections and Mb-scale human DNA assembly

Whole-genome synthesis provides a powerful approach for understanding and expanding organism function1-3. To build large genomes rapidly, scalably and in parallel, we need (1) methods for assembling megabases of DNA from shorter precursors and (2) strategies for rapidly and scalably replacing the genomic DNA of organisms with synthetic DNA. Here we develop bacterial artificial chromosome (BAC) stepwise insertion synthesis (BASIS)-a method for megabase-scale assembly of DNA in Escherichia coli episomes. We used BASIS to assemble 1.1 Mb of human DNA containing numerous exons, introns, repetitive sequences, G-quadruplexes, and long and short interspersed nuclear elements (LINEs and SINEs). BASIS provides a powerful platform for building synthetic genomes for diverse organisms. We also developed continuous genome synthesis (CGS)-a method for continuously replacing sequential 100 kb stretches of the E. coli genome with synthetic DNA; CGS minimizes crossovers1,4 between the synthetic DNA and the genome such that the output for each 100 kb replacement provides, without sequencing, the input for the next 100 kb replacement. Using CGS, we synthesized a 0.5 Mb section of the E. coli genome-a key intermediate in its total synthesis1-from five episomes in 10  days. By parallelizing CGS and combining it with rapid oligonucleotide synthesis and episome assembly5,6, along with rapid methods for compiling a single genome from strains bearing distinct synthetic genome sections1,7,8, we anticipate that it will be possible to synthesize entire E. coli genomes from functional designs in less than 2 months.

BiP/GRP78 Mediates ERAD Targeting of Proteins Produced by Membrane-Bound Ribosomes Stalled at the STOP-Codon

Translational stalling of ribosome bound to endoplasmic reticulum (ER) membrane requires an accurate clearance of the associated polypeptides, which is not completely understood in mammals. We characterized in mammalian cells the model of ribosomal stalling at the STOP-codon based on proteins tagged at the C-terminus with the picornavirus 2A peptide followed by a termination codon instead of the Proline (2A*). We exploited the 2A* stalling model to characterize the pathway of degradation of ER-targeted polypeptides. We report that the ER chaperone BiP/GRP78 is a new main factor involved. Moreover, degradation of the ER-stalled polypeptides required the activities of the AAA-ATPase VCP/p97, its associated deubiquitinylase YOD1, the ribosome-associated ubiquitin ligase Listerin and the proteasome. In human proteome, we found two human C-terminal amino acid sequences that cause similar stalling at the STOP-codon. Our data suggest that translational stalling at the ER membrane activates protein degradation at the interface of ribosomal- and ER-associated quality control systems.

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Ribosomal stalling at the STOP-codon causes degradation of the translated protein.

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Picornavirus 2A peptide and related sequences cause ribosome stalling at STOP-codon.

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BiP/GRP78 recognizes polypeptides produced by membrane-bound stalled ribosomes.

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ER-stalled polypeptides are disposed of through the ERAD pathway.

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BIP/GRP78 is a novel key player for ERAD targeting of stalled ribosomal peptides.

Engineered tobacco etch virus (TEV) protease active in the secretory pathway of mammalian cells

Tobacco etch virus protease (TEVp) is a unique endopeptidase with stringent substrate specificity. TEVp has been widely used as a purified protein for in vitro applications, but also as a biological tool directly expressing it in living cells. To adapt the protease to diverse applications, several TEVp mutants with different stability and enzymatic properties have been reported. Herein we describe the development of a novel engineered TEVp mutant designed to be active in the secretory pathway. While wild type TEVp targeted to the secretory pathway of mammalian cells is synthetized as an N-glycosylated and catalytically inactive enzyme, a TEVp mutant with selected mutations at two verified N-glycosylation sites and at an exposed cysteine was highly efficient. This mutant was very active in the endoplasmic reticulum (ER) of living cells and can be used as a biotechnological tool to cleave proteins within the secretory pathway. As an immediate practical application we report the expression of a complete functional monoclonal antibody expressed from a single polypeptide, which was cleaved by our TEVp mutant into the two antibody chains and secreted as an assembled and functional molecule. In addition, we show active TEVp mutants lacking auto-cleavage activity.

LSD1/PRMT6-targeting gene therapy to attenuate androgen receptor toxic gain-of-function ameliorates spinobulbar muscular atrophy phenotypes in flies and mice

Spinobulbar muscular atrophy (SBMA) is caused by CAG expansions in the androgen receptor gene. Androgen binding to polyQ-expanded androgen receptor triggers SBMA through a combination of toxic gain-of-function and loss-of-function mechanisms. Leveraging cell lines, mice, and patient-derived specimens, we show that androgen receptor co-regulators lysine-specific demethylase 1 (LSD1) and protein arginine methyltransferase 6 (PRMT6) are overexpressed in an androgen-dependent manner specifically in the skeletal muscle of SBMA patients and mice. LSD1 and PRMT6 cooperatively and synergistically transactivate androgen receptor, and their effect is enhanced by expanded polyQ. Pharmacological and genetic silencing of LSD1 and PRMT6 attenuates polyQ-expanded androgen receptor transactivation in SBMA cells and suppresses toxicity in SBMA flies, and a preclinical approach based on miRNA-mediated silencing of LSD1 and PRMT6 attenuates disease manifestations in SBMA mice. These observations suggest that targeting overexpressed co-regulators can attenuate androgen receptor toxic gain-of-function without exacerbating loss-of-function, highlighting a potential therapeutic strategy for patients with SBMA.

Selective targeting of proteins within secretory pathway for endoplasmic reticulum-associated degradation

The endoplasmic reticulum-associated degradation (ERAD) is a cellular quality control mechanism to dispose of misfolded proteins of the secretory pathway via proteasomal degradation. SEL1L is an ER-resident protein that participates in identification of misfolded molecules as ERAD substrates, therefore inducing their ER-to-cytosol retrotranslocation and degradation. We have developed a novel class of fusion proteins, termed degradins, composed of a fragment of SEL1L fused to a target-specific binding moiety located on the luminal side of the ER. The target-binding moiety can be a ligand of the target or derived from specific mAbs. Here, we describe the ability of degradins with two different recognition moieties to promote degradation of a model target. Degradins recognize the target protein within the ER both in secretory and membrane-bound forms, inducing their degradation following retrotranslocation to the cytosol. Thus, degradins represent an effective technique to knock-out proteins within the secretory pathway with high specificity.

Translational enhancement by base editing of the Kozak sequence rescues haploinsufficiency

A variety of single-gene human diseases are caused by haploinsufficiency, a genetic condition by which mutational inactivation of one allele leads to reduced protein levels and functional impairment. Translational enhancement of the spare allele could exert a therapeutic effect. Here we developed BOOST, a novel gene-editing approach to rescue haploinsufficiency loci by the change of specific single nucleotides in the Kozak sequence, which controls translation by regulating start codon recognition. We evaluated for translational strength 230 Kozak sequences of annotated human haploinsufficient genes and 4621 derived variants, which can be installed by base editing, by a high-throughput reporter assay. Of these variants, 149 increased the translation of 47 Kozak sequences, demonstrating that a substantial proportion of haploinsufficient genes are controlled by suboptimal Kozak sequences. Validation of 18 variants for 8 genes produced an average enhancement in an expression window compatible with the rescue of the genetic imbalance. Base editing of the NCF1 gene, whose monoallelic loss causes chronic granulomatous disease, resulted in the desired increase of NCF1 (p47phox) protein levels in a relevant cell model. We propose BOOST as a fine-tuned approach to modulate translation, applicable to the correction of dozens of haploinsufficient monogenic disorders independently of the causing mutation.

Synthetic genomics for curing genetic diseases

From the beginning of the genome sequencing era, it has become increasingly evident that genetics plays a role in all diseases, of which only a minority are single-gene disorders, the most common target of current gene therapies. However, the majority of people have some kind of health problems resulting from congenital genetic mutations (over 6000 diseases have been associated to genes, https://www.omim.org/statistics/geneMap) and most genetic disorders are rare and only incompletely understood. The vision and techniques applied to the synthesis of genomes may help to address unmet medical needs from a chromosome and genome-scale perspective. In this chapter, we address the potential therapy of genetic diseases from a different outlook, in which we no longer focus on small gene corrections but on higher-order tools for genome manipulation. These will play a crucial role in the next years, as they prelude to a much deeper understanding of the architecture of the human genome and a more accurate modeling of human diseases, offering new therapeutic opportunities.

Functional rescue of F508del-CFTR through revertant mutations introduced by CRISPR base editing

Cystic fibrosis (CF) is a life-shortening autosomal recessive disease caused by mutations in the CFTR gene, resulting in functional impairment of the encoded ion channel. F508del mutation, a trinucleotide deletion, is the most frequent cause of CF, affecting approximately 80% of persons with CF (pwCFs). Even though current pharmacological treatments alleviate the F508del-CF disease symptoms, there is no definitive cure. Here, we leveraged revertant mutations (RMs) in cis with F508del to rescue CFTR protein folding and restore its function. We developed CRISPR base editing strategies to efficiently and precisely introduce the desired mutations in the F508del locus. Both editing and CFTR function recovery were verified in CF cellular models, including primary epithelial cells derived from pwCFs. The efficacy of the CFTR recovery strategy was validated in cultures of pseudostratified epithelia from pwCF cells showing full recovery of ion transport. Additionally, we observed an additive effect by combining our strategy with small molecules that enhance F508del activity, thus paving the way to combinatorial therapies.