Featured Article: Electroporation-mediated gene delivery of surfactant protein B (SP-B) restores expression and improves survival in mouse model of SP-B deficiency

Transamniotic Delivery of Surfactant Protein B mRNA in a Healthy Model

INTRODUCTION

We sought to determine whether exogenous surfactant protein B (SPB) mRNA could be incorporated and translated by the fetal lung after simple transamniotic administration.

METHODS

Fetuses (n = 149) of twelve time-dated dams underwent intra-amniotic injections of either human SPB (hSPB) mRNA encapsulated into lipopolyplex (mRNA, n = 99) or lipopolyplex without mRNA (control; n = 50) on gestational day 17 (E17, term = E21-22). Lungs were screened for hSPB by enzyme-linked immunosorbent assay daily until term. Phosphatidylcholine (PC) (a surrogate for surfactant production) was measured in the amniotic fluid by fluorometric assay. Statistical analysis included nonparametric Wilcoxon rank sum test.

RESULTS

Significantly improved survival in the mRNA group compared to controls was observed at E18 (100% vs. 85.7%) and E20 (100% vs. 83.3%) (both p < 0.001). When controlled by mRNA-free injections, hSPB protein was detected in the mRNA group's lungs at E18, 19, and term (p = 0.002 to <0.001). Amniotic fluid PC levels were increased compared to control at term [285.9 (251.1, 363.9) μM versus 263.1 (222.8, 309.1) μM]; however, this did not reach significance (p = 0.33).

CONCLUSIONS

Encapsulated exogenous SPB mRNA can be incorporated and translated by fetal lung cells following intra-amniotic injection in a healthy rat model. Transamniotic mRNA delivery could become a novel strategy for perinatal surfactant protein replacement.

Single nucleotide polymorphisms in surfactant protein A1 are not associated with a lack of responsiveness to antenatal steroid therapy in a pregnant sheep model

Treatment with antenatal steroids (ANS) is standard practice for reducing the risk of respiratory distress in the preterm infant. Despite clear overall benefits when appropriately administered, many fetuses fail to derive benefit from ANS therapies. In standardized experiments using a pregnant sheep model, we have demonstrated that around 40% of ANS-exposed lambs did not have functional lung maturation significantly different from that of saline-treated controls. Surfactant protein A is known to play an important role in lung function. In this genotyping study, we investigated the potential correlation between polymorphisms in SFTPA1, messenger RNA and protein levels, and ventilation outcomes in animals treated with ANS. 45 preterm lambs were delivered 48 h after initial ANS therapy and 44 lambs were delivered 8 days after initial ANS therapy. The lambs were ventilated for 30 min after delivery. SFTPA1 mRNA expression in lung tissue was not correlated with arterial blood PaCO2 values at 30 min of ventilation in lambs delivered 48 h after treatment. SFTPA1 protein in lung tissue was significantly correlated with PaCO2 at 30 min of ventilation in lambs ventilated both 48 h and 8 days after ANS treatment. Six different single nucleotide polymorphisms (SNPs) in the Ovis aries SFTPA1 sequence were detected by Sanger Sequencing. No individual SNPs or SNP haplotypes correlated with alterations in PaCO2 at 30 min of ventilation or SFTPA1 protein levels in the lung. For the subset of animals analyzed in the present study, variable lung maturation responses to ANS therapy were not associated with mutations in SFTPA1.

Progress in Respiratory Gene Therapy

The prospect of gene therapy for inherited and acquired respiratory disease has energized the research community since the 1980s, with cystic fibrosis, as a monogenic disorder, driving early efforts to develop effective strategies. The fact that there are still no approved gene therapy products for the lung, despite many early phase clinical trials, illustrates the scale of the challenge: In the 1990s, first-generation non-viral and viral vector systems demonstrated proof-of-concept but low efficacy. Since then, there has been steady progress toward improved vectors with the capacity to overcome at least some of the formidable barriers presented by the lung. In addition, the inclusion of features such as codon optimization and promoters providing long-term expression have improved the expression characteristics of therapeutic transgenes. Early approaches were based on gene addition, where a new DNA copy of a gene is introduced to complement a genetic mutation: however, the advent of RNA-based products that can directly express a therapeutic protein or manipulate gene expression, together with the expanding range of tools for gene editing, has stimulated the development of alternative approaches. This review discusses the range of vector systems being evaluated for lung delivery; the variety of cargoes they deliver, including DNA, antisense oligonucleotides, messenger RNA (mRNA), small interfering RNA (siRNA), and peptide nucleic acids; and exemplifies progress in selected respiratory disease indications.

Gene Therapy Potential for Genetic Disorders of Surfactant Dysfunction

Pulmonary surfactant is critically important to prevent atelectasis by lowering the surface tension of the alveolar lining liquid. While respiratory distress syndrome (RDS) is common in premature infants, severe RDS in term and late preterm infants suggests an underlying genetic etiology. Pathogenic variants in the genes encoding key components of pulmonary surfactant including surfactant protein B (SP-B, SFTPB gene), surfactant protein C (SP-C, SFTPC gene), and the ATP-Binding Cassette transporter A3 (ABCA3, ABCA3 gene) result in severe neonatal RDS or childhood interstitial lung disease (chILD). These proteins play essential roles in pulmonary surfactant biogenesis and are expressed in alveolar epithelial type II cells (AEC2), the progenitor cell of the alveolar epithelium. SP-B deficiency most commonly presents in the neonatal period with severe RDS and requires lung transplantation for survival. SFTPC mutations act in an autosomal dominant fashion and more commonly presents with chILD or idiopathic pulmonary fibrosis than neonatal RDS. ABCA3 deficiency often presents as neonatal RDS or chILD. Gene therapy is a promising option to treat monogenic lung diseases. Successes and challenges in developing gene therapies for genetic disorders of surfactant dysfunction include viral vector design and tropism for target cell types. In this review, we explore adeno-associated virus (AAV), lentiviral, and adenoviral (Ad)-based vectors as delivery vehicles. Both gene addition and gene editing strategies are compared to best design treatments for lung diseases resulting from pathogenic variants in the SFTPB, SFTPC, and ABCA3 genes.

A human surfactant B deficiency air-liquid interface cell culture model suitable for gene therapy applications

Surfactant protein B (SPB) deficiency is a severe monogenic interstitial lung disorder that leads to loss of life in infants as a result of alveolar collapse and respiratory distress syndrome. The development and assessment of curative therapies for the deficiency are limited by the general lack of well-characterized and physiologically relevant in vitro models of human lung parenchyma. Here, we describe a new human surfactant air-liquid interface (SALI) culture model based on H441 cells, which successfully recapitulates the key characteristics of human alveolar cells in primary culture as evidenced by RNA and protein expression of alveolar cell markers. SALI cultures were able to develop stratified cellular layers with functional barrier properties that are stable for at least 28 days after air-lift. A SFTPB knockout model of SPB deficiency was generated via gene editing of SALI cultures. The SFTPB-edited SALI cultures lost expression of SPB completely and showed weaker functional barrier properties. We were able to correct this phenotype via delivery of a lentiviral vector pseudotyped with Sendai virus glycoproteins F/HN expressing SPB. We believe that SALI cultures can serve as an important in vitro research tool to study human alveolar epithelium, especially for the development of advanced therapy medicinal products targeting monogenic disorders.

Pulmonary gene delivery-Realities and possibilities

Delivery of genetic material to tissues in vivo is an important technique used in research settings and is the foundation upon which clinical gene therapy is built. The lung is a prime target for gene delivery due to a host of genetic, acquired, and infectious diseases that manifest themselves there, resulting in many pathologies. However, the in vivo delivery of genetic material to the lung remains a practical problem clinically and is considered the major obstacle needed to be overcome for gene therapy. Currently there are four main strategies for in vivo gene delivery to the lung: viral vectors, liposomes, nanoparticles, and electroporation. Viral delivery uses several different genetically modified viruses that enter the cell and express desired genes that have been inserted to the viral genome. Liposomes use combinations of charged and neutral lipids that can encapsulate genetic cargo and enter cells through endogenous mechanisms, thereby delivering their cargoes. Nanoparticles are defined by their size (typically less than 100 nm) and are made up of many different classes of building blocks, including biological and synthetic polymers, cell penetrant and other peptides, and dendrimers, that also enter cells through endogenous mechanisms. Electroporation uses mild to moderate electrical pulses to create pores in the cell membrane through which delivered genetic material can enter a cell. An emerging fifth category, exosomes and extracellular vesicles, may have advantages of both viral and non-viral approaches. These extracellular vesicles bud from cellular membranes containing receptors and ligands that may aid cell targeting and which can be loaded with genetic material for efficient transfer. Each of these vectors can be used for different gene delivery applications based on mechanisms of action, side-effects, and other factors, and their use in the lung and possible clinical considerations is the primary focus of this review.

Identification of AAV serotypes for lung gene therapy in human embryonic stem cell-derived lung organoids

Gene therapy is being investigated for a range of serious lung diseases, such as cystic fibrosis and emphysema. Recombinant adeno-associated virus (rAAV) is a well-established, safe, viral vector for gene delivery with multiple naturally occurring and artificial serotypes available displaying alternate cell, tissue, and species-specific tropisms. Efficient AAV serotypes for the transduction of the conducting airways have been identified for several species; however, efficient serotypes for human lung parenchyma have not yet been identified. Here, we screened the ability of multiple AAV serotypes to transduce lung bud organoids (LBOs)—a model of human lung parenchyma generated from human embryonic stem cells. Microinjection of LBOs allowed us to model transduction from the luminal surface, similar to dosing via vector inhalation. We identified the naturally occurring rAAV2 and rAAV6 serotypes, along with synthetic rAAV6 variants, as having tropism for the human lung parenchyma. Positive staining of LBOs for surfactant proteins B and C confirmed distal lung identity and suggested the suitability of these vectors for the transduction of alveolar type II cells. Our findings establish LBOs as a new model for pulmonary gene therapy and stress the relevance of LBOs as a viral infection model of the lung parenchyma as relevant in SARS-CoV-2 research.

New Directions in Pulmonary Gene Therapy

The lung has long been a target for gene therapy, yet efficient delivery and phenotypic disease correction has remained challenging. Although there have been significant advancements in gene therapies of other organs, including the development of several ex vivo therapies, in vivo therapeutics of the lung have been slower to transition to the clinic. Within the past few years, the field has witnessed an explosion in the development of new gene addition and gene editing strategies for the treatment of monogenic disorders. In this review, we will summarize current developments in gene therapy for cystic fibrosis, alpha-1 antitrypsin deficiency, and surfactant protein deficiencies. We will explore the different gene addition and gene editing strategies under investigation and review the challenges of delivery to the lung.

A lung tropic AAV vector improves survival in a mouse model of surfactant B deficiency

Surfactant protein B (SP-B) deficiency is an autosomal recessive disorder that impairs surfactant homeostasis and manifests as lethal respiratory distress. A compelling argument exists for gene therapy to treat this disease, as de novo protein synthesis of SP-B in alveolar type 2 epithelial cells is required for proper surfactant production. Here we report a rationally designed adeno-associated virus (AAV) 6 capsid that demonstrates efficiency in lung epithelial cell transduction based on imaging and flow cytometry analysis. Intratracheal administration of this vector delivering murine or human proSFTPB cDNA into SP-B deficient mice restores surfactant homeostasis, prevents lung injury, and improves lung physiology. Untreated SP-B deficient mice develop fatal respiratory distress within two days. Gene therapy results in an improvement in median survival to greater than 200 days. This vector also transduces human lung tissue, demonstrating its potential for clinical translation against this lethal disease.

Persistent Respiratory Distress in the Term Neonate: Genetic Surfactant Deficiency Diseases

Respiratory distress is one of the most common clinical presentations in newborns requiring admission to a Neonatal Intensive Care Unit (NICU). Many of these infants develop respiratory distress secondary to surfactant deficiency, which causes an interstitial lung disease that can occur in both preterm and term infants. Pulmonary surfactant is a protein and lipid mixture made by type II alveolar cells, which reduces alveolar surface tension and prevents atelectasis. The etiology of surfactant deficiency in preterm infants is pulmonary immaturity and inadequate production. Term infants may develop respiratory insufficiency secondary to inadequate surfactant, either from exposure to factors that delay surfactant synthesis (such as maternal diabetes) or from dysfunctional surfactant arising from a genetic mutation. The genetics of surfactant deficiencies are very complex. Some mutations are lethal in the neonatal period, while others cause a wide range of illness severity from infancy to adulthood. Genes that have been implicated in surfactant deficiency include SFTPA1, SFTPA2, SFTPB, SFTPC, and SFTPD (which encode for surfactant proteins A, B, C, and D, respectively); ABCA3 (crucial for surfactant packaging and secretion); and NKX2 (a transcription factor that regulates the expression of the surfactant proteins in lung tissue). This article discusses the interplay between the genotypes and phenotypes of newborns with surfactant deficiency to assist clinicians in determining which patients warrant a genetic evaluation.

Disruption of normal patterns of FOXF1 expression in a lethal disorder of lung development

BACKGROUND

Alveolar capillary dysplasia with misalignment of the pulmonary veins (ACDMPV) is a lethal disorder of lung development. ACDMPV is associated with haploinsufficiency of the transcription factor FOXF1, which plays an important role in the development of the lung and intestine. CNVs upstream of the FOXF1 gene have also been associated with an ACDMPV phenotype, but mechanism(s) by which these deletions disrupt lung development are not well understood. The objective of our study is to gain insights into the mechanisms by which CNVs contribute to an ACDMPV phenotype.

METHODS

We analysed primary lung tissue from an infant with classic clinical and histological findings of ACDMPV and harboured a 340 kb deletion on chromosome 16q24.1 located 250 kb upstream of FOXF1.

RESULTS

In RNA generated from paraffin-fixed lung sections, our patient had lower expression of FOXF1 than age-matched controls. He also had an abnormal pattern of FOXF1 protein expression, with a dramatic loss of FOXF1 expression in the lung. To gain insights into the mechanisms underlying these changes, we assessed the epigenetic landscape using chromatin immunoprecipitation, which demonstrated loss of histone H3 lysine 27 acetylation (H3K27Ac), an epigenetic mark of active enhancers, in the region of the deletion.

CONCLUSIONS

Together, these data suggest that the deletion disrupts an enhancer responsible for directing FOXF1 expression in the developing lung and provide novel insights into the mechanisms underlying a fatal developmental lung disorder.

Reversal of Surfactant Protein B Deficiency in Patient Specific Human Induced Pluripotent Stem Cell Derived Lung Organoids by Gene Therapy

Surfactant protein B (SFTPB) deficiency is a fatal disease affecting newborn infants. Surfactant is produced by alveolar type II cells which can be differentiated in vitro from patient specific induced pluripotent stem cell (iPSC)-derived lung organoids. Here we show the differentiation of patient specific iPSCs derived from a patient with SFTPB deficiency into lung organoids with mesenchymal and epithelial cell populations from both the proximal and distal portions of the human lung. We alter the deficiency by infecting the SFTPB deficient iPSCs with a lentivirus carrying the wild type SFTPB gene. After differentiating the mutant and corrected cells into lung organoids, we show expression of SFTPB mRNA during endodermal and organoid differentiation but the protein product only after organoid differentiation. We also show the presence of normal lamellar bodies and the secretion of surfactant into the cell culture medium in the organoids of lentiviral infected cells. These findings suggest that a lethal lung disease can be targeted and corrected in a human lung organoid model in vitro.

Recent Developments in mRNA-Based Protein Supplementation Therapy to Target Lung Diseases

Protein supplementation therapy using in vitro-transcribed (IVT) mRNA for genetic diseases contains huge potential as a new class of therapy. From the early ages of synthetic mRNA discovery, a great number of studies showed the versatile use of IVT mRNA as a novel approach to supplement faulty or absent protein and also as a vaccine. Many modifications have been made to produce high expressions of mRNA causing less immunogenicity and more stability. Recent advancements in the in vivo lung delivery of mRNA complexed with various carriers encouraged the whole mRNA community to tackle various genetic lung diseases. This review gives a comprehensive overview of cells associated with various lung diseases and recent advancements in mRNA-based protein replacement therapy. This review also covers a brief summary of developments in mRNA modifications and nanocarriers toward clinical translation.

Lessons learned from lung and liver in-vivo gene therapy: implications for the future

INTRODUCTION

Ex-vivo gene therapy has had significant clinical impact over the last couple of years and in-vivo gene therapy products are being approved for clinical use. Gene therapy and gene editing approaches have huge potential to treat genetic disease and chronic illness.

AREAS COVERED

This article provides a review of in-vivo approaches for gene therapy in the lung and liver, exploiting non-viral and viral vectors with varying serotypes and pseudotypes to target-specific cells. Antibody responses inhibiting viral vectors continue to constrain effective repeat administration. Lessons learned from ex-vivo gene therapy and genome editing are also discussed.

EXPERT OPINION

The fields of lung and liver in-vivo gene therapy are thriving and a comparison highlights obstacles and opportunities for both. Overcoming immunological issues associated with repeated administration of viral vectors remains a key challenge. The addition of targeted small molecules in combination with viral vectors may offer one solution. A substantial bottleneck to the widespread adoption of in-vivo gene therapy is how to ensure sufficient capacity for clinical-grade vector production. In the future, the exploitation of gene editing approaches for in-vivo disease treatment may facilitate the resurgence of non-viral gene transfer approaches, which tend to be eclipsed by more efficient viral vectors.