SUMMARY Lung alveoli, which are unique to air-breathing organisms, have been challenging to generate from pluripotent stem cells (PSCs) in part because there are limited model systems available to provide the necessary developmental roadmaps for in vitro differentiation. Here we report the generation of alveolar epithelial type 2 cells (AEC2s), the facultative progenitors of lung alveoli, from human PSCs. Using multicolored fluorescent reporter lines, we track and purify human SFTPC+ alveolar progenitors as they emerge from endodermal precursors in response to stimulation of Wnt and FGF signaling. Purified PSC-derived SFTPC+ cells form monolayered epithelial “alveolospheres” in 3D cultures without the need for mesenchymal support, exhibit self-renewal capacity, and display additional AEC2 functional capacities. Footprint-free CRISPR-based gene correction of PSCs derived from patients carrying a homozygous surfactant mutation (SFTPB121ins2) restores surfactant processing in AEC2s. Thus, PSC-derived AEC2s provide a platform for disease modeling and future functional regeneration of the distal lung.
In contrast to lethal neonatal RDS resulting from homozygous or compound heterozygous ABCA3 mutations, single ABCA3 mutations are overrepresented among European-descent infants ≥34 weeks' gestation with RDS and account for ~10.9% of the attributable risk among term and late preterm infants. Although ABCA3 mutations are individually rare, they are collectively common among European- and African-descent individuals in the general population.
ABSTRACT:The prevalence of the common mutations in the surfactant protein-B (121ins2), surfactant protein-C (I73T), and ATP-binding cassette member A3 (E292V) genes in populationbased or case-control cohorts of newborn respiratory distress syndrome (RDS) is unknown. We determined the frequencies of these mutations in ethnically diverse population and disease-based cohorts using restriction enzyme analysis (121ins2 and E292V) and a 5Ј nuclease assay (I73T) in DNA samples from population-based cohorts in Missouri, Norway, South Korea, and South Africa, and from a case-control cohort of newborns with and without RDS (n ϭ 420). We resequenced the ATP-binding cassette member A3 gene (ABCA3) in E292V carriers and computationally inferred ABCA3 haplotypes. The population-based frequencies of 121ins2, E292V, and I73T were rare (Ͻ0.4%). E292V was present in 3.8% of newborns with RDS, a 10-fold greater prevalence than in the Missouri cohort (p Ͻ 0.001). We did not identify other loss of function mutations in ABCA3 among patients with E292V that would account for their RDS. E292V occurred on a unique haplotype that was derived from a recombination of two common ABCA3 haplotypes. E292V was over-represented in newborns with RDS suggesting that E292V or its unique haplotype impart increased genetic risk for RDS. (Pediatr Res 63: 645-649, 2008)
Mutations in the ATP-binding cassette transporter A3 gene (ABCA3) result in severe neonatal respiratory distress syndrome and childhood interstitial lung disease. As most ABCA3 mutations are rare or private, determination of mutation pathogenicity is often based on results from in silico prediction tools, identification in unrelated diseased individuals, statistical association studies, or expert opinion. Functional biologic studies of ABCA3 mutations are needed to confirm mutation pathogenicity and inform clinical decision making. Our objective was to functionally characterize two ABCA3 mutations (p.R288K and p.R1474W) identified among term and latepreterm infants with respiratory distress syndrome with unclear pathogenicity in a genetically versatile model system. We performed transient transfection of HEK293T cells with wild-type or mutant ABCA3 alleles to assess protein processing with immunoblotting. We used transduction of A549 cells with adenoviral vectors, which concurrently silenced endogenous ABCA3 and expressed either wildtype or mutant ABCA3 alleles (p.R288K and p.R1474W) to assess immunofluorescent localization, ATPase activity, and organelle ultrastructure. Both ABCA3 mutations (p.R288K and p.R1474W) encoded proteins with reduced ATPase activity but with normal intracellular localization and protein processing. Ultrastructural phenotypes of lamellar body-like vesicles in A549 cells transduced with mutant alleles were similar to wild type. Mutant proteins encoded by ABCA3 mutations p.R288K and p.R1474W had reduced ATPase activity, a biologically plausible explanation for disruption of surfactant metabolism by impaired phospholipid transport into the lamellar body. These results also demonstrate the usefulness of a genetically versatile, human model system for functional characterization of ABCA3 mutations with unclear pathogenicity.Keywords: surfactant; childhood interstitial lung disease; neonatal respiratory distress; respiratory distress syndrome ATP-binding cassette transporter A3 (ABCA3), a member of a large family of proteins that hydrolyze ATP to transport substances across biologic membranes, is encoded by an 80-kb gene (ABCA3; NM_001089.2, Gene ID 21) and consists of 1,704 amino acids with two membranespanning domains and two nucleotide binding domains. ABCA3 is expressed in alveolar type II cells, localizes to the membranes of lamellar bodies, and transports phospholipids into the lamellar body, where surfactant, a complex protein-phospholipid mixture that reduces surface tension and prevents alveolar collapse at end expiration, is assembled and stored (1-3). ABCA3 is also required for lamellar body biogenesis (4).Biallelic loss-of-function (nonsense, frameshift) mutations in ABCA3 result in severe, progressive neonatal respiratory distress syndrome (RDS) that is lethal by 1 year of age without lung transplantation (5). However, for individuals with missense, T.C., F.V.W., A.H., B.P.H., and F.S.C.; drafting and revising the manuscript for important intellectual content-J.A.W., P.Y....
Rare or private, biallelic variants in the ABCA3 (ATP-binding cassette transporter A3) gene are the most common monogenic cause of lethal neonatal respiratory failure and childhood interstitial lung disease. Functional characterization of fewer than 10% of over 200 diseaseassociated ABCA3 variants (majority missense) suggests either disruption of ABCA3 protein trafficking (type I) or of ATPasemediated phospholipid transport (type II). Therapies remain limited and nonspecific. A scalable platform is required for functional characterization of ABCA3 variants and discovery of pharmacologic correctors. To address this need, we first silenced the endogenous ABCA3 locus in A549 cells with CRISPR/Cas9 genome editing. Next, to generate a parent cell line (A549/ABCA3 2/2 ) with a single recombination target site for genomic integration and stable expression of individual ABCA3 missense variant cDNAs, we used lentiviral-mediated integration of a LoxFAS cassette, FACS, and dilutional cloning. To assess the fidelity of this cell-based model, we compared functional characterization (ABCA3 protein processing, ABCA3 immunofluorescence colocalization with intracellular markers, ultrastructural vesicle phenotype) of two individual ABCA3 mutants (type I mutant, p.L101P; type II mutant, p.E292V) in A549/ABCA3 2/2 cells and in both A549 cells and primary, human alveolar type II cells that transiently express each cDNA after adenoviral-mediated transduction. We also confirmed pharmacologic rescue of ABCA3 variant-encoded mistrafficking and vesicle diameter in A549/ABCA3 2/2 cells that express p.G1421R (type I mutant). A549/ABCA3 2/2 cells provide a scalable, genetically versatile, physiologically relevant functional genomics platform for discovery of variant-specific mechanisms that disrupt ABCA3 function and for screening of potential ABCA3 pharmacologic correctors.
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