Unexplained neonatal respiratory distress due to congenital surfactant deficiency

ABCA3 c.838C>T (p.Arg280Cys, R280C) and c.697C>T (p.Gln233Ter, Q233X, Q233*) as Causative Variants for RDS: A Family Case Study and Literature Review

Background: Respiratory distress syndrome (RDS) is the primary cause of respiratory failure in preterm infants, but it also affects 5-7% of term infants. Dysfunctions in pulmonary surfactant metabolism, resulting from mutations of the lung surfactant genes, are rare diseases, ranging from fatal neonatal RDS to interstitial lung disease, associated with increased morbidity and mortality. This study aims to clarify the clinical significance of ABCA3 variants found in a specific family case, as existing data in the literature are inconsistent. Material and Methods: A family case report was conducted; targeted panel genetic testing identified a variant of the SFTPB gene and two variants of ABCA3 genes. Comprehensive research involving a systematic review of PubMed, Google Scholar databases, and genome browsers was used to clarify the pathogenicity of the two ABCA3 variants found in the index patient. Advanced prediction tools were employed to assess the pathogenicity of the two ABCA3 variants, ensuring the validity and reliability of our findings. Results: The index case exhibited fatal neonatal RDS. Genetic testing revealed the presence of the SFTPB p.Val267Ile variant, which was not previously reported but is a benign variant based on family genetic testing and history. Additionally, two ABCA3 gene variants were identified: c.697C>T, not yet reported, and c.838C>T. These variants were found to affect ABCA3 protein function and were likely associated with neonatal RDS. Prediction tools and data from nine other cases in the literature supported this conclusion. Conclusions: Based on in silico predictors, an analysis of the presented family, and cases described in the literature, it is reasonable to consider reclassifying the two ABCA3 variants identified in the index case as pathogenic/pathogenic. Reclassification will improve genetic counseling accuracy and facilitate correct diagnosis.

Interstitial lung disease in the newborn

Although relatively rare, interstitial lung diseases may present with respiratory distress in the newborn period. Most commonly these include developmental and growth disorders, disorders of surfactant synthesis and homeostasis, pulmonary interstitial glycogenosis, and neuroendocrine cell hyperplasia of infancy. Although the diagnosis of these disorders is sometimes made based on clinical presentation and imaging, due to the significant overlap between disorders and phenotypic variability, lung biopsy or, increasingly genetic testing is needed for diagnosis. These diseases may result in significant morbidity and mortality. Effective medical treatment options are in some cases limited and/or invasive. The genetic basis for some of these disorders has been identified, and with increased utilization of exome and whole genome sequencing even before lung biopsy, further insights into their genetic etiologies should become available.

Pulmonary Hypertension in Developmental Lung Diseases

Diverse genetic developmental lung diseases can present in the neonatal period with hypoxemic respiratory failure, often associated with with pulmonary hypertension. Intractable hypoxemia and lack of sustained response to medical management should increase the suspicion of a developmental lung disorder. Genetic diagnosis and lung biopsy are helpful in establishing the diagnosis. Early diagnosis can result in optimizing management and redirecting care if needed. This article reviews normal lung development, various developmental lung disorders that can result from genetic abnormalities at each stage of lung development, their clinical presentation, management, prognosis, and differential diagnoses.

Descriptive and Functional Genomics in Neonatal Respiratory Distress Syndrome: From Lung Development to Targeted Therapies

In this up-to-date study, we first aimed to highlight the genetic and non-genetic factors associated with respiratory distress syndrome (RDS) while also focusing on the genomic aspect of this condition. Secondly, we discuss the treatment options and the progressing therapies based on RNAs or gene therapy. To fulfill this, our study commences with lung organogenesis, a highly orchestrated procedure guided by an intricate network of conserved signaling pathways that ultimately oversee the processes of patterning, growth, and differentiation. Then, our review focuses on the molecular mechanisms contributing to both normal and abnormal lung growth and development and underscores the connections between genetic and non-genetic factors linked to neonatal RDS, with a particular emphasis on the genomic aspects of this condition and their implications for treatment choices and the advancing therapeutic approaches centered around RNAs or gene therapy.

Genetics of bronchopulmonary dysplasia: An update

Bronchopulmonary dysplasia (BPD) is a multi-factorial disease that results from multiple clinical factors, including lung immaturity, mechanical ventilation, oxidative stress, pulmonary congestion due to increasing cardiac blood shunting, nutritional and immunological factors. Twin studies have indicated that susceptibility to BPD can be strongly inherited in some settings. Studies have reported associations between common genetic variants and BPD in preterm infants. Recent genomic studies have highlighted a potential role for molecular pathways involved in inflammation and lung development in affected infants. Rare mutations in genes encoding the lipid transporter ATP-binding cassette, sub-family A, member 3 (ABCA3 gene) which is involved in surfactant synthesis in alveolar type II cells, as well as surfactant protein B (SFTPB) and C (SFTPC) can also result in severe form of neonatal-onset interstitial lung diseases and may also potentially affect the course of BPD. This chapter summarizes the current state of knowledge on the genetics of BPD.

LAMP3 deficiency affects surfactant homeostasis in mice

Lysosome-associated membrane glycoprotein 3 (LAMP3) is a type I transmembrane protein of the LAMP protein family with a cell-type-specific expression in alveolar type II cells in mice and hitherto unknown function. In type II pneumocytes, LAMP3 is localized in lamellar bodies, secretory organelles releasing pulmonary surfactant into the extracellular space to lower surface tension at the air/liquid interface. The physiological function of LAMP3, however, remains enigmatic. We generated Lamp3 knockout mice by CRISPR/Cas9. LAMP3 deficient mice are viable with an average life span and display regular lung function under basal conditions. The levels of a major hydrophobic protein component of pulmonary surfactant, SP-C, are strongly increased in the lung of Lamp3 knockout mice, and the lipid composition of the bronchoalveolar lavage shows mild but significant changes, resulting in alterations in surfactant functionality. In ovalbumin-induced experimental allergic asthma, the changes in lipid composition are aggravated, and LAMP3-deficient mice exert an increased airway resistance. Our data suggest a critical role of LAMP3 in the regulation of pulmonary surfactant homeostasis and normal lung function.

LAMP3 is a protein of unknown molecular function with highest expression in alveolar type II cells. In alveolar type II cells, LAMP3 localizes to lamellar bodies, specific lysosome-related organelles that play an important role in secreting pulmonary surfactant, a mixture of hydrophobic proteins and lipids lowering the surface tension between the gas and the liquid phase of the lung in order to prevent alveoli from collapsing. To decipher the physiological function of LAMP3, we generated Lamp3 knockout mice, which are viable and show no apparent phenotype. Under basal conditions, both the protein and lipid composition of pulmonary surfactant are altered, but do not affect the physiological function of the lung. However, under diseased conditions of experimental allergic asthma, changes in the lipid composition are aggravated and are associated with an impaired lung function, suggesting an important role of LAMP3 in the homeostasis of pulmonary surfactant.

ABCA3 mutations in adult pulmonary fibrosis patients: a case series and review of literature

PURPOSE OF REVIEW

The current review aims to recognize the variability in clinical presentation of adult patients with bi-allelic ABCA3 mutations, create more depth in ABCA3 mutations reported and highlight the influence of environmental factors on disease course.

RECENT FINDINGS

Mutations in ABCA3 are predominantly linked to neonatal and pediatric interstitial lung disease (ILD) with a minority surviving beyond puberty. Here, we present three patients with ABCA3 mutations who present with disease at the age of 19, 61 and 77. Moreover, we identified c.4451G>C (p.R1484P), c.1675G>A (p.G559R) and c.4745C>G (p.T1582S) as three novel ABCA3 mutations. In addition, we identified six additional patients with ABCA3 mutations in literature who reached an age above 18. Furthermore, we discuss the influence of infections, drugs and smoking on disease course.

SUMMARY

Although extremely rare, patients with bi-allelic mutations in ABCA3 may present at adulthood. Late onset of disease may be influenced by type of mutation or environmental factors.

ABCA3 gene mutations shape the clinical profiles of severe unexplained respiratory distress syndrome in late preterm and term infants

BACKGROUND

The majority of unexplained respiratory distress syndrome (URDS) cases in late preterm and term infants are caused by genetic abnormalities, with the most common of these being ABCA3 gene mutation. At present, it is unclear to neonatologists whether URDS patients with ABCA3 mutation have similar or more challenging clinical profiles to those without any defined genetic abnormalities. Our study aimed to answer this question by comparing the clinical characteristics of severe URDS patients with homozygous or compound heterozygous ABCA3 mutations, a single ABCA3 mutation, or no defined genetic abnormalities.

METHODS

This retrospective cohort study involved 39 late preterm and term infants with URDS underwent a clinical exome sequencing at a tertiary neonatal intensive care unit between January 2013 and December 2019. Based on the sequencing result, the study subjects were classified into the homozygous or compound heterozygous mutations, single ABCA3 mutation, or no defined genetic abnormalities groups. The major outcomes, including mortality, the age of symptom onset and development of severe RDS, and the radiological score, were compared between the groups.

RESULTS

A novel splicing site (c.3862+1G>C) was identified in one twin with homozygous expression. Patients with homozygous or compound heterozygous ABCA3 mutations exhibited symptom onset and development of severe respiratory distress syndrome (RDS) earlier than those with a single mutation or no genetic abnormalities (P<0.05). These patients also had higher mortality rates than those without genetic abnormalities (P=0.029). The total radiological scores were 51.14±4.91, 44.20±6.54, 35.91±4.42 for patients with homozygous or compound heterozygous mutations, a single mutation, and a wild-type gene, respectively, with significant differences between the groups observed by pairwise comparison (all P<0.05).

CONCLUSIONS

Late preterm or term infants with URDS due to homozygous or compound heterozygous ABCA3 mutations exhibited more challenging clinical profiles than those without genetic abnormalities. However, whether this relationship exists between patients with a single ABCA3 mutation and those without genetic abnormalities warrants further study.

Surfactant dysfunction disorder masquerading as meconium aspiration syndrome and persistent pulmonary hypertension of the newborn

About 10% of term neonates present with respiratory distress at birth. The most common aetiologies include transient tachypnoea of the newborn, pneumonia and meconium aspiration syndrome (MAS). Hyaline membrane disease (HMD) in a term infant occurs either as primary HMD, secondary surfactant deficiency or congenital surfactant dysfunction. A detailed history supported with appropriate radiological and laboratory investigations can help a clinician reach a diagnosis. We report a case of surfactant dysfunction disorder which presented as severe MAS and persistent pulmonary hypertension of the newborn. In the infant described, the significant history of a sibling death with severe neonatal respiratory disease led us to think of diffuse developmental lung diseases especially surfactant dysfunction syndromes. Exome sequencing detected a heterozygous missense variation in exon 21 of the ATP binding cassette protein member 3 (ABCA3) gene. Based on the clinical picture supported with the exome sequencing, a diagnosis of surfactant dysfunction disorder (ABCA3 deficiency) was confirmed.

Study on the Relationship Between Respiratory Distress Syndrome and SP-A1 (rs1059057) Gene Polymorphism in Mongolian Very Premature Infants

Aim: To study the relationship between rs1059057 polymorphism of pulmonary surfactant protein A1 (SP-A1) and respiratory distress syndrome (RDS) in Mongolian very premature infants. Methods: Applying the strategy of case-control study, 120 Mongolian RDS very premature infants (58 males and 62 females) in the western part of Inner Mongolia were selected as the case group, and 120 subjects of non-RDS very premature infants (56 males and 64 females) with the same nationality, same sex and similar gestational age were used as the control group. The single nucleotide polymorphism (SNP) site rs1059057 of SP-A1 was genotyped using polymerase chain reaction-single strand conformational polymorphism (PCR-SSCP). Results: Two genotypes, A/G and A/A, were detected at the SP-A1 rs1059057 locus in the western part of Inner Mongolia. In the case group, the frequencies of two genotypes were 53 and 47%, and the frequencies of A allele and G allele were 73 and 27%, respectively. In the control group, the frequencies of the two genotypes were 42 and 58%, and the frequencies of A allele and G allele were 79 and 21%, respectively. There was no significant difference in the genotype frequency of SP-A1 (rs1059057) locus between the case group and the control group (X 2 = 3.275, P > 0.05), and no significant difference in allele frequency between the case group and the control group (X 2 = 2.255, P > 0.05). Conclusion: The genotypes and allele frequencies of SP-A1 (rs1059057) locus were not associated with the incidence of RDS in Mongolian very premature infants in western Inner Mongolia.

Recessive missense LAMP3 variant associated with defect in lamellar body biogenesis and fatal neonatal interstitial lung disease in dogs

Neonatal interstitial lung diseases due to abnormal surfactant biogenesis are rare in humans and have never been reported as a spontaneous disorder in animals. We describe here a novel lung disorder in Airedale Terrier (AT) dogs with clinical symptoms and pathology similar to the most severe neonatal forms of human surfactant deficiency. Lethal hypoxic respiratory distress and failure occurred within the first days or weeks of life in the affected puppies. Transmission electron microscopy of the affected lungs revealed maturation arrest in the formation of lamellar bodies (LBs) in the alveolar epithelial type II (AECII) cells. The secretory organelles were small and contained fewer lamellae, often in combination with small vesicles surrounded by an occasionally disrupted common limiting membrane. A combined approach of genome-wide association study and whole exome sequencing identified a recessive variant, c.1159G>A, p.(E387K), in LAMP3, a limiting membrane protein of the cytoplasmic surfactant organelles in AECII cells. The substitution resides in the LAMP domain adjacent to a conserved disulfide bond. In summary, this study describes a novel interstitial lung disease in dogs, identifies a new candidate gene for human surfactant dysfunction and brings important insights into the essential role of LAMP3 in the process of the LB formation.

Genetic basis of surfactant dysfunction in Chinese children: A retrospective study

OBJECTIVE

To investigate the prevalence of surfactant dysfunction (SD) and the genotype distribution in Chinese childhood interstitial lung disease (chILD).

METHODS

From December 2013 to December 2016, whole exons and splicing regions of surfactant protein (SP)-B, SP-C, and adenosine triphosphate (ATP)-binding cassette subfamily A member 3 (ABCA3) were sequenced in chILD with unknown etiology in five children's medical centers of China. The sequencing was performed by Next-generation sequencing technique in a molecular genetics laboratory. The clinical and genetic data were reviewed retrospectively.

RESULTS

In total, 136 patients of age 3 months to 13 years (mean 12.5 ± 9.4 months) were recruited, among which 76 were males. Of the 136 cases of chILD, 13.2% (18 of 136) were diagnosed with SD. In these 18 SD cases, 15 had heterozygous SP-C deficiencies, two cases had compound heterozygous ABCA3 deficiencies, and no SP-B deficiency was identified. In SP-C deficiencies, there were six cases with p.I73T, 2 with p.I73N, 5 with p.V39L, 1 with c.417delA, and 1 case with IVS4, +1G>C. Two cases of ABCA3 mutation were heterozygous with c.1755delC and c.2890G>A; c.3913T>C (R1305W) and exon 13 to 18 deletion. One was negative by sequencing while diagnosed positive by pathology.

CONCLUSION

The proportion of genetic mutation of SD in chILD is 13.2% in China, of which SP-C deficiency is predominant. The mutation, SP-C p.V39L, was found to be relatively prevalent in China and warrants further investigation.

Bi-allelic missense ABCA3 mutations in a patient with childhood ILD who reached adulthood

The adenosine triphosphate (ATP)-binding cassette subfamily A member 3 (ABCA3) is a transmembrane glycoprotein that uses energy of ATP hydrolysis to transport phospholipids into the lamellar bodies of type 2 alveolar epithelial cells (AEC) and regulates lung surfactant homeostasis. More than 200 mutations have already been described in ABCA3, located on chromosome 16 [1, 2]. Patients present with a great heterogeneity of phenotypes, from lethal neonatal respiratory distress syndrome (RDS) to childhood and rarely adult interstitial lung disease (ILD) [3, 4]. ABCA3 mutations-related lung disease inheritance is autosomal recessive, as it requires two disease-causing (bi-allelic) mutations, one from each parent.

Children with ABCA3 mutations may survive beyond infancy and reach adulthood. Genetic mechanisms should always be examined in adult patients with childhood onset ILD and molecular analysis should be performed accordingly in specialised referral centres.http://bit.ly/2LzMNOE

Unusual long survival despite severe lung disease of a child with biallelic loss of function mutations in ABCA-3

Homozygous or compound heterozygous for frameshift or nonsense mutations in the ATP–binding cassette transporter A3 (ABCA3) is associated with neonatal respiratory failure and death within the first year of life without lung transplantation. We report the case of a newborn baby girl who developed severe respiratory distress soon after birth. She was diagnosed with compound heterozygous frameshift mutation of the ABCA3 gene. Despite extensive treatment (intravenous corticosteroids pulse therapy, oral corticosteroids, azithromycin, and hydroxychloroquine), she developed chronic respiratory failure. As the parents refused cardio-pulmonary transplantation and couldn't resolve to an accompaniment of end of life, a tracheostomy was performed resulting in continuous mechanical ventilation. A neurodevelopmental delay and an overall muscular dystrophy were noted. At the age of 5 years, after 2 episodes of pneumothorax, the patient died from severe respiratory failure. To our knowledge, this was the first case of a child with compound heterozygous frameshift mutation who posed such an ethical dilemma with a patient surviving till the age of five years.

Surfactant proteins gene variants in premature newborn infants with severe respiratory distress syndrome

OBJECTIVE

Genetic surfactant dysfunction causes respiratory failure in term and near-term newborn infants, but little is known of such condition in prematures. We evaluated genetic surfactant dysfunction in premature newborn infants with severe RDS.

PATIENTS AND METHODS

A total of 68 preterm newborn infants with gestational age ≤32 weeks affected by unusually severe RDS were analysed for mutations in SFTPB, SFTPC and ABCA3. Therapies included oxygen supplementation, nasal CPAP, different modalities of ventilatory support, administration of exogenous surfactant, inhaled nitric oxide and steroids. Molecular analyses were performed on genomic DNA extracted from peripheral blood and Sanger sequencing of whole gene coding regions and intron junctions. In one case histology and electron microscopy on lung tissue was performed.

RESULTS

Heterozygous previously described rare or novel variants in surfactant proteins genes ABCA3, SFTPB and SFTPC were identified in 24 newborn infants. In total, 11 infants died at age of 2 to 6 months. Ultrastructural analysis of lung tissue of one infant showed features suggesting ABCA3 dysfunction.

DISCUSSION

Rare or novel genetic variants in genes encoding surfactant proteins were identified in a large proportion (35%) of premature newborn infants with particularly severe RDS. We speculate that interaction of developmental immaturity of surfactant production in association with abnormalities of surfactant metabolism of genetic origin may have a synergic worsening phenotypic effect.

Monogenic diseases in respiratory medicine: Clinical perspectives

With the increasing awareness of genetics in respiratory medicine and improvements in molecular diagnostic techniques, many complicated and rare diseases in respiratory medicine can be diagnosed. Most respiratory diseases have no specific phenotype. However, the clinical spectrum of monogenic diseases in respiratory medicine varies, from pulmonary disease to other inherited disorders that involve the lung. The genes that mediate some of these diseases have been identified. Certain monogenic diseases remain poorly characterized clinically. Because of the specificity of the phenotype of respiratory disease, a future challenge will be to correlate the phenotype and genotype and understand its phenotypic variability. With the development of precision medicine, research on monogenic disorders has been intensive and vigorous. In this article, we provide a brief clinical introduction to monogenic diseases in pediatrics.

The Montreux definition of neonatal ARDS: biological and clinical background behind the description of a new entity

Acute respiratory distress syndrome (ARDS) is undefined in neonates, despite the long-standing existing formal recognition of ARDS syndrome in later life. We describe the Neonatal ARDS Project: an international, collaborative, multicentre, and multidisciplinary project which aimed to produce an ARDS consensus definition for neonates that is applicable from the perinatal period. The definition was created through discussions between five expert members of the European Society for Paediatric and Neonatal Intensive Care; four experts of the European Society for Paediatric Research; two independent experts from the USA and two from Australia. This Position Paper provides the first consensus definition for neonatal ARDS (called the Montreux definition). We also provide expert consensus that mechanisms causing ARDS in adults and older children-namely complex surfactant dysfunction, lung tissue inflammation, loss of lung volume, increased shunt, and diffuse alveolar damage-are also present in several critical neonatal respiratory disorders.

Metabolic Functions of the Lung, Disorders and Associated Pathologies

The primary function of the lungs is gas exchange. Approximately 400 million years ago, the Earth's atmosphere gained enough oxygen in the gas phase for the animals that emerged from the sea to breathe air. The first lungs were merely primitive air sacs with a few vessels in the walls that served as accessory organs of gas exchange to supplement the gills. Eons later, as animals grew accustomed to a solely terrestrial life, the lungs became highly compartmentalized to provide the vast air-blood surface necessary for O2 uptake and CO2 elimination, and a respiratory control system was developed to regulate breathing in accordance with metabolic demands and other needs. With the evolution and phylogenetic development, lungs were taking a variety of other specialized functions to maintain homeostasis, which we will call the non-respiratory functions of the lung and that often, and by mistake, are believed to have little or no connection with the replacement gas. In this review, we focus on the metabolic functions of the lung, perhaps the least known, and mainly, in the lipid metabolism and blood-adult lung vascular endothelium interaction. When these functions are altered, respiratory disorders or diseases appear, which are discussed concisely, emphasizing how they impact the most important function of the lungs: external respiration.

Comparison of efficacy and safety of two available natural surfactants in Iran, Curosurf and Survanta in treatment of neonatal respiratory distress syndrome: A randomized clinical trial

Introduction

The benefit of surfactant prescription for respiratory distress syndrome (RDS) has been approved. Curosurf and Survanta are two commonly used natural surfactants in Iran. Previous studies did not report priority for one of these two drugs. The present study aimed to compare the effectiveness and safety of Curosurf and Survanta in treatment of RDS.

Methods

In this randomized clinical trial, neonates were born with RDS diagnosis in two governmental and referral hospitals of Tehran (the capital of Iran) in 2014 were randomly selected. Neonates were randomly assigned into two groups receiving 100 mg/kg Curosurf or Survanta as soon as possible after randomization. Complications, mortality and needing the second dose were compared between the two groups.

Results

A total 112 patients with the mean gestational age of 32.59 ± 3.39 weeks were evaluated (56 patients in each group). There were no significant differences regarding birth weight, gestational age, delivery method, and parity between the two groups (P > 0.05). The complications were occurred in 18 neonates (32.1%) of Curosurf group and 20 neonates (35.7%) of Survanta group (RR = 0.922, 95% CI = 0.617–1.379). There were no significant differences regarding complications, mortality, and needing nasal CPAP and endotracheal tube between the two groups. In the neonates with gestational age of 29–32 weeks the IVH and NEC incidence were significantly more in Curosurf group compared to Survanta group (27.8% vs 0% and 22.3% vs 0%, P < 0.05).

Conclusion

There was no significant difference in complications or mortality between those two groups; however Curosurf was associated with less need of ET tube (in >32 birth weeks subgroup) and NCPAP (in 29–32 birth weeks subgroup) (p = 0.008). Further evaluations with longer follow-up duration are needed for comparing these two surfactants.

ABCA3, a key player in neonatal respiratory transition and genetic disorders of the surfactant system

Genetic disorders of the surfactant system are rare diseases with a broad range of clinical manifestations, from fatal respiratory distress syndrome (RDS) in neonates to chronic interstitial lung disease (ILD) in children and adults. ABCA3 [ATP-binding cassette (ABC), subfamily A, member 3] is a lung-specific phospholipid transporter critical for intracellular surfactant synthesis and storage in lamellar bodies (LBs). Its expression is developmentally regulated, peaking prior to birth under the influence of steroids and transcription factors. Bi-allelic mutations of the ABCA3 gene represent the most frequent cause of congenital surfactant deficiency, indicating its critical role in lung function. Mutations affect surfactant lipid and protein processing and LBs’ morphology, leading to partial or total surfactant deficiency. Approximately 200 mutations have been reported, most of which are unique to individuals and families, which makes diagnosis and prognosis challenging. Various types of mutations, affecting different domains of the protein, account in part for phenotype diversity. Disease-causing mutations have been reported in most coding and some non-coding regions of the gene, but tend to cluster in the first extracellular loop and the second nucleotide-binding domain (NBD), leading to defective glycosylation and trafficking defects and interfering with ATP binding and hydrolysis respectively. Mono-allelic damaging and benign variants are often subclinical but may act as disease modifiers in lung diseases such as RDS of prematurity or associate with mutations in other surfactant-related genes. Diagnosis is complex but essential and should combine pathology and ultrastructure studies on lung biopsy with broad-spectrum genetic testing of surfactant-related genes, made possible by recent technology advances in the massive parallel sequencing technology.

Lost after translation: insights from pulmonary surfactant for understanding the role of alveolar epithelial dysfunction and cellular quality control in fibrotic lung disease

Dating back nearly 35 years ago to the Witschi hypothesis, epithelial cell dysfunction and abnormal wound healing have reemerged as central concepts in the pathophysiology of idiopathic pulmonary fibrosis (IPF) in adults and in interstitial lung disease in children. Alveolar type 2 (AT2) cells represent a metabolically active compartment in the distal air spaces responsible for pulmonary surfactant biosynthesis and function as a progenitor population required for maintenance of alveolar integrity. Rare mutations in surfactant system components have provided new clues to understanding broader questions regarding the role of AT2 cell dysfunction in the pathophysiology of fibrotic lung diseases. Drawing on data generated from a variety of model systems expressing disease-related surfactant component mutations [surfactant proteins A and C (SP-A and SP-C); the lipid transporter ABCA3], this review will examine the concept of epithelial dysfunction in fibrotic lung disease, provide an update on AT2 cell and surfactant biology, summarize cellular responses to mutant surfactant components [including endoplasmic reticulum (ER) stress, mitochondrial dysfunction, and intrinsic apoptosis], and examine quality control pathways (unfolded protein response, the ubiquitin-proteasome system, macroautophagy) that can be utilized to restore AT2 homeostasis. This integrated response and its derangement will be placed in the context of cell stress and quality control signatures found in patients with familial or sporadic IPF as well as non-surfactant-related AT2 cell dysfunction syndromes associated with a fibrotic lung phenotype. Finally, the need for targeted therapeutic strategies for pulmonary fibrosis that address epithelial ER stress, its downstream signaling, and cell quality control are discussed.

Diseases of pulmonary surfactant homeostasis

Advances in physiology and biochemistry have provided fundamental insights into the role of pulmonary surfactant in the pathogenesis and treatment of preterm infants with respiratory distress syndrome. Identification of the surfactant proteins, lipid transporters, and transcriptional networks regulating their expression has provided the tools and insights needed to discern the molecular and cellular processes regulating the production and function of pulmonary surfactant prior to and after birth. Mutations in genes regulating surfactant homeostasis have been associated with severe lung disease in neonates and older infants. Biophysical and transgenic mouse models have provided insight into the mechanisms underlying surfactant protein and alveolar homeostasis. These studies have provided the framework for understanding the structure and function of pulmonary surfactant, which has informed understanding of the pathogenesis of diverse pulmonary disorders previously considered idiopathic. This review considers the pulmonary surfactant system and the genetic causes of acute and chronic lung disease caused by disruption of alveolar homeostasis.

Clinical and ultrastructural spectrum of diffuse lung disease associated with surfactant protein C mutations

Genetic defects of surfactant metabolism are associated with a broad range of clinical manifestations, from neonatal respiratory distress syndrome to adult interstitial lung disease. Early therapies may improve symptoms but diagnosis is often delayed owing to phenotype and genotype variability. Our objective was to characterize the cellular/ultrastructural correlates of surfactant protein C (SP-C) mutations in children with idiopathic diffuse lung diseases. We sequenced SFTPC - the gene encoding SP-C - SFTPB and ABCA3, and analyzed morphology, ultrastructure and SP expression in lung tissue when available. We identified eight subjects who were heterozygous for SP-C mutations. Median age at onset and clinical course were variable. None of the mutations were located in the mature peptide-encoding region, but were either in the pro-protein BRICHOS or linker C-terminal domains. Although lung morphology was similar to other genetic surfactant metabolism disorders, electron microscopy studies showed specific anomalies, suggesting surfactant homeostasis disruption, plus trafficking defects in the four subjects with linker domain mutation and protein misfolding in the single BRICHOS mutation carrier in whom material was available. Immunolabeling studies showed increased proSP-C staining in all cases. In two cases, amyloid deposits could be identified. Immunochemistry and ultrastructural studies may be useful for diagnostic purposes and for genotype interpretation.

Large ABCA3 and SFTPC deletions resulting in lung disease

RATIONALE

Mutations in genes encoding proteins important in the function and metabolism of pulmonary surfactant are recognized causes of lung disease. Clinical genetic testing is available for these disorders, but children with phenotypes consistent with surfactant dysfunction and no identifiable mutations in the known causative genes have been reported.

OBJECTIVES

To identify the mechanism(s) for lung disease in two children with the phenotype of surfactant dysfunction who had negative testing in clinical laboratories for gene mutations causing surfactant dysfunction.

METHODS

Amplicons spanning multiple exons of candidate genes were generated by polymerase chain reaction and sequenced.

MEASUREMENTS AND MAIN RESULTS

A 4,335-base deletion that included all of exon 12 of the gene encoding member A3 of the adenosine triphosphate-binding cassette transporter was identified in a full-term infant with respiratory failure. A 333-base deletion involving part of exon 4 and the adjacent intron of the gene encoding surfactant protein C was identified in a child with interstitial lung disease.

CONCLUSIONS

Large deletions are a cause of surfactant dysfunction disorders and may need to be sought for specifically in children whose phenotypes suggest these syndromes but in whom clinical genetic testing is unrevealing.

Genotype-phenotype correlations for infants and children with ABCA3 deficiency

RATIONALE

Recessive mutations in the ATP-binding cassette transporter A3 (ABCA3) cause lethal neonatal respiratory failure and childhood interstitial lung disease. Most ABCA3 mutations are private.

OBJECTIVES

To determine genotype-phenotype correlations for recessive ABCA3 mutations.

METHODS

We reviewed all published and unpublished ABCA3 sequence and phenotype data from our prospective genetic studies of symptomatic infants and children at Washington and Johns Hopkins Universities. Mutations were classified based on their predicted disruption of protein function: frameshift and nonsense mutations were classified as "null," whereas missense, predicted splice site mutations, and insertion/deletions were classified as "other." We compared age of presentation and outcomes for the three genotypes: null/null, null/other, and other/other.

MEASUREMENTS AND MAIN RESULTS

We identified 185 infants and children with homozygous or compound heterozygous ABCA3 mutations and lung disease. All of the null/null infants presented with respiratory failure at birth compared with 75% of infants with null/other or other/other genotypes (P = 0.00011). By 1 year of age, all of the null/null infants had died or undergone lung transplantation compared with 62% of the null/other and other/other children (P < 0.0001).

CONCLUSIONS

Genotype-phenotype correlations exist for homozygous or compound heterozygous mutations in ABCA3. Frameshift or nonsense ABCA3 mutations are predictive of neonatal presentation and poor outcome, whereas missense, splice site, and insertion/deletions are less reliably associated with age of presentation and prognosis. Counseling and clinical decision making should acknowledge these correlations.

Ultrastructural characterization of genetic diffuse lung diseases in infants and children: a cohort study and review

Pediatric diffuse lung diseases are rare disorders with an onset in the neonatal period or in infancy, characterized by chronic respiratory symptoms and diffuse interstitial changes on imaging studies. Genetic disorders of surfactant homeostasis represent the main etiology. Surfactant protein B and ABCA3 deficiencies typically cause neonatal respiratory failure, which is often lethal within a few weeks or months. Although heterozygous ABCA3 mutation carriers are mostly asymptomatic, there is growing evidence that monoallelic mutations may affect surfactant homeostasis. Surfactant protein C mutations are dominant or sporadic disorders leading to a broad spectrum of manifestations from neonatal respiratory distress syndrome to adult pulmonary fibrosis. The authors performed pathology and ultrastructural studies in 12 infants who underwent clinical lung biopsy. One carried a heterozygous SP-B mutation, 3 carried SP-C mutations, and 7 carried ABCA3 mutations (5 biallelic and 2 monoallelic). Optical microscopy made it possible to distinguish between surfactant-related disorders and other forms. One of the ABCA3 monoallelic carriers had morphological features of alveolar capillary dysplasia, a genetic disorder of lung alveolar, and vascular development. One patient showed no surfactant-related anomalies but had pulmonary interstitial glycogenosis, a developmental disorder of unknown origin. Electron microscopy revealed specific lamellar bodies anomalies in all SP-B, SP-C, and ABCA3 deficiency cases. In addition, the authors showed that heterozygous ABCA3 mutation carriers have an intermediate ultrastructural phenotype between homozygous carriers and normal subjects. Lung biopsy is an essential diagnostic procedure in unexplained diffuse lung disorders, and electron microscopy should be performed systematically, since it may reveal specific alterations in genetic disorders of surfactant homeostasis.

Null ABCA3 in humans: large homozygous ABCA3 deletion, correlation to clinical-pathological findings

A study was undertaken to analyze the clinical presentation, pulmonary function, and pathological features in two female siblings with neonatal pulmonary surfactant metabolism dysfunction, type 3 (MIM 610921). The clinical records of the siblings were examined; the genes encoding surfactant protein B (SFTPB), surfactant protein C (SFTPC), and ATP-binding cassette transporter 3 protein (ABCA3) were analyzed with direct sequencing and Southern blotting. The infants were homozygous for a 5,983 bp deletion in ABCA3 including exons 2-5 as well as the start AUG codon and a putative Golgi exit signal motif. Dense abnormalities of lamellar bodies at electron microscopy and absence of ABCA3 at immunohistochemical staining were in agreement with the presence of two null alleles. In addition, an increased lipid synthesis suggested a compensatory mechanism. The clinical course in the two sisters was influenced by different environmental factors like the time needed for molecular confirmation, the ventilatory assistance adopted, the occurrence of infections. A less aggressive clinical approach did not improve the course of the disease; the prognosis was always poor. Development of a fast molecular test, able to detect also structural variants, is needed.

Novel ABCA3 mutations as a cause of respiratory distress in a term newborn

We report here the case of a term female newborn that developed severe respiratory distress soon after birth. She was found to be a compound heterozygote for both novel mutations in the ABCA3 gene. ABCA3 deficiency should be considered in mature babies who develop severe respiratory distress syndrome.

Interstitial lung disease in infants: new classification system, imaging technique, clinical presentation and imaging findings

Interstitial lung disease (ILD) is defined as a rare, heterogeneous group of parenchymal lung conditions that develop primarily because of underlying developmental or genetic disorders. Affected infants typically present with clinical syndromes characterized by dyspnea, tachypnea, crackles and hypoxemia. Until recently, the understanding of ILD in infants has been limited largely owing to a lack of evidence-based information of underlying pathogenesis, natural history, imaging findings and histopathological features. However, ILD in infants is now better understood and managed because of (1) advances in imaging methods that result in rapid and accurate detection, (2) improved thoracoscopic techniques for lung biopsy, (3) a consensus regarding the pathological criteria for these particular lung conditions and (4) a new classification system based on the underlying etiology of ILD. This article reviews the new classification system, imaging technique, clinical presentation and imaging findings of ILD in infants. Specialized knowledge of this new classification system in conjunction with recognition of characteristic imaging findings of ILD in infants has great potential for early and accurate diagnosis, which in turn can lead to optimal patient management.

New coding in the International Classification of Diseases, Ninth Revision, for children's interstitial lung disease

The term "children's interstitial lung disease" (chILD) refers to a heterogeneous group of rare and diffuse lung diseases associated with significant morbidity and mortality. These disorders include neuroendocrine cell hyperplasia of infancy, pulmonary interstitial glycogenosis, surfactant dysfunction mutations, and alveolar capillary dysplasia with misalignment of pulmonary veins. Diagnosis can be challenging, which may lead to a delay in recognition and treatment of these disorders. Recently, International Classifications of Diseases, Ninth Revision codes have been added for several of the chILD disorders. The purpose of this article is to give an overview of the chILD disorders and appropriate diagnostic coding.

Heterozygosity for E292V in ABCA3, lung function and COPD in 64,000 individuals

BACKGROUND

Mutations in ATP-binding-cassette-member A3 (ABCA3) are related to severe chronic lung disease in neonates and children, but frequency of chronic lung disease due to ABCA3 mutations in the general population is unknown. We tested the hypothesis that individuals heterozygous for ABCA3 mutations have reduced lung function and increased risk of COPD in the general population.

METHODS

We screened 760 individuals with extreme pulmonary phenotypes and identified three novel (H86Y, A320T, A1086D) and four previously described mutations (E292V, P766S, S1262G, R1474W) in the ABCA3 gene. We genotyped the entire Copenhagen City Heart study (n = 10,604) to assess the clinical importance of these mutations. To validate our findings we genotyped an additional 54,395 individuals from the Copenhagen General Population Study.

RESULTS

In the Copenhagen City Heart Study individuals heterozygous for E292V had 5% reduced FEV₁ % predicted compared with noncarriers (t-test: p = 0.008), and an increased odds ratio for COPD of 1.9 (95% CI: 1.1-3.1). In contrast, the A1086D mutation was associated with increased FEVFEV₁ % predicted (p = 0.03). None of the other ABCA3 mutations associated with lung function or COPD risk in the Copenhagen City Heart Study. In the larger Copenhagen General Population Study, and in the two studies combined, E292V heterozygotes did not have reduced lung function or increased risk of COPD (p = 0.11-0.98), while this was the case for the positive controls, surfactant protein-B 121ins2 heterozygotes and α₁-antitrypsin ZZ homozygotes.

CONCLUSION

Our results indicate that partially reduced ABCA3 activity due to E292V is not a major risk factor for reduced lung function and COPD in the general population. This is an important finding as 1.3% in the Danish population has partially reduced ABCA3 function due to E292V.

An intronic ABCA3 mutation that is responsible for respiratory disease

INTRODUCTION

Member A3 of the ATP-binding cassette family of transporters (ABCA3) is essential for surfactant metabolism. Nonsense, missense, frameshift, and splice-site mutations in the ABCA3 gene (ABCA3) have been reported as causes of neonatal respiratory failure (NRF) and interstitial lung disease. We tested the hypothesis that mutations in noncoding regions of ABCA3 may cause lung disease.

METHODS

ABCA3-specific cDNA was generated and sequenced from frozen lung tissue from a child with fatal lung disease with only one identified ABCA3 mutation. ABCA3 was sequenced from genomic DNA prepared from blood samples obtained from the proband, parents, and other children with NRF.

RESULTS

ABCA3 cDNA from the proband contained sequences derived from intron 25 that would be predicted to alter the structure and function of the ABCA3 protein. Genomic DNA sequencing revealed a heterozygous C>T transition in intron 25 trans to the known mutation, creating a new donor splice site. Seven additional infants with an ABCA3-deficient phenotype and inconclusive genetic findings had this same variant, which was not found in 2,132 control chromosomes.

DISCUSSION

These findings support that this variant is a disease-causing mutation that may account for additional cases of ABCA3 deficiency with negative genetic studies.

Genetic interstitial lung disease

The interstitial lung diseases (ILDs), or diffuse parenchymal lung diseases, are a heterogeneous collection of more than 100 different pulmonary disorders that affect the tissue and spaces surrounding the alveoli. Patients affected by ILD usually present with shortness of breath or cough; for many, there is evidence of pulmonary restriction, decreased diffusion capacity, and radiographic appearance of alveolar and/or reticulonodular infiltrates. This article reviews the inherited ILDs, with a focus on the diseases that may be seen by pulmonologists caring for adult patients. The authors conclude by briefly discussing the utility of genetic testing in this population.

Surfactant dysfunction

Mutations in genes encoding proteins needed for normal surfactant function and metabolism cause acute lung disease in newborns and chronic lung disease in older children and adults. While rare these disorders are associated with considerable pulmonary morbidity and mortality. The identification of genes responsible for surfactant dysfunction provides clues for candidate genes contributing to more common respiratory conditions, including neonatal respiratory distress syndrome and lung diseases associated with aging or environmental insults. While clinical, imaging and histopathology features of these disorders overlap, certain features are distinctive for surfactant dysfunction. Natural histories differ depending upon the genes involved and a specific diagnosis is important to provide accurate information concerning prognosis and mode of inheritance. Diagnosis of surfactant dysfunction can be made by biopsy, but identification of the specific gene involved requires molecular genetic testing, which is non-invasive. Currently there are no effective medical treatments for surfactant dysfunction. Development of therapies is a priority for research, which may benefit patients with other lung diseases.

Molecular and cellular characteristics of ABCA3 mutations associated with diffuse parenchymal lung diseases in children

ABCA3 (ATP-binding cassette subfamily A, member 3) is expressed in the lamellar bodies of alveolar type II cells and is crucial to pulmonary surfactant storage and homeostasis. ABCA3 gene mutations have been associated with neonatal respiratory distress (NRD) and pediatric interstitial lung disease (ILD). The objective of this study was to look for ABCA3 gene mutations in patients with severe NRD and/or ILD. The 30 ABCA3 coding exons were screened in 47 patients with severe NRD and/or ILD. ABCA3 mutations were identified in 10 out of 47 patients, including 2 homozygous, 5 compound heterozygous and 3 heterozygous patients. SP-B and SP-C expression patterns varied across patients. Among patients with ABCA3 mutations, five died shortly after birth and five developed ILD (including one without NRD). Functional studies of p.D253H and p.T1173R mutations revealed that p.D253H and p.T1173R induced abnormal lamellar bodies. Additionally, p.T1173R increased IL-8 secretion in vitro. In conclusion, we identified new ABCA3 mutations in patients with life-threatening NRD and/or ILD. Two mutations associated with ILD acted via different pathophysiological mechanisms despite similar clinical phenotypes.

Altered surfactant homeostasis and recurrent respiratory failure secondary to TTF-1 nuclear targeting defect

Background

Mutations of genes affecting surfactant homeostasis, such as SFTPB, SFTPC and ABCA3, lead to diffuse lung disease in neonates and children. Haploinsufficiency of NKX2.1, the gene encoding the thyroid transcription factor-1 (TTF-1) - critical for lung, thyroid and central nervous system morphogenesis and function - causes a rare form of progressive respiratory failure designated brain-lung-thyroid syndrome. Molecular mechanisms involved in this syndrome are heterogeneous and poorly explored. We report a novel TTF-1 molecular defect causing recurrent respiratory failure episodes in an infant.

Methods

The subject was an infant with severe neonatal respiratory distress syndrome followed by recurrent respiratory failure episodes, hypopituitarism and neurological abnormalities. Lung histology and ultrastructure were assessed by surgical biopsy. Surfactant-related genes were studied by direct genomic DNA sequencing and array chromatine genomic hybridization (aCGH). Surfactant protein expression in lung tissue was analyzed by confocal immunofluorescence microscopy. For kinetics studies, surfactant protein B and disaturated phosphatidylcholine (DSPC) were isolated from serial tracheal aspirates after intravenous administration of stable isotope-labeled ²H₂O and ¹³C-leucine; fractional synthetic rate was derived from gas chromatography/mass spectrometry ²H and ¹³C enrichment curves. Six intubated infants with no primary lung disease were used as controls.

Results

Lung biopsy showed desquamative interstitial pneumonitis and lamellar body abnormalities suggestive of genetic surfactant deficiency. Genetic studies identified a heterozygous ABCA3 mutation, L941P, previously unreported. No SFTPB, SFTPC or NKX2.1 mutations or deletions were found. However, immunofluorescence studies showed TTF-1 prevalently expressed in type II cell cytoplasm instead of nucleus, indicating defective nuclear targeting. This pattern has not been reported in human and was not found in two healthy controls and in five ABCA3 mutation carriers. Kinetic studies demonstrated a marked reduction of SP-B synthesis (43.2 vs. 76.5 ± 24.8%/day); conversely, DSPC synthesis was higher (12.4 vs. 6.3 ± 0.5%/day) compared to controls, although there was a marked reduction of DSPC content in tracheal aspirates (29.8 vs. 56.1 ± 12.4% of total phospholipid content).

Conclusion

Defective TTF-1 signaling may result in profound surfactant homeostasis disruption and neonatal/pediatric diffuse lung disease. Heterozygous ABCA3 missense mutations may act as disease modifiers in other genetic surfactant defects.

Some ABCA3 mutations elevate ER stress and initiate apoptosis of lung epithelial cells

BACKGROUND

ABCA3 transporter (ATP-binding cassette transporter of the A subfamily) is localized to the limiting membrane of lamellar bodies, organelles for assembly and storage of pulmonary surfactant in alveolar epithelial type II cells (AECII). It transports surfactant phospholipids into lamellar bodies and absence of ABCA3 function disrupts lamellar body biogenesis. Mutations of the ABCA3 gene lead to fatal neonatal surfactant deficiency and chronic interstitial lung disease (ILD) of children. ABCA3 mutations can result in either functional defects of the correctly localized ABCA3 or trafficking/folding defects where mutated ABCA3 remains in the endoplasmic reticulum (ER).

METHODS

Human alveolar epithelial A549 cells were transfected with vectors expressing wild-type ABCA3 or one of the three ABCA3 mutant forms, R43L, R280C and L101P, C-terminally tagged with YFP or hemagglutinin-tag. Localization/trafficking properties were analyzed by immunofluorescence and ABCA3 deglycosylation. Uptake of fluorescent NBD-labeled lipids into lamellar bodies was used as a functional assay. ER stress and apoptotic signaling were examined through RT-PCR based analyses of XBP1 splicing, immunoblotting or FACS analyses of stress/apoptosis proteins, Annexin V surface staining and determination of the intracellular glutathion level.

RESULTS

We demonstrate that two ABCA3 mutations, which affect ABCA3 protein trafficking/folding and lead to partial (R280C) or complete (L101P) retention of ABCA3 in the ER compartment, can elevate ER stress and susceptibility to it and induce apoptotic markers in the cultured lung epithelial A549 cells. R43L mutation, resulting in a functional defect of the properly localized ABCA3, had no effect on intracellular stress and apoptotic signaling.

CONCLUSION

Our data suggest that expression of partially or completely ER localized ABCA3 mutant proteins can increase the apoptotic cell death of the affected cells, which are factors that might contribute to the pathogenesis of genetic ILD.

Fatal respiratory failure in a full-term newborn with two ABCA3 gene mutations: a case report

Genetic mutations associated with pulmonary surfactant protein deficiency are associated with diverse clinical phenotypes. Mutations of the surfactant protein B and C genes were the first to be described. In 2004, fatal surfactant deficiency in newborns due to mutations of the gene encoding the adenosine triphosphate-binding cassette transporter A3 (ABCA3) was first reported. Few cases of lethal adenosine triphosphate-binding cassette transporter A3 mutations have been described to date. In our report, we describe a full-term newborn that died because of respiratory failure secondary to an uncommon ABCA3 genetic configuration.

Fatal familial lung disease caused by ABCA3 deficiency without identified ABCA3 mutations

OBJECTIVE

To test the hypothesis that some functionally significant variants in the gene encoding member A3 of the ATP Binding Cassette family (ABCA3) are not detected using exon-based sequencing approaches.

STUDY DESIGN

The first of 2 female siblings who died from neonatal respiratory failure was examined for mutations with sequence analysis of all ABCA3 exons and known regulatory elements within the 5' untranslated region. Lung tissue from both siblings was immunostained for ABCA3 and examined with electron microscopy. Segregation of ABCA3 alleles was determined with analysis of polymorphisms in the parents and all children.

RESULTS

No mutations were identified with ABCA3 sequence analysis in the first affected infant. Affected siblings were concordant for their ABCA3 alleles, but discordant from those of their unaffected siblings. ABCA3 protein was not detectable with immunostaining in lung tissue samples from both affected infants. Electron microscopy demonstrated small, dense lamellar bodies, characteristically seen with ABCA3 mutations.

CONCLUSIONS

The segregation of ABCA3 alleles, absence of ABCA3 immunostaining, lung pathology, and ultrastructural findings support genetic ABCA3 deficiency as the cause of lung disease in these 2 infants, despite the lack of an identified genetic variant.

Genetic Basis of Children's Interstitial Lung Disease

Specific genetic causes for children's interstitial lung disease (chILD) have been identified within the past decade. These include deletions of or mutations in genes encoding proteins important in surfactant production and function (SP-B, SP-C, and ABCA3), surfactant catabolism (GM-CSF receptor), as well as transcription factors important for surfactant production (TTF1) or lung development (Fox F1), with heterozygous deletions or loss-of-function mutations of the latter resulting in alveolar capillary dysplasia (ACD) with misalignment of the pulmonary veins. Familial pulmonary fibrosis in adults may result from mutations in genes encoding components of telomerase and SP-A2. While not yet reported in children, the expression of these genes in alveolar type II epithelial cells supports a key role for the disruption of normal homeostasis in this cell type in the pathogenesis of interstitial lung disease. The identification of specific genetic causes for chILD now allows for the possibility of non-invasive diagnosis, and provides insight into basic cellular mechanisms that may allow the development of novel therapies.

Conditional deletion of Abca3 in alveolar type II cells alters surfactant homeostasis in newborn and adult mice

ATP-binding cassette A3 (ABCA3) is a lipid transport protein required for synthesis and storage of pulmonary surfactant in type II cells in the alveoli. Abca3 was conditionally deleted in respiratory epithelial cells (Abca3(Δ/Δ)) in vivo. The majority of mice in which Abca3 was deleted in alveolar type II cells died shortly after birth from respiratory distress related to surfactant deficiency. Approximately 30% of the Abca3(Δ/Δ) mice survived after birth. Surviving Abca3(Δ/Δ) mice developed emphysema in the absence of significant pulmonary inflammation. Staining of lung tissue and mRNA isolated from alveolar type II cells demonstrated that ∼50% of alveolar type II cells lacked ABCA3. Phospholipid content and composition were altered in lung tissue, lamellar bodies, and bronchoalveolar lavage fluid from adult Abca3(Δ/Δ) mice. In adult Abca3(Δ/Δ) mice, cells lacking ABCA3 had decreased expression of mRNAs associated with lipid synthesis and transport. FOXA2 and CCAAT enhancer-binding protein-α, transcription factors known to regulate genes regulating lung lipid metabolism, were markedly decreased in cells lacking ABCA3. Deletion of Abca3 disrupted surfactant lipid synthesis in a cell-autonomous manner. Compensatory surfactant synthesis was initiated in ABCA3-sufficient type II cells, indicating that surfactant homeostasis is a highly regulated process that includes sensing and coregulation among alveolar type II cells.

Chronic interstitial lung diseases in children

Interstitial lung diseases (ILDs) in children constitute a heterogeneous group of rare diseases that have been described and classified according to experiences and research in adults. However, pediatric pulmonologists have observed that the clinical spectrum is broader in children than in adults, and that many of these disorders have different courses and treatment responses. In addition, probably due to the various stages of lung development and maturation, new clinical forms have been described, particularly in infants. This has broadened the classification of ILDs in this age bracket. The understanding that neither the usual definition nor the standard classification of these disorders entirely apply to children has prompted multicenter studies designed to increase knowledge of these disorders, as well as to standardize diagnostic and therapeutic strategies. We have reviewed the conceptualization of ILDs in children, taking into consideration the particularities of this group of patients when using the criteria for the classification of these diseases in adults. We have also made a historical review of several multicenter studies in order to further understanding of the problem. We have emphasized the differences in the clinical presentation, in an attempt to highlight knowledge of newly described entities in young children. We underscore the need to standardize management of laboratory and radiological routines, as well as of lung biopsy processing, taking such knowledge into account. It is important to bear in mind that, among the recently described disorders, genetic surfactant dysfunction, which is often classified as an idiopathic disease in adults, should be included in the differential diagnosis of ILDs.

Inherited surfactant deficiency caused by uniparental disomy of rare mutations in the surfactant protein-B and ATP binding cassette, subfamily a, member 3 genes

OBJECTIVE

To characterize inheritance of homozygous, rare, recessive loss-of-function mutations in surfactant protein-B (SFTPB) or ATP binding cassette, subfamily A, member 3 (ABCA3) genes in newborns with lethal respiratory failure.

STUDY DESIGN

We resequenced genes from parents whose infants were homozygous for mutations in SFTPB or ABCA3. For infants with only 1 heterozygous parent, we performed microsatellite analysis for chromosomes 2 (SFTPB) and 16 (ABCA3).

RESULTS

We identified 1 infant homozygous for the g.1549C > GAA mutation (121ins2) in SFTPB for whom only the mother was heterozygous and 3 infants homozygous for mutations in ABCA3 (p.K914R, p.P147L, and c.806_7insGCT) for whom only the fathers were heterozygous. For the SP-B-deficient infant, microsatellite markers confirmed maternal heterodisomy with segmental isodisomy. Microsatellite analysis confirmed paternal isodisomy for the 3 ABCA3-deficient infants. Two ABCA3-deficient infants underwent lung transplantation at 3 and 5 months of age, respectively, and 2 infants died. None exhibited any nonpulmonary phenotype.

CONCLUSIONS

Uniparental disomy should be suspected in infants with rare homozygous mutations in SFTPB or ABCA3. Confirmation of parental carrier status is important to provide recurrence risk and to monitor expression of other phenotypes that may emerge through reduction to homozygosity of recessive alleles.

Genetic disorders of surfactant dysfunction

Mutations in the genes encoding the surfactant proteins B and C (SP-B and SP-C) and the phospholipid transporter, ABCA3, are associated with respiratory distress and interstitial lung disease in the pediatric population. Expression of these proteins is regulated developmentally, increasing with gestational age, and is critical for pulmonary surfactant function at birth. Pulmonary surfactant is a unique mixture of lipids and proteins that reduces surface tension at the air-liquid interface, preventing collapse of the lung at the end of expiration. SP-B and ABCA3 are required for the normal organization and packaging of surfactant phospholipids into specialized secretory organelles, known as lamellar bodies, while both SP-B and SP-C are important for adsorption of secreted surfactant phospholipids to the alveolar surface. In general, mutations in the SP-B gene SFTPB are associated with fatal respiratory distress in the neonatal period, and mutations in the SP-C gene SFTPC are more commonly associated with interstitial lung disease in older infants, children, and adults. Mutations in the ABCA3 gene are associated with both phenotypes. Despite this general classification, there is considerable overlap in the clinical and histologic characteristics of these genetic disorders. In this review, similarities and differences in the presentation of these disorders with an emphasis on their histochemical and ultrastructural features will be described, along with a brief discussion of surfactant metabolism. Mechanisms involved in the pathogenesis of lung disease caused by mutations in these genes will also be discussed.

Evaluation of human gene variant detection in amplicon pools by the GS-FLX parallel Pyrosequencer

Background

A new priority in genome research is large-scale resequencing of genes to understand the molecular basis of hereditary disease and cancer. We assessed the ability of massively parallel pyrosequencing to identify sequence variants in pools. From a large collection of human PCR samples we selected 343 PCR products belonging to 16 disease genes and including a large spectrum of sequence variations previously identified by Sanger sequencing. The sequence variants included SNPs and small deletions and insertions (up to 44 bp), in homozygous or heterozygous state.

Results

The DNA was combined in 4 pools containing from 27 to 164 amplicons and from 8,9 to 50,8 Kb to sequence for a total of 110 Kb. Pyrosequencing generated over 80 million base pairs of data. Blind searching for sequence variations with a specifically designed bioinformatics procedure identified 465 putative sequence variants, including 412 true variants, 53 false positives (in or adjacent to homopolymeric tracts), no false negatives. All known variants in positions covered with at least 30× depth were correctly recognized.

Conclusion

Massively parallel pyrosequencing may be used to simplify and speed the search for DNA variations in PCR products. Our results encourage further studies to evaluate molecular diagnostics applications.

Cerebropulmonary dysgenetic syndrome

Ventilatory treatment of neonatal respiratory distress often results in bronchopulmonary dysplasia from congenital surfactant deficiency due to mutants of transporter protein ABCA3. Association of this condition with other severe disorders in premature newborns has not heretofore been reported. A neonatal autopsy included an in vivo whole blood sample for genetic testing. Autopsy revealed severe interstitial pulmonary fibrosis at age 8 days with heterozygotic mutation p.E292V of ABCA3 and severe dystrophic retardation of cerebral cortex and cerebellum. Subsequently, 1300 archival neonatal autopsies, 1983-2006, were reviewed for comparable concurrent findings and bronchopulmonary dysplasia or retarded cerebral dystrophy lacking the other principal feature of this syndrome. Archival review revealed four similar cases and eight less so, without gene analysis. Further review for bronchopulmonary dysplasia revealed 59 cases, 1983-2006. Several other examples of similar retarded migration of germinal matrix and underdevelopment of cortical mantle, without pulmonary lesions of this type, were identified. The determination of an ABCA3 mutation in one case of severe pulmonary fibrosis with significant dystrophy of the brain and the identification of four highly similar archival cases and eight others with partial pathological findings supports the designation of an independent disorder, here referred to as the cerebropulmonary dysgenetic syndrome.

Population and disease-based prevalence of the common mutations associated with surfactant deficiency

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 population-based 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 5' 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 = 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 (<0.4%). E292V was present in 3.8% of newborns with RDS, a 10-fold greater prevalence than in the Missouri cohort (p < 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.