Inherited Surfactant Disorders W. Adam Gower, Susan E. Wert and Lawrence M. Nogee NeoReviews 2008;9;e458-e467 DOI: 10.1542/neo.9-10-e458

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NeoReviews is the official journal of the American Academy of Pediatrics. A monthly publication, it has been published continuously since 2000. NeoReviews is owned, published, and trademarked by the American Academy of Pediatrics, 141 Northwest Point Boulevard, Elk Grove Village, Illinois, 60007. Copyright © 2008 by the American Academy of Pediatrics. All rights reserved. Online ISSN: 1526-9906.

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Inherited Surfactant Disorders W. Adam Gower, MD,* Objectives After completing this article, readers should be able to: Susan E. Wert, PhD,† Lawrence M. Nogee, MD‡ 1. Recognize the clinical presentations resulting from inherited disorders of surfactant metabolism. 2. Describe the roles of surfactant protein (SP)-B, adenosine triphosphate-binding Author Disclosure cassette member A3 (ABCA3), and SP-C in surfactant function and metabolism. Drs Gower, Wert, and 3. Evaluate infants for suspected inherited surfactant disorders and recognize the Nogee have disclosed limitations of such studies. support by the 4. Delineate how rare disorders can provide insights into normal metabolism and the Eudowood pathogenesis of more common illnesses. Foundation, the National Institutes of Abstract Health (HL 54703 to Inherited disorders of surfactant metabolism are rare causes of respiratory disease in LMN), and the newborns but are associated with significant morbidity and mortality. This review Hartwell Foundation outlines the molecular basis and pathophysiology of the three currently identified (WAG). This single-gene disorders of surfactant metabolism as well as the clinical presentations and commentary does not evaluation of potentially affected infants. Implications for the understanding of contain a discussion normal surfactant metabolism and the potential roles of surfactant dysfunction mutations in more common neonatal disorders, such as respiratory distress syndrome, of an unapproved/ also are discussed. investigative use of a commercial product/ device. Introduction Pulmonary surfactant is the mixture of lipids and specific proteins that reduces surface tension at the air-liquid interface and prevents end-expiratory atelectasis. Lack of pulmo- nary surfactant due to decreased production from pulmonary immaturity is the principal cause of respiratory distress syndrome (RDS) in preterm infants. RDS is a relatively common disorder, affecting more than 50% of infants born at 30 weeks’ or less gestation, but modern methods of respiratory support and exogenous surfactant replacement Abbreviations therapy have aided in keeping mortality from RDS low. ABCA3: adenosine triphosphate-binding cassette Surfactant deficiency also may result from genetic mecha- member A3 nisms that disrupt the production of key proteins important DPPC: dipalmitoyl phosphatidylcholine in surfactant function and metabolism. Although these DSPC: disaturated phosphatidylcholine single-gene disorders are rare, their associated morbidity ILD: interstitial lung disease and mortality are very high. Thus, recognition of these PC: phosphatidylcholine disorders is important to counsel families properly concern- PG: phosphatidylglycerol ing prognosis, and because they are genetic in origin, recur- RDS: respiratory distress syndrome rence risk. In addition, the study of these disorders has SP: surfactant protein offered insight into normal aspects of surfactant metabolism TTF-1: thyroid transcription factor-1 and can provide clues to genes that may be important in influencing the risk for RDS in preterm infants. In this

*Eudowood Division of Pediatric Respiratory Sciences, Department of Pediatrics, Johns Hopkins University School of Medicine, Baltimore, Md. †Divisions of Neonatology and Pulmonary Biology, Department of Pediatrics, Cincinnati Children’s Hospital Medical Center & University of Cincinnati College of Medicine, Cincinnati, Ohio. ‡Eudowood Neonatal Pulmonary Division, Department of Pediatrics, Johns Hopkins University School of Medicine, Baltimore, Md. e458 NeoReviews Vol.9 No.10 October 2008 Downloaded from http://neoreviews.aappublications.org by J Michael Coleman on August 19, 2010 respiratory disorders inherited surfactant disorders

article, we review the known inherited disorders of sur- have been identified, as have transcription factors needed factant metabolism, focusing on their clinical features, for its expression, most notably thyroid transcription diagnosis, and implications for lung biology. factor 1 (TTF-1) (also known as Nkx2.1). (4) Many commonly occurring polymorphisms have been identi- Surfactant Function, Components, and fied in the SP-B gene, including one in the promoter Metabolism region that affects the level of SP-B gene transcription. Pulmonary surfactant is synthesized by alveolar type II (5) Another in exon 4 determines whether N-linked cells of the lung and stored within lamellar bodies, which glycosylation occurs in the aminoterminal domain of the are lysosomally derived organelles specific to these cells. proprotein (6) and, thus, may be functionally significant. Structurally, lamellar bodies consist of well-organized An extensively characterized variable nucleotide tandem concentric rings of lipid membrane surrounding an repeat region in intron 4 may be associated with a risk for electron-dense core, as observed by electron microscopy. RDS in conjunction with other variables. (7)(8) Surfactant is secreted by exocytosis after fusion of the Complete deficiency of SP-B results in lethal neonatal lamellar body with the plasma membrane. Surfactant respiratory disease. (9) The usual phenotype is a term may be either recycled through type II cells or catabo- infant who presents with symptoms and signs of respira- lized by alveolar macrophages. Phosphatidylcholine tory distress and diffuse lung disease radiographically, (PC) (also known as lecithin) is the principal phospho- resembling RDS in preterm infants. The initial severity of lipid in surfactant needed for lowering of surface tension, lung disease can be variable, with some children having in particular, dipalmitoyl PC (DPPC) or disaturated PC relatively mild symptoms in the first several postnatal (DSPC), which accounts for approximately half of sur- days, while others have rapid onset of hypoxemic respi- factant by weight. Precisely how surfactant becomes en- ratory failure progressing to the need for extracorporeal riched for DSPC is not known. membrane oxygenation. The disease is progressive, with Two low-molecular weight, extremely hydrophobic only transient improvement seen with surfactant replace- proteins, surfactant protein (SP) B and C, facilitate the ment therapy and modest improvement with corticoste- adsorption of surfactant lipids to the air-liquid interface roid treatment. Death from respiratory failure usually after secretion and are essential for surfactant’s surface occurs within 3 to 6 months despite maximal medical tension-lowering properties. Both SP-B and SP-C are therapy, and lung transplantation is currently the only found in varying amounts in the mammalian-derived effective treatment option. (10)(11) The disease is ex- exogenous surfactants used to treat preterm babies who tremely rare, although the exact incidence is unknown. have RDS. Surfactant also contains two larger, more As an autosomal recessive disorder, mutations on hydrophilic, structurally related glycoproteins, SP-A and both alleles are necessary for an infant to be affected by SP-D. These proteins are involved primarily in innate SP-B deficiency. One particular frameshift mutation immunity and immunomodulation, with more limited (121ins2) has been identified in multiple unrelated pa- roles in surface tension-lowering and metabolism. (1) tients who have the disorder. Approximately 1 in 1,000 Genetic diseases related to abnormalities in SP-A and individuals of Northern European descent are carriers for SP-D production have not yet been reported. this mutation, which has accounted for approximately 60% to 70% of the mutant SFTPB alleles identified to date Genetic Lung Disease Due to SP-B Deficiency and likely has a common ancestral origin (founder ef- SP-B is a 79-amino acid protein derived from a much fect). (12)(13) Multiple other mutations, including a larger precursor protein (proSP-B) encoded by a single large deletion, have been identified, all of which may be gene (SFTPB) on chromosome 2. Proteolytic cleavage of categorized as loss-of-function mutations. (10)(11) amino- and carboxyterminal domains from proSP-B (14)(15) Some mutations allow for production of yields the mature form of SP-B found in the airspaces. proSP-B that is unable to be processed efficiently to The final processing steps are specific to type II cells and mature SP-B, resulting in marked reduction or complete occur in lamellar bodies or their immediate precursors. absence of mature SP-B. Those children who have mu- (2)(3) tations that allow for some SP-B production (partial SFTPB contains ten coding exons, with mature SP-B deficiency) can survive well beyond the neonatal period, encoded in exons 6 and 7. SP-B expression is regulated although they are the exception. (16)(17) Transient developmentally, increasing dramatically in late gesta- deficiency in association with a mutation on only one tion. Sequences in the 5Ј untranslated region critical for allele also has been recognized. (18) These observations, the developmental and cell-specific regulation of SP-B along with experiments using genetically engineered

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mice in which SP-B production can be shut off, indicate are involved in lipid transport, and mutations in ABC that a critical level of SP-B expression is needed for genes are frequent causes of human genetic diseases. For normal lung function. (19) Thus, individuals whose abil- example, ABCA1 transports cholesterol, and its gene is ity to produce SP-B is impaired due to genetic mecha- mutated in Tangier disease; ABCA4 transports phos- nisms, either because of a loss-of-function mutation on phatidylethanolamine, and its gene is mutated in a form one allele or more common variants associated with of retinal degeneration; and ABCC7 is also known as decreased SP-B gene expression, may be at higher risk for CFTR, the gene mutated in cystic fibrosis. ABCA3 defi- lung disease if other factors, such as prematurity or ciency now has been recognized as the cause of lung inflammation, further limit their SP-B production. SP-B disease in both newborns who have an RDS-like pheno- is, therefore, a reasonable candidate gene for susceptibil- type and older children and adults who have forms of ity to neonatal RDS as well as lung injury later in life. interstitial lung disease (ILD). (24)(25)(26) Surfactant isolated from the lungs of SP-B-deficient ABCA3 is located on chromosome 16, spans more infants is abnormal in composition and is ineffective in than 60,000 bases, and contains 30 coding exons. More lowering surface tension. (20) Incomplete processing of than 90 different mutations have been reported in SP-C from its precursor protein to the mature form ABCA3, distributed among all of the coding exons. results in the secretion of large amounts of partially Most mutations are “private” and unique to a given processed SP-C intermediates with hydrophilic epitopes. family, and the degree of allelic heterogeneity has impor- (11) Accordingly, surfactant from SP-B-deficient infants tant implications for diagnostic genetic testing because is lacking in mature SP-C, and because the partially sequencing of all coding exons is needed to identify processed intermediates are not very surface-active, they affected patients. ABCA3 deficiency is an autosomal re- also inhibit its function. (21) Phospholipid composition cessive disorder. However, patients have been identified also is altered, with decreased amounts of PC and even who have a consistent phenotype and typical lung histo- more so of phosphatidylglycerol (PG). (20) Such sec- pathology findings but only one identifiable mutation. ondary alterations may be related to changes within the (25)(26)(27) Such patients may have functionally signif- SP-B-deficient type II cell, which lacks normal lamellar icant mutations in untranslated regions that were not bodies and contains disorganized, poorly lamellated ve- examined or could have major deletions, insertions, or sicular inclusions. (22) The lack of normally formed rearrangements of ABCA3 that would not be detected lamellar bodies may account for the incomplete process- by usual approaches. Therefore, the sensitivity of genetic ing of SP-C as the final steps occur in lamellar bodies. testing is less than 100% for all ABCA3 mutations. These observations indicate a fundamental intracellular Neonates who have ABCA3 deficiency may present role for SP-B (or proSP-B) in lamellar body biogenesis identically as those who have SP-B deficiency, exhibiting and surfactant packaging and at least partly explain the signs and symptoms of respiratory distress, pulmonary refractory nature to therapy. Incompletely processed hypertension, diffuse infiltrates radiographically, rapid SP-C may be a useful biomarker for diagnosis, but the progression to hypoxemic respiratory failure, and death sensitivity and specificity of such testing are currently despite maximal medical therapy. Although initial studies unknown, and analyses are confined to research labora- focused on identifying ABCA3 mutations in newborns tories. who had very severe disease, more recent experience indicates that the course of infants who have ABCA3 Genetic Lung Disease Due to ATP-binding deficiency is much more variable than that seen with Cassette Protein Member 3 those who have SP-B deficiency. Affected infants who (ABCA3) Deficiency have severe initial lung disease may show gradual im- ABCA3 is a member of the ABC family of transporters, provement and can be discharged from the hospital, transmembrane proteins that hydrolyze adenosine although symptoms and signs of lung disease usually triphosphate to move substrates across biologic mem- persist. Some affected infants have very mild or even no branes. The protein is comprised of 1,704 amino acids lung disease as neonates, presenting later with nonspe- and contains 12 membrane-spanning domains and two cific findings, including failure to thrive and digital club- nucleotide-binding domains. ABCA3 mRNA is ex- bing along with pulmonary signs and symptoms and a pressed in many tissues, including brain and kidney. In diagnosis of ILD. The finding of the same mutation, a the lungs, it is expressed primarily in type II cells, with substitution of valine for glutamic acid in codon 292 the protein localized to the limiting membranes of lamel- (E292V), in multiple unrelated older patients who have lar bodies. (23) Other members of the ABCA subfamily ILD suggests a genotype-phenotype correlation. (25) e460 NeoReviews Vol.9 No.10 October 2008 Downloaded from http://neoreviews.aappublications.org by J Michael Coleman on August 19, 2010 respiratory disorders inherited surfactant disorders

Children who have milder forms of ABCA3 deficiency tion has accounted for fewer than 10% of the mutant may have mutations that allow for some ABCA3 func- ABCA3 alleles identified to date, the contribution of tion, although this hypothesis has yet to be tested for- ABCA3 sequence variants to lung disease may be sub- mally. Disease results from loss-of-function mutations stantial. Interestingly, the E292V mutation was enriched leading to absent or decreased ABCA3 expression, ab- in a cohort of relatively mature (Ն28 weeks’ gestation) normal trafficking of ABCA3 within the type II cell, or infants who had RDS compared with controls, and in a decreased functional activity of ABCA3. (24)(28)(29) different study, a specific ABCA3 haplotype was associ- Because many affected children have mutations other ated with RDS risk. (33)(34) These findings support the than those that would be predicted to preclude ABCA3 hypothesis that ABCA3 variants may influence the risk expression completely, therapies that improve trafficking for RDS in preterm infants. of misfolded proteins through the endoplasmic reticu- lum or augment residual ABCA3 function may be of Genetic Lung Disease Due to SP-C Gene benefit. Glucocorticoids have been shown to increase (SFTPC) Mutations ABCA3 expression in vitro, (30) although clinical studies Respiratory symptoms due to mutations in SFTPC usu- assessing their efficacy for this disorder have not been ally do not present in neonates. Affected infants are less performed. likely to be encountered by neonatologists than those The precise role of ABCA3 in the lung and in surfac- who have SFTPB or ABCA3 mutations. The first re- tant metabolism is unknown. Because ABCA proteins ported patient who had an SFTPC mutation was evalu- transport lipids, ABCA3 may act by transporting lipids ated at birth due to a family history of ILD, but was not essential for surfactant function, such as DSPC, into recognized to be symptomatic until 6 weeks of age. (35) lamellar bodies. Analysis of lung fluid obtained at the Mice engineered to be SP-C-deficient do not exhibit time of lung transplantation in infants subsequently lung disease at birth, suggesting that SP-C is not critical demonstrated to be ABCA3-deficient revealed marked for perinatal adaptation, although SP-C-deficient mice decreases in both DSPC and PG, consistent with this do develop lung disease with increasing age in a strain- hypothesis. (31) The surface tension-lowering ability of dependent manner. (36) However, the presentation and the surfactant material isolated from ABCA3-deficient natural history of patients who have SFTPC mutations is children was markedly reduced compared with material highly variable and includes newborns who have severe, obtained from control infants transplanted for pulmo- rapidly evolving respiratory distress as well as adults who nary vascular disease or SP-B deficiency, which supports a experience the onset of ILD in the fifth or sixth decade. lack of functional surfactant as important in the patho- (37) The highly variable natural history of the disorder physiology of the lung disease resulting from ABCA3 complicates interpretation of single-patient studies of deficiency. (31) Although not a uniform finding, re- therapeutic interventions, such as therapeutic whole lung duced amounts of SP-B and impaired processing of lavage and hydroxychloroquine. (38)(39) Lung trans- proSP-B to mature SP-B have been observed in associa- plantation may be considered in cases of severe progres- tion with ABCA3 deficiency, both in human infants and sive disease. in a mouse model, which may aggravate surfactant defi- SP-C is encoded by a single small gene containing 6 ciency further. (24)(25)(32) A role for ABCA3 in lamel- exons located on the short arm of chromosome 8. The lar body biogenesis is supported by electron microscopic primary translation product, proSP-C, undergoes exten- findings in affected infants. Normal lamellar bodies are sive proteolysis to form the 35-amino acid hydrophobic absent or markedly reduced in number; instead, small, mature SP-C molecule that is encoded in exon 2 of the dense bodies that have eccentrically placed very electron- gene. Most mutations identified to date are located in the dense cores (giving a “fried-egg” appearance) are found region of the gene encoding the carboxyterminal portion in the cytoplasm of type II cells. (22)(24)(26) of proSP-C and only rarely involve the mature peptide An estimate of the incidence of ABCA3 deficiency is domain. A substitution of threonine for isoleucine in not available at this time. Given the relative frequency codon 73 (I73T) is the most commonly found mutation. with which this condition has been recognized since its (38)(40) In contrast to SP-B and ABCA3 deficiencies, a initial description compared with SP-B deficiency, it is mutation on one allele can result in lung disease, and probably the most common genetic cause of surfactant familial cases exhibit an autosomal dominant pattern of deficiency. The population frequency of the ABCA3 inheritance. Sporadic disease also may occur as the result E292V mutation recently was estimated at 1 in 275 of apparent spontaneous de novo mutations in the SP-C individuals in two populations. (33) Because this muta- gene. (38)(40)

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dicted by the specific SFTPC muta- tion. Relatives of affected individu- als may carry the same mutation and remain asymptomatic. ABCA3 may act as a disease-modifying gene in this disorder, as indicated by in- fants heterozygous for both an ABCA3 gene mutation and the Figure 1. Histopathology of neonatal lung. A. Normal neonatal lung histology with thin SFTPC I73T mutation having alveolar septa and airspaces devoid of debris. B. Histopathology of the lung from a child who had fatal SP-B deficiency demonstrates thickened alveolar septa and eosinophilic, more severe lung disease than fam- lipoproteinaceous material intermixed with large foamy alveolar macrophages (arrow) ily members who had the I73T mu- filling the alveolar spaces. C. Histopathology of the lung from a child who had fatal tation and were not carriers for an ABCA3 deficiency is similar to that for fatal SP-B deficiency, as shown in panel B. Original ABCA3 mutation. (45) .magnification is ؋10 Pathology Lung histopathology findings in The pathophysiology of lung disease due to SFTPC patients who have SFTPB, ABCA3, and SFTPC muta- mutations is complex and incompletely understood. tions may include nonspecific changes of lung injury SFTPC mutations are believed to result in misfolded due to prolonged mechanical ventilation and exposure proSP-C, which can elicit the unfolded protein response to hyperoxia. More characteristic findings include and accumulate in the endoplasmic reticulum of the type marked type II cell hyperplasia, foamy macrophages in the II cells, resulting in cellular stress and apoptosis in a toxic airspaces, and accumulation of granular, proteinaceous ma- gain-of-function mechanism. (41)(42)(43) Alterna- terial in the distal airspaces (Fig. 1). (9)(11)(26)(38)(46) tively, misfolded proSP-C may be targeted for degrada- The latter finding is similar to that observed in adult patients tion, and as proSP-C self-aggregates within the secretory who have alveolar proteinosis, a disorder caused by accu- pathway, causes wild type proSP-C to be degraded as mulation of surfactant due to impaired macrophage func- well, resulting in SP-C deficiency by a dominant negative tion as a result of autoantibodies directed against mechanism. (41)(42)(43)(44) Whether the specific mu- granulocyte-macrophage colony-stimulating factor. (47) tation determines which mechanism occurs in a given Although the term “congenital alveolar proteinosis” has patient or each can occur at different times in the same been used to characterize these disorders, this terminology individual, depending on the degree of SP-C gene ex- is potentially misleading. The amount of proteinosis mate- pression and other genetic and environmental factors, is rial observed varies and may even be absent, proteinosis is unknown. The severity of lung disease cannot be pre- not specific for the underlying gene disorder, and the mech- anisms for proteinosis differ from those in adults. The term “surfactant dysfunction” has been used to en- compass all three disorders related to genetic mechanisms disrupting sur- factant function, which cannot be distinguished from one another on the basis of routine histopathology alone. (48) Specific immunohisto- chemical stains may provide addi- tional information but currently are Electron microscopy of lamellar bodies in human alveolar type II cells. A. Figure 2. available primarily on a research ba- Well-developed, mature lamellar bodies (arrows) are found in alveolar type II cells from a sis. (11)(24)(25) Electron micro- normal lung. B. Large, disorganized, multivesicular bodies (arrows) are observed in alveolar type II cells from the lung of a child who had SP-B deficiency. C. Small, dense bodies that scopic findings (Fig. 2) can distin- have eccentrically placed, electron-dense inclusions, and tightly packed phospholipid guish between SP-B and ABCA3 lamellae (arrows) are observed in the alveolar type II cells from the lung of a child who deficiency and may be diagnostic, -has ABCA3 deficiency. For panels A and B, original magnification is ؋5,000; for panel C, and such studies should be per original magnification is ؋10,000. formed on lung tissue obtained by e462 NeoReviews Vol.9 No.10 October 2008 Downloaded from http://neoreviews.aappublications.org by J Michael Coleman on August 19, 2010 respiratory disorders inherited surfactant disorders

clude those encoding SP-A and SP-D and the enzymes needed to process proSP-B and proSP-C to their mature forms. Mice engi- neered to lack expression of SP-D develop lipid accumulations and emphysema. (50) These changes were observed starting at about 3 weeks after birth and progressed with age, suggesting that human dis- ease due to SP-D deficiency may be unlikely to present in the neonatal period. Napsin A and cathepsin H are enzymes expressed in type II pneumocytes that have potential roles in the processing of SP-B. (51)(52) Mutations in the genes encoding these proteins that dis- rupt their function might alter sur- Figure 3. Algorithm for genetic testing in term infants who have respiratory failure of unknown cause and a suspected inherited surfactant disorder as the cause. In babies of factant composition and lead to Northern European descent, screening first for the SFTPB 121ins2 mutation may allow for dysfunction and lung disease in the rapid diagnosis. If such test results are negative or the child is of a different ethnic newborn period. background, sequencing studies are needed. SFTPC should be evaluated in children who have a consistent phenotype in whom studies for SFTPB and ABCA3 are negative or in Diagnostic Evaluation -heterozygous. It is not possible to distinguish ge؍-/homozygous; ؉؍whom only one ABCA3 mutation is identified. ؉/؉ surfactant metabolism dysfunction, pulmonary. netic causes of surfactant deficiency؍SMDP from treatable or transient causes of biopsy or autopsy from infants suspected of having one of neonatal respiratory disease such as RDS, retained fetal these disorders. (22)(24)(26) lung liquid syndrome, or pneumonia purely on clinical or radiographic grounds. A lack of risk factors for RDS or Other Genetic Causes of Surfactant infection in a child who has diffuse parenchymal lung Dysfunction disease should raise suspicion for an inherited surfactant Haploinsufficiency for the gene encoding TTF-1, either disorder, and a family history of severe neonatal lung as a result of gene deletion or loss-of-function mutations disease should prompt evaluation. Disease due to SP-B on one allele, can cause a syndrome whose manifestations deficiency is relentlessly progressive; disease due to include neonatal respiratory distress, recurrent pulmo- ABCA3 deficiency may improve, but the symptoms usu- nary infection, hypothyroidism, and neurologic symp- ally do not resolve and radiographs do not clear com- toms. (49) Because TTF-1 is an important regulator of pletely. Newborns who have SFTPC mutations may SP-A, SP-B, and SP-C expression, neonatal lung disease show substantial improvement either with treatment or could result from delayed expression of SP-B and SP-C as a result of the natural history of the disease. In general, and chronic lung disease from reduced expression of all the longer that respiratory symptoms and diffuse radio- three proteins. However, neurologic disease (benign fa- graphic changes persist without a cause established, the milial chorea) in the absence of any pulmonary disease higher the likelihood of an inherited surfactant disorder. also has been observed in association with TTF-1 haplo- Analysis of lung fluid samples for surfactant compo- insufficiency. nents currently is only available in research laboratories. Children who have a phenotype consistent with an The sensitivity and specificity of such testing is unknown, inherited surfactant disorder but in whom no causative both for distinguishing inherited surfactant disorders mutations in SFTPB, ABCA3, or SFTPC were found from other diseases and for indicating which gene is have been reported, suggesting that mutations in other responsible. Genetic analyses provide the most definitive genes in the surfactant metabolic pathway can result in a means for establishing a specific diagnosis, and sequence similar phenotype. (26)(27)(48) Candidate genes in- analysis of all of the coding exons of SFTPB, ABCA3, and

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Table. Inherited Surfactant Deficiency Disorders

OMIM SMDP1 #265120 SMDP2 #610913 SMDP3 #610921 Locus SFTPB SFTPC ABCA3 Chromosomal location 2p12-p11.2 8p21 16p13.3 Protein SP-B SP-C ABCA3 Inheritance Autosomal recessive Autosomal dominant or sporadic Autosomal recessive Mechanism Loss-of-function Dominant negative or toxic gain-of-function Loss-of-function Age of onset Neonatal Infancy to adult > Neonatal Neonatal > Childhood Clinical Syndrome RDS ILD > RDS RDS or ILD Outcome Fatal without transplant Variable Severe variable

ILDϭinterstitial lung disease, OMIMϭOnline Mendelian Inheritance in Man, www.ncbi.nlm.nih.gov/sites/entrez?dbϭOMIM, RDSϭrespiratory distress syndrome, SMDPϭsurfactant metabolism dysfunction, pulmonary

SFTPC is available through certified diagnostic laborato- logic studies, tissue should be prepared for electron ries in the United Staters and Europe. A positive result microscopy. may obviate the need for lung biopsy. Limitations to genetic testing include cost and long turnaround times in Summary critically ill or rapidly deteriorating infants for whom a Inherited surfactant disorders are rare but important timely diagnosis is needed for management decisions. causes of neonatal mortality and morbidity. Considerable The interpretation of genetic studies also may be diffi- clinical overlap exists among the three known single- cult, particularly when rare missense variants not previ- gene disorders, with the spectrum of disease ranging ously known to be associated with lung disease are found, from severe neonatal respiratory failure to chronic respi- or if an SFTPB or ABCA3 mutation is identified on only ratory symptoms in older children and adults; salient one allele. features are summarized in the Table. These disorders Although genetic testing is not inexpensive (currently have provided insights into normal surfactant metabo- approximately United States $4,000 for all three genes), lism and indicated that these genes may be involved in its costs, both in dollars as well as risks to the patient, more common lung diseases, such as RDS and broncho- should be kept in perspective relative to those of per- pulmonary dysplasia. Mutations in genes encoding other forming a lung biopsy and of prolonged intensive care. proteins important in surfactant function and metabo- A sequential approach to genetic studies may be reason- lism may result in similar disease pictures. able, screening first for the SFTPB 121ins2 mutation in babies of Northern European descent, given its preva- ACKNOWLEDGMENTS. The authors wish to thank lence in this subpopulation, followed by sequence anal- their collaborators, including Janine Bullard, MD ysis for ABCA3 and SFTPC mutations (Fig. 3). If results (JHU); Timothy Weaver, PhD, and Jeffrey Whitsett, of such studies are negative and the index of suspicion MD (Cincinnati Children’s Hospital); Aaron Hamvas, remains high for a surfactant metabolic disorder, studies MD, and F. Sessions Cole, MD (Washington Univer- for SFTPC mutations can be obtained. Children who sity); and Michael Dean, PhD (NCI-Frederick). present with lung disease after the neonatal period should be evaluated initially for SFTPC mutations and subsequently for ABCA3 mutations if initial study results References are unrevealing. SP-B deficiency is very unlikely in chil- 1. Kuroki Y, Takahashi M, Nishitani C. Pulmonary collectins in innate immunity of the lung. Cell Microbiol. 2007;9:1871–1879 dren presenting beyond the neonatal period, and testing 2. Weaver TE. Synthesis, processing and secretion of surfactant for SFTPB mutations is unlikely to be revealing and not proteins B and C. Biochim Biophys Acta. 1998;1408:173–179 cost-effective in older children, with very rare exceptions. 3. Weaver TE, Conkright JJ. Function of surfactant proteins B and Finally, in cases where results of genetic studies are C. Annu Rev Physiol. 2001;63:555–578 unrevealing or ambiguous or when a more immediate 4. Bohinski RJ, Di Lauro R, Whitsett JA. The lung-specific surfac- tant protein B gene promoter is a target for thyroid transcription diagnosis is needed, lung biopsy may be indicated. factor 1 and hepatocyte nuclear factor 3, indicating common factors Guidelines for the handling and processing of such tissue for organ-specific gene expression along the foregut axis. Mol Cell have been published. (53) In addition to routine histo- Biol. 1994;14:5671–5681 e464 NeoReviews Vol.9 No.10 October 2008 Downloaded from http://neoreviews.aappublications.org by J Michael Coleman on August 19, 2010 respiratory disorders inherited surfactant disorders

5. Steagall WK, Lin JP, Moss J. The C/A(-18) polymorphism in of LBM180, a lamellar body limiting membrane protein of alveolar the surfactant protein B gene influences transcription and protein type II cells, as the ABC transporter protein ABCA3. J Biol Chem. levels of surfactant protein B. Am J Physiol Lung Cell Mol Physiol. 2002;277:22147–22155 2007;292:L448–L453 24. Brasch F, Schimanski S, Muhlfeld C, et al. Alteration of the 6. Wang G, Christensen ND, Wigdahl B, Guttentag SH, Floros J. pulmonary surfactant system in full-term infants with hereditary Differences in N-linked glycosylation between human surfactant ABCA3 deficiency. Am J Respir Crit Care Med. 2006;174: protein-B variants of the C or T allele at the single-nucleotide 571–580 polymorphism at position 1580: implications for disease. Biochem J. 25. Bullard JE, Wert SE, Whitsett JA, Dean M, Nogee LM. 2003;369:179–184 ABCA3 mutations associated with pediatric interstitial lung disease. 7. Hallman M, Haataja R. Genetic basis of respiratory distress Am J Respir Crit Care Med. 2005;172:1026–1031 syndrome. Front Biosci. 2007;12:2670–2682 26. Shulenin S, Nogee LM, Annilo T, Wert SE, Whitsett JA, Dean 8. Hamvas A, Wegner DJ, Trusgnich MA, et al. Genetic variant M. ABCA3 gene mutations in newborns with fatal surfactant defi- characterization in intron 4 of the surfactant protein B gene. Hum ciency. N Engl J Med. 2004;350:1296–1303 Mutat. 2005;26:494–495 27. Somaschini M, Nogee LM, Sassi I, et al. Unexplained neonatal 9. Nogee LM, de Mello DE, Dehner LP, Colten HR. Brief report: respiratory distress due to congenital surfactant deficiency. J Pedi- deficiency of pulmonary surfactant protein B in congenital alveolar atr. 2007;150:649–653 proteinosis. N Engl J Med. 1993;328:406–410 28. Cheong N, Madesh M, Gonzales LW, et al. Functional and 10. Nogee LM. Alterations in SP-B and SP-C expression in neo- trafficking defects in ABCA3 mutants associated with respiratory natal lung disease. Annu Rev Physiol. 2004;66:601–623 distress syndrome. J Biol Chem. 2006;281:9791–9800 11. Nogee LM, Wert SE, Proffit SA, Hull WM, Whitsett JA. Allelic 29. Matsumura Y, Ban N, Ueda K, Inagaki N. Characterization heterogeneity in hereditary surfactant protein B (SP-B) deficiency. and classification of ATP-binding cassette transporter ABCA3 mu- Am J Respir Crit Care Med. 2000;161:973–981 tants in fatal surfactant deficiency. J Biol Chem. 2006;281: 12. Hamvas A, Trusgnich M, Brice H, et al. Population-based 34503–34514 screening for rare mutations: high-throughput DNA extraction and 30. Yoshida I, Ban N, Inagaki N. Expression of ABCA3, a causative molecular amplification from Guthrie cards. Pediatr Res. 2001;50: gene for fatal surfactant deficiency, is up-regulated by glucocorti- 666–668 13. Tredano M, Cooper DN, Stuhrmann M, et al. Origin of the coids in lung alveolar type II cells. Biochem Biophys Res Commun. prevalent SFTPB indel g. 1549C Ͼ GAA (121ins2) mutation 2004;323:547–555 causing surfactant protein B (SP-B) deficiency. Am J Med Genet A. 31. Garmany TH, Moxley MA, White FV, et al. Surfactant com- 2006;140:62–69 position and function in patients with ABCA3 mutations. Pediatr 14. Tredano M, Griese M, de Blic J, et al. Analysis of 40 sporadic or Res. 2006;59:801–805 familial neonatal and pediatric cases with severe unexplained respi- 32. Cheong N, Zhang H, Madesh M, et al. ABCA3 is critical for ratory distress: relationship to SFTPB. Am J Med Genet A. 2003; lamellar body biogenesis in vivo. J Biol Chem. 2007;282: 119:324–339 23811–23817 15. Wegner DJ, Hertzberg T, Heins HB, et al. A major deletion in 33. Garmany TH, Wambach JA, Heins HB, et al. Population and the surfactant protein-B gene causing lethal respiratory distress. disease-based prevalence of the common mutations associated with Acta Paediatr. 2007;96:516–520 surfactant deficiency. Pediatr Res. 2008;63:645–649 16. Ballard PL, Nogee LM, Beers MF, et al. Partial deficiency of 34. Karjalainen MK, Haataja R, Hallman M. Haplotype analysis of surfactant protein B in an infant with chronic lung disease. Pediat- ABCA3: association with respiratory distress in very premature rics. 1995;96:1046–2052 infants. Ann Med. 2008;40:56–65 17. Dunbar AE 3rd, Wert SE, Ikegami M, et al. Prolonged survival 35. Nogee LM, Dunbar AE 3rd, Wert SE, Askin F, Hamvas A, in hereditary surfactant protein B (SP-B) deficiency associated with Whitsett JA. A mutation in the surfactant protein C gene associated a novel splicing mutation. Pediatr Res. 2000;48:275–782 with familial interstitial lung disease. N Engl J Med. 2001;344: 18. Klein JM, Thompson MW, Snyder JM, et al. Transient surfac- 573–579 tant protein B deficiency in a term infant with severe respiratory 36. Glasser SW, Detmer EA, Ikegami M, Na CL, Stahlman MT, failure. J Pediatr. 1998;132:244–248 Whitsett JA. Pneumonitis and emphysema in sp-C gene targeted 19. Melton KR, Nesslein LL, Ikegami M, et al. SP-B deficiency mice. J Biol Chem. 2003;278:14291–14298 causes respiratory failure in adult mice. Am J Physiol Lung Cell Mol 37. Nogee LM. Genetics of pediatric interstitial lung disease. Curr Physiol. 2003;285:L543–L549 Opin Pediatr. 2006;18:287–292 20. Beers MF, Hamvas A, Moxley MA, et al. Pulmonary surfactant 38. Brasch F, Griese M, Tredano M, et al. Interstitial lung disease metabolism in infants lacking surfactant protein B. Am J Respir Cell in a baby with a de novo mutation in the SFTPC gene. Eur Respir Mol Biol. 2000;22:380–391 J. 2004;24:30–39 21. Li J, Ikegami M, Na CL, et al. N-terminally extended surfac- 39. Rosen DM, Waltz DA. Hydroxychloroquine and surfactant tant protein (SP) C isolated from SP-B-deficient children has re- protein C deficiency. N Engl J Med. 2005;352:207–208 duced surface activity and inhibited lipopolysaccharide binding. 40. Cameron HS, Somaschini M, Carrera P, et al. A common Biochemistry. 2004;43:3891–3898 mutation in the surfactant protein C gene associated with lung 22. Edwards V, Cutz E, Viero S, Moore AM, Nogee L. Ultrastruc- disease. J Pediatr. 2005;146:370–375 ture of lamellar bodies in congenital surfactant deficiency. Ultra- 41. Beers MF, Mulugeta S. Surfactant protein C biosynthesis and struct Pathol. 2005;29:503–509 its emerging role in conformational lung disease. Annu Rev Physiol. 23. Mulugeta S, Gray JM, Notarfrancesco KL, et al. Identification 2005;67:663–696

NeoReviews Vol.9 No.10 October 2008 e465 Downloaded from http://neoreviews.aappublications.org by J Michael Coleman on August 19, 2010 respiratory disorders inherited surfactant disorders

42. Bridges JP, Wert SE, Nogee LM, Weaver TE. Expression of a disease in young children: application of a novel classification human surfactant protein C mutation associated with interstitial scheme. Am J Respir Crit Care Med. 2007;176:1120–1128 lung disease disrupts lung development in transgenic mice. J Biol 49. Pohlenz J, Dumitrescu A, Zundel D, et al. Partial deficiency Chem. 2003;278:52739–52746 of thyroid transcription factor 1 produces predominantly neuro- 43. Mulugeta S, Nguyen V, Russo SJ, Muniswamy M, Beers MF. logical defects in humans and mice. J Clin Invest. 2002;109: A surfactant protein C precursor protein BRICHOS domain mutation 469–473 causes endoplasmic reticulum stress, proteasome dysfunction, and 50. Yoshida M, Whitsett JA. Alveolar macrophages and emphy- caspase 3 activation. Am J Respir Cell Mol Biol. 2005;32:521–530 sema in surfactant protein-D-deficient mice. Respirology. 2006; 44. Wang WJ, Mulugeta S, Russo SJ, Beers MF. Deletion of exon 11(suppl):S37–S40 4 from human surfactant protein C results in aggresome formation 51. Brasch F, Ochs M, Kahne T, et al. Involvement of napsin A in and generation of a dominant negative. J Cell Sci. 2003;116: the C- and N-terminal processing of surfactant protein B in type-II 683–692 pneumocytes of the human lung. J Biol Chem. 2003;278: 45. Bullard JE, Nogee LM. Heterozygosity for ABCA3 mutations modifies the severity of lung disease associated with a surfactant 49006–49014 protein C gene (SFTPC) mutation. Pediatr Res. 2007;62:176–179 52. Ueno T, Linder S, Na CL, Rice WR, Johansson J, Weaver TE. 46. Tredano M, Griese M, Brasch F, et al. Mutation of SFTPC in Processing of pulmonary surfactant protein B by napsin and cathep- infantile pulmonary alveolar proteinosis with or without fibrosing sin H. J Biol Chem. 2004;279:16178–16184 lung disease. Am J Med Genet A. 2004;126:18–26 53. Langston C, Patterson K, Dishop MK, et al. A protocol for the 47. Trapnell BC, Whitsett JA, Nakata K. Pulmonary alveolar pro- handling of tissue obtained by operative lung biopsy: recommen- teinosis. N Engl J Med. 2003;349:2527–2539 dations of the chILD pathology co-operative group. Pediatr Dev 48. Deutsch GH, Young LR, Deterding RR, et al. Diffuse lung Pathol. 2006;9:173–180

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NeoReviews Quiz

5. Pulmonary surfactant is a mixture of lipids and specific proteins that reduces surface tension at the air- liquid interface and prevents end-expiratory atelectasis of the lung. Of the following, the principal constituent of surfactant needed for lowering surface tension is: A. Cholesterol. B. Phosphatidylcholine. C. Phosphatidylethanolamine. D. Phosphatidylglycerol. E. Sphingomyelin.

6. Surfactant protein B (SP-B) is a low-molecular weight, hydrophobic protein that facilitates the adsorption of surfactant lipids to the air-liquid interface after secretion, essential for surface tension-lowering properties of the surfactant. Genetic lung disease from SP-B deficiency is a rare, autosomal recessive disorder that results in progressive hypoxemic respiratory failure and death by 3 to 6 months of age. Of the following, the most common genetic mutation related to the SP-B gene in infants of Northern European descent involves: A. Frameshift mutation (121ins2). B. Nucleotide tandem repeats in intron 4. C. Polymorphism in exon 4. D. Polymorphism in promoter region. E. Thyroid transcription factor-1.

7. Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are transmembrane proteins that hydrolyze ATP to move substrates across biologic membranes. Mutations in ABC genes are frequent causes of human genetic diseases. Of the following, the deficiency of ABCA3 protein is most likely to cause: A. Cystic fibrosis. B. Hypothyroidism. C. Interstitial lung disease. D. Retinal degeneration. E. Tangier disease.

8. Surfactant protein C (SP-C) is a low-molecular weight, hydrophobic protein, derived by proteolysis of its precursor, proSP-C, the primary translational product of a gene located on chromosome 8. SP-C gene mutations can cause genetic lung disease. Of the following, the most accurate statement regarding genetic lung disease from SP-C gene mutations is that: A. Lung histopathology reveals hypoplasia of type II alveolar epithelial cells. B. Most familial cases exhibit an autosomal recessive pattern of inheritance. C. Most mutations occur in the carboxy-terminal portion of proSP-C. D. Respiratory symptoms typically manifest in the perinatal period. E. Severity of lung disease can be predicted by specific SP-C gene mutations.

NeoReviews Vol.9 No.10 October 2008 e467 Downloaded from http://neoreviews.aappublications.org by J Michael Coleman on August 19, 2010 Inherited Surfactant Disorders W. Adam Gower, Susan E. Wert and Lawrence M. Nogee NeoReviews 2008;9;e458-e467 DOI: 10.1542/neo.9-10-e458

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