Surfactant Biology and Pulmonary Fibrosis: A ChILD Shall Lead Them
Lawrence M. Nogee, M.D.
Professor of Pediatrics
Division of Neonatology
Department of Pediatrics
Johns Hopkins University School of Medicine
Baltimore, MD USA 21287
Contact information:
Mail: CMSC 6-104A
600 N. Wolfe Street
Baltimore, MD 21287 USA
Phone: 410 614-3355
Fax: 410 614-8388
E-mail:lnogee@jhmi.edu
Word count: 1751
Pulmonary surfactant is the mixture of lipids and proteins needed to reduce surface
tension in the alveolus and prevent end-expiratory atelectasis. Surfactant deficiency as a result of
pulmonary immaturity is the principal cause of the respiratory distress syndrome (RDS) observed
in prematurely born infants, and surfactant inactivation contributes to the pathophysiology of
acute RDS observed in older children and adults. Genetic mechanisms disrupting the normal
production and metabolism of surfactant components have been recognized as the cause for
some forms of children’s interstitial lung disease (ChILD) previously considered to be idiopathic
1
. While these diseases are rare, they are associated with considerable morbidity and mortality.
Understanding for the basis for these disorders also has implications for lung disease in older
children and adults. Surfactant components are synthesized, stored and secreted by alveolar type
II epithelial cells (AEC2s), and genetic mechanisms affecting the expression of surfactant
components may not only lead to functional surfactant deficiency, but serve as a source of
endogenous injury to AEC2s. These observations, along with studies on genetic causes of
pulmonary fibrosis in adults, support the hypothesis that chronic injury to AEC2s is an important
mechanism for pulmonary fibrosis across all age groups.
Surfactant is comprised of approximately 90% lipid by weight and 10% protein. Two low
molecular-weight, extremely hydrophobic proteins, Surfactant Proteins (SP-) B and C are
critically important in order to enhance the surface-tension lowering properties of surfactant
lipids. Both SP-B and SP-C are components of the mammalian-derived exogenous surfactant
preparations used to treat premature neonates with RDS. SP-B and SP-C are encoded on separate
genes, termed SFTPB and SFTPC respectively. Both are synthesized as larger precursor proteins
(proSP-B, proSP-C) that need to be proteolytically processed to the mature SP-B and SP-C
proteins found in surfactant. Surfactant also contains two larger, structurally related
glycoproteins, SP-A and SP-D. These proteins are part of the collectin family, containing
collagenous and lectin-like domains, and have important roles in innate immunity in the lung, as
well as roles in immunomodulation and surfactant metabolism. There are two genes for SP-A
(SFTPA1, SFTPA2) and a single gene for SP-D (SFTPD). Surfactant lipids and proteins are
packaged within the AEC2 in lysosomally-derived organelles called lamellar bodies, and
secreted by exocytosis of the lamellar bodies. A member of the ATP Binding Cassette (ABC)
family of transporter proteins, ABCA3, is highly expressed in AEC2s and localized to the
limiting membrane of lamellar bodies. ABC proteins hydrolyze ATP to transport substances
across biological membranes, and ABCA3 has a role in the transport of lipids critical for
surfactant function into the lamellar body. The expression of surfactant lipids and proteins is
developmentally regulated, with their expression increasing with advancing gestation. The
transcription factor, NKX2.1, also known as thyroid transcription factor 1 (TTF1), has an
essential role in the expression of the surfactant proteins and ABCA3.
Mutations in the genes encoding surfactant components have now been recognized as the
etiology for lung disease of variable onset and severity, ranging from fatal RDS in full-term
infants to pulmonary fibrosis in adults who may be asymptomatic until the 5th or 6th decade of
life. SP-B deficiency was the first recognized of these disorders, which results from loss-offunction mutations on both SFTPB alleles. Infants affected by this autosomal recessive disorder
generally present with hypoxemic respiratory failure in the immediate neonatal period and
diffuse lung disease that radiographically resembles RDS in premature infants. The disease is
generally progressive, with death usually occurring within three months despite maximal medical
support, and lung transplantation is the only effective therapy. The disorder is exceeding rare,
with an estimated incidence of < 1 in one million live births.
Aside from the lack of SP-B, secondary abnormalities are observed in other surfactant
components which also contribute to the pathophysiology of the disorder, including an inability
to fully process proSP-C to SP-C, and an abnormal surfactant lipid profile. Normally formed
lamellar bodies are not observed in SP-B deficient AEC2s by electron microscopy, which instead
contain large, organelles with vacuoles and poorly formed lamellae. Genetically engineered mice
in which SP-B production is able to be turned off as adults develop lung abnormalities when SPB levels fall to 20% to 30 % of control values, supporting the notion that there is a critical level
of SP-B needed for normal lung function. These findings, combined with the observations that
rare children with mutations resulting in partial deficiency can survive the newborn period and
the overlap of lung pathology findings in SP-B deficient children with those observed in some
forms of ChILD, specifically chronic pneumonitis of infancy and desquamative interstitial
pneumonia, led to the investigation of other components of the surfactant system in these
disorders.
Loss-of-function mutations on both alleles of the gene encoding ABCA3 (ABCA3) cause
diffuse lung disease that is variable in its onset and severity inherited as an autosomal recessive
disorder. Affected infants may present with an RDS-like phenotype similar to that of SP-B
deficient infants and have progressive, fatal disease. Others may have mild, transient neonatal
lung disease, thought to be either mild RDS, transient tachypnea or pneumonia, or have no
pulmonary symptoms in the neonatal period, but present later in life with pulmonary symptoms
along with clubbing, pectus excavatum, and/or failure to thrive 2. Prolonged survival is possible
with affected patients > 30 years of age identified. Such individuals have at least one mutation
predicted to result in some ABCA3 protein, compared to more severely affected infants with
clear-cut complete loss-of-function mutations, suggesting that genotype may be an important
predictor of outcome, with relatively milder phenotypes due to mutations that allow for some
retained ABCA3 function. A lack of normal lamellar bodies in AEC2s from ABCA3 deficient
infants which instead contain small, dense bodies with eccentrically placed electron dense cores
giving them a “fried-egg” appearance supports a role for ABCA3 in lamellar body biogenesis.
These observations coupled with the findings of abnormal phospholipids profiles in
bronchoalveolar lavage fluid obtained from ABCA3 deficient infants undergoing lung
transplantation and in lung tissue of ABCA3 null mice support a role for ABCA3 in transporting
surfactant lipids into lamellar bodies. The observations that some affected children had minimal
or no neonatal lung disease suggests that these children produced sufficient amounts of
surfactant perinatally, and that other factors are important in the pathophysiology of the chronic
lung disease due to ABCA3 deficiency. The incidence and prevalence of ABCA3 deficiency are
unknown, but the relative frequency with which ABCA3 deficient compared to SP-B deficient
infants have been identified suggest that it may be the most common cause of genetic surfactant
dysfunction.
Mutations on one SFTPC allele are sufficient to cause disease, and the mechanism is
thought to be due to a gain of toxic function, with mutations inducing misfolding of proSP-C.
The misfolded protein elicits the unfolded protein response, with secondary downstream events
resulting in direct toxicity to the AEC2, including apoptosis and secondary inflammation.
Sporadic disease may result from de novo mutations, or lung disease may be inherited as an
autosomal dominant trait with variable penetrance. The age of onset of disease is highly variable,
even amongst family members with the same mutation, ranging from presentation in infancy
(tachypnea, hypoxemia in room air, clubbing, failure to thrive) to the onset of dyspnea associated
with pulmonary fibrosis in the 5th or 6th decade of life. Neonatal lung disease due to SFTPC
mutations has been reported, but is unusual. Factors responsible for the variable severity of
disease remain largely unknown, although there may be interactions with mutations in other
surfactant-related genes, such as ABCA3, and viral infections. The incidence and prevalence of
lung disease due to SFTPC mutations are unknown. SFTPC mutations appear to be a rare cause
of idiopathic pulmonary fibrosis in adults, but suggest a role for toxicity to and loss of AEC2s as
possible primary events in the pathogenesis of pulmonary fibrosis.
Mutations resulting in loss-of-function or deletions of one copy (haploinsufficiency) of
the gene for TTF1 can cause lung disease with the phenotypes ranging from neonatal RDS to
ILD in older children and adults. As the protein is also expressed in the thyroid gland and the
central nervous system, affected patients may also exhibit signs of hypothyroidism or
neurological symptoms (Brain-Thyroid-Lung Syndrome) 3. Chorea is the most specific
neurological finding, but young infants may have non-specific findings of hypotonia and
developmental delay. Both isolated hypothyroidism and chorea have been reported due to TTF1
haploinsufficiency. Whether isolated pulmonary disease occurs and the full range of pulmonary
phenotypes due to TTF1 mutations remains to be determined. As TTF1 is important in the
expression of multiple surfactant-related genes in AEC2s, it is unknown if the lung disease
results from reduced expression of primarily one protein or combined effects.
Mutations in the genes encoding the components of telomerase have been recognized as
causes of familial pulmonary fibrosis in adults, and mutations in SFTPA2 were recently reported
in two kindreds with familial pulmonary fibrosis and lung adenocarcinoma 4, 5. Loss of telomere
length leads to death of replicating cells and AEC2s serve as progenitor cells for the lung
epithelium after injury as well as being responsible for surfactant production. Thus the linkage of
both of these genes to adult-onset pulmonary fibrosis further supports the hypothesis that injury
to the AEC2 cell may represent a primary event in the initiation of pulmonary fibrosis, as
exemplified by genetic causes of ChILD. Therapies aimed at improving surfactant homeostasis
and AEC2 function in ChILD patients may therefore have roles in adult lung disease as well.
The identification of genetic causes of ChILD allows for potential non-invasive diagnosis
by genetic testing and such testing is now available in clinical diagnostic laboratories.
Limitations of genetic testing include cost, the time needed to obtain results particularly in a
critically ill or rapidly deteriorating child, and unknown sensitivity of such testing. Current
approaches may miss mutations in untranslated regions as well as large deletions, insertions or
rearrangements, and the interpretation of results may not be straight-forward. In addition there
are likely to be mutations in as yet undiscovered genes that result in similar phenotypes. Correct
and timely genetic diagnosis will allow proper counseling of families regarding recurrence risk,
and potentially eliminate the need for lung biopsy and its associated risks and morbidity.
Currently there are no specific treatments available for ChILD due to genetic surfactant
dysfunction, and only anecdotal reports different therapies which are difficult to evaluate given
the variable natural history of some of the disorders. The evaluation of currently available and
development of novel therapies are important areas for future research.
References:
1.
Wert SE, Whitsett JA, Nogee LM. Genetic disorders of surfactant dysfunction. Pediatr
Dev Pathol 2009;12(4):253-74.
2.
Doan ML, Guillerman RP, Dishop MK, et al. Clinical, radiological and pathological
features of ABCA3 mutations in children. Thorax 2008;63(4):366-73.
3.
Guillot L, Carre A, Szinnai G, et al. NKX2-1 mutations leading to surfactant protein
promoter dysregulation cause interstitial lung disease in "Brain-Lung-Thyroid Syndrome". Hum
Mutat;31(2):E1146-62.
4.
Armanios MY, Chen JJ, Cogan JD, et al. Telomerase mutations in families with
idiopathic pulmonary fibrosis. N Engl J Med 2007;356(13):1317-26.
5.
Wang Y, Kuan PJ, Xing C, et al. Genetic defects in surfactant protein A2 are associated
with pulmonary fibrosis and lung cancer. Am J Hum Genet 2009;84(1):52-9.