Novel homozygous missense mutation in ABCA3 protein leading to severe respiratory distress in term infant
- 1 Neonatology, Madhukar Rainbow Children Hospital, Delhi, India
- 2 Genetics, Sir Ganga Ram Hospital, Delhi, India
- Correspondence to Dr Naveen Parkash Gupta; drgupta.naveen@gmail.com
Abstract
The term baby presented with respiratory distress with X-ray pictures consistent as hyaline membrane disease (HMD). Baby was ventilated and treated with surfactant. Because of the persistence of high ventilation needs with X-ray pictures consistent with HMD with a transient response to surfactant every time, the possibility of an inherited disorder of surfactant metabolism was kept. Whole-exome sequencing revealed a novel homozygous missense mutation in the gene for ATP binding cassette transporter protein A3. The baby died after 100 days of ventilation.
Background
Transient tachypnoea of neonate, surfactant deficiency, meconium aspiration syndrome and congenital pneumonia remains common causes of respiratory distress in a neonate. However, in a term baby with persistent respiratory distress syndrome (RDS) with clinical and X-ray picture suggestive of surfactant deficiency along with little or no response to exogenous surfactant, one should think about inherited disorders of surfactant metabolism or genetic disorders of surfactant dysfunction.1 Mutation in gene for ATP binding cassette transporter protein A3 (ABCA3) protein is the most common inherited disorder of surfactant metabolism.2 We present a case of a novel homozygous missense mutation in the gene for ABCA3 protein leading to hypoxemic respiratory failure in the term neonate.
Case presentation
Late preterm (36 weeks), appropriate for gestational age baby girl was delivered by caesarian section to a primigravida mother. She was weighing 2800 g at the time of birth. The couples were cousins and fourth-degree consanguineous. The pregnancy was followed as per protocol antenatally and the course was uneventful.
The baby cried immediately after birth, with no significant perinatal events. Apgar score was 8/10 and 9/10 at 1 and 5 min, respectively. The baby started having respiratory distress immediately after birth for which she was put on nasal continuous airway pressure (CPAP) and intubated later on. Chest radiograph revealed bilateral hazy lungs with reticulogranular pattern. Echocardiography of heart was suggestive of persistent pulmonary hypertension (right ventricular systolic pressure—40 mm Hg).
The baby was referred to a tertiary care centre at 24 hours of life. She was transported in an incubator with a ventilator (settings during transport were mean airway pressure (MAP) 10 cm H2O and fractional inspired oxygen (FIO2) 0.8). The baby remained haemodynamically stable during transport. At the time of admission, the baby was normothermic, her capillary refill time was 3 s and blood pressure was in the normal range. The baby was put on SLE 6000 ventilator on patient triggered ventialtion (PTV) mode, her initial requirements were MAP 12 cm H2O and FIO2 0.9.
Investigations
Number of chest radiographs were done during her stay as she was on ventilator for 100 days. Three relevant X-rays have been described in figure 1. When inherited disorder of surfactant metabolism was suspected, gene analysis was sent. Lung biopsy was also done and sent for histopathology and electron microscopy pending whole-exome sequencing report. Histopathology of lung biopsy showed diffuse type II cell hyperplasia with foamy macrophages (desquamative pattern) (figure 2).
X-rays of baby. Right most (A) is X-ray on day 1 of life. Centre (B) is X-ray on 3rd day of life revealing pneumomediastinum. Left most (C) is X-ray on 13th day of life showing consistent air bronchogram and low volume lungs.
Histopathology of lung tissue.
Electron microscopy
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Normal ciliary ultrastructure.
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Abnormal appearing lamellar bodies (LBs) in the alveolar epithelial cell cytoplasm, few showing poorly developed lamellae and others with eccentric osmiophilic inclusions.
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Cytosolic phagolysosomal bodies in histiocytic aggregates are seen in the lumina of few alveolar epithelial cells (figure 3).
Electron microscopy of lung tissue.
Both findings of histopathology and electron microscopy findings were pointing towards mutation in the gene encoding for ABCA3 protein.
Whole-exome sequencing revealed a homozygous missense mutation in exon 12 of the ABCA3 gene resulting in an amino acid change of leucine to proline at position 437 (p.Leu437Pro). Both parents were found to be heterozygous for the same mutation.
Differential diagnosis
At the time of admission, because of high ventilation needs and X-ray pictures suggestive of reticulogranular pattern, working diagnosis of respiratory distress syndrome (RDS) in term baby was kept in absence of a history of prolonged leaking of membranes and meconium. When high pressure needs on ventilation persisted with X-ray pictures consistent with RDS on the 14th day of life (DOL), differential diagnosis of inherited disorders of surfactant metabolism was kept and worked on.
Treatment
Given high ventilation needs with X-ray picture of hyaline membrane disease (figure 1A), surfactant was administered. The baby showed a response to surfactant and ventilation settings came down. At 36 hours of admission, the baby had deterioration in form of desaturations and hypotension. Ventilation settings were increased and inotropes were started after giving fluid bolus. Radiograph revealed pneumomediastinum (more on left side) (figure 1B). It was managed conservatively and got resolved after 3 days. The baby was shifted to high frequency ventilation on day 3 of life at the time of deterioration. She continued to remain on high frequency ventilation, whenever we tried to wean from high frequency ventilator (by decreasing mean airway pressures), her FIO2 needs went up. X-ray showed persistent hazy lungs (reticulogranular pattern) (figure 1C).
On D13 of life in view of not able to bring down mean airway pressure on high frequency and X-ray picture consistent with low volume lungs, a repeat dose of surfactant was administered. The baby again showed a response in form of decrease in ventilation needs.
At this time (DOL 13), baby had persistent ventilation needs, was showing transient response to surfactant every time, X-ray picture was consistent with hyaline membrane disease and sepsis workup was normal. So a possibility of inherited disorders of surfactant metabolism was considered. Initially, gene analysis for surfactant protein B (SFTPB) deficiency was sent which came normal on 28th DOL. The baby continued to require high frequency ventilation to maintain oxygenation. On DOL 29, blood sample was sent for whole-exome sequencing to rule out other inherited disorders of surfactant metabolism. Lung biopsy was also done subsequently pending whole-exome sequencing report. The specimen was sent for histopathology and electron microscopy examination.
The baby continued to remain on high frequency ventilation with MAP requirement of 15–18 cm H2O. Her oxygenation index used to be in the range of 18–24. Echocardiography showed tricuspid regurgitation with the pressure difference between right ventricle and atrium of approximately 40 mm Hg. Inhaled nitric oxide was started on 18th DOL at 20 ppm. Since there was no significant improvement in oxygenation, nitric oxide was tapered off and stopped on 23rd DOL. A trial of low-dose dexamethasone was given to facilitate extubation with no effect. The baby also remained on sildenafil for some days but no improvement was noticed.
Findings of histopathology and electron microscopy findings were pointing towards mutation in the gene encoding for ABCA3 protein.
Whole-exome sequencing revealed homozygous missense mutation in exon 12 of ABCA3 gene resulting in an amino acid change of leucine to proline at position 437 (p.Leu437Pro). Both parents were found to be heterozygous for same mutation. Parents were told about the nature of disease and outcome.
The baby started desaturating on D100 of life, had a cardiac arrest and could not be revived.
Outcome and follow-up
The baby died after 100 days of ventilation.
Discussion
Surfactant deficiency is one of the most common causes of RDS in preterm babies. On contrary, inherited disorders of surfactant metabolism or genetic disorders of surfactant dysfunction are single gene disorders due to mutations in genes encoding surfactant proteins which should be suspected in term neonate having unresolving respiratory distress with diffuse lung disease similar to hyaline membrane disease radiographically and little or transient response to administration of exogenous surfactant.1
Genes implicated in surfactant deficiency have been those encoding for SFTPB, SFTPC, ABCA3 protein and NKX2. Surfactant proteins, ABCA3 and NKX2 are involved in surfactant synthesis, packaging and secretion at various levels.3 Surfactant is synthesised in type II alveolar cells where it is stored as lamellar bodies (LBs). LBs are extruded in the lumen of alveoli by exocytosis where they form tubular myelin. Surfactant is recycled and secreted into the alveolar air space in a dynamic process to reduce surface tension in the lungs.3 Disruption in any of the above processes can lead to dysfunctional surfactant leading to severe respiratory distress.
Mutations in SFTPB and SFTPC primarily affect the folding of surfactant leading to accumulation in the golgi bodies and endoplasmic reticulum. Mutations in ABCA3 lead to abnormal LB formation while mutations in NKX-2 lead to abnormal transcription of the surfactant proteins leading to reduced levels.
SFTPB deficiency presents in the neonatal period as severe lung disease which is fatal without lung transplantation. SFTPC presents in infancy or childhood as interstitial lung disease. ABCA3 deficiency can present in the neonatal period or later on depending on the type of mutation. Overviews of these disorders have been described in table 1.
Overview of genetic disorders of surfactant dysfunction
| Type of disorder | SFTPB deficiency | SFTPC deficiency | ABCA3 deficiency |
| Mode of inheritance | Autosomal recessive | Autosomal dominant, de novo | Autosomal recessive |
| Most common mutation | 121ins2 (70%–80%) | E292V | |
| Function of protein | Packaging phospholipids into lamellar bodies Formation of tubular myelin Spreading of surfactant monolayer |
Enhance spreading of surfactant and participate in surfactant catabolism | Transport of lamellar bodies |
| Pulmonary presentations | Respiratory distress syndrome (RDS) in the neonatal period | Childhood Interstitial lung disease (ILD) Adult interstitial lung disease Neonatal RDS |
Neonatal RDS Childhood ILD |
| Course | Neonatal - lethal | Highly variable Survival until sixth decade reported |
Neonatal - lethal Variable severity in childhood |
| Treatment options | Supportive Lung transplant |
Supportive Corticosteroids Azithromycin Hydroxychloroquine Lung transplant |
Supportive Corticosteroids Hydroxychloroquine Lung transplant |
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ABCA3, ATP binding cassette transporter protein A3; SFTPB, surfactant proteins B.
Inherited disorders of surfactant metabolism are rare disorders. SFPTB deficiency has a predicted incidence of <1 in 1 000 000 live birth.4 One in 1000 individuals of northern European descent are carriers of 121ins2 mutation which accounts for approximately 60%–70% of mutant SFTPB alleles identified to date.5 6 The overall carrier frequency in the population for a deleterious ABCA3 mutation has been estimated to be between 1 in 33 and 1 in 70 individuals, predicting a disease incidence of between 1 in ~4400 and 1 in ~20 000.7 E292V is the most common ABCA3 mutation associated with childhood interstitial lung disease.8
Investigations in a suspected case of a genetic disorder of surfactant dysfunction
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Radiographic studies: X-ray pictures usually suggest diffuse lung disease similar to hyaline membrane disease in the neonatal period. High-resolution CT findings are usually inconclusive.
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Genetic testing: Next-generation sequencing panels that allow simultaneous analysis of multiple genes is considered investigation of choice.
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Lung biopsy: If genetic studies are unrevealing or ambiguous or when a more immediate diagnosis is needed, lung biopsy may be indicated. A biopsy specimen will be sent for histopathology and electron microscopy examination.
Approach
Step 1. Rule out other common causes of respiratory distress in term baby like transient tachypnoea of neonate, meconium aspiration syndrome, congenital pneumonia, RDS, pneumothorax and surgical causes.
Step 2. Think of Genetic disorders of surfactant dysfunction when respiratory failure persists in term baby with X-ray picture suggesting of hyaline membrane disease and there is transient or no response to exogenous surfactant administration. Also, look for a history of an affected sibling with similar disorders and consanguinity in parents.
Step3. Investigations
a. Genetic studies: If a baby is of northern European descent, screening first for the SFTPB 121ins2 mutation may allow for rapid diagnosis.
If the neonate is of different ethnic background or test results of SFTPB are negative, whole-exome sequencing is done.
b. Lung biopsy: If genetic studies are unrevealing or ambiguous or when a more immediate diagnosis is needed, lung biopsy may be indicated. A biopsy specimen is subjected to histopathology and electron microscopy examination. Routine histopathology may be suggestive for one of these disorders but do not differentiate one from another. Electron microscopy may distinguish one from another. In ABCA3 and SFPTB mutations, abnormal structure of LBs is found. In ABCA3, LBs appear small and dense with eccentrically placed inclusions. By contrast, LBs in SFPTB mutations appear disorganised and poorly lamellated with vesicular inclusions.
Step 4. Treatment: Supportive treatment in form of nutritional support, high frequency ventilation is needed for most of the babies. Pharmacological therapy includes hydroxychloroquine, azithromycin and corticosteroids with a variable response in SFPTC and ABCA3 deficiency. Lung transplant remains the only definitive treatment. The mortality and morbidity associated with lung transplantation in infants are considerable, with a 5-year survival rate of approximately 50%. One-year and 5-year survival rates post lung transplantation have been 83% and 56% in a single-centre study.9 In the same study, infants underwent lung transplant for SFPTB deficiency whereas children underwent it for SFPTC deficiency. In ABCA3 deficiency, age of lung transplantation depended on the age of presentation.
Outcome: Lung disease from SFPTB deficiency is universally fatal in the first few months without a lung transplant. Prognosis varies according to age of presentation in SFPTC and ABCA3 mutations. Earlier age at presentation carries worse prognosis in ABCA3 mutation as compared with later age at presentation.
Implications for future pregnancies: Disorder being autosomal recessive in nature, there is a 25% risk of recurrence in future pregnancies. Whole-exome sequencing in samples obtained from chorionic villous biopsy and amniocentesis can establish the diagnosis in utero and help parents in taking decisions regarding terminating the pregnancy.
ABCA3 deficiency: ABCA3 deficiency is the most common cause of inherited disorders of surfactant metabolism.2 It is a part of the ATP-binding cassette transporter family, which encodes a membrane protein located on chromosome 16p13.3,10 and expressed in the membrane of LBs in alveolar epithelial cells.11 Neonates with a non-functional ABCA3 protein cannot package or secrete surfactant because of defective glycosylation and other defects of trafficking.12 Protein is detectable in human fetuses around 26–27 weeks of gestation. When lung inflammation is present it may be detected as early as 23 weeks.3 More than 200 mutations have been reported with c.4545C>G (p.Tyr1515) homozygous mutation in exon 29 being the most common one.3 Geographical variation is being noticed with p.Tyr1515 mutation is the most common in the Middle East. The missense mutation involving glutamic acid replacing valine (p>Glu292 Val (E292V)) is the most common mutation in individuals of European descent.13 E292V mutation is usually described in older children with chronic interstitial lung disease.8 A case report from India reported c3703 +1 G>T mutation in ABCA3 gene.14 The most common pattern of inheritance is autosomal recessive, though cases of uniparental disomy do have been reported in the literature.15
The age of presentation varies in babies with ABCA3 mutation. A majority (81%) present at birth, 14% present at less than 1 year of age and only 5% present in childhood.16 Babies can be managed with conventional and high frequency ventilation and inhaled nitric oxide is used as a bridge to lung transplant. Gene studies, immunohistochemistry in bronchoalveolar lavage fluid and lung biopsy are the investigations needed on a case to case basis. Type of mutation can sometimes help in predicting outcome in these babies. On the basis of predictive disruption of protein function, recessive mutations have been classified.16 Frameshift and nonsense mutations were classified as ‘null’, whereas missense, predicted splice site mutations and insertions/deletions were classified as ‘others’. All of the null/null infants presented with respiratory failure at birth compared with 5% 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. Frameshift and nonsense mutations are associated with a severe presentation in the neonatal period and poor outcome as compared with missense and other mutations.16
Whole-exome sequencing in our case revealed homozygous missense variant identified in exon 12 of ABCA3 gene resulting in an aminoacid change of leucine to proline at amino acid 437 (OMIM 601615) which is inherited in an autosomal recessive fashion. This variant has not been described in Gnom AD exome/genome database populations. The mutation has also not been described in publicly available databases like ‘OMIM’, ‘Genereviews’,‘LOVD’. Both parents were tested and were found to be carriers for the same mutation. In spite of being a missense mutation, the baby presented with severe respiratory distress in the neonatal period and had a fatal course.
Learning points
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Late preterm or term baby with persistent respiratory distress out of proportion to gestational age in absence of other identifiable factors like meconium aspiration, pneumonia, persistent pulmonary hypertension of neonate, and congenital abnormalities should be screened for genetic disorders of surfactant metabolism.
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ATP binding cassette transporter protein A3 deficiency is the most common inherited disorder of surfactant metabolism. The majority presents in the neonatal period.
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Gene analysis helps us to find out mutation which may have prognostic value owing to varied clinical presentation.
Footnotes
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Contributors NPG is involved with making diagnosis and writing manuscript. AB and VM helped in clinical management of case RP was involved in genetics part related to case.
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Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
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Competing interests None declared.
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Patient consent for publication Parent/guardian consent obatined.
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Provenance and peer review Not commissioned; externally peer reviewed.
- © BMJ Publishing Group Limited 2020. No commercial re-use. See rights and permissions. Published by BMJ.
References
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