Interstitial lung diseases (ILDs) are a heterogeneous group of diseases characterized by widespread fibrotic and inflammatory abnormalities of the lung. Many forms of ILD are extremely rare, while other forms such as idiopathic pulmonary fibrosis (IPF) and sarcoidosis are seen commonly in general pulmonary practice. ILD refers to a heterogeneous collection of more than 200 distinct lung disorders that tend to be grouped together because they share clinical, radiographic, and pathologic features. These diseases have a broad spectrum of clinical presentations and manifestations ranging from lethal neonatal respiratory distress syndrome to adult chronic interstitial lung disease (ILD). In general, most interstitial lung diseases are characterized by four manifestations: (1) respiratory symptoms such as shortness of breath and cough, (2) specific chest radiographic abnormalities, (3) typical changes on pulmonary function tests in which the lung volume is decreased, and (4) characteristic microscopic patterns of inflammation and fibrosis [18,19,20].
Interstitial lung disease (ILD) in infants and children is associated with affected growth and high morbidity and mortality. All patients were recurrently admitted to hospital due to recurrent pneumonia, and they continued to have cough and tachypnea, and some of them exhibited failure to thrive later on .
Radiological investigations (chest X-ray) are the first approach as it is frequently abnormally diffused. But it has limited and low diagnostic specificity. Chest x-ray was performed for all studied 20 cases and showed patchy lung infiltrates. High-resolution computed tomography (HRCT), as a better diagnostic imaging technique for ILD children, helps avoid the need for surgical lung biopsy and is also useful in monitoring the therapeutic response. The most commonly observed findings in ILD by this technique are ground glass attenuation, honeycombing, and cyst formations. Ground glass attenuation opacities in CT chest were evident in all our patients and were our first clue for clinical diagnosis of ILD .
Echocardiography performed to the probands diagnosed 9 cases with pulmonary hypertension and 2 cases with congenital heart disease (patent ductus arteriosus (PDA) and patent foramen oval (PFO)); these findings were similar to a study, reporting a newly born female infant with neonatal pneumonia and PFO (1.9 mm). For some types of pediatric ILDs and few forms in adult ILDs, genetic causes have been identified and precise. Molecular diagnosis was used instead of the need for lung biopsies allowing definite diagnosis accessible to clinicians. Understanding the mechanisms of the idiopathic forms of interstitial lung disease is currently emerging. Studies of cells culture revealed a number of molecules and molecular pathways (such as transforming growth factor beta) that promote fibrosis [14, 23].
Previous studies reported genetic correlation of idiopathic pulmonary fibrosis (IPF) alterations in specific genes such as surfactant protein genes and telomerase. Further evidence for the role of genetics in interstitial lung disease comes from studies in patients with other disorders as sarcoidosis and Hermansky–Pudlak syndrome, where mutations in specific genes are associated with a higher incidence of lung fibrosis .
Egyptian study, conducted on 568 cases (191 males and 377 females) with mean age 44 ± 12 years from Upper Egypt, noted that IPF is the most common lung disease due to domestic air pollution, indoor exposures, and environmental factors .
Monoallelic mutations of the surfactant protein C gene (SFTPC) were associated with (ILD) in children and adults. When the SP-C gene is mutated, the precursor of surfactant protein C (proSP-C) is misfolded and accumulates within the ER and Golgi apparatus of AEC2s, leading to cellular injury and apoptosis .
In the present study, sequencing analysis of SFTPC gene within 20 ILD patients revealed five variants: c.42+35G>A (rs8192340), c.43-21T>C (rs13248346), c.436-26C>G (rs2070687), c.413C>A (rs4715), and c.557G>A (rs1124). A similar study was carried out on 760 Caucasian of Danish descents with different lung disease phenotypes as asthma, chronic obstructive pulmonary disease (COPD), and interstitial lung disease; they identified 18 variants including our five detected variants .
In patients with exonic mutations, the two previously reported SFPTC mutations in humans were described as acting in dominant fashion, with abrogation of SP-C synthesis (dominant negative effect). However, no deviation from the reference sequence was observed among patients harboring the variants predicted as benign after thorough SFTPC analysis, suggesting that either a cryptic SFTPC mutation segregated in this family, or that the SP-C deficiency was caused by a mutation at another genetic locus or by shared environmental exposures. Also, robust linkage disequilibrium was detected between the coding SNPs, T138N, and S186N by where the occurrences of the predictable haplotypes (for the allele frequencies of both SNPs) did not change considerably between patient and control groups. Remarkably, diverse histopathological types of pulmonary fibrosis were discovered in members of the same kindred harboring the same SFTPC mutation, and these diverse types were interpreted as possibly signifying pleiotropic expressions of the same genetic defect. SFTPC-associated familial forms of pulmonary fibrosis are characterized by reduced penetrance suggesting that further endogenous or exogenous elements might back the marked variety of pulmonary fibrosis predisposed by SFTPC mutations [13, 14, 27, 28].
In our study, single-nucleotide variants c.42+35G>A (rs8192340) and IVS4-8C>G (rs2070687), each was detected in two different patients (4 & 11); these variants were reported previously in 2 unrelated adult Dutch patients with familial pulmonary fibrosis though their mean ages at diagnosis were 54 years .
IVS4-8C>G (rs2070687) variant was reported in two American infants in two different studies; this variant was associated with persistent pulmonary hypertension (PPHN) and respiratory distress syndrome (RDS) symptoms [30, 31].
Sequencing analysis of (SFTPC) gene revealed two common missense variants p.T138N (rs4715) in exon 4 in five patients and S186N (rs1124) in exon 5 in four patients. p.T138N was predicted to be possibly damaging, affecting the domain of precursor SP-C (proSP-C) using in silico analysis, while the S186N variant was predicted to be polymorphism with no pathogenic effect .
The two common variants p.T138N and p.S186N in SFTPC gene were reported in two different studies, one in adult German (IPF) patients with age ranging from 24 to 61 years and the other in familial pulmonary fibrosis cases within population of Reunion Island, represented with unexplained respiratory distress (URD) [27, 28].
Moreover, the two variants p.T138N and p.S186N were described as minor alleles in two unrelated Finnish infants suffering respiratory distress syndrome (RDS) whose ages were less than 34 weeks with frequencies 0.20 and 0.22, respectively. Meanwhile, another study included 100 American patients assigned with familial pulmonary fibrosis, investigating the same two variants in frequencies of 0.255 and 0.318, respectively [33, 34].
Heterozygous SFTPC mutations concomitant, as well as heterozygous mutations in ATP-binding cassette transporter A3 (ABCA3) in infants with interstitial lung diseases (ILD), might likely led to development of clinical ILD .
Advances in understanding genetic factors contributing to ILD could outline some of the emerging roles of epigenetic modifications and give a vision about the progressed directions towards future prospects of genetically targeted therapies [18, 24].
Whole-exome sequencing (WES) has become an advanced approach to investigate rare alleles with direct functional consequences on protein products, affecting their pathways. WES was used to screen mutations concerned with surfactant proteins A and C (SFTPA2 and SFTPC) genes; these two genes were mainly related to cell adhesion and immune response, which might partially explain changes of gene expression involved in immune-related pathways in ILD .
Molecular studies were performed using the next-generation sequencing of surfactant dysfunction genes identified three mutations in surfactant protein-C gene (SFTPC) in 6 Chinese children with ILD symptoms, whose ages of onset ranged from 7 days to 15 months: I73Tin 4/6, D105G in 1/6, and Y113H in 1/6 patients .
Our study highlights five variants: c.42+35G>A (IVS1+35G>A) (rs8192340) and c.43-21T>C (IVS1-21T>C) (rs13248346) in intron 1, c.436-8C>G (IVS4-8C>G) (rs2070687) in intron 4, c.413C>A p.T138N (rs4715) in exon 4, and c.557G>Ap.S186N (rs1124) in exon 5 within SFTPC gene that were all associated with interstitial lung disease (ILD) in Egyptian children.