Key Points
Disease summary:
Idiopathic pulmonary fibrosis (IPF) is the most common form of the idiopathic interstitial pneumonias (IIPs), which are further grouped under the diffuse parenchymal lung diseases (DPLD), and commonly referred to as interstitial lung disease (ILD). IPF is a progressive lung disease that likely involves multiple interacting gene loci and has a median survival rate of 2 to 3 years.
Patients with IPF generally report the insidious onset of mild dyspnea, a nonproductive cough, and have evidence of restrictive lung function upon physiologic testing. The disease progresses at various rates and may have episodes of acute exacerbations.
Patients with IPF are over 50 years of age and are often current or former cigarette smokers, but the initiating stimulus is unknown.
Familial interstitial pneumonia (FIP), a subcategory of IIP, refers to the presence of two first-degree relatives with IIP. It is not known what percentage of an IIP population is familial, but it is estimated that up to 10% of cases have other family members with some form of IIP.
Differential diagnosis:
IIPs are diagnosed only after excluding known causes of ILD, such as medications, occupational and environmental factors, systemic autoimmune diseases, granulomatous ILDs such as sarcoidosis and other forms of interstitial lung diseases, for example, Langerhans cell histiocytosis or histiocytosis X, and eosinophilic pneumonia. IPF is further distinguished from other IIPs by the presence of bibasilar reticulonodular opacities and honeycombing consistent with usual interstitial pneumonia (UIP) on a high-resolution computed tomography (HRCT), although some cases of nonspecific interstitial pneumonia (NSIP) have also been associated with IPF.
Monogenic forms:
No monogenic forms have been found.
Family history:
Putative transmittance for FIP is autosomal dominant with reduced penetrance. No monogenic form is known, but a number of contributing genes have been identified with different associated risks for the offspring.
Twin studies:
None.
Environmental factors:
No environmental factors have been shown to trigger IPF (although some have been shown to cause pulmonary fibrosis). However, smoking cigarettes, chronic aspiration, some environmental pollutants, and medications have been correlated with the future development of IPF. In particular, smoking relatives of individuals with FIP have more than a twofold increase in the chance of developing ILD.
Gene variant associations:
Various associations exist, however the known disease-associated genetic variations in surfactant protein C (SPC) and telomerase genes account for less than 10% of cases of FIP (Table 118-1). Although a promoter variant in MUC5B occurs in approximately 20% of individuals in the normal population, this variant is observed in 60% to 70% of individuals with FIP or IPF and is associated with an increase in the production of MUC5B in the lung. Testing exists for the genetic variants in SPC, the telomerase genes, and MUC5B.
Pharmacogenomics:
No treatment specific for any genomic variants of IPF exists.
| Candidate Gene (Chromosome Location) | Associated Variant[effect on protein] | Relative Risk of Developing IPF | % of FIP Cases With Variant(s) in Gene | Frequency of Risk Allele in Controls | Putative Functional Significance | Associated Disease Phenotype | Genetic Testing |
|---|---|---|---|---|---|---|---|
MUC5B (11p15) | SNP [upregulation of MUC5B] rs35705950 | 8.0 heterozygous 27.0 homozygous | 59% | 19% | Promoter for MUC5B gene | IPF and FIP | Yes |
SFTPC (8p21) | Multiple variations, I73T mutation most common | Rare autosomal dominant | ~1% | Unclear | Inhibits synthesis of surfactant protein C | IPF and FIP | Yes |
TERT (5p15.3) | Various nonsense, missense codons, and splice-site variations | Rare autosomal dominant | ~5%-10% | Unclear | Loss of functionality of telomere reverse transcriptase | IPF and FIP | Yes |
TERC (3q21-q28) | Mutation in the pseudoknot region | Rare autosomal dominant | ~2% | Unclear | TERC is unable to provide template for telomere extension | IPF and FIP | Yes |
