Genetics of fibrosing lung diseases

Genetic studies in familial lung fibrosis have demonstrated an association with surfactant protein C genes: two mutations have been found resulting in protein misfolding and causing type-II epithelial cell injury. Remarkably, different histological patterns were observed in the affected subjects, suggesting the influence of modifier genes and/or environmental factors. Surfactant protein C gene variations have not, however, been associated with sporadic cases, i.e. idiopathic pulmonary fibrosis (IPF). Susceptibility to IPF probably involves a combination of polymorphisms related to epithelial cell injury and abnormal wound healing. To date, the genetic associations with IPF that have been reported in different cohorts include the genes encoding tumour necrosis factor (TNF; -308 adenine), interleukin-1 receptor antagonist (+2018 thymidine) and association with severity and progression (interleukin-6/TNF receptor II and transforming growth factor-beta 1 (TGFB1; +869 cytosine)), but none of these associations have been replicated by others. Unlike in IPF, immunological inflammation seems to be more prominent in the pathogenesis of scleroderma lung fibrosis, being an autoimmune disease with specific autoantibodies, such as antitopoisomerase antibodies, in patients with diffuse lung disease, and anticentromere antibodies, in patients with pulmonary vascular disease. Antitopoisomerase antibody positivity is associated with the carriage of human leukocyte antigen DRB1*11 and DPB1*1301 alleles, suggesting the recognition of a specific amino-acid motif. Extended haplotype analysis also supports the conclusion that TNF may be the primary association with anticentromere positivity. Intriguingly, associations with TGFB1 and genes involved in extracellular matrix homeostasis have been reported in this disease. In conclusion, significant steps forward have been taken in the understanding of the genetic contribution to fibrosing lung diseases, but major challenges lay ahead. It is the present authors' opinion that only a combined approach studying large numbers of familial and sporadic cases, all clinically well phenotyped, using multiple distinct cohorts, and genotyped according to relevant gene ontologies will be successful. It will be necessary to be particularly vigilant with regard to phenotype; the absence of very strong reproducible associations may be because of the rigidity of phenotype definition, coupled with the possibility that idiopathic pulmonary fibrosis may still be a heterogeneous group of diseases, despite the more rigid definition set out by the European Respiratory Society/American Thoracic Society statement.