Idiopathic pulmonary fibrosis (IPF) is a chronic lung disorder characterised by progressive lung parenchymal scarring, relentless lung function decline, and eventual respiratory failure. It is universally fatal with current treatments targeting downstream pathogenic pathways and only able to slow disease progression modestly. IPF is triggered by repeated microinjury to the alveolar epithelium and dysfunction of alveolar type 2 (AT2) cells, which in health produce pulmonary surfactant and act as progenitor cells to enable epithelial repair. AT2 cell dysfunction results in failed alveolar re-epithelisation and fibroblast recruitment, activation, and proliferation but the cellular mechanisms underlying this are poorly understood, precluding the development of therapeutics targeting early pathogenic events.

Inherited forms of pulmonary fibrosis, including those caused by pathogenic variants of surfactant protein C (SFTPC), offer an opportunity to study these early events. The commonest pathogenic mutation, SFTPC-I73T, results in aberrant accumulation of immature protein at the plasma membrane and in early endosomes resulting in a toxic gain-of-function phenotype. SFTPC-I73T mislocalisation reflects aberrant proprotein proteolytic cleavage, lack of ubiquitination and failed trafficking to lamellar bodies for packaging with other surfactant components. In this thesis, I sought to identify key trafficking factors required for SFTPC maturation in health and to understand the pathogenic consequences of SFTPC-I73T expression. Using targeted forward genetic screens, I identified the E3 ligase ITCH as a novel effector of SFTPC processing, and confirmed its importance in SFTPC maturation by developing a genetically manipulable alveolar organoid model in which to test the consequence of ITCH depletion. Using this novel organoid system, I then generated models of both inducible exogenous SFTPC-I73T expression and its endogenous expression using base editing. These heterozygous models reproduced key phenotypes observed in patient tissue and demonstrated numerous defective cellular pathways including altered apicobasal polarity, perturbed lumenogenesis, global endolysosomal dysfunction, and a markedly altered plasma membrane proteome and lipidome.

Together, I have forwarded our understanding of SFTPC handling in health, and the pathogenic consequences of its accumulation in disease. The aberrant cellular pathways I identified are likely to be relevant in sporadic IPF, as is the case with other monogenic inherited causes of pulmonary fibrosis. This could potentially offer novel therapeutic targets for addressing early pathogenic events.