Peter van Mourik

165 Rationale of the HIT-CF Organoid study BACKGROUND Cystic Fibrosis (CF) is a life shortening rare disease caused by loss-of-function mutations in the Cystic Fibrosis Transmembrane Conductance Regulator ( CFTR ) gene (1,2). Dysfunctional CFTR impairs epithelial chloride and bicarbonate transport, which leads to thickening of secretions in multiple organs, resulting in respiratory failure, endocrine dysfunction, liver cirrhosis, gastrointestinal disorders and early death 3 . CF is a monogenic recessive disease, and more than 2000 different variants in the CFTR gene have been described (genet.sickkids.on.ca ) 4 . The F508del mutation is present in ~80% of European CF patients, but 351 other variants have been confirmed as pathogenic 5 . The characteristics and disease liability of the remaining variants remain mostly unclear. They can affect CFTR function via distinct (combinations of) mechanisms and can result in different levels of CFTR dysfunction and clinical disease 1,6 . The recent introduction of so-called CFTR-modulators has caused a paradigm shift in the treatment of cystic fibrosis. Four different drug categories can increase CFTR protein availability and function, and act in a CFTR mutation specific fashion. Ivacaftor, the first CFTR modulator on the market, was identified as CFTR potentiator for patients with so-called gating mutations that are present in ~5% of CF patients. In these patients ivacaftor increases the open-probability of the CFTR channel, thereby improving the anion transport 7 . A combination of ivacaftor and the corrector lumacaftor proved to be effective in patients with the most common F508del mutation 8,9 . CFTR-correctors aim to correct impaired CFTR folding and trafficking to the cell surface 10,11 . Typically a potentiator with one or two correctors are needed to optimally re-establish the function of the complex CFTR-channel in patients with one or two F508del alleles 12,13 , and these combination treatments can lead to effective treatment for ~85% of the CF population 14,15 . The third type of CFTR modulators are CFTR-read-through agents that promote ribosomal read-through at premature termination codons (PTCs) and thus lead to full length CFTR protein production in patients with nonsense mutations 16 . CFTR- amplifiers increase CFTR mRNA availability and might act in a genotype agnostic fashion 17 . The therapeutic targets of read-through agents and CFTR-amplifiers still need to be assessed. Another interesting CFTR-modulating drug category are CFTR-stabilizers that aim to stabilize CFTR-protein activity at the cell membrane through different molecular targets 18,19 . CF patients with non-F508del and non-gating mutations (approximately 12000-13000 patients worldwide) are very heterogeneous and carry >2000 different ultra-rare and mostly uncharacterized CFTR mutations 8