Peter van Mourik

10 Chapter 1 Both FIS and SLA have only recently been developed, and technological expertise is currently available in a select number of laboratories. The CFTR dependency of FIS has been confirmed by independent other CF (e.g. De Boeck et al., Amaral et al.) and non-CF laboratories in intestinal 13 and non-intestinal tissues 14 . To further standardize multicenter research, biopsies can also be shipped to a centralized laboratory for processing, which is feasible within 48-72 hours after obtaining the biopsy. Growing organoid structures for FIS experiments takes approximately four weeks, and at that point organoids can be stored in liquid nitrogen for future use. When in culture, FIS and SLA assays can be performed weekly, and repeated measurements over several weeks can be performed to ensure reliability of measurements. WHAT IS THE CURRENT USE OF ORGANOIDS IN STUDY- ING CYSTIC FIBROSIS? Intestinal organoids are used to better understand how CFTR function or manipulation thereof by therapeutic interventions can impact on individual clinical phenotype (Figure 1). Additionally, the recently funded HIT-CF project (http://www.hitcf.org ) will use organoids to design clinical trials by pre-treatment stratification of in vitro responders. Clinical studies indicate that organoid FIS is relevant to characterize CFTR function of rare CFTR variants. Many of the currently known 2000 CFTR variants have not been characterized, which complicates the prediction of individual disease phenotype. Moreover, limited data regarding drug effects on rare genotypes prevents access to existing and new CFTR-mutation specific treatments. Pilot studies indicate that organoids could help both as individual biomarker of CFTR function to predict individual disease phenotype 15 , and could help characterize rare mutations for treatment response 7 . Organoids and clinical disease phenotype Organoid FIS reflects residual CFTR function and correlates with predicted phenotypic characteristics of the CF genotype 7,15 , and other biomarkers of CFTR function such as Sweat Chloride Concentration (SCC) 7,15 and intestinal current measurements (ICM) 15,16 . While Nasal Potential Difference (NPD) is another important clinical biomarker of CFTR function, no data on the correlation between organoid FIS and Nasal Potential Difference is currently available. The first study to directly compare organoid FIS and clinical phenotype prospectively included 34 newborns with CF. Newborns were clustered into low FIS or high FIS. Low