Rectal Organoid Morphology Analysis (ROMA): A Diagnostic Assay in Cystic Fibrosis

Abstract

Diagnosis of cystic fibrosis (CF) is not always straightforward, especially when sweat chloride concentration is intermediate and/or less than two disease-causing CFTR mutations can be identified. Physiological CFTR assays (nasal potential difference, intestinal current measurement) have been included in the diagnostic algorithm but are not always readily available or feasible (e.g., in infants). Rectal organoids are 3D structures that grow from stem cells isolated from crypts of a rectal biopsy when cultured under specific conditions. Organoids from non-CF subjects have a round shape and a fluid-filled lumen, as CFTR-mediated chloride transport drives water into the lumen. Organoids with defective CFTR function do not swell, retaining an irregular shape and having no visible lumen. Differences in morphology between CF and non-CF organoids are quantified in the 'Rectal Organoid Morphology Analysis' (ROMA) as a novel CFTR physiological assay. For the ROMA assay, organoids are plated in 96-well plates, stained with calcein, and imaged in a confocal microscope. Morphological differences are quantified using two indexes: The circularity index (CI) quantifies the roundness of organoids, and the intensity ratio (IR) is a measure of the presence of a central lumen. Non-CF organoids have a high CI and low IR compared to CF organoids. ROMA indexes perfectly discriminated 167 subjects with CF from 22 subjects without CF, making ROMA an appealing physiological CFTR assay to aid in CF diagnosis. Rectal biopsies can be routinely performed at all ages in most hospitals and tissue can be sent to a central lab for organoid culture and ROMA. In the future, ROMA might also be applied to test the efficacy of CFTR modulators in vitro. The aim of the present report is to fully explain the methods used for ROMA, to allow replication in other labs.