A clinical case of cystic fibrosis patient with pathogenic N1303K genotype variant with assessment of the CFTR channel function by intestinal current measurement and forskolin-induced swelling in rectal organoids

Rationale : Cystic fibrosis is a common monogenic disease related to pathogenic nucleotide sequence variants in the Cystic Fibrosis Transmembrane conductance Regulator ( CFTR ) (ABCC7) gene. The CFTR gene consists of 27 exons and is located in the 31.1 region on the long arm of chromosome 7 (7q31.1). The use of the sequencing method has led to the accumulation of new information about the diversity of genetic variants in cystic fibrosis. This information is important considering approaches to the development of targeted therapy for the disease, based on an individual genotype. No targeted therapy has been developed for the N1303K class II genetic variant. The function of the chloride channel in this mutation has not been compared with that in class II mutations like F508del. Materials and methods : We have analyzed medical files of a patient with cystic fibrosis and F508del/N1303K CFTR genotypes, including the results of rectal biopsy samples. The assessments included measurement of the intestinal potential difference and forskolin-induced swelling assay (FIS) in rectal organoids, with the results being analyzed in relation to the clinical data. Results : The results of intestinal current measurements (ICM) confirm that the N1303K genetic variant is “severe” and leads to the loss of the working CFTR protein, which is consistent with the clinical manifestations. The mean short circuit currency density (ΔISC) in response to amiloride (sodium channel stimulation) was -39 ± 1.22 µA/cm 2 , to forskolin (chloride channel stimulation) 3.83 ± 1.43 µA/cm 2 , to carbachol 6 ± 2.47 µA/cm 2 , and to histamine 8.5 ± 3.02 µA/cm 2 . FIS results indicate that the VX-770 potentiator and the VX-809 corrector have a weak effect on the stimulation of organoids by forskolin in the genetic variant N1303K: organoid swelling was non-significant (about 20% from their baseline size). Conclusion : The use of the ICM method and FIS assay in human intestinal organoids makes it possible to quantify the work of the CFTR protein and determine the in vitro effectiveness of targeted therapy in patients with cystic fibrosis. CFTR modulators are ineffective in patients with N1303K mutation in the compound-heterozygous condition with F508del, despite both pathogenic variants belong to class II.