Background:Cystic fibrosis (CF) is caused by a wide spectrum of mutations in the CF transmembrane conductance regulator (CFTR) gene, with some leading to non-classical clinical presentations. We present an integratedin vivo, in silicoandin vitroinvestigation of an individual with CF carrying the rare Q1291H-CFTRallele and the common F508del allele. At age 56 years, the participant had obstructive lung disease and bronchiectasis, qualifying for Elexacaftor/Tezacaftor/Ivacaftor (ETI) CFTR modulator treatment due to their F508del allele. Q1291HCFTRincurs a splicing defect, producing both a normally spliced but mutant mRNA isoform and a misspliced isoform with a premature termination codon, causing nonsense mediated decay. The effectiveness of ETI in restoring Q1291H-CFTR is largely unknown.Methods:We collected clinical endpoint measurements, including forced expiratory volume in 1 s percent predicted (FEV1pp) and body mass index (BMI), and examined medical history.In silicosimulations of the Q1291H-CFTR were compared to Q1291R, G551D, and wild-type (WT)-CFTR. We quantified relative Q1291HCFTRmRNA isoform abundance in patient-derived nasal epithelial cells. Differentiated pseudostratified airway epithelial cell models at air liquid interface were created and ETI treatment impact on CFTR was assessed by electrophysiology assays and Western blot.Results:The participant ceased ETI treatment after 3 months due to adverse events and no improvement in FEV1pp or BMI.In silicosimulations of Q1291H-CFTR identified impairment of ATP binding similar to known gating mutants Q1291R and G551D-CFTR. Q1291H and F508del mRNA transcripts composed 32.91% and 67.09% of total mRNA respectively, indicating 50.94% of Q1291H mRNA was misspliced and degraded. Mature Q1291H-CFTR protein expression was reduced (3.18% ± 0.60% of WT/WT) and remained unchanged with ETI. Baseline CFTR activity was minimal (3.45 ± 0.25 μA/cm2) and not enhanced with ETI (5.73 ± 0.48 μA/cm2), aligning with the individual’s clinical evaluation as a non-responder to ETI.Conclusion:The combination ofin silicosimulations andin vitrotheratyping in patient-derived cell models can effectively assess CFTR modulator efficacy for individuals with non-classical CF manifestations or rareCFTRmutations, guiding personalized treatment strategies and optimizing clinical outcomes.