Differential effects of ELX/TEZ/IVA on organ-specific CFTR function in two patients with the rare CFTR splice mutations c.273+1G>A and c.165-2A>G

Abstract

Introduction: Evidence for the efficiency of highly-effective triple-CFTR-modulatory therapy with elexacaftor/tezacaftor/ivacaftor (ETI), either demonstrated in clinical trials or by in vitro testing, is lacking for about 10% of people with cystic fibrosis (pwCF) with rare mutations. Comprehensive assessment of CFTR function can provide critical information on the impact of ETI on CFTR function gains for such rare mutations, lending argument of the prescription of ETI. The mutation c.165-2A>G is a rare acceptor splice mutation that has not yet been functionally characterized. We here describe the functional changes induced by ETI in two brothers who are compound heterozygous for the splice mutations c.273+1G>C and c.165-2A>G. Methods: We assessed the effects of ETI on CFTR function by quantitative pilocarpine iontophoresis (QPIT), nasal potential difference measurements (nPD), intestinal current measurements (ICM), β-adrenergic sweat secretion tests (SST) and multiple breath washout (MBW) prior to and 4 months after the initiation of ETI. Results: Functional CFTR analysis prior to ETI showed no CFTR function in the respiratory and intestinal epithelia and in the sweat gland reabsorptive duct in either brother. In contrast, β-adrenergic stimulated, CFTR-mediated sweat secretion was detectable in the CF range. Under ETI, both brothers continued to exhibit high sweat chloride concentration in QPIT, evidence of low residual CFTR function in the respiratory epithelia, but normalized β-adrenergically stimulated production of primary sweat. Discussion: Our results are the first to demonstrate that the c.165-2A>G/c.273+1G>C mutation genotype permits mutant CFTR protein expression. We showed organ-specific differences in the expression of CFTR and consecutive responses to ETI of the c.165-2A>G/c.273+1G>C CFTR mutants that are probably accomplished by non-canonical CFTR mRNA isoforms. This showcase tells us that the individual response of rare CFTR mutations to highly-effective CFTR modulation cannot be predicted from assays in standard cell cultures, but requires the personalized multi-organ assessment by CFTR biomarkers.

Keywords: CFTR rare mutations; c.165-2A>G; cystic fibrosis transmembrane conductance regulator (CFTR); elexacaftor/tezacaftor/ivacaftor; exon 3.

FIGURE 1

Effects of ETI on QPIT (A), LCI_2.5 (B), nPD basic potential (C), nPD Amiloride response (D), nPD cumulative depolarization response to chloride-free solution and isoproterenol (E), nPD Sermet Score (F), response to forskolin/IBMX and carbachol in ICM (G) and SST (H) in brother 1 (triangle) and brother 2 (circle) compared to the median effect on pwCF with one or two p.Phe508del alleles (white square) and reference values for healthy controls (HC, star). Dashed lines indicate published limits of normal for QPIT (Nährlich et al., 2013), LCI_2.5 (Wyler et al., 2021), nPD Sermet Score (Sermet-Gaudelus et al., 2010) and SST (Pallenberg et al., 2022).

FIGURE 2

Sweat bubble formation after β-adrenergic stimulation of the skin at baseline (A,B) and under ETI (C,D) in brother 1 (B1, left column) and brother 2 (B2, right column). Each dot represents the median bubble volume per time point as calculated by the AutoBuSTeD software (Pallenberg et al., 2022). The line shows the linear correlation, the slope corresponds to the sweat rate in nL/min.