Stability Prediction for Mutations in the Cytosolic Domains of Cystic Fibrosis Transmembrane Conductance Regulator

Effects of Point Mutations on the Thermal Stability of the NBD1 Domain of hCFTR

Cystic fibrosis (CF) is an autosomal recessive genetic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel. The first nucleotide-binding domain (NBD1) of the CFTR is considered to be a hotspot for CF-causing mutations, and some of these mutations compromise the domain's thermal stability as well as its interactions with other domains. The mechanisms by which such mutations exert their deleterious effects are important in the basic research of this complex disease as well as for the development of mutation-specific therapies. With this in mind, we studied two class-II, severe, CF-causing mutations, L467P and A559T, known to destabilize the domain by 19.3 and 10.7 °C, respectively, and to lead to a misfolded, nonfunctioning CFTR, by conducting microsecond-long molecular dynamics (MD) simulations at an elevated temperature of 410 K on L467P-NBD1 and A559T-NBD1 constructs. For comparison, similar simulations were also performed on the wild-type (WT) construct and on the 6SS-NBD1 and 2PT/M470V-NBD1 constructs, both bearing sets of stabilizing mutations that stabilize the domain by 17.5 and 8.2 °C, respectively. The resulting trajectories were analyzed using multiple metrics, leading to a good correlation between the experimental ΔTm values and the results of the simulations, as well as multiple experimental observations and results of previous modeling efforts. Specifically, our analyses point to specific regions within NBD1 that are substantially affected by the L467P and A559T mutations and, therefore, may play some role in their pathogenesis. Many of these regions are also known to be important for the proper folding and function of the full-length CFTR. Using time-dependent assignment of DSSP elements, we also found that the two mutants follow different disintegration pathways, that of L467P-NBD1 starting in region 464-471 which resides within the F1-like ATP-binding core subdomain and continues in regions 550-562 and 514-523 within the ABCα subdomain whereas that of A559T-NBD1 simultaneously starting at the 550-562 and 514-523 regions. We propose that the analyses presented in this work may pave the way toward the development of L467P and A559T-specific CF therapies and by extension to other mutation-specific therapies for CF and for other diseases involving mutations in NBDs of other proteins.

ATP-dependent thermoring basis for the heat unfolding of the first nucleotide-binding domain isolated from human CFTR

Traditionally, the thermostability of a protein is defined by a melting temperature, at which half of the protein is unfolded. However, this definition cannot indicate the structural origin of a heat-induced unfolding pathway. Here, the thermoring structures were studied on the ATP-dependent heat-induced unfolding of the first nucleotide-binding domain from the human cystic fibrosis transmembrane conductance regulator. The results showed that initial theoretical and experimental melting thresholds aligned well after three structural perturbations including the F508del mutation, the most common cause of cystic fibrosis. This alignment further demonstrated that the heat-induced unfolding process began with the disruption of the least-stable noncovalent interaction within the biggest thermoring along the single peptide chain. The C-terminal region, which was related to the least-stable noncovalent interaction and the ATP-dependent dimerization of two nucleotide-binding domains, emerged as a crucial determinant of the thermal stability of the isolated protein and a potential interfacial drug target to alleviate the thermal defect caused by the F508del mutation. This groundbreaking discovery significantly advances our understanding of protein activity, thermal stability, and molecular pathology.

Synthesis and Biological Evaluation of Pyrazole-Pyrimidones as a New Class of Correctors of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR)

Cystic fibrosis (CF) is caused by the functional expression defect of the cystic fibrosis transmembrane conductance regulator (CFTR) protein. Despite the recent success in CFTR modulator development, the available correctors only partially restore the F508del-CFTR channel function, and several rare CF mutations show resistance to available drugs. We previously identified compound 4172 that synergistically rescued the F508del-CFTR folding defect in combination with the existing corrector drugs VX-809 and VX-661. Here, novel CFTR correctors were designed by applying a classical medicinal chemistry approach on the 4172 scaffold. Molecular docking and three-dimensional quantitative structure-activity relationship (3D-QSAR) studies were conducted to propose a plausible binding site and design more potent and effective analogs. We identified three optimized compounds, which, in combination with VX-809 and the investigational corrector 3151, increased the plasma membrane density and function of F508del-CFTR and other rare CFTR mutants resistant to the currently approved therapies.

Rescue of Rare CFTR Trafficking Mutants Highlights a Structural Location-Dependent Pattern for Correction

Cystic Fibrosis (CF) is a genetic disease caused by mutations in the gene encoding the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) channel. Currently, more than 2100 variants have been identified in the gene, with a large number being very rare. The approval of modulators that act on mutant CFTR protein, correcting its molecular defect and thus alleviating the burden of the disease, revolutionized the field of CF. However, these drugs do not apply to all patients with CF, especially those with rare mutations-for which there is a lack of knowledge on the molecular mechanisms of the disease and the response to modulators. In this work, we evaluated the impact of several rare putative class II mutations on the expression, processing, and response of CFTR to modulators. Novel cell models consisting of bronchial epithelial cell lines expressing CFTR with 14 rare variants were created. The variants studied are localized at Transmembrane Domain 1 (TMD1) or very close to the signature motif of Nucleotide Binding Domain 1 (NBD1). Our data show that all mutations analyzed significantly decrease CFTR processing and while TMD1 mutations respond to modulators, those localized in NBD1 do not. Molecular modeling calculations confirm that the mutations in NBD1 induce greater destabilization of CFTR structure than those in TMD1. Furthermore, the structural proximity of TMD1 mutants to the reported binding site of CFTR modulators such as VX-809 and VX-661, make them more efficient in stabilizing the CFTR mutants analyzed. Overall, our data suggest a pattern for mutation location and impact in response to modulators that correlates with the global effect of the mutations on CFTR structure.

Triangulating variation in the population to define mechanisms for precision management of genetic disease

To understand mechanistically how the protein fold is shaped by therapeutics to inform precision management of disease, we developed variation-capture (VarC) mapping. VarC triangulates sparse sequence variation information found in the population using Gaussian process regression (GPR)-based machine learning to define the combined pairwise-residue interactions contributing to dynamic protein function in the individual in response to therapeutics. Using VarC mapping, we now reveal the pairwise-residue covariant relationships across the entire protein fold of cystic fibrosis (CF) transmembrane conductance regulator (CFTR) to define the molecular mechanisms of clinically approved CF chemical modulators. We discover an energetically destabilized covariant core containing a di-acidic YKDAD endoplasmic reticulum (ER) exit code that is only weakly corrected by current therapeutics. Our results illustrate that VarC provides a generalizable tool to triangulate information from genetic variation in the population to mechanistically discover therapeutic strategies that guide precision management of the individual.

Structural Comparative Modeling of Multi-Domain F508del CFTR

Cystic fibrosis (CF) is a rare genetic disease caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), an epithelial anion channel expressed in several vital organs. Absence of functional CFTR results in imbalanced osmotic equilibrium and subsequent mucus build up in the lungs-which increases the risk of infection and eventually causes death. CFTR is an ATP-binding cassette (ABC) transporter family protein composed of two transmembrane domains (TMDs), two nucleotide binding domains (NBDs), and an unstructured regulatory domain. The most prevalent patient mutation is the deletion of F508 (F508del), making F508del CFTR the primary target for current FDA approved CF therapies. However, no experimental multi-domain F508del CFTR structure has been determined and few studies have modeled F508del using multi-domain WT CFTR structures. Here, we used cryo-EM density data and Rosetta comparative modeling (RosettaCM) to compare a F508del model with published experimental data on CFTR NBD1 thermodynamics. We then apply this modeling method to generate multi-domain WT and F508del CFTR structural models. These models demonstrate the destabilizing effects of F508del on NBD1 and the NBD1/TMD interface in both the inactive and active conformation of CFTR. Furthermore, we modeled F508del/R1070W and F508del bound to the CFTR corrector VX-809. Our models reveal the stabilizing effects of VX-809 on multi-domain models of F508del CFTR and pave the way for rational design of additional drugs that target F508del CFTR for treatment of CF.

S945L-CFTR molecular dynamics, functional characterization and tezacaftor/ivacaftor efficacy in vivo and in vitro in matched pediatric patient-derived cell models

Cystic Fibrosis (CF) results from over 400 different disease-causing mutations in the CF Transmembrane Conductance Regulator (CFTR) gene. These CFTR mutations lead to numerous defects in CFTR protein function. A novel class of targeted therapies (CFTR modulators) have been developed that can restore defects in CFTR folding and gating. This study aimed to characterize the functional and structural defects of S945L-CFTR and interrogate the efficacy of modulators with two modes of action: gating potentiator [ivacaftor (IVA)] and folding corrector [tezacaftor (TEZ)]. The response to these modulators in vitro in airway differentiated cell models created from a participant with S945L/G542X-CFTR was correlated with in vivo clinical outcomes of that participant at least 12 months pre and post modulator therapy. In this participants' airway cell models, CFTR-mediated chloride transport was assessed via ion transport electrophysiology. Monotherapy with IVA or TEZ increased CFTR activity, albeit not reaching statistical significance. Combination therapy with TEZ/IVA significantly (p = 0.02) increased CFTR activity 1.62-fold above baseline. Assessment of CFTR expression and maturation via western blot validated the presence of mature, fully glycosylated CFTR, which increased 4.1-fold in TEZ/IVA-treated cells. The in vitro S945L-CFTR response to modulator correlated with an improvement in in vivo lung function (ppFEV₁) from 77.19 in the 12 months pre TEZ/IVA to 80.79 in the 12 months post TEZ/IVA. The slope of decline in ppFEV1 significantly (p = 0.02) changed in the 24 months post TEZ/IVA, becoming positive. Furthermore, there was a significant improvement in clinical parameters and a fall in sweat chloride from 68 to 28 mmol/L. The mechanism of dysfunction of S945L-CFTR was elucidated by in silico molecular dynamics (MD) simulations. S945L-CFTR caused misfolding of transmembrane helix 8 and disruption of the R domain, a CFTR domain critical to channel gating. This study showed in vitro and in silico that S945L causes both folding and gating defects in CFTR and demonstrated in vitro and in vivo that TEZ/IVA is an efficacious modulator combination to address these defects. As such, we support the utility of patient-derived cell models and MD simulations in predicting and understanding the effect of modulators on CFTR function on an individualized basis.