Drew Irion

Background: Cystic Fibrosis is a hereditary ion channel mutation that results in chronic upper respiratory infections and intestinal blockage¹. This condition impacts upwards of 30,000 people in the United States and approximately 60,000 people worldwide^3,4. The most common form of cystic fibrosis, delta F508, is a deletion of phenylalanine in the first nucleotide binding domain of the ion channel^2,5,6.  This deletion results in the misfolding of the channel which halts the outflow of chloride ions in cells. Without this normal flow, the mucous of the lungs and digestive tract becomes more viscous and provides an optimal breeding ground for bacteria⁶. This condition is normally diagnosed with newborn screening through immunoreactive trypsinogen assay¹. In 2012, the FDA approved the use of Cystic Fibrosis Transmembrane Regulator (CFTR) modulators, small molecules that improve the function of the ion channel in chloride transport. Before this, cystic fibrosis had mainly been treated symptomatically with a high incidence of mortality among patients⁴.

Objective: In this review, we will explore the molecular mechanisms by which cystic fibrosis occurs, and current therapies for the most common mutation.

Search Methods: The PubMed database was utilized to find research articles between 2017-2023 containing the search terms: “cystic fibrosis”, and “F508 deletion.”

Results: Studies indicate that the CFTR expression is most prevalent in secretory cells among both large and small epithelial cells. There was no notable difference between CFTR expression in normal lung tissue compared with individuals with cystic fibrosis. The gene expression was highest among ionocytes, however, these cell types were relatively rare in the lung⁸. When studying the structural integrity of the F508 CFTR ion channel, it was noted that instability was most present in the first nucleotide binding domain, and the junction where this domain interacted with the second transmembrane domain⁹. The misfolding of the channel in vivo leads to degradation through the ubiquitination pathway. The lysine chains K11 and K48 were best at recognizing the misfolded protein, whilst K33 and K63 had the opposite effect, and prolonged the halflife¹⁰. In attempts to avoid ubiquitination, and improve chloride transport, different combinations of the Trikafta drug were elicited. It was noted that the full triple drug therapy helped stabilize the ion channel conformation and facilitate the flow of chloride¹¹. Finally this drug was used in a double blinded study for patients with cystic fibrosis. Over the course of a four week period, those who had taken Trikafta had markedly increased expiratory volume and lower sweat chloride levels when compared to the control group¹².

Conclusions: Through these experiments we have seen that the expression of the CFTR gene is not universal, but instead localized to a smaller population of secretory cells. We also were able to see that the use of modulators facilitated folding of the channel into a wild type resembling structure. This was able to avoid ubiquitination and facilitate the transfer of chloride ions. In the human population it showed markedly improved results when compared to the placebo group. This review has shown the effectiveness of drugs working on the F508 mutation for patients with cystic fibrosis, however, this is not the only mutation that exists. It is my belief that with knowledge of location and structure of CFTR, other therapies that target less common mutations can be developed.

Works Cited:

1. Shteinberg M, Haq IJ, Polineni D, Davies JC. Cystic fibrosis. Lancet. 2021 Jun 5;397(10290):2195-2211. doi: 10.1016/S0140-6736(20)32542-3. PMID: 34090606.
2. Hwang TC, Sheppard DN. Gating of the CFTR Cl- channel by ATP-driven nucleotide-binding domain dimerisation. J Physiol. 2009 May 15;587(Pt 10):2151-61. doi: 10.1113/jphysiol.2009.171595. Epub 2009 Mar 30. PMID: 19332488; PMCID: PMC2697289.
3. Bergeron C, Cantin AM. Cystic Fibrosis: Pathophysiology of Lung Disease. Semin Respir Crit Care Med. 2019 Dec;40(6):715-726. doi: 10.1055/s-0039-1694021. Epub 2019 Oct 28. PMID: 31659725.
4. Hwang TC, Braakman I, van der Sluijs P, Callebaut I. Structure basis of CFTR folding, function and pharmacology. J Cyst Fibros. 2022 Oct 7:S1569-1993(22)00688-9. doi: 10.1016/j.jcf.2022.09.010. Epub ahead of print. PMID: 36216744.
5. Vernon RM, Chong PA, Lin H, Yang Z, Zhou Q, Aleksandrov AA, Dawson JE, Riordan JR, Brouillette CG, Thibodeau PH, Forman-Kay JD. Stabilization of a nucleotide-binding domain of the cystic fibrosis transmembrane conductance regulator yields insight into disease-causing mutations. J Biol Chem. 2017 Aug 25;292(34):14147-14164. doi: 10.1074/jbc.M116.772335. Epub 2017 Jun 27. PMID: 28655774; PMCID: PMC5572908.
6. Endres TM, Konstan MW. What Is Cystic Fibrosis? JAMA. 2022 Jan 11;327(2):191. doi: 10.1001/jama.2021.23280. PMID: 35015036.
7. De Boeck K. Cystic fibrosis in the year 2020: A disease with a new face. Acta Paediatr. 2020 May;109(5):893-899. doi: 10.1111/apa.15155. Epub 2020 Jan 22. PMID: 31899933.
8. Okuda K, Dang H, Kobayashi Y, Carraro G, Nakano S, Chen G, Kato T, Asakura T, Gilmore RC, Morton LC, Lee RE, Mascenik T, Yin WN, Barbosa Cardenas SM, O’Neal YK, Minnick CE, Chua M, Quinney NL, Gentzsch M, Anderson CW, Ghio A, Matsui H, Nagase T, Ostrowski LE, Grubb BR, Olsen JC, Randell SH, Stripp BR, Tata PR, O’Neal WK, Boucher RC. Secretory Cells Dominate Airway CFTR Expression and Function in Human Airway Superficial Epithelia. Am J Respir Crit Care Med. 2021 May 15;203(10):1275-1289. doi: 10.1164/rccm.202008-3198OC. PMID: 33321047; PMCID: PMC8456462.
9. McDonald EF, Woods H, Smith ST, Kim M, Schoeder CT, Plate L, Meiler J. Structural Comparative Modeling of Multi-Domain F508del CFTR. Biomolecules. 2022 Mar 18;12(3):471. doi: 10.3390/biom12030471. PMID: 35327663; PMCID: PMC8946492.
10. Wu Q, Henri YT, Yao R, Yu L, Zhang B, Wang Z, Ma X, Zhao G, Hou X. Opposite regulation of F508del-CFTR biogenesis by four poly-lysine ubiquitin chains In vitro. Biochim Biophys Acta Proteins Proteom. 2022 Jun 1;1870(6):140792. doi: 10.1016/j.bbapap.2022.140792. Epub 2022 May 13. PMID: 35569794.
11. Fiedorczuk K, Chen J. Molecular structures reveal synergistic rescue of Δ508 CFTR by Trikafta modulators. Science. 2022 Oct 21;378(6617):284-290. doi: 10.1126/science.ade2216. Epub 2022 Oct 20. PMID: 36264792; PMCID: PMC9912939.
12. Middleton PG, Mall MA, Dřevínek P, Lands LC, McKone EF, Polineni D, Ramsey BW, Taylor-Cousar JL, Tullis E, Vermeulen F, Marigowda G, McKee CM, Moskowitz SM, Nair N, Savage J, Simard C, Tian S, Waltz D, Xuan F, Rowe SM, Jain R; VX17-445-102 Study Group. Elexacaftor-Tezacaftor-Ivacaftor for Cystic Fibrosis with a Single Phe508del Allele. N Engl J Med. 2019 Nov 7;381(19):1809-1819. doi: 10.1056/NEJMoa1908639. Epub 2019 Oct 31. PMID: 31697873; PMCID: PMC7282384.