Cystic Fibrosis (CF)

Cystic fibrosis (CF) is the most frequently occurring rare genetic disease among Caucasians, affecting more than 30,000 individuals in the United States and 80,000 globally.^1,2 Inheritance is autosomal recessive. CF is characterized by the secretion of thick, viscous mucus, which accumulates and causes dysfunction in multiple organs, especially those of the gastrointestinal, pulmonary, and genitourinary systems.²

Pathophysiology

Normal chloride channel proteins are embedded throughout the apical membranes of epithelial cells throughout the body, particularly in the respiratory and intestinal tracts.³ They regulate the movement of chloride and sodium ions, as well as water, across the epithelial cell membranes.^1-3 

CF results from genetic abnormalities in the cystic fibrosis transmembrane conductance regulator gene (CFTR). Mutant genes subsequently encode abnormal chloride channel or CFTR protein. Most known CFTR mutations affect 3 or fewer nucleotides and include frameshift, amino acid substitution, splice-site, and nonsense mutations.¹ 

Classes of CFTR Mutations

More than 2000 different mutations in the CFTR gene have been divided into 6 classes according to the type of dysfunction they cause.⁴ 

Class I mutations, which cause defective protein synthesis, are nonsense, splice-site, or frameshift mutations that end the mRNA sequence prematurely. An incomplete mRNA sequence is not translated into a complete and functional protein product. Class I mutations result in complete absence of the CFTR protein and account for 2% to 5% of cases of CF.⁴ Examples of class I mutations include G542X, W1282X, and 621 + 1GtoT.⁵

The predominant CFTR mutation, which is responsible for two-thirds of cases of CF globally, is F508del, a class II mutation. In this mutation, the deletion of 3 base pairs in the CFTR gene results in defective post-translational protein processing.^1,4 An absence of the amino acid phenylalanine at position 508 on the chloride channel protein leads to protein misfolding during post-translational processing.¹ 

Misfolding affects the protein labeling required for intracellular transport to the correct location in the cell membrane. Abnormal protein is prematurely destroyed within the Golgi apparatus,⁴ although an insignificant amount may reach the cell surface.⁵ Exocrine pancreatic insufficiency frequently develops in individuals with the F508del mutation, in whom meconium ileus is common.⁴ Another example of a class II mutation is N1303K.⁵

Class III mutations are associated with disordered channel regulation, which results from nonfunctional proteins in the cell membrane with diminished activity related to intracellular signaling.⁴ Class III mutations cause amino acid substitutions that affect channel regulation, typically leading to decreased channel opening. Examples of class III mutations are G551D and R560T.⁵

Class IV mutations result in defective chloride conductance. Although the protein is correctly placed in the cell membrane, the rate of chloride ion flow and the duration of channel activation are both abnormal.⁴ Examples of class IV mutations resulting in amino acid substitutions are R117H and R347P.⁵

Class V mutations result in insufficient concentrations of proteins with normal structure. Because of the reduced efficiency of CFTR mRNA splicing, a smaller amount is spliced properly. Examples of class V mutations are 3849 + 10kb CtoT and 2789 + 5GtoA.⁵ 

Class VI mutations destabilize the mature CFTR protein at the cell surface.⁵ The unstable protein is quickly degraded, so that the number of CFTR channels in the cell membrane is reduced.⁴ Examples of class VI mutations are Q1412X, N287Y, and possibly dF508.⁵

Studies have demonstrated that the failure of cyclic adenosine monophosphate (cAMP) to activate chloride channel proteins also disrupts the transport of chloride ions from inside the cell to the outside surface.^3,6 

Organ Dysfunction 

When sodium and chloride ion transport is disrupted, the balance of salt and water on the surface of epithelial cells in the gut or lungs is also disrupted. Properly transported chloride ions attract a water layer to the exterior surface of the cell. The water layer allows cilia, tiny hairlike structures on the cell surface, to sweep back and forth to clear any mucus secretions.⁷ 

In CF, all mutations of the CFTR gene decrease chloride secretion and increase sodium reabsorption. Water follows sodium back into the cellular space,⁴ so that the cell surface is not hydrated. The result is thickening of the mucus secretions covering the epithelial linings of organs and thickening of exocrine gland secretions. Hyperviscous mucus is a primary contributor to most of the symptoms of CF. Obstruction by thickened plugs of mucus cause dysfunction at multiple sites, particularly the sinuses, lungs, pancreas, liver, gallbladder, intestines, and sweat glands.^4,7 

Sweat Gland Dysfunction

Salt (NaCl) reabsorption is significantly altered in individuals with CF. Normally, as sweat travels through a sweat duct, most of the NaCl is reabsorbed. Reabsorption is driven primarily by a powerful concentration gradient, and sodium ions flow passively from the duct into the cell lumen through epithelial sodium channels in the apical membrane.⁸ 

From here, the basolateral sodium-potassium pump actively pumps 3 positively charged sodium ions out of the cell into the bloodstream in exchange for 2 positively charged potassium ions. This process contributes to a slightly negative electrical potential inside the cell.^8,9 

The passive diffusion of negatively charged chloride ions from inside the sweat duct through activated CFTR channel proteins in the epithelium is promoted by 2 factors. The slightly negative electrical potential inside the cell repels the similarly charged chloride ions outward, and the strong attraction to the positively charged sodium ions draws the chloride ions out from the duct. Chloride follows sodium; however, water is retained in the duct, generating diluted sweat for effective thermoregulation without a significant loss of electrolytes.^8,9 

In the sweat ducts of persons with CF, the passive diffusion of chloride is interrupted by the deactivated CFTR protein, preventing chloride from following sodium and creating a net negative charge inside the sweat duct. The negative charge attracts the positively charged sodium back into the duct, so that the retention of both sodium and chloride leads to a high salt concentration in the sweat of individuals with CF.^8,9 The extremely high salt concentration in the sweat of patients with CF (>100 mM vs 20-30 mM in normal persons) is the single most reliable biomarker of CF.⁸ 

References

1. Lubamba B, Dhooghe B, Noel S, Leal T. Cystic fibrosis: insight into CFTR pathophysiology and pharmacotherapy. Clin Biochem. 2012;45(15):1132-1144. doi:10.1016/j.clinbiochem.2012.05.034
2. Brown SD, White R, Tobin P. Keep them breathing: cystic fibrosis pathophysiology, diagnosis, and treatment. JAAPA. 2017;30(5):23-27. doi:10.1097/01.JAA.0000515540.36581.92
3. Anderson MP, Sheppard DN, Berger HA, Welsh MJ. Chloride channels in the apical membrane of normal and cystic fibrosis airway and intestinal epithelia. American Journal of Physiology-Lung Cellular and Molecular Physiology. 1992;263(1):L1-L14. doi:10.1152/ajplung.1992.263.1.L1
4. Yu E, Sharma S. Cystic fibrosis. StatPearls [Internet]. Updated August 11, 2021. Accessed January 6, 2022.
5. Hull J. Cystic fibrosis transmembrane conductance regulator dysfunction and its treatment. J R Soc Med. 2012;105(Suppl 2):S2-S8. doi:10.1258/jrsm.2012.12s001
6. Pereira MMC, Parker J, Stratford FLL, McPherson M, Dormer RL. Activation mechanisms for the cystic fibrosis transmembrane conductance regulator protein involve direct binding of cAMP. Biochem J. 2007;405(Pt 1):181-189. doi:10.1042/BJ20061879
7. Basics of the CFTR protein. Cystic Fibrosis Foundation. Accessed January 6, 2022.
8. Cystic fibrosis. Notes for human genome, J Wine, 2003. Accessed January 6, 2022.
9. Reddy MM, Stutts MJ. Status of fluid and electrolyte absorption in cystic fibrosis. Cold Spring Harb Perspect Med. 2013;3(1):a009555. doi:10.1101/cshperspect.a009555

Reviewed by Kyle Habet, MD, on 1/19/2022.

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Maria Arini Lopez, PT, DPT, CSCS, CMTPT, CIMT

Maria Arini Lopez, PT, DPT, CSCS, CMTPT, CIMT is a freelance medical writer and Doctor of Physical Therapy from Maryland. She has expertise in the therapeutic areas of orthopedics, neurology, chronic pain, gastrointestinal dysfunctions, and rare diseases especially Ehlers Danlos Syndrome.