The Cellular Mechanism of CF: The Basics
Cystic fibrosis was ushered into the spotlight during President Barack Obama’s 2015 State of the Union Address as he highlighted the advances in CF therapy, which have served as a model for the President’s precision medicine initiative. Precision medicine paves the way for the development of therapies that are tailored to the patient’s unique genetic makeup and drugs like Kayldeco, which was originally developed toward the G551D mutation in CF, reveals the promise of these mutation specific therapies with both gains in lung function and reduced levels of infection in patients.
Precision medicine and its relevance to novel CF therapies provides an increased incentive for patients and families to educate themselves and learn their specific CF mutation(s). Programs are in place, such as the CF Mutation Analysis Program (MAP), which provides free genotyping to CF patients to learn which CFTR (the gene responsible for cystic fibrosis) mutations they possess.
Education and understanding of the mutations in CF are important not only to appropriately match available therapies for physical healing but also to understand the process that contributes to an inextricable part of a patient and who they are. In the years leading up to high school, I often would feel an isolation from friends, family and doctors because I did not have a language to communicate or understand the way CF was affecting me. As I began to learn the biology of the disease I slowly developed a vocabulary that helped me to understand a bit of the way I was feeling as well as a point of connection to talk to those whom I had previously experienced a disconnect.
A wealth of literature exists regarding the science and mechanism of CF. I have found the review, “Cystic Fibrosis Genetics: From Molecular Understanding to Clinical Application,” published in Nature Reviews in November 2014 by Dr. Garry R. Cutting, M.D., particularly helpful to my own understanding of the disease and I will relay of bit of the information to increase an understanding of the genetics and mechanism of the disease. The information I provide here is in no way an exhaustive review but only touches on a few parts of the cellular mechanism underlying CF.
The Cellular Mechanism
To begin to understand cystic fibrosis it is important to understand the root cause that leads to the clinical manifestations of the disease. The development of CF results from a misfolded or improperly functioning protein known as the cystic fibrosis conductance regulator (CFTR). The protein works in the apical membrane of epithelial cells in organs throughout the body as a chloride ion channel, which, as its name suggests, allows for the passage of chloride ions out of the cell. This movement attracts sodium ions across the cell membrane followed by a subsequent flow of water to the cell’s exterior. The passage of water is key because it hydrates and thins mucous so it can be properly cleared from organ passages, specifically in the lungs.
In CF, the absence, or dysfunction of this CFTR channel inhibits the flow of water and leaves mucous dehydrated and thickened, making it difficult to move with normal ciliary clearance. The immobilized, or stuck mucous then creates a nice, little niche for bacteria to grow, which gives rise to the classic manifestations of the disease including, chronic pulmonary infections, inflammation and decay of lung function. However, it should be noted that the thickened mucous plays a role in creating complications not only in the lung but other parts of the body, such as the sinuses, liver, pancreas, intestine and male reproductive tract.
Cystic Fibrosis Conductance Regulator (CFTR) Protein & Mutations
As previously mentioned, the CFTR protein serves as a gate at the cell surface, which opens to allow chloride ions to cross the cell membrane. The chloride channel is an ATP-binding cassette (ABC) transporter and is comprised of three distinct domains or parts, which include two nucleotide-binding domains (NBD 1 and 2), two membrane-spanning domains (MSD 1 and 2), and a regulatory domain (R domain). The NBDs bind ATP, which provide the energy necessary to open and close the channel. The MSDs then help to anchor the channel securely in the cell membrane so that it stays at the cell surface. Additionally, the R domain allows for phosphorylation which generally regulates the opening and closing of the channel.
The structure of the CFTR, although seemingly abstract and uninteresting, is important because mutations in this gene, responsible for CF, can occur in any of these three regions resulting in two primary defects: a chloride channel, which is not in the proper shape and therefore cannot insert into the apical membrane, or a channel that does not open and close properly on the membrane. Either situation may result in reduced water flow and as previously described will create thick mucous. The review by Dr. Cutting points out that the these two defects create a disturbance in the cell leading to CF by affecting “the quantity and/or function of CFTR at the cell membrane.”
Since the discovery of the CFTR gene twenty-five years ago, nearly 2,000 mutations have been identified within the gene. A record of the known mutations can be found in the Cystic Fibrosis Mutation Database. Among the known mutations, Dr. Cutting cites, “40% are predicted to cause substitution of a single amino acid, 36% are expected to alter RNA processing (including nonsense, frameshift and mis-splicing variants), ~3% involve large rearrangements of CFTR, and 1% affects promoter regions; 14% seem to be neutral variants and the effect of the remaining 6% is unclear.”
Genetic Modifiers: Variance in Disease Severity
Any family with two children can attest to the fact that the severity of the disease can vary greatly from one child to the other, which may seem strange since children have the same parents and receive the same set of mutations for the disease. In my family, my younger sister and I both were homozygous (two copies) for the F508del mutation, however, my younger sister recently passed from rejection following her double lung transplant.
The variance that can be seen in the severity of the disease can be partially attributed to what are known as “genetic modifiers;” which are genes other than CFTR that can either increase or lessen the severity of CF when expressed in patients. Twin studies have been used to observe these differences. “By comparing clinical measures of affected twin pairs when they lived together to the same measures after they moved apart,” Dr. cutting, in his review, notes that, “~50% of the difference in lung function measures could be attributed to genetic modifiers.”
To better identify and understand the role genetic modifiers The North American Cystic Fibrosis Gene Modifier Consortium was established. The continued identification of modifiers through this consortium serves to grow a greater understanding of the pathophysiology of the disease as well as identify potential targets for the development of future CF therapies.