The cystic fibrosis transmembrane conductance regulator ($CFTR$) gene codes for a protein essential for maintaining fluid balance across many surfaces in the body. When this gene contains errors, the resulting protein is either missing or dysfunctional, causing the genetic disorder Cystic Fibrosis (CF). This condition primarily affects the lungs, pancreas, and digestive system. CF results from a failure to regulate the movement of salt and water, leading to the buildup of thick, sticky mucus.
Normal Function of the CFTR Protein
The CFTR protein functions as a microscopic, gated channel situated on the surface of cells in organs that produce mucus, sweat, saliva, and digestive juices. Its role is to allow negatively charged chloride ions to flow out of the cell and into the surrounding fluid layer. This controlled movement of chloride ions regulates the flow of water across the cell membrane.
Water follows the movement of chloride ions out of the cell, maintaining a thin, slippery layer of fluid on the cell surface. This fluid layer ensures the mucus remains thin enough to be moved easily. In the airways, for example, this water allows the hair-like cilia to sweep the mucus away, clearing debris and pathogens. When the CFTR channel malfunctions, water transport is disrupted, and the resulting dehydrated mucus becomes thick and obstructs ducts and passageways throughout the body.
How Gene Changes Cause Protein Failure
The CFTR gene is the instruction manual for building the CFTR protein, a complex structure made of over 1,400 amino acids. A mutation is a permanent “typo” in this genetic code, causing the cell to build a protein with an altered or incorrect structure. The location and type of DNA error determine the severity of the resulting disease.
The most common error is the Delta F508 ($\Delta$F508) mutation, a deletion of three DNA building blocks that results in the loss of the amino acid phenylalanine at position 508. This change causes a catastrophic misfolding of the protein structure. The cell’s quality control system, housed in the endoplasmic reticulum, recognizes the misshapen $\Delta$F508 protein as defective. Because the protein is structurally unstable, this system tags it for destruction before it can move to the cell surface to function as a channel.
Categorizing CFTR Mutations
CFTR mutations are classified into six functional classes based on where the defect occurs. This classification dictates which targeted therapies, known as CFTR modulator drugs, might be effective. Class I mutations, such as G542X, are the most severe because they introduce a premature stop signal, preventing the cell from producing any functional CFTR protein.
Class II mutations, including the common $\Delta$F508, cause a protein processing defect where the protein misfolds and is destroyed before reaching the cell membrane. Class III mutations, like G551D, involve a gating defect; the protein reaches the surface, but the channel gate does not open properly, blocking the flow of ions. Class IV mutations, exemplified by R117H, reach the surface and open, but the channel pore is narrowed, leading to reduced flow of chloride ions. Class V and Class VI mutations result in a reduced quantity of functional protein at the cell surface, either due to reduced synthesis or accelerated breakdown.
Identifying CFTR Mutations
Genetic testing detects CFTR mutations and is performed in three main contexts: newborn screening, carrier screening, and diagnostic testing. All U.S. newborns are screened for CF, often starting with a blood test measuring immunoreactive trypsinogen (IRT). IRT is an enzyme typically elevated in infants with CF. If IRT levels are high, a second tier of testing analyzes the baby’s DNA for a panel of common CFTR mutations.
Carrier screening is offered to prospective parents to determine if they carry a CFTR mutation, which puts their child at risk. These tests examine DNA samples from a blood draw or cheek swab, looking for a panel of common mutations, such as the 23 recommended by the American College of Medical Genetics. When initial screening is inconclusive or symptoms are present, full gene sequencing may be performed to look for any of the more than 2,000 known mutations in the entire CFTR gene.