Cystic Fibrosis

Cystic fibrosis (CF) is a genetic disorder that alters salt and water balance in the body. It affects multiple organs, especially the lungs and digestive system.

More than 10 million people in the US carry a cystic fibrosis-causing gene variation, but most do not know it. Early diagnosis leads to better outcomes, so cystic fibrosis is tested for in most newborn genetic screening panels. Starting treatment right away can prevent lung damage and improve nutrition, leading to a much longer and healthier life.

The affected gene in cystic fibrosis is CFTR (cystic fibrosis transmembrane conductance regulator) on chromosome 7. The gene codes for the CFTR protein. People with cystic fibrosis have two non-working copies (alleles) of the gene, and so they make little or no CFTR protein.

The job of the CFTR protein is to help maintain a healthy balance of salt and water. It does this by moving chloride ions (from sodium chloride, or salt) out of cells. This process is key for maintaining a healthy layer of mucus inside the lungs, digestive tract, and other organs.

People with cystic fibrosis inherit two non-working copies (alleles) of the CFTR gene: one from each parent. Without CFTR protein, cells are not able to balance salt and water properly. The normally thin fluids that line certain organs become thick and sticky, leading to the effects of cystic fibrosis.

From the perspective of having the genetic disorder, cystic fibrosis follows an autosomal recessive inheritance pattern. It takes two non-working alleles to cause the disorder.

From the perspective of the CFTR protein that is made, a person's two CFTR alleles are co-dominant. Protein is made from both.

The CFTR gene is switched off in most cell types. It is on at high levels in cells that line the lungs and digestive organs, especially the gallbladder, pancreas, and intestines. The CFTR gene is on at lower levels in cells that line certain reproductive organs in both men and women, and in sweat glands in the skin.

The cells that make CFTR protein form a barrier between the body and the spaces inside of it. In the lungs, that space holds air; in the pancreas and gallbladder, it holds digestive juices; in the small intestine, it holds food that is being digested.

CFTR protein sits in the plasma membrane of the cells that make it, making a connection between the inside of the cells and the outside space.

Healthy CFTR protein moves chloride ions between the inside of a cell and the space that surrounds it. Its job is to keep salt and water in balance. This balance is key to maintaining a healthy layer of mucus between the body and the outside world.

In the lungs, the mucus traps debris—things like dust, bacteria, and viruses. Cilia wave back and forth to carry this debris back out of the body. In the pancreas and gallbladder, the right balance of salt and water helps to keep digestive juices flowing. CFTR protein also maintains water and salt balances in the intestines and liver. In sweat glands, it helps re-absorb salt from sweat.

When CFTR protein is not working, salt and water get out of balance. The mucus layer in the lungs is thick and sticky. Chloride is not re-absorbed from sweat, making it extra salty. Mucus can clog the pancreas, making digestive juices too thick to flow. Thickened fluids similarly affect other organs, keeping them from working properly. All of these effects lead to the symptoms and features of cystic fibrosis.

The effects of cystic fibrosis can be very different from person to person. They fall on a spectrum from mild to severe, and they may involve one or more organ systems.

Babies born with cystic fibrosis are often small. Most have thick mucus in their lungs, which causes coughing and wheezing from an early age. Because the mucus traps bacteria, people with cystic fibrosis tend to get one lung infection after another. These can lead to pneumonia or bronchitis. Before treatments were available, these infections were often deadly.

In the digestive system, thickened mucus can prevent the absorption of nutrients from food. Without treatment, this can cause low weight, slow growth, and greasy stools.

Problems with salt regulation in sweat can lead to dehydration and salt imbalances. Extra-salty skin is a defining characteristic of cystic fibrosis. In fact, a sweat test—which measures chloride levels on the skin—is the most common way that cystic fibrosis is diagnosed.

People with cystic fibrosis have two non-working CFTR alleles. They may have two copies of the same allele, or two different alleles. There are many CFTR alleles that can cause cystic fibrosis, and each on codes for a protein that works a little differently. These differences in protein function are part of why the effects of cystic fibrosis vary.

Some CFTR alleles produce no CFTR protein at all. Some code for CFTR protein that never makes it to the cell membrane, so it cannot do its job. Other alleles code for CFTR proteins that make it to the cell membrane, but they do not work very well.

The amount of CFTR protein a person makes, and how well that protein can move chloride, affects a person's symptoms. People who cannot move chloride at all tend to be sicker, while people who can move some chloride tend to have fewer symptoms. Those who have just one healthy CFTR allele have near-normal levels of CFTR protein function, and no symptoms of cystic fibrosis.

The graph shows the amount of chloride on the skin of people with different CFTR allele combinations. This test can estimate how well a person's CFTR proteins are working. Yet it does not always predict how severe their symptoms will be. The protein may work better or worse in other tissues. Other factors come into play as well (see below).

Between every two people with cystic fibrosis, there are differences in the details. Even when they have the same CFTR alleles. Often symptoms appear before a baby is one year old, but not always. People vary in which organs are affected, and in severity of symptoms. Symptoms change over time—most often getting worse with age. However, this can happen at different rates.

Some of these differences are due to variations in other genes. For example, there are other genes that affect how the lungs or the pancreas work. Many genes influence how well the immune system responds to infection. Still other genes are needed to repair damaged tissue. Variations in these genes may make cystic fibrosis better or worse.

Environmental factors can also cause differences in symptoms. Air pollution and cigarette smoke can make lung symptoms worse. Good nutrition and access to healthcare can help people stay healthier.