Cystic fibrosis, CF, is the most common inherited disorder that leads to shortened life expectancy,
especially among people of Northern European origin.
CF is caused by mutations in the CF transmembrane conductance regulator,
CFTR, gene. The CFTR protein encoded by this gene functions as a chloride channel,
allowing passage of chloride ions across cell membranes. Because chloride drives water movement,
CFTR plays an important role in regulation of water and salt balance in secreted fluids such
as mucus and sweat. Mutations in CFTR lead to production of THICKER mucus in various organs,
causing obstruction and other problems. CF patients also have saltier sweat.
Cystic fibrosis is inherited in an autosomal recessive manner.
It occurs when a child inherits one mutation of the CFTR gene from each parent. The parents
typically do not have the disease but are carriers – they carry one mutated gene and one normal gene.
More than 2000 different CFTR mutations have been identified so far, with the most common – deletion
of the amino acid phenylalanine at position 508 - accounting for about 70% of all cases.
Different mutations affect gene function to different degrees.
The severity of the disease depends on which mutations a patient has.
The most severe form may manifest in the first days of life, when the meconium – the first
stool of the newborn - is so thick and sticky that it is unable to pass, resulting in bowel
obstruction – an emergency that requires immediate attention. On the other hand, patients with a
milder CF may not show symptoms until their late childhood or even adulthood. Usually, the later
the symptoms manifest, the milder the disease. CF is a multi-system disorder but it affects
mainly the digestive, respiratory, and urogenital organs. While respiratory insufficiency is
the leading cause of mortality, digestive symptoms are usually the first to appear.
In the digestive system, obstruction of bile ducts may lead to poor fat absorption and
possible liver damage. Thick secretions may also block the pancreatic duct, preventing pancreatic
enzymes from reaching the intestine. In addition to poor digestion, this leads to accumulation
of digestive enzymes in the pancreas, which destroy pancreatic tissue, causing inflammation,
or pancreatitis. Damage of the pancreas may also result in shortage of beta-cells that
produce insulin, causing insulin-dependent diabetes – a very common complication.
Other digestive symptoms include abdominal distension, intestinal obstruction,
and intussusception, in which a segment of the intestine slides inside its adjacent segment.
In the lung, thick mucus cannot be readily cleared and becomes an environment for bacteria to grow
and cause infection. Chronic infections, together with the resulting overwhelming inflammatory
response, eventually lead to airway destruction. Thick mucus may also block the airways.
Common respiratory symptoms include chronic cough, wheezing, and recurrent pneumonia.
In the urogenital system, CF may cause infertility in men and decreased fertility in women.
CF is part of the newborn screening test in many countries. The screening typically involves an
immunoreactive trypsinogen test and a DNA test for common CFTR mutations. If the result is positive,
a sweat test for chloride content is performed to confirm the diagnosis.
Once diagnosed, chest radiographs, pulmonary function tests and arterial blood gas analysis,
may be used to follow disease progression. There is no definitive cure for CF but
early diagnosis can help manage the condition and increase lifespan.
A number of medications and therapies are available to help prevent or control lung
infections, clear or loosen mucus from the lungs, prevent or treat intestinal blockage,
and improve digestion. Lung transplantation maybe required for severe lung damage.
Recently, new modulator molecules have been developed to directly interact with the
mutated CFTR protein to correct its folding and facilitate its function. Notably, a combination
of 3 medications has been proven to effectively manage CF caused by the F-508 deletion.