Cystic Fibrosis: Screening, Testing, Ethics
Diane Seibert PhD, MS, WHCNP, ANP, CRNP
Melissa H. Fries MD, Colonel, USAF, MC
An Ancient Disease, a New Understanding: Description of the Problem
Parents of children with cystic fibrosis have recognized for centuries that the salty taste of their children was a sign of trouble. Folk tales arising during the middle ages in Central and Northern Europe warned “woe to that child which when kissed on the forehead tastes salty. He is bewitched and soon must die” (Halvorsen 2003). Many generations passed, however, before a full picture of the disorder of cystic fibrosis (CF) and the reason behind the salty taste emerged. Dr Dorothy Andersen, a pathologist at Columbia University, provided the first comprehensive pathological description of the disease in 1938. That year, the median survival for a child with CF was 12 months, and many children died so young that making the connection between respiratory disease, failure to thrive, and pancreatic disorders was virtually impossible. Andersen autopsied 49 infants and children who died from respiratory disorders characterized by thick, tenacious mucous (Andersen 1938). She found a consistent pattern of disease; all of the children had signs of pancreatic damage, chronic lung infection,
and malnutrition. Dr Andersen is credited with giving the disease its name, calling it “cystic fibrosis of the pancreas” because of the small cysts and fibrotic changes she noted in the pancreas.
and malnutrition. Dr Andersen is credited with giving the disease its name, calling it “cystic fibrosis of the pancreas” because of the small cysts and fibrotic changes she noted in the pancreas.
Advances in the treatment of CF are phenomenal, highlighting the impact that basic science research can have on improving survival and quality of life for thousands of individuals. As antibiotics and vaccines against common respiratory pathogens became available in the 1950s and 1960s, targeted treatment for CF began. For the first time, a child born with CF could be expected to survive to early adolescence (Boat 1997). When the cellular defect became understood in the early 1980s, therapy was further refined and longevity improved again. In 1989, when the genetic defect was identified and early diagnosis became possible, lifespan increased again. A child born with CF in 2005 has a median life expectancy of 40 years, and CF is now considered a manageable, chronic disease, not a childhood death sentence.
Why does the CF gene remain in the human genome, if individuals homozygous for the CF gene have such a poor survival rate? The answer may lie in some protective effect for having one copy (but not two) of the CF gene. Studies (Hansson 1988; Baxter, Goldhill, Hardcastle, Hardcastle, and Taylor 1988; Rodman and Zamudio 1991) suggest that CF heterozygotes may be more resistant to bacterial toxin-mediated diarrhea, possibly protecting carriers from cholera or other chloride-ion-secreting diarrheas. This protective effect may be particularly important for infants. It has also been hypothesized that CF heterozygote individuals may have a genetic protection against typhoid fever, because S. typhi enters the body through GI epithelial cells, using CFTR chloride channels (Pier, Grout, Zaidi, Meluleni, Mueschenborn, Banting, et al. 1998). Whatever the benefit for carriers of a single copy of a CF mutation, a double dose of the more severe form of CF mutation has, until recently, been lethal at a very young age.
Physiology
The symptoms of CF result from a genetic defect that inactivates a large protein molecule called the cystic fibrosis transmembrane regulator (CFTR). This protein is responsible for the structure and function of epithelial chloride channels, which, when functioning normally, permit chloride channels to open and close. Typically, when chloride ions move from one side of a membrane to the other, sodium ions follow, creating an osmotic “pull” that draws water along with them. In the normal lung, chloride ions move from the interstitial tissues into the airway, pulling water in with them. This water thins pulmonary mucous, allowing cilia to sweep bacteria
and mucous up and out of the lung. Normal chloride channel activity is critical to the effective functioning of epithelial cells in many body systems, but it is particularly important in lung and gastrointestinal tissues. It is the effect on the sweat glands that leads to the increased amount of surface sodium chloride, leading to the salty taste of the affected child’s skin.
and mucous up and out of the lung. Normal chloride channel activity is critical to the effective functioning of epithelial cells in many body systems, but it is particularly important in lung and gastrointestinal tissues. It is the effect on the sweat glands that leads to the increased amount of surface sodium chloride, leading to the salty taste of the affected child’s skin.
The CFTR gene was identified and sequenced in 1989. Located on the long arm of chromosome 7 (7q31.2), it contains approximately 250,000 base pairs and 27 exons (coding regions of the gene). To date, more than 1,300 CFTR mutations have been described, but the most common as well as the most serious mutation is a deletion of a three nucleotides encoding for phenylalynine (F) at the 508th position (DF508). Of all the patients in the United States with “classic” CF, 70% have a DF508 mutation. The wide range of mutation possibilities and severity of dysfunction is responsible for the range of symptom severity that can be seen with CF (Zeitlin 1999; Selvadurai, McKay, Blimkie, Cooper, Mellis, and van Asperen 2002). Individuals who are homozygous for DF508 or compound heterozygotes for two severe CF mutations usually present with the hallmark signs of “classic” cystic fibrosis first described by Dr Andersen: chronic respiratory infection, fat malabsorption due to inadequate pancreatic enzyme function, male infertility, and high sweat chloride levels (Knowles and Durie 200310).
Cystic Fibrosis Mutations
Cystic fibrosis mutations are classified according to how the mutation affects the CFTR protein. Class I, II, and III mutations are the most severe and typically cause symptoms associated with “classic CF,” while Class IV and V are less severe because at least some chloride ions reach their target organs. Class I mutations are typically nonsense or frameshift mutations (E69X, W401X, etc.) that block CFTR protein synthesis, resulting in a complete absence of CFTR. Class II mutations (DF508, N1303K), are usually caused by a deletion or missense mutation which produces a protein that is unable to migrate from the endoplasmic reticulum to the cell membrane. In a class II mutation, CFTR protein is made, but it cannot get to its site of action. Class III (G551D) mutations are missense or substitution mutations which disrupt the regulation of CFTR at the level of the cell membrane; the protein is made, is transported, but doesn’t work once it arrives. Class IV (R117H, R347P) mutations are usually missense mutations that alter the amount of chloride ions that move through the channel, reducing the overall volume. Class V (A445E) mutations are caused either by missense or alternative splicing, resulting in a reduction in the overall production of CFTR protein.
Features of Classic Cystic Fibrosis
Respiratory Tract
In the normal lung, chloride ions move freely across the epithelial cell wall via the chloride channel (from the airway epithelial cell into the mucous lining). As sodium ions move into the cell, chloride moves out and into the mucous layer in the airway, creating a concentration gradient which attracts water molecules, resulting in a thinning of pulmonary secretions, allowing for mobilization out of the airway. When the chloride channel is non-functional, pulmonary mucous becomes dehydrated and secretions become thick and tenacious. In the absence of negatively charged chloride ions, mucosal pH is also altered, creating a bacteria-friendly environment. Interestingly, it has also been hypothesized that the functioning CFTR proteins protect against Pseudomonas aeruginosa infection. Studies have shown that a lipopolysaccharide (LPS) in the cell membrane of P. aeruginosa is recognized, bound, and destroyed by CFTR protein. Lung cells with the ΔF508 mutation fail to bind and destroy P. aeruginosa. So, in addition to providing a healthy pulmonary environment, CFTR seems to be a critical element in lung’s immune response to bacterial infection, particularly P. aeruginosa (Schroder, Lee, Yacono Cannon, Gerçeker, Golan, et al. 2002).
Digestive Tract
Chloride channels function in the gut by allowing pancreatic proenzymes to move from the pancreas into the bowel. When CFTR proteins are absent, proenzymes build up in the pancreatic ducts resulting initially in fat malabsorption and eventually in the destruction of the pancreas. Symptoms associated with the pancreatic enzyme insufficiency include diarrhea, malnutrition resulting in poor weight gain, and slowed linear growth. Malabsorption often begins at or shortly after birth and failure to thrive may prompt a CF workup (a series of diagnostic procedures that may indicate the presence of a specific condition, such as CF) and eventual diagnosis. As mentioned, the pancreatic damage continues throughout life, and many individuals will develop Cystic fibrosis-related diabetes mellitus (CFRD) during the second decade of life. As the pancreatic damage accumulates, physiologic changes progress from reduced insulin secretion and insulin resistance (Ratjen and Doring 2003) to a total loss of pancreatic function. A recent study has shown that poor linear growth in the first decade of life may, in fact, be related to an early reduction in insulin production (Ripa, Robertson, Cowley, Harris, Masters, and Cotterill 2002). In a study of 18 children ages 9.5 to 15, 20% had impaired glucose tolerance and among those
with clinical normal glucose tolerance tests, insulin secretion was impaired in 65%. These results raise the question about whether insulin therapy should be considered in children with CF prior to the onset of CFRD (Ripa et al. 2002).
with clinical normal glucose tolerance tests, insulin secretion was impaired in 65%. These results raise the question about whether insulin therapy should be considered in children with CF prior to the onset of CFRD (Ripa et al. 2002).
Manifestations of Non-Classic Cystic Fibrosis
CF mutations in classes other than I, II, and III (A445E, etc.) may result in a partial loss of CFTR function and thus a variety of non-classic symptoms. They may have mild or atypical disease in childhood, develop “classic” CF symptoms in adulthood, present in adulthood for an infertility workup related to congenital bilateral absence of the vas deferens (CBAVD), or remain completely asymptomatic. The relationship between CFTR, other genes, and the environment are only now beginning to be appreciated. These relationships can play a significant role in the expression of the disease; children living in a smoke-filled environment, for example, often have more rapidly progressive pulmonary disease. Danish studies have shown that individuals with a CFTR mutation and a mannose-binding lectin mutation on chromosome 10 experience more rapidly progressive lung disease (Burke 2003). Finally, for reasons that are not well understood, the phenotypic expression of even “classic” CF (DF508/DF508) varies widely, from severe disease to total absence of symptoms.
What Is the Incidence?
Cystic fibrosis is one of the most common and well-documented autosomal recessive diseases in the United States. It is estimated that over 10 million people in the United States (1 in 25-29 European Caucasians) carry a CF mutation and are completely unaware of it. These asymptomatic CF carriers are healthy, but carry one normal (or “wild type”) gene and one mutated CF gene. When two asymptomatic carriers reproduce, the odds that their two mutated genes will assort together is 1:4, giving them a 25% chance of having a child with CF. This happens frequently enough that in the United States almost 1,000 people are born with cystic fibrosis each year—about 1:3,300 live births.
The Cystic Fibrosis Foundation (CFF), one of the oldest and most active disease-specific genetic support groups, was founded in 1955. The CFF supports the National Cystic Fibrosis Patient Registry, a comprehensive database with information on nearly all the 30,000 Americans living with cystic fibrosis (Cystic Fibrosis Foundation 2006), and certifies 117 Cystic Fibrosis Care Centers across the United States. The care centers provide data to the national registry annually on each one
of their patients, providing critical data about patient demographics, diagnostic symptoms, genotype, survival status, primary causes of death, clinical (pulmonary) status, microbiology, anthropometric measurements, number of hospitalizations, insurance coverage, etc. (Beker, Russek-Cohen, and Fink 2001). Each patient is assigned a unique identifier, so if they are seen at a different center, they are not lost to follow-up, and information is not duplicated. The registry, established in 1955, has set a goal to recruit, track, and report on the health status of all Americans with cystic fibrosis.
of their patients, providing critical data about patient demographics, diagnostic symptoms, genotype, survival status, primary causes of death, clinical (pulmonary) status, microbiology, anthropometric measurements, number of hospitalizations, insurance coverage, etc. (Beker, Russek-Cohen, and Fink 2001). Each patient is assigned a unique identifier, so if they are seen at a different center, they are not lost to follow-up, and information is not duplicated. The registry, established in 1955, has set a goal to recruit, track, and report on the health status of all Americans with cystic fibrosis.
