Case Study # 36

Cystic Fibrosis and Genetic Analysis
- Dr David Ramsay MD
- Dr Marie L. Borum, MD, EdD, MPH
- Dr David Jager MD

Learning Objectives

1. Explain the epidemiology of Cystic Fibrosis (CF)
2. Review the mode of inheritance and the genes involved with Cystic Fibrosis
3. Describe the pathophysiology of Cystic Fibrosis
4. Describe the typical presentation and disease progression of Cystic Fibrosis
5. Describe the more common gastrointestinal complications experiences by Cystic Fibrosis patients
6. Review the diagnosis of Cystic Fibrosis
7. Create a plan for genetic counseling of family members for patients with Cystic Fibrosis
8. Review the current and future treatment options

Pretest Questions

1. What is the typical initial presentation of neonates with cystic fibrosis?
   a. Pyloric stenosis
   b. Intussusception
   c. Volvulus
   d. Omphalocele
   e. Meconium Ileus
   f. Gastroschisis
2. Which of the following vitamins is classically malabsorpted by cystic fibrosis patients?
   a. Vitamin B1
   b. Vitamin B6
   c. Vitamin B12
   d. Vitamin C
   e. Vitamin E
3. What is the prevalence of cystic fibrosis in the Caucasian population in both North America and Europe?
   a. 1/30
   b. 1/300
   c. 1/3,000
   d. 1/30,000
   e. 1/300,000
   f. 1/3,000,000
4. Which of the following is not one of the more common gastrointestinal effects of cystic fibrosis?
   a. failure of the exocrine pancreas to secrete bicarbonate and enzymes
   b. retention of biliary secretions leading to focal biliary cirrhosis
   c. failure of the gallbladder epithelium to secrete salt and water leading to chronic cholecystitis and cholelithiasis
   d. failure of hepatocytes to oxidize free fatty acids leading to steatosis
   e. failure of the intestinal epithelium to flush secreted mucins and other macromolecules
5. Which of the following is a classic pathologic organism found in the sputum microbiology of cystic fibrosis patients?
   a. Burkholderia cepacia
   b. Moraxella catarrhalis
   c. Legionella pneumoniae
   d. Nocardia asteroides
   e. Actinomyces israelii

Answers:  1) e, 2) e, 3) c, 4) d, 5) a

Case Study

A 21 year old patient presents to your office with complaints of abdominal discomfort and constipation.  You have known this patient for many years.  He was diagnosed with cystic fibrosis (CF) shortly after birth when he developed a meconium ileus.   During his childhood, he had several cases of pneumonia and required hospitalization on multiple occasions for IV antibiotic treatment.  He also has a history of alternating periods of diarrhea and constipation.   His only surgery to date is an appendectomy three years ago.  He tries to stay away from irritants such as cigarette smoke, and he takes pancreatic enzyme replacements, fat soluble vitamins, and DNase to help break up respiratory tract mucus.  He has no other medical history.  His family history is significant for one of his four siblings diagnosed with cystic fibrosis.  There is no other history of CF in his family.  Over the past few weeks he complains of progressive abdominal pain after meals, and he has strained more during bowel movements.  He states he has not had a bowel movement for several days.  He denies fever, wheezing, shortness of breath, or chest pain, but he states he has a great deal of abdominal pain, nausea with occasional emesis, and distention.  On physical exam you notice the patient appears to be in a good amount of pain.  His pulmonary exam is significant for mild to moderate wheezes throughout all lung fields but the patient has good air movement.  On abdominal exam, you note a distended abdomen, high pitch bowel sounds, and tenderness to palpation without guarding or rebound.  The rest of the physical exam is within normal limits.  On abdominal plain film, you notice distended small bowel loops.

Explain the epidemiology and prevalence of Cystic Fibrosis:

            Approximately 1/3000 lives birth in the Caucasian population of both North America and Europe are affected by certain mutations in ion transport making cystic fibrosis one of the most common genetic disorders.  Although the highest rates of incidence are in Caucasians and Ashkenazi Jews, the disease is seen in a wide range of racial subgroups.  For example, the rate of detection in Hispanics is 1/9500 live births, and the rate of detection in African-Americans is 1/17,000 live births.  The median survival is 29 years for females and 32 years for males (1). 

Review the mode of inheritance and the genes involved with Cystic Fibrosis:

Cystic fibrosis is an autosomal recessive disease resulting from mutations on the CF transmembrane conductance regulator (CFTR) gene located on chromosome 7.  Although there are over 1000 reported mutations on the gene that have been associated with disease, 70% of known cystic fibrosis patients have a specific mutation called Delta F508 which is a 3-base pair deletion resulting in an absence of phenylalanine at amino acid position 508 of the CFTR gene.  There are different levels of disease in patients with the same genotype leading to speculation other factors, such as the environment, may play a role in disease progression (2).  Many cystic fibrosis patients will have more than one mutation in the CFTR gene, however, this finding usually does not correlate with disease progression or prognosis.
. 

Explain the pathophysiology of Cystic Fibrosis:

          Patients with Cystic Fibrosis manifest disease in numerous different organs including the lungs, liver, biliary tract, pancreas, small and large bowels, reproductive tract, and sweat glands.   The CFTR gene encodes proteins that conducts chloride across the apical membranes of polarized epithelia and regulates the activity of ENaC, an epithelial sodium channel (2).  Since these proteins affect ion (and therefore water) flow across epithelial membranes, secretion and absorption are impaired in cystic fibrosis patients.  The result is specific to the affected organ system. 

In the lungs, there is increased Na+ absorption and decreased Cl- secretion.  The mucus produced by the epithelium is devoid of salt and water.  The periciliary water volume is also reduced.  Since the formation and clearance of mucus is a primary defense mechanism against bacteria, the mucus is less effective against bacteria due to increased adhesion to the airway surface and failure to clear.  Therefore infection of the airways tends to be present in the mucus layer where antibiotics have poor penetration rather than the airspace (1). 

In the gastrointestinal tract there is a failure to secrete bicarbonate and Cl- in the pancreas leading to a lack of secretion and alkalization as well as retention of pancreatic enzymes.  These enzymes tend to digest the pancreas over time and lead to chronic pancreatitis and diarrhea.  Approximately 85-90% to cystic fibrosis patients will require pancreatic enzyme replacement.  In the intestine, there is a decrease of Cl- and water secretion in intestinal epithelial cells leading to failure to secrete mucin and other macromolecules.  The result is meconium ileus in neonates and constipation in adults.  Other common complications are small and large bowel obstruction due to dry bulky stools, malabsorption of fat soluble vitamins (A, D, E, K), and appendicitis.  In the hepatobiliary system, there is defective Cl- and water secretion in the hepatic duct and gallbladder epithelium which leads to retention of biliary secretions, focal biliary cirrhosis, chronic cholecystitis, and cholelithiasis.

In the genitourinary tract, defective liquid secretion leads to obstruction of the vas deferens and azoospermia as well as occasional obstruction of the fallopian tubes.  Women have thickened vaginal secretions that can obstruct sperm motility.    
Cystic fibrosis patients with different mutations may present with various levels of dysfunction.  It is believed the specific CFTR genotype is poorly correlated with the level of pulmonary disease.  As mentioned earlier, many patients with the same genotype will have various presentations suggesting there are other genetic or environmental factors that have a role in pathogenesis.

Explain the typical presentation and disease progression of Cystic Fibrosis:
         
Cystic fibrosis can present in a number of different ways.   Two-thirds of patients in the United States are diagnosed in childhood.  7% are diagnosed as adults.  Classically, the first signs of disease occur in the first days of life when a newborn fails to pass stool.  Examination will lead to the diagnosis of meconium ileus which is found in 10-20% of cystic fibrosis newborns.  Small and large intestine obstruction as well as constipation are common in children.  Another presentation in children is failure to thrive.  This finding may be due to pancreatic obstruction or malabsorption of fat soluble vitamins.  CF is the most common heritable cause of pancreatic insufficiency (3).  As children age, many will need pancreatic enzyme supplementation including insulin injections for diabetes.  Early respiratory manifestations include productive cough and pulmonary function tests that are consistent with obstructive airway disease.  Pulmonary complications before one year of age have a poor prognosis (4).  Panopacification of the paranasal sinuses are present in 90-100% of patients older than 8 months of age.  Nasal polyps are seen in 10-32% of patients.  As children age they will often develop lung infections and chronic bronchitis with or without bronchiectasis.  Common pathogens include Staphylococcus aureus, Haemophilus influenzae, Burkholderia cepacia, Aspergillus, and Pseudomonas species. 

As adults, cystic fibrosis patients will present with numerous pneumonias, bronchitis, and pneumothorax, however, GI complaints, DM, and infertility are common presentations.  Adults will have various gastrointestinal pathology including focal biliary cirrhosis, chronic cholecystitis, cholelithiasis.  The mean age of presentation of liver disease is around 7–9 years of age and prevalence increases as patients age, reaching a peak prevalence around 9% in 16–20 year olds (2).  Pancreatitis, appendicitis, intestinal obstruction, and constipation leading to a buildup of mucin and other macromolecules are other common GI problems.  Adults with CF are often unable to conceive due to destruction of the vas deferens, obstruction of fallopian tubes, or blockage of sperm or the cervical os by thickened mucus.  More than 95% of male patients are azoospermic and 20% of women are infertile.  Of those who are able to conceive, 90% of pregnancies will produce viable infants with women able to breast feed normally (1).   

More than 95% of CF patients die from complications of pulmonary function that leads to infection.  As treatment has improved, the life-expectancy has increased.  Over 38% of CF patients are now adults and 13% are now past the age of 30.  As mentioned earlier, the median survival is over 32 years for males and 29 years for females (1). 
         

How do you diagnose Cystic Fibrosis?

            Since there are so many different mutations, the primary method of diagnosis is based on clinical presentation and confirmatory testing.

The diagnosis of cystic fibrosis (CF) is established in individuals with the following (5):

• One or more characteristic phenotypic features of CF
  AND
• Evidence of an abnormality in cystic fibrosis transmembrane conductance regulator (CFTR) function based upon ONE of the following:
• Presence of two disease-causing mutations in the CFTR gene
  OR
• Two abnormal quantitative pilocarpine iontophoresis sweat chloride values (>60 mEq/L)
  OR
• Transepithelial nasal potential difference (NPD) measurements characteristic of CF

The diagnosis of CF may be made in the absence of phenotypic features of CF in the following settings:

• Confirmed diagnosis of CF in a sib and an abnormal sweat chloride value or the presence of the same two disease-causing mutations in the CFTR gene as identified in an affected sib
• Diagnosis in a newborn screening program (based on the presence of two disease-causing mutations in the CFTR gene OR abnormal sweat chloride value). In 2002, 12.8% of newly diagnosed individuals were identified through newborn screening
• In utero diagnosis by presence of two disease-causing mutations in the CFTR gene. In 2002, 4.0% of newly diagnosed individuals were identified by prenatal diagnosis.

A Cl- concentration >60-70 meq/L strongly suggests the diagnosis of CF.  At least 50 mg of sweat must be collected within a 45-minute period for the test to be valid so collecting sufficient sweat is frequently problematic in young infants.  The transepithelial nasal potential difference (NPD) measures the absence of functional CFTR at the apical surface with resultant alterations in chloride efflux and sodium transport which produces an abnormal electrical potential difference across epithelial surfaces.  Most infants with CF have elevated blood levels of immunoreactive trypsin (IRT), which can be quantified by radioimmunoassay or by an enzyme–linked immunoassay.  The test allows detection of at least 95 percent of newborns with CF and can be used if sweat chloride testing is not feasible.  However, the rates of false positive and false negative results for IRT testing are relatively high.  Genetic testing is available for many genotypes.  Most diagnostic laboratories in the United States screen for 20 to 30 of the most common mutations, which identifies approximately 90 percent of CF chromosomes.

 Create a plan for genetic counseling of family members for patients with Cystic Fibrosis:

          As always with testing for genetic disease, all of the implications need to be discussed with the patient and family before they consent to testing.  There are well known psychological and financial risks to any type of genetic testing.  The National Institutes of Health Consensus Development Conference Statement on genetic testing for cystic fibrosis recommends genetic testing should be offered to the following groups (4):

• adults with a positive family history of CF should have testing at any time
• newborn siblings of patients with CF as well as other siblings who exhibit atypical symptoms should be tested. However, testing of minors for the purpose of identifying carrier status is not recommended
• partners of people with CF
• couples currently planning a pregnancy, particularly in high risk populations such as Caucasians or Ashkenazi Jews
• couples seeking prenatal care

The panel also recommended providing education of cystic fibrosis to all people who undergo genetic testing.  The panel did not recommend offering CF genetic testing to the general population.  The panel also did not recommend routine screening of newborns as there are no studies showing improved pulmonary outcomes for diagnosis earlier than the current average (4).  It should be noted some studies have shown benefit for patients diagnosed in infancy compared to childhood.  Some states have instituted mandatory screening based on the recommendation of the CDC in 2004 that screening programs were justified if local resources were available based on better growth and improved cognitive development of cystic fibrosis children in screened patient populations (6).

What are the current and future treatment options for CF patients?

          Due to the numerous manifestations of the disease, careful attention must be given to all of the affected organ systems. 

Pulmonary- Antibiotics should be given for signs of airway infection, however, there has been little proven benefit for the use of prophylactic antibiotics in asymptomatic patients.  Due to its prevalence, antibiotic choice should include coverage for Pseudomonas sp.  IV antibiotics should be considered for any infection except for mild exacerbations.  Treatment for chronic obstruction includes the use of bronchodilators, such as beta-agonists and anticholenergics.  DNase can decrease the viscosity of sputum by cleaving long strands of DNA into smaller segments and thereby help clear obstructed airways, and long term trials have shown DNase extends time between pulmonary exacerbations.  Hypertonic saline has showed benefit when used with bronchodilators, and home exercise may help pulmonary function. Lung transplantation has a two year survival that exceeds 60% and survival is similar to those who undergo transplantation for other disease.  The transplanted lungs do not display the CF phenotype.  Most patients who receive lung transplants die from graft rejections instead of infection. 

Gastrointestinal- Nutrition is very important for CF patients and supplementation of pancreatic enzymes and fat-soluble vitamins can help maintain good health.  Treatment for diabetes is often necessary with insulin injection.  Obstructions can be treated with barium enema or surgery if severe.  Biliary disease is managed in the same manner as the general population.  Biliary sludge or frank obstruction, and associated hepatic inflammation, are treated with oral ursodiol.  Liver transplant may become necessary and has a >50% two-year survival.  There are also increased risks of osteopenia related to Vitamin D and calcium deficiencies that require screening of bone density starting in adolescence.

Genitourinary- Men who wish to conceive can have microsurgical epididymal sperm aspiration and intracytoplasmic sperm injection. 

            Future treatments will involve gene therapy with the hope of replacing or circumventing the loss of CFTR function, however there are currently no proven treatments involving gene therapy. 

Patient Resources

Cystic Fibrosis Foundation
http://www.cff.org/home/

Medline Plus: Cystic Fibrosis
http://www.nlm.nih.gov/medlineplus/cysticfibrosis.html

Cystic Fibrosis Support Community
http://www.cysticfibrosis.com/

Genetics Home Reference for Cystic Fibrosis
http://ghr.nlm.nih.gov/condition=cysticfibrosis

Cystic Fibrosis Carrier Testing Information
http://www.acog.org/from_home/wellness/cf001.htm

References 

1. Kasper D, Braunwald E, Fauci A, et al. Harrison’s Principles of Internal Medicine. 16thed, McGraw Hill, 2005, 1543-46.

2. Cutting GR. Modifier genetics: cystic fibrosis. Review of Genomics & Human Genetics. 6:237-60, 2005.

3. Cohn JA, Mitchell RM, Jowell PS. The impact of cystic fibrosis and PSTI/SPINK1 gene mutations on susceptibility to chronic pancreatitis. Clinics in Laboratory Medicine. 25(1):79-100, 2005 Mar.

4. Anonymous. Genetic testing for cystic fibrosis. National Institutes of Health Consensus Development Conference Statement on genetic testing for cystic fibrosis. Archives of Internal Medicine. 159(14):1529-39, 1999 Jul 26.

5. Moskowitz S, Gibson R, MD, Sternen D, et al.  GeneReviews: CFTR Related Disorders. http://www.genetests.org/query?mim=602421 2005 Aug 24.

6. Grosse SD, Boyle CA, Botkin JR, et al. Newborn screening for cystic fibrosis: evaluation of benefits and risks and recommendations for state newborn screening programs. MMWR Recomm Rep 2004; 53:1.