Latest in Cystic Fibrosis

Abstract

Cystic Fibrosis (CF) is a genetic disease affecting multiple organs. There are about 30,000 patients with CF in the United States, resulting from mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene, as well as its protein product. The life expectancy of CF patients has increased steadily over recent years, with the current expectation being for them to live into their late 30s. This is due to increased understanding, and therapeutic advances in the CF treatment armamentarium.

Historical Time line

Dorothy Andersen, MD, a faculty member at Columbia University, New York, first described the symptoms of CF in 1938; since then, remarkable progress has been made towards understanding the pathogenesis of the disease.¹ The sweat test, which measures the chloride content of sweat, which was originally developed by Paul di Sant’Agnese, MD, of Columbia University in 1953, remains the gold standard for diagnosing CF. The predicted median age of survival for patients with CF has now increased to 37 years from 20 years, in early 1980s. The defective CFTR gene, which is responsible for CF, was first discovered in 1989 by Francis Collins, MD, PhD, John Riordan, MD, and Lap-Chee Tsui, PhD. This breakthrough has led researchers to understand other genetically complex diseases, and has helped us understand the mechanism of CF at its structural level.

Epidemiology

CF is the most common fatal autosomal recessive disease among Caucasian population affecting one in 2,500–3,500 live births. However, it can occur in all races, affecting one in 17,000 African-American infants. There are 30,000 people in the US with CF and 10 million are carriers of the defective CF gene. Approximately 1,000 new cases of CF are diagnosed each year, and more than 70% of patients are diagnosed by the age of two years. Currently, more than 45% of the CF patient population is age 18 years or older. The predicted median age of survival for a person with CF is in the late 30s.²

Pathogenesis

CF occurs as a result of mutations along the long arm of chromosome 7 that encode for a protein called CFTR, which is expressed in the epithelial cells of airways, pancreatic duct, intestines, sweat gland ducts and reproductive duct. CFTR is a 1480 amino acid protein that contains two membrane-spanning domains and six membrane-spanning alpha helixes, portions of which form a chloride-conductance pore (See Figure 1). Channel activity is governed by two nucleotide-binding domains. The regulatory domain is a site of protein kinase A phosphorylation. The delta F508 mutation, which is the most common mutation, occurs on the surface of nucleotide-binding domain 1 due to omission of phenylalanine.³ See Figure 2.

Figure 1. Hypothesized Structure of CFTR.

The protein contains 1480 amino acids and a number of discrete globular and transmembrane domains. Activation of CFTR relies on phosphorylation, particularly through protein kinase A but probably involving other kinases as well. Channel activity is governed by the two nucleotide-binding domains, which regulate channel gating. The carboxyl terminal (consisting of threonine, arginine, and leucine [TRL]) of CFTR is anchored through a PDZ-type–binding interaction with the cytoskeleton and is kept in close approximation (dashed arrows) to a number of important proteins. These associated proteins influence CFTR functions, including conductance, regulation of other channels, signal transduction, and localization at the apical plasma membrane. Each membrane-spanning domain contains six membrane-spanning alpha helixes, portions of which form a chloride-conductance pore. The regulatory domain is a site of protein kinase A phosphorylation. The common ΔF508 mutation occurs on the surface of nucleotide-binding domain 1.

Figure 2. Categories of CFTRMutations.

Classes of defects in the CFTR gene include the absence of synthesis (class I); defective protein maturation and premature degradation (class II); disordered regulation, such as diminished ATP binding and hydrolysis (class III); defective chloride conductance or channel gating (class IV); a reduced number of CFTR transcripts due to a promoter or splicing abnormality (class V); and accelerated turnover from the cell surface (class VI).

Modifier Genes

There are many modifier genes responsible for the variations in pulmonary manifestations of CF patients who are homozygous for delta F 508 mutation.

TGFb1 (transforming growth factor beta 1) variants are implicated with severe lung disease.⁴ A recent study has discovered two new loci on chromosome 11p13 and chromosome 20q13 as CF pulmonary disease modifiers.⁵

CF Guidelines for Diagnosis of Cystic Fibrosis in Newborns through Older Adults

The Cystic Fibrosis Foundation (CF Foundation) has proposed the diagnostic criteria that require one or more characteristic phenotypic features (See Table 1), a history of CF in a sibling, or a positive newborn screening test.⁶ The laboratory evidence of CFTR dysfunction must be established with an increased sweat chloride concentration by pilocarpine iontophoresis on two or more occasions, a demonstration of abnormal nasal epithelial ion transport, or by identification of two CF genetic mutations. Sweat Chloride test should be the initial test performed in a suspected case of CF and is the gold standard for the diagnosis. It is abnormal in more than 90% of the diagnosed patients, although a normal sweat chloride value cannot be used as the sole criterion for ruling out the diagnosis of CF. The sweat chloride test is considered to be positive when it is more than 60 mmol/L, and it is borderline when the result is between 40 – 60 mmol/L. A sweat chloride value below 40 mmol/L is normal. The nasal potential difference (PD) in vivo showing the characteristic bioelectric abnormalities, is performed mostly in research centers.

Table I.

Phenotypic Features Consistent with a Diagnosis of CF

1. Chronic sinopulmonary disease, manifested by: Persistent colonization/infection with typical CF pathogens, including Staphylococcus aureus, nontypeable Haemophilus influenzae, mucoid and nonmucoid Pseudomonas aeruginosa, Stenotrophomonas maltophilia, and Burkholderia cepacia Chronic cough and sputum production Persistent chest radiograph abnormalities (eg, bronchiectasis, atelectasis, infiltrates, hyperinflation) Airway obstruction, manifested by wheezing and air-trapping Nasal polyps; radiographic or CT abnormalities of the paranasal sinuses Digital clubbing

2. Gastrointestinal and nutritional abnormalities, including: Intestinal: meconium ileus, distal intestinal obstruction syndrome, rectal prolapse Pancreatic: PI, recurrent acute pancreatitis, chronic pancreatitis, pancreatic abnormalities on imaging Hepatic: prolonged neonatal jaundice, chronic hepatic disease manifested by clinical or histological evidence of focal biliary cirrhosis or multilobular cirrhosis Nutritional: failure to thrive (protein-calorie malnutrition), hypoproteinemia and edema, complications secondary to fat-soluble vitamin deficiencies

3. Salt loss syndromes: acute salt depletion, chronic metabolic alkalosis

4. Genital abnormalities in males, resulting in obstructive azoospermia

Modified from Rosenstein B, Cutting G. The diagnosis of cystic fibrosis: a consensus statement. Cystic Fibrosis Foundation Consensus Panel. J Pediatr 1998;132:589–95. Used with permission.

There are currently 1893 mutations listed in CFTR mutation database. Only 23 have been demonstrated by direct or empirical evidence to cause sufficient loss of CFTR function to confer CF disease, and a screening test for checking these common mutations can be used for diagnostic purposes (See Table 2). These mutations account for the defects in both CFTR genes in 85% of the CF population, with the resultant severe loss of CFTR function usually causing pancreatic insufficiency and pulmonary complications. However, in some patients, a complete sequencing of the CF gene is necessary to establish the diagnosis.

Table II.

Recommended panel of CF-causing mutations

Missense, deletion, stop mutations			Splicing, frameshift mutations
G85E	I507del	R560T	621+1G>T	2789+5G>A
R117H	F508del	R1162X	711+1G>T	3120+1G>A
R334W	G542X	W1282X	1717–1G>A	3659delC
R347P	G551D	N1303K	1898+1G>A	3849+10kbC>T
A455E	R553X		2184delA

Revised from the mutation panel for population screening for CF developed by the ACMG.⁷⁷

Additional or alternative mutations present at significant frequencies in an ethnic population served by an NBS program maybe added.

Newborn screening depends on the initial identification of high values of immunoreactive trypsinogen (IRT) in the blood of the newborn.

CFTR-Related Diseases

The number of people with CFTR dysfunction is growing and such patients present with male infertility, recurrent pancreatitis, chronic sinusitis, and primary sclerosing cholangitis.² The sweat chloride test is not useful as a diagnostic test in these situations, and CFTR mutation testing should be referred to a CF geneticist since non-diagnostic variants may pose a clinical and diagnostic challenge.

Microbiology

The prevalence of Pseudomonas aeruginosa (P. aeruginosa) and Burkholderia cepacia (B. cepacia) complex have decreased over the last decade and the prevalence of Methicillin resistant Staphylococcus aureus (MRSA) and Stenotrophomonas maltophilia have increased.⁷

Treatment

CF care centers have a team of CF physicians, respiratory therapists, social workers, dietitians, and nurses who function as coordinators. The CF Foundation recommends that patients be evaluated at least once a year by respiratory therapists, social workers, and dietitians, as part of a comprehensive treatment plan.⁸

The CDC’s Advisory Committee on Immunization Practices, recommends influenza vaccination for all patients with CF over six months of age. As Cystic Fibrosis Related Diabetes (CFRD) is a frequently associated complication, the CF Foundation guidelines on CFRD recommend annual screening with an oral glucose tolerance test to begin at 10 years of age in all CF patients.⁹

Chronic Therapy for Maintenance of Lung Health

The CF Foundation recommends therapy based on the severity of the lung disease, which is expressed as a percentage of the predicted forced expiratory volume in 1 second (FEV1). The lung function is considered normal if FEV1 is more than 90% of predicted, mildly impaired if it is 70–89% of predicted, moderately impaired if it is 40–69% of predicted, and severely impaired, if less than 40% predicted value.¹⁰

Aerosolized Antibiotics

P. aeruginosa is the most common organism responsible for deterioration of lung function. Aerosolized antibiotics are indicated for the initial eradication of the organism as well as for chronic suppression of the infection.¹¹ The CF Foundation strongly recommends the chronic use of inhaled tobramycin to improve lung function, and reduce exacerbations in patients with CF, six years of age and older, who have P. aeruginosa persistently present in cultures of the airways. The net benefit is substantial in such patients with moderate to severe disease.¹⁰ Inhaled tobramycin at a dose of 300 mg is used twice daily in repeated cycles of 28 days on and 28 days off medication.

In 2010, the U.S. Food and Drug Administration approved inhaled Aztreonam solution for the treatment of CF patients with P. aeruginosa infection. Studies have demonstrated improvement in FEV1, with decrease in the density of P. aeruginosa in sputum, making inhaled Aztreonam an effective therapy for patients with CF who have chronic airway infection.¹² The usual dose is 75 mg three times a day through a special nebulizer that takes less than five minutes to deliver the medication. Inhaled Aztreonam is also used in repeated cycles of 28 days on and 28 days off medication. It may be used in the patients who are intolerant to inhaled Tobramycin, and also as a rotating antibiotic with inhaled Tobramycin, to maintain lung function.

There is not enough evidence to recommend the chronic use of other inhaled antibiotics like Colistin, Gentamicin, and Ceftazidime.

Recombinant Human DNase (dornase alfa)

Dornase alfa, the first biotech medication developed for CF, breaks down free DNA in mucus and decreases the viscosity of airway secretions, thereby helping patients with airway clearance.¹³ The CF Foundation recommends the chronic use of dornase alfa to improve lung function and reduce exacerbations.¹⁰ The net benefit with the usual recommended nebulized dose of 2.5 mg once daily is substantial in patients with moderate to severe disease.

Hypertonic Saline

Hypertonic saline (HS) inhalation increases the hydration of airway surfaces and improves the rheological properties of surface mucus by water diffusion, hence facilitating airway clearance. Studies have shown a significant reduction in acute exacerbation of CF lung disease as well as improvement in lung function with the use of hypertonic saline 7% twice daily.^14,15 Commonly observed side effects are cough and bronchospasm. This medication is well tolerated if pretreated with an inhaled beta agonist.

Macrolide Antibiotics

The beneficial effects of Azithromycin use in patients with CF are probably due to antimicrobial and anti-inflammatory properties. Clinical trials have shown a significant reduction in acute pulmonary exacerbations, along with improvement in lung function.¹⁶ The exclusion criteria used in these clinical trials were a history of positive sputum culture for Burkholderia cepacia complex or non tuberculous mycobacteria and abnormal liver function tests. In these trials, Azithromycin was used in 250 or 500 mg doses three times a week, or as 250 mg dose daily.

Bronchodilators-beta 2 Adrenergic Receptor Agonists

The CF Foundation recommends the chronic use of inhaled beta 2-adrenergic receptor agonists to improve lung function in patients with CF. ¹⁰

Role of Inhaled Corticosteroids

The CF Foundation recommends against the routine use of inhaled corticosteroids to improve pulmonary function, or to decrease pulmonary exacerbations for patients with CF without asthma, or allergic bronchopulmonary aspergillosis (ABPA) as there is no significant benefit.¹⁰

Airway Clearance Therapy (ACT)

CF patients face difficulty in clearing pathogenic organisms from the lung due to dehydration of airway surface liquids and defective mucociliary clearance, which leads to chronic lung infection and inflammation. ACT is recommended for all CF patients, however no particular form of ACT has been proven to be superior to others. Some forms of ACT include chest physiotherapy, huff coughing, high frequency chest compression (HFCC) popularly known as vest therapy, Internal Percussion Ventilation (IPV), and hand held devices like positive expiratory pressure (PEP), acapella, and flutter valves. Aerobic exercise is recommended as adjunctive therapy for ACT. The prescription for ACT should be customized based on age, severity of lung disease, and patient preference.¹⁷

Acute Exacerbation of Cystic Fibrosis Lung Disease

The CF Foundation registry 2010 has reported that the national average of acute exacerbations is 44.7% with one or more pulmonary exacerbations in patients 18 years and older.⁸ The CF Foundation has developed guidelines to treat acute exacerbations of CF lung disease; however, there is no clear definition of a pulmonary exacerbation. The clinical manifestations of an exacerbation usually include shortness of breath, worsening of cough with increased quantity of sputum, chest pain, loss of appetite, weight loss, and decreased lung function (FEV1).¹⁸ The major impacts of an acute exacerbation are poor quality of life, decreased survival due to progressive reduction in pulmonary function, and increase in overall health care costs. Prevention and appropriate management of acute exacerbations could lead to better quality of life and improvement in survival of CF patients. Failure to return to baseline lung function after exacerbation, is related to multiple factors, like Medicaid insurance, pancreatic insufficiency and female sex, as well as infections with P. aeruginosa, B. cepacia, MRSA and ABPA.^19,20

The CF Foundation recommendations for an acute exacerbation include continuation of chronic pulmonary therapies with an increase in airway clearance therapy. Intravenous antibiotics are commonly used for the treatment of acute exacerbation of CF lung disease. Hospitalization is a better option to treat an acute exacerbation, with optimal resources and support, although consideration can be given to starting (or continuing) intravenous antibiotics at home. The standard of care is to treat Pseudomonas infection during an acute exacerbation of pulmonary disease with the combination of two antibiotics, especially in patients with more severe disease.¹⁸ The treatment of an acute exacerbation with a single antibiotic maybe adequate in CF patients with a milder disease. Therapeutic regimens that include once a day aminoglycoside are preferable to multiple dosings per day. There is significant variability in the duration of antibiotic therapy in different CF care centers. The CF Foundation has stated that there is insufficient evidence to recommend an optimal duration of antibiotic treatment of an acute exacerbation of pulmonary disease. The usual practice is to treat patients with intravenous antibiotics for 14–21 days. The median duration (hospital and home IV) of hospital length of stay for treatment of pulmonary exacerbation in patients 18 years and older as per the CF Foundation registry 2010 is 14.7 days. Improvement in lung function during treatment of an acute exacerbation appears to plateau after 7 to 10 days of therapy. It is common practice to start the patient on intravenous antibiotics initially and switch to oral route for remainder of the course of therapy.

Many patients will need a higher level of nutrition during an exacerbation, and patients with CFRD require adjustment of insulin dosing for optimal control of blood sugars. Physicians have to pay close attention to adverse effects, especially to renal dysfunction resulting from antibiotic therapy.

Lung Transplantation

CF patients who have undergone lung transplantation tend to have a more favorable long-term survival than patients with other pulmonary conditions, due to their younger age and the lack of the concurrent comorbid factors in an older population. CF patients usually undergo double lung transplantation. Most centers decline patients with documented growth of B. cepacia on sputum culture. Survival among lung transplant recipients with CF is better as compared to patients with other pulmonary disorders, like idiopathic pulmonary hypertension, sarcoidosis, alpha-1 antitrypsin-deficiency emphysema, chronic obstructive pulmonary disease and idiopathic pulmonary fibrosis.²¹ The criteria for referral to lung transplantation include an FEV₁ below 30% predicted or a rapid decline in FEV₁ (particularly in young female patients), exacerbation of pulmonary disease requiring ICU stay, increasing frequency of exacerbations requiring antibiotic therapy, recurrent pneumothorax, and recurrent hemoptysis that is not controlled by embolization.²² The decision to transplant patients is determined by severity of lung disease, increase in oxygen requirement, hypercapnia, functional status, and pulmonary hypertension.

Nutrition and Gastrointestinal

There is a strong correlation between nutritional status and pulmonary function in CF patients. Addressing the nutritional status is essential for the maintenance of lung health in CF patients. Patients’ height and weight should be measured and their body-mass index (BMI) calculated at every clinic visit; the goal established by the CF Foundation Nutrition Guidelines is a BMI of 22 for females and 23 for males.²³ Nutritional supplements either orally or via a gastrostomy tube should be considered in those patients with growth or weight deficits, in addition to their usual dietary intake for children and adults. Pancreatic enzyme replacement therapy is indicated for all patients with pancreatic insufficiency based on clinical presentation with steatorrhea, abdominal pain and failure to thrive or a low fecal elastase level.

New Therapies

The CF Foundation has developed a therapeutic pipeline to improve the clinical course of CF and search for a cure.¹ A recent breakthrough involves directly targeting the defective CFTR.^24,25 VX-770 (Ivacaftor) is a CFTR potentiator and it has increased the activity of defective cell-surface CFTR protein in vitro, especially on cells with the G551D-CFTR mutation. At least one of the G551D mutations are present in approximately 4–5% of CF patients. VX-770 is administered orally and keeps phosphorylated G551D-CFTR channels open for longer time at the cell surface, and results in the increase of the ion flow across the epithelial apical membranes, that is why it called a potentiator.

A recently published, randomized, double-blind, placebo-controlled trial to evaluate Ivacaftor in subjects 12 years of age or older with CF and at least one G551D-CFTR mutation, showed a 10.6% improvement in the percent of predicted FEV 1 from baseline than the placebo group; this beneficial effect on lung function was observed by week two and lasted through weeks 48.²⁶ Ivacaftor also reduced pulmonary exacerbations by 55% and improved quality of life and weight gain at 48 weeks when compared to subjects receiving placebo. The concentration of sweat chloride, a marker of CFTR function, decreased by 48 mmol per liter from baseline to week 48 in subjects receiving Ivacaftor when compared to the group receiving placebo.

A phase 2 trial with VX-809 in subjects with CF who have the delta F508l mutation has been completed and there are no safety concerns. The treatment group demonstrated a decrease in sweat chloride, however, there was no change in lung function. VX-809 is known as a “corrector,” and it is designed to increase the trafficking of CFTR to the cell surface. Ongoing research will study the effectiveness and safety of the combination of VX 809 and VX770 in subjects with CF who have either one or two copies of the F508del-CFTR mutation.

Conclusion

Survival in patients with CF has improved steadily over the last few decades due to effective therapies that have resulted in improved clinical manifestations. Twenty-two years after the discovery of the CF gene, although elusive, there is hope of finding a cure for CF.

Biography

Ravi P. Nayak, MD, FCCP, is Associate Professor of Internal Medicine, in the Division of Pulmonary, Critical Care, and Sleep Medicine at Saint Louis University School of Medicine.

Contact: nayakrp@slu.edu