Cystic Fibrosis Flashcards
What chromosome has Cystic Fibrosis been mapped to?
Chromosome 7
What method of inheritance does CF follow?
Autosomal recessive inheritance
Approximately how many live births does CF occur in?
Affects approximately 1 in 2,500 livebirths
Describe the clinical features of classic CF.
It is a complex multi-system disease.
CF primarily affects mucus-producing cells in the exocrine glands.
Major effects on:
- the epithelia of the respiratory tract
- the function of the exocrine pancreas
- the intestine
- the hepatobiliary system
- The male genital tract may also be affected.
Many additional complications are also associated with CF.
What is the major cause of morbidity and mortality in CF?
Pulmonary disease is the major cause of morbidity and mortality in CF.
What problems can arise due to the effects of CF on the GI tract?
The GI tract can be affected leading to pancreatic insufficiency because of the blocked tubes. Malabsorption of nutrients occurs in the great majority of patients with CF.
A blocked intestine (Meconium ileus) occurs in 10-20% of newborns diagnosed with CF. This is a major complication for newborn children with cystic fibrosis.
What feature may lead you to conclude a diagnosis of CF?
One or more phenotypic features:
- Chronic sinopulmonary disease
- GI/nutritional abnormalities
- Obstructive azoospermia (no sperm in ejaculate)
- Salt-loss syndrome
Plus 1,2 or 3
1) . 2 disease causing mutations in CFTR
2) . 2x sweat tests with CI>60mEq/l
3) . Transepithelial nasal potential difference characteristic of CF
What forms of disease are variants in the CFTR gene associated with?
Mutations in the CFTR gene are associated with both classical and non-classical CF-related diseases.
Classical CF: obstructive lung disease, bronchiectasis, exocrine pancreatic insufficiency, elevated sweat chloride (>60mM), infertility in males due to CBAVD.
Non-Classical CF: Chronic pulmonary disease +/- pancreatic disease +/- elevated sweat chloride +/- CBAVD - there is a spectrum of disease and at the far end individuals may present with no symptoms other than CBAVD
CBAVD: Congenital bilateral absence of the vas deferens
How may non-classical CF present?
Non-Classical CF:
Chronic pulmonary disease with or without pancreatic disease, with or without elevated sweat chloride, with or without CBAVD.
There is a spectrum of disease and at the far end individuals may present with no symptoms other than CBAVD
What features are required in order to make a diagnosis of CBAVD?
- Azoospermia
- Absence of the vas deferens on palpatation
- Mutation in 1 or 2 alleles
CBAVD represents 1.2-1.7% of male infertility and 80% will have at least 1 mutation in CFTR.
How is CF managed?
CF is managed aggressively and this has resulted in huge improvements in the lifespans of individuals in recent decades.
To manage the respiratory symptoms:
- physiotherapy
- Oral, inhaled or IV antibiotics
- Bronchidilators
- Mucolytic agents
- Anti-inflammatory agents
- Home oxygen therapy
- Lung or heart-lung transplantation
- Lifestyle (exercise)
To manage GI symtoms:
- High calorie, high fat diet
- Supplemental feeding
- Oral pancreatic enzyme replacement therapy (pancreatin)
- Tube feeding
To manage CBAVD:
- Assisted reproductive technology
The future:
- Gene therapy
- Protein assist/repair
How can the respiratory symptoms of CF be managed?
To manage the respiratory symptoms:
- physiotherapy
- Oral, inhaled or IV antibiotics
- Bronchidilators
- Mucolytic agents
- Anti-inflammatory agents
- Home oxygen therapy
- Lung or heart-lung transplantation
- Lifestyle (exercise)
How can the GI symptoms of CF be managed?
To manage GI symtoms:
- High calorie, high fat diet
- Supplemental feeding
- Oral pancreatic enzyme replacement therapy (pancreatin)
- Tube feeding
How can the CBAVD be managed?
Via the utilisation of assisted reproductive technology.
What future therapies are in the pipeline for CF management?
The future:
- Gene therapy
- Protein assist/repair
What is the long-term outlook for CF patients currently?
- Median survival is currently 31.6 years and rising
- If they are pancreatic sufficient (
Describe the specific location and features of the CFTR gene.
- The CFTR gene is located at 7q31.2
- The CFTR gene spans 190kb of genomic DNA
- The CFTR gene contains 27 exons and consists of 1480 amino acids
Describe the CFTR protein and its function.
Within the cell, the CFTR protein sits within the cell membrane and its main function is in secretory epithelial cells. The role of the protein is in homeostasis in maintaining the salt balance between cells. It is a salt channel and it regulates the movement of chloride ions (Cl-) through the cell. The regulation of chloride ions is controlled by 3 domains in the protein. In order to function normally CFTR requires ATP binding to the nucleotide binding domains (of which there are 2) and phosphorylation of a regulatory domain.
Describe the types of CF-causing CFTR mutations usually found in the CFTR gene.
The majority of mutations found in the CFTR gene are single nucleotide changes. They are usually found in exons, splice sites, or in specific intronic areas.
The delta508 mutation (3nt deletion causing a loss of phenylalanine residue) accounts for approximately 70% of white UK mutations. Approximately 30 mutations account for over 90% of total CF mutations identified. However, there are over 1900 rare mutations in the Toronto CF Reference database.
How do Class 1 CFTR mutations affect the CFTR protein?
Class 1 mutations affect translation. For example, the G542X mutation causes a premature stop codon and therefore leads to truncation of the CFTR protein.
How do Class 2 CFTR mutations affect the CFTR protein?
Class 2 mutations affect protein maturation. For example, the common delta508 mutations causes incorrect folding and susceptibility to proteolysis.
How do Class 3 CFTR mutations affect the CFTR protein?
Class 3 mutations affect the function of the protein at the cell surface. The protein is correctly lodged at the cell surface but cannot be activated. For example, the G551D mutation affects the nucleotide binding domain.
How do Class 4 CFTR mutations affect the CFTR protein?
Class 4 mutations cause a decrease in chloride conductance. For example, the R1347P mutation affects the transmembrane segment of the protein.
How do Class 5 CFTR mutations affect the CFTR protein?
Class 5 mutations cause a decrease in the amount of synthesis in the CFTR protein. For example, mutations affecting splicing efficiency such as polyT repeats in intron 8.
Which classes of mutations are generally associated with a severe/classic CF phenotype?
Class 1, 2 and some 3 are generally associated with a more severe phenotype/classic CF
Which classes of mutations are generally associated with a milder phenotype?
Class 4, 5 and some 3 are generally associated with a milder phenotype
Why is it thought that the phenotype of CFTR-RD is so variable?
It is thought that the phenotype is variable because in addition to the CFTR mutations, other modifier genes and environmental influences also have an effect on the phenotype displayed.
Outline the process of gene therapy for CF using compacted DNA
The process uses single molecules of plasmid DNA with the CFTR gene in compacted into nanoparticles. These particles produce no immune response or significant side effects. There is evidence of gene transfer for several days (transient, 6-28 days). The efficiency of gene transfer is increased if delivery is conducted using liposomes. There is now ongoing work in animals to look at the viability of this in animals.
How does the drug PTC124 function?
It has been known for some time that read through of stop codons can be facilitated by the antibiotic gentamicin. PTC124 is an analog of gentamicin and in trials it compares favourably with gentamicin (well tolerated, non-toxic). It promotes read-through of premature stop codons (such as W128X). PTC124 has been of particular interest in Israel where W1282X accounts for 60% of CF mutations in Ashkenazi Jews.
Describe how the developmental CF drug VX770 works.
VX770 acts as a potentiator and has been shown to increase gating activity in studies using the mutations G551D and G1349D which are both class 3 mutations. Patients have shown up to a 10% increase in their lung function.
Summarise a few of the CF drugs that are currently under development.
PTC124 - fixes some of the Class 1 mutations by allowing stop codon read through.
VX809 - interferes with the chaperone machinery and fixes some Class 2 mutations.
VX770 - aids in gating defects of Class 3 mutations.
Future therapy may well be mutation specific.
What does the CF drug PTC124 do?
PTC124 - fixes some of the Class 1 mutations by allowing stop codon read through.
What does the CF drug VX809 do?
VX809 - interferes with the chaperone machinery and fixes some Class 2 mutations.
What does the CF drug VX770 do?
VX770 - aids in gating defects of Class 3 mutations.
Summary:
- CFTR mutations cause a disease spectrum
- The primary pathology of CF is the loss of salt movement and homeostasis leading to viscous mucus (lung pathology/pancreatic malfunction) and developmental abnormalities (CBAVD)
- CF is a serious condition with improving management
- Several drugs that target specific mutations are currently being developed and are in trial stages
In which other gene have mutations been identified in some cases of non-classical CF where no CFTR mutations have been identified?
Mutations in the SCN1 gene have been found in non-classic CF cases where ni CFTR mutations have been identified (diagnostic utility of test not determined).
What is the UK incidence of CF?
The UK incidence is about 1 in 2000 to 1 in 3000
What is the UK carrier frequency of CF?
The UK carrier frequency is about 1 in 22 to 1 in 27
In the past CF patients rarely had children. The new mutation rate is very low for CFTR but the frequency of CF remains stable. What are some of the suggested reasons for this?
It is postulated that CF must have/have had some selective advantage over normal homozygotes. There is some debate over what this advantage may be. One theory is that the CFTR membrane chloride channel is required for Salmonelli typhi to enter intestinal epithelial cells. Therefore, heterozygotes may be resistant to typhoid fever and/or to cholera. For the present CF gene frequency to be maintained even in the absence of fresh mutations heterozygotes need to have only 2.3% more surviving children than wild type homozygotes.
What is meant by the term locus homogeneity?
Locus homogeneity means that only 1 gene is involved and only 1 gene therefore needs to be tested (as is regarded to be the case with CF).
Describe allelic heterogeneity in CF.
There is a lot of allelic heterogeneity in the CFTR gene. More than 1500 different mutations in the CFTR gene have been reported. These mutations are loss of function mutations including small deletions/insertions, nonsense, splice site and missense mutationsm and in addition large deletions/duplications (whole exon / multi exon) which account for about 2% of CFTR mutations.
What types of mutations are usually found in the CFTR gene?
There is a lot of allelic heterogeneity in the CFTR gene. More than 1500 different mutations in the CFTR gene have been reported. These mutations are loss of function mutations including small deletions/insertions, nonsense, splice site and missense mutationsm and in addition large deletions/duplications (whole exon / multi exon) which account for about 2% of CFTR mutations.
Why is it impractical to perform a gene screen on the whole CFTR gene?
Given the size of the gene it is relatively time consuming and expensive to perform a gene screen of the coding region for CF mutations. In addition, some mutations are deeply intronic such as the 3849+10kbC>T mutation. Just sequencing the coding region of the gene would not detect mutations such as this.
