Genetics of Cystic Fibrosis Flashcards

1
Q

Defne Cystic fibrosis

A

Multi-organ genetic disorder of the CFTR gene

Autosomal recessive

  • Developed world: lung disease main cause of morbidity and mortatlity*
  • Less-developed: Intestinal main cause of mortality (aless availbility of pancreatic enzymes)*
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2
Q

Describe the function of the Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) protein

A

Lets Cl- ions flow into the lumen.

Inhibits eNaC (so ↓Na movement from lumen so that H2O follows Cl-)

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3
Q

Describe the consequences of a faulty CFTR

A

Less Cl- movement into the lumen

ENaC is active, so Na moves into the cell from lumen

H2O follows Na

Mucus dries up and becomes thick (Low volume hypothesis)

Cilia are not standing up straight

↓Mucociliary clearance

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4
Q

Name two hypotheses to explain the pathology of CF

A
  1. Low Volume Hypothesis
  2. Bicarbonate hypothesis
  • High salt hypothesis - old and disproven*
  • Aletered macrophage and neutrophil activity - jury still out (leads to impaired bacterial killing and clearance)*
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5
Q

Explain the bicarbonate hypothesis in the pathology of CF

A

CFTR allows HCO3- movement into the lumen to raise the pH of the mucus - created optimum pH environment

Mutated CFTR leads to ↓HCO3- into the lumen, therefore ↓pH (more acidic) and denaturing anti-bacterial proteins

There is dysregulation of the Airway Surface Liquid (ASL) Compartment

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6
Q

Explain the Low Volume Hypothesis in the pathology of CF

A

Mutated CFTR leads to no Cl- movement into lumen and dysregulation of ENaC (activates ENaC).

Na moves in from lumen into cell and H2O follows

Mucus dries up and there is inadequate airway surface liquid for the cilia to beat

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7
Q

Apart from potential low volume hypothesis, what else could be making the mucus thicker?

A

Infections are cleared by neutrophils mainly in CF.

Neutrophil death releases genomic DNA which makes mucus thick and sticky,

DNAse degrades the genetic material but it is expensive

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8
Q

Describe the risk of CF mutation in caucasians

Currently >1800 mutations identified for CF

A

1 in 25 (30)

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9
Q

Name some types of mutations

A

Missense

Frameshift

Splicing

Nonsense

In frame insertino/deletion

Large insertion/deletion

Promotor

Sequence Variation

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10
Q

Explain a missense mutation

A

Changing of a nucleotide in a sequence causing an amino acid change

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11
Q

Explain a frameshift mutation

A

An insertion or deletion causes a shift in the reading frame so that the codons are read in different triplets, coding for different amino acids

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12
Q

Explain a splicing mutation

A

The insertion, deletion of exons/introns to form a completely different amino acid sequence

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13
Q

Explain a nonsense mutation

A

The incorporation of a STOP codon

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14
Q

Sickel cell anaemia give a heterozygous advantage against malaria. What does CF give a heterozygous advantage to?

A

Cholera (potentiatlly)

Due to the involvement of Cl- channels in the pathogenesis of the watery diarrhoea

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15
Q

What is the most common mutation for CF and explain what the notation means

A

ΔF508

Deletion of three nucleotides at position 508, leading to a frameshift mutation (phenylalanine)

NBD1 and NBD2 involved in hydrolysis of ATP -> needed to transport ions across channel

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16
Q

Describe the normal processing of CFTR

A

Transcription from DNA

Translation by ribosomes

Post-translational modifications in golgi

Packaged into vesicles

Incorporated into the cell membrane

17
Q

Describe and explain the 6 molecular consequences of CFTR mutations

A
  • Class I
    • No synthesis
    • Big deletions
  • Class II
    • Block in processing
    • Protein made in ER, but does NOT get to golgi
    • ΔF508
  • Class III
    • Block in regulation
    • R-domain is mutated
    • Ivacaftor (potentiator)
  • Class IV
    • Altered conductance
    • Gets to membrane but no ion movement
  • Class V
    • Reduced synthesis
    • TF not binding too well?
  • Class VI
    • Increased recycling
    • Functional protein, but recycled too quickly
18
Q

ΔF508 has a 79% prevelance and G551D has 2.17%. Other mutations are < 1%. Explain the implicatoins of studying a disease with mutation prevelance like CF.

A

Less cases and therefore, RCT would not be useful to evaluate the effects of novel treatment.

Currently can use proof that the novel treatment treats a type of class mutations.

19
Q

Describe the founder effect in terms of population genetics and CF.

A

The loss of genetic variation when a new population is established by a very small group from the whole population

W1282X is a nonsense (STOP) mutation found in 51% of Ashkazi Jews who have CF.

20
Q

Describe the mechanistic basis of the sweat test in CF

A

Non-functioning CFTR in duct cells

Less re-absorption of Cl- from the gland secretions

Hypertonic sweat

21
Q

FEV1 is often used as a primary outcome in respiratory research. Describe why it isn’t the most useful?

A

Effort-dependent

We use FEV1 anyway as it is the best we have at the moment

22
Q

Ivacaftor (Kalydeco) is a good CFTR _____ but is only effective in 5% of patients. It’s main positive effects were:

  1. ↓_____
  2. ↓time to first pulmonary exacerbation
  3. ↑_____
  4. ↑_____

One method looked at taking _____ biopsy and observe en ex-vivo effect on the Cl- conductance. This is used for class _____ mutations.

A

Ivacaftor (Kalydeco) is a good CFTR potentiator but is only effective in 5% of patients. It’s main positive effects were:

  1. Sweat chloride
  2. ↓time to first pulmonary exacerbation
  3. 10-12% FEV1
  4. Weight

One method looked at taking intestinal biopsy and observe en ex-vivo effect on the Cl- conductance. This is used for class III mutations.

23
Q

_____ is a CFTR protein corrector, which aims to treat class _____ mutations by increasing modificationand processing of proteins in the golgi. Trials showed changes in sweat chloriide but no differences in lung function.

A

Lumacaftor is a CFTR protein corrector, which aims to treat class II mutations by increasing modificationand processing of proteins in the golgi. Trials showed changes in sweat chloriide but no differences in lung function​.

24
Q

Ivacafotr and Lumacaftor was observed in combination and the results is as shown below. Interpret the data from the Phase IIa study and conclude whether it is useful or not.

A

Combination therapy shows significant increase in the change in FEV1 compared to placebo

No difference between high and low dose Lumacaftor

Not useful as the increase in FEV1 was only 2%

25
Q

Describe the mechanism of action of ataluren

A

Targets genetic transcription to ignore a mutated stop codon

Especially useful for Jewish population due to high prevalance of Stop codon mutations

26
Q

Describe two other alternative therapies for CF

A
  1. ENaC inhibitors
  2. Calcium activated Cl- channels
27
Q

Describe whether there is a good genotype-phenotype correlation in:

Lungs

Pancreas

A

Lungs - NO

Pancreas - YES (Class I-III severe, Class IV-VI mild-moderate)

28
Q

The same genetic mutation can occur but with different lung disease severity. What can affect the disease severity of CF?

A
  1. Other genetic factors
    1. Modifier genes
    2. Lower baseline of sodium transport
  2. Environmental
    1. Smoking
    2. Recurrent infections
  3. Socio-economic
    1. Adherance
    2. Access
29
Q

Describe the phenotype seen in CFTR KO mice

A

Intestinal disease but no lung disease