CASE 3 Flashcards
what is CF? demographics and epidemiology
- an inherited disease
- 1/2500 births
- inherited in an autosomal recessive fashion
- most common genetic disease in the UK caucasian population
- carrier frequency = 1/25
- P(both parents are carriers) = 1/625
- median lifespan = 49.1 years
what is the chance of having a child with CF when one parent is a carrier and one parent has CF?
50%
what is the chance of having a child with CF when both parents are carriers?
25%
where is the mutation found in CF and what is the most common mutation?
= found on chromosome 7 in the CFTR gene
- F508del = a deletion of 3 adjacent base pairs at the 508th codon - loss of a phenylalanine
role of CFTR protein
a chloride ion channel in the cell membrane of epithelial tissue cells
normal use of CFTR protein
- upon activation, the CFTR protein channel opens
- causes intracellular chloride ions to leave the cell
- as a result, the extra cellular environment becomes negative
- therefore intracellular sodium ions leave the cell via sodium channels
- reduces the level of intracellular NaCl
- creates a water potential, so water moves out via osmosis, improving the quality of cellular mucous secretions
- under normal conditions, the secretions aren’t thick as they have sufficient water content
how is CFTR activated and what kind of protein is it?
- ABC protein — have a modular structure with 4 conserved domains
- has an additional 5th domain called the Regulatory domain which regulates the activity of the protein
- when it is phosphorylated the channel can open
- also requires the binding of ATP to the protein (binds to ATP-binding domains)
mutated CFTR protein in sweat glands
defective channel = inability to reabsorb chloride —> reduced reabsorptions of sodium ions and water —> excess loss of salt and elevated levels of NaCl in sweat
mutated CFTR proteins in all other exocrine glands (+ how does it lead to organ damage)
defective channel = inability to transport chloride out of cells —> reduced secretion of chloride and water —> accumulation of intracellular chloride —> more sodium ions are reabsorbed —> more water reabsorbed —> formation of hyperviscous mucous —> accumulation of secretions and blockage of small passages of affected organs —> chronic inflammation and remodelling —> organ damage
how does a defective CFTR protein lead to inflammation?
the thick mucus provides a prime environment for the growth of bacteria
give examples of CF treatment
- diet supplemented with high-calorie/proteins foods — high energy to compensate for the increased energy demand
- given pancreatic extract/enzymes
- large doses of vitamins A,D and K and liberal use of salt
- daily chest physiotherapy
- maintain breast-feeding for as long as possible — boosts infant’s immunity, provides them with the much needed calorific nutrition
- maintenance of good hygiene
what does daily chest physiotherapy entail?
- postural drainage = trachea is inclined downwards and below the affected chest area — drains fluid from lungs
- percussion and vibrations = clapping of chest with percussion cups — helps loosen and mobilise secretions
name some symptoms of CF
- excess coughing
- hypoproteinemia
- anaemia
- fatty, foul-smelling stools
- ravenous appetite but poor weight gain — thin
- salty-tasting skin due to higher levels of chloride in sweat
- frequent coughing and wheezing
- difficulty in breathing as mucus clogs airways
what is hypoproteinemia and why do patients with CF develop it? what does it result in?
= decrease of protein in the blood (less is digested as the thick music blocks the pancreas from secreting digestive enzymes)
- results in swelling (odema) due to the accumulation of fluid in tissues —> INFECTION
- a result of malabsorption
how does CF affect the pancreas and what is the consequence?
build up of thick mucus prevents the pancreas from secreting digestive enzymes —> malnutrition and poor growth
how does CF cause pancreatic disease?
- the pancreatic ducts become blocked with mucus (pancreatic secretions are dehydrated and thick)
- pancreas continues to produce digestive enzymes
- these abundant enzymes damage the pancreatic tissue —> fibrosis of pancreas until it is no longer able to produce enough enzymes to properly digest food
what is the effect of the pancreas no longer being able to digest food properly?
- malabsorption of nutrients — malnutrition
- lowered absorption of fats = diarrhoea, weight loss, malnutrition
what is the 2nd most common mutation leading to CF?
= missense mutation changing Glycine at position 551 to an aspartate reside
= G5510
= reduces chances of the CFTR channel opening
name 5 methods of diagnosis for CF
- chest x-ray — looks for lung infections and dilated airways filled with mucus
- sweat test — measures amount of NaCl in sweat
- blood test — blood cells screened for CF gene
- screening test
- analysis of faeces — tests for pancreatic enzymes
what is an abnormal sweat test result, indicating CF?
> 60 mmol/l
describe pseudomonas aeruginosa
- gram negative
- bacillus (rod) shaped
- ‘opportunistic pathogen’
- produces toxic proteins that enter and kill host cells
- resistant to many antibiotics
symptoms of pseudomonas aerugiosa infection
- inflammation
- sepsis
- fever
- elevated respiratory and heart rates
what is meconium ileus? what are the symptoms? what can it lead to?
= obstruction of the bowel caused by thick abnormal meconium
- distension of the abdomen or vomiting
- can lead to bowel perforation, a twisting of the vowel, or inflammation and infection of the abdominal cavity
(98% of babies with MI have CF)
how are mutations expressed?
mutations are expressed as proteins — happens through the protein synthesis of the mutated gene
describe the stages of transcription
- DNA helix unwinds using RNA helicase
- one of the strands acts as a template strand
- RNA polymerase binds to the DNA of the gene at the promoter
- RNA polymerase always builds in the 5’ to 3’ direction (binds at the 3’ end)
- complementary base pairing occurs
- RNA strands have U instead of T
- RNA polymerase stops transcribing when it reaches the terminator
- splicing occurs — introns removed from pre-mRNA by small nuclear ribonucleoproteins (snRNPs)
- mRNA formed, leaves nucleus through nuclear pore
describe the stages of translation
- mRNA molecule attaches to a ribosome
- there are free molecules or tRNA in the cytoplasm, carrying anti codons and a specific amino acid binding site
- tRNA molecules bind with their specific amino acids and bring them to the mRNA on the ribosome
- anticodon on the tRNA molecule pairs with its complementary codon on the mRNA
- ribosome moves to next codon
- peptide bond forms between amino acids
- translation stops when a stop codon is reached
describe the process of PCR gene amplification
- heat to 95 degrees — breaks H bonds, forming single-stranded DNA
- add free DNA nucleotides and DNA polymerase
- add primers
- cool mixture to 55 degrees — allows primers to bond and form short lengths of double stranded DNA at either end of the sample
- DNA polymerase extends the primer DNA to synthesise the complementary DNA sequence
- temp raised to 72 degrees = optimum temp for DNA polymerase
- DNA polymerase adds free nucleotides and amplification ends when it reaches the other end
- subsequent heat denaturation of the double-stranded DNA, followed by annealing of the same primer sequences to the resulting single-stranded DNA, will result in the synthesis of further copies of the target DNA.
what can DNA probes be labelled with?
- a radioactive marker — location can be seen on exposure to photographic film
- a fluorescent marker that emits a colour on exposure to UV light (FISH)
what can DNA probes be used for?
- locating a specific gene for genetic engineering
- identifying the same gene on different genomes
- identifying the presence of absence of an allele for a particular genetic disease
what is genetic counselling?
a communication process that deals with the human problems associated with the occurrence of risk of occurrence, of a genetic disorder in a family. The process aims to help the individual or family to understand, choose, and adjust