Cycstic fibrosis Flashcards
How is cycstic fibrosis inherited
Autosomal recessive
One of the most common autosomal recessive in caucasion population
Describe a referal form for cyctic fibrosis
6 yr old girl
High amount of salt in sweat
Recurrent chest infections
Had fetal echogenic bowel
Genetic testing please
How many people does CF affect?
CF affects 1 in 2500 live births (carrier frequency of 1 in 25)
Whats the life expectancy of CF patients?
Life expectancy ~42 years
Increase from 31 years (15 years ago)
Expected that children born today with CF will live to 50 years
What is the gene that causes CF
CFTR gene
Cystic fibrosis transmembrane conductance regulator
What is the role of CFTR gene?
Forms chloride channel in membrane of secretory epithelial cells
These cells produce mucus, sweat, saliva, tears, & digestive enzymes
Transports negatively chloride ions into and out of cells
Helps control the movement of water in tissues
Necessary for production of thin, freely flowing mucus
Lubricates & protects the lining of the airways, digestive system, reproductive system, & others
What do mutations of CFTR gene cause?
Result is defective chloride transport - don’t get good control of movement of water as you should
Mutations in CFTR gene 🡪 defective chloride transport
Abnormally thick mucus in epithelia of organs 🡪 Multi-organ effects
How is CF a multisystem disorder?
Pulmonary
Pancreatic
Gastro-intestinal
Reproductive (males)
Describe the pulmonary features
Responsible for at least 80% of CF-related deaths
Thick mucus blocks airways 🡪 chronic obstructive airway disease
Leads to bronchiectasis - Persistent and progressive dilation of bronchi or bronchioles in lungs
Increased susceptibility to chronic respiratory infections,
Describe the reproductive features
> 95% males with CF are infertile
Obstructive azoospermia (no sperm in semen)
Congenital absence of the vas deferens (CBAVD) – tubes that carry sperm out of testes
Can be seen as an isolated feature (CFTR spectrum)
Females are fertile, though some evidence of abnormal cervical mucus contributing to infertility
Represents 1.2-1.7% of male infertility
Diagnostic tests for cystic fibrosis
Sweat (chloride) test
Sweat (chloride) test
CF patients have high amount of salt in sweat
Uses Quantitative pilocarpine iontophoresis
(pilocarpine stimulates sweating, collect sweat using filter paper, measure chloride)
Positive = increased Chlorine in sweat
Done in Biochemistry departments
24hr result
An old diagnostic test for CF - but still used!
CF referralls for CF for genetic testing: Diagnostic
Diagnostic Recurrent chest infection
Failure to Thrive
Chronic pancreatitis
Fetal echogenic bowel
Meconium ileus
CF referralls for CF for genetic testing: : Family
Carrier and predictive testing
Prenatal
CF referralls for CF for genetic testing: Infertility
Azoospermia / Oligospermia / OATS
Gamete donors
CF referralls for CF for genetic testing: Newborn screening
Raised IRT on NBS bloodspots
Describe CFTR gene
Autosomal recessive
CFTR gene located at 7q31
>2000 mutations, throughout 24 exons
Describe CF genetics
Most CFTR variants are missenses & small deletions
deltaF508 or F508 is most common variant (70% of variants)
More correctly called p.Phe508del
A 3 nucleotide deletion causing loss of a phenylalanine (Phe or F) amino acid in exon 10
c.1521_1523delCTT
Mutation types and frequencies also vary between populations
Why such a high frequency of CFTR heterozygotes?
Genetic defects leading to reduced CFTR protein expression or chloride transport capacity lead to cystic fibrosis
Conversely…
Overstimulation of CFTR in intestinal epithelial cells by bacterial toxins leads to secretory diarrhoea
Having less CFTR may provide a selective advantage
Toxins (such as cholera) activate the protein kinases responsible for “priming” the CFTR channel
- Much larger world health problem: 3 million deaths per year of children the age of 5
GENETIC TESTING
- Sequence whole gene
Or
- Test for specific most common variants?
And
Follow up whole gene sequnece if needed?
Elucigene CF-EU2v1 assay
Fluorescent ARMS PCR (amplification refractory mutation system)
50 mutations panel
CF-EU2V1 ASSAY
Primer sequence specific for variant
If variant not present, primer doesn’t bind
No PCR product
Variant present – primer binds – PCR product#
Also primer for wildtype
Look at slides for CF-EU2V1 assay results
P.(ARG117HIS)(;)(PHE508DEL) COMPOUND HETEROZYGOUS
What does this mean for the patient?
Positive for two CFTR mutations:
Arg117His
Phe508del
Confirms diagnosis of Cystic Fibrosis
Are they are the same or different allele?
Recommend carrier status of parents be checked
What does autosomal recessive mean?
If both parents are carriers
25% risk of affected child
50% risk of carrier child
