medical education

1. CYSTIC FIBROSIS GENETICS AND
PATHOGENESIS
PRABHAKAR K

2. • INTRODUCTION
• HISTORICAL PERSPECTIVES
• EPIDEMIOLOGY
• GENETICS
• STRUCTURE OF CFTR GENE
• TYPES OF CF MUTATIONS
• FUNCTION OF CF GENE
• PATHOGENESIS

3. INTRODUCTION
• Cystic fibrosis (Mucoviscidosis) is an inherited
multi system disorder of ion transport that
affects fluid secretion in exocrine glands and in
the epithelial lining of the respiratory ,
gastrointestinal, and reproductive tracts.
• The name cystic fibrosis refers to the
characteristic scarring (fibrosis ) and cyst
formation with in pancreas, first recognized in
the 1938.

4. HISTORICAL PERSPECTIVES
• 1650 – Infants & children with meconium ileus and
characteristic pancreatic and lung disease.
• 1938 -Dorothy Andersen, coined the term cystic
fibrosis.
• 1940 -Andersen and Hodges, an autosomal recessive
inheritance pattern
• 1945 –Farber , disease of exocrine glands &
mucoviscidosis.

5. • 1953 - Di Sant'Agnese colleagues, investigated
salt depletion in children with Cf
Gibson and Cooke, Pilocarpine iontophoresis
method for sweat testing.
• 1970 – CF syndrome &complications
• 1980 - Abnormalities of both sodium and
chloride transport by CF respiratory epithelia
• 1985-1987 : Using restriction fragment-length
polymorphism analysis, located the CFTR gene on
the long arm of chromosome 7

6. EPIDEMIOLOGY
• 1:2500 and 1:17,000 live births in white and
black populations respectively.
Incidence Race
1 in 3000 Caucasians
1 in 6000 Hispanics
1 in 10,000 African Americans

7. CF IN INDIAN SCENARIO
• Cystic fibrosis was thought to be very rare in
India.
• The precise incidence of cystic fibrosis among
Indians is unknown.
• The incidence in migrant population in the
USA has been estimated to be 1in 40000 and
in the UK between 1 in 10000 to 12000.

9. GENETICS
• CF is an autosomal recessive trait, resulting
from mutations at a single gene (CFTR) locus
on the long arm of chromosome 7q31.2.
• The gene CFTR code for protein cystic fibrosis
trans membrane conductance regulator
(CFTR)

12. • Most common mutation – 3 base pair deletion
(phenyl alanine) in exon 10 from position
(F508) .
• This mutation accounts for 66 % of CF
mutations.
• However, more than 1800 CF mutations have
now been reported

15. STRUCTURE OF CFTR GENE
• The CF gene spans approximately 230 kb of DNA.
•
• Contains 27 exons
• Contains 1480 AA.
• Molecular weight is 170 kD, but CFTR migrates in
its mature form as an approximately 180-kD
glycoprotein due to variable N glycosylation.

16. • It has five domains
1.Two membrane spanning domains (each
with six α -helices )
2.Two nucleotide (ATP) binding domains
(NBD)
3.One Regulatory domain with phoshorylation
sites (Protein kinase A &C)

18. TYPES OF MUTATION
• The mutations can be grouped into six classes
based on their effect on the CFTR protein.
Class Mutation
CLASSIC TYPE
Class I Defective protein synthesis Ex: Gly 542X
Class II Abnormal protein folding, processing, and trafficking ,F508 mutation
Class III Defective regulation
NON CLASSIC
Class IV Decreased conductance
Class V Reduced abundance
Class VI Altered function in regulation of ion channels

20. • Frequency of the F 508 mutation
Population Cystic fibrosis chromosomes (%)
North American Whites 76
North American Hispanics 46
United Kingdom 74
Spain 49
Italy 43
Ashkenazi Jews 30
India 19 -56

22. • Genetic and Environmental Modifiers.
• Although cystic fibrosis remains one of the
best-known examples of the“one gene, one
disease” axiom, there is now considerable
evidence that genes other than CFTR modify
the frequency and severity of certain organ-
specific manifestations , especially
• Pulmonary manifestations and
• Neonatal meconium ileus.

23. NON CFTR GENE MODIFIERS

24. FUNCTION OF CFTR GENE
 CFTR regulates multiple additional ion
channels and cellular processes
• Initially characterized as a chloride
conductance channel.

25. • The epithelial sodium channel (ENaC),
• Transport of bicarbonate ions (SLC26),
• Inwardly rectified potassium channels (Kir6.1)
• Gap junction channels.
Cellular processes
• ATP transport
• mucus secretion

26. PATHOGENESIS
• CFTR is an integral membrane protein that
functions as an epithelial anion channel.
• It act as a conduit for chloride and bicarbonate
transport across plasma membranes of epithelial
tissues, with direction of ion flow dependent on
the electrochemical driving force.

27. • Gating of CFTR involves conformational cycling
between an open and closed configuration &
augmented by hydrolysis of adenosine tri
phosphate (ATP).

28. • Decreased secretion of chloride and water by
airway epithelial cells results in dehydrated
mucus.
• Increased reabsorption of sodium.
• Increased epithelium sodium channel activity
alone alters regulation of ions and water,
resulting in mucus obstruction of airways.

31. • Along respiratory mucosa, CFTR is necessary
to provide sufficient depth of the peri ciliary
fluid layer (PCL), allowing normal ciliary
extension and muco-ciliary transport.
• CFTR-deficient airway cells exhibit depleted
PCL, causing ciliary collapse and failure to
clear overlying mucus.

33. • CFTR transports bicarbonate (SLC26)
• loss of CFTR function may result in
acidification of the small intestinal lumen
and, possibly, the airway lining fluid.

35. • CFTR may also function in intracellular
membranes (e.g., endoplasmic reticulum,
endosomes,and clathrin-coated vesicles).
• A consequence of the altered function of CFTR
in intracellular membranes may be the
mislocalization of glycosyltransferases.

36. • Increased sulfation of respiratory mucins, with
decreased sialylation and increased
fucosylation of both secreted and membrane
glycoproteins.
• Altered glycosylation of airway glycoproteins
may significantly impact bacterial epithelial
interactions and innate immune functions in
the lung.

37. • loss of CFTR function negatively impacts
innate immunity and accentuates
inflammation.
• Absence of CFTR function is associated with
impaired bacterial killing in vitro and defective
function of antimicrobials including
humanbeta-defensin 1 and lysozyme.
• Increased IL-8 production and decreased IL-10

38. • The presence of excessive, unopposed
neutrophil elastase in the airway cleaves
complement and immunoglobulins,potentially
interfering with bacterial opsonization.
• CF airways have increased oxidant stress due
to neutrophilic inflammation and reduced
antioxidants such as glutathione.

39. • Finally, tissues in CF have an increased ratio of
arachidonic acid metabolites to
docosahexaenoic acid metabolites reflecting
an increase in inflammatory lipids in affected
tissues, and decreased levels of lipoxin, an
anti-inflammatory lipid mediator in airway
surface fluid.
• Together, these factors synergistically increase
the inflammatory milieu in the airways in CF.

40. • In other exocrine glands characterized by
reduced mucus transport .
Ex: Pancreatic acini and ducts,
Liver & Bile canaliculi,
Intestinal lumen,
Reproductive system
• In these tissues, a driving force for apical
chloride and bicarbonate secretion is believed
to promote CFTR-mediated fluid and
electrolyte release into the lumen.

41. • Failure of this mechanism disrupts normal
hydration and transport of glandular secretion
and is widely viewed as a proximate cause of
ductular obstruction, with concomitant tissue
injury.

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