CYSTIC FIBROSIS TRANSMEMBRRANE RECEPTOR

Introduction

History

This gene was discovered after year of intensive research by Rommens JM,

Iannuzzi MC, et al. identification of the cystic fibrosis gene : chromosome

walking & jumping Science 1989.

In 1938 Dorothy Hansine Andersen published an article, "Cystic Fibrosis of

the Pancreas and Its Relation to Celiac Disease: a Clinical and Pathological

Study," in the American Journal of Diseases of Children.

First to describe the characteristic cystic fibrosis of the pancreas and to

correlate it with the lung and intestinal disease prominent in CF. 

First hypothesized that CF was a recessive disease and first used pancreatic

enzyme replacement to treat affected children.

Protein Function: The normal CFTR protein product is a

chloride channel protein found in membranes of cells

that line passageways of the lungs, liver, pancreas,

intestines, reproductive tract, and skin.

Associated Disorders: Defective versions of this protein,

caused by CFTR gene mutations, can lead to the

development of CF and congenital bilateral aplasia of

the vas deferens (CBAVD).

CFTR Role In Body fig1

CFTRFollow

osmotic method

Sweat glands

lungs

Pancreas

Intestine

kidneys

Defect in gene encoding CFTR Leading to reduction in chloride

transport

Regulate

d by this

transport

Absorptive epithelia use similar transporter &

channels

Fig 2 Classification of CFTR mutations. CFTR mutations are classified into six classes according to their effect on CFTR function. Class I mutations inhibit biosynthesis, while Class II mutations affect protein processing. Milder mutations such as Class III, IV, and VI impair CFTR channel function and Class V mutations affect gene expression, adapted from Allen (1999).

Classification of CFTR mutations.

Presentation of Disease

Colon

Pancreas

Sticky mucus secretion

Ducts are filled with sticky mucus. Scaring of tissue.

Mucous in the airways cannot be easily cleared from the lungs.

ABC Transporters

These are responsible for transporting small foreign molecule

(like drugs & toxins) especially out of cells i.e. exsorption (&

thus called efflux pump) which make them clinically important.

A classical e.g. of ABC is P-glycoprotein which is responsible

for pumping hydrophobic drug especially anticancer drugs out

of cells

All presence of large quantity of these protein thus makes the

cells resistant to a verity of drugs used in cancer chemotherapy

a phenomenon is called multi drug resistance.

Structure & organization of ABC Basic unit of an ABC transport consists of four core

domains. Protein consist of an aqueous pore , formed by the

TMDs with large opening at the extracellular face of the membrane

NBDs (nucleotide binding domain) are at the cytoplasmic face of the membrane are in close apposition to the TMDs & possibly partly buried in the bilayer membrane

NBDs

Lipid bilayerLipid

bilayer

pore

TM

Mechanism of Transport

PBP(Periplasmic binding proteins) substrate can

be considered as the PBP substrate interacting

at the extracellular face of the membrane

Substrate get released from PBP-substrate

complex

Conformational changes takes place in the TMD

& is transmitted to the NBDs to initiate the ATP

hydrolysis which further leads to the

conformational changes to NBDs.

Cystic Fibrosis Transmembrane Conductance Regulator

Cystic fibrosis transmembrane conductance regulator (CFTR) is a protein that in humans is encoded by the CFTR gene.

CFTR is an ABC transporter-class ion channel that transports chloride and thiocyanate ions across epithelial cell membranes. Mutations of the CFTR gene affect functioning of the chloride ion channels in these cell membranes, leading to cystic fibrosis and congenital absence of the vas deferens.

Molecular Genetics and Gene Function

The gene that encodes the CFTR protein is found on the human chromosome 7, on the long arm at position q31.2. from base pair 116,907,253 to base pair 117,095,955. 

Protein Function and Biochemistry

CFTR controls chloride ion movement in and out of the cell.

Protein Function Continued

STRUCTURE OF CFTR GENE

CFTR is a glycoprotein with 1480 amino acids. The protein consists of five domains. There are two transmembrane domains, each with six spans of alpha helices.

These are each connected to a nucleotide binding domain (NBD) in the cytoplasm. The first NBD is connected to the second transmembrane domain by a regulatory "R" domain that is a unique feature of CFTR, not present in other ABC transporters.

The ion channel only opens when its R-domain has been phosphorylated by PKA and ATP is bound at the NBDs. The carboxyl terminal of the protein is anchored to the cytoskeleton by a PDZ-interacting domain.

LOCATION AND FUNCTION The CFTR is found in the epithelial cells of many

organs including the  lung, liver, pancreas, digestive tract, reproductive tract, and skin and sweat glands.

CFTR functions as a cAMP activated  ATP -gated anion channel, increasing the conductance for certain anions (e.g. Cl–) to flow down their electrochemical gradient. ATP-driven conformational changes in CFTR open and close a gate to allow transmembrane flow of anions down theirelectrochemical gradient.

CFTR defects result in reduced transport of sodium chloride and sodium thiocyanate in the reabsorptive duct and saltier sweat. This was the basis of a clinica.lly important sweat test  for  cystic fibrosis.

3D Image of Protein

When a CFTR protein with the delta F508 mutation reaches the ER, the quality-control mechanism of this cellular component recognizes that the protein is folded incorrectly and marks the defective protein for degradation. As a result, delta F508 never reaches the cell membrane.

People who are homozygous for delta F508 mutation tend to have the most severe symptoms of cystic fibrosis due to critical loss of chloride ion transport.

This upsets the sodium and chloride ion balance needed to maintain the normal, thin mucus layer that is easily removed by cilia lining the lungs and other organs. The sodium and chloride ion imbalance creates a thick, sticky mucus layer that cannot be removed by cilia and traps bacteria, resulting in chronic infections.

Regulation of CFTR

Two separate processes control the gating of CFTR:

1) Phosphorylation

2) Binding and hydrolysis of ATP

Phosphorylation is necessary for activation, but it is not sufficient.

After phosphorylation, gating between the closed and open states is

controlled by ATP hydrolysis

CYSTIC FIBROSIS

Cystic fibrosis also known as mucoviscidosis,is an autosomal recessive genetic disorder that affects mainly the lungs ,and also the pancreas, liver, and intestine.

It is characterised by abnormal transport of chloride and sodium across an epithelium leading to thick viscous secretions.

the name cystic fibrosis refers to the characteristic scarring (fibrosis) and cyst formation within the pancreas, first recognized in the Andersen DH (1938)

Difficulty breathing due to lung infections that are treated with antibiotics and other medications.

Salty tasting skin. poor growth and poor weight gain despite

normal food intake. accumulation of thick, sticky mucus, frequent

chest infections, and coughing or shortness of breath. 

Males can be infertile due to congenital absence of the vas deferens.

 Symptoms often appear in infancy and childhood, such as bowel obstruction due to meconium ileus in newborn babies.

Gastrointestinal malabsorption.

Cystic fibrosis is a heterogeneous recessive genetic disorder with features that reflect mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene.

Classic cystic fibrosis is characterized by chronic bacterial infection of the airways and sinuses, fat maldigestion due to pancreatic exocrine insufficiency, infertility in males due to obstructive azoospermia, and elevated concentrations of chloride in sweat.

Patients with nonclassic cystic fibrosis have at least one copy of a mutant gene that confers partial function of the CFTR protein, and such patients usually have no overt signs of maldigestion because some pancreatic exocrine function is preserved.

CLINICAL FEATURES

CAUSES

CF is caused by a mutation in the gene for the protein cystic fibrosis transmembrane conductance regulator (CFTR). This protein is required to regulate the components of sweat, digestive fluids, and mucus.

CFTR regulates the movement of chloride and sodium ions across epithelial membranes, such as the alveolar epithelia located in the lungs.

Most people without CF have two working copies of the CFTR gene, and both copies must be missing for CF to develop, due to the disorder's recessive nature. CF develops when neither copy works normally (as a result of mutation) and therefore has autosomal recessive inheritance.

CF is caused by a mutation in the gene cystic fibrosis transmembrane conductance regulator (CFTR). The most common mutation, ΔF508, is a deletion (Δ signifying deletion) of three nucleotides that results in a loss of the amino acid phenylalanine (F) at the 508th position on the protein.

This mutation accounts for 2/3rd of cf cases worldwide.

Pathophysiology of CF

In cystic fibrosis a loss of functional CFTR chloride

channels leads to defective cAMP-stimulated

chloride transport in most epithelia this defect result

in decreased chloride secretion & increased

absorption

Defective electrolyte transport is thought to alter the

volume or composition of the fluid secreted by the

pancreas, hepatobiliary tree , reproductive tract

sweat gland & airways.

LUNGS : Disease results from clogging of the airways due to mucus build-up, decreased mucociliary clearance, and resulting inflammation causing injury and stucrural changes.

Staphylococcus aureus, Haemophilus influenzae, and Pseudomonas aeruginosa are the three most common organisms causing chronic lung infections in CF patients.

Other symptoms :coughing up blood (hemoptysis) high blood pressure in the lung (pulmonary

hypertension)

Gastrointestinal : Prior to prenatal and newborn screening, CF was often diagnosed when a newborn infant failed to pass feces (meconium). Meconium may completely block the intestines and cause serious illness. This condition, called meconium ileus, occurs in 5–10% of newborns with CF.

The thick mucus seen in the lungs has a counterpart in thickened secretions from the pancreas, an organ responsible for providing digestive juices that help break down food. These secretions block the exocrine movement of the digestive enzymes into the duodenum and result in irreversible damage to the pancreas, often with painful inflammation (pancreatitis)

The lack of digestive enzymes leads to difficulty absorbing nutrients with their subsequent excretion in the feces, a disorder known as malabsorption, leading to loss of fat soluble vitamins A,D,E and K.

Endocrine : The pancreas contains the islets of Langerhans, which are responsible for making insulin, a hormone that helps regulate blood glucose. Damage of the pancreas can lead to loss of the islet cells, leading to a type of diabetes . This cystic fibrosis-related diabetes (CFRD) shares characteristics that can be found in type 1 and type 2 diabetics, and is one of the principal nonpulmonary complications of CF.

Vitamin D is involved in calcium and phosphate regulation . Poor uptake of vitamin D from the diet because of malabsorption can lead to the bone disease osteoporosis in which weakened bones are more susceptible to fractures

In addition, people with CF often develop clubbing of their fingers and toes due to the effects of chronic illness and low oxygen in their tissues.

Fig : Clubbing in the fingers of a person with cystic fibrosis

DIAGNOSIS

Individuals with cystic fibrosis can be diagnosed before birth by genetic testing, or by a sweat test in early childhood.

The newborn screen initially measures

for raised blood concentration of 

immunoreactive trypsinogen.

Ultimately, lung transplantation is often

necessary as CF worsens.

CURE The major goal in treating CF is to clear the abnormal and excess secretions and control infections in the lungs, and to prevent obstruction in the intestine.

TREATMENT : The only way to cure CF would be to use gene therapy to replace the defective gene or to give the patient the normal form of the protein before symptoms cause permanent damage. •For patients with advanced stages of the disease, a lung transplant operation may be necessary.

Treatment for cystic fibrosis

1. Gastrointestinal system

A. Pancreatic enzyme supplement

Cotazym

Creon

Ilozyme

Ku-zyme

Pancrease

ultraseMT12

Treatment for cystic fibrosis

B. Vitamin supplementation

Multivitamin tablets

C. Prevention & treatment of cirrhosis

Ursodeoxycholic acid

Treatment for cystic fibrosis

2. Cardiovascular system

Vasodilators Inotropic Diuretics

3. Pulmonary system

A. Anti obstructive therapy

0.9% sodium chloride solution is inhaled to liquefy pulmonary secretion

recombinant human dna has been approved for use in cf to reduces the

viscosity of CF sputum

Theophylline (bronchodilators)

B. Anti inflammatory therapy

C. Antibiotic therapy

1. Oral antibiotics

Trimethoprim-sulfamethoxazole Cephalexin

ciprofloxacin amoxicillin-clavulanic acid

2. Inhaled antibiotics

Colistin Gentamicin or tobramycin

Polymixin –B

3. Parenteral antibiotics

Amikacin

Azlocillin

Aztreonam

Colistin

Imipenem

piperacillin

GENE THERAPY

Gene therapy is the use of normal DNA to "correct" for the damaged genes that cause disease.

In the case of CF, gene therapy involves inhaling a spray that delivers normal DNA to the lungs.

The goal is to replace the defective CF gene in the lungs to cure CF or slow the progression of the disease.

Recent Drugs used for Treatment of CF : The Pulmonary Therapies Committee of

Cystic Fibrosis Foundation recommends long-term use of hypertonic saline for patients with cystic fibrosis aged 6 years or older to improve lung function and to reduce the number of exacerbations.

The cystic fibrosis transmembrane conductance regulator (CFTR), Ivacaftor (Kalydeco), was approved by the FDA in January 2012.

In March 2013, the FDA approved Tobramycin inhalation powder for the treatment of CF patients with P aeruginosa. The powder is inhaled twice daily for 28 days; treatment is then stopped for 28 days before resuming.