What causes the condition Cystic Fibrosis?

Cystic Fibrosis is caused by a mutation in the CFTR gene. The mutation causes a small change in the DNA's base sequence which codes for a different sequence of amino acids and therefore as a result, the protein synthesised is faulty.

Normally, a healthy CFTR protein, that is located in the plasma membrane of ciliated epithelial cells lining the respiratory tract, transports chloride ions out of the cell and into the mucus by active transport. These chloride ions are crucial in the lowering of the water potential of the mucus. Water molcules will then move into the the mucus by osmosis, thinning the mucus and allowing it to be moved up (by the cilia) and away from the lungs.

The faulty CFTR protein however, is the incorrect shape so it cannot transport cloride ions out of the cell into the mucus.Therefore the water potential of the mucus is not lowered and water does not move out of the cell into the mucus. This causes the mucus to become thick and sticky, making it difficult for the cilia to move the mucus upwards and away from the lungs. The mucus (containing harmful pathogens) can then enter the lungs and that is why people with Cystic Fibrosis are prone to lung infections.