Cystic fibrosis is an inherited disorder affecting specific channel proteins in cell-surface membranes. In this article we will look at how it is inherited, how chloride ion transport is affected, and the symptoms which develop as a consequence.
Inheritance of cystic fibrosis
Cystic fibrosis is an inherited recessive disorder, meaning it is caused by a recessive allele. Therefore both parents must either be carrying the recessive allele (be heterozygous) or have cystic fibrosis themselves in order for their offspring to have a chance of having cystic fibrosis. Genetic diagrams can be used to work the chances of somebody having the disease based on the parent’s genotypes. Head over to the inheritance series to revise inheritance and recessive alleles.
The CFTR channel protein
The gene which is mutated in cystic fibrosis codes for the cystic fibrosis transmembrane conductance regulator (CFTR) protein. It has a long name, but basically it is a chloride ion channel protein. The mutation causes this protein to be less efficient at chloride ion transport. Normally, chloride ions diffuse out of cells (such as epithelial cells in the respiratory system) through these channel proteins and enter the mucus outside. As a consequence, the water potential of the mucus is lowered and water moves into the mucus by osmosis, keeping it fairly thin and watery. If chloride ion transport is reduced or stopped, there is not a water potential gradient and the movement of water into mucus cannot happen. The mucus becomes thick and sticky. This is the cause of the problems associated with cystic fibrosis.
Symptoms and problems
Cystic fibrosis can affect any area of the body where mucus is produced:
- In the respiratory system, mucus normally lines the airways to trap microorganisms. It is moved up to the throat by cilia on the epithelial cells so that it can be swallowed. If the mucus is thick and sticky, it is not able to be moved so it builds up in the airways and can cause blockages, coughing, and infections (microorganisms are not moved up to the throat). A blockage in the airways means that the surface area for gas exchange is reduced which can cause breathlessness.
- In the male reproductive system, the vas deferens (the tubes between the testes and penis) are often completely absent. Therefore the man is infertile.
- In the female reproductive system, the cervical mucus is thick and sticky which can prevent sperm from entering the uterus.
- In the digestive system, the tube between the pancreas and the small intestine can become blocked with mucus. This reduces the ability to digest food because digestive enzymes cannot reach the small intestine. To make this worse, the small intestine is also lined with thick sticky mucus, reducing the efficiency of nutrient absorption.
Summary
- Cystic fibrosis is an inherited recessive disorder.
- A mutation in the gene coding for the CFTR protein means that chloride ion transport is hindered and mucus becomes thick and sticky.
- The respiratory system, reproductive system, and digestive system can all be affected.
