cystinosin, lysosomal cystine transporter
The CTNS gene provides instructions for making a protein called cystinosin. This protein is located in the membrane of lysosomes, which are compartments in the cell that digest and recycle materials. Proteins digested inside lysosomes are broken down into smaller building blocks, called amino acids. The amino acids are then moved out of lysosomes by transport proteins. Cystinosin is a transport protein that specifically moves the amino acid cystine out of the lysosome.
More than 80 different mutations that are responsible for causing cystinosis have been identified in the CTNS gene. The most common mutation is a deletion of a large part of the CTNS gene (sometimes referred to as the 57-kb deletion), resulting in the complete loss of cystinosin. This deletion is responsible for approximately 50 percent of cystinosis cases in people of European descent. Other mutations result in the production of an abnormally short protein that cannot carry out its normal transport function. Mutations that change very small regions of the CTNS gene may allow the transporter protein to retain some of its usual activity, resulting in a milder form of cystinosis.