Cystinosis is a condition characterized by accumulation of the amino acid cystine (a building block of proteins) within cells. Excess cystine damages cells and often forms crystals that can build up and cause problems in many organs and tissues. The kidneys and eyes are especially vulnerable to damage; the muscles, thyroid, pancreas, and testes may also be affected.

There are three distinct types of cystinosis. In order of decreasing severity, they are nephropathic cystinosis, intermediate cystinosis, and non-nephropathic or ocular cystinosis.

Nephropathic cystinosis begins in infancy, causing poor growth and a particular type of kidney damage (renal Fanconi syndrome) in which certain molecules that should be reabsorbed into the bloodstream are instead eliminated in the urine. The kidney problems lead to the loss of important minerals, salts, fluids, and many other nutrients. The loss of nutrients impairs growth and may result in soft, bowed bones (hypophosphatemic rickets), especially in the legs. The nutrient imbalances in the body lead to increased urination, thirst, dehydration, and abnormally acidic blood (acidosis). By about the age of 2, cystine crystals may be present in the clear covering of the eye (cornea). The buildup of these crystals in the eye causes pain and an increased sensitivity to light (photophobia). Untreated children will experience complete kidney failure by about the age of 10. Other signs and symptoms that may occur in untreated people, especially after adolescence, include muscle deterioration, blindness, inability to swallow, diabetes, thyroid and nervous system problems, and an inability to father children (infertility) in affected men.

The signs and symptoms of intermediate cystinosis are the same as nephropathic cystinosis, but they occur at a later age. Intermediate cystinosis typically becomes apparent in affected individuals in adolescence. Malfunctioning kidneys and corneal crystals are the main initial features of this disorder. If intermediate cystinosis is left untreated, complete kidney failure will occur, but usually not until the late teens to mid-twenties.

People with non-nephropathic or ocular cystinosis typically experience photophobia due to cystine crystals in the cornea, but usually do not develop kidney malfunction or most of the other signs and symptoms of cystinosis. Due to the absence of severe symptoms, the age at which this form of cystinosis is diagnosed varies widely.

All three types of cystinosis are caused by mutations in the CTNS gene. Mutations in this gene lead to a deficiency of a transporter protein called cystinosin. Within cells, this protein normally moves cystine out of the lysosomes, which are compartments in the cell that digest and recycle materials. When cystinosin is defective or missing, cystine accumulates and forms crystals in the lysosomes. The buildup of cystine damages cells in the kidneys and eyes and may also affect other organs.

Normal Function

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.

Health Conditions Related to Genetic Changes

Cystinosis

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.

Other Names for This Gene

• CTNS-LSB
• CTNS_HUMAN
• Cystinosis
• PQLC4

Genomic Location

• Poor growth in infancy (failure to thrive)
• Vomiting
• Excessive urination (polyuria)
• Excessive thirst (polydipsia)
• Sensitivity to light (photophobia)
• Soft bones (rickets)
• Inability to absorb nutrients through the kidneys (Fanconi syndrome)
• Kidney failure

• Nephrologist (kidney specialist)
• Ophthalmologist (eye specialist)
• Neurologist (nerve specialist)

Cystinosis affects approximately 1 in 100,000 to 200,000 newborns worldwide. The incidence is higher in the province of Brittany, France, where the disorder affects 1 in 26,000 individuals.