Cystinosis

Medical quality assurance by Dr. Albrecht Nonnenmacher, MD at December 8, 2016
StartDiseasesCystinosis

Cystinosis is an extremely rare genetic disorder that impacts roughly 500 people, primarily children, in the United States and approximately 2,000 individuals across the world. The condition is a metabolic disease involving the buildup of the amino acid, cystine in the cells. As the condition is so rare, research and resources available to treat the disease are lacking.

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Definition & Facts

The condition is an autosomal recessive genetic disease, meaning that both parents must be genetic carriers of the abnormal gene for a child to inherit the condition. Even so, the parents do not exhibit any symptoms of the condition, which makes it difficult to predict.

When an individual does in fact inherit the disease, it is characterized by the accumulation of cystine within the cells. This excess cystine damages the cells and can often form crystals that cause buildup and damage to the body’s organs and tissues. The kidneys and eyes are most vulnerable to damage; however, the thyroid, pancreas, testes, and muscles can also be affected.

There are three different types of cystinosis, each with a different degree of severity. Nephropathic cystinosis is the most severe; intermediate; and non-nephropathic/ocular cystinosis being slightly less severe.

Symptoms & Complaints

The types of cystinosis often present with the same symptoms only they occur at different ages and to a different degree depending on the type.

Nephropathic cystinosis begins in infancy and causes poor growth as well as a specific kind of kidney damage, known as renal Fanconi syndrome.

This damage causes the body to eliminate important molecules in the urine rather than reabsorbing them, contributing to the loss of important minerals, salt, fluids, and other nutrients. This lack of nutrients contributes to impaired growth and may cause soft, bowed bones. Additionally, the imbalance of nutrients leads to frequent urination, thirst, dehydration, and acidic blood.

Some individuals may even experience the apparent and painful buildup of crystals in the eyes. Untreated children typically even experience complete kidney failure by the time they reach the age of 10. Additional symptoms that untreated individuals experience include muscle deterioration, blindness, diabetes, thyroid disease and neurological disorders, trouble swallowing, and infertility.

Intermediate cystinosis is commonly marked by the same symptoms as nephropathic cystinosis; however, the symptoms do not develop until a later age. This condition typically does not become apparent in individuals until adolescence. Kidney problems and corneal crystals are the main characteristics of this disorder. If left untreated, an affected individual will likely experience complete kidney failure in his late teens to mid-20s.

Individuals with non-nephropathic or ocular cystinosis primarily experience photophobia or increased sensitivity to light, due to the buildup of cystine crystals in the cornea. Most individuals with this condition do not exhibit the additional kidney issues or other signs and symptoms.

Causes

Cystinosis is an inherited condition that is passed on from parent to child. The condition is caused by genetic mutations involving defects of the CTNS gene. The recessive gene may lie dormant for a generation or more before two people with the defective gene have a child together. There is a 25 percent chance of a couple who are both cystinosis carriers having a child with the condition.

The symptoms and ailments associated with the condition are caused by the body’s inability to effectively process and remove the amino acid cystine. Typically, proteins are degraded within the lysosomes of cells and then the amino acids are transported out of the cell via specific transporters. However, the transporter for cystine is defective in children with cystinosis.

The amino acid enters the cell and then has no way out. Because of the defective transportation, the cell crystallizes, resulting in premature cell death. As the condition progresses and causes more and more cell death, it slowly begins destroying the kidneys, liver, eyes, muscles, brain, and other tissues. The cystine content in the cells of an individual with the condition are typically 50 to 100 times the normal value.

Diagnosis & Tests

The condition is typically diagnosed following a thorough examination of the child and an assessment for predominant features in affected individuals. Common features that a physician looks for in diagnosis include:

Additionally, a number of laboratory tests and/or additional assessments may be conducted for a definitive diagnosis. Some of the common tests include:

Treatment & Therapy

This condition is typically treated symptomatically. Dysfunction of the kidneys requires an increased intake of fluids as well as replacing electrolytes to prevent dehydration. Common electrolyte replacements include sodium bicarbonate, potassium citrate, and sodium citrate. Phosphates and vitamin D supplements are also administered to correct the deficiency of phosphate in the kidneys. 

Other treatments have also been found to begin removing cystine from the cells. Cysteamine is a medication that depletes the cells of cystine and lowers the level of the amino acid within the body. This treatment has been found effective for preventing or at least delaying renal failure as well as improving growth in children. Research indicates that oral cysteamine should also be taken by post-transplant cystinosis patients.

There are solutions of cysteamine that can dissolve cystine crystals accumulating in the eyes. Treatment with this drug should be started as soon as the diagnosis is given and cystine crystals have been found in the corneas.

Prevention & Prophylaxis

While there aren't any preventative measures against cystinosis, genetic testing can help prospective parents understand the risk in passing the condition on to their child and/or to prepare for having a child with the condition.

More research needs to be done on exactly how to predict the condition, but genetic testing is able to identify preexisting defective genes and other concerns that may contribute to conditions such as cystinosis.