Cystinosis is a lysosomal storage disease characterized by an abnormal accumulation of the amino acid cystine in the cells of the body. Also called cystine storage disease, it is an inborn error of metabolism that leads to the deposition of cystine crystal in various tissues such as the bone marrow, the cornea, the kidney and the liver.
Types of cystinosis
There are three distinct types of cystinosis and these include:
Nephropathic cystinosis is the most severe form of the condition. This form begins in infancy and causes rickets, liver enlargement, and kidney malfunction. The infant may fail to grow at the expected rate and may urinate excessively. The deposits in the kidney eventually cause Fanconi syndrome which describes the defective reabsorption of amino acids, salts, sugar and water in the kidneys. Children who are not treated for the condition often die from kidney failure by the age of 10.
This form of the condition is much less severe than nephropathic cystinosis and mainly only affects the eyes. The accumulation of cystine crystals in the cornea can cause photophobia which is a visual hypersensitivity to light.
Intermediate cystinosis is also a severe form of the condition and is similar to nephropathic cystinosis. However, this form of the disease has a later onset, usually effecting adolescents and causing complete kidney failure somewhere between the ages of 15 and 25.
Cause and prevalence
Cystinosis is inherited in an autosomal recessive fashion and is estimated to affected 1 in every 100,000 to 200,000 individuals. All three forms of cystinosis are associated with mutations in a gene called CTNS which codes for a protein that usually transports cystine out of the lysosomes inside cells.
Pathology, diagnosis and treatment
The cystine crystals accumulate in a large range of tissues and organs including the kidneys, conjunctiva, cornea, liver, spleen, lymph nodes, muscle, brain, thyroid, intestines and bone marrow.
Under polarizing prisms, the crystals appear hexagonal, rhombohedral or varied and polymorphous in shape. The kidneys develop a narrowing in the proximal renal tubule called the swan-neck deformity, which predisposes to the development of Fanconi syndrome. Diagnosis is often based on the symptoms of this syndrome (increased excretion of water, salts, sugar, amino acids in the urine) along with genetic testing.
There are two main approaches to the treatment of cystinosis which are symptomatic therapy and specific therapy. Symptomatic therapy involves treating the Fanconi syndrome and replacing the lost fluids and electrolytes, calcium, amino acids and glucose. Specific treatment involves the use of cysteamine, an aminothiol that depletes cystine levels over time and eases the condition. The early cysteamine treatment is started, the greater the chance of preventing the progression of renal damage.