Cystinosis is a rare, genetic, metabolic disease in which an amino acid (one of several molecules that are linked together to make proteins1) called cystine, accumulates in cells in the body. The excess cystine damages cells and often forms crystals that can build up in and damage many organs and tissues, particularly the kidneys and the eyes.2 Formation of cystine crystals within the corneas, known as corneal crystals, is a common symptom of cystinosis and can begin to appear between 18 months and two years of age.
Cystinosis is thought to affect about 2,000 people worldwide, including approximately 500 people in the United States.3 It is an autosomal recessive genetic disease, meaning that both parents carry the abnormal gene that causes the condition, even though they do not show any signs or symptoms of the disease.2
Causes of Cystinosis
Cystinosis results from mutations (defects) in a gene known as CTNS. Mutations in this gene lead to a deficiency in a transporter protein (a protein that can go completely through a cell membrane4) called cystinosin.
Normally, cystinosin removes cystine out of the lysosomes, which are compartments of the cell that digest and recycle materials. When cystinosin is defective or missing, cystine accumulates and forms crystals in the lysosomes, damaging cells in the kidneys and eyes and possibly affecting other organs.2
Types of Cystinosis
There are three types of cystinosis: infantile (also known as nephropathic) cystinosis, which is usually diagnosed during infancy; intermediate (also known as late-onset) cystinosis, which typically appears during adolescence; and non-nephropathic (also known as ocular) cystinosis, which generally lacks severe symptoms and can be diagnosed at a wide range of ages.2
Symptoms of the Disease
The symptoms of infantile cystinosis, the most severe and most common form of the disease, can appear as early as six months of age, and typically include poor growth, renal tubular Fanconi syndrome (a serious type of kidney damage in which certain nutrients and minerals that should be reabsorbed in the bloodstream are instead eliminated in the urine), rickets (soft, bowed bones), excessive thirst and urination, dehydration, and acidosis (abnormally acidic blood).
Between 18 months and two years of age, cystine crystals may appear in the cornea (the clear covering of the eye). The buildup of these crystals can cause eye pain and photophobia (excessive sensitivity to light). Left untreated, infantile cystinosis can cause kidney failure by about 10 years of age. Other complications of untreated cystinosis, especially after adolescence, may include muscle deterioration, blindness, difficulty swallowing, diabetes, thyroid and nervous system problems, and infertility (an inability to father children) in affected males.2,5
Intermediate cystinosis has the same signs and symptoms as infantile cystinosis, though they appear at a later age. Without treatment, individuals with intermediate cystinosis will experience complete kidney failure by their late teens to mid-20s.2
Individuals with non-nephropathic cystinosis typically have photophobia resulting from cystine crystals in the cornea, but usually do not experience kidney problems.2