Definition of Tyrosinemia

Tyrosinemia: A genetic disorder involving the metabolism of the amino acid tyrosine characterized by abnormally high levels of tyrosine in blood (hypertyrosinemia) and urine (tyrosinuria). There are several different forms of tyrosinemia.

The classic form, tyrosinemia type I, is due to deficiency of an enzyme called fumarylacetoacetic hydrolase, the last enzyme in the tyrosine pathway. Although generally rare, its incidence is greatly elevated in Norway, Finland, and Quebec.

Tyrosinemia type I is inherited as an autosomal recessive disorder and causes cirrhosis of the liver before 6 months of age and, untreated, leads to death from liver failure. A more chronic form of the disease is characterized by progressive cirrhosis of the liver, a renal syndrome (with loss of phosphate into the urine causing rickets of renal origin and growth failure) and recurring neurologic crises. Untreated, it usually leads to death by age 10 from liver failure or hepatocarcinoma (cancer of the liver).

A diet with a special formula that lacks the amino acids phenylalanine and tyrosine can slow the inevitable progression of the disorder. A drug called NTBC that inhibits the tyrosine metabolic pathway can reverse the symptoms of tyrosinemia and help the liver and kidney tissue to return to normal. Liver transplantation was used more in the past since it provided a better long-term outcome than diet alone, but treatment with NTBC is more effective.

The usual treatment of tyrosinemia today is NTBC plus diet. This permits greater than 90% survival. Infants with tyrosinemia and cancer of the liver need a liver transplant to survive.

Tyrosinemia type II is due to deficiency of the enzyme tyrosine transaminase. It is characterized by the deposition of crystals of tyrosine in the skin and eyes. Thickened areas (keratoses) on the palms and soles become painful and ulcers develop in the cornea. There is often mental retardation in tyrosinemia type II. It is inherited in an autosomal recessive manner.