Tyrosinemia is a genetic disorder characterized by elevated blood levels of the amino acid tyrosine, a building block of most proteins. Tyrosinemia is caused by the shortage (deficiency) of one of the enzymes required for the multistep process that breaks down tyrosine. If untreated, tyrosine and its byproducts build up in tissues and organs, which leads to serious medical problems.
There are three types of tyrosinemia. Each has distinctive symptoms and is caused by the deficiency of a different enzyme. Type I tyrosinemia, the most severe form of this disorder, is caused by a shortage of the enzyme fumarylacetoacetate hydrolase. Symptoms usually appear in the first few months of life and include failure to gain weight and grow at the expected rate (failure to thrive), diarrhea, vomiting, yellowing of the skin and whites of the eyes (jaundice), cabbage-like odor, and increased tendency to bleed (particularly nosebleeds). Type I tyrosinemia can lead to liver and kidney failure, problems affecting the nervous system, and an increased risk of liver cancer.
Type II tyrosinemia is caused by a deficiency of the enzyme tyrosine aminotransferase. This form of the disorder can affect the eyes, skin, and mental development. Symptoms often begin in early childhood and include excessive tearing, abnormal sensitivity to light (photophobia), eye pain and redness, and painful skin lesions on the palms and soles. About 50 percent of individuals with type II tyrosinemia have some degree of intellectual disability.
Type III tyrosinemia is a rare disorder caused by a deficiency of the enzyme 4-hydroxyphenylpyruvate dioxygenase. Characteristic features include intellectual disability, seizures, and periodic loss of balance and coordination (intermittent ataxia).
Mutations in the FAH, TAT, and HPD genes can cause tyrosinemia types I, II, and III, respectively.
In the liver, enzymes break down tyrosine in a five step process, resulting in molecules that are either excreted by the kidneys or used to produce energy or make other substances in the body. The FAHgene provides instructions for the fumarylacetoacetate hydrolase enzyme, which is responsible for the final step of tyrosine breakdown. The enzyme produced from the TAT gene, called tyrosine aminotransferase enzyme, is involved at the first step in the process. The HPD gene provides instructions for making the 4-hydroxyphenylpyruvate dioxygenase enzyme, which is responsible for the second step.
Mutations in the FAH, TAT, or HPD gene cause a decrease in the activity of one of the enzymes in the breakdown of tyrosine. As a result, tyrosine and its byproducts accumulate to toxic levels, which can cause damage and death to cells in the liver, kidneys, nervous system, and other organs.
Worldwide, tyrosinemia type I affects about 1 in 100,000 individuals. This type is more common in Norway where 1 in 60,000 to 74,000 individuals are affected. Tyrosinemia type I is even more common in Quebec, Canada where it occurs in about 1 in 16,000 individuals. In the Saguenay-Lac St. Jean region of Quebec, tyrosinemia type I affects 1 in 1,846 people.
Tyrosinemia type II occurs in fewer than 1 in 250,000 individuals worldwide. Tyrosinemia type III is very rare; only a few cases have been reported.