Tyrosinemia
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 mental retardation.
Type III tyrosinemia is a rare disorder caused by a
deficiency of the enzyme 4-hydroxyphenylpyruvate dioxygenase.
Characteristic features include mild mental retardation,
seizures, and periodic loss of balance and coordination
(intermittent ataxia).
About 10 percent of newborns have temporarily elevated
levels of tyrosine. In these cases, the cause is not
genetic. The most likely causes are vitamin C deficiency
or immature liver enzymes due to premature birth.
Worldwide, type I tyrosinemia affects about 1 person
in 100,000. This type of tyrosinemia is much more common
in Quebec, Canada. The overall incidence in Quebec is
about 1 in 16,000 individuals. In the Saguenay-Lac St.
Jean region of Quebec, type I tyrosinemia affects 1
person in 1,846.
Type II tyrosinemia occurs in fewer than 1 in 250,000
individuals. Type III tyrosinemia is very rare; only
a few cases have been reported.
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