Phenylketonuria
Phenylketonuria (PKU) is an inherited error of metabolism caused
by a deficiency in the enzyme phenylalanine hydroxylase. Loss of this enzyme
results in mental retardation, organ damage, unusual posture and can, in cases
of maternal PKU, severely compromise pregnancy.
Classical PKU is an autosomal recessive disorder, caused by
mutations in both alleles of the gene for phenylalanine hydroxylase (PAH),
found on chromosome 12. In the body, phenylalanine hydroxylase converts the
amino acid phenylalanine to tyrosine, another amino acid. Mutations in both
copies of the gene for PAH means that the enzyme is inactive or is less efficient,
and the concentration of phenylalanine in the body can build up to toxic levels.
In some cases, mutations in PAH will result in a phenotypically mild form
of PKU called hyperphenylalanemia. Both diseases are the result of a variety
of mutations in the PAH locus; in those cases where a patient is heterozygous
for two mutations of PAH (ie each copy of the gene has a different mutation),
the milder mutation will predominate.
A form of PKU has been discovered in mice, and these model organisms
are helping us to better understand the disease, and find treatments against
it. With careful dietary supervision, children born with PKU can lead normal
lives, and mothers who have the disease can produce healthy children.
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