PHENYLKETONURIA:
It is an inborn error of metabolism caused by defective phenylalanine
metabolism.
It is of 5 types:
- Type 1, classic phenylketonuria: Due to deficiency of
phenylalanine hydroxylase. - Type 2 and type 3: Due to deficiency of Dihydrobiopterin
reductase (chromosome 4). - Type 4 and type 5: Due to defective Dihydrobiopterin
biosynthesis.
It is an autosomal recessive disorder caused by mutation in short
arm of chromosome 12. Frequency of phenylketonuria is 1 in 1,500
all over the world and in India, it is seen in 1 in 25,000 live births.
Biochemical abnormalities:
Phenylalanine cannot be converted to tyrosine. So, phenylalanine
accumulates. When Phenylalanine level in blood is elevated, alternate
minor pathways are opened. Compounds like Phenylketone (phenylpyruvate),
phenyllactate and phenylacetate are found in urine.
Clinical manifestations:
PKU child is mentally retarded usually IQ of 50 is seen. About
20% inmates of lunatic asylum may have PKU.
Often clinical manifestations seen as Agitation, hyperactivity, tremors and convulsions. Phenylalanine interferes with
neurotransmitter synthesis that leads to common clinical features.
Hypopigmentation which is a common clinical finding is due to the inhibition of enzyme
tyrosinase.
Phenylacetate excreted in sweat causes mousy odor of the body.
Laboratory Diagnosis:
- Blood phenylalanine level: Normal level is 1 mg/dL. In PKU, the level is
20 mg/dL. This may be demonstrated by chromatography. Tandem
mass spectroscopy is the most reliable test.
- Guthrie test is a rapid screening test -Guthrie’s test detects phenylalanine in the blood of newborns, enabling for early diagnosis of PKU
- Ferric chloride test: Urine of the patient contains phenylketones
about 500–3000 mg/day. This could be detected by adding a drop of
ferric chloride to the urine. A transient blue-green color is a positive
test. But this is a less reliable test. - DNA probes – diagnose the defects in enzymes
phenylalanine hydroxylase and dihydrobiopterin reductase.
Treatment:
Early detection is very important.
About 5 units of IQ are lost for each 10-week delay in starting the treatment for phenylketonuria.
The treatment is to provide a diet containing low phenylalanine
(10–20 mg/kg body weight per day).
Food based on tapioca (cassava) will have low phenyl-
alanine content.
This special diet with low phenylalanine content is to be continued during the first
decade of life; after which the child can have a normal diet. Life-long
compliance of special diet is advised, though not mandatory.
Female child when becomes adult and become pregnant, again special diet is to be given, because the brain development of the fetus will be affected by increased phenylalanine
level.
