Phenylketonuria

INTRODUCTION:
  • Autosomal recessive metabolic genetic disorder
  • Mutation in the gene for phenylalanine hydroxylase(PAH).
  • Phenylalanine hydroxylase which converts phenylalanine to tyrosine is deficient in phenylketonuria.
  • When PAH activity is reduced, phenylalanine accumulates and is converted into phenylpyruvate(phenylketone), which can be detected in the urine.
  • Untreated PKU can lead to intellectual disability, seizures, and other serious medical problems.
  • Patients who are diagnosed early and maintain a strict diet can have a normal life span with normal mental development.
  • PKU is rare – it is estimated to affect 1 in every 10,000 babies.
ETIOLOGY:
  • Autosomal recessive disorder caused by mutation in PAH gene
  • Located on 12th chromosome.
  • A carrier does not have symptoms of the disease, but can pass on the defective gene to his or her children.
SYMPTOMS:
  • Most babies with phenylketonuria appear healthy at birth.
  • Symptoms usually only develop due to complications that arise if the condition is not treated properly.
  • If it isn't treated, damage to the brain and nervous system can lead to:
  1. Mental Retardation
  2. learning disabilities
  3. behavioural difficulties
  4. epilepsy
  • Often have lighter skin, hair, and eyes than brothers or sisters without the disease.
  • Other symptoms include:
  1. Eczema
  2. Recurrent vomiting
  3. Jerking movements in arms and legs
  4. Enamel Hypoplasia
  5. Tremors
  6. Mood disorders
  7. Microcephaly,
DIAGNOSIS:
  • Screening on blood samples during the first week of life.
  • Ferric chloride Test: Phenyl ketones in urine can be detected by this test. A transient blue-green color is a positive test.
  • Nitroprusside Test is used for the colour reaction of amino acid Sulfhydryl group (Cysteine.)
  • Urine test : Guthrie test 
  • Mousy odour urine 
  • If a diagnosis of PKU is confirmed, the child will need regular blood tests to measure levels of phenylalanine in their blood and assess how well they are responding to treatment.
TREATMENT:
  • Limiting the substrate for deficient enzyme
  • Phenylketonuria (PKU) can be successfully treated with a low-protein diet and dietary supplements.
  • The diet must be strictly followed.
  • Those who continue the diet into adulthood have better physical and mental health.
Exam Question
  • In Phenylketonuria the main aim of first line therapy is Limiting the substrate for deficient enzyme
  • Microcephaly, blue eyes, fair skin, and mental retardation in a 4 year old girl with a positive ferric chloride test is indicative of Phenylketonuria 
  • A child brought with seizures and impairment of development is diagnosed having phenylketonuria. Restriction of substrate of the deficient enzyme in the diet is the initial line of treatment in this childv The primary deficiency in the disease phenylketonuria occurs in the synthesis of Tyrosine
  • Phenylketonuria is detected by Guthrie test 
  • Incomplete oxidation of phenylpyruvic acid results in Phenylketonuria
  • Mousy odour urine is seen in Phenylketonuria
  • Enzyme deficient in phenylketonuria is Phenylalanine hydroxylase
  • In phenylketonuria FeCl3 test with urine gives green color
  • Severe mental retardation,Enamel hypoplasia & Vomiting in early infancy is seen in Phenylketonuria
  • Brain damage in phenylketonuria is due to accumulation of Phenylalanine
  • Phenylketonuria is Autosomal recessive disorder

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