6-Pyruvoyl-Tetrahydropterin Synthase Deficiency

6-pyruvoyl-tetrahydropterin synthase (PTPS) deficiency is one of the causes of malignant hyperphenylalaninemia due to tetrahydrobiopterin deficiency. Not only does tetrahydrobiopterin deficiency cause hyperphenylalaninemia, it is also responsible for defective neurotransmission of monoamines because of malfunctioning tyrosine and tryptophan hydroxylases, both tetrahydrobiopterin-dependent hydroxylases.

Clinical description

When left untreated, the deficiency causes neurological signs at age 4 or 5 months, although clinical signs are often obvious from birth. The principal symptoms include psychomotor retardation, tonus disorders, convulsions, drowsiness, irritability, abnormal movements, hyperthermia, hypersalivation and difficulty swallowing.

Diagnostic methods

PTPS deficiency should be suspected in all infants with a positive neonatal screening test for phenylketonuria, especially when hyperphenylalaninemia is moderate. The most effective way to diagnose the disorder is to measure pteridine levels in urine and to confirm the result by measuring neurotransmitters 5-hydroxyindolacetic acid (5-HIAA) and homovanillic acid (HVA) in cerebrospinal fluid and with an oral tetrahydrobiopterin-loading test (20 mg/kg).

Genetic counseling

PTPS deficiency is an autosomal recessive genetic disorder.

Management and treatment

Treatment attempts to bring phenylalaninemia levels back to normal (diet with restricted phenylalanine intake or prescription of tetrahydrobiopterin) and to restore normal monoaminergic neurotransmission by administering precursors (L-dopa/carbidopa and 5-hydroxytryptophan).