Inherited disorders affecting dopamine and serotonin: critical neurotransmitters derived from aromatic amino acids

Many inherited disorders affecting aromatic amino acid metabolism have been described. This review will concentrate on the defects that lead to deficiencies of dopamine and serotonin within the central nervous system. Phenylalanine hydroxylase, tyrosine hydroxylase, and tryptophan hydroxylase all require tetrahydrobiopterin (BH4) as a cofactor. Inherited defects that reduce the concentration of BH4, therefore, in general, lead to phenylketonuria and to deficiencies of dopamine and serotonin, as tyrosine hydroxylase and tryptophan hydroxylase are the rate-limiting enzymes required for the synthesis of these neurotransmitters. Primary inherited defects of tyrosine hydroxylase and aromatic l-amino acid decarboxylase have also been described. The clinical phenotypes are very similar to those observed in patients with defects of BH4 metabolism. Differential diagnosis is critical as treatment is different in each of the disorders. To date, a primary deficiency of tryptophan hydroxylase has not been described; when it finally is, the clinical phenotype might surprise us, as many groups around the world have been searching for such a defect for a long time.