Tetrahydrobiopterin as an alternative treatment for mild phenylketonuria.
BACKGROUND
Hyperphenylalaninemia is a common inherited metabolic disease that is due to phenylalanine hydroxylase deficiency, and at least half the affected patients have mild clinical phenotypes. Treatment with a low-phenylalanine diet represents a substantial psychosocial burden, but alternative treatments have not been effective.


METHODS
To explore the therapeutic efficacy of tetrahydrobiopterin, we performed a combined phenylalanine-tetrahydrobiopterin loading test and analyzed the in vivo rates of [13C]phenylalanine oxidation in 38 children with phenylalanine hydroxylase deficiency (age range, 1 day to 17 years). We assessed whether responsiveness to tetrahydrobiopterin was associated with specific genotypes, and we mapped mutations using a structural model of the phenylalanine hydroxylase monomer.


RESULTS
In 27 (87 percent) of 31 patients with mild hyperphenylalaninemia (10 patients) or mild phenylketonuria (21 patients), tetrahydrobiopterin significantly lowered blood phenylalanine levels. Phenylalanine oxidation was significantly enhanced in 23 of these 31 patients (74 percent). Conversely, none of the seven patients with classic phenylketonuria had a response to tetrahydrobiopterin as defined in this study. Long-term treatment with tetrahydrobiopterin in five children increased daily phenylalanine tolerance, allowing them to discontinue their restricted diets. Seven mutations (P314S, Y417H, V177M, V245A, A300S, E390G, and IVS4-5C-->G) were classified as probably associated with responsiveness to tetrahydrobiopterin, and six mutations (A403V, F39L, D415N, S310Y, R158Q, and I65T) were classified as potentially associated. Four mutations (Y414C, L48S, R261Q, and I65V) were inconsistently associated with this phenotype. Mutations connected to tetrahydrobiopterin responsiveness were predominantly in the catalytic domain of the protein and were not directly involved in cofactor binding.


CONCLUSIONS
Tetrahydrobiopterin responsiveness is common in patients with mild hyperphenylalaninemia phenotypes. Responsiveness cannot consistently be predicted on the basis of genotype, particularly in compound heterozygotes.
Muntau
Ania C
AC
Research Center, Department of Biochemical Genetics and Molecular Biology, Dr. von Hauner Children's Hospital, Ludwig Maximilians University, Munich, Germany.
Röschinger
Wulf
W
Habich
Matthias
M
Demmelmair
Hans
H
Hoffmann
Björn
B
Sommerhoff
Christian P
CP
Roscher
Adelbert A
AA
D000293:Adolescent; D001708:Biopterins / Q000031:analogs & derivatives* / Q000494:pharmacology / Q000627:therapeutic use*; D002648:Child; D002675:Child, Preschool; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D008968:Molecular Conformation; D020125:Mutation, Missense; D010084:Oxidation-Reduction / Q000187:drug effects; D010649:Phenylalanine / Q000097:blood / Q000378:metabolism*; D010651:Phenylalanine Hydroxylase / Q000737:chemistry / Q000235:genetics; D010661:Phenylketonurias / Q000145:classification / Q000188:drug therapy* / Q000235:genetics / Q000378:metabolism; D011446:Prospective Studies
D001708:Biopterins; D010649:Phenylalanine; D010651:Phenylalanine Hydroxylase; C003402:sapropterin