![[Feedback]](/icons/banner/feedbackACT.gif){kind=icon}
![[For Subscribers]](/icons/banner/subscriberACT.gif){kind=icon}
![[Archive]](/icons/banner/archiveACT.gif){kind=icon}
![[Search]](/icons/banner/searchACT.gif){kind=icon}
![[Contents]](/icons/banner/tocACT.gif){kind=icon}
|
|
|
|
|
||
4 Department of Health and Human Performance, University of Houston, Houston, TX 77204; 5 Departments of Pediatrics-Genetics and Biochemistry and Molecular Biology, University of Texas Medical Branch, Galveston, TX 77555; 6 John F. Kennedy Institute, Glostrup 2600, Denmark; and 7 University of Texas-Health Science Centre, Houston, TX 77030
* To whom correspondence should be addressed. E-mail: rmatalon{at}utmb.edu.
 |
ABSTRACT |
|---|
 TOP ABSTRACT IntroductionDiscussion and ConclusionLITERATURE CITED |
|---|
|
Introduction |
|---|
|
TOPABSTRACTIntroductionDiscussion and ConclusionLITERATURE CITED |
|---|
Classification of patients with PKU is sometimes confusing, but for purposes of convenience it is helpful to use peak level of blood Phe before treatment begins.
The classification "classical" (severe) PKU is based on historic level of blood Phe >1200 µmol/L. Such patients require more rigid dietary Phe restriction. Patients with "atypical" (mild) PKU have peak blood Phe <1200 µmol/L on a normal diet. These patients tend to have an easier time getting their blood Phe concentration in the range advocated by the NIH Consensus Conference report (2). Patients with "benign" hyperphenylalaninemia (HPA) have blood Phe concentration <600 µmol/L while on a normal diet that can be controlled without specific dietary treatment.
It is important to rule out other causes of elevated blood Phe, such as defects in the production or regeneration of the cofactor tetrahydrobiopterin (BH4). This cofactor is involved in the hydroxylation of Phe to tyrosine by PAH. However, the cofactor BH4 is also utilized in the hydroxylation reactions of tyrosine, and tryptophan. The inability to hydroxylate these amino acids leads to deficient levels of the neurotransmitters L-DOPA, from tyrosine, and serotonin, from tryptophan. Treatment of cofactor defects is completely different from for PKU. Patients with PKU are able to make adequate amounts of BH4 and have normal regeneration of BH4.
The genotype of most patients with PKU correlates with their phenotype, but the genotype is not always predictive and clinical correlation is always important. The enzyme PAH is a tetramer, with each monomer made of 452 amino acids, consisting of 3 domains: regulatory, catalytic, and tetramerization. The mutations of PAH have been found throughout all the domains of the enzyme. There have been >500 mutations described that lead to defects with varying severity in the activity of the enzyme PAH (3). These mutations can be deletions of parts of the gene, insertion of additional bases, missense mutations that change an amino acid on the protein, splicing defects where the reading of the message changes, and nonsense mutations where the protein is truncated or shortened. Most of the mutations are numbered according the amino acid on the protein chain. A common mutation among Caucasians is the change of arginine (R) at position 408 of PAH to tryptophan (W). This is a common severe mutation and it is indicated as R408W. When the residual activity of the enzyme is known, a mutation is classified as severe, moderate, or mild according to the enzyme activity. An example of a common mild mutation is Y414C, where tyrosine (Y) at position 414 of PAH is changed to cysteine (C).
Most patients with PKU are compound heterozygotes for 2 different mutations, contributing to the biochemical and clinical heterogeneity of the disease. The genotype gives an idea of what to expect regarding clinical response of the patient. Two severe mutations (severe/severe) would be expected to lead to a severe phenotype, whereas with a severe/mild genotype, a milder course is expected. Genotypes generally predict the phenotype; however, this is not always the rule. Although mutation analysis is important, the treatment of the patient with PKU needs to be individualized and blood Phe levels regularly monitored regardless of the genotype.
Treatment of PKU requires dietary restriction of Phe. High protein foods are restricted and a medical food is used to provide adequate dietary protein with additional tyrosine and no Phe. The Phe content of food is counted daily and the diet prescription is in milligrams of Phe per day. Dietary treatment of PKU is recommended for life with treatment goals of blood Phe concentrations between 120–360 µmol/L through 12 y of age and after 13 y of age, blood Phe concentration 120–900 µmol/L is considered acceptable, although blood concentration of Phe between 120 and 600 µmol/L is preferred (2). The diet is very restrictive and it is difficult for adolescents and adults with PKU to achieve the desired blood Phe goals. Therefore, other methods to improve dietary treatment of PKU are desired (3–6).
Kure et al. (7) found a decrease in blood Phe with oral administration of BH4 in patients with PKU. This was a rather startling finding, because these patients had normal levels of BH4. This observation was confirmed and replicated by many centers that treat PKU (8–16). One reason for the favorable response to BH4 is that mild mutations have some residual PAH activity and additional BH4 enhances the PAH activity and increases the half-life of the mutant enzyme. There have been reports of variations in response to oral BH4 in patients with the same mutations (same genotype) (17). The number of patients with PKU responding favorably to BH4 has varied. A study of 87 newborns with HPA, found that only 3 had a temporary decrease in blood Phe with oral BH4 (18), whereas Matalon et al. (16) reported favorable response in 
50% of patients studied in the US. This report summarizes findings on the response rate of patients with PKU to BH4 in the US where there may be higher response rate because of the heterogeneous population.
BH4 response in U.S. patients
Loading response. A pilot study in the United States was undertaken to determine how many patients with PKU would respond to an oral loading dose of 10 mg/kg BH4. There were 37 patients with PKU, 20 females and 17 males, including 5 children ages 5–14 y who participated. Twenty-four subjects had the designation of classical or severe PKU (blood Phe >1200 µmol/L), 10 had atypical or mild PKU (blood Phe 360–1199 µmol/L), and 3 had benign HPA (blood Phe <360 µmol/L) who did not require diet restrictions. Subjects had mutations in PAH identified via automated DNA sequencing (3,19). Mutations were identified and named according to the convention utilized by the phenylalanine hydroxylase locus knowledgebase in which codon 1 is the initiation codon (20). Patients were given a single oral dose of BH4, 10 mg/kg purchased from Schircks Laboratories. Blood Phe was determined at 0 and 24 h. Eighteen of 37 subjects (49%) responded with a significant decrease in blood Phe concentration 24 h after the 10 mg/kg BH4 load. These patients are shown in Table 1. Additionally, there were 4 patients (11%) who had a decrease of blood Phe following 10 mg/kg BH4 in the range of 17–26%, a level considered adequate by the NIH Consensus Conference (Table 2) (16). Other responsive subjects after the pilot project are shown in Table 3. Some of these were patients with benign HPA and were not on dietary treatment. The U.S. findings are similar to reports from Europe and Japan where BH4 was given in higher doses (7,11).
|
|
|
Response of patients with PKU to ascending or multiple doses of BH4. Another study was done to determine changes in blood Phe following ascending single doses of BH4 with 10, 20, and 40 mg/kg and to evaluate multiple daily doses, for 7 d each, of 10 and 20 mg/kg BH4 in 20 patients with PKU. We found 10 patients with PKU that responded to a single dose of 10 mg BH4/kg (50%) and 14 patients responded to 20 mg/kg (70%). There was no increased response rate in patients taking 40 mg/kg BH4. The lowest blood Phe concentration was found in patients after 1 wk of BH4 treatment (21).
|
Discussion and Conclusion |
|---|
|
TOPABSTRACTIntroductionDiscussion and ConclusionLITERATURE CITED |
|---|
In Europe, BH4 is used in a dose of 20 mg/kg (22). In our studies, the response rate increased to 70% when the dose of 20 mg/kg BH4 was used compared with 50% when 10 mg/kg was given. Increasing the dosage of BH4 in patients with PKU may further improve treatment and increase the number of patients that respond to BH4.
The use of mutation analysis in classifying severity of PKU is useful. In most cases, 2 severe mutations indicate a patient that will require a more restricted dietary Phe intake and their blood Phe concentrations will be sensitive to small increases in dietary intake of Phe. Patients with classical PKU are less likely to have positive response to BH4. If patients have a combination of a severe and a mild mutation, they typically can eat up to twice the amount of dietary Phe compared with severe/severe genotype patients. Additionally, blood Phe usually remains relatively constant even when small discrepancies in dietary Phe intake occur. These patients most typically respond positively to BH4. Therefore, mutation analysis can be important for identifying potential patients who can benefit from BH4. However, due to the uniqueness of every individual, an actual BH4 challenge is needed to ascertain responsiveness and nonresponsiveness. Again, different combinations of mutations found in the melting pot of the United States and epigenetic factors often lead to surprises between prediction and actual response to BH4.
Mutations F39L, I65T, and R68S are in the regulatory domain and the other mutations are in the catalytic domain with the exception of Y414C, which is in the tetramerization domain. All the mutants in this review had >30% residual PAH activity when the enzyme was tested following expression, except V388M, which had 23% residual activity (23). The kinetics and enzyme half-life of 15 responsive mutations have been reported earlier and indicate that the response to BH4 is multifactorial (23).
The favorable response to additional BH4 can be due to improved binding of the cofactor BH4 to the mutant enzyme, because the mutant enzyme requires more BH4 (Km mutations). Other factors may provide protective effects of BH4 on the mutant enzyme, resulting in an increase in its half-life; improved enzyme activity can be caused by prevention of misfolding. In the 15 mutations studied, only 3 were clear Km mutations. These were F39L, I65T, and L308F (23). There can be increased stability and correction of the Km of the mutant enzyme in the same patient. The increase of the half-life of some mutant enzymes suggests a protective "chaperone" effect of BH4. Mutation F39L is clearly a Km mutation, but also BH4 shows a chaperone effect for this mutation (23,24).
The addition of BH4 to the management of PKU is new and exciting. When all PKU patients are tested for response to BH4, it appears that at least 50% will be responders. The heterogeneity of the population in the US and the large number of mutations is the likely cause for the results of those early studies. There will be patients who will require only BH4 (monotherapy); these are usually very mild PKU patients. Most of the other BH4 responders will require a combination of BH4 and some dietary Phe restriction. So the addition of BH4 will add another parameter for better compliance in the management of PKU.
|
FOOTNOTES |
|---|
2 Supported in part by grants from NIH, HD023148; FDA, FD-R002600; and Biomarin Pharmaceutical.
3 Author disclosures: K. Michals-Matalon and R. Matalon, travel expenses to attend meeting paid by Ajinomoto Company, Inc.; G. Bhatia, F. Guttler, and S. K. Tyring, no conflicts of interest.
8 Abbreviations used: BH4, tetrahydrobiopterin; HPA, hyperphenylalaninemia; PAH, phenylalanine hydroxylase; Phe, phenylalanine; PKU, phenylketonuria.
|
LITERATURE CITED |
|---|
|
TOPABSTRACTIntroductionDiscussion and ConclusionLITERATURE CITED |
|---|
1. Scriver CR, Kaufman S. Hyperphenylalaninemias: phenylalanine hydroxylase deficiency. In: Scriver CR, Beaudet AL, Sly WS, Valle D, editors. The metabolic and molecular bases of inherited disease. New York: McGraw-Hill; 1980. p. 1667–1724.
2. NIH. Phenylketonuria: screening and management. NIH Consensus Statement October 16–18, 2000. An Pediatr (Barc). 2001;108:972–82.
3. Scriver CR, Hurtubise M, Konecki D, Phommarinh M, Prevost L, Erlandsen H, Stevens R, Waters PJ, Ryan S, McDonald D, et al. Hum. Mutat. 2003;21:333–44.[Medline]
4. Guttler F. Hyperphenylalaninemia. Diagnosis and classification of the various types of phenylalanine hydroxylase deficiency in children. Acta Paediatr Scand. 1980;280:1–80.
5. Bickel H, Gerrard AJ, Hickman EM. Influence of phenylalanine intake on phenylketonuria. Lancet. 1953;2:812–3.
6. Recommendations on the dietary management of phenylketonuria. Report of Medical Research Council Working Party on Phenylketonuria. Arch Dis Child. 1993;68:426–7.
7. Kure S, Hou DC, Ohura T, Iwamoto H, Suzuki S, Sugiyama N, Sakamoto O, Fujii K, Matsubara Y, et al. Tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency: a novel clinical entity. J Pediatr. 1999;135:375–8.[Medline]
8. Blau N, Trefz F. Tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency: possible regulation of gene expression in a patient with the homozygous L48S mutation. Mol Genet Metab. 2002;75:186–7.[Medline]
9. Lassker U, Zschocke J, Blau N, Santer R. Tetrahydrobiopterin responsiveness in phenylketonuria. Two new cases and a review of molecular genetic findings. J Inherit Metab Dis. 2002;25:65–70.[Medline]
10. Muntau AC, Roschinger W, Habich M, Demmelmeir P, Hoffmann B, Sommerhoff CP, Roscher AA. Tetrahydrobiopterin as an alternative treatment for mild phenylketonuria. N Engl J Med. 2002;347:2122–32.
11. Shintaku H, Kure S, Ohura T, Okano Y, Ohwada M, Sugiyama N, Sakura N, Yoshida I, Yoshino M, et al. Long-term treatment and diagnosis of tetrahydrobiopterin-responsive hyperphenylalaninemia with a mutant phenylalanine hydroxylase gene. Pediatr Res. 2004;55:425–30.[Medline]
12. Desviat LR, Perez B, Belanger-Quintana A, Castro M, Aguado C, Sanchez A, Garcia MJ, Martinez-Pardo M, Ugarte M. Tetrahydrobiopterin responsiveness: results of the BH4 loading test in 31 Spanish PKU patients and correlation with their genotype. Mol Genet Metab. 2004;83:157–62.[Medline]
13. Leuzzi V, Carducci C, Carducci C, Chiarotti F, Artiola C, Giovanniello T, Antonozzi I. The spectrum of phenylalanine variations under tetrahydrobiopterin load in subjects affected by phenylalanine hydroxylase deficiency. J Inherit Metab Dis. 2006;29:38–46.[Medline]
14. Hennermann JB, Vetter B, Kulozik AE, Monch E. Long-term treatment with tetrahydrobiopterin increases phenylalanine tolerance in children with severe phenotype of phenylketonuria. Mol Genet Metab. 2005;86: Suppl 1:S86–90.[Medline]
15. Spaapen LJ, Bakker JA, Velter C, Loots W, Rubio-Gozalbo ME, Forget PP, Dorland L, De Koning TJ, Poll-The BT, et al. Tetrahydrobiopterin-responsive hyperphenylalaninemia (HPA) in Dutch neonates. J Inherit Metab Dis. 2001;24:352–8.[Medline]
16. Matalon R, Koch R, Michals-Matalon K, Moseley K, Surendran S, Tyring S, Erlandsen H, Gamez A, Stevens RC, et al. Biopterin responsive phenylalanine hydroxylase deficiency. Genet Med. 2004;6:27–32.[Medline]
17. Lindner M, Hass D, Zschocke J, Burgard P. Tetrahydrobiopterin responsiveness in phenylketonuria differs between patients with the same genotype. Mol Genet Metab. 2001;73:104–6.[Medline]
18. Weglage J, Grenzebach M, von Teeffelen-Heithoff A, Marquardt T, Feldmann R, Denecke J, Godde D, Koch HG. Tetrahydrobiopterin responsiveness in a large series of phenylketonuria patients. J Inherit Metab Dis. 2002;25:321–2.[Medline]
19. Guldberg P, Henriksen KF, Guttler F. Molecular analysis of phenylketonuria in Denmark: 99% of the mutations detected by denaturing gradient gel electrophoresis. Genomics. 1993;17:141–6.[Medline]
20. Scriver CR, Waters PJ, Sarkissian C, Ryan S, Prevost L, Cote D, Novak J, Teebi S, Nowacki PM. PAHdb: a locus-specific knowledgebase. Hum Mutat. 2000;15:99–104.[Medline]
21. Matalon R, Michals-Matalon K, Koch R, Grady J, Tyring S, Stevens RC. Response of patients with phenylketonuria in the US to tetrahydrobiopterin. Mol Genet Metab. 2005;86:S17–21.[Medline]
22. Trefz F, Aulehla-Scholz C, Blau N. Successful treatment of phenylketonuria with tetrahydrobiopterin [letter to editor]. Eur J Pediatr. 2001;160:315.[Medline]
23. Erlandsen H, Pey AL, Gamez A, Perez B, Desviat LR, Aguado C, Koch R, Surendran S, Tyring S, et al. Correction of kinetic and stability defects by tetrahydrobiopterin in phenylketonuria patients with certain phenylalanine hydroxylase mutations. Proc Natl Acad Sci USA. 2004;101:16903–8.
24. Erlandsen H, Stevens RC. A structural hypothesis for BH4 responsiveness in patients with mild forms of hyperphenylalaninemia and phenylketonuria. J Inherit Metab Dis. 2001;24:213–30.[Medline]
This article has been cited by other articles:
|
|
 |
|
  K. Takai Introduction to the Transdisciplinary International Conference on Aromatic Amino Acids and Related Substances: Chemistry, Biology, Medicine, and Application J. Nutr., June 1, 2007; 137(6): 1501S - 1503S. [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
