Tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency in Dutch neonates

Developmental delay and non-phenylketonuria (PKU) hyperphenylalaninemia in DNAJC12 deficiency: Case and approach

BACKGROUND

Hyperphenylalaninemia is a biomarker for several monogenic neurotransmitter disorders where the body cannot metabolise phenylalanine to tyrosine. Biallelic pathogenic variants in DNAJC12, co-chaperone of phenylalanine, tyrosine, and tryptophan hydroxylases, leads to hyperphenylalaninemia and biogenic amines deficiency.

METHODS AND RESULTS

A male firstborn to non-consanguineous Sudanese parents had hyperphenylalaninemia 247 µmol/L [reference interval (RI) < 200 µmol/L] at newborn screening. Dried blood spot dihydropteridine reductase (DHPR) assay and urine pterins were normal. He had severe developmental delay and autism spectrum disorder without a notable movement disorder. A low phenylalanine diet was introduced at two years without any clinical improvements. Cerebrospinal fluid (CSF) neurotransmitters at five years demonstrated low homovanillic acid (HVA) 0.259 µmol/L (reference interval (RI) 0.345-0.716) and 5-hydroxyindoleaetic acid (5HIAA) levels 0.024 µmol/L (reference interval (RI) 0.100-0.245). Targeted neurotransmitter gene panel analysis identified a homozygous c.78 + 1del variant in DNAJC12. At six years, he was commenced on 5-hydroxytryptophan 20 mg daily, and his protein-restricted diet was liberalised, with continued good control of phenylalanine levels. Sapropterin dihydrochloride 7.2 mg/kg/day was added the following year with no observable clinical benefits. He remains globally delayed with severe autistic traits.

CONCLUSIONS

Urine, CSF neurotransmitter studies, and genetic testing will differentiate between phenylketonuria, tetrahydrobiopterin or DNAJC12 deficiency, with the latter characterised by a clinical spectrum ranging from mild autistic features or hyperactivity to severe intellectual disability, dystonia, and movement disorder, normal DHPR, reduced CSF HIAA and HVA. DNAJC12 deficiency should be considered early in the differential workup of hyperphenylalaninemia identified from newborn screening, with its genotyping performed once deficiencies of phenylalanine hydroxylase (PAH) and tetrahydrobiopterin (BH4) have been biochemically or genetically excluded.

Sapropterin dihydrochloride for phenylketonuria

BACKGROUND

Phenylketonuria results from a deficiency of the enzyme phenylalanine hydroxylase. Dietary restriction of phenylalanine keeps blood phenylalanine concentration low. Most natural foods are excluded from diet and supplements are used to supply other nutrients. Recent publications report a decrease in blood phenylalanine concentration in some patients treated with sapropterin dihydrochloride. We examined the evidence for the use of sapropterin dihydrochloride to treat phenylketonuria. This is an update of a previously published Cochrane Review. 

OBJECTIVES

To assess the safety and efficacy of sapropterin dihydrochloride in lowering blood phenylalanine concentration in people with phenylketonuria.

SEARCH METHODS

We identified relevant trials from the Group's Inborn Errors of Metabolism Trials Register. Date of last search: 11 August 2014.We also searched ClinicalTrials.gov and Current controlled trials. Last search: 4 September 2014We contacted the manufacturers of the drug (BioMarin Pharmaceutical Inc.) for information regarding any unpublished trials.

SELECTION CRITERIA

Randomized controlled trials comparing sapropterin with no supplementation or placebo in people with phenylketonuria due to phenylalanine hydroxylase deficiency.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed trials and extracted outcome data.

MAIN RESULTS

Two placebo-controlled trials were included. One trial administered 10 mg/kg/day sapropterin in 89 children and adults with phenylketonuria whose diets were not restricted and who had previously responded to saproterin.This trial measured change in blood phenylalanine concentration. The second trial screened 90 children (4 to 12 years) with phenylketonuria whose diet was restricted, for responsiveness to sapropterin. Forty-six responders entered the placebo-controlled part of the trial and received 20 mg/kg/day sapropterin. This trial measured change in both phenylalanine concentration and protein tolerance. Both trials reported adverse events. The trials showed an overall low risk of bias; but both are Biomarin-sponsored. One trial showed a significant lowering in blood phenylalanine concentration in the sapropterin group (10 mg/kg/day), mean difference -238.80 μmol/L (95% confidence interval -343.09 to -134.51); a second trial (20 mg/kg/day sapropterin) showed a non-significant difference, mean difference -51.90 μmol/L (95% confidence interval -197.27 to 93.47). The second trial also reported a significant increase in phenylalanine tolerance, mean difference18.00 mg/kg/day (95% confidence interval 12.28 to 23.72) in the 20 mg/kg/day sapropterin group.

AUTHORS' CONCLUSIONS

There is evidence of short-term benefit from using sapropterin in some people with sapropterin-responsive forms of phenylketonuria; blood phenylalanine concentration is lowered and protein tolerance increased. There are no serious adverse events associated with using sapropterin in the short term.There is no evidence on the long-term effects of sapropterin and no clear evidence of effectiveness in severe phenylketonuria.

Long-term follow-up of patients with phenylketonuria treated with tetrahydrobiopterin: a seven years experience

BACKGROUND

Phenylketonuria (PKU) is an autosomal recessive disorder caused by the deficiency of phenylalanine hydroxylase that catalyzes the conversion of phenylalanine to tyrosine, using tetrahydrobiopterin (BH4) as coenzyme. Besides dietary phenylalanine restriction, new therapeutic options are emerging, such as the treatment with BH4 in subgroups of PKU patients responding to a loading test with BH4.

METHODS

A no-profit open-label interventional trial with long-term oral BH4 therapy, sponsored by the Italian Medicines Agency (AIFA), was performed in a group of 17 PKU patients resulted as BH4 responders among 46 subjects analyzed for BH4-responsiveness (prot. FARM5MATC7). We report on efficacy and safety data of BH4 therapy and analyze factors predicting BH4-responsiveness and long-term response to BH4. A BH4-withdrawal test was used as a proof of the efficacy of long-term therapy with BH4.

RESULTS

Forty-four percent of the patients responded to the 48 h-long loading test with BH4. All the phenotypic classes were represented. Genotype was the best predictor of responsiveness, along with lower phenylalanine levels at diagnosis, higher tolerance and lower phenylalanine/tyrosine ratio before the test. In BH4 responder patients, long-term BH4 therapy resulted safe and effective in increasing tolerance while maintaining a good metabolic control. The BH4 withdrawal test, performed in a subset of patients, showed that improved tolerance was directly dependent on BH4 assumption. Tolerance to phenylalanine was re-evaluated in 43.5% of patients and was longitudinally analyzed in 5 patients.

CONCLUSIONS

Long-term treatment with BH4 is safe and effective in increasing tolerance to phenylalanine. There is real need to assess the actual tolerance to phenylalanine in PKU patients to ameliorate quality of life, improve nutritional status, avoiding unnecessarily restricted diets, and interpret the effects of new therapies for PKU.

Tetrahydrobiopterin responsiveness in phenylketonuria: prediction with the 48-hour loading test and genotype

BACKGROUND

How to efficiently diagnose tetrahydrobiopterin (BH4) responsiveness in patients with phenylketonuria remains unclear. This study investigated the positive predictive value (PPV) of the 48-hour BH4 loading test and the additional value of genotype.

METHODS

Data of the 48-hour BH4 loading test (20 mg BH4/kg/day) were collected at six Dutch university hospitals. Patients with ≥30% phenylalanine reduction at ≥1 time points during the 48 hours (potential responders) were invited for the BH4 extension phase, designed to establish true-positive BH4 responsiveness. This is defined as long-term ≥30% reduction in mean phenylalanine concentration and/or ≥4 g/day and/or ≥50% increase of natural protein intake. Genotype was collected if available.

RESULTS

177/183 patients successfully completed the 48-hour BH4 loading test. 80/177 were potential responders and 67/80 completed the BH4 extension phase. In 58/67 true-positive BH4 responsiveness was confirmed (PPV 87%). The genotype was available for 120/177 patients. 41/44 patients with ≥1 mutation associated with long-term BH4 responsiveness showed potential BH4 responsiveness in the 48-hour test and 34/41 completed the BH4 extension phase. In 33/34 true-positive BH4 responsiveness was confirmed. 4/40 patients with two known putative null mutations were potential responders; 2/4 performed the BH4 extension phase but showed no true-positive BH4 responsiveness.

CONCLUSIONS

The 48-hour BH4 loading test in combination with a classified genotype is a good parameter in predicting true-positive BH4 responsiveness. We propose assessing genotype first, particularly in the neonatal period. Patients with two known putative null mutations can be excluded from BH4 testing.

Testing for tetrahydrobiopterin responsiveness in patients with hyperphenylalaninemia due to phenylalanine hydroxylase deficiency

INTRODUCTION

Pharmacological levels of the phenylalanine hydroxylase enzyme cofactor, tetrahydrobiopterin (BH4), reduce plasma phenylalanine levels in some patients with phenylketonuria (PKU), providing the first pharmacological therapy for PKU. Responsiveness to this therapy must be determined empirically through a BH4 loading test or trial. The authors have analyzed the loading tests currently in use in light of the numerous factors that can influence their results. Sapropterin dihydrochloride is a stable, synthetic form of BH4 approved for treatment of PKU in responsive patients.

METHODS

An expert panel identified evidence from published reports of clinical experience. Reports of research involving at least 25 patients and published in English were considered.

RESULTS

In all, 14 studies met both criteria; eight employing the sapropterin dihydrochloride preparation from Schircks Laboratories and six the sapropterin dihydrochloride preparation from Biomarin/Merck Serono.

CONCLUSION

The arbitrary responsiveness definition of a >30% reduction in blood phenylalanine appears to be a good compromise between sensitivity and specificity for the initial screening test. However, individual patient characteristics should be considered when interpreting results, especially in patients with low baseline phenylalanine levels.

Sapropterin dihydrochloride for phenylketonuria

BACKGROUND

Phenylketonuria results from a deficiency of the enzyme phenylalanine hydroxylase. Dietary restriction of phenylalanine keeps blood phenylalanine concentration low. Most natural foods are excluded from diet and supplements are used to supply other nutrients. Recent publications report a decrease in blood phenylalanine concentration in some patients treated with sapropterin dihydrochloride. We examined the evidence for the use of sapropterin dihydrochloride to treat phenylketonuria.  

OBJECTIVES

To assess the safety and efficacy of sapropterin dihydrochloride in lowering blood phenylalanine concentration in people with phenylketonuria.

SEARCH METHODS

We identified relevant trials from the Group's Inborn Errors of Metabolism Trials Register. Date of last search: 29 June 2012.We also searched ClinicalTrials.gov and Current controlled trials. Last search: 23 July 2012.We contacted the manufacturers of the drug (BioMarin Pharmaceutical Inc.) for information regarding any unpublished trials.

SELECTION CRITERIA

Randomized controlled trials comparing sapropterin with no supplementation or placebo in people with phenylketonuria due to phenylalanine hydroxylase deficiency.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed trials and extracted outcome data.

MAIN RESULTS

Two placebo-controlled trials were included. One trial administered 10 mg/kg/day sapropterin in 89 children and adults with phenylketonuria whose diets were not restricted and who had previously responded to saproterin.This trial measured change in blood phenylalanine concentration. The second trial screened 90 children (4 to 12 years) with phenylketonuria whose diet was restricted, for responsiveness to sapropterin. Forty-six responders entered the placebo-controlled part of the trial and received 20 mg/kg/day sapropterin. This trial measured change in both phenylalanine concentration and protein tolerance. Both trials reported adverse events. The trials showed an overall low risk of bias; but both are Biomarin-sponsored. One trial showed a significant lowering in blood phenylalanine concentration in the sapropterin group (10 mg/kg/day), mean difference -238.80 μmol/L (95% confidence interval -343.09 to -134.51); a second trial (20 mg/kg/day sapropterin) showed a non-significant difference, mean difference -51.90 μmol/L (95% confidence interval -197.27 to 93.47). The second trial also reported a significant increase in phenylalanine tolerance, mean difference18.00 mg/kg/day (95% confidence interval 12.28 to 23.72) in the 20 mg/kg/day sapropterin group.

AUTHORS' CONCLUSIONS

There is evidence of short-term benefit from using sapropterin in some patients with sapropterin-responsive forms of phenylketonuria; blood phenylalanine concentration is lowered and protein tolerance increased. There are no serious adverse events associated with using sapropterin in the short term.There is no evidence on the long-term effects of sapropterin and no clear evidence of effectiveness in severe phenylketonuria.

Long-term pharmacological management of phenylketonuria, including patients below the age of 4 years

BH4 therapy is an advancement in the treatment of phenylketonuria, reducing blood phenylalanine (phe) levels and increasing tolerance to natural proteins of responding patients. We report the results of 16 patients undergoing long-term BH4 treatment. Responding patients to BH4 was usually based on 24-h loading tests; a ≥30% decrease in blood phe was considered a positive response. Weekly loading made it possible to identify an additional "slow responder." The 16 responders constitute 24.6% of patients who completed the trial (87.5% of responders in mild hyperphenylalaninemia, 38.1% in mild PKU, and 2.8% in classical PKU).Mean dose of BH4 used was 9.75 ± 0.9 mg/kg per day, during a mean of 62 months. Age at treatment start was below 4 years in seven patients; five of which begun treatment during their first month since birth. All but one patient showed good treatment compliance; six continue on BH4 monotherapy without dietary phe restriction; six showed an increase in phe tolerance of 24-55%; and in the five patients who received treatment since the neonatal period an increase in phe tolerance following the phase of maximum growth has persisted. None of the patients showed side effects except one whom vomiting at the beginning of the treatment.Testing at the time of diagnosis in the neonatal period is very appropriate, and if there is a positive response, the patient can be treated with BH4 from onset with the advantage of being able to continue breast-feeding.

Effect of tetrahydrobiopterin on Phe/Tyr ratios and variation in Phe levels in tetrahydrobiopterin responsive PKU patients

BACKGROUND

Whilst a reduction in blood phenylalanine (Phe) levels is essential in patients with PKU, a decrease in Phe/Tyrosine (Tyr) ratio and fluctuations in blood Phe levels over time have been recently associated with improved neuropsychological outcome. The aim of this study was to identify if Tetrahydrobiopterin (BH(4)) offers additional benefit based on the assumption that these 2 factors are beneficial.

METHOD

Since 2002, 9 patients identified through NBS as BH(4) responsive (BH(4) group) and 25 non-responsive patients (non-BH(4) group) produced a total of 1384 and 4415 samples, respectively, for analysis. Statistical analysis was performed to compare mean and median Phe levels, Tyr levels and Phe/Tyr ratios in BH(4) and non BH(4) responsive patients.

RESULTS

Variations in blood Phe levels were greater in the non-BH(4) group (BH(4): median 338 μmol/L, 95% Confidence Interval (CI) 329-346, mean: 358 μmol/L, CI 350-366; non-BH(4): median 338 μmol/L CI 332-344, mean: 370 μmol/L CI 364-376). Variations in blood Tyr levels were slightly greater in the non-BH(4) group: (BH(4): median 59 μmol/L CI 58-61, mean 67 CI 66-69; non-BH(4): median 62 μmol/L CI 61-63, mean 70 CI 69-71). The variation in Phe/Tyr ratios was greater in the non-BH(4) group (mean 6.12, CI 5.9-6.3) than in the BH(4) group (Mean 5.44, CI 5.3-5.6), particularly at blood Phe levels >600 μmol/L.

CONCLUSION

BH(4) responsive patients have smaller variations in blood Phe levels and tighter Phe/Tyr ratios than non-BH(4) responsive patients, particularly at high blood Phe levels. If decreased fluctuations in Phe levels and a decreased Phe/Tyr ratio are indeed neuro-protective, then BH(4) responsiveness is advantageous over diet alone in PKU. Neuropsychological testing in patients who have been treated with BH(4) long term may be able to ascertain the clinical benefit of these biochemical findings.

The interplay between genotype, metabolic state and cofactor treatment governs phenylalanine hydroxylase function and drug response

The discovery of a pharmacological treatment for phenylketonuria (PKU) raised new questions about function and dysfunction of phenylalanine hydroxylase (PAH), the enzyme deficient in this disease. To investigate the interdependence of the genotype, the metabolic state (phenylalanine substrate) and treatment (BH(4) cofactor) in the context of enzyme function in vitro and in vivo, we (i) used a fluorescence-based method for fast enzyme kinetic analyses at an expanded range of phenylalanine and BH(4) concentrations, (ii) depicted PAH function as activity landscapes, (iii) retraced the analyses in eukaryotic cells, and (iv) translated this into the human system by analyzing the outcome of oral BH(4) loading tests. PAH activity landscapes uncovered the optimal working range of recombinant wild-type PAH and provided new insights into PAH kinetics. They demonstrated how mutations might alter enzyme function in the space of varying substrate and cofactor concentrations. Experiments in eukaryotic cells revealed that the availability of the active PAH enzyme depends on the phenylalanine-to-BH(4) ratio. Finally, evaluation of data from BH(4) loading tests indicated that the patient's genotype influences the impact of the metabolic state on drug response. The results allowed for visualization and a better understanding of PAH function in the physiological and pathological state as well as in the therapeutic context of cofactor treatment. Moreover, our data underscore the need for more personalized procedures to safely identify and treat patients with BH(4)-responsive PAH deficiency.

Phenylketonuria as a model for protein misfolding diseases and for the development of next generation orphan drugs for patients with inborn errors of metabolism

The lecture dedicated to Professor Horst Bickel describes the advances, successes, and opportunities concerning the understanding of the biochemical and molecular basis of phenylketonuria and the innovative treatment strategies introduced for these patients during the last 60 years. These concepts were transferred to other inborn errors of metabolism and led to significant reduction in morbidity and to an improvement in quality of life. Important milestones were the successful development of a low-phenylalanine diet for phenylketonuria patients, the recognition of tetrahydrobiopterin as an option to treat these individuals pharmacologically, and finally market approval of this drug. The work related to the discovery of a pharmacological treatment led metabolic researchers and pediatricians to new insights into the molecular processes linked to mutations in the phenylalanine hydroxylase gene at the cellular and structural level. Again, phenylketonuria became a prototype disorder for a previously underestimated but now rapidly expanding group of diseases: protein misfolding disorders with loss of function. Due to potential general biological mechanisms underlying these disorders, the door may soon open to a systematic development of a new class of pharmaceutical products. These pharmacological chaperones are likely to correct misfolding of proteins involved in numerous genetic and nongenetic diseases.

Sapropterin dihydrochloride for phenylketonuria

BACKGROUND

Phenylketonuria results from a deficiency of the enzyme phenylalanine hydroxylase. Dietary restriction of phenylalanine keeps blood phenylalanine concentration low. Most natural foods are excluded from diet and supplements are used to supply other nutrients. Recent publications report a decrease in blood phenylalanine concentration in some patients treated with sapropterin dihydrochloride. We examined the evidence for the use of sapropterin dihydrochloride to treat phenylketonuria.

OBJECTIVES

To assess the safety and efficacy of sapropterin dihydrochloride in lowering blood phenylalanine concentration in people with phenylketonuria.

SEARCH STRATEGY

We identified relevant trials from the Group's Inborn Errors of Metabolism Trials Register. Last search:07 May 2010.We also searched ClinicalTrials.gov and Current controlled trials. Last search: 01 September 2009.We contacted the manufacturers of the drug (BioMarin Pharmaceutical Inc.) for information regarding any unpublished trials.

SELECTION CRITERIA

Randomized controlled trials comparing sapropterin with no supplementation or placebo in people with phenylketonuria due to phenylalanine hydroxylase deficiency.

DATA COLLECTION AND ANALYSIS

Two authors independently assessed trials and extracted outcome data.

MAIN RESULTS

Two placebo-controlled trials were included. One trial administered 10 mg/kg/day sapropterin in 89 children and adults with phenylketonuria whose diets were not restricted and who had previously responded to saproterin.This trial measured change in blood phenylalanine concentration. The second trial screened 90 children (4 to 12 years) with phenylketonuria whose diet was restricted, for responsiveness to sapropterin. Forty-six responders entered the placebo-controlled part of the trial and received 20 mg/kg/day sapropterin. This trial measured change in both phenylalanine concentration and protein tolerance. Both trials reported adverse events. The trials showed an overall low risk of bias; but both are Biomarin-sponsored. One trial showed a significant lowering in blood phenylalanine concentration in the sapropterin group (10 mg/kg/day), mean difference -238.80 mumol/L (95% confidence interval -343.09 to -134.51); a second trial (20 mg/kg/day sapropterin) showed a non-significant difference, mean difference -51.90 mumol/L (95% confidence interval -197.27 to 93.47). The second trial also reported a significant increase in phenylalanine tolerance, mean difference18.00 mg/kg/day (95% confidence interval 12.28 to 23.72) in the 20 mg/kg/day sapropterin group.

AUTHORS' CONCLUSIONS

There is evidence of short-term benefit from using sapropterin in some patients with sapropterin-responsive forms of phenylketonuria; blood phenylalanine concentration is lowered and protein tolerance increased. There are no serious adverse events associated with using sapropterin in the short term.There is no evidence on the long-term effects of sapropterin and no clear evidence of effectiveness in severe phenylketonuria.

Effect of BH(4) supplementation on phenylalanine tolerance

BACKGROUND

Tetrahydrobiopterin (BH(4)) is a potential new orphan drug for the treatment of some patients with phenylketonuria (PKU), mostly mild forms. Numerous studies have confirmed this finding and BH(4)-responsiveness may be predicted to some extent from the corresponding genotype.

AIM

To investigate the response to BH(4) loading test, the phenylalanine hydroxylase (PAH) mutations and the long-term therapeutic efficacy of BH(4) in patients with PKU, and to better define BH(4)-responsive patients according to phenylalanine (Phe) levels and dietary phenylalanine tolerance.

METHODS

30 Italian PKU patients (age range: 6 months-24 years; 12 female, 18 male) were included in this retrospective study. Eleven out of 30 patients presented with Phe levels below 450 micromol/L and 19 patients with Phe levels between 450 and 900 micromol/L. In the second group, we investigated the effect of long-term (6 months-7 years) oral administration of BH(4) on blood Phe levels and daily Phe tolerance.

RESULTS

In all patients with initial blood Phe levels <450 micromol/L (n = 11), BH(4) loading test was positive, but no treatment was introduced. In 12 out of 19 patients with blood Phe levels >450 micromol/L and positive at BH(4) loading, the treatment with BH(4) (10 mg/kg per day) was initiated. Before BH(4) treatment, Phe tolerance was less than 700 mg/day in all patients except for one (patient no. 9), increasing to 2-3-fold (from 498 +/- 49 to 1475 +/- 155 mg/day) on BH(4) treatment. In these patients the amino acid mixture supplementation was stopped and the diet was a combination of low-protein foods and natural proteins, mostly from animal sources.

CONCLUSION

Long-term BH(4) substitution (up to 7 years) in a group of moderate PKU patients allowed a substantial relaxation of the dietary restrictions or even replacement of the diet with BH(4) without any adverse effects.

Sapropterin dihydrochloride, 6-R-L-erythro-5,6,7,8-tetrahydrobiopterin, in the treatment of phenylketonuria

Sapropterin dihydrochloride, 6-R-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4) is being introduced in the US for treatment of phenylketonuria (PKU). This compound has been in use in Europe to treat mild forms of PKU. Tetrahydrobiopterin is the cofactor in the hydroxylation reaction of the three aromatic amino acids phenylalanine, tyrosine and tryptophan. It is also involved in other reactions, which are not the focus of this review. The cofactor BH4 is synthesized in many tissues in the body. The pathway of BH4 biosynthesis is complex, and begins with guanosine triphosphate (GTP). The first reaction that commits GTP to form pterins is GTP cyclohydrolase. Several reactions follow resulting in the active cofactor BH4. During the hydroxylation reaction BH4 is oxidized to quinonoid-BH2, which is recycled by dihydropteridine reductase, resulting in the active cofactor. It was discovered that some patients with PKU had a decline in blood phenylalanine after oral intake of BH4. This response to BH4 is not the result of change in the synthesis or regeneration of the cofactor, but rather an effect on the mutant enzyme phenylalanine hydroxylase either by accommodating the higher K(m) of the mutant enzyme or by acting as a chaperone for the mutant enzyme. This response has become of intense interest in the treatment of PKU.

Effects of tetrahydrobiopterin and phenylalanine on in vivo human phenylalanine hydroxylase by phenylalanine breath test

BH(4) administration results in the reduction of blood phenylalanine level in patients with tetrahydrobiopterin (BH(4))-responsive phenylalanine hydroxylase (PAH) deficiency. The mechanism underlying BH(4) response remains unknown. Here, we studied the effects of BH(4) and phenylalanine on in vivo PAH activity of normal controls using the phenylalanine breath test (PBT) by converting l-[1-(13)C] phenylalanine to (13)CO(2). Phenylalanine oxidation rates were expressed as Delta(13)C ((13)CO(2)/(12+13)CO(2), per thousand) and cumulative recovery rates over 120min (CRR(120), %; total amount of (13)CO(2)/the administered dose of (13)C-phenylalanine). Under physiological conditions of blood phenylalanine, BH(4) administration reduced the Delta(13)C peak from 40.8 per thousand to 21.6 per thousand and CRR(120) from 16.9% to 10.2%. Under high blood phenylalanine conditions, administration of BH(4) increased the Delta(13)C peak from 30.7 per thousand to 46.0 per thousand, while the CRR(120) was similar between phenylalanine (19.9%) and phenylalanine+BH(4) (21.1%) groups. Corrected Delta(13)C and CRR(120) were calculated against serum phenylalanine levels to remove the effects of phenylalanine loading. After BH(4) administration, the corrected Delta(13)C peak increased from 82.7 per thousand to 112.6 per thousand, while the corrected CRR(120) was similar (47.6% and 45.6%). These results indicate that phenylalanine worked as a regulator of in vivo PAH by serving as both a substrate and an activator for the enzyme. Excessive dosages of BH(4) inhibited PAH under normal phenylalanine conditions and activated PAH under conditions of high phenylalanine. The regulation system is therefore designed to maintain phenylalanine levels in the human body. Appropriate BH(4) supplementation must be reviewed in patients with BH(4)-responsive PAH deficiency.

Assessment of tetrahydrobiopterin (BH4) responsiveness in phenylketonuria

OBJECTIVE

To determine the prevalence of and identify subjects with phenylketonuria (PKU; phenylalanine hydroxylase deficiency) responsive to 6R-tetrahydrobiopterin (BH4) and to establish selection criteria for potential treatment with BH4.

STUDY DESIGN

Blood phenylalanine levels from 557 newborns and children with various degrees of PKU (blood phenylalanine, 301 to 4743 micromol/L) challenged with BH4 (20 mg/kg of body weight) were analyzed at 8 and 24 hours after BH4 administration. The 2 modalities were compared in terms of phenylalanine reduction.

RESULTS

The overall prevalence of BH4 responsiveness within patients with PKU for blood phenylalanine reductions of 20%, 30%, 40%, and 50% was 48%, 38%, 31%, and 24%, respectively, using the 8-hour modus and 55%, 46%, 41%, and 33%, respectively, using the 24-hour modus. Using the 30% cutoff, BH4 responsiveness was similar regardless of the modality in patients with mild hyperphenylalaninemia (79% to 83% responders), mild PKU (49% to 60% responders), and classical PKU (7% to 10% responders).

CONCLUSIONS

BH4 responsiveness is more prevalent than was previously assumed, particularly in patients with mild hyperphenylalaninemia and mild PKU. Depending on the severity of hyperphenylalaninemia, selection criteria for the potential treatment with BH4 may range from 20% to 40% blood phenylalanine reduction after 24 hours.

Response of phenylketonuria to tetrahydrobiopterin

A favorable response, indicated by decline of blood phenylalanine (Phe) in patients with phenylketonuria (PKU), to orally administered 6-R-L-erythro-5, 6, 7, 8-tetrahydrobiopterin (BH4) has been reported in many countries following the first publication in 1999. In this review, we describe the experience in the United States with PKU patients and their response to BH4. A significant response to BH4 is arbitrarily considered as a decrease of 30% or greater of blood Phe concentration 24 h after administration of BH4. In our studies, 18 of 37 patients with PKU (49%) responded to oral BH4 by >30% decrease in blood Phe concentration. Four PKU patients responded with a decrease of blood Phe concentration between 17.3 and 26.3%. It is suggested that patients with sufficient response to BH4 are candidates who will benefit from BH4 as it becomes available for PKU management. In a separate trial, 20 patients with PKU were screened with ascending doses of BH4: 10, 20, and 40 mg/kg. A favorable response was found in 10 subjects (50%) after 10 mg/kg BH4 and 14 subjects (70%) after 20 mg/kg BH4. There was no additional advantage to 40 mg/kg BH4. A 1-wk trial with 10 and 20 mg/kg BH4 in the same 20 patients showed blood Phe concentrations lowest after 7 d of BH4. The BH4-responsive patients were genotyped and most were compound heterozygotes with 1 mild mutation on 1 allele, responsible for the increase of the residual activity of Phe hydroxylase when BH4 was added. Individuals with the same genotype exhibit different responses upon administration of BH4, attributed to epigenetic factors, such as the metabolic makeup of the individual. Patients with PKU, regardless of their genotype or classification, need to be screened for response to BH4. The majority of patients are identified by 10 mg/kg BH4.

Pharmacokinetics of orally administered tetrahydrobiopterin in patients with phenylalanine hydroxylase deficiency

The oral loading test with tetrahydrobiopterin (BH(4)) is used to discriminate between variants of hyperphenylalaninaemia and to detect BH(4)-responsive patients. The outcome of the loading test depends on the genotype, dosage of BH(4), and BH(4) pharmacokinetics. A total of 71 patients with hyperphenylalaninaemia (mild to classic) were challenged with BH(4) (20 mg/kg) according to different protocols (1 x 20 mg or 2 x 20 mg) and blood BH(4) concentrations were measured in dried blood spots at different time points (T(0), T(2), T(4), T(8), T(12), T(24), T(32) and T(48 h)). Maximal BH(4) concentrations (median 22.69 nmol/g Hb) were measured 4 h after BH(4) administration in 63 out of 71 patients. Eight patients presented with maximal BH(4) concentrations approximately 44% higher at 8 h than at 4 h. After 24 h, BH(4) blood concentrations dropped to 11% of maximal values. This profile was similar using different protocols. The following pharmacokinetic parameters were calculated for BH(4) in blood: t (max) = 4 h, AUC (T(0-32)) = 370 nmol x h/g Hb, and t (1/2) for absorption (1.1 h), distribution (2.5 h), and elimination (46.0 h) phases. Maximal BH(4) blood concentrations were not significantly lower in non-responders and there was no correlation between blood concentrations and responsiveness. Of mild PKU patients, 97% responded to BH(4) administration, while one was found to be a non-responder. Only 10/19 patients (53%) with Phe concentrations of 600-1200 mumol/L responded to BH(4) administration, and of the patients with the severe classical phenotype (blood Phe > 1200 mumol/L) only 4 out of 17 patient responded. An additional 36 patients with mild hyperphenylalaninaemia (HPA) who underwent the combined loading test with Phe+BH(4) were all responders. Slow responders and non-responders were found in all groups of HPA.

Three-year audit of the hyperphenylalaninaemia/phenylketonuria spectrum in Victoria

AIMS

To determine the prevalence, the types and severity of hyperphenylalaninaemia (including phenylketonuria (PKU)) in Victoria and to report on a new treatment modality of PKU.

METHODS

We reviewed the medical records of all patients diagnosed with high blood phenylalanine levels by newborn screening between November 2001 and October 2004.

RESULTS

We identified 17 newborn babies with high levels of blood phenylalanine (total samples: 190,835). Dihydrobiopterin reductase deficiency was excluded in all babies. Five babies had persistent phenylalanine levels of 200-300, and do not receive any dietary or pharmaceutical therapy. One baby was diagnosed as having pyruvoyl tetrahydro-pterin synthase deficiency. Following reports of tetrahydrobiopterin (BH(4))-responsive PKU, we have performed a BH(4) load (20 mg/kg, 6R-5,6,7,8-tetrahydro-L-biopetrin dehydrochloride; Schricks Laboratories, Jona, Switzerland) in 10 newborn babies with PKU (one baby with a phenylalanine level of 2600 micromol/L was started on diet without prior load). Three babies had a significant response to BH(4) (>35% decrease in phenylalanine level). Protein restriction (1.2 g/kg/day) and introduction of phenylalanine-free formula, in addition to BH(4) treatment, were necessary in one patient. The other patients maintain good metabolic control with BH(4) treatment only (at approximately 11 mg/kg/day) and an intake of 2-3 g protein per day. Of the nine babies who are on a full PKU diet, three have high phenylalanine tolerance (consistently >40 mg/kg/day).

CONCLUSION

There is a spectrum of severity of hyperphenylalaninaemia in the population. The detection of BH(4)-responsive PKU patients offers them a less restrictive dietary regimen and an improved quality of life, and may enable near normal life-style in adolescence.

The spectrum of phenylalanine variations under tetrahydrobiopterin load in subjects affected by phenylalanine hydroxylase deficiency

A fall in blood phenylalanine (Phe) after tetrahydrobiopterin (BH(4)) administration is a common trait in phenylalanine hydroxylase (PAH, EC 1.14.16.1) deficiency (McKusick 261600). To explore the extent and biological correlates of this phenomenon we studied: (a) the spectrum of BH(4) response in patients with PAH deficiency; (b) the variability of BH(4) response according to the severity of the biochemical phenotype; and (c) the variability of the response to BH(4) in subjects with the same genotype. Fifty PAH-deficient subjects (age 1 month-35 years) were enrolled for the study (5 with mild hyperphenylalaninaemia (MHPHE), 15 with mild phenylketonuria (MPKU) and 30 with classic phenylketonuria (CPKU) and underwent an identical schedule of blood samplings 24 h before and after oral BH(4) challenge (6(R)-BH4, 20 mg/kg per day), leaving Phe intake unchanged. The effect of BH(4) on blood Phe concentration was evaluated according to the percent decrease of Phe during the 24 h following the challenge (criterion a), and as variation exceeding the individual variability of blood Phe (criterion b). The number of BH(4)-responders according to criterion b was 31 (including all the 14 detected by criterion a): 17 out of 30 CPKU (57%), 9 out of 15 MPKU (60%), and all the MHPHE subjects (chi(2) = 3.45, df = 2, p = 0.178). The effect of BH(4) showed a large interindividual variability unrelated to diagnostic classification, basal value of blood Phe, maximum percentage of Phe reduction, Phe intake, and genotype. Some inconsistencies were found in patients with identical genotype. The first responsive case homozygous for the severe R408W mutation was found. Two new mutations, Y387X and G352C, were identified (the former was BH(4)-responsive), and the responsiveness of three already reported mutations (R261Q, D338Y, T92I) was substantiated.

[Tetrahydrobiopterin therapy for hyperphenylalaninemia due to phenylalanine hydroxylase deficiency. When and how?]

INTRODUCTION

Some patients with hyperphenylalaninemia due to phenylalanine hydroxylase deficiency respond with a variable decrease in plasma phenylalanine levels after oral tetrahydrobiopterin (BH4) administration and are then able to tolerate higher dietary phenylalanine intake or even to discontinue a phenylalanine-restricted diet. BH4-sensitive patients are usually identified by means of a BH4 loading test, but consensus on the methodology of this test and the interpretation of its results is lacking. Consequently, a simple tool to identify which patients are likely candidates for this treatment and how they will progress in the long-term is required.

MATERIAL AND METHODS

A combined oral BH4 loading test with phenylalanine (100 mg/kg) and BH4 (20 mg/kg) was performed in 20 patients with hyperphenylalaninemia under dietary phenylalanine restriction.

RESULTS

Independently of the genotype, the result was positive in all the 9 patients whose maximum phenylalanine level at diagnosis was below 815 nmol/ml. Currently, they are under treatment with tetrahydrobiopterin doses of 7-15 mg/kg/day. All these patients have been able to increase their oral phenylalanine intake. Six are currently following a normal diet and the remaining three are close to reaching this goal. None of the patients with a maximum phenylalanine level at diagnosis higher than 938 nmol/ml responded to the BH4 loading test.

CONCLUSIONS

The maximum phenylalanine level at diagnosis seems to be a simple and reliable method to predict response to BH4 treatment. A high percentage of BH4-sensitive patients are able to discontinue a phenylalanine-restricted diet after long-term tetrahydrobiopterin treatment.

Long-term treatment of patients with mild and classical phenylketonuria by tetrahydrobiopterin

Tetrahydrobiopterin (BH4), the natural cofactor of phenylalanine hydroxylase (EC 1.14.16.1), can reduce blood phenylalanine (Phe) in BH4 sensitive patients with hyperphenylalaninemia (McKuisick 261600). We report on the long-term treatment of eight patients with mild and classical phenylketonuria (blood Phe levels maximum blood Phe levels between 771 and 1500 micromol/L) using BH4 at a dosage of 8-12 mg/kg BW per day. In all patients reduction of blood Phe was >30% after BH4 loading test. Three patients were treated from birth by BH4 only, five after initial low Phe dietary treatment. Seven of them continue to be on BH4 treatment only, one has a relaxed low protein diet. No side effects could be observed (longest observation time 5 years), somatic and psychomotor development were normal. The main problem of BH4 treatment is finding an optimal dosage at different ages and an under special conditions like infectious diseases. There is evidence that in some patients BH4 treatment may allow a more relaxed low protein diet showing positive effects on weight gain and quality of life. Further controlled studies are necessary not only to rule out any side effects but also for optimizing treatment strategies with BH4 treatment in mild phenylketonuria.

Tetrahydrobiopterin-responsive phenylketonuria: the New South Wales experience

Recent studies have shown that a subgroup of phenylketonuric patients respond to high doses of BH4 (20 mg/kg) by a decrease of plasma phenylalanine. A clinically significant response has been defined as a decrease in phenylalanine by more than 30% within 24 h, after a BH4 challenge. We report our experience with 37 patients diagnosed with hyperphenylalaninemia, mild, moderate, or classical Phenylketonuria (PKU) using a seven day combined BH4 and phenylalanine load. Nine of the 37 patients responded with a 30% decrease in their phenylalanine levels in the first 8 h of treatment. A total of 17 patients (46%) had a decrease of at least 30% during the study period. This study confirms that a significant number of patients with mild to moderate PKU will respond to a BH4 load. Furthermore, it confirms that the seven-day phenylalanine test is more sensitive in detecting BH4 responsive patients.

Tetrahydrobiopterin responsiveness in patients with phenylketonuria

OBJECTIVES

To investigate the BH4 response in a group of patients with phenylketonuria (PKU) in order to offer this alternative treatment to the responsive patients.

DESIGN AND METHODS

The 24-h-long Phe/BH4 loading test was performed on 64 PKU patients requiring dietary treatment.

RESULTS

All patients with mild-PKU and 75% of patients with moderate-PKU were BH4 responsive, while only 11% of classic-PKU patients showed good/partial response (P < 0.0001). The percentages of Phe decrease after the BH4 loading test were significantly different in the three PKU phenotypes (mild PKU: 67.9 +/- 18.7; moderate PKU: 37.4 +/- 16.8; and classical PKU: 21.9 +/- 13.7; ANOVA with Bonferroni correction: P < 0.0001). We report four mutations (P147S, D222G, P275S, and P362T) not previously associated with BH4 responsiveness, all of them combined with mutations with zero predicted residual activity.

CONCLUSION

Both the percentage of Phe decrease and the Phe value achieved 24 h after BH4 loading are valuable data in predicting a response. We report four mutations not previously associated with BH4 responsiveness.

In vivo studies of phenylalanine hydroxylase by phenylalanine breath test: diagnosis of tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency

Tetrahydrobiopterin (BH4)-responsive phenylalanine hydroxylase (PAH) deficiency is characterized by reduction of blood phenylalanine level after a BH4-loading test. Most cases of BH4-responsive PAH deficiency include mild phenylketonuria (PKU) or mild hyperphenylalaninemia (HPA), but not all patients with mild PKU respond to BH4. We performed the phenylalanine breath test as reliable method to determine the BH4 responsiveness. Phenylalanine breath test quantitatively measures the conversion of L-[1-13C] phenylalanine to 13CO2 and is a noninvasive and rapid test. Twenty Japanese patients with HPA were examined with a dose of 10 mg/kg of 13C-phenylalanine with or without a dose of 10 mg . kg(-1) . d(-1) of BH4 for 3 d. The phenylalanine breath test [cumulative recovery rate (CRR)] could distinguish control subjects (15.4 +/- 1.5%); heterozygotes (10.3 +/- 1.0%); and mild HPA (2.74%), mild PKU (1.13 +/- 0.14%), and classical PKU patients (0.29 +/- 0.14%). The genotypes in mild PKU cases were compound heterozygotes with mild (L52S, R241C, R408Q) and severe mutations, whereas a mild HPA case was homozygote of R241C. CRR correlated inversely with pretreatment phenylalanine levels, indicating the gene dosage effects on PKU. BH4 loading increased CRR from 1.13 +/- 0.14 to 2.95 +/- 1.14% (2.6-fold) in mild PKU and from 2.74 to 7.22% (2.6-fold) in mild HPA. A CRR of 5 to 6% reflected maintenance of appropriate serum phenylalanine level. The phenylalanine breath test is useful for the diagnosis of BH4-responsive PAH deficiency and determination of the optimal dosage of BH4 without increasing blood phenylalanine level.

Wild-type phenylalanine hydroxylase activity is enhanced by tetrahydrobiopterin supplementation in vivo: an implication for therapeutic basis of tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency

We previously proposed a novel disease entity, tetrahydrobiopterin (BH4)-responsive phenylalanine hydroxylase (PAH) deficiency, in which administration of BH4 reduced elevated levels of serum phenylalanine [J. Pediatr. 135 (1999) 375-378]. Subsequent reports indicate that the prevalence of BH4-responsive PAH deficiency is much higher than initially anticipated. Although growing attention surrounds treatment with BH4, little is known about the mechanism of BH4 responsiveness. An early report indicates that BH4 concentration in rat liver was 5 microM where Km for BH4 of rat PAH was estimated to be 25 microM in an oxidation experiment using a liver slice, suggesting relative insufficiency of BH4 in liver in vivo. In the present study, we developed a breath test for mice using [1-13C]phenylalanine in order to examine the BH4 responsiveness of normal PAH in vivo. The reliability of the test was verified using BTBR mice and its mutant strain lacking PAH activity, Pahenu2. BH4 supplementation significantly enhanced 13CO2 production in C57BL/6 mice when phenylalanine was pre-loaded. Furthermore, BH4 apparently activated PAH in just 5 min. These observations suggest that submaximal PAH activity occurs at the physiological concentrations of BH4 in vivo, and that PAH activity can be rapidly enhanced by supplementation with BH4. Thus, we propose a possible hypothesis that the responsiveness to BH4 in patients with PAH deficiency is due to the fact that suboptimal physiological concentrations of BH4 are normally present in hepatocytes and the enhancement of the residual activity may be associated with a wide range of mutations.

The metabolic and molecular bases of tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency

About two-thirds of all mild phenylketonuria (PKU) patients are tetrahydrobiopterin (BH4)-responsive and thus can be potentially treated with BH4 instead of a low-phenylalanine diet. Although there has been an increase in the amount of information relating to the diagnosis and treatment of this new variant of PKU, very little is know about the mechanisms of BH4-responsiveness. This review will focus on laboratory investigations and possible molecular and structural mechanisms involved in this process.

Long-term treatment and diagnosis of tetrahydrobiopterin-responsive hyperphenylalaninemia with a mutant phenylalanine hydroxylase gene

A novel therapeutic strategy for phenylketonuria (PKU) has been initiated in Japan. A total of 12 patients who met the criteria for tetrahydrobiopterin (BH(4))-responsive hyperphenylalaninemia (HPA) with a mutant phenylalanine hydroxylase (PAH) (EC 1.14.16.1) gene were recruited at 12 medical centers in Japan between June 1995 and July 2001. Therapeutic efficacy of BH(4) was evaluated in single-dose, four-dose, and 1-wk BH(4) loading tests followed by long-term BH(4) treatment, and also examined in relation to the PAH gene mutations. The endpoints were determined as the percentage decline in serum phenylalanine from initial values after single-dose (>20%), four-dose (>30%), and 1-wk BH(4) (>50%) loading tests. Patients with mild PKU exhibiting decreases in blood phenylalanine concentrations of >20% in the single-dose test also demonstrated decreases of >30% in the four-dose test. The 1-wk test elicited BH(4) responsiveness even in patients with poor responses in the shorter tests. Patients with mild HPA, many of whom carry the R241C allele, responded to BH(4) administration. No clear correlation was noted between the degree of decrease in serum phenylalanine concentrations in the single- or four-dose tests and specific PAH mutations. The 1-wk test (20 mg/kg of BH(4) per day) is the most sensitive test for the diagnosis of BH(4)-responsive PAH deficiency. Responsiveness apparently depends on mutations in the PAH gene causing mild PKU, such as R241C. BH(4) proved to be an effective therapy that may be able to replace or liberalize the phenylalanine-restricted diets for a considerable number of patients with mild PKU.

A hypothesis on the biochemical mechanism of BH(4)-responsiveness in phenylalanine hydroxylase deficiency

We describe six children with tetrahydrobiopterin (BH(4)) responsive phenylalanine hydroxylase (PAH) deficiency. All patients carry two mutant alleles in the PAH gene. Cofactor deficiency was excluded. The effect of BH(4) administration was studied by correlating different oral BH(4) doses with plasma phenylalanine levels under defined protein intake. Our results indicate that oral BH(4) supplementation may be used as long-term treatment for individuals with BH(4)-responsive PAH deficiency, either without or in combination with a less restrictive diet. Previous in vitro studies have demonstrated that BH(4) inhibits PAH tetramers but activates PAH dimers. This may indicate, that BH(4)-responsiveness results from BH(4) induced stabilization of mutant PAH dimers. In addition, interindividual differences in the cellular folding apparatus may determine the tertiary structure and the amount of mutant PAH dimers and hence may account for divergent BH(4)-responsiveness reported for the same PAH genotype.

Phenylketonuria: genotype-phenotype correlations based on expression analysis of structural and functional mutations in PAH

When analyzed in the context of the phenylalanine hydroxylase (PAH) three-dimensional structure, only a minority of the PKU mutations described world-wide affect catalytic residues. Consistent with these observations, recent data point to defective folding and subsequent aggregation/degradation as a predominant disease mechanism for several mutations. In this work, we use a combined approach of expression in eukaryotic cells at different temperatures and a prokaryotic system with co-expression of chaperonins to elucidate and confirm structural consequences for 18 PKU mutations. Three mutations are located in the amino terminal regulatory domain and 15 in the catalytic domain. Four mutations were found to abolish the specific activity in all conditions. Two are catalytic mutations (Y277D and E280K) and two are severe structural defects (IVS10-11G>A and L311P). All the remaining mutations (D59Y, I65T, E76G, P122Q, R158Q, G218V, R243Q, P244L, R252W, R261Q, A309V, R408Q, R408W, and Y414C) are folding defects causing reduced stability and accelerated degradation, although some of them probably affect residues involved in regulation. In these cases, we have demonstrated that the amount of mutant PAH protein and residual activity could be modulated by in vitro experimental conditions, and therefore the observed in vivo metabolic variation may be explained by interindividual variation in the quality control systems. The results derived provide an experimental framework to define the mutation severity relating genotype to phenotype. They also explain the observed inconsistencies for some mutations in patients with similar genotype and different phenotypes.

BH4-sensitive hyperphenylalaninemia: new case and review of literature

We report a patient with BH(4)-sensitive phenylketonuria. In neonatal screening, phenylalanine levels above 10 mg/dl were detected. In the tetrahydrobiopterin- (BH(4)) loading test, phenylalanine concentrations in serum fell significantly. Dihydropteridine reductase activity in blood, pterines, and neurotransmitters in cerebrospinal fluid, as well as pterines in urine were all normal. Mutation analysis revealed compound-heterozygosity for the mutations R408W and K320N. Under BH(4)-supplementation without a specific phenylalanine-reduced diet, phenylalanine-concentrations are in the therapeutic range and our patient developed normally.

Tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency, state of the art

Since 1999 an increasing number of patients with phenylalanine hydroxylase (PAH) deficiency are reported to be able to decrease their plasma phenylalanine (Phe) concentrations after a 6R-tetrahydrobiopterin (BH(4)) challenge. The majority of these patients have mild PKU or MHP (mild hyperphenylalaninemia) and harbour at least one missense mutation in the PAH gene associated with this phenotype. The rate of decrease and the lowest achieved Phe level vary between patients with different genotypes but appears to be similar in patients with the same genotype. A number of the mutations associated with BH(4)-responsiveness have been studied in an 'in vitro' eukaryotic cell expression system leading to biosynthesis of a mutant PAH enzyme with some residual activity. Patients bearing mutations that cause severe structural distortion in the expressed protein (loss of function mutations), leading to undetectable PAH activity, are not responsive to BH(4). These observations suggest that residual PAH activity (in vitro) is a prerequisite for BH(4)-responsiveness. However, an in vitro residual PAH activity is not a guarantee for in vivo BH(4)-responsiveness. Mechanisms behind this responsiveness could be relieve of decreased binding affinity for BH(4), BH(4)-mediated increase of PAH gene expression or stabilization of the mutant enzyme protein by BH(4). BH(4)-responsive PAH-deficient patients have only been reported since 1999. For the western countries this is explained by the fact that the manufacturer changed the diastereoisomeric purity of the BH4 preparation from 69% of the natural 6R-BH4 (31% of 6S-BH4) to 99.5% 6R-BH4. The new findings on BH(4)-responsiveness may be of clinical relevance because these patients can be treated with BH(4) with concomitant relief or withdrawal of the burdensome PKU diet. These observations warrant further clinical studies to assess efficacy, optimal dosage, and safety of BH(4) treatment in this group. The data strongly emphasize the necessity of the BH(4) loading test in patients detected in the newborn PKU screening.

Danger of high-protein dietary supplements to persons with hyperphenylalaninaemia

A 16-year-old adolescent with mild hyperphenylalaninaemia was given a high-protein 'body building' supplement twice daily, causing headaches, decreased school performance and mild depression. All symptoms disappeared after cessation of the supplement. The phenylalanine hydroxylase mutation H170D/IVS1nt5G>T was found to be responsive to tetrahydrobiopterin with significant decrease in blood phenylalanine concentration and increase in tyrosine blood content. A brain phenylalanine level of 0.5 mmol/L was initially documented, which decreased to the normal carrier range of 0.2 mmol/L within one month of discontinuance of the protein supplement. At present, the patient is on a normal diet without phenylalanine restriction.

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.

High frequency of tetrahydrobiopterin-responsiveness among hyperphenylalaninemias: a study of 1,919 patients observed from 1988 to 2002

Tetrahydrobiopterin (BH(4))-responsive hyperphenylalaninemia (HPA) is a recently described variant of phenylalanine hydroxylase deficiency. In contrast to patients with classical phenylketonuria, these patients respond to BH(4) loading tests (20mg/kg) with decrease of plasma phenylalanine levels 4 and 8 h after administration and they can be treated with BH(4) monotherapy. We retrospectively evaluated 1,919 loading tests from 33 different countries performed in our laboratory between 1988 and 2002 of which 278 loading tests were performed with 6R-BH(4), which is about 33% more active than the formerly used 6R,S-BH(4). The loading tests were performed between the ages of one week and 4.6 years, using 2.6-30.0 mg 6R,S- or 6R-BH(4)/kg. Plasma phenylalanine levels before the test ranged from 121 to 4,705 micromol/L. We calculated the phenylalanine "hydroxylation rate" 4 and 8 h after BH(4) administration and plotted the slope of the hydroxylation rate against the phenylalanine levels at time 0. The slope was greater than 3.75 in 65, 74, 33, 17, 0, and 10% of patients with basal phenylalanine levels of 120-400, 400-800, 800-1,200, 1,200-1,600, 1,600-2,200, and >2,200 micromol/L, respectively, when loaded with 20 mg 6R-BH(4)/kg (p>0.0001). This is 5-20 times higher compared with tests using 6R,S-BH(4) or lower doses of BH(4). More than 70% of patients with mild HPA (<800 micromol/L) are found to be BH(4) responders. Therapy with BH(4) (approximately 10mg/kg/day) was initiated in several patients instead of a low-phenylalanine diet, resulting in much better treatment compliance. Our data further demonstrate that BH(4) loading tests can only distinguish between BH(4) responders and non-responders. To differentiate between BH(4) and phenylalanine hydroxylase deficiencies additional tests are essential.

Two novel genetic lesions and a common BH4-responsive mutation of the PAH gene in Italian patients with hyperphenylalaninemia

Hyperphenylalaninemia (HPA), due to a deficiency of phenylalanine hydroxylase (PAH) enzyme, is caused by mutations in the PAH gene. Molecular analysis in 23 Italian patients with PAH deficiency identified two novel (P281R, L287V) and 20 previously described genetic lesions in the PAH gene. The detection of the A403V amino acid substitution in combination with null mutations in patients with BH4-responsive PAH deficiency leads us to correlate it with BH4 responsiveness.

Tetrahydrobiopterin monotherapy for phenylketonuria patients with common mild mutations

The effect of tetrahydrobiopterin (BH(4)) administration was studied in three infants with BH(4) responsive phenylalanine hydroxylase (PAH) deficiency by correlating different BH(4) doses with plasma phenylalanine levels under defined protein intake.

High-dose vitamin therapy stimulates variant enzymes with decreased coenzyme binding affinity (increased K(m)): relevance to genetic disease and polymorphisms

As many as one-third of mutations in a gene result in the corresponding enzyme having an increased Michaelis constant, or K(m), (decreased binding affinity) for a coenzyme, resulting in a lower rate of reaction. About 50 human genetic dis-eases due to defective enzymes can be remedied or ameliorated by the administration of high doses of the vitamin component of the corresponding coenzyme, which at least partially restores enzymatic activity. Several single-nucleotide polymorphisms, in which the variant amino acid reduces coenzyme binding and thus enzymatic activity, are likely to be remediable by raising cellular concentrations of the cofactor through high-dose vitamin therapy. Some examples include the alanine-to-valine substitution at codon 222 (Ala222-->Val) [DNA: C-to-T substitution at nucleo-tide 677 (677C-->T)] in methylenetetrahydrofolate reductase (NADPH) and the cofactor FAD (in relation to cardiovascular disease, migraines, and rages), the Pro187-->Ser (DNA: 609C-->T) mutation in NAD(P):quinone oxidoreductase 1 [NAD(P)H dehy-drogenase (quinone)] and FAD (in relation to cancer), the Ala44-->Gly (DNA: 131C-->G) mutation in glucose-6-phosphate 1-dehydrogenase and NADP (in relation to favism and hemolytic anemia), and the Glu487-->Lys mutation (present in one-half of Asians) in aldehyde dehydrogenase (NAD + ) and NAD (in relation to alcohol intolerance, Alzheimer disease, and cancer).

Mental illness in mild PKU responds to biopterin

A 25-year-old woman with mild hyperphenylalaninemia developed disabling depression and panic attacks. The mutations on the phenylalanine hydroxylase gene indicated that she might be responsive to tetrahydrobiopterin therapy. Mutation analyses were performed by the John F. Kennedy Institute in Glostrup, Denmark. The response to tetrahydrobiopterin therapy was impressive at an oral dose of 50 mg twice a day. A 25-year-old woman with mild hyperphenylalaninemia due to a PAH mutation of IVS12nt1g-->a/E390G has been treated for 1 year with BH4 therapy. A maintenance dosage of only 100 mg/day has resulted in significant improvement of depression and panic attacks, with discontinuation of psychotropic medication.