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

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.

New evidence for assessing tetrahydrobiopterin (BH(4)) responsiveness

OBJECTIVE

To evaluate the protocol we propose for detecting BH(4)-responsive patients and the possibility of delimiting more precisely the population to be tested.

METHODS

We recruited 102 phenylketonuric patients on a phenylalanine (Phe)-restricted diet. The initial stage of the protocol was a 24-h BH(4) loading test involving Phe loading and subsequent ingestion of the cofactor, a 50% fall in blood Phe levels being considered a positive response. The non-responders at this stage then completed a one-week therapeutic test combining BH(4) administration and daily protein intake meeting recommended dietary allowances, to assess whether the 24-h test had detected all responders.

RESULTS

The 24-h test detected almost all BH(4) responders (30.3% of the 99 patients included in the analysis), with just two patients (2.0%) subsequently responding positively to the therapeutic test. The 24-h test did not give any false positive results.

CONCLUSIONS

The 24-h BH(4) loading test is clinically useful for screening phenylketonuric patients. Specifically, 95% of patients with Phe levels <700 μmol/L, and none with Phe levels >1500 μmol/L were BH(4)-responsive. Given these results, we conclude that patients with Phe levels<700 μmol/L or>1500 μmol/L probably do not need to be tested, prioritising the identification of BH(4)-responsiveness among individuals with intermediate Phe concentrations, between the aforementioned values. Additionally, our results suggest that the therapeutic test only needs to be performed in cases where the reduction in blood Phe levels after cofactor administration is within the range 40%-50%.

Optimized loading test to evaluate responsiveness to tetrahydrobiopterin (BH4) in Brazilian patients with phenylalanine hydroxylase deficiency

INTRODUCTION

Recent studies showed that phenylalanine (Phe) plasma concentrations may decrease in some patients with hyperphenylalaninemia (HPA) due to phenylalanine hydroxylase (PAH) deficiency, after the administration of tetrahydrobiopterin (BH(4)).

OBJECTIVE

To determine responsiveness to a single dose of BH(4) administered according to an innovative protocol using a combined Phe and BH(4) loading test in Brazilian phenylketonuria (PKU) patients.

METHODS

Patient age should be ≥ 4 years, and median Phe plasma concentration ≤ 600 μmol/L when following dietary restrictions. Participants received a simple Phe loading test using 100mg/kg L-Phe (Test 1) and a combined Phe+BH(4) loading test using 100mg/kg L-Phe and 20mg/kg/BH(4) (Test 2). Blood samples were collected at baseline and 3, 11 and 27 h after Phe ingestion (T0, T1, T2 and T3). Responsiveness was defined as: criterion A: plasma Phe reduction of ≥ 30% at T1 and T2 for Tests 1 and 2; criterion B: plasma Phe reduction of ≥ 30% at T1 and T3 for Tests 1 and 2; and criterion C: at least 30% difference of the areas under the Phe curve for Tests 1 and 2.

RESULTS

Eighteen patients (median age 12 yrs; 8 classical PKU; 10 mild PKU) participated in the study. Six patients (2 classical PKU; 4 mild PKU) were classified as responsive according to at least one of the criteria. Responsiveness was concordant when criteria A + B we compared with criterion C (kappa = 0.557; p = 0.017). Of the patients whose genotype was available (n = 16), six had data about BH(4)-responsiveness genotypes described in the literature, which were in agreement with our findings.

CONCLUSION

The comparison of simple Phe loading and combined Phe + BH(4) loading seems to be an optimal method to evaluate responsiveness to BH(4) in patients with good metabolic control.

Adjusting diet with sapropterin in phenylketonuria: what factors should be considered?

The usual treatment for phenylketonuria (PKU) is a phenylalanine-restricted diet. Following this diet is challenging, and long-term adherence (and hence metabolic control) is commonly poor. Patients with PKU (usually, but not exclusively, with a relatively mild form of the disorder) who are responsive to treatment with pharmacological doses of tetrahydrobiopterin (BH4) have either lower concentrations of blood phenylalanine or improved dietary phenylalanine tolerance. The availability of a registered formulation of BH4 (sapropterin dihydrochloride, Kuvan®) has raised many practical issues and new questions in the dietary management of these patients. Initially, patients and carers must understand clearly the likely benefits (and limitations) of sapropterin therapy. A minority of patients who respond to sapropterin are able to discontinue the phenylalanine-restricted diet completely, while others are able to relax the diet to some extent. Care is required when altering the phenylalanine-restricted diet, as this may have unintended nutritional consequences and must be undertaken with caution. New clinical protocols are required for managing any dietary change while maintaining control of blood phenylalanine, ensuring adequate nutrition and preventing nutritional deficiencies, overweight or obesity. An accurate initial evaluation of pre-sapropterin phenylalanine tolerance is essential, and the desired outcome from treatment with sapropterin (e.g. reduction in blood phenylalanine or relaxation in diet) must also be understood by the patient and carers from the outset. Continuing education and support will be required thereafter, with further adjustment of diet and sapropterin dosage as a young patient grows.

Significance of genotype in tetrahydrobiopterin-responsive phenylketonuria

BACKGROUND

The value of genotyping to identify tetrahydrobiopterin-responsive (BH(4)-responsive) patients with phenylalanine hydroxylase (PAH) deficiency is a matter of debate.

METHODS

We reviewed 250 cases of patients with PAH deficiency, using published data from 198 cases and unpublished data from 52 cases of patients attending our own clinic. Patients underwent analyses for BH(4) load and genetic mutations. Partial and full BH(4) responses were defined as a 10-29% decrease and a >or=30% decrease from baseline in blood phenylalanine levels, respectively. BH(4)-responsive alleles were identified from BH(4)-responsive patients as either homozygous for a specific allele or compound heterozygous for that allele with a null mutation.

RESULTS

Most inconsistencies between observed genotype and BH(4) response were associated with mutations in the regulatory domain of PAH (p.R68S, p.I65T, p.L48S and p.F39C), where 20/62 alleles (32.2%) were non-responsive. In the catalytic domain (mutations p.Y414C, p.R261Q, p.E390G, p.A300S, p.R241C, p.A403V and p.V388M), only 8/125 alleles (6.4%) were non-responsive. Seven patients had a genotype with two BH(4)-responsive alleles resulting in no response or only a partial response to BH(4). Ten patients had identical genotypes but inconsistent responses in BH(4) load.

CONCLUSIONS

These results show that BH(4) non-responsiveness is associated with genotype. However, patients with mutations in the regulatory domain show inconsistent results. In patients with two responsive alleles, non-responsiveness may be related to negative inter-allelic complementation. In patients with the same genotype and inconsistent results for BH(4) load, external factors such as intestinal absorption of BH(4), catabolic conditions or other genetic factors may be responsible. Further in vitro studies are necessary to clarify the genotype-phenotype correlation in patients with BH(4)-responsive PKU.

BH4 responsiveness associated to a PKU mutation with decreased binding affinity for the cofactor

BACKGROUND

Tetrahydrobiopterin (BH4), cofactor of phenylalanine hydroxylase, can be used to treat a subset of phenylketonuria (PKU) patients as it results in a reduction in blood phenylalanine levels. The molecular basis of the response appears to be multifactorial.

METHOD

A standard BH4 loading test (20 mg/kg) was performed. Genotyping was performed by DGGE and sequencing analysis. Expression analysis of the D129G mutation was performed in E. coli (expression as fusion protein MBP-PAH) and in a human hepatoma cell line with an N-terminal FLAG epitope.

RESULTS

We report the positive response and long-term treatment of a patient functionally hemizygous for the D129G mutation in the phenylalanine hydroxylase gene. Expression in the prokaryotic system revealed partial activity and a decreased binding affinity for BH4 of the mutant protein. In the eukaryotic system the mutant protein shows reduced stability.

CONCLUSION

The D129G mutation which confers a BH4-responsive phenotype, has a decreased binding affinity for BH4.