A structural hypothesis for BH4 responsiveness in patients with mild forms of hyperphenylalaninaemia and phenylketonuria

Phenotypic correlates of structural and functional protein impairments resultant from ALDH5A1 variants

To investigate the genotype-to-protein-to-phenotype correlations of succinic semialdehyde dehydrogenase deficiency (SSADHD), an inherited metabolic disorder of γ-aminobutyric acid catabolism. Bioinformatics and in silico mutagenesis analyses of ALDH5A1 variants were performed to evaluate their impact on protein stability, active site and co-factor binding domains, splicing, and homotetramer formation. Protein abnormalities were then correlated with a validated disease-specific clinical severity score and neurological, neuropsychological, biochemical, neuroimaging, and neurophysiological metrics. A total of 58 individuals (1:1 male/female ratio) were affected by 32 ALDH5A1 pathogenic variants, eight of which were novel. Compared to individuals with single homotetrameric or multiple homo and heterotetrameric proteins, those predicted not to synthesize any functional enzyme protein had significantly lower expression of ALDH5A1 (p = 0.001), worse overall clinical outcomes (p = 0.008) and specifically more severe cognitive deficits (p = 0.01), epilepsy (p = 0.04) and psychiatric morbidity (p = 0.04). Compared to individuals with predictions of having no protein or a protein impaired in catalytic functions, subjects whose proteins were predicted to be impaired in stability, folding, or oligomerization had a better overall clinical outcome (p = 0.02) and adaptive skills (p = 0.04). The quantity and type of enzyme proteins (no protein, single homotetramers, or multiple homo and heterotetramers), as well as their structural and functional impairments (catalytic or stability, folding, or oligomerization), contribute to phenotype severity in SSADHD. These findings are valuable for assessment of disease prognosis and management, including patient selection for gene replacement therapy. Furthermore, they provide a roadmap to determine genotype-to-protein-to-phenotype relationships in other autosomal recessive disorders.

Phenotypic Correlates of Structural and Functional Protein Impairments Resultant from ALDH5A1 Variants

Objective

To investigate the genotype-to-protein-to-phenotype correlations of succinic semialdehyde dehydrogenase deficiency (SSADHD), an inherited metabolic disorder of γ-aminobutyric acid catabolism.

Methods

Bioinformatics and in silico mutagenesis analyses of ALDH5A1 variants were performed to evaluate their impact on protein stability, active site and co-factor binding domains, splicing, and homotetramer formation. Protein abnormalities were then correlated with a validated disease-specific clinical severity score and neurological, neuropsychological, biochemical, neuroimaging, and neurophysiological metrics.

Results

A total of 58 individuals (1:1 male/female ratio) were affected by 32 ALDH5A1 pathogenic variants, eight of which were novel. Compared to individuals with single homotetrameric or multiple homo and heterotetrameric proteins, those predicted not to synthesize any functional enzyme protein had significantly lower expression of ALDH5A1 (p = 0.001), worse overall clinical outcomes (p = 0.008) and specifically more severe cognitive deficits (p = 0.01), epilepsy (p = 0.04) and psychiatric morbidity (p = 0.04). Compared to individuals with predictions of having no protein or a protein impaired in catalytic functions, subjects whose proteins were predicted to be impaired in stability, folding, or oligomerization had a better overall clinical outcome (p = 0.02) and adaptive skills (p = 0.04).

Conclusions

The quantity and type of enzyme proteins (no protein, single homotetramers, or multiple homo and heterotetramers), as well as their structural and functional impairments (catalytic or stability, folding, or oligomerization), contribute to phenotype severity in SSADHD. These findings are valuable for assessment of disease prognosis and management, including patient selection for gene replacement therapy. Furthermore, they provide a roadmap to determine genotype-to-protein-to-phenotype relationships in other autosomal recessive disorders.

A noncoding RNA modulator potentiates phenylalanine metabolism in mice

The functional role of long noncoding RNAs (lncRNAs) in inherited metabolic disorders, including phenylketonuria (PKU), is unknown. Here, we demonstrate that the mouse lncRNA Pair and human HULC associate with phenylalanine hydroxylase (PAH). Pair-knockout mice exhibited excessive blood phenylalanine (Phe), musty odor, hypopigmentation, growth retardation, and progressive neurological symptoms including seizures, which faithfully models human PKU. HULC depletion led to reduced PAH enzymatic activities in human induced pluripotent stem cell-differentiated hepatocytes. Mechanistically, HULC modulated the enzymatic activities of PAH by facilitating PAH-substrate and PAH-cofactor interactions. To develop a therapeutic strategy for restoring liver lncRNAs, we designed GalNAc-tagged lncRNA mimics that exhibit liver enrichment. Treatment with GalNAc-HULC mimics reduced excessive Phe in Pair -/- and Pah R408W/R408W mice and improved the Phe tolerance of these mice.

Milestones in treatments for inborn errors of metabolism: Reflections on Where chemistry and medicine meet

From Sir Archibald Garrod's initial description of the tetrad of albinism, alkaptonuria, cystinuria, and pentosuria to today, the field of medicine dedicated to inborn errors of metabolism has evolved from disease identification and mechanistic discovery to the development of therapies designed to subvert biochemical defects. In this review, we highlight major milestones in the treatment and diagnosis of inborn errors of metabolism, starting with dietary therapy for phenylketonuria in the 1950s and 1960s, and ending with current approaches in genetic manipulation.

In silico screening and molecular dynamics simulation of deleterious PAH mutations responsible for phenylketonuria genetic disorder

Phenylketonuria (PKU) is a genetic disorder that if left untreated can lead to behavioral problems, epilepsy, and even mental retardation. PKU results from mutations within the phenylalanine-4-hydroxylase (PAH) gene that encodes for the PAH protein. The study of all PAH causing mutations is improbable using experimental techniques. In this study, a collection of in silico resources, sorting intolerant from tolerant, Polyphen-2, PhD-SNP, and MutPred were used to identify possible pathogenetic and deleterious PAH non-synonymous single nucleotide polymorphisms (nsSNPs). We identified two variants of PAH, I65N and L311P, to be the most deleterious and disease causing nsSNPs. Molecular dynamics (MD) simulations were carried out to characterize these point mutations on the atomic level. MD simulations revealed increased flexibility and a decrease in the hydrogen bond network for both mutants compared to the native protein. Free energy calculations using the MM/GBSA approach found that BH4 , a drug-based therapy for PKU patients, had a higher binding affinity for I65N and L311P mutants compared to the wildtype protein. We also identify important residues in the BH4 binding pocket that may be of interest for the rational drug design of other PAH drug-based therapies. Lastly, free energy calculations also determined that the I65N mutation may impair the dimerization of the N-terminal regulatory domain of PAH.

Neuropsychological Profile of Children with Early and Continuously Treated Phenylketonuria: Systematic Review and Future Approaches

OBJECTIVE

To provide a comprehensive systematic review of the literature by examining studies published on all cognitive aspects of children with early and continuously treated phenylketonuria (ECT-PKU) included in the databases Medline, PsycINFO, and PsycARTICLE.

METHOD

In addition to a classical approach, we summarized methodology and results of each study in order to discuss current theoretical and methodological issues. We also examined recent advances in biochemical markers and treatments of PKU, with implications for future research on metabolic control and its role as a determinant of neuropsychological outcome.

RESULTS

Consistent with previous reviews, the hypothesis of a specific and central executive impairment in children with ECT-PKU was suggested. However, findings are inconclusive regarding the nature of executive impairments as well as their specificity, impact on everyday life, persistence over time, and etiology.

CONCLUSION

Given the current state of the science, we suggest future directions for research that utilizes a developmental and integrative approach to examine the effects of recent advances in biochemical markers and treatment of PKU. (JINS, 2019, 25, 624-643).

Relationship between genotype, phenylalanine hydroxylase expression and in vitro activity and metabolic phenotype in phenylketonuria

Residual phenylalanine hydroxylase (PAH) activity is the main determinant of the metabolic phenotype in phenylketonuria (PKU). The genotypic heterogeneity of PKU, involving >1000 PAH variants and over 2500 different genotypes, makes genotype-based phenotype prediction challenging. While a relationship between PAH variants and the metabolic phenotype is well established, we questioned the importance of PAH expression and residual in vitro activity for the metabolic phenotype. Thirty-four PAH variants (p.F39 L, p.A47V, p.D59Y, p.I65S, p.R68G, p.R68S, p.E76G, p.A104D, p.D143G, p.R155H, p.R176L, p.V190A, p.G218 V, p.R241C, p.R243Q, p.P244L, p.R252W, p.R261Q, p.E280K, p.R297H, p.A300S, p.I306V, p.A309V, p.L311P, p.A313T, p.L348 V, p.V388 M, A403V, p.R408Q, p.R408W, p.R413P, p.D415N, p.Y417H, and p.A434D) were transiently transfected into COS-7 cells, and expression of PAH was investigated. Expression patterns were compared with in vitro PAH activity and allelic phenotype values (APVs). In vitro PAH activity was significantly higher (p < .01) in variants associated with mild hyperphenylalaninemia (PAH activity = 52.1 ± 8.5%; APV = 6.7-10.0) than that in classic PKU variants (PAH activity = 21.1 ± 7.0%; APV = 0-2.7). Mild PKU variants (PAH activity = 40.2 ± 7.6%; APV = 2.8-6.6) were not significantly different from mild hyperphenylalaninemia, but there was a difference (p < .048) compared with classic PKU phenotypes.

Development of the US English version of the phenylketonuria - quality of life (PKU-QOL) questionnaire

Background

Phenylketonuria (PKU) is a rare genetic disorder caused by a defect in the metabolism of phenylalanine (PHE) resulting in elevated blood and brain PHE levels, and leading to cognitive, emotional, and psychosocial problems. The phenylketonuria – quality of life (PKU-QOL) questionnaire was the first self-administered disease-specific instrument developed to assess the impact of PKU and its treatment on the health-related quality of life (HRQL) of patients and their caregivers. Available in four versions (child, adolescent, adult and parent), the PKU-QOL was simultaneously developed and validated in seven countries [i.e., France, Germany, Italy, The Netherlands, Spain, Turkey and the United Kingdom (UK)]. The objectives of our study were to develop and linguistically validate the PKU-QOL questionnaire for use in the United States (US).

Methods

The UK versions served as a basis for the development of the US English PKU-QOL questionnaire. The linguistic validation process consisted of 4 steps: 1) adaptation of the UK versions into US English by a translator native of US English and living in the US; 2) a clinician review; 3) cognitive interviews with patients and caregivers to test the appropriateness, understandability and clarity of the US translations; and 4) two proof-readings.

Results

The adaptation from UK to US English revealed the usual syntactic and idiomatic differences between the two languages, such as differences in: 1) Spelling, e.g., “dietician” (UK) vs. “dietitian” (US), or “mum” (UK) vs. “mom” (US); 2) Syntax or punctuation; and 3) Words/expressions use, e.g., “holidays” (UK) vs. “vacation” (US), or “biscuits” (UK) vs. “crackers” (US). The major issue was cultural, and consisted of using a different terminology to describe PKU treatment throughout the questionnaires. The clinician, with the patients and the caregivers, during the interviews suggested to replace “supplement and amino-acid mixture” or “supplements” with “medical formula.” This wording was later changed to “medical food” to be consistent with the terminology used in current US published guidelines.

Conclusions

The translation of the UK English PKU-QOL questionnaire into US English did not raise critical semantic and cultural issues. The PKU-QOL will be valuable for US healthcare providers in individualizing treatment and managing patients with PKU.

Long-term safety and efficacy of sapropterin: the PKUDOS registry experience

The Phenylketonuria (PKU) Demographics, Outcomes and Safety (PKUDOS) registry is designed to provide longitudinal safety and efficacy data on subjects with PKU who are (or have been) treated with sapropterin dihydrochloride. The PKUDOS population consists of 1189 subjects with PKU: N = 504 who were continuously exposed to sapropterin from date of registry enrollment, N = 211 who had intermittent exposure to the drug, and N = 474 with some other duration of exposure. Subjects continuously exposed to sapropterin showed an average 34% decrease in blood phenylalanine (Phe)--from 591 ± 382 μmol/L at baseline to 392 ± 239 μmol/L (p = 0.0009) after 5 years. This drop in blood Phe was associated with an increase in dietary Phe tolerance [from 1000 ± 959 mg/day (pre-sapropterin baseline) to 1539 ± 840 mg/day after 6 years]. Drug-related adverse events (AEs) were reported in 6% of subjects, were mostly considered non-serious, and were identified in the gastrointestinal, respiratory, and nervous systems. Serious drug-related AEs were reported in ≤ 1% of subjects. Similar safety and efficacy data were observed for children<4 years. Long-term data from the PKUDOS registry suggest that sapropterin has a tolerable safety profile and that continuous use is associated with a significant and persistent decrease in blood Phe and improvements in dietary Phe tolerance.

Structural features of the regulatory ACT domain of phenylalanine hydroxylase

Phenylalanine hydroxylase (PAH) catalyzes the conversion of L-Phe to L-Tyr. Defects in PAH activity, caused by mutations in the human gene, result in the autosomal recessively inherited disease hyperphenylalaninemia. PAH activity is regulated by multiple factors, including phosphorylation and ligand binding. In particular, PAH displays positive cooperativity for L-Phe, which is proposed to bind the enzyme on an allosteric site in the N-terminal regulatory domain (RD), also classified as an ACT domain. This domain is found in several proteins and is able to bind amino acids. We used molecular dynamics simulations to obtain dynamical and structural insights into the isolated RD of PAH. Here we show that the principal motions involve conformational changes leading from an initial open to a final closed domain structure. The global intrinsic motions of the RD are correlated with exposure to solvent of a hydrophobic surface, which corresponds to the ligand binding-site of the ACT domain. Our results strongly suggest a relationship between the Phe-binding function and the overall dynamic behaviour of the enzyme. This relationship may be affected by structure-disturbing mutations. To elucidate the functional implications of the mutations, we investigated the structural effects on the dynamics of the human RD PAH induced by six missense hyperphenylalaninemia-causing mutations, namely p.G46S, p.F39C, p.F39L, p.I65S, p.I65T and p.I65V. These studies showed that the alterations in RD hydrophobic interactions induced by missense mutations could affect the functionality of the whole enzyme.

The effects of tetrahydrobiopterin (BH4) treatment on brain function in individuals with phenylketonuria

Phenylketonuria (PKU) is a rare genetic condition characterized by an absence or mutation of the PAH enzyme, which is necessary for the metabolism of the amino acid phenylalanine into tyrosine. Recently, sapropterin dihydrochloride, a synthetic form of tetrahydrobiopterin (BH₄), has been introduced as a supplemental treatment to dietary phe control for PKU. Very little is known regarding BH₄ treatment and its effect on brain and cognition. The present study represents the first examination of potential changes in neural activation in patients with PKU during BH₄ treatment. To this end, we utilized an n-back working memory task in conjunction with functional magnetic resonance imaging (fMRI) to evaluate functional brain integrity in a sample of individuals with PKU at three timepoints: Just prior to BH₄ treatment, after 4 weeks of treatment, and after 6 months of treatment. Neural activation patterns observed for the PKU treatment group were compared with those of a demographically-matched sample of healthy non-PKU individuals who were assessed at identical time intervals. Consistent with past research, baseline evaluation revealed impaired working memory and atypical brain activation in the PKU group as compared to the non-PKU group. Most importantly, BH₄ treatment was associated with improvements in both working memory and brain activation, with neural changes evident earlier (4-week timepoint) than changes in working memory performance (6-month timepoint).

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We examine working memory and neural activation in patients with PKU at baseline.

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We track behavioral and neural changes related to BH4 treatment in the patients.

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BH4 treatment associated with improvement in neural activity at 4-week timepoint.

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BH4 treatment associated with improvement in working memory at 6-month timepoint.

Hyperphenylalaninemia in the Czech Republic: genotype-phenotype correlations and in silico analysis of novel missense mutations

BACKGROUND

Hyperphenylalaninemia (HPA) is one of the most common inherited metabolic disorders caused by deficiency of the enzyme phenylalanine hydroxylase (PAH). HPA is associated with mutations in the PAH gene, which leads to reduced protein stability and/or impaired catalytic function. Currently, almost 700 different disease-causing mutations have been described. The impact of mutations on enzyme activity varies ranging from classical PKU, mild PKU, to non-PKU HPA phenotype.

METHODS

We provide results of molecular genetic diagnostics of 665 Czech unrelated HPA patients, structural analysis of missense mutations associated with classical PKU and non-PKU HPA phenotype, and prediction of effects of 6 newly discovered HPA missense mutations using bioinformatic approaches and Molecular Dynamics simulations.

RESULTS

Ninety-eight different types of mutations were indentified. Thirteen of these were novel (6 missense, 2 nonsense, 1 splicing, and 4 small gene rearrangements). Structural analysis revealed that classical PKU mutations are more non-conservative compared to non-PKU HPA mutations and that specific sequence and structural characteristics of a mutation might be critical when distinguishing between non-PKU HPA and classical PKU mutations. The greatest impact was predicted for the p.(Phe263Ser) mutation while other novel mutations p.(Asn167Tyr), p.(Thr200Asn), p.(Asp229Gly), p.(Leu358Phe), and p.(Ile406Met) were found to be less deleterious.

Prevalence of tetrahydrobiopterine (BH4)-responsive alleles among Austrian patients with PAH deficiency: comprehensive results from molecular analysis in 147 patients

Phenylketonuria (PKU, MIM 261600) is an autosomal recessive disorder caused by mutations of the phenylalanine hydroxylase gene (PAH, GenBank U49897.1, RefSeq NM_000277). To date more than 560 variants of the PAH gene have been identified. In Europe there is regional distribution of specific mutations. Due to recent progress in chaperone therapy, the prevalence of BH4-responsive alleles gained therapeutic importance. Here we report the mutational spectrum of PAH deficiency in 147 unrelated Austrian families. Overall mutation detection rate was 98.6 %. There was a total of 62 disease-causing mutations, including five novel mutations IVS4 + 6T>A, p.H290Y, IVS8-2A>G, p.A322V and p.I421S. The five most prevalent mutations found in patients were p.R408W, IVS12 + 1G>A, p.R261Q, p.R158Q and IVS2 + 5G>C. Neonatal phenylalanine levels before treatment were available in 114/147 patients. Prediction of BH4-responsiveness in patients with full genotypes was exclusively made according to published data. Among the 133 patients needing dietary treatment, 28.4 % are expected to be BH4 "non-responsive", 4.5 % are highly likely BH4-responsive, 35.8 % are probably BH4-responsive while no interpretation was possible for 31.3 %. The mutation data reflect the population history of Austria and provide information on the likely proportion of Austrian PKU patients that may benefit from BH4-therapy.

Phenylalanine hydroxylase deficiency: molecular epidemiology and predictable BH4-responsiveness in South Portugal PKU patients

Hyperphenylalaninemia (HPA, OMIM #261600), which includes phenylketonuria (PKU), is caused by mutations in the gene encoding phenylalanine hydroxylase (PAH), being already described more than 600 different mutations. Genotype-phenotype correlation is a useful tool to predict the metabolic phenotype, to establish the better tailored diet and, more recently, to assess the potential responsiveness to BH(4) therapy, a current theme on PKU field. The aim of this study was the molecular analysis of the PAH gene, evaluation of genotype-phenotype relationships and prediction of BH(4)-responsiveness in the HPA population living in South Portugal. We performed the molecular characterization of 83 HPA patients using genomic DNA extracted from peripheral blood samples or Guthrie cards. PAH mutations were scanned by PCR amplification of exons and related intronic boundaries, followed by direct sequence analysis. Intragenic polymorphisms were determined by PCR-RFLP analysis. The results allowed the full characterization of 67 patients. The mutational spectrum encompasses 34 distinct mutations, being the most frequent IVS10nt-11G>A (14.6%), V388M (10.8%), R261Q (8.2%) and R270K (7.6%), which account for 46% of all mutant alleles. Moreover, 12 different haplotypes were identified and most mutations were associated with a single one. Notably, more than half of the 34 mutations belong to the group of more than 70 mutations already identified in BH(4)-responsive patients, according to BIOPKU database. Fifty one different genotypic combinations were found, most of them in single patients and involving a BH(4)-responsive mutation. In conclusion, a significant number (30-35%) of South Portugal PKU patients may potentially benefit from BH(4) therapy which, combined with a less strict diet, or eventually in special cases as monotherapy, may contribute to reduce nutritional deficiencies and minimize neurological and psychological dysfunctions.

Unresponsiveness to tetrahydrobiopterin of phenylalanine hydroxylase deficiency

Conflicting results have been reported concerning the efficacy of tetrahydrobiopterin (BH4), the cofactor of phenylalanine hydroxylase, for reducing phenylalanine (Phe) concentration in phenylketonuria (PKU). We aimed to test quantitatively the effects of BH4 in PKU patients. Seven fully characterized patients were selected among a population of 130 PKU subjects as harboring PKU mutations predicted as BH4 responsive and previously considered responsive to a cofactor challenge. They received a simple Phe (100 mg/kg) and 2 combined Phe (100 mg/kg) and BH4 (20 mg/kg) oral loading tests. Cofactor was administered either before or after the amino acid. The concentrations of Phe, tyrosine (Tyr), and biopterin were measured over 24 hours after loading. The comparative analysis of the loading tests showed that in all patients plasma Phe concentrations peaked within 3 hours, and fell within 24 hours by about 50% in benign, 20% in mild, and 15% in severe phenylalanine hydroxylase deficiency regardless of BH4 administration. A consistent or moderate increase of plasma Tyr, again independent of the cofactor challenge, was observed only in the less severe forms of PAH deficiency. Mean blood biopterin concentration increased 6 times after simple Phe and 34 to 39 times after combined loading tests. The administration of BH4 does not alter Phe and Tyr metabolism in PKU patients. The clearance of plasma Phe after oral loading and, as well as Tyr production, is not related to cofactor challenge but to patient's phenotype. The assessment of BH4 responsiveness by the methods so far used is not reliable, and the occurrence of BH4-responsive forms of PKU still has to be definitely proven.

Tetrahydrobiopterin responsiveness after extended loading test of 12 Danish PKU patients with the Y414C mutation

Phenylketonuria (PKU) is an inherited metabolic disease characterized by phenylalanine (Phe) accumulation due to defects in the enzyme phenylalanine hydroxylase (PAH). Phe accumulation can lead to cognitive impairment. Some individuals with PKU respond to tetrahydrobiopterin (BH4) treatment, the natural cofactor of PAH, by a reduction in blood Phe concentrations.We tested 12 patients with PKU, 8-29 years of age, all carrying the common Y414C mutation in the PAH gene. Three were homozygous and nine were compound heterozygous, with the second mutation being a putative null mutation. During the study period, genuine protein was increased to approximately 1 g/kg. The patients were treated with 20, 10, and 5 mg BH4/kg/day for 1 week on each dose, starting with 20 mg/kg. A positive response was defined as a decline in blood Phe>30%. Blood Phe was measured four times a week. Nonresponding children were excluded from the study. Eleven of 12 patients had a positive response with 20 mg/kg, 5/10 responded on 10 mg/kg, and 1/9 on 5 mg/kg. Two were late responders, with a response on 20 mg/kg after >48 h. We could confirm the previously reported inconsistent responsiveness of Y414C in the nine heterozygous patients, whereas the three homozygous patients had early median Phe declines of 73%, 51%, and 27%, respectively, on the three different doses. The varying responses despite uniform trial conditions and genotypes may be due to individual differences in BH4 absorption or metabolism. No side effects were observed.

Sapropterin: a review of its use in the treatment of primary hyperphenylalaninaemia

Sapropterin dihydrochloride (Kuvan), hereafter referred to as sapropterin, is a synthetic formulation of the active 6R-isomer of tetrahydrobiopterin, a naturally occurring cofactor for phenylalanine hydroxylase. In the EU, sapropterin is approved for the treatment of hyperphenylalaninaemia in patients >or=4 years of age with tetrahydrobiopterin-responsive phenylketonuria (PKU) and in adults and children with tetrahydrobiopterin deficiency who have been shown to be responsive to such treatment. In the US, it is approved to reduce blood phenylalanine levels in patients with hyperphenylalaninaemia due to tetrahydrobiopterin-responsive PKU. Oral sapropterin effectively lowers blood phenylalanine levels in a proportion of patients with PKU; to date, there are no published efficacy trials of the specific sapropterin formulation under review in patients with tetrahydrobiopterin deficiency. Sapropterin was well tolerated in patients with PKU, although longer-term tolerability data are required. Sapropterin is the first non-dietary treatment for patients with PKU that has been shown in randomized, double-blind trials to be effective in lowering blood phenylalanine levels. Thus, sapropterin provides a promising treatment option for patients with PKU who are tetrahydrobiopterin-responsive. PHARMACOLOGICAL PROPERTIES: The mechanism of action of sapropterin in lowering blood phenylalanine levels in patients with PKU has not been fully elucidated, but appears to be related, in part, to its effect in augmenting and stabilizing mutant phenylalanine hydroxylases, resulting in increased clearance of phenylalanine from the body. In tetrahydrobiopterin deficiency, its mechanism of action is presumed to be secondary to replacement of endogenous tetrahydrobiopterin. In healthy adults, orally-administered sapropterin is absorbed into the bloodstream, reaching maximum concentrations in 3-4 hours. It has a mean elimination half-life of approximately 4 hours in healthy adults and, based on a population pharmacokinetic study, 6.7 hours in patients with tetrahydrobiopterin-responsive PKU. Age, from 9 to 49 years, had no effect on key pharmacokinetic parameters. THERAPEUTIC EFFICACY: In an 8-day screening study in patients aged >or=8 years with PKU, approximately 20% of patients responded to sapropterin 10 mg/kg/day (i.e. were tetrahydrobiopterin responsive). Tetrahydrobiopterin-responsive patients from this study were entered into a randomized, double-blind, placebo-controlled trial in which they received sapropterin 10 mg/kg/day or placebo. At the end of 6 weeks of treatment, sapropterin recipients experienced a significant 28% decrease from baseline in mean blood phenylalanine level, while there was no significant change in placebo recipients. The difference in mean blood phenylalanine level between sapropterin and placebo groups was statistically significant at -245 micromol/L. In an extension of this trial, significantly greater reductions in blood phenylalanine levels were observed with sapropterin dosages of 10 and 20 mg/kg/day than with sapropterin 5 mg/kg/day (each dose administered for 2 weeks), indicating a dose dependent effect. During 12 weeks of treatment with the sapropterin dosage individualized to the patient according to the earlier response to sapropterin 5, 10 or 20 mg/kg/day, reductions in plasma phenylalanine were observed in all dosage groups. In a randomized, double-blind trial in children aged 4-12 years with tetrahydrobiopterin-responsive PKU, patients treated with sapropterin 20 mg/kg/day had reduced blood phenylalanine levels after 3 weeks of treatment. Over the full 10-week trial, sapropterin and placebo recipients experienced a significantly increased tolerance to dietary phenylalanine (20.9 mg/kg/day in sapropterin and 2.9 mg/kg/day in placebo recipients).

TOLERABILITY

Sapropterin was well tolerated in patients with PKU. In clinical trials in patients with PKU, the following adverse events were identified: headache, rhinorrhoea (both at a frequency of >or=10%), pharyngolaryngeal pain, nasal congestion, cough, diarrhoea, vomiting, abdominal pain and hypophenylalaninaemia (all at a frequency of >or=1% to <10%). There were no serious adverse events that were thought to be related to sapropterin treatment.

Five human phenylalanine hydroxylase proteins identified in mild hyperphenylalaninemia patients are disease-causing variants

Hyperphenylalaninemia is a group of autosomal recessive disorders caused by a wide range of phenylalanine hydroxylase (PAH) gene variants. To study the effects of mutations on PAH activity, we have reproduced five mutations (p.N223Y, p.R297L, p.F382L, p.K398N and p.Q419R) that we recently identified in a population of Southern Italy. Transient expression of mutant full-length cDNAs in human HEK293 cells yielded PAH variants whose l-phenylalanine hydroxylase activity was between 40% and 70% that of the wild-type enzyme. Moreover, Western blot analysis revealed a 50-kD monomer in all mutants thereby indicating normal synthesis of the mutant proteins. Because of the clinical mild nature of the phenotypes we performed an in vivo BH4 loading test. This was positive in all tested patients, which indicates that they are likely to respond to the coenzyme in vivo. We also analysed the environment of each mutation site in the available crystal structures of PAH by using molecular graphics tools. The structural alteration produced by each mutation was elucidated and correlated to the mutated properties of the mutant enzymes. All the data obtained demonstrate the disease-causing nature of the five novel variants.

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.

Molecular genetics of tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency

Mutations in the phenylalanine hydroxylase (PAH) gene result in phenylketonuria (PKU). Tetrahydrobiopterin (BH(4))-responsive hyperphenylalaninemia has been recently described as a variant of PAH deficiency caused by specific mutations in the PAH gene. It has been suggested that BH(4)-responsiveness may be predicted from the corresponding genotypes. Data from BH(4) loading tests indicated an incidence of BH(4)-responsiveness of >40% in the general PKU population and >80% in mild PKU patients. The current project entailed genotype analysis of 315 BH(4)-responsive patients tabulated in the BIOPKUdb database and comparison with the data from the PAHdb locus-specific knowledgebase, as well as with previously published PAH mutations for several European countries, Northern China, and South Korea. We identified 57 mutations, presenting with a substantial residual PAH activity (average approximately 47%), presumed to be associated with BH(4)-responsiveness. More than 89% of patients are found to be compound heterozygotes. The three most common mutations found in >5% of BH(4)-responsive patients are p.A403 V, p.R261Q, and p.Y414C. Using the Hardy-Weinberg formula the predicted average frequency of BH(4)-responsiveness in European populations was calculated to be 55% (range 17-79%, lowest in Baltic countries and Poland and highest in Spain), 57% in Northern China, and 55% for South Korea. The genotype-predicted prevalence of BH(4)-responsiveness was higher than prevalence data obtained from BH(4) loading tests. Inconsistent results were observed for mutations p.L48S, p.I65 T, p.R158Q, p.R261Q, and p.Y414C. Our data suggest that BH(4)-responsiveness may be more common than assumed and to some extent may be predicted or excluded from the patient's genotype.

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.

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.

Molecular epidemiology of phenylalanine hydroxylase deficiency in Southern Italy: a 96% detection rate with ten novel mutations

Hyperphenylalaninemia (HPA) comprises a group of autosomal recessive disorders mainly caused by phenylalanine hydroxylase (PAH) gene mutations. We investigated PAH mutations in 126 HPA patients from Southern Italy who were identified in a neonatal screening program. The promoter, coding and exon-flanking intronic sequences of the PAH gene were amplified and sequenced. Mutations were identified in 240/249 alleles (detection rate: 96.4%). We found 60 gene variants; the most frequent were p.R261Q (15.7% of alleles), p.A403V (11.6% of alleles) and c.1066-11G > A (8.8% of alleles). The remaining mutations were rare, and ten are novel. This mutation epidemiology differs from that reported for Northern Italy and other European countries. We also identified several discordant genotype/phenotype correlations. About two-thirds of all mild phenylketonuria patients showed at least one tetrahydrobiopterin (BH4)-responsive mutation, and are thus candidates for a customized therapeutic approach.

Evaluation of electrospray-tandem mass spectrometry for the detection of phenylketonuria and other rare disorders

Since a few years ESI-MS/MS has been employed for the simultaneous detection of a wide range of inborn errors of metabolism. The screening center North at the Hamburg University Medical Center processes 40-50,000 samples per year. To assess current developments in neonatal screening, the Northern German Working Group on Neonatal Screening consisting of health care providers, metabolic centers, and screening laboratories was founded. Based on current literature and experience four categories of diseases were established. The first three categories were recommended for screening under constant scientific evaluation, while glutaric aciduria II, beta-ketothiolase deficiency, short-chain acylCoA dehydrogenase deficiency, and homocystinuria were not included in the screening program. In contrast, general screening for phenylketonuria (PKU) remains undisputed and MS/MS screening reduced false positives by simultaneously detecting phenylalanine and tyrosine. Recently, (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (BH(4))-sensitive PKU has been discovered. We were able to demonstrate that BH(4) treatment without dietary restrictions may be sufficient for certain BH(4)-responsive PKU patients. In general, MS/MS provides a potential to rapidly screen for a wide variety of rare metabolic disorders but a close cooperation between scientists and metabolic doctors is required to constantly evaluate results in terms of improving the outcome of patients.

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.

Kinetic and stability analysis of PKU mutations identified in BH4-responsive patients

From all the different molecular mechanisms put forward to explain the basis of BH4 responsiveness in PKU patients, a clear picture is now emerging based on the results from expression studies performed with a number of missense mutations identified in patients with a positive response in BH4 loading tests. Two of the proposed mechanisms, namely decreased binding affinity of the mutant proteins for the natural cofactor and stabilization effect of BH4, have been confirmed for several PKU mutations and the results are reviewed here. The actual view supports a multifactorial basis of the response, highlighting the necessity of detailed in vitro characterization of each mutant PAH protein. Several of the confirmed molecular mechanisms may be operating simultaneously, as exemplified in the data presented, and this may result in different degrees of BH4 responsiveness.

Incidence of BH4-responsiveness in phenylalanine-hydroxylase-deficient Italian patients

BACKGROUND

Hyperphenylalaninemia (HPA) is an inherited metabolic disorder due to deficiency of the enzyme phenylalanine hydroxylase (PAH) or its cofactor tetrahydrobiopterin (BH4). BH4-responsiveness in PAH-deficient HPA is a recently described characteristic of most milder phenotypes. BH4-responsive patients show reduction of plasma phenylalanine (phe) levels after oral administration of BH4.

AIM

Determination of the incidence of BH4-responsiveness among a non-selected, cohort population of PAH-deficient hyperphenylalaninemic patients and evaluation of phenotype-genotype correlations.

PATIENTS AND METHODS

All patients born in Lombardy (Italy) between January 2000 and December 2004, and affected by HPA (107 patients) were classified after BH4 loading test, analysis of urinary pterins, and determination of DHPR activity in blood, and investigated for BH4-responsiveness. 6R-BH4 (20 mg/kg) was administered orally as a single dose and plasma samples were obtained at time-points 0, 4, 8, and 24 h after BH4 administration. In patients with basal plasma phe levels

RESULTS

BH4 significantly lowered blood phe levels in 91 (85%) of 107 patients affected by PAH-deficient HPA. Most responsive patients were affected by mild HPA (77%), a smaller percentage by mild (7%) and moderate (7%) phenylketonuria (PKU). One patient with classical PKU was responsive to BH4. Eighteen mutations were found to be associated to the BH4-responsive phenotype.

CONCLUSIONS

BH4-responsiveness is shown by a consistent number of PAH-deficient hyperphenylalaninemic patients and seems to be common in milder phenotypes. Genotype is not the only factor determining BH4-responsiveness.

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MESH Headings

• Biopterins/analogs & derivatives
• Biopterins/pharmacology
• Biopterins/therapeutic use
• Child, Preschool
• Cohort Studies
• DNA Mutational Analysis
• Genotype
• Humans
• Infant
• Italy
• Mutation
• Phenylalanine/blood
• Phenylalanine Hydroxylase/deficiency
• Phenylketonurias/blood
• Phenylketonurias/drug therapy
• Phenylketonurias/genetics

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Affiliation(s)

Laura Fiori Department of Paediatrics, San Paolo Hospital, University of Milan, Italy.
Betina Fiege
Enrica Riva
Marcello Giovannini

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Long-term treatment with tetrahydrobiopterin increases phenylalanine tolerance in children with severe phenotype of phenylketonuria

Hyperphenylalaninemia caused by phenylalanine hydroxylase (PAH) deficiency requires lifelong rigorous diet starting in early infancy to prevent severe neurodevelopmental handicap. In a considerable number of children with mild hyperphenylalaninemia, long-term tetrahydrobiopterin (BH4) treatment significantly improves phenylalanine (phe) tolerance, but it has never been investigated in classic phenylketonuria (PKU). We performed a BH4-loading test in 40 consecutive infants with phe serum concentrations exceeding 240 microM, who had been detected by newborn screening programs. Eighteen out of 40 infants were found to be BH4 responsive. Five of them, responding to the neonatal BH4-loading test, showed a phe tolerance of less than 20 mg/kg/day and a phe pretreatment level of >1000 microM. They were treated with BH4 (20 mg/kg/day) over a period of 24 months. All five children had a sustained response to BH4, allowing substantial easing of dietary restrictions. Before BH4 treatment daily phe tolerance was 18-19 mg/kg, increasing to 30-80 mg/kg on BH4 treatment and decreasing again to 12-17 mg/kg after termination of BH4 treatment. Mutation analysis revealed compound heterozygosity for a putative null and a variant PAH mutation in four patients and homozygosity for a variant PAH mutation in one patient. We conclude that BH4 sensitivity is not restricted to mild hyperphenylalaninemia and that long-term BH4 treatment may also improve phenylalanine tolerance in a considerable number of children with a more severe PKU phenotype.

Clinical and nutritional evaluation of phenylketonuric patients on tetrahydrobiopterin monotherapy

The clinical, nutritional, and neuropsychological data of 11 mild/moderate PKU patients after one year of treatment with BH4 are evaluated. BH4 monotherapy was introduced at 5 mg/kg/day in 14 PKU patients. In 11/14 patients, Phe tolerance increased significantly from 356+/-172 to 1546+/-192 mg/day (p=0.004), and special PKU formula was gradually reduced until complete removal. In them, mean plasma Phe concentrations remained below 360 micromol/L at 5 mg BH4/kg/day (7 mg/kg/day in one patient). BH4 therapy was stopped in three patients (V388M/P362T and R243Q/IVS10-11G>A genotypes) because it was not possible to improve Phe tolerance and to remove formula intake. Serum micronutrients were not significantly different at the start of treatment and at one year follow-up, except for selenium, which increased significantly after one year of therapy (p=0.017). Anthropometric, and nutritional measurements were within the age- and sex-specific percentiles for a healthy population after one year therapy. Neuropsychological follow-up indicated that intelligence scores persisted within normal limits. In terms of patients' genotype, we confirmed that the P275S mutation combined with R408W was associated with long-term BH4 responsiveness, while the combination of P362T/V388M, and R243Q/IVS10-11G>A resulted in poor metabolic control in long-term BH4 therapy. In summary, our data confirm that BH4 is a safe, and effective therapy in a selected group of mild, and moderate PKU patients who respond to the BH4 loading test. Low doses of BH4 in monotherapy permit withdrawal of the special formula and guarantee a good clinical and nutritional outcome with no adverse side effects in PKU 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.

Tetrahydrobiopterin protects phenylalanine hydroxylase activity in vivo: implications for tetrahydrobiopterin-responsive hyperphenylalaninemia

The natural cofactor of phenylalanine hydroxylase (PAH), tetrahydrobiopterin (BH4), regulates the enzyme activity as well as being essential in catalysis. BH4-responsive PAH deficiency is a variant of hyperphenylalaninemia or phenylketonuria (PKU) caused by mutations in the human PAH gene that respond to oral BH4 loading by stimulating enzyme activity and therefore lowering serum phenylalanine. Here, we showed in a coupled transcription-translation in vitro assay that upon expression in the presence of BH4, wild-type PAH enzyme activity was enhanced. We then investigated the effect of BH4 on PAH activity in transgenic mice that had a complete or partial deficiency in the endogenous cofactor biosynthesis. The rate of hepatic PAH enzyme activity increased significantly with BH4 content without affecting gene expression or Pah-mRNA stability. These results indicate that BH4 has a chaperon-like effect on PAH synthesis and/or is a protecting cofactor against enzyme auto-inactivation and degradation also in vivo. Our findings thus contribute to the understanding of the regulation of PAH by its cofactor BH4 on an additional level and provide a molecular explanation for cofactor-responsive PKU.

Mechanisms underlying responsiveness to tetrahydrobiopterin in mild phenylketonuria mutations

A subtype of phenylalanine hydroxylase (PAH) deficiency that responds to cofactor (tetrahydrobiopterin, BH4) supplementation has been associated with phenylketonuria (PKU) mutations. The underlying molecular mechanism of this responsiveness is as yet unknown and requires a detailed in vitro expression analysis of the associated mutations. With this aim, we optimized the analysis of the kinetic and cofactor binding properties in recombinant human PAH and in seven mild PKU mutations, i.e., c.194T>C (p.I65T), c.204A>T (p.R68S), c.731C>T (p.P244L), c.782G>A (p.R261Q), c.926C>T (p.A309V), c.1162G>A (p.V388M), and c.1162G>A (p.Y414C) expressed in E. coli. For p.I65T, p.R68S, and p.R261Q, we could in addition study the equilibrium binding of BH4 to the tetrameric forms by isothermal titration calorimetry (ITC). All the mutations resulted in catalytic defects, and p.I65T, p.R68S, p.P244L, and most probably p.A309V, showed reduced binding affinity for BH4. The possible stabilizing effect of the cofactor was explored using a cell-free in vitro synthesis assay combined with pulse-chase methodology. BH4 prevents the degradation of the proteins of folding variants p.A309V, p.V388M, and p.Y414C, acting as a chemical chaperone. In addition, for wild-type PAH and all mild PKU mutants analyzed in this study, BH4 increases the PAH activity of the synthesized protein and protects from the rapid inactivation observed in vitro. Catalase and superoxide dismutase partially mimic this protection. All together, our results indicate that the response to BH4 substitution therapy by PKU mutations may have a multifactorial basis. Both effects of BH4 on PAH, i.e., the chemical chaperone effect preventing protein misfolding and the protection from inactivation, may be relevant mechanisms of the responsive phenotype.

Thermodynamic characterization of the binding of tetrahydropterins to phenylalanine hydroxylase

Phenylalanine hydroxylase (PAH) is the key enzyme in the catabolism of L-Phe. The natural cofactor of PAH, 6R-tetrahydrobiopterin (BH4), negatively regulates the enzyme activity in addition to being an essential cosubstrate for catalysis. The analogue 6-methyltetrahydropterin (6M-PH4) is effective in catalysis but does not regulate PAH. Here, the thermodynamics of binding of BH4 and 6M-PH4 to human PAH have been studied by isothermal titration calorimetry. At neutral pH and 25 degrees C, BH4 binds to PAH with higher affinity (Kd = 0.75 +/- 0.18 microM) than 6M-PH4 (Kd = 16.5 +/- 2.7 microM). While BH4 binding is a strongly exothermic process (DeltaH = -11.8 +/- 0.4 kcal/mol) accompanied by an entropic penalty (-TDeltaS = 3.4 +/- 0.4 kcal/mol), 6M-PH4 binding is both enthalpically (DeltaH = -3.3 +/- 0.3 kcal/mol) and entropically (-TDeltaS = -3.2 kcal/mol) driven. No significant changes in binding affinity were observed in the 5-35 degrees C temperature range for both pterins at neutral pH, but the enthalpic contribution increased with temperature rendering a heat capacity change (DeltaCp) of -357 +/- 26 cal/mol/K for BH4 and -63 +/- 12 cal/mol/K for 6M-PH4. Protons do not seem to be taken up or released upon pterin binding. Structure-based energetics calculations applied on the molecular dynamics simulated structures of the complexes suggest that in the case of BH4 binding, the conformational rearrangement of the N-terminal tail of PAH contribute with favorable enthalpic and unfavorable entropic contributions to the intrinsic thermodynamic parameters of binding. The entropic penalty is most probably associated to the reduction of conformational flexibility at the protein level and disappears for the L-Phe activated enzyme. The calculated energetic parameters aid to elucidate the molecular mechanism for cofactor recognition and the regulation of PAH by the dihydroxypropyl side chain of BH4.

The molecular basis of phenylketonuria in Koreans

Phenylketonuria (PKU) is an inborn error of metabolism that results from a deficiency of phenylalanine hydroxylase (PAH). We characterized the PAH mutations of 79 independent Korean patients with PKU or hyperphenylalaninemia. PAH nucleotide sequence analysis revealed 39 different mutations, including ten novel mutations. The novel mutations consisted of nine missense mutations (P69S, G103S, N207D, T278S, P281A, L293M, G332V, S391I, and A447P) and a novel splice site variant (IVS10-3C>G). R243Q, IVS4-1G>A, and E6-96A>G were the most prevalent mutations, as they accounted for 32% of the total mutant alleles in this study. Although some common characteristics of allele frequency and distribution were identified among oriental populations, several distinctive characteristics were revealed in Korean patients. Although the R413P allele is the most prevalent form (30.5%) in Japanese, we detected it in only five chromosomes from 158 independent chromosomes (3.2%). The A259T allele, which has not yet been found in oriental populations, was frequently found in this study. We also observed that tetrahydrobiopterin (BH4) responsiveness was associated with specific genotypes (R53H, R241C, and R408Q), suggesting there are some correlations between phenotype and genotype.

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.

A Mass Spectrometry Plate Reader: Monitoring Enzyme Activity and Inhibition with a Desorption/Ionization on Silicon (DIOS) Platform

A surface-based laser desorption/ionization mass spectrometry assay that makes use of Desorption/Ionization on Silicon Mass Spectrometry (DIOS-MS) has been developed to monitor enzyme activity and enzyme inhibition. DIOS-MS has been used to characterize inhibitors from a library and then to monitor their activity against selected enzyme targets, including proteases, glycotransferase, and acetylcholinesterase. An automated DIOS-MS system was also used as a high-throughput screen for the activity of novel enzymes and enzyme inhibitors. On two different commercially available instruments, a sampling rate of up to 38 inhibitors per minute was accomplished, with thousands of inhibitors being monitored. The ease of applying mass spectrometry toward developing enzyme assays and the speed of surface-based assays such as DIOS for monitoring inhibitor effectiveness and enzyme activity makes it attractive for a broad range of screening applications.

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.

Biopterin responsive phenylalanine hydroxylase deficiency

PURPOSE

Phenylketonuria (PKU) is an autosomal recessive disorder caused by mutations in the phenylalanine hydroxylase (PAH) gene. There have been more than 400 mutations identified in the PAH gene leading to variable degrees of deficiency in PAH activity, and consequently a wide spectrum of clinical severity. A pilot study was undertaken to examine the response to 6-R-l-erythro-5,6,7,8-tetrahydrobiopterin (BH4) in patients with atypical and classical PKU.

METHODS

PAH gene mutation analysis was performed using denaturing gradient gel electrophoresis and gene sequencing. Patients with classical, atypical, or mild PKU were orally given BH4 10 mg/kg. Blood phenylalanine and tyrosine levels were determined using tandem MS/MS at 0 hours, 4 hours, 8 hours, and 24 hours intervals.

RESULTS

Thirty-six patients were given a single oral dose of 10 mg/kg of BH4. Twenty one patients (58.33%) responded with a decrease in blood phenylalanine level. Of the patients that responded, 12 were classical, 7 atypical, and 2 mild. The mean decline in blood phenylalanine at 24 hours was > 30% of baseline. There were 15 patients who did not respond to the BH4 challenge, 14 of those had classical and one had atypical PKU. Mapping the mutations that responded to BH4 on the PAH enzyme showed that mutations were in the catalytic, regulatory, oligomerization, and BH4 binding domains. Five patients responding to BH4 had mutations not previously identified.

CONCLUSION

The data presented suggest higher than anticipated number of PKU mutations respond to BH4, and such mutations are on all the domains of PAH.

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.

PAHdb 2003: what a locus-specific knowledgebase can do

PAHdb, a legacy of and resource in genetics, is a relational locus-specific database (http://www.pahdb.mcgill.ca). It records and annotates both pathogenic alleles (n = 439, putative disease-causing) and benign alleles (n = 41, putative untranslated polymorphisms) at the human phenylalanine hydroxylase locus (symbol PAH). Human alleles named by nucleotide number (systematic names) and their trivial names receive unique identifier numbers. The annotated gDNA sequence for PAH is typical for mammalian genes. An annotated gDNA sequence is numbered so that cDNA and gDNA sites are interconvertable. A site map for PAHdb leads to a large array of secondary data (attributes): source of the allele (submitter, publication, or population); polymorphic haplotype background; and effect of the allele as predicted by molecular modeling on the phenylalanine hydroxylase enzyme (EC 1.14.16.1) or by in vitro expression analysis. The majority (63%) of the putative pathogenic PAH alleles are point mutations causing missense in translation of which few have a primary effect on PAH enzyme kinetics. Most apparently have a secondary effect on its function through misfolding, aggregation, and intracellular degradation of the protein. Some point mutations create new splice sites. A subset of primary PAH mutations that are tetrahydrobiopterin-responsive is highlighted on a Curators' Page. A clinical module describes the corresponding human clinical disorders (hyperphenylalaninemia [HPA] and phenylketonuria [PKU]), their inheritance, and their treatment. PAHdb contains data on the mouse gene (Pah) and on four orthologous mutant mouse models and their use (for example, in research on oral treatment of PKU with the enzyme phenylalanine ammonia lyase [EC 4.3.1.5]).

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.

How PAH gene mutations cause hyper-phenylalaninemia and why mechanism matters: insights from in vitro expression

Mutations in the human PAH gene, which encodes phenylalanine hydroxylase are associated with varying degrees of hyperphenylalaninemia (HPA). The more severe of these manifest as a classic metabolic disease--phenylketonuria (PKU). In vitro expression analysis of PAH mutations has three major applications: 1) to confirm that a disease-associated mutation is genuinely pathogenic, 2) to assess the severity of a mutation's impact, and 3) to examine how a mutation exerts its deleterious effects on the PAH enzyme, that is, to elucidate the molecular mechanisms involved. Data on expression analysis of 81 PAH mutations in multiple in vitro systems is summarized in tabular form online at www.pahdb.mcgill.ca. A review of these findings points in particular to a prevalent general mechanism that appears to play a major role in the pathogenicity of many PAH mutations. Amino acid substitutions promote misfolding of the PAH protein monomer and/or oppose the correct assembly of monomers into the native tetrameric enzyme. The resulting structural aberrations trigger cellular defenses, provoking accelerated degradation of the abnormal protein. The intracellular steady-state levels of the mutant PAH enzyme are therefore reduced, leading to an overall decrease in phenylalanine hydroxylation within cells and thus to hyperphenylalaninemia. There is considerable scope for modulation of the enzymic and metabolic phenotypes by modification of the cellular handling--folding, assembly, and degradation--of the mutant PAH protein. This has major implications, both for our understanding of genotype-phenotype correlations and for the development of novel therapeutic approaches.

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.

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.

Phenylketonuria: an update

Phenylketonuria is a flagship inborn error of metabolism and has been at the forefront of our growing understanding, diagnosis, and treatment of this family of disorders. In this article, the current understanding of its diagnosis, treatment, and complex molecular biology and physiology is reviewed. Recent papers exploring newer and less well-delineated areas of cofactor supplementation and genetic and epigenetic modification of the genotypic expression are presented. The excitement surrounding the continued exploration of the hyperphenylalaninemias is emphasized.