Newborn screening and genetic features of patients with hyperphenylalaninemia in a southern Chinese population

BACKGROUND AND AIMS

Hyperphenylalaninemia (HPA) is the most common congenital amino acid metabolism-related defect, but its incidence differs substantially between northern and southern China. We aimed to elucidate the incidence, proportion, and genetic features of HPA in a southern Chinese population.

MATERIALS AND METHODS

We analyzed the HPA screening results for 580,460 newborns from 2014 to 2021.

RESULTS

Of the 296 newborns who tested HPA positive, 56 were diagnosed with HPA, including 47 with phenylalanine hydroxylase deficiency and nine with tetrahydrobiopterin deficiency (BH4D). HPA incidence was estimated to be 1:10,365 newborns. All patients had elevated Phe and Phe/Tyr levels. Thirty-three PAH variants and five PTS variants were detected in HPA patients; 80.6 % PAH variants and 100 % PTS variants were classified as pathogenic or likely pathogenic. In silico tools predicted the remaining variants to be damaging. PAH variants clustered in exons 3, 5, 7, 11, and 12 and PTS variants clustered in exons 2 and 5. The most common PAH variants were c.158G > A (p.R53H, 22.3 %) and c.721C > T (p.R241C, 14.9 %). The most common PTS variants were c.155A > G (p.N52S, 50.0 %) and c.259C > T (p.P87S, 33.3 %).

CONCLUSION

Newborn screening is an effective method for early detection of HPA, but differential diagnosis of BH4D is necessary.

The effect of casein glycomacropeptide versus free synthetic amino acids for early treatment of phenylketonuria in a mice model

Introduction

Management of phenylketonuria (PKU) is mainly achieved through dietary control with limited intake of phenylalanine (Phe) from food, supplemented with low protein (LP) food and a mixture of free synthetic (FS) amino acids (AA) (FSAA). Casein glycomacropeptide (CGMP) is a natural peptide released in whey during cheese making by the action of the enzyme chymosin. Because CGMP in its pure form does not contain Phe, it is nutritionally suitable as a supplement in the diet for PKU when enriched with specific AAs. Lacprodan^® CGMP-20 (= CGMP) used in this study contained only trace amounts of Phe due to minor presence of other proteins/peptides.

Objective

The aims were to address the following questions in a classical PKU mouse model: Study 1, off diet: Can pure CGMP or CGMP supplemented with Large Neutral Amino Acids (LNAA) as a supplement to normal diet significantly lower the content of Phe in the brain compared to a control group on normal diet, and does supplementation of selected LNAA results in significant lower brain Phe level?. Study 2, on diet: Does a combination of CGMP, essential (non-Phe) EAAs and LP diet, provide similar plasma and brain Phe levels, growth and behavioral skills as a formula which alone consist of FSAA, with a similar composition?.

Material and methods

45 female mice homozygous for the Pah^enu2 mutation were treated for 12 weeks in five different groups; G1(N-CGMP), fed on Normal (N) casein diet (75%) in combination with CGMP (25%); G2 (N-CGMP-LNAA), fed on Normal (N) casein diet (75%) in combination with CGMP (19,7%) and selected LNAA (5,3% Leu, Tyr and Trp); G3 (N), fed on normal casein diet (100%); G4 (CGMP-EAA-LP), fed on CGMP (70,4%) in combination with essential AA (19,6%) and LP diet; G5 (FSAA-LP), fed on FSAA (100%) and LP diet. The following parameters were measured during the treatment period: Plasma AA profiles including Phe and Tyr, growth, food and water intake and number of teeth cut. At the end of the treatment period, a body scan (fat and lean body mass) and a behavioral test (Barnes Maze) were performed. Finally, the brains were examined for content of Phe, Tyr, Trp, dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), serotonin (5-HT) and 5-hydroxyindole-acetic acid (5-HIAA), and the bone density and bone mineral content were determined by dual-energy x-ray absorptiometry.

Results

Study 1: Mice off diet supplemented with CGMP (G1 (N-CGMP)) or supplemented with CGMP in combination with LNAA (G2 (N-CGMP-LNAA)) had significantly lower Phe in plasma and in the brain compared to mice fed only casein (G3 (N)). Extra LNAA (Tyr, Trp and Leu) to CGMP did not have any significant impact on Phe levels in the plasma and brain, but an increase in serotonin was measured in the brain of G2 mice compared to G1. Study 2: PKU mice fed with mixture of CGMP and EAA as supplement to LP diet (G4 (CGMP-EAA-LP)) demonstrated lower plasma-Phe levels but similar brain- Phe levels and growth as mice fed on an almost identical combination of FSAA (G5 (FSAA-LP)).

Conclusion

CGMP can be a relevant supplement for the treatment of PKU.

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.

BH4-deficient hyperphenylalaninemia in Russia

A timely detection of patients with tetrahydrobiopterin (BH₄) -deficient types of hyperphenylalaninemia (HPABH₄) is important for assignment of correct therapy, allowing to avoid complications. Often HPABH₄ patients receive the same therapy as phenylalanine hydroxylase (PAH) -deficiency (phenylketonuria) patients—dietary treatment—and do not receive substitutive BH₄ therapy until the diagnosis is confirmed by molecular genetic means. In this study, we present a cohort of 30 Russian patients with HPABH₄ with detected variants in genes causing different types of HPA. Family diagnostics and biochemical urinary pterin spectrum analyses were carried out. HPABH₄A is shown to be the prevalent type, 83.3% of all HPABH₄ cases. The mutation spectrum for the PTS gene was defined, the most common variants in Russia were p.Thr106Met—32%, p.Asn72Lys—20%, p.Arg9His—8%, p.Ser32Gly—6%. We also detected 7 novel PTS variants and 3 novel QDPR variants. HPABH₄ prevalence was estimated to be 0.5–0.9% of all HPA cases in Russia, which is significantly lower than in European countries on average, China, and Saudi Arabia. The results of this research show the necessity of introducing differential diagnostics for HPABH₄ into neonatal screening practice.

Maternal PKU: Defining phenylalanine tolerance and its variation during pregnancy, according to genetic background

BACKGROUND AND AIMS

Phenylketonuria (PKU)-affected women may become pregnant, and dietary phenylalanine (Phe) intake must be adjusted according to Phe tolerance. We report our experience with maternal PKU in relation to genotype PKU heterogeneity.

METHODS AND RESULTS

A total of 10 pregnancies in 7 PKU women (7 different genotypes) were followed up as part of personalized care. Phe tolerance during preconception and pregnancy was assessed by strict dietary control and weekly Phe measurement (blood spots) in relation to genotype. Most women had stopped PKU diet during childhood or adolescence and six pregnancies were unplanned; a phenylalanine-restricted diet was reinstituted soon after conception. Women were classified according to their Phe levels at birth screening and genotype. Phe tolerance increased systematically in the course of pregnancy in all cases, but the increase was different in subjects with classic PKU (cPKU) when compared with cases with mild hyperphenylalaninemia (mHPA), both on average (+297 mg/day in cPKU vs. 597 in mHPA; P = 0.017) and as percentage (+107% in cPKU vs. +17% in mHPA). Notably, Phe tolerance also varied in the same women in the course of different pregnancies, when body weight gain was also different. Two newborns from the same cPKU mother (unplanned pregnancies on free diet) were affected by congenital alterations.

CONCLUSIONS

Several factors influence metabolic phenotype in maternal PKU, to an unpredictable extent even in the same woman. The number of maternal PKU cases is growing in dedicated Nutrition Units, and the burden associated with careful management of this condition for the health care system should be adequately considered.

Metabolomics analysis reveals perturbations of cerebrocortical metabolic pathways in the Pahenu2 mouse model of phenylketonuria

AIMS

Phenylketonuria (PKU), which is caused by mutations in the phenylalanine hydroxylase (PAH) gene, is one of the most common inherited diseases of amino acid metabolism. Phenylketonuria is characterized by an abnormal accumulation of phenylalanine and its metabolites in body fluids and brain tissues, subsequently leading to severe brain dysfunction. Various pathophysiological and molecular mechanisms underlying brain dysfunction in PKU have been described. However, the metabolic changes and their impacts on the function of cerebral cortices of patients with PKU remain largely unknown.

METHODS

We measured the levels of small molecule metabolites in the cerebrocortical tissues of PKU mice and wild-type control mice using liquid chromatography-mass spectrometry (LC-MS)-based metabolome analysis. Differential metabolites were further subjected to metabolic pathway and enrichment analysis.

RESULTS

Metabolome analysis revealed 35 compounds among 143 detected metabolites were significantly changed in PKU mice as compared to those in their wild-type littermates. Metabolic pathway and enrichment analysis of these differential metabolites showed that multiple metabolic pathways, including phenylalanine, tyrosine, and tryptophan biosynthesis; valine, leucine, and isoleucine biosynthesis; alanine, aspartate, and glutamate metabolism; purine metabolism; arginine and proline metabolism and methionine metabolism, were impacted in the cerebral cortices of PKU mice.

CONCLUSIONS

The data revealed that multiple metabolic pathways in cerebral cortices of PKU mice were disturbed, suggesting that the disturbances of the metabolic pathways might contribute to neurological or neurodevelopmental dysfunction in PKU, which could thus provide new insights into brain pathogenic mechanisms in PKU as well as mechanistic insights for better understanding the complexity of the metabolic mechanisms of the brain dysfunction in PKU.

5-year retrospective analysis of patients with phenylketonuria (PKU) and hyperphenylalaninemia treated at two specialized clinics

BACKGROUND

Phenylketonuria (PKU) is an autosomal recessive disease caused by mutations in the PAH gene, resulting in deficiency of phenylalanine hydroxylase (PAH), an enzyme that converts phenylalanine (Phe) to tyrosine (Tyr). The purpose of this study was to capture real-world data associated with managing PKU under current standard of care and to characterize a representative population for a planned gene therapy trial.

METHODS

A retrospective chart review was conducted at two U.S. clinics for individuals 10-40 years old diagnosed with PKU-related hyperphenylalaninemia (HPA). Demographics, medical history, treatments and blood Phe data were collected from electronic medical records spanning a five-year period ending in November 2017.

RESULTS

152 patients were enrolled (65.8% had classical PKU). Although >95% of patients were prescribed a Phe-restricted diet, blood Phe concentrations remained substantially elevated, particularly in patients diagnosed with classical PKU. As the Phe threshold was lowered (Phe < 600, 360, 120 or 30 μmol/L), the number of patients with consecutive lab values below the threshold decreased, suggesting that many patients' Phe levels are inadequately controlled. 62.5% of patients were reported as having a history of at least one neuropsychiatric comorbidity, and adults were more likely than adolescents (69.5% vs. 54.3%). 92 of 98 PAH genotypes collected were distinct mutations; the 6 null-null genotypes were associated with classical PKU. Overall the demographics and clinical data were consistent across both sites.

CONCLUSION

Despite dietary restrictions, mean Phe concentrations were > 360 μmol/L (a level considered well-controlled based on current U.S. treatment guidelines) for mild, moderate, and classical PKU patients. There remains an unmet need for therapies to control Phe concentrations.

State-of-the-Art 2019 on Gene Therapy for Phenylketonuria

Phenylketonuria (PKU) is considered to be a paradigm for a monogenic metabolic disorder but was never thought to be a primary application for human gene therapy due to established alternative treatment. However, somewhat unanticipated improvement in neuropsychiatric outcome upon long-term treatment of adults with PKU with enzyme substitution therapy might slowly change this assumption. In parallel, PKU was for a long time considered to be an excellent test system for experimental gene therapy of a Mendelian autosomal recessive defect of the liver due to an outstanding mouse model and the easy to analyze and well-defined therapeutic end point, that is, blood l-phenylalanine concentration. Lifelong treatment by targeting the mouse liver (or skeletal muscle) was achieved using different approaches, including (1) recombinant adeno-associated viral (rAAV) or nonviral naked DNA vector-based gene addition, (2) genome editing using base editors delivered by rAAV vectors, and (3) by delivering rAAVs for promoter-less insertion of the PAH-cDNA into the Pah locus. In this article we summarize the gene therapeutic attempts of correcting a mouse model for PKU and discuss the future implications for human gene therapy.

Retrospective Study of Patients with Hyperphenylalaninemia- Experience from a Tertiary Care Center in Pakistan

OBJECTIVE

To assess the clinical and biochemical features as well as outcome of hyperphenylalaninemia patients.

Methods

The descriptive retrospective study was conducted at the Aga Khan University Hospital, Karachi, and comprised data from January 2013 to February 2017 of plasma amino acid analysed at the Biochemical Genetic Laboratory of patients with phenylalanine levels >120 umol/L. Medical charts of patients registered with the Metabolic Clinics were reviewed, while outside referrals were contacted by telephone to collect data on a pre-structured questionnaire. Data was analysed using SPSS 21.

RESULTS

Of the 18 patients, 13(72%) were males. Overall median age was 606 days (interquartile range: 761) and median phenylalanine levels were 1280 (interquartile range: 935) umol/L. Phenylalanine hydroxylase deficiency was present in 5(28%) patients while 3(16.6%) had defects in the metabolism or regeneration of tetrahydrobiopterin. The most common clinical features was intellectual deficit and seizures 14(78%) each, followed by lighter hair colour 10(55.5%) and hypotonia 11(61%). High treatment cost was the leading reason for cessation of therapy in 7(39%) followed by refusal by patient's family 5(28%).

CONCLUSIONS

Most hyperphenylalaninemia cases were diagnosed late when intellectual disability had already developed.

Long-term dietary intervention with low Phe and/or a specific nutrient combination improve certain aspects of brain functioning in phenylketonuria (PKU)

INTRODUCTION

In phenylketonuria (PKU), a gene mutation in the phenylalanine metabolic pathway causes accumulation of phenylalanine (Phe) in blood and brain. Although early introduction of a Phe-restricted diet can prevent severe symptoms from developing, patients who are diagnosed and treated early still experience deficits in cognitive functioning indicating shortcomings of current treatment. In the search for new and/or additional treatment strategies, a specific nutrient combination (SNC) was postulated to improve brain function in PKU. In this study, a long-term dietary intervention with a low-Phe diet, a specific combination of nutrients designed to improve brain function, or both concepts together was investigated in male and female BTBR PKU and WT mice.

MATERIAL & METHODS

48 homozygous wild-types (WT, +/+) and 96 PKU BTBRPah2 (-/-) male and female mice received dietary interventions from postnatal day 31 till 10 months of age and were distributed in the following six groups: high Phe diet (WT C-HP, PKU C-HP), high Phe plus specific nutrient combination (WT SNC-HP, PKU SNC-HP), PKU low-Phe diet (PKU C-LP), and PKU low-Phe diet plus specific nutrient combination (PKU SNC- LP). Memory and motor function were tested at time points 3, 6, and 9 months after treatment initiation in the open field (OF), novel object recognition test (NOR), spatial object recognition test (SOR), and the balance beam (BB). At the end of the experiments, brain neurotransmitter concentrations were determined.

RESULTS

In the NOR, we found that PKU mice, despite being subjected to high Phe conditions, could master the task on all three time points when supplemented with SNC. Under low Phe conditions, PKU mice on control diet could master the NOR at all three time points, while PKU mice on the SNC supplemented diet could master the task at time points 6 and 9 months. SNC supplementation did not consistently influence the performance in the OF, SOR or BB in PKU mice. The low Phe diet was able to normalize concentrations of norepinephrine and serotonin; however, these neurotransmitters were not influenced by SNC supplementation.

CONCLUSION

This study demonstrates that both a long-lasting low Phe diet, the diet enriched with SNC, as well as the combined diet was able to ameliorate some, but not all of these PKU-induced abnormalities. Specifically, this study is the first long-term intervention study in BTBR PKU mice that shows that SNC supplementation can specifically improve novel object recognition.

Prevalence of comorbid conditions among adult patients diagnosed with phenylketonuria

BACKGROUND

Phenylalanine hydroxylase (PAH) deficiency, otherwise known as phenylketonuria (PKU), is an inborn error of metabolism that requires treatment to be initiated in the newborn period and continued throughout life. Due to the challenges of treatment adherence and the resulting cumulative effects of high and labile blood phenylalanine, PKU exerts a significant burden of disease. Retrospective studies using large databases allow for unique perspectives on comorbidities associated with rare diseases. An evaluation of comorbidities across various organ systems is warranted to understand the disease burden in adult patients.

OBJECTIVES

The aim of this insurance claim-based observational study was to assess the prevalence of comorbid conditions across various organ systems (e.g. dermatological, renal, respiratory, gastrointestinal, hematological, and others) among adult PKU patients compared with matched controls from the general population.

METHODS

This retrospective, case-controlled study selected patients from United States insurance claims databases from 1998 to 2014 using International Classification of Diseases, Ninth Revision (ICD-9) codes for diagnosis of PKU. The date of first diagnosis during the study period was index date and this was not necessarily the first time the patient was diagnosed with PKU. Cases were matched with a 1:5 ratio with general population (non-PKU controls) on age, sex, race, geographic location, duration of time in the database and insurance type. Prevalence and prevalence ratio (PR) calculations for comorbidities across various organ systems among adults (≥20 years old) with PKU were compared with the general population (non-PKU controls). The conditions were selected based on complications associated with PKU and feedback from clinicians treating PKU patients.

RESULTS

A total of 3691 PKU patients and 18,455 matched, non-PKU controls were selected, with an average age of 35 years. The mean healthcare costs incurred by the PKU patients during baseline, were approximately 4 times that of the controls ($4141 vs $1283; p < .0001). The prevalence rates of comorbidities across various organ systems during the follow-up period were significantly higher for those with PKU than in the control group. After adjusting for baseline characteristics, the adjusted prevalence ratios (PR) of 15 conditions studied (asthma, alopecia, urticaria, gallbladder disease, rhinitis, esophageal disorders, anemia, overweight, GERD, eczema, renal insufficiency, osteoporosis, gastritis/esophagitis and kidney calculus) were all above PR = 1.24 and significantly higher for the PKU cohort (p ≤ .001). The highest adjusted PR were for renal insufficiency with hypertension (PR [95% CI]: 2.20 [1.60-3.00]; p < .0001) and overweight (PR [95%CI]: 2.06 [1.85-2.30]; p < .0001).

CONCLUSIONS

The prevalence of selected comorbidities across several organ systems is significantly higher among PKU patients than for general population controls. Regular screening for common co-morbidities may be warranted as part of PKU management.

Mutational spectrum of the phenylalanine hydroxylase gene in patients with phenylketonuria in the central region of China

Phenylketonuria (PKU, OMIM 261600) caused by phenylalanine hydroxylase (PAH) deficiency is an autosomal recessive disease that is characterized by abnormalities of phenylalanine metabolism. In this study, a total of 77 patients, originating from the central region of China and who were diagnosed with PAH deficiency at the third affiliated hospital of Zhengzhou University, were enrolled in this study. The 13 exons and 12 flanking introns of the PAH gene were analyzed by Sanger sequencing and next generation sequencing. The sequencing data were aligned to the hg19, PAHvdb and HGMD databases to characterize the genotypes of PKU patients, and genotype-phenotype correlations and BH4 responsiveness predictions were performed using BIOPKUdb. In total, 149 alleles were characterized among the 154 PKU alleles. These mutations were located in exons 2-13, and intron 12 of the PAH gene, with a relative frequency of ≥5%, for EX6-96A>G, p.R241C, p.R243Q, p.V399V and p.R53H. Additionally, a novel variant, p.D84G, was identified. The genotype correlated with clinical symptoms in 33.3-100% of the cases, depending on the disease severity, and BH4 responsiveness predictions show that only five patients with MHP-PKU and one patient with Mild-PKU were predicted to be BH4 responsive. In conclusion, we have characterized the mutational spectrum of PAH in the central region of China and have identified a novel mutation. The hotspot mutation information might be useful for screening, diagnosis and treatment of PKU.

DNA methylation in the pathophysiology of hyperphenylalaninemia in the PAH(enu2) mouse model of phenylketonuria

Phenylalanine hydroxylase deficient phenylketonuria (PKU) is the paradigm for a treatable inborn error of metabolism where maintaining plasma phenylalanine (Phe) in the therapeutic range relates to improved clinical outcomes. While Phe is the presumed intoxicating analyte causal in neurologic damage, the mechanism(s) of Phe toxicity has remained elusive. Altered DNA methylation is a recognized response associated with exposure to numerous small molecule toxic agents. Paralleling this effect, we hypothesized that chronic Phe over-exposure in the brain would lead to aberrant DNA methylation with secondary influence upon gene regulation that would ultimately contribute to PKU neuropathology. The PAH(enu2) mouse models human PKU with intrinsic hyperphenylalaninemia, abnormal response to Phe challenge, and neurologic deficit. To examine this hypothesis, we assessed DNA methylation patterns in brain tissues using methylated DNA immunoprecipitation and paired end sequencing in adult PAH(enu2) animals maintained under either continuous dietary Phe restriction or chronic hyperphenylalaninemia. Heterozygous PAH(enu2/WT) litter mates served as controls for normal Phe exposure. Extensive repatterning of DNA methylation was observed in brain tissue of hyperphenylalaninemic animals while Phe restricted animals displayed an attenuated pattern of aberrant DNA methylation. Affected gene coding regions displayed aberrant hypermethylation and hypomethylation. Gene body methylation of noncoding RNA genes was observed and among these microRNA genes were prominent. Of particular note, observed only in hyperphenylalaninemic animals, was hypomethylation of miRNA genes within the imprinted Dlk1-Dio3 locus on chromosome 12. Aberrant methylation of microRNA genes influenced their expression which has secondary effects upon the expression of targeted protein coding genes. Differential hypermethylation of gene promoters was exclusive to hyperphenylalaninemic PAH(enu2) animals. Genes with synaptic involvement were targets of promoter hypermethylation that resulted in down-regulation of their expression. Gene dysregulation secondary to abnormal DNA methylation may be contributing to PKU neuropathology. These results suggest drugs that prevent or correct aberrant DNA methylation may offer a novel therapeutic option to management of neurological symptoms in PKU patients.

Long-term BH4 (sapropterin) treatment of children with hyperphenylalaninemia - effect on median Phe/Tyr ratios

BACKGROUND

Phenylalanine hydroxylase deficiency causes various degrees of hyperphenylalaninemia (HPA). Tetrahydrobiopterin (BH4; sapropterin) reduces phenylalanine (Phe) levels in responders, enabling relaxation of dietary therapy. We aimed to assess long-term effects of BH4 treatment in HPA patients.

METHODS

Nine pre-pubertal BH4 responsive children were treated with BH4 for at least 2 years. The median dietary tolerance to Phe and levels of blood Phe, tyrosine (Tyr), zinc, selenium and vitamin B12 and anthropometric measurements, in the 2 years periods before and after the introduction of BH4 treatment were analyzed and compared. Adverse effects of BH4 were assessed.

RESULTS

The daily Phe tolerance had tripled, from pretreatment median value of 620 mg (IQR 400-700 mg) to 2000 (IQR 1000-2000 mg) after 2 years of follow up (p<0.001). The median blood Phe levels during the 2 years period before introducing BH4 did not change significantly during the 2 years on therapy (from 200 μmol/L; IQR 191-302 to 190 μmol/L; IQR 135-285 μmol/L), but the median blood Phe/Tyr ratio had lowered significantly from pre-treatment value 4.7 to 2.4 during the 2 years on therapy (p=0.01). Median zinc, selenium, vitamin B12 levels and anthropometric measurements did not change while on BH4 therapy (p=NS). No adverse effects were noticed.

CONCLUSIONS

BH4 therapy enabled patients much higher dietary Phe intakes, with no noticeable adverse effects. Median blood Phe and Tyr levels, median zinc, selenium, vitamin B12 levels and anthropometric measurements did not change significantly on BH4 therapy, but median Phe/Tyr ratios had lowered.

Tetrahydrobiopterin (BH4) responsiveness in neonates with hyperphenylalaninemia: a semi-mechanistically-based, nonlinear mixed-effect modeling

Neonatal loading studies with tetrahydrobiopterin (BH4) are used to detect hyperphenylalaninemia due to BH4 deficiency by evaluating decreases in blood phenylalanine (Phe) concentrations post BH4 load. BH4 responsiveness in phenylalanine hydroxylase (PAH)-deficient patients introduced a new diagnostic aspect for this test. In older children, a broad spectrum of different levels of responsiveness has been described. The primary objective of this study was to develop a pharmacodynamic model to improve the description of individual sensitivity to BH4 in the neonatal period. Secondary objectives were to evaluate BH4 responsiveness in a large number of PAH-deficient patients from a neonatal screening program and in patients with various confirmed BH4 deficiencies from the BIODEF database. Descriptive statistics in patients with PAH deficiency with 0-24-h data available showed that 129 of 340 patients (37.9%) had a >30% decrease in Phe levels post load. Patients with dihydropteridine reductase deficiency (n = 53) could not be differentiated from BH4-responsive patients with PAH deficiency. The pharmacologic turnover model, "stimulation of loss" of Phe following BH4 load, fitted the data best. Using the model, 193 of 194 (99.5%) patients with a proven BH4 synthesis deficiency or recycling defect were classified as BH4 sensitive. Among patients with PAH deficiency, 216 of 375 (57.6%) patients showed sensitivity to BH4, albeit with a pronounced variability; PAH-deficient patients with blood Phe <1200 μmol/L at time 0 showed higher sensitivity than patients with blood Phe levels >1200 μmol/L. External validation showed good correlation between the present approach, using 0-24-h blood Phe data, and the published 48-h prognostic test. Pharmacodynamic modeling of Phe levels following a BH4 loading test is sufficiently powerful to detect a wide range of responsiveness, interpretable as a measure of sensitivity to BH4. However, the clinical relevance of small responses needs to be evaluated by further studies of their relationship to long-term response to BH4 treatment.

Protein substitute for children and adults with phenylketonuria

BACKGROUND

Phenylketonuria is an inherited metabolic disorder characterised by an absence or deficiency of the enzyme phenylalanine hydroxylase. The aim of treatment is to lower blood phenylalanine concentrations to the recommended therapeutic range to prevent developmental delay and support normal growth. Current treatment consists of a low-phenylalanine diet in combination with a protein substitute which is free from or low in phenylalanine. Guidance regarding the use, dosage, and distribution of dosage of the protein substitute over a 24-hour period is unclear, and there is variation in recommendations among treatment centres. This is an update of a Cochrane review first published in 2005, and previously updated in 2008.

OBJECTIVES

To assess the benefits and adverse effects of protein substitute, its dosage, and distribution of dose in children and adults with phenylketonuria who are adhering to a low-phenylalanine diet.

SEARCH METHODS

We searched the Cochrane Cystic Fibrosis and Genetic Disorders Group Trials Register which consists of references identified from comprehensive electronic database searches and hand searches of relevant journals and abstract books of conference proceedings. We also contacted manufacturers of the phenylalanine-free and low-phenylalanine protein substitutes for any data from published and unpublished randomised controlled trials.Date of the most recent search of the Group's Inborn Errors of Metabolism Trials Register: 03 April 2014.

SELECTION CRITERIA

All randomised or quasi-randomised controlled trials comparing: any dose of protein substitute with no protein substitute; an alternative dosage; or the same dose, but given as frequent small doses throughout the day compared with the same total daily dose given as larger boluses less frequently.

DATA COLLECTION AND ANALYSIS

Both authors independently extracted data and assessed trial quality.

MAIN RESULTS

Three trials (69 participants) are included in this review. One trial investigated the use of protein substitute in 16 participants, while a further two trials investigated the dosage of protein substitute in a total of 53 participants. Due to issues with data presentation in each trial, described in full in the review, formal statistical analyses of the data were impossible. Investigators will be contacted for further information.

AUTHORS' CONCLUSIONS

No conclusions could be drawn about the short- or long-term use of protein substitute in phenylketonuria due to the lack of adequate or analysable trial data. Additional data and randomised controlled trials are needed to investigate the use of protein substitute in phenylketonuria. Until further evidence is available, current practice in the use of protein substitute should continue to be monitored with care.

Molecular epidemiology and BH4-responsiveness in patients with phenylalanine hydroxylase deficiency from Galicia region of Spain

Knowledge of hyperphenylalaninemia (HPA) mutational spectrum in a population allows in many cases an accurate prediction of the phenotype and tetrahydrobiopterin (BH4) responsiveness, thus selecting an adequate treatment. In this work, we have performed the molecular characterization of 105 HPA patients from Galicia, in the northwest region of Spain, evaluating their phenotype and BH4 response. The mutational spectrum analysis showed 47 distinct mutations in 89 families, 37 of them (78.7%) corresponding to missense mutations. Six mutations account for 47.2% of all the investigated alleles, each one with a frequency ≥ 5% (IVS10-11G>A, p.R261Q, p.V388M, p.R176L, p.E280K, p.A300S). The most prevalent HPA mutations in Galicia are the common Mediterranean mutation IVS10-11G>A and p.R261Q, with frequencies of 13.8% and 10.5%, respectively. One novel mutation (p.K361Q; c.1081A>C) was also reported. Although a good genotype-phenotype correlation is observed, there is no exact correlation for some genotypes involving mutations p.R261Q, p.I65T or IVS10-11G>A. Forty seven patients were monitored for post-challenge BH4, establishing genotype-based predictions of BH4-responsiveness in all of them. All phenylketonuric patients with 2 nonresponsive mutations were unresponsive to BH4 and patients with mutations previously associated with BH4 responsiveness in the two alleles had a clear positive response to the test, with the exception of 5 patients with mutations p.R261Q, p.I65T and p.R68S. Our study supports a similar degree of heterogeneity of the HPA mutation spectrum in Galicia compared to reported data from Southern Europe. Patients carrying null mutations in both alleles showed the highest degree of concordance with the most severe phenotypes. Genotype is a good predictor of BH4 response.

Pitfalls in phenylalanine loading test in the diagnosis of dopa-responsive dystonia

Phenylalanine (Phe) loading test is a useful tool in the differential diagnosis of dopa-responsive dystonia due to autosomal dominant or recessive GTP cyclohydrolase I (GTPCH) deficiency or autosomal recessive sepiapterin reductase (SR) deficiency. In these patients hepatic phenylalanine hydroxylase system is compromised due to subnormal tetrahydrobiopterin (BH(4)) levels and hydroxylation of phenylalanine (Phe) to tyrosine (Tyr) is reduced with elevated Phe/Tyr ratio 1-2 h after oral Phe administration (100 mg/kg bw) administration. In healthy persons there is only a modest increase in Tyr production and blood Phe normalizes after 4 h. We report on a challenge with Phe (100 mg/kg bw) in a patient with dopa-responsive dystonia while on therapy with BH(4) and l-dopa. During Phe challenge Phe concentration remained below the normal range while a transient mild hypertyrosinemia was observed, leading to an extremely low Phe/Tyr ratio. A repeated test, after BH(4) withdrawal, reversed the findings and resulted normal. These data suggest activation of hepatic phenylalanine hydroxylase by BH(4). Thus, the Phe loading test should not be performed during substitution with BH(4).

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.

Long-term treatment with tetrahydrobiopterin in phenylketonuria: treatment strategies and prediction of long-term responders

Tetrahydrobiopterin (BH4) responsive phenylketonuria has been described more than 10 years ago. However, criteria for the identification of long-term BH4 responsive patients are not yet established. 116 patients with phenylketonuria, aged 4-18 years, were screened for potential long-term BH4 responsiveness by at least two of the following criteria: positive neonatal BH4 loading test, putative BH4 responsive genotype, and/or milder phenotype. Patients had to be on permanent dietary treatment. 23 patients fulfilled these criteria and were tested for long-term BH4 responsiveness: 18/23 were long-term BH4 responsive, 5/23 were not. On long-term BH4 treatment over a period of 48 ± 27 months in a dose of 14.9 ± 3.3mg/kg/day phenylalanine tolerance was increased from 452 ± 201 mg/day to 1593 ± 647 mg/day, corresponding to a mean increase of 1141 ± 528 mg/day. Dietary phenylalanine intake was increased stepwise according to a clear defined protocol. In 8/18 patients, diet was completely liberalized; 10/18 patients still received phenylalanine-free amino acid formula with 0.63 ± 0.23 g/kg/day. The most predictive value for long-term BH4 responsiveness was the combination of pretreatment phenylalanine of < 1200 μmol/L, pretreatment phenylalanine/tyrosine ratio of <15, phenylalanine/tyrosine ratio of <15 on treatment, phenylalanine tolerance of >20mg/kg/day at age 3 years, positive neonatal BH4 loading, and at least one putative BH4 responsive mutation (p = 0.00024). Our data show that long-term BH4 responsiveness may be predicted already during neonatal period by determining maximum pretreatment phenylalanine and phenylalanine/tyrosine concentrations, neonatal BH4 loading and PAH genotype. A clear defined protocol is necessary to install long-term BH4 treatment.

Assessment of tetrahydrobiopterin (BH(4))-responsiveness and spontaneous phenylalanine reduction in a phenylalanine hydroxylase deficiency population

A BH(4) loading test was performed in 36 patients from 34 unrelated families. The patients had 29 different genotypes, and previous data on only eight of them were found in the BIOPKU database. Thirteen patients were classified as classic PKU (35.1%), 14 as mild PKU (37.8%) and 9 as MHP (27.0%). Blood Phe levels were shown to reach a plateau after three full days of increased natural protein ingestion. Measuring the 24-hour blood Phe levels (T(-24), T(-16), T(0)) on the fourth day of increased protein ingestion before BH(4) administration showed that within 24h Phe on average increased by 2.4% in MHP patients, decreased by 2.7% in mild PKU patients and increased by 9.7% in classic PKU patients (NS for all comparisons); Phe only slightly decreased in responders by 0.2% but increased in non-responders by 7.8% (P>0.05). Altogether, 16 of 36 (44.4%) patients represented by 12 of 29 (41.4%) different genotypes were proven to be BH(4) responders, and four (10.8%) were slow-responders. Responders were 6/9 (66.7%) MHP patients, 10/14 (71.4%) mild PKU patients and 0/13 classic PKU patients. Twenty of the 29 (68.9%) genotypes harbored at least one mutation with a known PRA of 10% or more but only 11 (55%) of them were BH(4)-responsive. Spontaneous reduction of blood Phe levels within 24h on the fourth day of natural protein loading was observed only in mild PKU patients and was shown not to be an important part of the BH(4)-response. 73.3% of genotypes containing at least one allele with a PRA of at least 30% were found to be BH(4) responsive; a PRA of at least 15.5% was needed for the responder genotype in our population.

The spectrum of mutations identified in Cypriot patients with phenylalanine hydroxylase deficiency detected through neonatal screening

OBJECTIVES

The purpose of this study was to identify the mutations responsible for phenylalanine hydroxylase deficiency in Cypriot patients detected through neonatal screening.

DESIGN AND METHODS

Analysis of the PAH gene was performed by direct sequencing of the patients' genomic DNA, MLPA analysis and real-time PCR.

RESULTS

Among 22 independent alleles thirteen previously described mutations were detected (detection rate 100%), all in compound heterozygosity: p.Arg395Gly (18.2%), c.168+5G>C (13.6%), p.EX3del (9%), c.1066-11G>A (9%), p.Ala403Val (9%), p.Glu178Gly (9%), p.Ser70Pro (4.5%), p.Arg241His (4.5%), p.Phe55fs (4.5%), p.Arg158Gln (4.5%), p.Asp222Gly (4.5%), p.Ala300Ser (4.5%), p.Pro225Thr (4.5%). Of the ten different genotypes, three have been previously reported to be associated with a mild clinical phenotype and to respond to tetrahydrobiopterin (BH₄) administration.

CONCLUSIONS

Marked genetic heterogeneity was found in Cypriot patients with hyperphenylalaninemia with two mutations accounting for 32% of the alleles. Most of the mutations detected have been found in other European and Mediterranean populations.

Novel pharmacological chaperones that correct phenylketonuria in mice

Phenylketonuria (PKU) is caused by inherited phenylalanine-hydroxylase (PAH) deficiency and, in many genotypes, it is associated with protein misfolding. The natural cofactor of PAH, tetrahydrobiopterin (BH(4)), can act as a pharmacological chaperone (PC) that rescues enzyme function. However, BH(4) shows limited efficacy in some PKU genotypes and its chemical synthesis is very costly. Taking an integrated drug discovery approach which has not been applied to this target before, we identified alternative PCs for the treatment of PKU. Shape-focused virtual screening of the National Cancer Institute's chemical library identified 84 candidate molecules with potential to bind to the active site of PAH. An in vitro evaluation of these yielded six compounds that restored the enzymatic activity of the unstable PAHV106A variant and increased its stability in cell-based assays against proteolytic degradation. During a 3-day treatment study, two compounds (benzylhydantoin and 6-amino-5-(benzylamino)-uracil) substantially improved the in vivo Phe oxidation and blood Phe concentrations of PKU mice (Pah(enu1)). Notably, benzylhydantoin was twice as effective as tetrahydrobiopterin. In conclusion, we identified two PCs with high in vivo efficacy that may be further developed into a more effective drug treatment of PKU.

Regular exercise prevents oxidative stress in the brain of hyperphenylalaninemic rats

Phenylketonuria (PKU) is caused by deficiency of phenylalanine hydroxylase, leading to accumulation of phenylalanine and its metabolites. Clinical features of PKU patients include mental retardation, microcephaly, and seizures. Oxidative stress has been found in these patients, and is possibly related to neurophysiopatology of PKU. Regular exercise can leads to adaptation of antioxidant system, improving its capacity to detoxification reactive species. The aim of this study was to verify the effects of regular exercise on oxidative stress parameters in the brain of hyperphenylalaninemic rats. Animals were divided into sedentary (Sed) and exercise (Exe) groups, and subdivided into saline (SAL) and hyperphenylalaninemia (HPA). HPA groups were induced HPA through administration of alpha-methylphenylalanine and phenylalanine for 17 days, while SAL groups (n = 16-20) received saline. Exe groups conducted 2-week aerobic exercise for 20 min/day. At 18th day, animals were killed and the brain was homogenized to determine thiobarbituric acid reactives substances (TBA-RS) content, superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) activities. Soleus muscles were collected to determine glycogen content as a marker of oxidative adaptation. Exe groups showed enhanced glycogen content. HPA condition caused an increase in TBA-RS and SOD, and reduces CAT and GPx. Exercise was able to prevent all changes seen in the HPA group, reaching control values, except for SOD activity. No changes were found in the ExeSAL group compared to SedSAL. Hyperphenylalaninemic rats were more responsive to the benefits provided by regular exercise. Physical training may be an interesting strategy to restore the antioxidant system in HPA.

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.

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.

New insights into tetrahydrobiopterin pharmacodynamics from Pah enu1/2, a mouse model for compound heterozygous tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency

Phenylketonuria (PKU), an autosomal recessive disease with phenylalanine hydroxylase (PAH) deficiency, was recently shown to be a protein misfolding disease with loss-of-function. It can be treated by oral application of the natural PAH cofactor tetrahydrobiopterin (BH(4)) that acts as a pharmacological chaperone and rescues enzyme function in vivo. Here we identified Pah(enu1/2) bearing a mild and a severe mutation (V106A/F363S) as a new mouse model for compound heterozygous mild PKU. Although BH(4) treatment has become established in clinical routine, there is substantial lack of knowledge with regard to BH(4) pharmacodynamics and the effect of the genotype on the response to treatment with the natural cofactor. To address these questions we applied an elaborate methodological setup analyzing: (i) blood phenylalanine elimination, (ii) blood phenylalanine/tyrosine ratios, and (iii) kinetics of in vivo phenylalanine oxidation using (13)C-phenylalanine breath tests. We compared pharmacodynamics in wild-type, Pah(enu1/1), and Pah(enu1/2) mice and observed crucial differences in terms of effect size as well as effect kinetics and dose response. Results from in vivo experiments were substantiated in vitro after overexpression of wild-type, V106A, and F263S in COS-7 cells. Pharmacokinetics did not differ between Pah(enu1/1) and Pah(enu1/2) indicating that the differences in pharmacodynamics were not induced by divergent pharmacokinetic behavior of BH(4). In conclusion, our findings show a significant impact of the genotype on the response to BH(4) in PAH deficient mice. This may lead to important consequences concerning the diagnostic and therapeutic management of patients with PAH deficiency underscoring the need for individualized procedures addressing pharmacodynamic aspects.

Phenylketonuria management from an European perspective: a commentary

Phenylketonuria is discussed from an European perspective, addressing the need of common definitions of terms commonly used, the need of a world-wide guideline on the diagnosis and treatment of phenylketonuria, the differences between existing European guidelines, and day-to-day care, further directives for the near future, and changing the concept from compliance to concordance, in which patients have a more clearly defined responsibility.

Pahenu1 is a mouse model for tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency and promotes analysis of the pharmacological chaperone mechanism in vivo

The recent approval of sapropterin dihydrochloride, the synthetic form of 6[R]-l-erythro-5,6,7,8-tetrahydrobiopterin (BH(4)), for the treatment of phenylketonuria (PKU) as the first pharmacological chaperone drug initiated a paradigm change in the treatment of monogenetic diseases. Symptomatic treatment is now replaced by a causal pharmacological therapy correcting misfolding of the defective phenylalanine hydroxylase (PAH) in numerous patients. Here, we disclose BH(4) responsiveness in Pah(enu1), a mouse model for PAH deficiency. Loss of function resulted from loss of PAH, a consequence of misfolding, aggregation, and accelerated degradation of the enzyme. BH(4) attenuated this triad by conformational stabilization augmenting the effective PAH concentration. This led to the rescue of the biochemical phenotype and enzyme function in vivo. Combined in vitro and in vivo analyses revealed a selective pharmaceutical action of BH(4) confined to the pathological metabolic state. Our data provide new molecular-level insights into the mechanisms underlying protein misfolding with loss of function and support a general model of pharmacological chaperone-induced stabilization of protein conformation to correct this intracellular phenotype. Pah(enu1) will be essential for pharmaceutical drug optimization and to design individually tailored therapies.

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.

Blood phenylalanine concentrations in patients with PAH-deficient hyperphenylalaninaemia off diet without and with three different single oral doses of tetrahydrobiopterin: assessing responsiveness in a model of statistical process control

Tetrahydrobiopterin (BH(4)) cofactor loading is a standard procedure to differentiate defects of BH(4) metabolism from phenylalanine hydroxylase (PAH) deficiency. BH(4) responsiveness also exists in PAH-deficient patients with high residual PAH activity. Unexpectedly, single cases with presumed nil residual PAH activity have been reported to be BH(4) responsive, too. BH(4) responsiveness has been defined either by a >or=30% reduction of blood Phe concentration after a single BH(4) dose or by a decline greater than the individual circadian Phe level variation. Since both methods have methodological disadvantages, we present a model of statistical process control (SPC) to assess BH(4) responsiveness. Phe levels in 17 adult PKU patients of three phenotypic groups off diet were compared without and with three different single oral dosages of BH(4) applied in a double-blind randomized cross-over design. Results are compared for >or=30% reduction and SPC. The effect of BH(4) by >or=30% reduction was significant for groups (p < 0.01) but not for dose (p = 0.064), with no interaction of group with dose (p = 0.24). SPC revealed significant effects for group (p < 0.01) and the interaction for group with dose (p < 0.05) but not for dose alone (p = 0.87). After one or more loadings, seven patients would be judged to be BH(4) responsive either by the 30% criterion or by the SPC model, but only three by both. Results for patients with identical PAH genotype were not very consistent within (for different BH(4) doses) and between the two models. We conclude that a comparison of protein loadings without and with BH(4) combined with a standardized procedure for data analysis and decision would increase the reliability of diagnostic results.

Genotype-predicted tetrahydrobiopterin (BH4)-responsiveness and molecular genetics in Croatian patients with phenylalanine hydroxylase (PAH) deficiency

Specific mutations in the gene encoding phenylalanine hydroxylase (PAH), located on chromosome 12q22-24.1, are linked to tetrahydrobiopterin (BH4; sapropterin)-responsive phenylketonuria (PKU). Diagnosis is usually done through the newborn screening for PKU, followed by a BH4 loading test. So far, more than 60 mutant alleles, presenting with a substantial residual PAH activity (average approximately 47%), were identified in more than 500 patients worldwide. We investigated the predictive value of BH4-responsive PAH mutations in Croatian population. From a group of 127 PKU patients, 62 were selected (based on the genotype) as potentially BH4-responsive and 39 loaded with BH4 (20 mg/kg). The overall frequency of BH4-responsiveness (>30% blood phenylalanine reduction within 24 h) was 36% (14 out of 39 patients with 23 different genotypes), significantly less than expected. The best responders were patients with mild hyperphenylalaninemia (4/4; 100%), followed by mild PKU (8/9; 89%), and classical PKU (2/26; 8%). The most common BH(4)-responsive genotypes were p.E390G/p.R408W and p.P281L/p.E390G. These genotypes correspond for approximately >30% residual PAH activity. The p.E390G mutation was 100% associated with BH4-responsiveness, regardless of the second allele (p.R408W, p.P281L, p.F55Lfs, p.L249P). With regard to the predicted relative PAH activity of recombinantly expressed mutant alleles, there was a significant (p<0.002) difference between BH4-responders and non-responders. In a general Croatian PKU population, disease-causing mutations were identified on 226 alleles (99%). There were 35 different mutations: 21 missense, 8 splice site, 3 nonsense, 2 single nucleotide deletions, and 1 in-frame deletion. Four mutations are reported for the first time: p.E76D, p.L333P, p.G346E, and IVS8-2A>G. Five mutations accounted for over two-thirds of investigated alleles: p.L48S, p.R261Q, p.P281L, p.E390G, and p.R408W. Thus, the Croatian PKU population seems to be more homogenous than some other Mediterranean or Central European populations. This study reveals the importance of a full genotype for the prediction of BH4-responsiveness. In contrast to previous assumption and with exception of the p.E390G mutation, single allele mutations are not reliable for the selection of potential PKU candidates for pharmacological therapy with BH4.

Protein substitute for children and adults with phenylketonuria

BACKGROUND

Phenylketonuria is an inherited metabolic disorder characterised by an absence or deficiency of the enzyme phenylalanine hydroxylase. The aim of treatment is to lower blood phenylalanine concentrations to the recommended therapeutic range to prevent developmental delay and support normal growth. Current treatment consists of a low-phenylalanine diet in combination with a protein substitute which is free from or low in phenylalanine. Guidance regarding the use, dosage, and distribution of dosage of the protein substitute over a 24-hour period is unclear, and there is variation in recommendations among treatment centres.

OBJECTIVES

To assess the benefits and adverse effects of protein substitute, its dosage, and distribution of dose in children and adults with phenylketonuria who are adhering to a low-phenylalanine diet.

SEARCH STRATEGY

We searched the Cochrane Cystic Fibrosis and Genetic Disorders Group Trials Register which consists of references identified from comprehensive electronic database searches and hand searches of relevant journals and abstract books of conference proceedings. We also contacted manufacturers of the phenylalanine-free and low-phenylalanine protein substitutes for any data from published and unpublished randomised controlled trials.Date of the most recent search of the Group's Trials Register: April 2008.

SELECTION CRITERIA

All randomised or quasi-randomised controlled trials comparing: any dose of protein substitute with no protein substitute; an alternative dosage; or the same dose, but given as frequent small doses throughout the day compared with the same total daily dose given as larger boluses less frequently.

DATA COLLECTION AND ANALYSIS

Both authors independently extracted data and assessed trial quality.

MAIN RESULTS

Three trials (69 participants) are included in this review. One trial investigated the use of protein substitute in 16 participants, while a further two trials investigated the dosage of protein substitute in a total of 53 participants. Due to issues with data presentation in each trial, described in full in the review, formal statistical analyses of the data were impossible. Investigators are being contacted for further information.

AUTHORS' CONCLUSIONS

No conclusions could be drawn about the short- or long-term use of protein substitute in phenylketonuria due to the lack of adequate or analysable trial data. Additional data and randomised controlled trials are needed to investigate the use of protein substitute in phenylketonuria. Until further evidence is available, current practice in the use of protein substitute should continue to be monitored with care.

Evaluation of neonatal BH4 loading test in neonates screened for hyperphenylalaninemia

BACKGROUND

The outcome in phenylketonuria is related to the early diagnosis and management due to neonatal screening.

AIMS

To assess the interest of tetrahydrobiopterin (BH4) loading test and phenylalanine hydroxylase (PAH) genotyping in the management of neonates with hyperphenylalaninemia (HPA).

STUDY DESIGN

We evaluate the effectiveness of a BH4 loading test (20 mg/kg) in ten neonates screened for HPA. We evaluated the time required to reach a target plasma Phenylalanine (Phe) level below 300 micromol/l. We compared these ten BH4-loaded patients to the 10 previous neonates non-loaded with BH4. In all these patients, the PAH genotype was determined.

RESULTS

One loaded patient had biopterin synthesis deficiency and has been retrieved from statistical analysis. All others patients have PAH deficiency. Between the BH4 loaded group (L) and the BH4 non-loaded group (NL), a statistically significant difference was observed in the average time required to reached the target Phe level (13.56 +/- 4.30 (L) vs. 20.6 +/- 7.59 days (NL) [p < 0.02]). Results of the genotyping from all but one of these 19 patients indicated that among all mutations present in this patient population, there were 4 known PAH mutations associated with BH4 responsiveness (p.R261Q, the p.V388 M, the p.E390G and the p.Y414C). These mutations were found in 4 non-loaded and 6 loaded patients. Two patients had a more than 90% reduction in their plasma Phe level within 24 h after the load. One of these patients had a PTPS deficiency. The other fully responsive patient (p.Y414C and IVS10-11G>A) has been treated with BH4 from birth with an excellent metabolic control for three years now.

CONCLUSION

BH4 loading test improves the management of HPA. It allows an immediate identification of the children fully responsive to BH4. Our results therefore suggest the incorporation of BH4 loading test in the management of neonates screened for HPA.

Loss of function in phenylketonuria is caused by impaired molecular motions and conformational instability

A significant share of patients with phenylalanine hydroxylase (PAH) deficiency benefits from pharmacological doses of tetrahydrobiopterin (BH(4)), the natural PAH cofactor. Phenylketonuria (PKU) is hypothesized to be a conformational disease, with loss of function due to protein destabilization, and the restoration of enzyme function that is observed in BH(4) treatment might be transmitted by correction of protein misfolding. To elucidate the molecular basis of functional impairment in PAH deficiency, we investigated the impact of ten PAH gene mutations identified in patients with BH(4)-responsiveness on enzyme kinetics, stability, and conformation of the protein (F55L, I65S, H170Q, P275L, A300S, S310Y, P314S, R408W, Y414C, Y417H). Residual enzyme activity was generally high, but allostery was disturbed in almost all cases and pointed to altered protein conformation. This was confirmed by reduced proteolytic stability, impaired tetramer assembly or aggregation, increased hydrophobicity, and accelerated thermal unfolding--with particular impact on the regulatory domain--observed in most variants. Three-dimensional modeling revealed the involvement of functionally relevant amino acid networks that may communicate misfolding throughout the protein. Our results substantiate the view that PAH deficiency is a protein-misfolding disease in which global conformational changes hinder molecular motions essential for physiological enzyme function. Thus, PKU has evolved from a model of a genetic disease that leads to severe neurological impairment to a model of a treatable protein-folding disease with loss of function.

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.

Sapropterin dihydrochloride (Kuvan/phenoptin), an orally active synthetic form of BH4 for the treatment of phenylketonuria

Phenylketonuria (PKU) and mild hyperphenylalaninemia (HPA) are genetic disorders characterized by a deficiency in phenylalanine hydroxylase (PAH), resulting in intellectual impairment if not treated with dietary restriction of phenylalanine intake. Sapropterin dihydrochloride (Kuvan) is an orally active synthetic form of (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4; a cofactor for PAH) that has received Orphan Drug status and Fast Track designation for the treatment of PKU. Phase II and III clinical data demonstrated that Kuvan was a safe and effective therapy in selected patients with HPA and mild-to-moderate PKU who responded to a BH4 loading test. Based on the clinical data, BioMarin Pharmaceutical Inc has estimated that Kuvan could be a potential treatment option for 30 to 50% of the estimated 50,000 patients in the developed world who have been diagnosed with PKU. According to Thomson Scientific's Strategic Drugs Database (SDdb), the worldwide consensus forecast values for Kuvan are approximately US $120, 190 and 260 million for 2008, 2009 and 2010, respectively.

Effects and clinical significance of tetrahydrobiopterin supplementation in phenylalanine hydroxylase-deficient hyperphenylalaninaemia

In recent years several studies on tetrahydrobiopterin (BH4)-responsive phenylalanine hydroxylase (PAH) deficiency have been published. The molecular mechanisms of BH4 responsiveness are not conclusively understood, but there is evidence that BH4 responsiveness in hyperphenylalaninaemia (HPA) depends on the patient's genotype and residual PAH activity. As a BH4 preparation will soon obtain marketing approval as an alternative treatment for phenylketonuria (PKU), it is particularly important to evaluate this treatment and to define criteria to identify patients with a potential benefit from it. Most of the patients found to be BH4-responsive suffered from mild PKU or mild hyperphenylalaninaemia (MHP) and some of these would not be treated at all in many countries. Of patients with moderate and classic forms of PKU, only a few were classified as responders and the clinical significance of the effect size may be small.

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.

Analysis of the effect of tetrahydrobiopterin on PAH gene expression in hepatoma cells

Tetrahydrobiopterin (BH4)-responsive phenylalanine hydroxylase (PAH) deficiency is a recently recognized variant of phenylketonuria, with a probable multifactorial molecular basis. In this study we have investigated the effect of BH4 on PAH gene expression in human hepatoma. Our results show that increased BH4 levels result in an enhancement of PAH activity and PAH protein, due to longer turnover rates, while PAH mRNA levels remain unchanged. This was confirmed for mutant PAH proteins (A309V, V388M and Y414C) associated to in vivo BH4 responsiveness, validating previous studies. We can conclude that there is no effect of the cofactor on PAH gene transcription, probably being the chemical chaperone effect of BH4 stabilizing mutant PAH proteins the major underlying mechanism of the response.

Phénylcétonurie

Phenylketonuria is a hereditary metabolic disease, characterized by deficiency of phenylalanine hydroxylase, an enzyme necessary for the transformation of phenylalanine into tyrosine. Untreated, phenylketonuria leads to mental retardation, sometimes profound, as well as hypopigmentation. Dietary phenylalanine restriction allows patients to lead almost normal lives. Phenylalanine is toxic to fetal development and severe disorders occur in the children of women whose phenylketonuria is untreated during pregnancy. These women must be informed that they must plan pregnancy and begin dietary restrictions in the preconceptional period. France has set up routine neonatal screening in view of the incidence of this disease (1/17000 in France) and the existence of effective treatment. Since 1970, approximately 1600 infants with phenylketonuria have thus been diagnosed and treated. Strict metabolic control is necessary during the first 10 years of life, after which the diet can be progressively enlarged. Dietary restriction must resume before any pregnancy. Advances in treatment: a study published in 2002 showed that some patients deficient in phenylalanine hydroxylase are sensitive to pharmacological doses of tetrahydrobiopterin (BH4), a cofactor of this 'enzyme essential to the transformation of phenylalanine into tyrosine. Some patients treated by this cofactor have normal levels of phenylalanine intake. While only a few patients have so far received this alternative treatment, intermediate and long-term experiments are currently being evaluated.

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.