Molecular Genetics and Genotype-Based Estimation of BH4-Responsiveness in Serbian PKU Patients: Spotlight on Phenotypic Implications of p.L48S

High-Resolution Haplotyping of the PAH Gene Enables Early Gestation Noninvasive Prenatal Diagnosis of Phenylketonuria and Evolution Analysis of Recurrent Pathogenic Variations

BACKGROUND

The clinical performance of RHDO-based NIPD for PKU during early gestation remains under-evaluated. Furthermore, studies focused on SNP loci obtained by next-generation sequencing to analyze the genetic evolution of pathogenic variations in PKU is limited.

METHODS

Maternal peripheral blood, along with proband and paternal samples, was collected between 7 and 12 weeks of gestation. The PAH gene and surrounding high heterozygosity SNPs were targeted for enrichment and sequencing. Fetal genotypes were inferred using RHDO-based NIPD. High-resolution PAH haplotypes were used for the analysis of two common pathogenic variants in the Chinese population: c.728G>A and c.1238G>C.

RESULTS

Sixty one PKU families participated with an average fetal fraction of 6.08%. The median gestational age was 8+6 weeks. RHDO-based NIPD successfully identified fetal genotypes in 59 cases (96.72%, 59/62). Two cases failed because of insufficient informative SNPs. In addition, a recombination event was assessed in one fetus of 59 cases. Six, and three haplotypes were identified for c.728G>A(p.Arg243Gln) and c.1238G>C(p.Arg413Pro), respectively. Hap_3 and hap_8 were identified as the ancestral haplotypes for these pathogenic variants, with other haplotypes arising from mutations or recombination based on these ancestral haplotypes.

CONCLUSIONS

This study validates the feasibility of an RHDO-based assay for NIPD of PKU in early pregnancy and introduces its application in the demonstration of founder effects in recurrent pathogenic variations, offering new insights into the evolutionary analysis of PAH variations.

The Spectrum of Disease-Associated Alleles in Countries with a Predominantly Slavic Population

There are more than 260 million people of Slavic descent worldwide, who reside mainly in Eastern Europe but also represent a noticeable share of the population in the USA and Canada. Slavic populations, particularly Eastern Slavs and some Western Slavs, demonstrate a surprisingly high degree of genetic homogeneity, and, consequently, remarkable contribution of recurrent alleles associated with hereditary diseases. Along with pan-European pathogenic variants with clearly elevated occurrence in Slavic people (e.g., ATP7B c.3207C>A and PAH c.1222C>T), there are at least 52 pan-Slavic germ-line mutations (e.g., NBN c.657_661del and BRCA1 c.5266dupC) as well as several disease-predisposing alleles characteristic of the particular Slavic communities (e.g., Polish SDHD c.33C>A and Russian ARSB c.1562G>A variants). From a clinical standpoint, Slavs have some features of a huge founder population, thus providing a unique opportunity for efficient genetic studies.

Tetrahydrobiopterin responsiveness in Phenylalanine hydroxylase deficient patients from North-east of Iran: Genotype-phenotype correlation, identification of a novel mutation and 7 new responsive genotypes

Phenylalanine hydroxylase enzyme defects result in a hereditary metabolic disorder called phenylketonuria. Sapropterin (tetrahydrobiopterin) is one of the treatment strategies for this disorder. Even though a correlation between genotype and BH4 responsiveness was established by earlier studies, a subset of mutations often presented inconsistent responses and/or phenotypes. Different genetic background is one of the potential reasons for this fact. In this study, the genotype of a total of 34 PAH deficient patients from Khorasan-Razavi providence in the north-east of Iran was obtained. Among this patients, 21 individuals took the 24 h and 48 h BH4 loading test and if the result was positive, their Phenylalanine tolerance was assessed. It is the first study of its type in patients from Iran to evaluate genotype role in predicting the most probable responsive individuals. The known pathogenic variant p.R169P and the novel variant p. Leu72_Asp75delinsTyr were first classified as responsive.Seven genotypes were reported as responsive for the first time. All patients carrying at least one pathogenic variant, which was previously reported as BH4 responsive, respond to BH4. Three patients with p.L48S, p.R261Q and p.A309V pathogenic variants were exceptions. There was no certain statistical correlation between genotype and response. Genotype and phenotype were significantly correlated and majority of patients with mild phenotype carried at least one non-null pathogenic variant. In Khorasan-Razavi province of Iran, patients with at least one non-null mutation are most probable to demonstrate mild phenotype and respond to BH4 phenotype.

Genetic etiology and clinical challenges of phenylketonuria

This review discusses the epidemiology, pathophysiology, genetic etiology, and management of phenylketonuria (PKU). PKU, an autosomal recessive disease, is an inborn error of phenylalanine (Phe) metabolism caused by pathogenic variants in the phenylalanine hydroxylase (PAH) gene. The prevalence of PKU varies widely among ethnicities and geographic regions, affecting approximately 1 in 24,000 individuals worldwide. Deficiency in the PAH enzyme or, in rare cases, the cofactor tetrahydrobiopterin results in high blood Phe concentrations, causing brain dysfunction. Untreated PKU, also known as PAH deficiency, results in severe and irreversible intellectual disability, epilepsy, behavioral disorders, and clinical features such as acquired microcephaly, seizures, psychological signs, and generalized hypopigmentation of skin (including hair and eyes). Severe phenotypes are classic PKU, and less severe forms of PAH deficiency are moderate PKU, mild PKU, mild hyperphenylalaninaemia (HPA), or benign HPA. Early diagnosis and intervention must start shortly after birth to prevent major cognitive and neurological effects. Dietary treatment, including natural protein restriction and Phe-free supplements, must be used to maintain blood Phe concentrations of 120-360 μmol/L throughout the life span. Additional treatments include the casein glycomacropeptide (GMP), which contains very limited aromatic amino acids and may improve immunological function, and large neutral amino acid (LNAA) supplementation to prevent plasma Phe transport into the brain. The synthetic BH4 analog, sapropterin hydrochloride (i.e., Kuvan®, BioMarin), is another potential treatment that activates residual PAH, thus decreasing Phe concentrations in the blood of PKU patients. Moreover, daily subcutaneous injection of pegylated Phe ammonia-lyase (i.e., pegvaliase; PALYNZIQ®, BioMarin) has promised gene therapy in recent clinical trials, and mRNA approaches are also being studied.

Untreated PKU patients without intellectual disability: SHANK gene family as a candidate modifier

Phenylketonuria (PKU) is an inborn error of metabolism caused by variants in the phenylalanine hydroxylase (PAH) gene and it is characterized by excessively high levels of phenylalanine in body fluids. PKU is a paradigm for a genetic disease that can be treated and majority of developed countries have a population-based newborn screening. Thus, the combination of early diagnosis and immediate initiation of treatment has resulted in normal intelligence for treated PKU patients. Although PKU is a monogenic disease, decades of research and clinical practice have shown that the correlation between the genotype and corresponding phenotype is not simple at all. Attempts have been made to discover modifier genes for PKU cognitive phenotype but without any success so far.

We conducted whole genome sequencing of 4 subjects from unrelated non-consanguineous families who presented with pathogenic mutations in the PAH gene, high blood phenylalanine concentrations and near-normal cognitive development despite no treatment. We used cross sample analysis to select genes common for more than one patient. Thus, the SHANK gene family emerged as the only relevant gene family with variants detected in 3 of 4 analyzed patients. We detected two novel variants, p.Pro1591Ala in SHANK1 and p.Asp18Asn in SHANK2, as well as SHANK2:p.Gly46Ser, SHANK2:p.Pro1388_Phe1389insLeuPro and SHANK3:p.Pro1716Thr variants that were previously described. Computational analysis indicated that the identified variants do not abolish the function of SHANK proteins. However, changes in posttranslational modifications of SHANK proteins could influence functioning of the glutamatergic synapses, cytoskeleton regulation and contribute to maintaining optimal synaptic density and number of dendritic spines.

Our findings are linking SHANK gene family and brain plasticity in PKU for the first time. We hypothesize that variant SHANK proteins maintain optimal synaptic density and number of dendritic spines under high concentrations of phenylalanine and could have protective modifying effect on cognitive development of PKU patients.

Manipulation of a cation-π sandwich reveals conformational flexibility in phenylalanine hydroxylase

Phenylalanine hydroxylase (PAH) is an allosteric enzyme that maintains phenylalanine (Phe) below neurotoxic levels; its failure results in phenylketonuria, an inborn error of amino acid metabolism. Wild type (WT) PAH equilibrates among resting-state (RS-PAH) and activated (A-PAH) conformations, whose equilibrium position depends upon allosteric Phe binding. The RS-PAH conformation of WT rat PAH (rPAH) contains a cation-π sandwich involving Phe80 that cannot exist in the A-PAH conformation. Phe80 variants F80A, F80D, F80L, and F80R were prepared and evaluated using native PAGE, size exclusion chromatography, ion exchange behavior, intrinsic protein fluorescence, enzyme kinetics, and limited proteolysis, each as a function of [Phe]. Like WT rPAH, F80A and F80D show allosteric activation by Phe while F80L and F80R are constitutively active. Maximal activity of all variants suggests relief of a rate-determining conformational change. Limited proteolysis of WT rPAH (minus Phe) reveals facile cleavage within a 4-helix bundle that is buried in the RS-PAH tetramer interface, reflecting dynamic dissociation of that tetramer. This cleavage is not seen for the Phe80 variants, which all show proteolytic hypersensitivity in a linker that repositions during the RS-PAH to A-PAH interchange. Hypersensitivity is corrected by addition of Phe such that all variants become like WT rPAH and achieve the A-PAH conformation. Thus, manipulation of Phe80 perturbs the conformational space sampled by PAH, increasing sampling of on-pathway intermediates in the RS-PAH and A-PAH interchange. The behavior of the Phe80 variants mimics that of disease-associated R68S and suggests a molecular basis for proteolytic susceptibility in PKU-associated human PAH variants.

Biophysical characterization of full-length human phenylalanine hydroxylase provides a deeper understanding of its quaternary structure equilibrium

Dysfunction of human phenylalanine hydroxylase (hPAH, EC 1.14.16.1) is the primary cause of phenylketonuria, the most common inborn error of amino acid metabolism. The dynamic domain rearrangements of this multimeric protein have thwarted structural study of the full-length form for decades, until now. In this study, a tractable C29S variant of hPAH (C29S) yielded a 3.06 Å resolution crystal structure of the tetrameric resting-state conformation. We used size-exclusion chromatography in line with small-angle X-ray scattering (SEC-SAXS) to analyze the full-length hPAH solution structure both in the presence and absence of Phe, which serves as both substrate and allosteric activators. Allosteric Phe binding favors accumulation of an activated PAH tetramer conformation, which is biophysically distinct in solution. Protein characterization with enzyme kinetics and intrinsic fluorescence revealed that the C29S variant and hPAH are otherwise equivalent in their response to Phe, further supported by their behavior on various chromatography resins and by analytical ultracentrifugation. Modeling of resting-state and activated forms of C29S against SAXS data with available structural data created and evaluated several new models for the transition between the architecturally distinct conformations of PAH and highlighted unique intra- and inter-subunit interactions. Three best-fitting alternative models all placed the allosteric Phe-binding module 8-10 Å farther from the tetramer center than do all previous models. The structural insights into allosteric activation of hPAH reported here may help inform ongoing efforts to treat phenylketonuria with novel therapeutic approaches.

Genotypes of 2579 patients with phenylketonuria reveal a high rate of BH4 non-responders in Russia

Phenylalanine hydroxylase (PAH) deficiency is responsible for most cases of phenylketonuria (PKU). Furthermore, numerous studies on BH4-sensitive PAH deficiency have been conducted. To date, BH4, a cofactor of PAH, has not been used to treat PKU in Russia.Genotype data of patients with PKU can be used to predict their sensitivity to BH4 therapy. A cohort of 2579 patients with PKU from Russia was analyzed for 25 common PAH gene mutations using custom allele-specific multiplex ligation-dependent probe amplification-based technology. A mutation detection rate of 84.1% chromosomes was accomplished. Both pathogenic alleles were identified in 73.1% of patients. The most frequent pathogenic variants were p.Arg408Trp (50.9%), p.Arg261Gln (5.3%), p.Pro281Leu (3.5%), IVS12+1G>A (3.1%), IVS10-11G>A (2.6%), and p.Arg158Leu (2.4%). The exact boundaries of a PAH exon 5 deletion were defined as EX5del4154ins268 (c.442-2913_509+1173del4154ins268). Severe phenotypes prevailed in the cohort, and classical PKU was observed in 71.8% cases. Due to the genotype-based prediction, 55.9% of the probands were non-responders to the BH4-treatment, and 20.2% were potential responders. Analysis of genotype data is useful to predict BH4 response in PKU patients. The high rate of non-responders among Russian patients was due to the high allele frequency of severe PAH mutations.

Functional Characterization of Novel Phenylalanine Hydroxylase p.Gln226Lys Mutation Revealed Its Non-responsiveness to Tetrahydrobiopterin Treatment in Hepatoma Cellular Model

Treatment with tetrahydrobiopterin (BH4) is the latest therapeutic option approved for patients with phenylketonuria (PKU)-one of the most frequent inborn metabolic diseases. PKU or phenylalanine hydroxylase (PAH) deficiency is caused by mutations in the PAH gene. Given that some PAH mutations are responsive to BH4 treatment while others are non-responsive, for every novel mutation that is discovered it is essential to confirm its pathogenic effect and to assess its responsiveness to a BH4 treatment in vitro, before the drug is administered to patients. We found a c.676C>A (p.Gln226Lys) mutation in the PAH gene in two unrelated patients with PKU. The corresponding aberrant protein has never been functionally characterized in vitro and its response to BH4 treatment is unknown. Computational analyses proposed that glutamine at position 226 is an important, evolutionary conserved amino acid while the substitution with lysine probably disturbs tertiary protein structure and impacts posttranslational PAH modifications. Using hepatoma cellular model, we demonstrated that the amount of mutant p.Gln226Lys PAH detected by Western blot was only 1.2% in comparison to wild-type PAH. The addition of sepiapterin, intracellular precursor of BH4, did not increase PAH protein yield thus marking p.Gln226Lys as BH4-non-responsive mutation. Therefore, computational, experimental, and clinical data were all in accordance showing that p.Gln226Lys is a severe pathogenic PAH mutation. Its non-responsiveness to BH4 treatment in hepatoma cellular model should be considered when deciding treatment options for PKU patients carrying this mutation. Consequently, our study will facilitate clinical genetic practice, particularly genotype-based stratification of PKU treatment.

Analysis of the genotype-phenotype correlation in patients with phenylketonuria in mainland China

Mutations in the gene encoding phenylalanine hydroxylase (PAH) are associated with various degrees of phenylketonuria (PKU). The aim of our study was to define the genotype-phenotype correlations of mutations in the PAH gene that cause phenylketonuria (PKU) among the Chinese mainland population. Mutations in the PAH gene were analysed by next-generation sequencing, and a genotype-phenotype correlation analysis was performed in 1079 patients. Fifteen “null + null” genotypes, including four homoallelic and eleven heteroallelic genotypes, were clearly associated with classic PKU. Five functionally hemizygous (p.E280K, p.R252Q, p.E56D, p.S310F and p.T372R) and four compound heterozygous (p.T278I/p.S359L, p.R408W/p.R243Q, p.F161S/p.R243Q and p.F161S/p.R413P) genotypes were clearly associated with classic PKU. Ten functionally hemizygous genotypes, p.G257V, p.R158W, p.L255S, p.G247V, p.F161S, p.R158Q, p.V388M, p.I65T, p.I324N and p.R400K, were frequently associated with classic PKU. Three functionally hemizygous genotypes, p.P147L, p.I95del and p.F331S, and four compound heterozygous genotypes, p.G257V/p.R408Q, p.A434D/p.R413P, p.R243Q/p.A47E and p.R241C/p.G239D, were consistently correlated with mild PKU. Three functionally hemizygous genotypes, p.H107R, p.Q419R and p.F392I, and nine compound heterozygous genotypes (p.G312V/p.R241C, p.R243Q/p.V230I, p.R243Q/p.A403V, p.R243Q/p.Q419R, p.R243Q/p.R53H, p.R243Q/p.H107R, p.R241C/p.R408Q, p.R241C/p.H220P and p.R53H/p.R400K) were consistent with mild hyperphenylalaninaemia (MHP). Our study provides further support for the hypothesis that the PAH genotype is the main factor that determines the phenotype of PKU.

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.

New protein structures provide an updated understanding of phenylketonuria

Phenylketonuria (PKU) and less severe hyperphenylalaninemia (HPA) constitute the most common inborn error of amino acid metabolism, and is most often caused by defects in phenylalanine hydroxylase (PAH) function resulting in accumulation of Phe to neurotoxic levels. Despite the success of dietary intervention in preventing permanent neurological damage, individuals living with PKU clamor for additional non-dietary therapies. The bulk of disease-associated mutations are PAH missense variants, which occur throughout the entire 452 amino acid human PAH protein. While some disease-associated mutations affect protein structure (e.g. truncations) and others encode catalytically dead variants, most have been viewed as defective in protein folding/stability. Here we refine this view to address how PKU-associated missense variants can perturb the equilibrium among alternate native PAH structures (resting-state PAH and activated PAH), thus shifting the tipping point of this equilibrium to a neurotoxic Phe concentration. This refined view of PKU introduces opportunities for the design or discovery of therapeutic pharmacological chaperones that can help restore the tipping point to healthy Phe levels and how such a therapeutic might work with or without the inhibitory pharmacological chaperone BH4. Dysregulation of an equilibrium of architecturally distinct native PAH structures departs from the concept of "misfolding", provides an updated understanding of PKU, and presents an enhanced foundation for understanding genotype/phenotype relationships.

The Spectrum of PAH Mutations and Increase of Milder Forms of Phenylketonuria in Sweden During 1965-2014

Newborn screening (NBS) for phenylketonuria (PKU) which has a continuum of disease severities has been performed for more than 50 years. The screening method has undergone a continuous development with not only improvements of the positive predictive value but also identification of milder forms of the disease. With the introduction of genetic testing the confirmation of the diagnosis has improved. The Swedish NBS is centralized to one laboratory, which also performs confirmatory testing.Here we present the results of NBS for PKU in Sweden during 1965-2014 describing an increase in diagnosed patients and a shift in the spectrum of phenylalanine hydroxylase (PAH) mutations towards an increasing heterogeneity. Milder mutations common in southern Europe and the Middle East together with lowering of the recall level for phenylalanine (Phe) have led to a shift towards milder phenotypes among the patients identified by the screening program. The inclusion of a Phe and tyrosine (Tyr) ratio as an additional marker has improved the positive predictive value to the present 0.92. Also discussed is what impact earlier sampling has had on the prediction of disease severity, concluding that the shift of age at sampling from 72 to 48 h does not increase the risk of missing patients in need of treatment.

New PAH gene promoter KLF1 and 3'-region C/EBPalpha motifs influence transcription in vitro

Phenylketonuria (PKU) is a metabolic disease caused by mutations in the phenylalanine hydroxylase (PAH) gene. Although the PAH genotype remains the main determinant of PKU phenotype severity, genotype-phenotype inconsistencies have been reported. In this study, we focused on unanalysed sequences in non-coding PAH gene regions to assess their possible influence on the PKU phenotype. We transiently transfected HepG2 cells with various chloramphenicol acetyl transferase (CAT) reporter constructs which included PAH gene non-coding regions. Selected non-coding regions were indicated by in silico prediction to contain transcription factor binding sites. Furthermore, electrophoretic mobility shift assay (EMSA) and supershift assays were performed to identify which transcriptional factors were engaged in the interaction. We found novel KLF1 motif in the PAH promoter, which decreases CAT activity by 50 % in comparison to basal transcription in vitro. The cytosine at the c.-170 promoter position creates an additional binding site for the protein complex involving KLF1 transcription factor. Moreover, we assessed for the first time the role of a multivariant variable number tandem repeat (VNTR) region located in the 3'-region of the PAH gene. We found that the VNTR3, VNTR7 and VNTR8 constructs had approximately 60 % of CAT activity. The regulation is mediated by the C/EBPalpha transcription factor, present in protein complex binding to VNTR3. Our study highlighted two novel promoter KLF1 and 3'-region C/EBPalpha motifs in the PAH gene which decrease transcription in vitro and, thus, could be considered as PAH expression modifiers. New transcription motifs in non-coding regions will contribute to better understanding of the PKU phenotype complexity and may become important for the optimisation of PKU treatment.

Molecular characterisation of phenylketonuria in a Chinese mainland population using next-generation sequencing

Phenylketonuria (PKU) is an inherited autosomal recessive disorder of phenylalanine metabolism, mainly caused by a deficiency of phenylalanine hydroxylase (PAH). The incidence of various PAH mutations differs among race and ethnicity. Here we report a spectrum of PAH mutations complied from 796 PKU patients from mainland China. The all 13 exons and adjacent intronic regions of the PAH gene were determined by next-generation sequencing. We identified 194 different mutations, of which 41 are not reported before. Several mutations reoccurred with high frequency including p.R243Q, p.EX6-96A > G, p.V399V, p.R241C, p.R111*, p.Y356*, p.R413P, and IVS4-1G > A. 76.33% of mutations were localized in exons 3, 6, 7, 11, 12. We further compared the frequency of each mutation between populations in northern and southern China, and found significant differences in 19 mutations. Furthermore, we identified 101 mutations that are not reported before in Chinese population, our study thus broadens the mutational spectrum of Chinese PKU patients. Additionally, 41 novel mutations will expand and improve PAH mutation database. Finally, our study offers proof that NGS is effective, reduces screening times and costs, and facilitates the provision of appropriate genetic counseling for PKU patients.

Phenylketonuria screening and management in southeastern Europe - survey results from 11 countries

BACKGROUND

We aimed to assess the current state of PKU screening and management in the region of southeastern Europe.

METHODS

A survey was performed involving all identified professionals responsible for the PKU management in the 11 countries from South-Eastern region of Europe (Albania, Bulgaria, Bosnia and Herzegovina, Croatia, Kosovo, Macedonia, Moldova, Montenegro, Romania, Serbia, Slovenia). The questionnaire was designed to assess the characteristics regarding PKU management in three main areas: nation-wide characteristics, PKU screening, and characteristics of the PKU management in the responding centre. It consisted of 56 questions. The distribution and collection of the questionnaires (via e-mail) was taking place from December 2013 to March 2014.

RESULTS

Responses from participants from 11 countries were included; the countries cumulative population is approx. 52.5 mio. PKU screening was not yet introduced in 4 of 11 countries. Reported PKU incidences ranged from 1/7325 to 1/39338 (and were not known for 5 countries). National PKU guidelines existed in 5 of 11 countries and 7 of 11 countries had PKU registry (registries included 40 to 194 patients). The number of PKU centers in each country varied from 1 to 6. Routine genetic diagnostics was reported in 4 of 11 countries. Most commonly used laboratory method to assess phenylalanine levels was fluorometric. Tetrahydrobiopterine was used in only 2 of 11 countries. Most frequently, pediatricians were caring for the patients. Dietitian was a member of PKU team in only 4 of 11 countries, while regular psychological assessments were performed in 6 of 11 countries. Patient's PKU society existed in 7 of 11 countries.

CONCLUSIONS

The region of southeastern Europe was facing certain important challenges of PKU screening and management. Neonatal PKU screening should be introduced throughout the region. Furthermore, PKU management was falling behind internationally established standards-of-care in many aspects.

Sapropterin dihydrochloride for the treatment of hyperphenylalaninemias

INTRODUCTION

Phenylketonuria (PKU) is caused by mutation of the enzyme, phenylalanine (Phe) hydroxylase (PAH). The hyperphenylalaninemia characteristic of PKU causes devastating neurological damage if not identified and treated at birth with a Phe-restricted diet. Sapropterin dihydrochloride, a pharmaceutical formulation of the natural cofactor for PAH (6R-tetrahydrobiopterin; BH4), is now available for the management of hyperphenylalaninemia in some PKU patients, including BH4 deficiencies. Sapropterin dihydrochloride improves dietary Phe tolerance in about 20% of patients with PKU.

AREAS COVERED

This evaluation describes the identification of patients suitable for treatment of sapropterin dihydrochloride, together with its indications, therapeutic properties and efficacy. Furthermore, the article reviews its safety and tolerability in patients with PKU or BH4 deficiency.

EXPERT OPINION

A reduction in blood Phe of at least 30% occurred in ∼ 20 - 30% of sapropterin-treated PKU patients (mostly with milder forms of PKU). Treatment with sapropterin resulted in clinically significant and sustained reductions in blood Phe concentrations and increased dietary Phe tolerance in well-designed clinical studies in PKU patients who responded to BH4. Successful treatment with sapropterin may lead to a relaxation of the Phe-restricted diet, although continued monitoring of blood Phe is required. Sapropterin was well tolerated.