Phenylketonuria: a review of current and future treatments.
ABSTRACT: Phenylketonuria (PKU) is an autosomal recessive inborn error of metabolism caused by a deficiency in the hepatic enzyme phenylalanine hydroxylase (PAH). If left untreated, the main clinical feature is intellectual disability. Treatment, which includes a low Phe diet supplemented with amino acid formulas, commences soon after diagnosis within the first weeks of life. Although dietary treatment has been successful in preventing intellectual disability in early treated PKU patients, there are major issues with dietary compliance due to palatability of the diet. Other potential issues associated with dietary therapy include nutritional deficiencies especially vitamin D and B12. Suboptimal outcomes in cognitive and executive functioning have been reported in patients who adhere poorly to dietary therapy. There have been continuous attempts at improving the quality of medical foods including their palatability. Advances in dietary therapy such as the use of large neutral amino acids (LNAA) and glycomacropeptides (GMP; found within the whey fraction of bovine milk) have been explored. Gene therapy and enzyme replacement or substitution therapy have yielded more promising data in the recent years. In this review the current and possible future treatments for PKU are discussed.
Project description:Phenylketonuria (PKU) was the first disorder in which severe neurocognitive dysfunction could be prevented by dietary treatment. However, despite this effect, neuropsychological outcome in PKU still remains suboptimal and the phenylalanine-restricted diet is very demanding. To improve neuropsychological outcome and relieve the dietary restrictions for PKU patients, supplementation of large neutral amino acids (LNAA) is suggested as alternative treatment strategy that might correct all brain biochemical disturbances caused by high blood phenylalanine, and thereby improve neurocognitive functioning.As a proof-of-principle, this study aimed to investigate all hypothesized biochemical treatment objectives of LNAA supplementation (normalizing brain phenylalanine, non-phenylalanine LNAA, and monoaminergic neurotransmitter concentrations) in PKU mice.C57Bl/6 Pah-enu2 (PKU) mice and wild-type mice received a LNAA supplemented diet, an isonitrogenic/isocaloric high-protein control diet, or normal chow. After six weeks of dietary treatment, blood and brain amino acid and monoaminergic neurotransmitter concentrations were assessed.In PKU mice, the investigated LNAA supplementation regimen significantly reduced blood and brain phenylalanine concentrations by 33% and 26%, respectively, compared to normal chow (p<0.01), while alleviating brain deficiencies of some but not all supplemented LNAA. Moreover, LNAA supplementation in PKU mice significantly increased brain serotonin and norepinephrine concentrations from 35% to 71% and from 57% to 86% of wild-type concentrations (p<0.01), respectively, but not brain dopamine concentrations (p = 0.307).This study shows that LNAA supplementation without dietary phenylalanine restriction in PKU mice improves brain biochemistry through all three hypothesized biochemical mechanisms. Thereby, these data provide proof-of-concept for LNAA supplementation as a valuable alternative dietary treatment strategy in PKU. Based on these results, LNAA treatment should be further optimized for clinical application with regard to the composition and dose of the LNAA supplement, taking into account all three working mechanisms of LNAA treatment.
Project description:Many phenylketonuria (PKU) patients cannot adhere to the severe dietary restrictions as advised by the European PKU guidelines, which can be accompanied by aggravated neuropsychological impairments that, at least in part, have been attributed to brain monoaminergic neurotransmitter deficiencies. Supplementation of large neutral amino acids (LNAA) to an unrestricted diet has previously been shown to effectively improve brain monoamines in PKU mice of various ages. To determine the additive value of LNAA supplementation to a liberalized phenylalanine-restricted diet, brain and plasma monoamine and amino acid concentrations in 10 to 16-month-old adult C57Bl/6 PKU mice on a less severe phenylalanine-restricted diet with LNAA supplementation were compared to those on a non-supplemented severe or less severe phenylalanine-restricted diet. LNAA supplementation to a less severe phenylalanine-restricted diet was found to improve both brain monoamine and phenylalanine concentrations. Compared to a severe phenylalanine-restricted diet, it was equally effective to restore brain norepinephrine and serotonin even though being less effective to reduce brain phenylalanine concentrations. These results in adult PKU mice support the idea that LNAA supplementation may enhance the effect of a less severe phenylalanine-restricted diet and suggest that cerebral outcome of PKU patients treated with a less severe phenylalanine-restricted diet may be helped by additional LNAA treatment.
Project description:Phenylketonuria (PKU; also known as phenylalanine hydroxylase (PAH) deficiency) is an autosomal recessive disorder of phenylalanine metabolism, in which especially high phenylalanine concentrations cause brain dysfunction. If untreated, this brain dysfunction results in severe intellectual disability, epilepsy and behavioural problems. The prevalence varies worldwide, with an average of about 1:10,000 newborns. Early diagnosis is based on newborn screening, and if treatment is started early and continued, intelligence is within normal limits with, on average, some suboptimal neurocognitive function. Dietary restriction of phenylalanine has been the mainstay of treatment for over 60 years and has been highly successful, although outcomes are still suboptimal and patients can find the treatment difficult to adhere to. Pharmacological treatments are available, such as tetrahydrobiopterin, which is effective in only a minority of patients (usually those with milder PKU), and pegylated phenylalanine ammonia lyase, which requires daily subcutaneous injections and causes adverse immune responses. Given the drawbacks of these approaches, other treatments are in development, such as mRNA and gene therapy. Even though PAH deficiency is the most common defect of amino acid metabolism in humans, brain dysfunction in individuals with PKU is still not well understood and further research is needed to facilitate development of pathophysiology-driven treatments.
Project description:Phenylketonuria (PKU) is an inherited metabolic disease characterized by abnormally high concentrations of the essential amino acid L-phenylalanine (Phe) in blood plasma caused by reduced activity of phenylalanine hydroxylase (PAH). While numerous studies have shown association between high plasma Phe concentration and intellectual impairment, it is not clear whether increased Phe fluctuations also observed in PKU affect the brain as well. To investigate this, time-resolved in vivo data on Phe and competing large neutral amino acid (LNAA) concentrations in neurons are needed, but cannot be acquired readily with current methods. We have used in silico modeling as an alternative approach to characterize the interactive dynamics of Phe and competing LNAAs (CL) in the neurovascular unit (NVU). Our results suggest that plasma Phe fluctuations can propagate into the NVU cells and change there the concentration of LNAAs, with the highest magnitude of this effect observed at low frequency and high amplitude-to-mean ratio of the plasma Phe concentration fluctuations. Our model further elucidates the effect of therapeutic LNAA supplementation in PKU, showing how abnormal concentrations of Phe and CL in the NVU move thereby toward normal physiologic levels.
Project description:BACKGROUND:Phenylketonuria (PKU) is due to a defective hepatic enzyme, phenylalanine (Phe) hydroxylase. Transport of the precursor amino acids from blood into the brain for serotonin and dopamine synthesis is reported to be inhibited by high blood Phe concentrations. Deficiencies of serotonin and dopamine are involved in neurocognitive dysfunction in PKU. OBJECTIVE:(1) To evaluate the effects of sapropterin (BH4) and concurrent use of large neutral amino acids (LNAA) on the peripheral biomarkers, melatonin and dopamine with the hypothesis they reflect brain serotonin and dopamine metabolism. (2) To evaluate synergistic effects with BH4 and LNAA. (3) To determine the effects of blood Phe concentrations on the peripheral biomarkers concentrations. METHODS:Nine adults with PKU completed our study consisting of four 4-week phases: (1) LNAA supplementation, (2) Washout, (3) BH4 therapy, and (4) LNAA with BH4 therapy. An overnight protocol measured plasma amino acids, serum melatonin, and 6-sulfatoxymelatonin and dopamine in first void urine after each phase. RESULTS:(1) Three out of nine subjects responded to BH4. A significant increase of serum melatonin levels was observed in BH4 responders with decreased blood Phe concentration. No significant change in melatonin, dopamine or Phe levels was observed with BH4 in the subjects as a whole. (2) Synergistic effects with BH4 and LNAA were observed in serum melatonin in BH4 responders. (3) The relationship between serum melatonin and Phe showed a significant negative slope (p = 0.0005) with a trend toward differing slopes among individual subjects (p = 0.066). There was also a negative association overall between blood Phe and urine 6-sulfatoxymelatonin and dopamine (P = 0.040 and 0.047). CONCLUSION:Blood Phe concentrations affected peripheral monoamine neurotransmitter biomarker concentrations differently in each individual with PKU. Melatonin levels increased with BH4 therapy only when blood Phe decreased. Monitoring peripheral neurotransmitter metabolites may assist in optimizing individualized treatment in PKU.
Project description:The standard treatment for phenylketonuria (PKU) is a lifelong low-phenylalanine (Phe) diet, supplemented with Phe-free protein substitutes; however, adult patients often show poor adherence to therapy. Alternative treatment options include the use of large neutral amino acids (LNAA). The aim of this study was to determine the Phe, tyrosine (Tyr), and Phe/Tyr ratio in a cohort of sub-optimally controlled adult patients with classical PKU treated with a new LNAA formulation. Twelve patients received a Phe-restricted diet plus a slow-release LNAA product taken three times per day, at a dose of 1 g/kg body weight (mean 0.8 ± 0.24 g/kg/day), over a 12-month period. The product is in a microgranulated formulation, which incorporates all amino acids and uses sodium alginate as a hydrophilic carrier to prolong its release. This LNAA formulation provides up to 80% of the total protein requirement, with the rest of the protein supplied by natural food. Patients had fortnightly measurements of Phe and Tyr levels over a 12-month period after the introduction of LNAA. All patients completed the 12-month treatment period. Overall, adherence to the new LNAA tablets was very good compared with a previous amino acid mixture, for which taste was a major complaint by patients. Phe levels remained unchanged (p = 0.0522), and Tyr levels increased (p = 0.0195). Consequently, the Phe/Tyr ratio decreased significantly (p < 0.05) in the majority of patients treated. In conclusion, LNAA treatment increases Tyr levels in sub-optimally controlled adult PKU patients, while offering the potential to improve their adherence to treatment.
Project description:Phenylketonuria (PKU) is an inborn error of metabolism of the amino acid phenylalanine. It is an autosomal recessive disorder with a rate of incidence of 1 in 10,000 in Caucasian populations. Mutations in the phenylalanine hydroxylase (PAH) gene are the major cause of PKU, due to the loss of the catalytic activity of the enzyme product PAH. Newborn screening for PKU allows early intervention, avoiding irreparable neurological damage and intellectual disability that would arise from untreated PKU. The current primary treatment of PKU is the limitation of dietary protein intake, which in the long term may be associated with poor compliance in some cases and other health problems due to malnutrition. The only alternative therapy currently approved is the supplementation of BH4, the requisite co-factor of PAH, in the orally-available form of sapropterin dihydrochloride. This treatment is not universally available, and is only effective for a proportion (estimated 30%) of PKU patients. Research into novel therapies for PKU has taken many different approaches to address the lack of PAH activity at the core of this disorder: enzyme replacement via virus-mediated gene transfer, transplantation of donor liver and recombinant PAH protein, enzyme substitution using phenylalanine ammonia lyase (PAL) to provide an alternative pathway for the metabolism of phenylalanine, and restoration of native PAH activity using chemical chaperones and nonsense read-through agents. It is hoped that continuing efforts into these studies will translate into a significant improvement in the physical outcome, as well as quality of life, for patients with PKU.
Project description:Monoamine neurotransmitter deficiency has been implicated in the etiology of neuropsychiatric symptoms associated with chronic hyperphenylalaninemia in phenylketonuria (PKU). Two proposed explanations for neurotransmitter deficiency in PKU include first, that chronically elevated blood L-phenylalanine (Phe) inhibits the transport of L-tyrosine (Tyr) and L-tryptophan (Trp), the substrates for dopamine and serotonin synthesis respectively, into brain. In the second hypothesis, elevated Phe competitively inhibits brain tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH) activities, the rate limiting steps in dopamine and serotonin synthesis. Dietary supplementation with large neutral amino acids (LNAA) including Tyr and Trp has been recommended for individuals with chronically elevated blood Phe in an attempt to restore amino acid and monoamine homeostasis in brain. As a potential alternative treatment approach, we demonstrate that pharmacologic inhibition of Tyr degradation through oral administration of nitisinone (NTBC) yielded sustained increases in blood and brain Tyr, decreased blood and brain Phe, and consequently increased dopamine synthesis in a murine model of PKU. Our results suggest that Phe-mediated inhibition of TH activity is the likely mechanism of impaired dopamine synthesis in PKU. Pharmacologic inhibition of Tyr degradation may be a promising adjunct therapy for CNS monoamine neurotransmitter deficiency in hyperphenylalaninemic individuals with PKU.
Project description:Notwithstanding the success of the traditional dietary phenylalanine restriction treatment in phenylketonuria (PKU), the use of large neutral amino acid (LNAA) supplementation rather than phenylalanine restriction has been suggested. This treatment modality deserves attention as it might improve cognitive outcome and quality of life in patients with PKU. Following various theories about the pathogenesis of cognitive dysfunction in PKU, LNAA supplementation may have multiple treatment targets: a specific reduction in brain phenylalanine concentrations, a reduction in blood (and consequently brain) phenylalanine concentrations, an increase in brain neurotransmitter concentrations, and an increase in brain essential amino acid concentrations. These treatment targets imply different treatment regimes. This review summarizes the treatment targets and the treatment regimens of LNAA supplementation and discusses the differences in LNAA intake between the classical dietary phenylalanine-restricted diet and several LNAA treatment forms.
Project description:Hyperphenylalaninemia (HPA) can be classified into phenylketonuria (PKU) which is caused by mutations in the phenylalanine hydroxylase (PAH) gene, and BH4 deficiency caused by alterations in genes involved in tetrahydrobiopterin (BH4) biosynthesis pathway. Dietary restriction of phenylalanine is considered to be the main treatment of PKU to prevent irreversible intellectual disability. However, the same dietary intervention in BH4 deficiency patients is not as effective, as BH4 is also a cofactor in many neurotransmitter syntheses.We utilized next generation sequencing (NGS) technique to investigate four unrelated Thai patients with hyperphenylalaninemia.We successfully identified all eight mutant alleles in PKU or BH4-deficiency associated genes including three novel mutations, one in PAH and two in PTS, thus giving a definite diagnosis to these patients. Appropriate management can then be provided.This study identified three novel mutations in either the PAH or PTS gene and supported the use of NGS as an alternative molecular genetic approach for definite diagnosis of hyperphenylalaninemia, thus leading to proper management of these patients in Thailand.