Treatable neurotransmitter deficiency in mild phenylketonuria

Toxic Metabolites and Inborn Errors of Amino Acid Metabolism: What One Informs about the Other

In inborn errors of metabolism, such as amino acid breakdown disorders, loss of function mutations in metabolic enzymes within the catabolism pathway lead to an accumulation of the catabolic intermediate that is the substrate of the mutated enzyme. In patients of such disorders, dietarily restricting the amino acid(s) to prevent the formation of these catabolic intermediates has a therapeutic or even entirely preventative effect. This demonstrates that the pathology is due to a toxic accumulation of enzyme substrates rather than the loss of downstream products. Here, we provide an overview of amino acid metabolic disorders from the perspective of the ‘toxic metabolites’ themselves, including their mechanism of toxicity and whether they are involved in the pathology of other disease contexts as well. In the research literature, there is often evidence that such metabolites play a contributing role in multiple other nonhereditary (and more common) disease conditions, and these studies can provide important mechanistic insights into understanding the metabolite-induced pathology of the inborn disorder. Furthermore, therapeutic strategies developed for the inborn disorder may be applicable to these nonhereditary disease conditions, as they involve the same toxic metabolite. We provide an in-depth illustration of this cross-informing concept in two metabolic disorders, methylmalonic acidemia and hyperammonemia, where the pathological metabolites methylmalonic acid and ammonia are implicated in other disease contexts, such as aging, neurodegeneration, and cancer, and thus there are opportunities to apply mechanistic or therapeutic insights from one disease context towards the other. Additionally, we expand our scope to other metabolic disorders, such as homocystinuria and nonketotic hyperglycinemia, to propose how these concepts can be applied broadly across different inborn errors of metabolism and various nonhereditary disease conditions.

Consensus guideline for the diagnosis and treatment of tetrahydrobiopterin (BH4) deficiencies

BACKGROUND

Tetrahydrobiopterin (BH4) deficiencies comprise a group of six rare neurometabolic disorders characterized by insufficient synthesis of the monoamine neurotransmitters dopamine and serotonin due to a disturbance of BH4 biosynthesis or recycling. Hyperphenylalaninemia (HPA) is the first diagnostic hallmark for most BH4 deficiencies, apart from autosomal dominant guanosine triphosphate cyclohydrolase I deficiency and sepiapterin reductase deficiency. Early supplementation of neurotransmitter precursors and where appropriate, treatment of HPA results in significant improvement of motor and cognitive function. Management approaches differ across the world and therefore these guidelines have been developed aiming to harmonize and optimize patient care. Representatives of the International Working Group on Neurotransmitter related Disorders (iNTD) developed the guidelines according to the SIGN (Scottish Intercollegiate Guidelines Network) methodology by evaluating all available evidence for the diagnosis and treatment of BH4 deficiencies.

CONCLUSION

Although the total body of evidence in the literature was mainly rated as low or very low, these consensus guidelines will help to harmonize clinical practice and to standardize and improve care for BH4 deficient patients.

The effects of sapropterin on urinary monoamine metabolites in phenylketonuria

BACKGROUND

Sapropterin dihydrochloride (BH4, tetrahydrobiopterin) can lower plasma phenylalanine (Phe) concentrations for a subset of patients with phenylketonuria (PKU), an inborn error of metabolism. Studies suggest that monoamine neurotransmitter concentrations are low in PKU patients. Sapropterin functions as a cofactor for hydroxylases specific to Phe, tyrosine, and tryptophan metabolism, pathways essential for catecholamine and serotonin synthesis.

OBJECTIVE

The objective of this study is to determine the impact of sapropterin on monoamine neurotransmitter status in patients with PKU.

DESIGN

58 PKU subjects were provided 20 mg/kg of sapropterin for 1 month. Those who responded with at least a 15% decrease in plasma Phe received sapropterin for 1 year, while Non-responders discontinued it. After an additional 3 months, Responders who demonstrated increased Phe tolerance and decreased medical food dependence were classified as Definitive, whereas Responders unable to liberalize their diet without compromising plasma Phe control were identified as Provisional. At study visits, patients provided blood for plasma amino acids, 3-day diet records, and 12-hour urine samples analyzed for epinephrine (E), dopamine (DA), dihydroxyphenylacetate (DOPAC), homovanillic acid (HVA), 3-methoxytyramine (3MT), serotonin (5HT), and 5-hydroxyindole acetic acid (5HIAA) using HPLC with electrochemical detection.

RESULTS

Compared with healthy non-PKU controls, subjects with PKU had significantly lower baseline concentrations of DA, HVA, 3MT, 5HT, and 5HIAA (p < 0.001 for all). Medical food protein intake had a direct association with DA, HVA, 5HT, and 5HIAA during the study (p < 0.05 for all), while plasma Phe had an inverse association with these markers (p < 0.01 for all). DOPAC was also associated with plasma Phe throughout the year (p = 0.035), although not at baseline. Patients with PKU had a significant increase in HVA (p = 0.015) after 1 month of sapropterin. When stratifying by Responder and Non-Responder status, significance of HVA increase in Non-responders (p = 0.041) was confirmed, but not in Responders (p = 0.081). A declining trend in urinary 5HIAA, significant only after controlling for plasma Phe (p = 0.019), occurred for Definitive Responders during the 1-year study.

CONCLUSION

Urinary monoamine concentrations are low in patients with PKU and are influenced by oral sapropterin and medical food intake, highlighting the importance of these therapies to neurotransmitter metabolism in phenylketonuria.

In vivo catecholaminergic metabolism in the medial prefrontal cortex of ENU2 mice: an investigation of the cortical dopamine deficit in phenylketonuria

OBJECTIVE

Phenylketonuria (PKU) is an inherited metabolic disease characterized by plasma hyperphenylalaninemia and several neurological symptoms that can be controlled by rigorous dietetic treatment. The cellular mechanisms underlying impaired brain functions are still unclear. It has been proposed, however, that phenylalanine interference in cognitive functions depends on impaired dopamine (DA) transmission in the prefrontal cortical area due to reduced availability of the precursor tyrosine. Here, using Pah(enu2) (ENU2) mice, the genetic murine model of PKU, we investigated all metabolic steps of catecholamine neurotransmission within the medial preFrontal Cortex (mpFC), availability of the precursor tyrosine, synthesis and release, to find an easy way to reinstate normal cortical DA neurotransmission.

METHODS AND RESULTS

Analysis of blood and brain levels of tyrosine showed reduced plasma and cerebral levels of tyrosine in ENU2 mice. Western blot analysis demonstrated deficient tyrosine hydroxylase (TH) protein levels in mpFC of ENU2 mice. Cortical TH activity, determined in vivo by measuring the accumulation of l-3,4-dihydroxyphenylalanine (L-DOPA) in mpFC after inhibition of L-aromatic acid decarboxylase with NSD-1015, was reduced in ENU2 mice. Finally, a very low dose of L-DOPA, which bypasses the phenylalanine-inhibited metabolic steps, restored DA prefrontal transmission to levels found in healthy mice.

CONCLUSION

The data suggests that a strategy of using tyrosine supplementation to treat PKU is unlikely to be effective, whereas small dose L-DOPA administration is likely to have a positive therapeutic effect.

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.

5-Hydroxytryptophan rescues serotonin response to stress in prefrontal cortex of hyperphenylalaninaemic mice

Adult early treated hyperphenylalaninaemic patients can show specific deficits of prefrontal cortical functions. The development of additional therapeutic strategies for these patients requires the understanding of the mechanisms involved in phenylalanine-dependent impairment of fronto-cortical functions. We tested the hypothesis of phenylalanine interference with aminergic neurotransmission in the prefrontal cortex by evaluating, in vivo, amine release in adult Pah(enu2) mice, the genetic model of phenylketonuria. Mice of healthy background responded to a psychogenic stressor with the classic time-dependent increase of norepinephrine, dopamine and serotonin release from prefrontal cortical terminals. Neither the dopaminergic nor the serotoninergic responses were observable in the Pah(enu2) mice. Temporary reduction of circulating phenylalanine, by phenylalanine-free diet without amino- acid supplement, promoted recovery of the serotonin response only, demonstrating direct interference with serotonin synthesis in the mature brain. Evaluation of different steps of serotonin synthesis in the prefrontal cortex of hyperphenylalaninaemic mice demonstrated inhibition of cortical tryptophan hydroxylase activity. Finally, systemic administration of 5-hydroxytryptophan, the product of tryptophan hydroxylase activity, allowed frontal cortical serotonin response to stress in hyperphenylalaninaemic mice. Collectively, these results demonstrate that hyperphenylalaninaemia interferes with the ability of the mature prefrontal cortex to respond to psychological challenges, point to serotonin synthesis as the target of phenylalanine interference, and support the use of 5-hydroxytryptophan in lifelong treatment of hyperphenylalaninaemic subjects.

Production and Peripheral Roles of 5-HTP, a Precursor of Serotonin

Serotonin (5-hydroxytryptamine [5-HT]) has been implicated in a variety of physiological and pathological functions. Multiple steps of enzyme reactions enable biosynthesis of 5-HT. The first and rate-limiting step of the reaction is the synthesis of 5-hydroxy-L-tryptophan (5-HTP) from L-tryptophan. This step is dictated by an enzyme, tryptophan hydroxylase (TPH). TPH requires 6R-L-erythro-5,6,7,8-tetrahydrobiopterin (BH4) as a co-substrate of TPH. 5-HTP has been simply regarded as a precursor of 5-HT and it is believed that the biological significance of 5-HTP is essentially ascribed to the production of 5-HT. However, recent works shed light on the specific functions of 5-HTP in the periphery. In this review article, we focus on the specific roles of exogenous 5-HTP as well as the endogenous 5-HTP in the gut epithelial cells. Since systemic treatment with 5-HTP is applied to patients with lower 5-HT levels, the studies on the specific role of 5-HTP might create an opportunity to explore the effects of exogenously-applied 5-HTP in the gut in man.

Brain dysfunction in phenylketonuria: is phenylalanine toxicity the only possible cause?

In phenylketonuria, mental retardation is prevented by a diet that severely restricts natural protein and is supplemented with a phenylalanine-free amino acid mixture. The result is an almost normal outcome, although some neuropsychological disturbances remain. The pathology underlying cognitive dysfunction in phenylketonuria is unknown, although it is clear that the high plasma concentrations of phenylalanine influence the blood-brain barrier transport of large neutral amino acids. The high plasma phenylalanine concentrations increase phenylalanine entry into brain and restrict the entry of other large neutral amino acids. In the literature, emphasis has been on high brain phenylalanine as the pathological substrate that causes mental retardation. Phenylalanine was found to interfere with different cerebral enzyme systems. However, apart from the neurotoxicity of phenylalanine, a deficiency of the other large neutral amino acids in brain may also be an important factor affecting cognitive function in phenylketonuria. Cerebral protein synthesis was found to be disturbed in a mouse model of phenylketonuria and could be caused by shortage of large neutral amino acids instead of high levels of phenylalanine. Therefore, in this review we emphasize the possibility of a different idea about the pathogenesis of mental dysfunction in phenylketonuria patients and the aim of treatment strategies. The aim of treatment in phenylketonuria might be to normalize cerebral concentrations of all large neutral amino acids rather than prevent high cerebral phenylalanine concentrations alone. In-depth studies are necessary to investigate the role of large neutral amino acid deficiencies in brain.

CRHR1-dependent effects on protein expression and posttranslational modification in AtT-20 cells

Corticotropin-releasing hormone (CRH) plays a major role in coordinating the organism's stress response, including the activity of the hypothalamic-pituitary-adrenocortical axis. The molecular underpinnings of CRH-dependent signal transduction mechanisms in the anterior pituitary have not yet been revealed in detail. In order to dissect the signal transduction cascades activated by CRH receptor type 1, a comparative proteome approach was performed in vitro utilizing murine corticotroph AtT-20 cells. Alterations in protein expression and posttranslational modification in response to CRH stimulation were studied by 2D gel electrophoresis. Selected candidates were analyzed by immunoblotting and quantitative real-time PCR. The differential analyses revealed proteins regulated or modified related to diverse cellular processes. Amongst others we identified alterations in PRKAR1A, the regulatory subunit of protein kinase A; in PGK1 and PGAM1, key regulators of glycolysis; and in proteins involved in proteasome-mediated proteolysis, PSMC2 and PSMA3. These results offer novel entry points to molecular mechanisms underlying stress responses elicited via the hypothalamic-pituitary-adrenocortical axis.

Reduced availability of brain amines during critical phases of postnatal development in a genetic mouse model of cognitive delay

Serotonin (5-HT), dopamine (DA) and noradrenaline (NE) play important roles in brain postnatal maturation. Therefore, deficits in brain availability of biogenic amines during critical developmental phases might underlie neurodevelopmental disturbances associated with cognitive impairment. To test this hypothesis we evaluated brain availability of 5-HT, DA and NE, of their immediate precursors 5-hydroxytryptophan and 3,4-dihydroxy-l-phenylalanine, and of large neutral amino acids phenylalanine, tyrosine and tryptophan, in developing PahEnu2 mice, the genetic model of Phenylketonuria (PKU) a cause of severe cognitive delay. We found deficits of brain amine levels in PKU pups between day 14 and 35 of postnatal life, when pups of the healthy background showed developmental peak increases of amines and precursors. 5-HT deficits were most pronounced, were unrelated with brain availability of the amino acid precursor tryptophan, but overlapped with peak brain phenylalanine concentrations and reduced availability of 5-HT direct precursor 5-hydroxytryptophan. These results identify a critical window of brain amine availability susceptible to disturbances in a genetic mouse model of pathological neurodevelopment and suggest a mechanism of interference with brain aminergic synthesis in PKU and non-PKU hyperphenylalaninemia.

Phenylalanine hydroxylase gene in psychiatric patients: screening and functional assay of mutations

BACKGROUND

Reports relating phenylalanine kinetics and metabolism to psychiatric disorders led us to undertake the comprehensive screening of the phenylalanine hydroxylase (PAH) coding region and functional testing of discovered mutations in a sample of psychiatric patients and healthy control subjects.

METHODS

Genomic DNA from psychiatric patients and control subjects was assayed for sequence variants in all PAH coding regions and splice junctions. In vivo functional analysis of mutations was conducted by assessing the kinetics and conversion to tyrosine of a standardized phenylalanine dose and by measuring fasting pterin levels.

RESULTS

A known missense mutation was observed in a schizoaffective subject, and a novel missense mutation was discovered in four subjects with schizophrenia and one normal subject. The schizoaffective patient heterozygous for the known A403V mutation showed the lowest rate of phenylalanine kinetics and lowest conversion to tyrosine in the patient sample. The four schizophrenic patients heterozygous for the novel K274E mutation showed significantly decreased phenylalanine kinetics, reduced conversion to tyrosine, and increased synthesis of the PAH cofactor tetrahydrobiopterin compared with schizophrenic subjects without the mutation.

CONCLUSIONS

The study findings suggest that larger scale studies are warranted to test the relationship of the PAH genotype with a psychiatric phenotype.

Tetrahydrobiopterin-responsive phenylalanine hydroxylase deficiency, state of the art

Since 1999 an increasing number of patients with phenylalanine hydroxylase (PAH) deficiency are reported to be able to decrease their plasma phenylalanine (Phe) concentrations after a 6R-tetrahydrobiopterin (BH(4)) challenge. The majority of these patients have mild PKU or MHP (mild hyperphenylalaninemia) and harbour at least one missense mutation in the PAH gene associated with this phenotype. The rate of decrease and the lowest achieved Phe level vary between patients with different genotypes but appears to be similar in patients with the same genotype. A number of the mutations associated with BH(4)-responsiveness have been studied in an 'in vitro' eukaryotic cell expression system leading to biosynthesis of a mutant PAH enzyme with some residual activity. Patients bearing mutations that cause severe structural distortion in the expressed protein (loss of function mutations), leading to undetectable PAH activity, are not responsive to BH(4). These observations suggest that residual PAH activity (in vitro) is a prerequisite for BH(4)-responsiveness. However, an in vitro residual PAH activity is not a guarantee for in vivo BH(4)-responsiveness. Mechanisms behind this responsiveness could be relieve of decreased binding affinity for BH(4), BH(4)-mediated increase of PAH gene expression or stabilization of the mutant enzyme protein by BH(4). BH(4)-responsive PAH-deficient patients have only been reported since 1999. For the western countries this is explained by the fact that the manufacturer changed the diastereoisomeric purity of the BH4 preparation from 69% of the natural 6R-BH4 (31% of 6S-BH4) to 99.5% 6R-BH4. The new findings on BH(4)-responsiveness may be of clinical relevance because these patients can be treated with BH(4) with concomitant relief or withdrawal of the burdensome PKU diet. These observations warrant further clinical studies to assess efficacy, optimal dosage, and safety of BH(4) treatment in this group. The data strongly emphasize the necessity of the BH(4) loading test in patients detected in the newborn PKU screening.

Danger of high-protein dietary supplements to persons with hyperphenylalaninaemia

A 16-year-old adolescent with mild hyperphenylalaninaemia was given a high-protein 'body building' supplement twice daily, causing headaches, decreased school performance and mild depression. All symptoms disappeared after cessation of the supplement. The phenylalanine hydroxylase mutation H170D/IVS1nt5G>T was found to be responsive to tetrahydrobiopterin with significant decrease in blood phenylalanine concentration and increase in tyrosine blood content. A brain phenylalanine level of 0.5 mmol/L was initially documented, which decreased to the normal carrier range of 0.2 mmol/L within one month of discontinuance of the protein supplement. At present, the patient is on a normal diet without phenylalanine restriction.

Deficits in brain serotonin synthesis in a genetic mouse model of phenylketonuria

Although hyperphenylalaninemia causes neurological disturbances and mental retardation, the neuropathological effects of phenylalanine excess are still poorly understood. Brain serotonin depletion may play a major role in such disturbances and is a possible target for feasible pharmacotherapies. In the present study, we investigated hyperphenylalaninemia-related brain serotonin depletion using a genetic mouse model of phenylketonuria, the Pah(enu2) mutant. Mutant mice showed severe depletion of whole brain serotonin, a mild reduction in the brain level of tryptophan, its amino acid precursor, and major deficits in the brain level of 5-hydroxytryptophan, the second rate-limiting factor in serotonin synthesis. These results suggest that interference with brain 5-hydroxytryptophan synthesis may be the major cause of serotonin deficits in hyperphenylalaninemia.

Diagnosis and treatment of neurotransmitter-related disorders

Neurotransmitter disorders constitute a spectrum of neurologic conditions that share several clinical features depending on the severity and pattern of neurotransmitter deficiency or excess. These uncommon conditions can be suspected based on their clinical features, and several can be confirmed by cerebrospinal fluid analysis of neurotransmitters and their metabolites. Certain disorders, such as autosomal dominant dopa-responsive dystonia caused by GTP cyclohydrolase deficiency, or Segawa syndrome, respond dramatically to medical therapy. This article summarizes current knowledge regarding the clinical manifestations, diagnosis, and treatment of these important disorders.

Two novel genetic lesions and a common BH4-responsive mutation of the PAH gene in Italian patients with hyperphenylalaninemia

Hyperphenylalaninemia (HPA), due to a deficiency of phenylalanine hydroxylase (PAH) enzyme, is caused by mutations in the PAH gene. Molecular analysis in 23 Italian patients with PAH deficiency identified two novel (P281R, L287V) and 20 previously described genetic lesions in the PAH gene. The detection of the A403V amino acid substitution in combination with null mutations in patients with BH4-responsive PAH deficiency leads us to correlate it with BH4 responsiveness.

Mental illness in mild PKU responds to biopterin

A 25-year-old woman with mild hyperphenylalaninemia developed disabling depression and panic attacks. The mutations on the phenylalanine hydroxylase gene indicated that she might be responsive to tetrahydrobiopterin therapy. Mutation analyses were performed by the John F. Kennedy Institute in Glostrup, Denmark. The response to tetrahydrobiopterin therapy was impressive at an oral dose of 50 mg twice a day. A 25-year-old woman with mild hyperphenylalaninemia due to a PAH mutation of IVS12nt1g-->a/E390G has been treated for 1 year with BH4 therapy. A maintenance dosage of only 100 mg/day has resulted in significant improvement of depression and panic attacks, with discontinuation of psychotropic medication.