Blood-brain barrier phenylalanine transport and individual vulnerability in phenylketonuria

Current Landscape on Development of Phenylalanine and Toxicity of its Metabolites - A Review

Phenylalanine, an essential amino acid, is the "building block" of protein. It has a tremendous role in different aspects of metabolic events. The tyrosine pathway is the prime one and is typically used to degrade dietary phenylalanine. Phenylalanine exceeds its limit in bodily fluids and the brain when the enzyme, phenylalanine decarboxylase, phenylalanine transaminase, phenylalanine hydroxylase (PAH) or its cofactor tetrahydrobiopterin (BH4) is deficient causes phenylketonuria, schizophrenia, attentiondeficit/ hyperactivity disorder and another neuronal effect. Tyrosine, an amino acid necessary for synthesizing the pigments in melanin, is produced by its primary metabolic pathway. Deficiency/abnormality in metabolic enzymes responsible for the catabolism pathway of Phenylalanine causes an accumulation of the active intermediate metabolite, resulting in several abnormalities, such as developmental delay, tyrosinemias, alkaptonuria, albinism, hypotension and several other undesirable conditions. Dietary restriction of the amino acid(s) can be a therapeutic approach to avoid such undesirable conditions when the level of metabolic enzyme is unpredictable. After properly identifying the enzymatic level, specific pathophysiological conditions can be managed more efficiently.

Neurological and imaging phenotypes of adults with untreated phenylketonuria: new cases and literature review

OBJECTIVES

Phenylketonuria (PKU) is the most prevalent congenital disease of amino acid metabolism. Neurological manifestations usually complicate PKU in untreated adult patients. This study describes neurological and imaging phenotypes of adult patients with untreated PKU.

METHODS

We investigated a cohort of 320 unrelated adult patients with suspected genetic leukoencephalopathies using whole-exome sequencing (WES). We analyzed the phenotypic features of adult PKU patients in our cohort and summarized cases reported in the literature.

RESULTS

We identified 10 patients in our cohort and 12 patients in the literature, who presented with neurological manifestations and were diagnosed with PKU in adulthood. Approximately 60% of these patients had onset of clinical features in adulthood. The most common neurological symptoms of patients presenting in adulthood were cognitive disturbance and spastic paralysis, followed by vision loss, cerebellar ataxia, weakness of limbs, and seizure. This differed from that of patients presenting with PKU features in childhood, who consistently had mental retardation with various neurological complications emerging during a broad age range. Imaging findings were similar between patients presenting with clinical features in childhood compared with adulthood, comprising symmetric periventricular white matter hyperintense on T2-weighted imaging and diffusion-weighted imaging predominantly in the parietal and occipital lobes. Also, normal brain imaging and diffuse leukoencephalopathies were observed in both patient groups.

CONCLUSION

PKU with clinical features presenting in adulthood is an atypical subtype and should be considered during diagnosis of adults with neurological symptoms and leukoencephalopathy. DWI seems to be most helpful to distinguish patients with PKU. Additionally, we demonstrate that PKU constitutes a part (3.1%) of adult genetic leukoencephalopathies.

Phenylketonuria and the brain

Classic phenylketonuria (PKU) is caused by defective activity of phenylalanine hydroxylase (PAH), the enzyme that coverts phenylalanine (Phe) to tyrosine. Toxic accumulation of phenylalanine and its metabolites, left untreated, affects brain development and function depending on the timing of exposure to elevated levels. The specific mechanisms of Phe-induced brain damage are not completely understood, but they correlate to phenylalanine levels and on the stage of brain growth. During fetal life, high levels of phenylalanine such as those seen in maternal PKU can result in microcephaly, neuronal loss and corpus callosum hypoplasia. Elevated phenylalanine levels during the first few years of life can cause acquired microcephaly, severe cognitive impairment and epilepsy, likely due to the impairment of synaptogenesis. During late childhood, elevated phenylalanine can cause alterations in neurological functioning, leading to ADHD, speech delay and mild IQ reduction. In adolescents and adults, executive function and mood are affected, with some of the abnormalities reversed by better control of phenylalanine levels. Altered brain myelination can be present at this stage. In this article, we review the current knowledge about the consequences of high phenylalanine levels in PKU patients and animal models through different stages of brain development and its effect on cognitive, behavioural and neuropsychological function.

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.

The rs113883650 variant of SLC7A5 (LAT1) gene may alter brain phenylalanine content in PKU

Functional alteration of the LAT1 amino acid transporter may be responsible for interindividual differences in cerebral phenylalanine content and the lack of intellectual disability in some patients with untreated phenylketonuria. We assessed the effect of the common variant rs113883650 of the SLC7A5 (LAT1) gene on brain phenylalanine content, as measured with use of magnetic resonance spectroscopy. Our results suggest that the presence of this variant could influence the amount of phenylalanine in the brain.

Untreated PKU Patients without Intellectual Disability: What Do They Teach Us?

Phenylketonuria (PKU) management is aimed at preventing neurocognitive and psychosocial dysfunction by keeping plasma phenylalanine concentrations within the recommended target range. It can be questioned, however, whether universal plasma phenylalanine target levels would result in optimal neurocognitive outcomes for all patients, as similar plasma phenylalanine concentrations do not seem to have the same consequences to the brain for each PKU individual. To better understand the inter-individual differences in brain vulnerability to high plasma phenylalanine concentrations, we aimed to identify untreated and/or late-diagnosed PKU patients with near-normal outcome, despite high plasma phenylalanine concentrations, who are still alive. In total, we identified 16 such cases. While intellectual functioning in these patients was relatively unaffected, they often did present other neurological, psychological, and behavioral problems. Thereby, these “unusual” PKU patients show that the classical symptomatology of untreated or late-treated PKU may have to be rewritten. Moreover, these cases show that a lack of intellectual dysfunction despite high plasma phenylalanine concentrations does not necessarily imply that these high phenylalanine concentrations have not been toxic to the brain. Also, these cases may suggest that different mechanisms are involved in PKU pathophysiology, of which the relative importance seems to differ between patients and possibly also with increasing age. Further research should aim to better distinguish PKU patients with respect to their cerebral effects to high plasma phenylalanine concentrations.

Optimising amino acid absorption: essential to improve nitrogen balance and metabolic control in phenylketonuria

It has been nearly 70 years since the discovery that strict adherence to a diet low in phenylalanine prevents severe neurological sequelae in patients with phenylalanine hydroxylase deficiency (phenylketonuria; PKU). Today, dietary treatment with restricted phenylalanine intake supplemented with non-phenylalanine amino acids to support growth and maintain a healthy body composition remains the mainstay of therapy. However, a better understanding is needed of the factors that influence N balance in the context of amino acid supplementation. The aim of the present paper is to summarise considerations for improving N balance in patients with PKU, with a focus on gaining greater understanding of amino acid absorption, disposition and utilisation. In addition, the impact of phenylalanine-free amino acids on 24 h blood phenylalanine/tyrosine circadian rhythm is evaluated. We compare the effects of administering intact protein v. free amino acid on protein metabolism and discuss the possibility of improving outcomes by administering amino acid mixtures so that their absorption profile mimics that of intact protein. Protein substitutes with the ability to delay absorption of phenylalanine and tyrosine, mimicking physiological absorption kinetics, are expected to improve the rate of assimilation into protein and minimise fluctuations in quantitative plasma amino acid levels. They may also help maintain normal glycaemia and satiety sensation. This is likely to play an important role in improving the management of patients with PKU.

Can untreated PKU patients escape from intellectual disability? A systematic review

Background

Phenylketonuria (PKU) is often considered as the classical example of a genetic disorder in which severe symptoms can nowadays successfully be prevented by early diagnosis and treatment. In contrast, untreated or late-treated PKU is known to result in severe intellectual disability, seizures, and behavioral disturbances. Rarely, however, untreated or late-diagnosed PKU patients with high plasma phenylalanine concentrations have been reported to escape from intellectual disability. The present study aimed to review published cases of such PKU patients.

Methods

To this purpose, we conducted a literature search in PubMed and EMBASE up to 8th of September 2017 to identify cases with 1) PKU diagnosis and start of treatment after 7 years of age; 2) untreated plasma phenylalanine concentrations ≥1200 μmol/l; and 3) IQ ≥80. Literature search, checking reference lists, selection of articles, and extraction of data were performed by two independent researchers.

Results

In total, we identified 59 published cases of patients with late-diagnosed PKU and unexpected favorable outcome who met the inclusion criteria. Although all investigated patients had intellectual functioning within the normal range, at least 19 showed other neurological, psychological, and/or behavioral symptoms.

Conclusions

Based on the present findings, the classical symptomatology of untreated or late-treated PKU may need to be rewritten, not only in the sense that intellectual dysfunction is not obligatory, but also in the sense that intellectual functioning does not (re)present the full picture of brain damage due to high plasma phenylalanine concentrations. Further identification of such patients and additional analyses are necessary to better understand these differences between PKU patients.

Electronic supplementary material

The online version of this article (10.1186/s13023-018-0890-7) contains supplementary material, which is available to authorized users.

[Phenotypic and molecular characterization of a Colombian family with phenylketonuria]

INTRODUCTION

Phenylketonuria is a metabolic disorder characterized by severe neurological involvement and behavioral disorder, whose early diagnosis enables an effective treatment to avoid disease sequelae, thus changing the prognosis. Objective: To characterize a family with phenylketonuria in Colombia at clinical, biochemical and molecular levels. Materials and methods: The population consisted of seven individuals of a consanguineous family with four children with suggestive symptoms of phenylketonuria. After signing an informed consent, blood and urine samples were taken for colorimetric tests and high performance liquid and thin layer chromatographies. DNA extraction and sequencing of the 13 exons of the PAH gene were performed in all subjects. We designed primers for each exon with the Primer 3 software using automatic sequencing equipment Abiprism 3100 Avant. Sequences were analyzed using the SeqScape, v2.0, software. Results: We described the clinical and molecular characteristics of a Colombian family with phenylketonuria and confirmed the presence of the mutation c.398_401delATCA. We established a genotype-phenotype correlation, highlighting the interesting clinical variability found among the affected patients despite having the same mutation in all of them. Conclusions: Early recognition of this disease is very important to prevent its neurological and psychological sequelae, given that patients reach old age without diagnosis or proper management.

Phenylketonuria: brain phenylalanine concentrations relate inversely to cerebral protein synthesis

In phenylketonuria, elevated plasma phenylalanine concentrations may disturb blood-to-brain large neutral amino acid (LNAA) transport and cerebral protein synthesis (CPS). We investigated the associations between these processes, using data obtained by positron emission tomography with l-[1-(11)C]-tyrosine ((11)C-Tyr) as a tracer. Blood-to-brain transport of non-Phe LNAAs was modeled by the rate constant for (11)C-Tyr transport from arterial plasma to brain tissue (K1), while CPS was modeled by the rate constant for (11)C-Tyr incorporation into cerebral protein (k3). Brain phenylalanine concentrations were measured by magnetic resonance spectroscopy in three volumes of interest (VOIs): supraventricular brain tissue (VOI 1), ventricular brain tissue (VOI 2), and fluid-containing ventricular voxels (VOI 3). The associations between k3 and each predictor variable were analyzed by multiple linear regression. The rate constant k3 was inversely associated with brain phenylalanine concentrations in VOIs 2 and 3 (adjusted R(2)=0.826, F=19.936, P=0.021). Since brain phenylalanine concentrations in these VOIs highly correlated with each other, the specific associations of each predictor with k3 could not be determined. The associations between k3 and plasma phenylalanine concentration, K1, and brain phenylalanine concentrations in VOI 1 were nonsignificant. In conclusion, our study shows an inverse association between k3 and increased brain phenylalanine concentrations.

Age-related psychophysiological vulnerability to phenylalanine in phenylketonuria

BACKGROUND

Phenylketonuria (PKU) is caused by the inherited defect of the phenylalanine hydroxylase enzyme, which converts phenylalanine (Phe) into tyrosine (Tyr). Neonatal screening programs and early treatment have radically changed the natural history of PKU. Nevertheless, an increased risk of neurocognitive and psychiatric problems in adulthood remains a challenging aspect of the disease. In order to assess the vulnerability of complex skills to Phe, we explored: (a) the effect of a rapid increase in blood Phe levels on event-related potentials (ERP) in PKU subjects during their second decade of life; (b) the association (if existing) between psychophysiological and neurocognitive features.

METHODS

Seventeen early-treated PKU subjects, aged 10-20, underwent ERP [mismatch negativity, auditory P300, contingent negative variation (CNV), and Intensity Dependence of Auditory Evoked Potentials] recording before and 2 h after an oral loading of Phe. Neurocognitive functioning, historical and concurrent biochemical values of blood Phe, Tyr, and Phe/Tyr ratio, were all included in the statistical analysis.

RESULTS

Event-related potential components were normally detected in all the subjects. In subjects younger than 13 CNV amplitude, W2-CNV area, P3b latency, and reaction times in motor responses were negatively influenced by Phe-loading. Independently from the psychophysiological vulnerability, some neurocognitive skills were more impaired in younger patients. No correlation was found between biochemical alterations and neurocognitive and psychophysiological findings.

CONCLUSION

The vulnerability of the emerging neurocognitive functions to Phe suggests a strict metabolic control in adolescents affected by PKU and a neurodevelopmental approach in the study of neurocognitive outcome in PKU.

A comparison of phenylketonuria with attention deficit hyperactivity disorder: do markedly different aetiologies deliver common phenotypes?

Phenylketonuria (PKU) is a well-defined metabolic disorder arising from a mutation that disrupts phenylalanine metabolism and so produces a variety of neural changes indirectly. Severe cognitive impairment can be prevented by dietary treatment; however, residual symptoms may be reported. These residual symptoms appear to overlap a more prevalent childhood disorder: Attention Deficit/Hyperactivity Disorder (ADHD). However, the aetiology of ADHD is a vast contrast to PKU: it seems to arise from a complex combination of genes; and it has a substantial environmental component. We ask whether these two disorders result from two vastly different genotypes that converge on a specific core phenotype that includes similar dysfunctions of Gray's (Gray, 1982) Behavioural Inhibition System (BIS), coupled with other disorder-specific dysfunctions. If so, we believe comparison of the commonalities will allow greater understanding of the neuropsychology of both disorders. We review in detail the aetiology, treatment, neural pathology, cognitive deficits and electrophysiological abnormalities of PKU; and compare this with selected directly matching aspects of ADHD. The biochemical and neural pathologies of PKU and ADHD are quite distinct in their causes and detail; but they result in the disorder in the brain of large amino acid levels, dopamine and white matter that are very similar and could explain the overlap of symptoms within and between the PKU and ADHD spectra. The common deficits affect visual function, motor function, attention, working memory, planning, and inhibition. For each of PKU and ADHD separately, a subset of deficits has been attributed to a primary dysfunction of behavioural inhibition. In the case of ADHD (excluding the inattentive subtype) this has been proposed to involve a specific failure of the BIS; and we suggest that this is also true of PKU. This accounts for a substantial proportion of the parallels in the superficial symptoms of both disorders and we see this as linked to prefrontal, rather than more general, dysfunction of the BIS.

Biochemical correlates of neuropsychiatric illness in maple syrup urine disease

Maple syrup urine disease (MSUD) is an inherited disorder of branched chain amino acid metabolism presenting with neonatal encephalopathy, episodic metabolic decompensation, and chronic amino acid imbalances. Dietary management enables survival and reduces risk of acute crises. Liver transplantation has emerged as an effective way to eliminate acute decompensation risk. Psychiatric illness is a reported MSUD complication, but has not been well characterized and remains poorly understood. We report the prevalence and characteristics of neuropsychiatric problems among 37 classical MSUD patients (ages 5-35 years, 26 on dietary therapy, 11 after liver transplantation) and explore their underlying mechanisms. Compared with 26 age-matched controls, MSUD patients were at higher risk for disorders of cognition, attention, and mood. Using quantitative proton magnetic resonance spectroscopy, we found lower brain glutamate, N-acetylaspartate (NAA), and creatine concentrations in MSUD patients, which correlated with specific neuropsychiatric outcomes. Asymptomatic neonatal course and stringent longitudinal biochemical control proved fundamental to optimizing long-term mental health. Neuropsychiatric morbidity and neurochemistry were similar among transplanted and nontransplanted MSUD patients. In conclusion, amino acid dysregulation results in aberrant neural networks with neurochemical deficiencies that persist after transplant and correlate with neuropsychiatric morbidities. These findings may provide insight into general mechanisms of psychiatric illness.

Neurocognitive functioning in adults with phenylketonuria: results of a long term study

OBJECTIVES

A controlled long-term study was performed to assess the neurological and neuropsychological performance in adult patients with early-treated phenylketonuria (PKU).

METHODS

We investigated 57 patients with early-treated classical PKU aged 19 to 41 years (mean age 31 years) and 46 matched healthy controls, matched for age and socioeconomic status. Patients and controls were assessed for their intelligence quotient (IQ), and attention and information-processing abilities. Magnetic resonance imaging (MRI) of the brain was performed in all patients. Neuropsychological assessments and MRI were repeated at a five-year-follow-up.

RESULTS

In the five-year interval IQ, information processing and attention of patients and controls remained constant. At both assessment times the IQ scores were significantly lower in patients compared to controls. Older adult patients (>32 years) showed poorer information processing and attention at both assessment times compared to young adult patients (<32 years) and controls. IQ, information processing and attention showed no correlation to imaging results but were significantly correlated to blood phenylalanine (Phe) levels in patients' childhood and adolescence, and Phe levels had been higher in the adolescent years of older adult patients.

CONCLUSIONS

Cognitive performance in adult patients with early-treated PKU does not seem to be subject to deterioration observable in a five-year interval. Neuropsychological assessment in adults with PKU revealed neurocognitive impairment particularly in older adult patients. This seems to refer to an early relaxation of diet that was recommended when the older patients were adolescents. Results indicate a benefit of dietary control during adolescence in PKU.

Phenylalanine hydroxylase deficiency

Phenylalanine hydroxylase deficiency is an autosomal recessive disorder that results in intolerance to the dietary intake of the essential amino acid phenylalanine. It occurs in approximately 1:15,000 individuals. Deficiency of this enzyme produces a spectrum of disorders including classic phenylketonuria, mild phenylketonuria, and mild hyperphenylalaninemia. Classic phenylketonuria is caused by a complete or near-complete deficiency of phenylalanine hydroxylase activity and without dietary restriction of phenylalanine most children will develop profound and irreversible intellectual disability. Mild phenylketonuria and mild hyperphenylalaninemia are associated with lower risk of impaired cognitive development in the absence of treatment. Phenylalanine hydroxylase deficiency can be diagnosed by newborn screening based on detection of the presence of hyperphenylalaninemia using the Guthrie microbial inhibition assay or other assays on a blood spot obtained from a heel prick. Since the introduction of newborn screening, the major neurologic consequences of hyperphenylalaninemia have been largely eradicated. Affected individuals can lead normal lives. However, recent data suggest that homeostasis is not fully restored with current therapy. Treated individuals have a higher incidence of neuropsychological problems. The mainstay of treatment for hyperphenylalaninemia involves a low-protein diet and use of a phenylalanine-free medical formula. This treatment must commence as soon as possible after birth and should continue for life. Regular monitoring of plasma phenylalanine and tyrosine concentrations is necessary. Targets of plasma phenylalanine of 120-360 μmol/L (2-6 mg/dL) in the first decade of life are essential for optimal outcome. Phenylalanine targets in adolescence and adulthood are less clear. A significant proportion of patients with phenylketonuria may benefit from adjuvant therapy with 6R-tetrahydrobiopterin stereoisomer. Special consideration must be given to adult women with hyperphenylalaninemia because of the teratogenic effects of phenylalanine. Women with phenylalanine hydroxylase deficiency considering pregnancy should follow special guidelines and assure adequate energy intake with the proper proportion of protein, fat, and carbohydrates to minimize risks to the developing fetus. Molecular genetic testing of the phenylalanine hydroxylase gene is available for genetic counseling purposes to determine carrier status of at-risk relatives and for prenatal testing.

Tackling frontal lobe-related functions in PKU through functional brain imaging: a Stroop task in adult patients

BACKGROUND

Profound mental retardation in phenylketonuria (PKU) can be prevented by a low phenylalanine (Phe) diet. However, even patients treated early have inconsistently shown deficits in several frontal lobe-related neuropsychological tasks such as the widely accepted Stroop task. The goal of this study was to investigate whether adult patients exhibit altered brain activation in Stroop-related locations in comparison to healthy controls and if an acute increase in blood Phe levels in patients has an effect on activation patterns.

METHODS

Seventeen male, early-treated patients with classic PKU (mean ± SD age: 31.0 ± 5.2 years) and 15 male healthy controls (32.1 ± 6.4 years) were compared using a color-word matching Stroop task in a functional magnetic resonance imaging (fMRI) study at 3T. Participants were scanned twice, and an oral Phe load (100 mg/kg body weight) was administered to patients prior to one of the fMRI sessions (placebo-controlled). Activity in brain regions that are known to be involved in Stroop tasks was assessed.

RESULTS

PKU patients exhibited poorer accuracy in incongruent trials. Reaction times were not significantly different. There were no consistent differences in BOLD activations in Stroop-associated brain regions. The oral Phe administration had no significant effect on brain activity.

CONCLUSIONS

Neither a generally slower task performance nor distinctively altered functioning of brain networks involved in a task representing a subset of dopamine-dependent executive functions could be proven. Decreased accuracy and inconsistent findings in posterior areas necessitate further study of frontal-lobe functioning in PKU patients in larger study samples.

Biological and social influences on cognitive control processes dependent on prefrontal cortex

Cognitive control functions ("executive functions" [EFs] such as attentional control, self-regulation, working memory, and inhibition) that depend on prefrontal cortex (PFC) are critical for success in school and in life. Many children begin school lacking needed EF skills. Disturbances in EFs occur in many mental health disorders, such as ADHD and depression. This chapter addresses modulation of EFs by biology (genes and neurochemistry) and the environment (including school programs) with implications for clinical disorders and for education. Unusual properties of the prefrontal dopamine system contribute to PFC's vulnerability to environmental and genetic variations that have little effect elsewhere. EFs depend on a late-maturing brain region (PFC), yet they can be improved even in infants and preschoolers, without specialists or fancy equipment. Research shows that activities often squeezed out of school curricula (play, physical education, and the arts) rather than detracting from academic achievement help improve EFs and enhance academic outcomes. Such practices may also head off problems before they lead to diagnoses of EF impairments, including ADHD. Many issues are not simply education issues or health issues; they are both.

Challenges and pitfalls in the management of phenylketonuria

Despite recent advances in the management of phenylketonuria and hyperphenylalaninemia, important questions on the management of this disorder remain unanswered. Consensus exists on the need for neonatal screening and early treatment, yet disagreement persists over threshold levels of blood phenylalanine for starting treatment, target blood phenylalanine levels, and the management of older patient groups. The mainstay of treatment is a phenylalanine-restricted diet, but its application varies between and within countries. Beyond diet treatment, there is a lack of consensus on the use of newer treatments such as tetrahydrobiopterin. Although neonatal screening and early treatment has meant that most well-treated children grow up with near-normal IQ scores, the effect of relaxing metabolic control on cognitive and executive function later in life is still not fully understood. Although it is clear from the available literature that the active control of blood phenylalanine levels is of vital importance, there are other treatment-related factors that affect outcome. A uniform and firmly evidence-based approach to the management of phenylketonuria is required.

What we know that could influence future treatment of phenylketonuria

Phenylketonuria (PKU), a Mendelian autosomal recessive phenotype (OMIM 261600), is an inborn error of metabolism that can result in impaired postnatal cognitive development. The phenotypic outcome is multifactorial in origin, based both in nature, the mutations in the gene encoding the L-phenylalanine hydroxylase enzyme, and nurture, the nutritional experience introducing L-phenylalanine into the diet. The PKU story contains many messages including a framework to appreciate the complexity of this disease where phenotype reflects both locus-specific and genomic components. This knowledge is now being applied in the development of patient-specific therapies.

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.

Reduced cerebral fluoro-L-dopamine uptake in adult patients suffering from phenylketonuria

Deficiency of phenylalanine hydroxylase activity in phenylketonuria (PKU) causes an excess of phenylalanine (Phe) throughout the body, predicting impaired synthesis of catecholamines in the brain. To test this hypothesis, we used positron emission tomography (PET) to measure the utilization of 6-[18F]fluoro-L-DOPA [corrected] (FDOPA) in the brain of adult patients suffering from PKU and in healthy controls. Dynamic 2-h long FDOPA emission recordings were obtained in seven adult PKU patients (five females, two males; age: 21 to 27 years) with elevated serum Phe levels, but lacking neurologic deficits. Seven age-matched, healthy volunteers were imaged under identical conditions. The utilization of FDOPA in striatum was calculated by linear graphical analysis (k3S, min(-1)), with cerebellum serving as a nonbinding reference region. The time to peak activity in all brain time-radioactivity curves was substantially delayed in the PKU patients relative to the control group. The mean magnitude of k3S in the striatum of the PKU patients (0.0052+/-0.0004 min(-1)) was significantly lower than in the control group (0.0088+/-0.0009 min(-1)) (P<0.001). There was no significant correlation between individual serum Phe levels and k3S. The unidirectional clearance of FDOPA to brain was impaired in adult patients suffering from PKU, presumably reflecting the competitive inhibition of the large neutral amino acid carrier by Phe. Assuming this competition to be spatially uniform, the relationship between striatum and cerebellum time-activity curves additionally suggests inhibition of DOPA efflux, possibly also due to competition from Phe. The linear graphical analysis shows reduced k3S in striatum, indicating reduced DOPA decarboxylase activity.

Pathogenesis of CNS involvement in disorders of amino and organic acid metabolism

Inherited disorders of amino and organic acid metabolism have a high cumulative frequency, and despite heterogeneous aetiology and varying clinical presentation, the manifestation of neurological disease is common. It has been demonstrated for some of these diseases that accumulating pathological metabolites are directly involved in the manifestation of neurological disease. Various pathomechanisms have been suggested in different in vitro and in vivo models including an impairment of brain energy metabolism, an imbalance of excitatory and inhibitory neurotransmission, altered transport across the blood-brain barrier and between glial cells and neurons, impairment of myelination and disturbed neuronal efflux of metabolic water. This review summarizes recent knowledge on pathomechanisms involved in phenylketonuria, glutaric aciduria type I, succinic semialdehyde dehydrogenase deficiency and aspartoacylase deficiency with examples, highlighting general as well as disease-specific concepts and their putative impact on treatment.

The PAH gene, phenylketonuria, and a paradigm shift

"Inborn errors of metabolism," first recognized 100 years ago by Garrod, were seen as transforming evidence for chemical and biological individuality. Phenylketonuria (PKU), a Mendelian autosomal recessive phenotype, was identified in 1934 by Asbjörn Fölling. It is a disease with impaired postnatal cognitive development resulting from a neurotoxic effect of hyperphenylalaninemia (HPA). Its metabolic phenotype is accountable to multifactorial origins both in nurture, where the normal nutritional experience introduces L-phenylalanine, and in nature, where mutations (>500 alleles) occur in the phenylalanine hydroxylase gene (PAH) on chromosome 12q23.2 encoding the L-phenylalanine hydroxylase enzyme (EC 1.14.16.1). The PAH enzyme converts phenylalanine to tyrosine in the presence of molecular oxygen and catalytic amounts of tetrahydrobiopterin (BH4), its nonprotein cofactor. PKU is among the first of the human genetic diseases to enter, through newborn screening, the domain of public health, and to show a treatment effect. This effect caused a paradigm shift in attitudes about genetic disease. The PKU story contains many messages, including: a framework on which to appreciate the complexity of PKU in which phenotype reflects both locus-specific and genomic components; what the human PAH gene tells us about human population genetics and evolution of modern humans; and how our interest in PKU is served by a locus-specific mutation database (http://www.pahdb.mcgill.ca; last accessed 20 March 2007). The individual Mendelian PKU phenotype has no "simple" or single explanation; every patient has her/his own complex PKU phenotype and will be treated accordingly. Knowledge about PKU reveals genomic components of both disease and health.

MR Spectroscopy of Metabolic Disorders

The application of MR spectroscopy (MRS) in pediatric brain disorders yields valued information on pathologic processes, such as ischemia, demyelination, gliosis, and neurodegeneration. Because these processes manifest in inborn errors of metabolism, the purposes of this article are to (1) describe the spectral changes that are associated with the relatively common metabolic disorders, with summaries of known spectroscopic features of these disorders; (2) offer suggestions for recognition and distinction of disorders; and (3) provide general guidelines for MRS implementation. Although many conditions have a similar presentation, MRS offers valuable information for the individual patient in diagnosis and therapy when integrated fully into the clinical setting.

Human LAT1 single nucleotide polymorphism N230K does not alter phenylalanine transport

The deleterious effects on the brain of phenylketonuria are caused by the saturation of the blood-brain barrier large neutral amino acid transporter type 1 (LAT1) by high plasma phenylalanine concentrations. There is only one known single nucleotide polymorphism (SNP) of the open reading frame of human LAT1, N230K. Site-directed mutagenesis of the wild type human LAT1 cDNA replicated the N230K SNP, and the corresponding cloned RNA encoding either the wild type or N230K human LAT1 were injected into frog oocytes. The kinetics of phenylalanine transport via either form of the human LAT1 was not significantly different. Similarly, there was no difference in the kinetics of phenylalanine transport via the wild type rabbit LAT1 or the corresponding K226N mutant of rabbit LAT1. These studies demonstrate that the only known SNP in the open reading frame of human LAT1 has no effect on the kinetics of large neutral amino acid transport via this carrier.

1H MR chemical shift imaging detection of phenylalanine in patients suffering from phenylketonuria (PKU)

Short echo time single voxel methods were used in previous MR spectroscopy studies of phenylalanine (Phe) levels in phenylketonuria (PKU) patients. In this study, apparent T2 relaxation time of the 7.3-ppm Phe multiplet signal in the brain of PKU patients was assessed in order to establish which echo time would be optimal. 1H chemical shift imaging (CSI) examinations of a transverse plain above the ventricles of the brain were performed in 10 PKU patients and 11 persons not suffering from PKU at 1.5 T, using four echo times (TE 20, 40, 135 and 270 ms). Phe was detectable only when the signals from all CSI voxels were summarized. In patients suffering from PKU the T2 relaxation times of choline, creatine and N-acetyl aspartate (NAA) were similar to those previously reported for healthy volunteers (between 200 and 325 ms). The T2 of Phe in brain tissue was 215 +/- 120 ms (standard deviation). In the PKU patients the brain tissue Phe concentrations were 141 +/- 69 microM as opposed to 58 +/- 23 microM in the persons not suffering from PKU. In the detection of Phe, MR spectroscopy performed at TE 135 or 270 ms is not inferior to that performed at TE 20 or 40 ms (all previous studies). Best results were obtained at TE=135 ms, relating to the fact that at that particular TE, the visibility of a compound with a T2 of 215 ms still is good, while interfering signals from short-TE compounds are negligible.

Developmental regulation of the rabbit blood-brain barrier LAT1 large neutral amino acid transporter mRNA and protein

The expression of the blood-brain barrier (BBB) LAT1 large neutral amino acid transporter mRNA and protein was investigated in development in rabbits. The BBB LAT1 mRNA was down-regulated with postnatal development. However, the BBB immunoreactive LAT1 protein was unchanged in postnatal development, despite an up-regulation of the BBB GLUT1 glucose transporter protein during this period. The dissociation between LAT1 protein and mRNA levels in development is consistent with posttranscriptional regulation of BBB LAT1 gene expression.

Site-directed mutagenesis of rabbit LAT1 at amino acids 219 and 234

The availability of amino acids in the brain is regulated by the blood-brain barrier (BBB) large neutral amino acid transporter type 1 (LAT1) isoform, which is characterized by a high affinity (low Km) for substrate large neutral amino acids. The hypothesis that brain amino acid transport activity can be altered with single nucleotide polymorphisms was tested in the present studies with site-directed mutagenesis of the BBB LAT1. The rabbit has a high Km LAT1 large neutral amino acid transporter, as compared to the low Km neutral amino acid transporter at the human or rat BBB. The rabbit LAT1 was cloned from a rabbit brain capillary cDNA library. Alignment of the amino acid sequences of rabbit, human, and rat LAT1 revealed two radical amino acid residues that differ in the rabbit relative to the rat or human LAT1. The G219D mutation had a modest effect on the Km and Vmax of tryptophan transport via cloned rabbit LAT1 in frog oocytes, but the W234L variant reduced the Km by 64% and the Vmax by 96%. Conversely, LAT1 transport of either tryptophan or phenylalanine was nearly normalized when the double mutation W234L/G219D variant was produced. These studies show that marked changes in the affinity and capacity of the LAT1 are caused by single nucleotide polymorphisms and that phenotype can be restored with a double mutation.

Metabolic disorders and mental retardation

The metabolic and anatomical substrate of most forms of mental retardation is not known. Because the basis of normal brain function is not sufficiently understood, the basis of abnormal function is understood poorly. Even in disorders where the fundamental biochemical defect is known, such as phenylketonuria (PKU) and other enzyme defects, the exact basis for brain dysfunction is uncertain. The outcome for treated PKU, galactosemia, homocystinuria, and lysosomal disorders is not yet optimal. The various forms of nonketotic hyperglycinemia often respond poorly to current therapy. Less familiar disorders, with or without seizures, such as deficient synthesis of serine or creatine and impaired glucose transport into the brain, and disorders with variable malformations, such as Smith-Lemli-Opitz (SLO) syndrome and the congenital disorders of glycosylation (CDGs), may initially be thought to be a nonspecific form of developmental delay. Less familiar disorders, with or without seizures and disorders with variable malformations may initially be thought to be a nonspecific form of developmental delay. Simple tests of urine, blood, and cerebrospinal fluid may lead to a diagnosis, accurate genetic counseling, and better treatment. Metabolic brain imaging (magnetic resonance spectroscopy (MRS)) has also helped to reveal biochemical abnormalities within the brain.

Different presentations of late-detected phenylketonuria in two brothers with the same R408W/R111X genotype in the PAH gene

Although the clinical heterogeneity of phenylketonuria (PKU) is well established, some questions about this condition remain. Subjects from the same family who share the same mutations in the phenylalanine hydroxylase (PAH) gene are expected to display similar disease courses, and therefore, when blood phenylalanine (Phe) levels, genotype and dietary treatment are all similar, differences in patient outcomes require additional explanations. The present authors describe two entirely different courses of late-detected PKU in two brothers with the same R408W/R111X genotype in the PAH gene. The older sibling was diagnosed with PKU at the age of 4 years and given treatment. His IQ was 97 at 26 years of age and moderate involvement of periventricular white matter was detected. The younger brother was diagnosed with PKU at the age of 11 months and given treatment. His IQ was < 25 at 22 years of age and severe dysmyelination changes were found by magnetic resonance imaging. The differences in the courses of the disease between these two brothers appear to be related to variations in their blood-brain barriers.

Phenylalanine can be detected in brain tissue of healthy subjects by 1H magnetic resonance spectroscopy

Transport of phenylalanine (Phe) and the other large neutral amino acids across the blood-brain barrier plays a crucial role in the pathogenesis of phenylketonuria (PKU). Thus, investigation of Phe transport kinetics by means of proton magnetic resonance spectroscopy (1H MRS) became an important research area in the mid 1990s. As 1H MRS measurements of brain phenylalanine are restricted to tissue concentrations above 100-150 micromol/kg wet weight, this approach was possible only in PKU patients, and comparison with healthy controls was not achieved. Using standardized single-dose oral Phe loading in three healthy subjects, it was shown that Phe values increase steeply, peak at about 1 h post load, and decrease thereafter. In a single case study, repetitive Phe loading was then performed to achieve a plateau of high blood Phe concentrations for several hours. It was demonstrated that detection and monitoring of brain Phe concentrations is feasible by means of 1H MRS. This approach constitutes a prerequisite for describing carrier kinetics in health.

Individual blood-brain barrier phenylalanine transport in siblings with classical phenylketonuria

Recent studies indicate that individual blood-brain transport characteristics of phenylalanine may lead to different clinical outcomes in phenylketonuria (PKU) patients in spite of comparable dietary control. To check these preliminary data, we investigated four pairs of siblings with classical PKU (and identical genotype) using in vivo nuclear magnetic resonance spectroscopy in the course of an oral phenylalanine load (100 mg/kg body weight). Patients' brain phenylalanine concentrations were different in spite of similar blood levels. Interindividual variations of the apparent transport Michaelis constant, Kt,app, ranged from 0.10 to 0.84 mmol/L. The ratio of the maximal transport velocity, Tmax, over the intracerebral consumption rate, Vmet, varied between 2.61 and 14.0. Siblings with lower values for Kt,app, higher values for Tmax/Vmet, and higher concurrent brain phenylalanine levels showed a lower IQ and a higher degree of cerebral white matter abnormalities. The results indicate that blood-brain barrier transport characteristics and the resultant brain phenylalanine levels are causative factors for the individual clinical outcome in PKU.

Experimental hyperphenylalaninemia provokes oxidative stress in rat brain

Tissue accumulation of L-phenylalanine (Phe) is the biochemical hallmark of human phenylketonuria (PKU), an inherited metabolic disorder clinically characterized by mental retardation and other neurological features. The mechanisms of brain damage observed in this disorder are poorly understood. In the present study we investigated some oxidative stress parameters in the brain of rats with experimental hyperphenylalaninemia. Chemiluminescence, total radical-trapping antioxidant potential (TRAP), superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) activities were measured in the brain of the animals. We observed that chemiluminescence is increased and TRAP is reduced in the brain of hyperphenylalaninemic rats. Similar data were obtained in the in vitro experiments using Phe at various concentrations. CAT activity was significantly inhibited by Phe in vitro and in vivo, whereas GSH-Px activity was reduced in vivo but not in vitro and SOD activity was not altered by any treatment. The results indicate that oxidative stress may be involved in the neuropathology of PKU. However, further studies are necessary to confirm and extend our findings to the human condition and also to determine whether an antioxidant therapy may be of benefit to these patients.

Individual blood-brain barrier phenylalanine transport determines clinical outcome in phenylketonuria

Different clinical outcomes in spite of comparable dietary controls are well known in patients with phenylketonuria. Currently, reasons for this phenomenon are unknown. Kinetic investigations in 15 patients with classic phenylketonuria were performed using in vivo nuclear magnetic resonance spectroscopy before and after an oral phenylalanine load (100 mg/kg body weight). Patients' brain phenylalanine concentrations were quite different in spite of similar blood phenylalanine levels. Interindividual variations of the apparent transport Michaelis constant, K(t,app), covered a range from 0.10 to 1.03 mmol/L. The ratio of the maximal transport velocity, Tmax, over the intracerebral consumption rate, Vmet, varied between 2.61 and 14.0. Both parameters as well as the preload brain phenylalanine levels correlated significantly with the degree of cerebral white matter abnormalities on magnetic resonance images. Correlations of K(t,app), Tmax/Vmet, and the preload brain phenylalanine levels with patients' intelligence scores approached significance. In conclusion, blood-brain barrier phenylalanine transport characteristics and the resultant brain phenylalanine levels seem to be causative factors for the individual clinical outcome in phenylketonuria. This observation may lead to individual dietary recommendations in the future.

Treatable neurotransmitter deficiency in mild phenylketonuria

The authors describe a case of neurologic involvement in mild hyperphenylalaninemia (HPA), not due to tetrahydrobiopterin (BH(4)) deficiency, with low levels of monoamine neurotransmitter metabolites in CSF. The combined BH(4)-Phe loading test suggested a BH(4) response, confirmed by clinical improvement after BH(4) therapy. Molecular study revealed a compound heterozygosity of the phenylalanine hydroxylase alleles: a mild HPA-associated mutation (T380M) and the new mutation D151E. This case demonstrates that even mild HPA, generally considered a benign disorder, may present neurologic impairment.

Assessment of adult phenylketonuria

Phenylketonuria (PKU) has been detected on the newborn screening programme since the 1960s. Although it is recognised that dietary treatment is successful in avoiding the severe mental retardation associated with untreated PKU, the long-term outcome for adults remains unclear. The Medical Research Council recommends that the diet be followed for life. This paper discusses the relevance of the findings of neurological deterioration, neuropsychological problems and brain imaging in adults with PKU. It suggests an approach to follow-up for adults with PKU including neurological assessments, awareness of nutritional deficiencies, educational requirements and the risks of maternal PKU.

Garrod's foresight; our hindsight

Archibald Edward Garrod introduced a paradigm, new for its day, in medicine: Biochemistry is dynamic and different from the static nature of organic chemistry. It led him to think about metabolic pathways and to recognize that variation in Mendelian heredity could explain an 'inborn error of metabolism'. At the time, Garrod had no idea about the nature of a gene. Genes are now well understood, genomes are being described for one organism after another (including H. sapiens) and it is understood that genomes 'speak biochemistry (not phenotype)'. Accordingly, in the era of genomics, biochemistry and physiology become the bases of functional genomics and it is possible to appreciate why 'nothing in biology makes sense without evolution' (and nothing in medicine will make sense without biology). Mendelian, biochemical and molecular genetics together have revealed what lies behind the four canonical inborn errors described by Garrod (albinism, alkaptonuria, cystinuria and pentosuria). Both older and newer ideas in genetics, new tools for applying them, and renewed respect for the clinician-scientist will enhance our understanding of the human biological variation that accounts for variant states of health and overt disease; an 'unsimple' phenotype (phenylketonuria) is used to illustrate in some detail. What can be known and what ought to be done with knowledge about human genetics to benefit individuals, families and communities (society) is both opportunity and challenge.

Normal clinical outcome in untreated subjects with mild hyperphenylalaninemia

ABSTRACT There is international consensus that patients with phenylalanine (Phe) levels <360 microM on a free diet do not need Phe-lowering dietary treatment whereas patients with levels >600 microM do. Clinical outcome of patients showing Phe levels between 360 and 600 microM in serum on a free nutrition has so far only been assessed in a small number of cases. Therefore, different recommendations exist for patients with mild hyperphenylalaninemia. We investigated in a nationwide study 31 adolescent and adult patients who persistently displayed serum Phe levels between 360 and 600 microM on a normal nutrition with a corresponding genotype. Because of limited accuracy of measurements, Phe levels should be looked on as an approximation, but not as an absolute limit in every instance. In addition to serum Phe levels, the assessment program consisted of comprehensive psychological testing, magnetic resonance imaging of the head, (1)H magnetic resonance spectroscopy, and genotyping. We found a normal intellectual (intelligence quotient, 103 +/- 15; range, 79-138) and educational (school performance and job career) outcome in these subjects as compared with healthy control subjects (intelligence quotient, 104 +/- 11; range, 80-135). Magnetic resonance imaging revealed no changes of cerebral white matter in any patient, and (1)H magnetic resonance spectroscopy revealed brain Phe levels below the limit of detection (<200 microM). In the absence of any demonstrable effect, dietary treatment is unlikely to be of value in patients with mild hyperphenylalaninemia and serum Phe levels <600 microM on a free nutrition, and should no longer be recommended. Because of a possible late-onset phenylketonuria, Phe levels of untreated patients should be monitored carefully at least during the first year of life. Nevertheless, problems of maternal phenylketonuria should still be taken into account.

Variability of blood-brain ratios of phenylalanine in typical patients with phenylketonuria

Blood-brain ratios (BBR) of phenylalanine (Phe) were determined by quantitative in vivo 1H magnetic resonance spectroscopy (1H-MRS) in 17 adult patients with early-treated phenylketonuria who were randomly selected from a sample of 75 adults. Measurements were performed in all patients during steady-state conditions. The BBR showed a unimodal distribution with a mean of 4.0 (range 3.3 to 4.5). Blood-brain ratios were comparable for subgroups of patients with genotypes classified as severe, moderate, or mild and for patients on different types of diets. Brain Phe concentrations showed a strong linear correlation with blood Phe values (r = 0.93, P < 0.001). There were no saturation effects for blood Phe values up to 1.8 mmol/L, and a local regression analysis did not confirm increasing BBR for increasing blood Phe values. The intellectual outcome (Wechsler Adult Intelligence Scale) was correlated with long-term dietary control (r = -0.65, P < 0.05), fluctuation of blood Phe values during treatment (r = -0.60, P < 0.05), and concurrent blood and brain Phe concentration. The severity of white matter changes visible on magnetic resonance images (MRI) was increased with high blood and brain Phe concentrations but failed to reach statistical significance. No correlation was found between BBR values, intelligence quotient, and MRI grade. Based on the assumption that BBR show intraindividual stability, the current data do not support the hypothesis that blood-brain barrier transport of Phe is a key explanatory factor for outcome variability in the vast majority of "typical" patients with phenylketonuria.

Blood-brain phenylalanine relationships in persons with phenylketonuria

OBJECTIVES

Clinicians caring for persons with phenylketonuria (PKU) have been perplexed by the occasional normal individual with the classical biochemical profile consistent with the diagnosis of PKU. Usually untreated subjects with the biochemical profile of blood phenylalanine (Phe) levels >1200 micromol/L are severely mentally retarded and may have neurological findings. Preliminary reports have recently appeared suggesting that low brain Phe levels, in comparison with elevated blood Phe levels, account for the occurrence of these occasional unaffected individuals with the biochemical profile consistent with PKU.

METHOD

Magnetic resonance imaging/magnetic resonance spectroscopy was used to measure brain Phe content compared with simultaneously obtained blood Phe levels determined on the amino acid analyzer. This comparison was obtained in 5 normal non-PKU persons, 4 carriers of the gene causing PKU, and in 29 individuals with the proven form of the disorder.

RESULTS

Blood-brain measurements in 5 normal persons ranged from.051 to.081 mmol/L, with a mean of.058 mmol/L. Their simultaneously measured brain levels of Phe ranged from.002 to.15 mmol/L, with a mean of.09 mmol/L. Similar measurements were obtained in 4 carriers of the gene causing PKU. Their blood levels varied between.068 and.109 mmol/L, with a mean of.091 mmol/L and simultaneously obtained brain levels of Phe varied between.06 and.21 mmol/L, with a mean of.11 mmol/L. Twenty subjects with a mean IQ of 104 exhibited a mean blood level of 1.428 mmol/L and a simultaneous mean brain level of.23 mmol/L, whereas 9 persons with a mean IQ of 98.7 exhibited a mean blood Phe level of 1.424 and a mean brain Phe level of.64 mmol/L. The correlation between blood and brain levels was not significant.

CONCLUSION

In usual cases, intellectually normal persons who have never been treated but who have a biochemical profile consistent with classical PKU exhibit lower brain levels of Phe. Such individuals are exceptional and may not need the vigorous restriction of their blood Phe levels that is required by the majority of persons with PKU.

MR spectroscopy: a powerful tool for investigating brain function and neurological diseases

Magnetic resonance spectroscopy (MRS) has attracted much attention in recent years and has become an important tool to study in vivo particular biochemical aspects of brain disorders. Since the proton is the most sensitive stable nucleus for MRS, and since almost all metabolites contain hydrogen atoms, investigation by in vivo 1H MRS provides chemical information on tissue metabolites, thus enabling a non-invasive assessment of changes in brain metabolism underlying several brain diseases. In this review a brief description of the basic principles of MRS is given. Moreover, we provide some explanations on the techniques and technical problems related to the use of 1H MRS in vivo including water suppression, localization, editing, quantitation and interpretation of 1H spectra. Finally, we discuss the more recent advancement in three major areas of neurological diseases: brain tumors, multiple sclerosis, and inborn errors of metabolism.

Clinical significance of brain phenylalanine concentration assessed by in vivo proton magnetic resonance spectroscopy in phenylketonuria

Recent studies using in vivo proton magnetic resonance spectroscopy (1H MRS) have suggested that plasma phenylalanine (Phe) may not be a reliable indicator of brain Phe level in subjects with phenylketonuria (PKU). Interindividual variation in cerebral Phe can contribute to the phenotypic variability of the disease. We report the results of the direct assessment of brain Phe by 1H MRS in 10 off-diet PKU patients (aged 15.5-30.5 years), 4 detected and treated early, 6 late. In a single patient, brain Phe was evaluated before and 15 days after diet discontinuation. FLAIR MRI and 1H MRS were performed in the same setting by a 1.5 T clinical MR scanner. MR images were scored according to the extent of the lobar white-matter hyperintensity. Brain 1H MRS Phe signal (resonating at 7.36 ppm) was evaluated as a ratio to the creatine+phosphocreatine signal. Brain Phe was correlated with clinical, biochemical and MRI findings. Results were as follows. (1) An abnormal concentration of brain Phe was detected in all 10 PKU subjects (ranging from 0.030 to 0.074), associated with a wide interindividual variability of concurrent plasma Phe (ranging from 724 to 2800 micromol/L). (2) In late-detected subjects, brain Phe concentration correlated with clinical phenotype better than did plasma Phe. The discrepancy between brain and plasma Phe was relevant from a clinical point of view in two cases: in one, a late-detected patient with normal mental development, a high level of plasma Phe was associated with a relatively low concentration of brain Phe; in the other, a late-detected subject with severe neurological impairment, a very high level of brain Phe was associated with plasma Phe compatible with the diagnosis of mild PKU. (3) White-matter alterations were detected in all patients. FLAIR MRI sequences disclosed an involvement of optic chiasma and tracts in 7 subjects. No correlation was found between white-matter alterations and concurrent brain Phe concentrations. (4) In the only case assessed under different intake of Phe, the relevant increase of brain Phe paralleled the concurrent increase of plasma Phe, showing that 1H MRS can be a useful tool in evaluating the individual vulnerability of PKU patients to different values of plasma Phe.

Proton NMR chemical shifts and coupling constants for brain metabolites

Proton NMR chemical shift and J-coupling values are presented for 35 metabolites that can be detected by in vivo or in vitro NMR studies of mammalian brain. Measurements were obtained using high-field NMR spectra of metabolites in solution, under conditions typical for normal physiological temperature and pH. This information is presented with an accuracy that is suitable for computer simulation of metabolite spectra to be used as basis functions of a parametric spectral analysis procedure. This procedure is verified by the analysis of a rat brain extract spectrum, using the measured spectral parameters. In addition, the metabolite structures and example spectra are presented, and clinical applications and MR spectroscopic measurements of these metabolites are reviewed.