Tyrosine requirements in children with classical PKU determined by indicator amino acid oxidation

Tyrosine (Tyr) is an essential amino acid in phenylketonuria (PKU) because of the limited hydroxylation of phenylalanine (Phe) to Tyr. The recommended intakes for Tyr in PKU are at least five times the recommended phenylalanine intakes. This suggests that Phe and Tyr contribute approximately 20 and 80%, respectively, of the aromatic amino acid (AAA) requirement (REQ). In animals and normal humans, dietary Tyr was shown to spare 40-50% of the Phe requirement, proportions that reflect dietary and tissue protein composition. We tested the hypothesis that the Tyr REQ in PKU would account for 45% of the total AAA REQ by indicator amino acid oxidation (IAAO). Tyr REQ was determined in five children with PKU by examining the effect of varying dietary Tyr intake on lysine oxidation and the appearance of (13)CO(2) in breath (F(13)CO(2)) under dietary conditions of adequate energy, protein (1.5 g x kg(-1) x day(-1)), and phenylalanine (25 mg x kg(-1) x day(-1)). Lysine oxidation and F(13)CO(2) were determined using a primed 4-h oral equal-dose infusion of L-[1-(13)C]lysine. Lysine oxidation and F(13)CO(2) decreased linearly as Tyr intake increased, to a break point that was interpreted as the mean dietary Tyr requirement (16.3 and 19.2 mg x kg(-1) x day(-1), respectively). At Tyr intakes of >16.3 and 19.2 mg x kg(-1) x day(-1), lysine oxidation and F(13)CO(2), respectively, were low and constant. This represents 40.4 and 44.4%, respectively, of the total AAA intake. The current recommendations for Tyr intake in PKU patients appear to be overestimated by a factor of approximately 5. This study is the first application of the IAAO technique in a pediatric population and in humans with an inborn error of metabolism.

Visual event-related potentials in children with phenylketonuria

Visual event-related potentials (ERPs) were examined in 16 children (aged 5-14 y) with phenylketonuria (PKU) and 16 age- and sex-matched controls. Lifetime median measures of phenylalanine (Phe) were 230-460 micromol/l. The most recent Phe levels were 56-624 micromol/l. ERPs were recorded whilst the children performed a discrimination task. All stimuli were square wave gratings degree, which appeared for 33 ms. A response to an infrequent grating that differed in orientation or spatial frequency was required. The older children with PKU had a delay in the first peak (P1) of the ERP, and age-related changes in the amplitude of P1. There was attenuation of the second peak across age groups in PKU. There was no evidence of reduced response accuracy or longer reaction times in children with PKU. Latencies of the cognitive P3 were not delayed in PKU. The delayed early peaks are consistent with previous studies that have shown delayed visual evoked potentials in PKU. The lack of differences in reaction time and P3 may be due to relatively good Phe control in children with PKU, or to the simplicity of the task. Suggestions are made for future ERP studies of PKU.

Bone mineral density in patients with phenylketonuria

Dual energy X-ray absorptiometry was performed in 44 patients with phenylketonuria (PKU) aged 6-29 y. The phenylalanine-restricted diet was based on a low-protein diet in combination with phenylalanine-free amino acid mixtures and phenylalanine-low casein hydrolysate in 32 patients. The 10 oldest patients were supplemented only with casein hydrolysate, and the youngest child received only the amino acid mixture. One patient has recently come off the diet. Bone mineral density (BMD) of the lumbar spine and total BMD were measured and expressed as Z-score, i.e. the difference between the BMD of the patient and the average BMD of sex- and age-matched controls divided by the standard deviation of the control group. Normal BMD was found in 24 (54%) patients. Lumbar spine BMD was decreased in 20 patients and total BMD was decreased in 14 patients. Z-scores of -1to 2.5 were found in 14 patients (32%) and Z-scores of <-2.5 in 6 patients (14%). No significant correlation was found between total or lumbar spine BMD and daily intake of phenylalanine from natural sources in the low-protein diet or the amount of phenylalanine-free amino acid mixtures per kg of body weight. A significant negative correlation was observed between both total and lumbar spine BMD Z-scores and the amount of casein hydrolysate supplementation per kg of body weight (r = - 0.45; y = 0.07 - 0.69x; p < 0.01). Long-lasting dietary restriction in patients with PKU may increase the risk of late complications of dietary therapy, such as osteoporosis or trace element deficiency. O Bone mineral density, osteoporosis, phenylalanine-low diet, phenylketonuria

Regression of neuropsychological deficits in early-treated phenylketonurics during adolescence

Even early-treated phenylketonuric patients suffer from phenylalanine-associated (mild) neuropsychological impairment. To date it is still unclear whether patients' deficits show a progression on ageing. This unsolved question seems to be an important aspect in the still ongoing debate about how long and how strictly the patients should be maintained on diet. Twenty early-treated (20 +/- 10, 9-30 days) adolescent phenylketonurics (10 boys, 10 girls) and 20 healthy controls, matched for age, sex and IQ, were investigated twice at a mean ages of 11 and 14 years for their IQ (Culture Fair Intelligence Test-Scale 2; CFT-20), fine motor abilities (Motor Performance Task), sustained (Test d2) and selective attention (Stroop-Task). At the first test, examinations revealed significant blood phenylalanine-correlated neuropsychological deficits in PKU patients. In spite of raised blood phenylalanine concentrations during the following 3 years and significantly elevated concurrent blood phenylalanine concentrations, the repeated measurements revealed a significant decrease of patients' deficits compared to controls. Clinical-neurological status of patients and controls was normal at both test times. The results indicate a decreased vulnerability of PKU-patients with respect to their neuropsychological functioning against elevated blood phenylalanine levels on ageing.

Large neutral amino acids block phenylalanine transport into brain tissue in patients with phenylketonuria

Large neutral amino acids (LNAAs), including phenylalanine (Phe), compete for transport across the blood-brain barrier (BBB) via the L-type amino acid carrier. Accordingly, elevated plasma Phe impairs brain uptake of other LNAAs in patients with phenylketonuria (PKU). Direct effects of elevated brain Phe and depleted LNAAs are probably major causes for disturbed brain development and function in PKU. Competition for the carrier might conversely be put to use to lower Phe influx when the plasma concentrations of all other LNAAs are increased. This hypothesis was tested by measuring brain Phe in patients with PKU by quantitative 1H magnetic resonance spectroscopy during an oral Phe challenge with and without additional supplementation with all other LNAAs. Baseline plasma Phe was approximately 1,000 micromol/l and brain Phe was approximately 250 micromol/l in both series. Without LNAA supplementation, brain Phe increased to approximately 400 micromol/l after the oral Phe load. Electroencephalogram (EEG) spectral analysis revealed acutely disturbed brain activity. With concurrent LNAA supplementation, Phe influx was completely blocked and there was no slowing of EEG activity. These results are relevant for further characterization of the LNAA carrier and of the pathophysiology underlying brain dysfunction in PKU and for treatment of patients with PKU, as brain function might be improved by continued LNAA supplementation.

Reversal of hypopigmentation in phenylketonuria mice by adenovirus-mediated gene transfer

Phenylketonuria (PKU) is caused by deficiency of phenylalanine hydroxylase (PAH) in the liver. Patients with PKU show increased L-phenylalanine in blood, which leads to mental retardation and hypopigmentation of skin and hair. As a step toward gene therapy for PKU, we constructed a replication-defective, E1/E3-deleted recombinant adenovirus harboring human PAH cDNA under the control of a potent CAG promoter. When a solution containing 1.2 x 10(9) plaque-forming units of the recombinant adenovirus was infused into tail veins of PKU model mice (Pah(enu2)), predominant expression of PAH activity was observed in the liver. The gene transfer normalized the serum phenylalanine level within 24 h. However, it also provoked a profound host immune response against the recombinant virus; as a consequence, the biochemical changes lasted for only 10 d and rechallenge with the virus failed to reduce the serum phenylalanine concentration. Administration of an immunosuppressant, FK506, to mice successfully blocked the host immune response, prolonged the duration of gene expression to more than 35 d, and allowed repeated gene delivery. We noted a change in coat pigmentation from grayish to black after gene delivery. The current study is the first to demonstrate the reversal of hypopigmentation, one of the major clinical phenotypes of PKU in mice as well as in humans, by adenovirus-mediated gene transfer, suggesting the feasibility of gene therapy for PKU.

Guanosine triphosphate cyclohydrolase I deficiency: a rare cause of hyperphenylalaninemia

Tetrahydrobiopterin (BH4) deficiencies are a heterogeneous group of disorders caused by a defect in two of the three enzymes involved in its biosynthesis or in the two recycling enzymes. Except for the deficiency of dehydratase, an enzyme catalyzing a reaction in the recycling pathway, all other variants of BH4 deficiency are characterized by developmental delay, progressive neurological deterioration, hypokinesis, drooling, swallowing difficulty, truncal hypotonia, increased limb tone, myoclonus and brisk deep tendon reflexes. A deficiency of guanosine triphosphate cyclohydrolase I (GTPCH), the first enzyme in the biosynthetic pathway of BH4, is described in a 14-month-old male infant with hyperphenylalaninemia, developmental delay, hypertonia of the extremities, seizures, feeding difficulties, and vomiting. Urinary pteridine screening revealed very low levels of neopterin and biopterin which was highly suggestive of GTPCH deficiency. Low cerebrospinal fluid concentrations of 5-hydroxyindoleacetic acid (5HIAA) and homovanillic acid concentrations, together with no detectable neopterin and decreased concentrations of biopterin and folate, agreed with the diagnosis of GTPCH deficiency. Subsequently measured neopterin and biopterin synthesis in cytokine-stimulated skin fibroblasts confirmed GTPCH deficiency, albeit indirectly. The patient showed marked improvement on a low-protein low-phenylalanine diet with neurotransmitter precursor administration. The favorable outcome in this patient clearly shows that not only newborns with elevated phenylalanine levels but also older children with neurological signs and symptoms should be screened for a BH4 deficiency in order to have maximum benefit of the treatment.

Impairment of callosal and corticospinal system function in adolescents with early-treated phenylketonuria: a transcranial magnetic stimulation study

The transcranial activation and the conduction properties of corticospinal and callosal neurons were investigated in 12 early-treated adolescents (aged 17.3, SD 3.5 years; range 14-27 years) with phenylketonuria (PKU) by focal transcranial magnetic stimulation (fTMS) of the motor cortex. The patients had no functionally relevant motor disturbances in daily life or on clinical testing. Corticospinally mediated excitatory (response thresholds, amplitudes, central motor latencies) and inhibitory [duration of postexcitatory inhibition (PI)] effects of fTMS were investigated in contralateral hand muscles. Transcallosal inhibition (TI) (onset latency, duration, transcallosal latency) of tonic electromyographic (EMG) activity was tested in ipsilateral muscles. Peripheral motor latencies were determined for responses elicited by magnetic stimulation over cervical nerve roots. Ten normal subjects served as controls. Since in all PKU patients, central and peripheral motor latencies were normal, no neurophysiological indication of a demyelination of corticospinal or peripheral motor fibres was found. However, cortical thresholds of corticospinally mediated responses were increased (52.1, SD 11.6% versus 35.0, SD 7.4% of maximum stimulator output; P < 0.05; n = 24 hands) and their amplitudes reduced (2.9, SD 1.4 mV versus 6.1, SD 1.5 mV, P < 0.05). The duration of PI was shortened (132, SD 53 ms versus 178, SD 57 ms; P < 0.05). TI was absent in 37.5% of the investigated hands or tended to be weak. When TI was present, its onset latencies (38.0, SD 3.6 ms versus 34.7, SD 3.3 ms) and transcallosal latencies were prolonged (18.5, SD 3.8 ms versus 14.8, SD 3.2 ms), while its duration was normal. These abnormal excitatory and inhibitory effects of fTMS suggest a reduced susceptibility of cortical excitatory and inhibitory neuronal structures compatible with a loss of neurons or a rarefication of their dendrites.

Benefits of normalizing plasma phenylalanine: impact on behaviour and health. A case report

An elderly man with mental retardation who had never received dietary treatment for his phenylketonuria was placed on a phenylalanine-restricted diet. Social skills and walking gait improved and a new interest in the objects in his environment developed spontaneously. A 2-year analysis of diet, blood plasma phenylalanine levels and behavioural state indicated that small differences in phenylalanine intake impacted his well-being. Of significant note, leg tremor and spasm that precipitated severe self-injury were only reversible when plasma blood phenylalanine concentrations were titrated to near normal ranges and daily phenylalanine intake was strictly controlled. This case may offer a potential explanation for some of the late treatment failures that have been reported and suggest new avenues to explore in the late treatment of PKU.

Bone mineral status in children with phenylketonuria under treatment

Bone mineral status was assessed in 48 children with phenylketonuria (PKU) (20M, 28 F, aged 2.5-17 y). Bone density was measured in the distal third of the right forearm using single photon absorptiometry and was expressed as +/-SD with respect to age- and gender-matched controls. Serum calcium (Ca), magnesium (Mg), phosphorus (P), alkaline phosphatase (ALP), parathyroid hormone and 25-hydroxyvitamin D were measured in morning samples. The ratios of urinary Ca/creatinine (UCa/UCr), UP/UCr, UMg/UCr and hydroxyproline (OH-Pr)/UCr were calculated in urine samples collected over a period of 3 h. Patients' data were compared with those of 50 controls (22 M, 28 F, aged 3-15 y). The data showed severe osteopenia (below -2SD) in 22/48 patients. Bone loss was more prominent in patients over 8 y old. Bone density correlated significantly with age (r=-0.56,p < 0.001) and with Phe (r=-0.49, p < 0.007) but did not correlate with the other biochemical indices studied. Comparing PKU children with controls, significantly higher serum calcium and magnesium (p=0.04, p < 0.001, respectively), lower ALP (p=0.01), higher UCa/UCr ratio (p < 0.001), lower UP/UCr (p < 0.001) and lower UOH-Pr/UCr (p < 0.001) were found. Dietary compliance was poor in patients over the age of 8y, as only 3/22 of < or = 8y had mean serum phenylalanine >10mgdl(-1), in contrast to 21/26 in the older group. It is clear from the data that osteopenia is commonly found in PKU patients from early life. The biochemical data indicate a metabolic state of low bone turnover in PKU patients. In conclusion, a better, more restricted diet may correct osteopenia.

Nutrient intake and growth of infants with phenylketonuria undergoing therapy

BACKGROUND

Because of reports of poor growth, a study was conducted for 6 months in 35 infants with classic phenylketonuria diagnosed during the neonatal period who were fed Phenex-1 Amino Acid Modified Medical Food With Iron (Ross Products Division, Columbus, OH, U.S.A.).as their primary protein source.

METHODS

Diet diaries and anthropometric measures were obtained monthly as part of a larger study in which nutrition status was evaluated.

RESULTS

In 6-month-old infants, mean percentiles for crown-heel length (59.14+/-4.31 SEM), head circumference (63.88+/-4.50) and weight (71.51+/-4.25) were normal. Mean (+/- SEM) daily intake of medical food was 79+/-4 g; protein and energy intakes were 17.3+/-0.6 g and 2772+/-75.6 kJ (660+/-18 kcal). Mean daily phenylalanine and tyrosine intakes per kilogram of body weight were 40+/-1 mg and 219+/-9 mg. Intakes of protein, energy, and tyrosine were positively correlated with crown-heel length, head circumference, and weight at 3 months of study. Overall plasma phenylalanine and tyrosine concentrations during the 6-month study were 297+/-41 micromol/l and 58+/-5 micromol/l, respectively. Neither plasma phenylalanine nor tyrosine concentration was correlated with growth.

CONCLUSION

Phenex-1 supports normal growth when fed in adequate amounts. These data support those of the Medical Research Council Working Party on Phenylketonuria for 3 g/kg per day of amino acids from medical food.

Factors affecting cognitive, motor, behavioral and executive functioning in children with phenylketonuria

We administered measures of cognitive, frontal lobe (executive), behavioral and motor functioning to 18 children with classical phenylketonuria, aged 12-101 months, in order to determine the relationship of age, current and lifetime average phenylalanine levels, and individual variation (standard deviation of lifetime average levels) to these functions. On measures of cognitive function, in children > or = 3 y of age lower current phenylalanine levels were associated with higher cognitive functioning. On a behavioral temperament scale designed for normal children, we found that higher current and average phenylalanine levels correlated with more difficult temperament. Motor function was also poorer in children with phenylketonuria, and was most impaired in children with current phenylalanine levels >360 micromol/l. We also identified a previously unreported correlation between increased individual variation and poorer executive function performance, a finding that may raise new management concerns about level fluctuations. Maintenance of phenylalanine levels <360 micromol/l may be necessary for optimal performance in children with phenylketonuria.

Expression of Ca2+ channels from rat brain with model phenylketonuria in Xenopus oocytes

Ca2+ channels expressed in Xenopus oocytes using mRNA purified from the brain of the rats subjected to chronic treatment with l-phenylalanine in order to model conditions typical for the congenital disease called phenylketonuria (PKU) were studied using double microelectrode technique. The amplitude of Ca2+ channel currents (IBa, 40 mM Ba2+ as a charge carrier) directed in the oocytes by mRNA from the brain of the animals with model PKU was significantly smaller compared to the control animals (145+/-23 nA vs. 270+/-38 nA, p<0.025) while the voltage-dependence of both currents was similar and typical for that of high voltage-activated (HVA) Ca2+ channels. No evidence for the expression of low voltage-activated Ca2+ channels were found. The decrease of the overall HVA Ba2+ current under model PKU occurred primarily at the expense of the decaying, omega-conotoxin-sensitive component which accounted for about 64% of the total current amplitude in control, and apparently was associated with the activity of the expressed N-type Ca2+ channels. omega-Aga-IVA-sensitive, P/Q component of IBa that contributed not more than 10% to the total current in control showed no change under PKU conditions. In addition to the decreased amplitude, Ba2+ current from model PKU animals showed accelerated run-down during prolonged recording (50%/h compared to 15%/h in control). Our data suggest that hyperphenylalaninemic conditions affect the expression of preferentially N-type Ca2+ channels via the reduction of their specific mRNA content as well as influence the type and manner of channels regulation. The underexpression of N-type Ca2+ channels is consistent with the decrease in the overall number of synaptic contacts during PKU and may be one of the factors contributing to the severe damage of the brain function.

Prefrontal cortex cognitive deficits in children treated early and continuously for PKU

To begin to study the importance of dopamine for executive function abilities dependent on prefrontal cortex during early childhood, the present investigation studied children in whom we predicted reduced dopamine in prefrontal cortex but otherwise normal brains. These are children treated early and continuously for the metabolic disorder phenylketonuria (PKU). Untreated PKU is the most common biochemical cause of mental retardation. The root problem is an inability to convert one amino acid, phenylalanine (Phe), into another, tyrosine (Tyr), the precursor of dopamine. Phe levels in the bloodstream soar; Tyr levels fall. Treatment with a diet low in Phe reduces the Phe:Tyr imbalance but cannot eliminate it. We hypothesized that the resultant modest elevation in the ratio of Phe to Tyr in the blood, which results in slightly less Tyr reaching the brain, uniquely affects the cognitive functions dependent on prefrontal cortex because of the special sensitivity of prefrontally projecting dopamine neurons to small decreases in Tyr. In a 4-year longitudinal study, we found that PKU children whose plasma Phe levels were three to five times normal (6-10 mg/dl) performed worse than other PKU children with lower Phe levels, matched controls, their own siblings, and children from the general population on tasks that required the working memory and inhibitory control abilities dependent on dorsolateral prefrontal cortex. The impairment was as evident in our oldest age range (3 1/2-7 years) as it was in the youngest (6-12 months). The higher a child's Phe level, the worse that child's performance. Girls were more adversely affected than boys. The deficit appears to be selective, affecting principally one neural system, since even PKU children with Phe levels three to five times normal performed well on the 13 control tasks. Clinical implications for the treatment of PKU and other neurodevelopmental disorders are discussed.

Intellectual, neurologic, and neuropsychologic outcome in untreated subjects with nonphenylketonuria hyperphenylalaninemia. German Collaborative Study on Phenylketonuria

Based on the serum phenylalanine levels under free diet patients with hyperphenylalaninemia are classified as "classical" (>1200 micromol/L), "mild" (600-1200 micromol/L), or "non-phenylketonuria (PKU)-hyperphenylalaninemia" (<600 micromol/L). Recent studies revealed intellectual, neurologic, and neuropsychologic deficits as well as abnormalities of cerebral white matter (magnetic resonance imaging, MRI) in patients with early and adequately treated PKU. In addition deficits in IQ were reported for a group of 4-y-old patients with untreated mild PKU and non-PKU hyperphenylalaninemia (serum phenylalanine levels below 900 micromol/L). As a consequence, a lifelong diet with serum phenylalanine levels below 400 micromol/L was recommended even for those patients with serum phenylalanine levels remaining consistently between 400 and 600 micromol/L. Generally patients with non-PKU hyperphenylalaninemia were not treated, as a normal outcome was suspected, but the clinical development of patients with non-PKU hyperphenylalaninemia was not so far studied systematically. We assessed 28 untreated patients with non-PKU hyperphenylalaninemia (age: mean = 21.8, SD = 4.2 y) for IQ, school and job career, clinical-neurologic development, fine motor performances, selective and sustained attention, as well as for frontal lobe-dependent "executive functions." In addition, cranial MRI was obtained in 10 of these patients. Compared with healthy control subjects, matched for age, sex, and socioeconomic status, the patients reached normal results in all clinical and psychometric tests. Cranial MRI revealed no abnormalities. Additionally, no significant correlations between serum phenylalanine levels and test results were obtained. In the absence of any demonstrative effect, treatment is unlikely to be of significant effect in patients with non-PKU hyperphenylalaninemia.

Does impaired growth of PKU patients correlate with the strictness of dietary treatment? National Dutch PKU Steering Committee

To assess whether growth retardation in patients with phenylketonuria (PKU) is related to the strictness of their dietary treatment, the relationship between Z scores for height up to 3 y of age and different indices of dietary control in 103 early treated Dutch PKU patients was studied. As indices of dietary control, the mean phenylalanine (Phe) concentration, the frequency of plasma Phe concentrations < 200 and < 120 micromol/l, and the standard deviation of the individual plasma Phe concentrations were studied. These measures of the dietary control were divided into quartiles. The mean Z score of the studied patients showed a decrease of 0.18/y (SD 0.36). No statistically significant difference between any of the quartiles of the studied indices with growth retardation was found. None of the used indices of dietary control based on plasma Phe concentrations showed a relationship between different degrees of the strictness of dietary treatment with growth retardation in Dutch PKU patients.

Mesoprefrontal dopaminergic neurons: can tyrosine availability influence their functions?

The dopamine (DA) neurons projecting to the prefrontal cortex (PFC) are thought to be involved in working memory, stress response, and the pathogenesis of schizophrenia. In this commentary, we review the current evidence supporting a precursor tyrosine dependence of these mesoprefrondal DN neurons. Several studies in rats employing different experimental paradigms [i.e. experimental diabetes and early-treated phenylketonuria (PKU) model] have shown that reduced tyrosine levels in brain can affect markedly the physiology and functions of these DA neurons. However, supplemental tyrosine is effective in enhancing functional transmitter outflow from mesoprefrontal DA neurons only under conditions where their physiological activity is enhanced and DA synthesis and release are uncoupled from intrinsic regulatory controls. Recent studies in humans have also suggested that variations in brain tyrosine levels can affect significantly higher cortical functions subserved by the PFC. In early-treated PKU patients with mildly reduced tyrosine levels, marked impairments in cognitive functions dependent on the dorsolateral PFC could be detected. In drug-treated schizophrenic patients, supplemental tyrosine was shown to have a disruptive effects on the smooth-pursuit eye movement performance task. Furthermore, tyrosine administration was effective in restoring impaired working memory in humans following cold stress paradigm, as assessed by a computer-based delayed matching to-sample memory task. These human studies, together with the current evidence obtained from animal experiments, suggest that the functions of the mesoprefrontal DA neurons can, under certain circumstances, be readily influenced by the availability of the precursor tyrosine.

[Effect of dietary measures on body weight and height of children with phenylketonuria in East Germany]

Body length and body weight of 90 patients with PKU (48 girls and 42 boys) were compared during the years up to 1990 and thereafter. The patients got Berlophen up to 1990 for at least 3 years, and Milupa PKU or PAM (SHS) for 3 years thereafter. The data were compared with the percentiles according to Prader, the significance was calculated with the Chi2-Test. Body length and weight are significantly reduced during the first decade of life compared with normal population. Body length reaches normal values at the age of 12 years, body weight at 9 years respectively. The change of the diet after the german unification had no influence onto body weight and length. The retardation of body length, however, was more marked in patients with very strict dietary control compared to patients with more often dietary faults. The limited availability of essential nutrients seems to be the cause for the somatic retardation of the patients with PKU in the eastern part of germany.

Large daily fluctuations in plasma tyrosine in treated patients with phenylketonuria

In patients with phenylketonuria (PKU), extra tyrosine supplementation is advocated in addition to tyrosine-enriched amino acid mixtures. PKU patients have low fasting plasma tyrosine concentrations, but little is known about tyrosine fluctuations during the day. Plasma tyrosine concentrations were studied in 12 PKU patients in response to a test without breakfast and to three tests with different tyrosine contents in breakfast and lunch: 0%/30%, 25%/30%, 50%/10%, and 75%/10% tests, reflecting the protein consumption at breakfast and lunch, respectively. Prolonged fasting resulted in a small decrease in the already low overnight fasting plasma tyrosine concentrations. Breakfast and lunch with 25% and 30% of the daily tyrosine intake resulted in both lower than normal and higher than normal tyrosine concentrations. The 50%/10% and 75%/10% tests resulted in excessively high plasma tyrosine concentrations in most patients. Therefore, both lower than normal and higher than normal postprandial plasma tyrosine concentrations were found in treated PKU patients, even if the daily tyrosine intake was distributed evenly. When there was a large fractional tyrosine intake from one meal, very high plasma tyrosine concentrations were found. Therefore, strict control of plasma tyrosine is necessary if tyrosine supplementation is considered in addition to the tyrosine-enriched amino acid mixtures.

Phenylketonuria: findings at MR imaging and localized in vivo H-1 MR spectroscopy of the brain in patients with early treatment

PURPOSE

To characterize white matter changes in early-treated phenylketonuria (PKU) with magnetic resonance (MR) imaging and hydrogen-1 MR spectroscopy and to correlate these findings to biochemical control and brain function.

MATERIALS AND METHODS

Fifty-one patients aged 12-33 years underwent T1-, T2-, and proton-density-weighted MR imaging and testing of intelligence, visual evoked potentials (VEPs), and neuropsychologic status (29 adult patients only). H-1 MR spectroscopy was performed in eight patients to determine brain metabolite concentrations, including phenylalanine (PHE) concentration, and brain compartmentation.

RESULTS

MR imaging revealed a high frequency of supra- and infratentorial abnormalities. MR imaging grade, which was based on areas of high signal intensity on T2-weighted images, showed statistically significant correlation with long-term biochemical control and neuropsychologic test results but not with intelligence quotient or VEPs. H-1 MR spectroscopy revealed normal metabolite levels, except for increased PHE levels. It also showed enlarged cerebrospinal fluid-like compartments in affected white matter, related to plasma and brain concentrations of PHE and MR imaging grades.

CONCLUSION

A synergistic use of MR imaging and MR spectroscopy may help elucidate both the pathogenesis of brain dysfunction and clinical treatment policies in PKU.

Evidence for the importance of dopamine for prefrontal cortex functions early in life

There is considerable evidence that dorsolateral prefrontal cortex subserves critical cognitive abilities even during early infancy and that improvement in these abilities is evident over roughly the next 10 years. We also know that (a) in adult monkeys these cognitive abilities depend critically on the dopaminergic projection to prefrontal cortex and (b) the distribution of dopamine axons within dorsolateral prefrontal cortex changes, and the level of dopamine increases, during the period that infant monkeys are improving on tasks that require the cognitive abilities dependent on prefrontal cortex. To begin to look at whether these cognitive abilities depend critically on the prefrontal dopamine projection in humans even during infancy and early childhood we have been studying children who we hypothesized might have a selective reduction in the dopaminergic innervation of prefrontal cortex and a selective impairment in the cognitive functions subserved by dorsolateral prefrontal cortex. These are children treated early and continuously for the genetic disorder, phenylketonuria (PKU). In PKU the ability to convert the amino acid, phenylalanine (Phe), into another amino acid, tyrosine (Tyr), is impaired. This causes Phe to accumulate in the bloodstream to dangerously high levels and the plasma level of Tyr to fall. Widespread brain damage and severe mental retardation result. When PKU is moderately well controlled by a diet low in Phe (thus keeping the imbalance between Phe and Tyr in plasma within moderate limits) severe mental retardation is averted, but deficits remain in higher cognitive functions. In a four-year longitudinal study we have found these deficits to be in the working memory and inhibitory control functions dependent upon dorsolateral prefrontal cortex in PKU children with plasma Phe levels 3-5 times normal. The fact that even infants showed these impairments suggests that dopaminergic innervation to prefrontal cortex is critical for the proper expression of these abilities even during the first year of life. To test the hypothesis about the underlying biological mechanism we have created the first animal model of early and continuously treated PKU. As predicted, the experimental animals had reduced levels of dopamine and the dopamine metabolite, homovanillic acid (HVA), in prefrontal cortex and showed impaired performance on delayed alternation, a task dependent on prefrontal cortex function. Noradrenaline levels were unaffected; however some reduction in serotonin levels and in dopamine levels outside the prefrontal cortex was found. If prefrontal cortex functions are vulnerable in children with a moderate plasma Phe:Tyr imbalance because of the special properties of the dopamine neurons that project to prefrontal cortex, then other dopamine neurons that share those same properties should also be vulnerable in these children. The dopamine neurons in the retina share these properties (i.e. unusually high firing and dopamine turnover rates), and we have found that PKU children with plasma Phe levels 3-5 times normal are impaired in their contrast sensitivity, a behavioural measure sensitive to retinal dopamine levels.

Untreated non-phenylketonuric-hyperphenylalaninaemia: intellectual and neurological outcome

The intellectual, neurological, and neuropsychological outcome of patients with non-phenylketonuric-hyperphenylalaninaemia (PKU-HPA) (serum phenylalanine levels under free diet < 600 mumol/l) has not been systematically studied so far. We therefore tested 28 patients (mean age = 21.8, SD = 4.2 years) for IQ (WAIS-R/WISC-R), school performance, job career, clinical neurological examination, fine motor performance (motor performance task), and selective and sustained attention (stroop task, Dot Pattern Exercise from the Sonneville visual attention task). In addition, cranial MRI (1.5 T unit) was obtained in 10 of these patients. Clinical-neurological examination revealed no significant abnormalities in the non-PKU-HPA patients. They also had a normal IQ (mean = 101.9, SD = 13.6). Compared to their healthy siblings, they attended a normal school and had a normal job career. The motor performance task revealed no deficits in fine motor abilities. The patients performed normally in the stroop task and the dot pattern exercise. Their MRIs were normal. Our results indicate that patients with non-PKU-HPA are not at risk for developing intellectual, neurological, and neuropsychological impairment, as described for patients with treated mild or classical phenylketonuria. From this point of view a dietary treatment is not necessary in patients with hyperphenylalaninaemia.