Progress in experimental phenylketonuria: a critical review

Phenylalanine Monitoring via Aptamer-Field-Effect Transistor Sensors

Determination of the amino acid phenylalanine is important for lifelong disease management in patients with phenylketonuria, a genetic disorder in which phenylalanine accumulates and persists at levels that alter brain development and cause permanent neurological damage and cognitive dysfunction. Recent approaches for treating phenylketonuria focus on injectable medications that efficiently break down phenylalanine but sometimes result in detrimentally low phenylalanine levels. We have identified new DNA aptamers for phenylalanine in two formats, initially as fluorescent sensors and then, incorporated with field-effect transistors (FETs). Aptamer-FET sensors detected phenylalanine over a wide range of concentrations (fM to mM). para-Chlorophenylalanine, which inhibits the enzyme that converts phenylalanine to tyrosine, was used to induce hyperphenylalaninemia during brain development in mice. Aptamer-FET sensors were specific for phenylalanine versus para-chlorophenylalanine and differentiated changes in mouse serum phenylalanine at levels expected in patients. Aptamer-FETs can be used to investigate models of hyperphenylalanemia in the presence of structurally related enzyme inhibitors, as well as naturally occurring amino acids. Nucleic acid-based receptors that discriminate phenylalanine analogs, some that differ by a single substituent, indicate a refined ability to identify aptamers with binding pockets tailored for high affinity and specificity. Aptamers of this type integrated into FETs enable rapid, electronic, label-free phenylalanine sensing.

Some morphological changes in the rat thyroid gland during experimental hyperphenylalaninemia

BACKGROUND

Little is known about the effect of phenylketonuria on the thyroid gland. In the present study, this problem was investigated by using a defined experimental model of hyperphenylalaninemia (HPA).

METHODS

The experimental group was subjected to an HPA regimen (Matsuo and Hommes, 1988. Neurochem. Res., 13:867-870) from the 5th day of postnatal development. The pups were decapitated on the 7th, 14th, 21st, 28th, and 35th days. The thyroid glands were fixed in Bouin's fluid and routinely embedded in paraffin. The staining techniques used were Mallory-Slinchenko's method, toluidin blue, silver impregnation of the basement membrane, immunohistochemical staining of the proliferating cell nuclear antigen (PCNA), and neuron-specific enolase (NSE).

RESULTS

The size of the follicles was less than that in the control group. There were no substantial changes in the epitheliomer structures. In almost all of the treated groups, a reduction in the number of PCNA+, NSE+, and mast cells was observed until the 28th day. On the 28th day of HPA, the level of mast cell degranulation was higher (61%) than that in the control group. On the 35th day, these parameters began to reach normal levels. From the 28th day, degenerative changes in the thyroid glands of treated animals were observed in the NSE+ cells.

CONCLUSIONS

The HPA condition mainly has an influence on the number and structure of the NSE+ cells of the thyroid gland. One may assume that under HPA the increase in mast cell degranulation plays a significant role in the normalisation of the parameter of the thyroid gland.

Treatment variables and intellectual outcome in children with classic phenylketonuria. A single-center-based study

Records of 46 patients with classic phenylketonuria (PKU) were used to determine treatment variables associated with intellectual outcome. Patients comprised three groups: phenylalanine-restricted diet started 1) after 3 months and loss of diet control at a mean age of 7 years, 2) before age 3 months and loss of diet control at a mean age of 5 years, and 3) before age 3 months and through a mean age of 11 years. All underwent IQ testing during the diet; groups 1 and 2 were retested at a mean of six years off the diet. On the diet, groups 2 and 3 had higher IQs than group 1; group 3 IQ was also higher than IQ off diet in groups 1 and 2. After discontinuing the diet, group 2 IQs decreased significantly. Predictors of IQ in group 1 were age at loss of diet control and percentage of phenylalanine concentrations > 15 mg/dL; in group 2, mean phenylalanine concentrations and age at loss of diet control. Predictors of changes in group 1 IQs were global degree of dietary control and percentage of phenylalanine concentrations > 15 mg/dL; in group 2, phenylalanine concentrations of < 3 mg/dL and age at start of diet. Group 1 patients with phenylalanine concentrations < 3 mg/dL or > 15 mg/dL achieved no IQ gain by continuing the diet after age 7 years. Thus, intellectual prognosis is best for PKU patients who start a phenylalanine-restricted diet early and continue through age 12 years.

Cognitive profile of rats exposed to lactational hyperphenylalaninemia: correspondence with human mental retardation

The present study was designed to provide further information on the enduring cognitive effects of experimental phenylketonuria (PKU) in rats, produced by the administration of alpha-methylphenylalanine and phenylalanine on postnatal days 3-21. These rats evidenced: (1) impaired learning set formation, (2) stimulus perseveration, particularly after an error, and (3) difficulty in utilizing the less salient features of their environment in mastering discrimination problems. In contrast, long-term memory function and the ability to form simple associations did not differ from controls. This pattern of intact and impaired cognitive functions bears remarkable similarity to that of mentally retarded humans and neonatally hyperphenylalaninemic rhesus monkeys, thus affirming the use of rats to study mental retardation. In addition, possible reasons for the mildness of the impairments commonly observed in animal models of severe mental retardation syndromes are discussed. We suggest that transfer of learning paradigms that assess the animal's ability to use information acquired in other problems are more likely to uncover significant cognitive impairments in such models than are procedures that test only the animals' ability to solve a single problem.

The control of 5-hydroxytryptamine and dopamine synthesis in the brain: a theoretical approach

The transport of the eight amino acids (phenylalanine, tyrosine, tryptophan, valine, leucine, isoleucine, histidine and methionine) using the large neutral amino acid transporter of the blood-brain barrier (BBB) has been calculated using published kinetic data. The fate of the amino acids has been followed from blood to interstitial space, to cell and through metabolism which included, for tyrosine and tryptophan, the hydroxylases. The system was analysed in terms of flux control coefficients. Since the summation theorem did not hold, the system clearly behaved as a non-homogeneous system. At physiological levels of these eight amino acids, the largest contribution to the control of the flux of tyrosine is given by the hydroxylase step, followed by the diffusional component of the transport across the BBB. For tryptophan it is the hydroxylase step, followed by the carrier-mediated transport across the BBB. For the other amino acids it is the metabolism, followed by the diffusional component of the BBB transport. These parameters for tyrosine and tryptophan were determined at increased levels of blood phenylalanine, tyrosine or histidine. The flux through tryptophan hydroxylase can be affected by high blood levels of tyrosine and histidine to values also observed in hyperphenylalaninaemia. Since hypertyrosinaemia (type II) and hyperhistidinaemia are not associated with mental retardation, it is concluded that interference with transport across the BBB of tyrosine and tryptophan, as well as the flux through tryptophan hydroxylase leading to the synthesis of 5-hydroxytryptamine, do not contribute to the cause of permanent brain dysfunction in hyperphenylalaninaemia. It can be calculated that addition of tyrosine to the diet to raise the blood tyrosine level in phenylketonuria patients may have a beneficial effect for the synthesis of neurotransmitters derived from tyrosine.

The role of the blood-brain barrier in the aetiology of permanent brain dysfunction in hyperphenylalaninaemia

Calculations on the rate of entry of the neutral amino acids into the brain via the blood-brain barrier show that a considerable decrease in this rate, particularly for tryptophan and tyrosine, takes place in histidinaemia and tyrosinaemia, type II. These conditions are, however, not associated with mental retardation. It is therefore concluded that effects at the blood-brain barrier alone do not provide an adequate explanation for the aetiology of permanent brain dysfunction in hyperphenylalaninaemia.

Pattern reversal visual evoked potentials in phenylketonuria

The pathogenesis of brain dysfunction in phenylketonuria (PKU) is still under investigation. Hyperphenylalaninaemia results in increased turnover of myelin. In order to demonstrate the derangement of myelinization in PKU we studied the visual evoked potentials (VEP) in 14 PKU patients and in 20 normal subjects. VEP findings were correlated with the metabolic control of the disease and with the electroencephalographic findings. VEP were more sensitive than the EEG in detecting a neurological dysfunction. VEP are influenced by dietary control and are normal only in children with good metabolic control.

Amino acid depletion in the blood and brain tissue of hyperphenylalaninemic rats is abolished by the administration of additional lysine: a contribution to the understanding of the metabolic defects in phenylketonuria

The elevated phenylalanine concentration in the blood of untreated phenylketonuric children is known to be paralleled by decreased concentrations of other amino acids in the blood and brain tissue. Due to the low availability of other large, neutral amino acids in the brain, protein synthesis in, and the normal development of, the brain are disturbed. A similar effect is observed in suckling rats rendered hyperphenylalaninemic by the daily injection of phenylalanine plus alpha-methylphenylalanine, an in vivo inhibitor of the phenylalanine-hydroxylating pathway in the liver. In this study, the simultaneous injection of lysine is shown to prevent the depletion of amino acids from the blood and brain tissue, and the retardation of brain growth, in suckling hyperphenylalaninemic rats. It is suggested that both amino acids, phenylalanine and lysine, are important rate-limiting substrates for the rapid protein anabolism of developing tissues. In the presence of an excess of phenylalanine, other amino acids, and in relation to its requirement during the phase of hyperplastic growth in particular lysine, are less available from the circulation and limit phenylalanine-stimulated protein synthesis in developing tissues. The supplementation of lysine to developing hyperphenylalaninemic rats prevents the consequences of this effect, i.e., the depletion of amino acids in the blood, and therefore, in the brain tissue, and the retardation of brain growth.

Analysis of learning in retarded monkeys

Sixteen rhesus monkeys were either subjected to diets high in phenylalanine or parachlorophenyalanine either pre- or postpartum; there were 16 controls. Subjects were tested after removal from the PKU diet on a series of learning tasks. No support was found for suggestions that PKU monkeys a do worse if task difficulty is increased or b have an attentional or short-term memory storage deficit. Equivocal support was found suggesting that c PKU monkeys may have stronger initial biases and d do not attend to the relevant dimension. The most support was found for the hypothesis that e PKU subjects are more emotional which would account for a disruption in performance following negative reinforcement, and difficulty in changing an initial or a learned response pattern. The literature on learning in human PKU and induced PKU in animals is reviewed.

Dietary effects on rhesus social behavior: altered amino acid diets

Twenty-one monkeys continuously fed one of five diets high in tyrosine, histidine, alanine, glycine, or tryptophan between the ages of 1 and 12 months were compared with 20 controls. Social behavior either (a) while on the diet and tested in familiar groups of four or (b) while on a normal diet and paired with unfamiliar monkeys showed no effects of the high amino acid diets, confirming a previous analysis of learning ability.

Partial alleviation of phenylalanine toxicity in the chick by supplemental dietary tryptophan

Studies were conducted to determine the physiological effects of feeding diets containing high levels of phenylalanine to chicks. Marked reductions in weight gain and feed conversion were observed in birds fed a diet containing 2.52% L-phenylalanine and .47% L-tyrosine. Excess dietary phenylalanine caused elevations of phenylalanine and tyrosine in serum and brain, whereas cerebral concentrations of free isoleucine, leucine, and valine were generally decreased. Supraoptimal amounts of glycine, arginine, and tryptophan were fed in an attempt to alleviate the toxic symptoms observed in birds fed high phenylalanine diets. Only tryptophan supplementation proved successful in partially alleviating the growth-depressive effects of phenylalanine toxicity. Both serum and brain levels of tryptophan were increased by tryptophan supplementation. Results of two radiotracer studies with 14C-tryptophan suggested that brain uptake of tryptophan was inhibited by hyperphenylalaninemia.

Inhibition by L-phenylalanine of tryptophan transport by synaptosomal plasma membrane vesicles: implications in the pathogenesis of phenylketonuria

Phenylalanine is accumulated in the genetically linked deficiency phenylketonuria. The effect of L-phenylalanine on the transport of tryptophan was studied using membrane vesicles from rat-brain synaptosomes. Phenylalanine at similar concentrations to those found in phenylketonuric patients competitively inhibits tryptophan uptake, with a Ki of the same order as the Km for tryptophan. This inhibition could be responsible for the depletion of serotonin found in phenylketonuria.