More on untreated maternal hyperphenylalaninemia

Blood-brain barrier carrier-mediated transport and brain metabolism of amino acids

The transport of neutral amino acids through the brain capillary endothelial wall, which makes up the blood-brain barrier (BBB) in vivo, is an important control point for the overall regulation of cerebral metabolism, including protein synthesis and neurotransmitter production. The Michaelis-Menten kinetics of BBB amino acid transport have been investigated in vivo with the brain uptake index (BUI) technique, and in vitro with the isolated human brain capillary preparation. The only amino acid that is albumin-bound is tryptophan, and the majority of albumin-bound tryptophan in the plasma is available for transport through the BBB via an enhanced dissociation mechanism that operates at the surface of the brain capillary endothelium. The availability in brain of amino acids is predicted from the BBB Km values to be sharply influenced by supra-physiological concentrations of phenyalanine in the 200-500 microM range. Moreover, the measurement of cerebral protein synthesis with an internal carotid artery perfusion technique and HPLC-based measurements of aminoacyl-transfer RNA specific activities shows an inverse relationship between cerebral protein synthesis and plasma phenyalanine concentrations in the 200-500 microM range. These findings indicate the neurotoxicity of hyperphenylalninemia is not restricted to the phenylketonuria range of approximately 2000 microM, but is exerted in the supra-physiological range of 200-500 microM.

Maternal phenylketonuria: a metabolic teratogen

The maternal phenylketonuria (PKU) syndrome refers to the teratogenic effects of PKU during pregnancy. These effects include mental retardation, microcephaly, congenital heart disease, and intrauterine growth retardation. In untreated pregnancies wherein the mother has classic PKU with a blood phenylalanine level > or = 1,200 microM (20 mg/dl), the frequencies of these abnormalities in offspring are exceedingly high, approaching 75-90% for microcephaly and mental retardation and 15% for congenital heart disease. There is a dose response relationship with progressively lower frequencies of these abnormalities at lower phenylalanine levels, both in the pregnancies of women with variants of PKU and in treated classic PKU pregnancies. The pathogenesis of this syndrome is unknown; it may be related to inhibition by phenylalanine of large neutral amino acid transport across the placenta or to direct toxicity of phenylalanine and/or a phenylalanine metabolite in certain fetal organs. A mouse model for PKU now exists, and studies of maternal PKU in this model are in progress. The treatment of maternal PKU consists of biochemical control through a phenylalanine restricted diet during pregnancy. The best results are obtained with diet initiation before conception or no later than the earliest weeks of pregnancy. Women with PKU and their families require much psychosocial support to meet the strict requirements of a maternal PKU pregnancy, including compliance with a difficult diet. With such compliance, however, it seems that bearing normal or near normal offspring is possible.

Maternal mild hyperphenylalaninemia: results of treated and untreated pregnancies in two sisters

The outcomes of mild hyperphenylalaninemia (MHP) in three children of two sisters were compared. The IQ of the child from an untreated pregnancy was 105; the developmental quotients of the two infant offspring from treated and untreated pregnancies were 122 and 114, respectively. The IQ of the sister with untreated MHP was 101; that of the sister who received dietary treatment for MHP during infancy was 90. Thus MHP and maternal MHP appear to have been clinically inconsequential in this family.

Decreases in brain protein synthesis elicited by moderate increases in plasma phenylalanine

Large increases in plasma phenylalanine concentration (greater than 1 mM) adversely affect brain function and inhibit cerebral protein synthesis. The threshold hypothesis predicts that moderate increases in plasma phenylalanine concentrations below a 1 mM threshold have no adverse effects on brain function or protein synthesis. Using a new in situ internal carotid artery perfusion technique and measurement of the amino acyl-tRNA specific activities by high performance liquid chromatography (HPLC) separation of phenylisothiocyanate (PITC) derivatized amino acids, the present studies demonstrate a linear decrease in cerebral protein synthesis in vivo in proportion to moderate increases in plasma phenylalanine. A 50% inhibition of brain protein synthesis in vivo is observed at a plasma phenylalanine concentration of approximately 0.40 mM. Since moderate increases in plasma phenylalanine concentrations may be achieved in humans with ingestion of phenylalanine, the present findings should be considered in evaluating the safety of liberal and selective increases in dietary phenylalanine.

Newborn screening for hyperphenylalaninemia on day 5: is 240 mumol/liter the most appropriate cut-off level?

In most countries, including the UK, Sweden, Japan, and the FRG, blood samples for newborn screening are not taken until Day 5, in contrast to Day 3 in the United States. This results in fundamentally different conditions with respect to the amount of phenylalanine ingested by the newborn until the day of sampling. Nevertheless those countries who routinely screen on Day 5 have adopted the U.S. cut-off level of 240 mumol/liter (4 mg/dl). This study is the first to compare, via direct evaluation of a regional screening program, whether a higher or a lower cut-off level would be more appropriate for phenylalanine screening on Day 5. In a prospective study on phenylalanine screening with 132,638 infants (cut-off level 120 mumol/liter) the number of recalls was 152. Among them 14 cases with a permanent disorder of phenylalanine metabolism were detected. With the officially recommended cut-off level of 240 mumol/liter as well as with the 360 mumol/liter level used in some centers, the recall rate would have been lower, specificity higher, and the predictive values of a positive result higher. On the other hand sensitivity would have been significantly lower (only 80%). Because in newborn screening the crucial point is sensitivity, our data suggest that in phenylalanine screening done on Day 5 cut-off levels of 240 or 360 mumol/liter are less appropriate than a lower cut-off level. With higher cut-off levels a large proportion of patients (one-fifth) with a permanent disorder of phenylalanine metabolism would be missed.

The New England Maternal PKU Project: identification of at-risk women

Young women with phenylketonuria (PKU) are at risk for bearing children with mental retardation, microcephaly, heart defects, and low birthweight. These effects may be prevented if a low phenylalanine diet is maintained prior to and throughout pregnancy. This report describes the procedures of the New England Regional Maternal PKU Project for identifying and locating this population of at-risk women. Newborn screening records, routine umbilical cord blood screening, and PKU Clinic records provided most of the identifying information. We identified 235 women with hyperphenylalaninemia, ages 12 to 44 years. Of these, 183 had PKU or atypical PKU while 52 had non-PKU mild hyperphenylalaninemia. The 235 women represent 88 per cent of the expected number of women with hyperphenylalaninemia in New England. We identified more than the expected number of those with PKU but only 57 per cent of the expected number with mild hyperphenylalaninemia. Developing a national registry, as well as screening women who utilize birth control clinics or prenatal clinics, may be helpful. Implementing routine umbilical cord blood screening programs may be beneficial in efforts to identify women with hyperphenylalaninemia who have had a child and may want more children in the future.

Timing of strict diet in relation to fetal damage in maternal phenylketonuria. An international collaborative study by the MRC/DHSS Phenylketonuria Register

64 infants born to women with phenylketonuria (PKU) were grouped according to the mother's dietary treatment in pregnancy. 17 infants, whose mothers were receiving a strict low phenylalanine diet and had blood phenylalanine concentrations below 600 mumol/l at the time of conception, had normal birthweights and head circumferences and no malformations. In the 29 infants whose mothers were receiving either a relaxed diet or a normal diet at conception but who started a strict diet at some time during pregnancy, birthweights and head circumferences were below those in healthy infants and there was an excess of malformations; the findings closely resembled those for the 18 infants whose mothers received no treatment during pregnancy. The birthweights and head circumferences of the 64 infants were inversely related to the maternal phenylalanine concentrations around the time of conception. Only a strict diet started before conception is likely to prevent fetal damage.

Maternal phenylketonuria (PKU)--a review

This review points out the very high incidence of damage to the fetus in untreated maternal phenylketonuria (PKU). In classical cases, 92% of the offspring are mentally retarded, 73% have microcephaly, 40% are growth retarded at birth, and 12% have congenital anomalies. Less severe types of PKU and its variants and patients treated with a low phenylalanine diet during pregnancy have a much lower incidence of these defects in their offspring. Very promising results have been obtained in a small number of preconception and early first trimester treated patients under very strict dietary control. Nutrition of the mother and fetus is a major concern during the application of this restrictive diet and must be monitored closely to avoid fetal damage from malnutrition. A 7-year collaborative study of maternal PKU began in November 1984 in the US and Canada, but even in this well publicized study, many patients are presenting late for treatment. It is suggested that premarital and/or prenatal screening for maternal PKU should be initiated for the next generation.

Pre-conceptual dietary management for maternal phenylketonuria

Experience of preconceptual dietary management of maternal hyperphenylalaninaemia is limited. Two Edinburgh mothers with hyperphenylalaninaemia and borderline IQ have received dietary advice to maintain normal plasma Phe concentration both before conception and during pregnancy, in an attempt to prevent foetal abnormality. Satisfactory compliance was only obtained after intensive in-patient instruction and the diets were maintained for long periods prior to conception. Intellectual and psychological factors were very important and the availability of "same day" biochemical reports useful in encouraging the mothers. Both infants have normal Phe concentrations and satisfactory growth and development parameters at present. Further follow-up continues but these cases provide further evidence in favour of the use of preconceptual dietary advice in the management of maternal hyperphenylalaninaemia.