hyperphenylalaninemia (PHP). For each of these specimens there were four control cord-blood specimens from in ants born on the same day and,generally, ...
Proc. Natl. Acad. Sci. USA Vol. 77, No. 10, pp. 6175-6178, October 1980
Medical Sciences
Cord-blood tyrosine levels in the full-term phenylketonuric fetus and the "justification hypothesis" (phenylalanine/pleiotropy/mental retardation/placenta)
CHARLES R. SCRIVER*t, DAVID E. C. COLE*, SALLY A. HOUGHTONt, HARVEY L. ANDRE GRENIERt§, AND CLAUDE LABERGEt§
LEVY*,
*The Medical Research Council Genetics Group and Departments of Biology and Pediatrics, McGill University, Montreal, Quebec, Canada; tState Laboratory Institute, Massachusetts Department of Public Health and Department of Neurology, Harvard Medical School, Boston, Massachusetts 02130; §Centre Hospitalier Universitaire Laval et l'Universit6 Laval, and tRWseau Provincial de MWdecine Genftique, Ministere des Affaires Sociales, Gouvernement du Quebec, Quebec,
Canada
Communicated by Arno G. Motulsky, July 11, 1980
ABSTRACT The "justification hypothesis" attributes mental retardation in phenylketonuria (PKU) to an inability of the heterozygous mother to deliver an appropriate amount of tyrosine to the PKU fetus who, in turn, is unable to correct for this deficiency because of its genetic constitution. We tested this hypothesis by measuring concentrations of tyrosine and phenylalanine in cord blood obtained at delivery from nine infants with PKU and five infants with persistent (non-PKU) hyperphenylalaninemia (PHP). For each of these specimens there were four control cord-blood specimens from in ants born on the same day and, generally, in the same hospital. PKU and PHP groups were similar with respect to cord-blood tyrosine and phenylalanine values. There was no biologically significant deficiency of tyrosine in cord blood of the pooled PKU and PHP groups (54 i 10 sM, mean ± SD) compared with controls (61 + 16 IsM, P = 0.13). On the other hand, phenylalanine in cord blood of the pooled PKU and PHP groups was significantly increased (144 + 30 uM, mean + SD) compared with controls (128 + 24, P = 0.004) The magnitude o the ifferences in cord-blood tyrosine and phenylalanine between control and PKU subjects are so small that it is unlikely that they have any consequences for physical and mental development. The justification hypothesis, as it pertains to blood tyrosine at term, is not upheld.
Bessman cited phenylketonuria (PKU) as a paradigm for his "justification hypothesis" (1, 2). The hypothesis indicts tyrosine deprivation rather than excessive accumulation of phenylalanine as the important event in the pathogenesis of mental retardation in PKU. Indeed, PKU is presented as a "deficiency disease" in this revisionist view. The justification hypothesis predicts that the homozygous PKU fetus is in double jeopardy with respect to tyrosine availability: first, it is dependent on maternal metabolism which, in the heterozygous phenotype, cannot regulate tyrosine delivery to the fetus as effectively as it can in the normal homozygous mother; second, the fetus cannot readily repair this presumed deficiency because of its own inability to convert phenylalanine to tyrosine. Therefore, a hypothetical deficiency of tyrosine in utero is presumed to limit fetal brain growth and to vitiate postnatal treatment. The justification hypothesis has been used also to explain some cases of nonspecific mental retardation. It is assumed that female heterozygotes bearing the PKU allele(s) and representing about 2% of Caucasian childbearing women could harm their non-PKU offspring with tyrosine deficiency in utero. Tests of the hypothesis (3-5), have compared only the distribution of either IQ scores or PKU heterozygosity in at-risk and normal populations; no direct test by measurement of blood tyrosine levels in the at-risk fetus has yet been made. The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact.
We present data on fetal blood tyrosine and phenylalanine levels in subjects at term who subsequently expressed either the PKU or persistent hyperphenylalaninemia (PHP) phenotype. We found no deficiency of tyrosine in cord blood of these subjects. Nonetheless, an abnormal phenotype was detectable even in the term fetus with either PKU or PHP: the blood phenylalanine concentration was increased. MATERIALS AND METHODS We studied the fetal phenotype in cord blood at delivery of 14 subjects with persistent postnatal hyperphenylalaninemia. These subjects were identified by routine newborn screening for PKU in the Massachusetts Metabolic Disorders Screening Program (6). Cord-blood specimens are collected routinely on filter paper (Schleicher & Schuell 908, prepared as Form CBF 1OOM-178-14 2636) in the cord-blood screening component of the Massachusetts Program. The cord-blood specimens for the 14 hyperphenylalaninemic subjects were retrieved in November 1979 from the repository maintained at room temperature at the State Laboratory Institute in Massachusetts. Each experimental sample was matched with a set of four control cordblood specimens (n = 56) obtained on the same day from infants born in the same hospital or in one nearby. Control subjects were all screened at discharge from the nursery and shown to be euphenylalaninemic. The cord-blood filter-paper specimens were coded and sent to the Reseau Provincial de Medecine Genetique (RPMG) (7) where they were processed without knowledge of the sample phenotype; tyrosine and phenylalanine were measured as a routine operation by the semi-automated fluorometric methods of Grenier and Laberge (8) and McCaman and Robins (9), respectively. All samples were processed on 1 day. The data were inspected to identify, if possible, the hyperphenylalaninemic subjects; then the code was broken and statistical analysis of the data was performed according to standard methods (10, 11). The hyperphenylalaninemic subjects were clearly classifiable according to prevailing criteria (12): nine had PKU with blood phenylalanine levels >1 mM when consuming a normal diet and required dietary restriction of phenylalanine to 2.3 SD from the mean. This finding could reflect an independent biological variable-namely, impaired tyrosine oxidation in the
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Medical Sciences: Scriver et al.
newborn as an expression of normal ontogeny (12). After the appropriate statistical treatment of these deviant tyrosine values, control and experimental populations had comparable variance and under these conditions no statistical difference in cordblood tyrosine levels could be identified. Cord-blood phenylalanine concentration was higher in the population that developed postnatal hyperphenylalaninemia. We emphasize: the phenotype was an excess of phenylalanine rather than a deficiency of tyrosine, a finding that confirms earlier anecdotal observations (23, 24). An increased cord-blood phenylalanine value in the hyperphenylalaninemic infants could reflect either the maternal phenotype or the fetal genotype. Kang and Paine (23) and Berry et al. (24) both observed that plasma phenylalanine concentration is higher in pregnant heterozygotes than in presumably homozygous normal women during pregnancy. Because there is a transplatental gradient of phenylalanine in favor of the fetus (12), cord-blood phenylalanine concentration in the mutant homozygous fetus is likely to be higher than in the fetus carried by a nonheterozygous mother in whom the plasma phenylalanine is not increased. The studies by Kang and Paine (23) and Berry et al. (24) confirm this to be the case. Moreover, Kang and Paine (23) did not observe a higher cord-blood phenylalanine value in the PKU fetus compared with the non-PKU fetus born to heterozygous mothers. In other words, maternal rather than fetal genotype appears to influence blood phenylalanine in the term fetus, and this influence is seen more in phenylalanine excess than in tyrosine deficiency. Lastly, it should be noted that maternal serum tyrosine is normally depressed in pregnancy (25). It may be further depressed in the obligate heterozygote during pregnancy but data are lacking on this point. The fetus at term apparently reflects the maternal tyrosine environment but PKU and normal infants are not different in this respect. We are grateful to Dr. Sergio Pena and others for helpful advice and criticism during preparation of the manuscript. This study was supported by grants from the Medical Research Council of Canada to the Medical Genetics Group, from the Quebec Ministry of Social Affairs (Quebec Network of Genetic Medicine), from the Health Services and Mental Health Administration (U.S. Public Health Service, Grant
Proc. Natl. Acad. Sci. USA 77 (1980)
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