PEDIATRICS Vol. 106 No. 5 November 2000, pp. 1136

COMMENTARY:
Benefits of Mutation Analysis and Examination of Brain Phenylalanine Levels in the Management of Phenylketonuria

Phenylketonuria (PKU) was first described in 1934 by Følling in Norway¹; however, the actual gene defect was not elucidated until 1983 by Woo and colleagues.² During the past 17 years, progress in assessing the usefulness of DNA analyses in the management of PKU has been obscured by the numerous mutations of the phenylalanine hydroxylase (PAH) gene that have been described to date.³ The fact that routine mutational analysis for PKU is unavailable in the United States at present is attributable to several factors, but speaks forcefully to the lack of appreciation of the usefulness of DNA technology by professionals as well as that of the general public. Although mutation analyses potentially represent new powerful techniques in a wide variety of medical disorders, such as cancer, societal attitudes have been influential in delaying implementation of the resources necessary for clinical application of these new methodologies. In the case of PKU, such delays are understandable because of the need to establish a scientific background of data, which demonstrates the value of mutational analysis of the PAH gene to the practicing clinician, thereby justifying the cost to parents, as well as funding agencies. The pioneering work of Scriver and colleagues in developing the Phenylalanine Hydroxylase Locus Database³ and the work of various laboratories in different parts of the world, such as Europe,^4-7 the United States,⁸ and Australia⁹ has now developed to the stage where certain guidelines are needed. Thus clinicians caring for patients with PKU can more easily provide appropriate family guidance.¹⁰ This is especially needed at the present time because funding agencies are unaware of their usefulness.

BACKGROUND

The first large reports on the correlations between PAH mutation genotypes and metabolic PKU phenotypes appeared in 1997-1998^7-11 and consisted of approximately 1000 patients with different forms of PKU. The data were interpreted as showing that the PAH genotype was the chief determinant of the metabolic phenotype in the majority of persons with PKU. In one of these studies,⁷ each of more than 100 different PAH mutations was assigned to 1 of 4 metabolic phenotypic categories; classical, moderate PKU, mild PKU, and mild hyperphenylalaninemia. Patients with genotypes corresponding to the latter category did not appear to require treatment with the phenylalanine-restricted diet. Unfortunately, intellectual assessments were not available in these persons to document a relationship between the metabolic phenotype and intellectual outcome because this publication was a collaborative effort by clinicians located in 7 different countries. The participating clinics with varying approaches to the treatment of persons with PKU and the use of differing methods of intellectual assessments at different ages made this impossible. Furthermore, a prior publication in 1997¹⁰ based on data collected from 72 adults was unable to verify a strong relationship between a metabolic phenotype and intellectual outcome. This result was explained by assuming that dietary intervention obviated the effect of the metabolic phenotype on intellectual outcome.

In a recent study of 222 hyperphenylalaninemic females enrolled in the Maternal PKU Collaborative Study, Güttler et al¹² reported not only a significant relationship between the mutational genotype and the biochemical phenotype, but also a significant relationship between genotype and cognitive performance (IQ) measured by the Wechsler Adult Intelligence Scale-Revised. The conclusion was that IQ decreases with increasing severity of genotype. Females with 2 mutations associated with severe PKU or mutations associated with moderate PKU had a mean IQ of 83. In contrast, females who had inherited a mutation associated with mild PKU had a mean IQ of 96 (P = .0001) and females who had inherited a mutation associated with mild hyperphenylalaninemia showed a mean IQ of 99 (P < .0001). In addition, the treatment history was shown to significantly affect IQ outcome. Treatment instituted neonatally and continued for more than 6 years of life produced a significantly higher maternal IQ (P = .02) than average for each genetic group.¹² In addition, females who were late treated (>1 month) or untreated demonstrated IQ scores that were lower than the average for their respective groups. Surprisingly, 5 late-treated women with 2 mutations associated with severe PKU and with a mean assigned phenylalanine level of 1680 µmol/L (27 mg%) exhibited a mean IQ of 96, and 4 late-treated women with mutations associated with moderate PKU and a mean assigned phenylalanine level of 1380 µmol/L (23 mg%) demonstrated a mean IQ of 94. Such exceptions have also been observed by others¹³ and were recently suggested to be caused by the occasional lack of correlation between blood phenylalanine levels and brain phenylalanine levels.¹⁴ These startling results are based on magnetic resonance imaging/magnetic resonance spectroscopy developed by Ross and associates¹⁵ and by the clinical follow-up studies by Weglage et al.¹³ Recently, Moats et al¹⁴ have confirmed that low brain levels of phenylalanine levels are associated with improved IQ outcome.

Collectively, the aforementioned results confirm the need for continued lifelong ingestion of the medical foods in the large majority of persons in all 3 of the major groups of phenylketonuric individuals, and provide an understanding for the few persons with classical PKU, who have achieved a good outcome despite the lack of treatment. For those participants with PKU with physiologically low blood brain transport of phenylalanine the present data may remove the onus of very restrictive blood phenylalanine levels (120-360 µmol/L) now recommended. For the remaining participants with PKU, brain phenylalanine levels may be reduced by the administration of large neutral amino acids (valine, isoleucine, leucine, tyrosine, and tryptophan), which compete with phenylalanine for transport across the blood-brain barrier.^16-19 Several studies have found that dietary supplementation with these amino acids produce improvement in neurologic, cognitive, and behavior measures.^20,21

CONCLUSION

Adults with PKU having 2 severe mutations on the PAH allele should remain on a phenylalanine-restricted diet for a lifetime, but variability of blood phenylalanine control should be individually assessed. Magnetic resonance imaging/magnetic resonance spectroscopy may be helpful in determining an appropriate level of control, but more research is needed to prove this conclusively.

ACKNOWLEDGMENT

The work was supported by funding from the National Institute of Child Health and Human Development and the Danish Medical Research Council, Copenhagen, Denmark.

Richard Koch, MD
Division of Medical Genetics
Department of Pediatrics
Children's Hospital, Los Angeles
University of Southern California School of Medicine
Los Angeles, CA 90027

Flemming Güttler, MD, PhD
John F. Kennedy Institute
GL Landevej 7
DK-2600, Glostrup, Denmark

FOOTNOTES

Received for publication Feb 22, 2000; accepted Feb 22, 2000.

Reprint requests to (R.K.) Maternal PKU Collaborative Study, Children's Hospital/Los Angeles, Mail Stop 73, Division of Medical Genetics, 4650 Sunset Blvd, Los Angeles, CA 90027. E-mail: rkoch{at}earthlink.net

ABBREVIATIONS

PKU, phenylketonuria; PAH, phenylalanine hydroxylase.

REFERENCES

1. Følling A Über Ausscheidung von Phenylbrenztraubensäure in den Harn als Stoffwechselanomalie in Verbindung mit Imbezillität. Ztschr Physiol Chem. 1934; 227:169-176
2. Woo SLC, Lidsky AS, Güttler F, Chandra T, Robson KJH Cloned human phenylalanine hydroxylase gene permits prenatal diagnosis and carrier detection of classical phenylketonuria. Nature. 1983; 306:151-155 [CrossRef][Medline]
3. Scriver CR, Nowacki PM, Byck S, Prevost L. The PAH Mutation Analysis Consortium Database. Available at: http://www.mcgill.ca/pahdb. 1999
4. Svensson E, von Döbeln U, Eisensmith RC, Hagenfeldt L, Woo SLC Relation between genotype and phenotype in Swedish phenylketonuria and hyperphenylalaninemia patients. Eur J Pediatr. 1993; 152:132-139 [CrossRef][Medline]
5. Romano V, Guldberg P, Güttler F, PAH deficiency in Italy: correlation of genotype to phenotype in the Sicilian population. J Inher Metab Dis. 1996; 19:15-24 [CrossRef][Medline]
6. Desviat LR, Perez B, Garcia MJ, Relationship between mutation genotype and biochemical phenotype in a heterogeneous Spanish phenylketonuria population. Eur J Hum Genet. 1997; 5:196-202 [Medline]
7. Guldberg P, Rey F, Zschocke J, A European multicenter study of phenylalanine hydroxylase deficiency: classification of 105 mutations and a general system for genotype-based prediction of metabolic phenotype. Am J Hum Genet. 1998; 63:71-79 [CrossRef][Medline]
8. Eisensmith RC, Martinez DR, Kuzmin AI, Molecular basis of phenylketonuria and a correlation between genotype and phenotype in a heterogeneous southeastern US population. Pediatrics. 1996; 97:512-516 [Abstract/Free Full Text]
9. Ramus SJ, Forrest SM, Pitt DB, Saleeba JA, Cotton RG Comparison of genotype and intellectual phenotype in untreated PKU patients. J Med Genet. 1993; 30:401-405 [Abstract/Free Full Text]
10. Koch R, Fishler K, Azen C, Guldberg P, Güttler F The relationship of genotype to phenotype in phenylalanine hydroxylase deficiency. Biochem Mol Med. 1997; 60:92-101 [CrossRef][Medline]
11. Kayaalp E, Treacy E, Waters PJ, Byck S, Nowacki P, Scriver CR. Human PAH mutation and hyperphenylalaninemia phenotypes: a metanalysis of genotype-phenotype correlations. Am J Hum Genet. 1997:1309-1317
12. Güttler F, Azen C, Guldberg P, Relationship among genotype, biochemical phenotype, and cognitive performance in females with phenylalanine hydroxylase deficiency. Report from the Maternal PKU Collaborative Study. Pediatrics. 1999; 104:258-262 [Abstract/Free Full Text]
13. Weglage J, Moller HE, Wiedermann D, Cipcic-Schmidt S, Zschocke J, Ullrich K In vivo NMR spectroscopy in patients with phenylketonuria: clinical significance of interindividual differences in brain phenylalanine concentrations. J Inherit Metab Dis. 1998; 21:81-82 [CrossRef][Medline]
14. Moats RA, Koch R, Moseley K, Brain phenylalanine concentration in the management of adults with phenylketonuria. J Inherit Metab Dis. 2000; 23:7-14 [CrossRef][Medline]
15. Ross BD, Freeman DM, Chan L Phosphorus metabolites by NMR. Adv Exp Med Biol. 1984; 178:455-464 [Medline]
16. Pardridge WM Blood-brain barrier carrier-mediated transport and brain metabolism of amino acids. Neurochem Res. 1998; 23:635-644 [CrossRef][Medline]
17. Andersen AE, Avins L Lowering brain phenylalanine levels by giving other large neutral amino acids. A new experimental therapeutic approach to phenylketonuria. Arch Neurol. 1976; 33:684-686 [Abstract/Free Full Text]
18. Pietz J, Kreis R, Rupp A, Large neutral amino acids block phenylalanine transport into brain tissue in patients with phenylketonuria. J Clin Invest. 1999; 103:1169-1178 [Medline]
19. Berry HK, Bofinger MK, Hunt MM, Phillips PJ, Guilfoile MB Reduction of cerebrospinal fluid phenylalanine after oral administration of valine, isoleucine, and leucine. Pediatr Res. 1982; 16:751-755 [Medline]
20. Jordan MK, Brunner RL, Hunt MM, Berry HK Preliminary support for the oral administration of valine, isoleucine and leucine for phenylketonuria. Dev Med Child Neurol. 1985; 27:33-39 [Medline]
21. Berry HK, Brunner RL, Hunt MM, White PP Valine, isoleucine, and leucine. A new treatment for phenylketonuria. Am J Dis Child. 1990; 144:539-543 [Abstract/Free Full Text]