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Phenylketonuria

  1. George C-T Jiang1,
  2. George J Yohrling IV2,
  3. Kent E Vrana3

Published Online: 19 APR 2001

DOI: 10.1038/npg.els.0002006

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How to Cite

Jiang, G. C.-T., IV, G. J. Y. and Vrana, K. E. 2001. Phenylketonuria. eLS. .

Author Information

  1. 1

    Wake Forest University School of Medicine, Winston-Salem, North Carolina, USA

  2. 2

    Wake Forest University School of Medicine, Winston-Salem, North Carolina, USA

  3. 3

    Wake Forest University School of Medicine, Winston-Salem, North Carolina, USA

Publication History

  1. Published Online: 19 APR 2001

Introduction

  1. Top of page
  2. Introduction
  3. Clinical Signs and Conditions
  4. Classical Phenylketonuria
  5. Nonclassical Phenylketonuria
  6. Future Treatments for Phenylketonuria
  7. Summary
  8. Further Reading

Metabolic disorders can adversely affect the body's natural homeostatic or steady state and lead to chemical imbalances and severe pathological conditions. Phenylketonuria is such an example in which the normal conversion of the dietary amino acid phenylalanine to tyrosine is blocked. The resulting build-up of phenylalanine and its metabolites in young patients produces a number of severe side effects including intellectual impairment and cutaneous changes. This article reviews the autosomal recessive metabolic disorder of phenylketonuria, covering clinical diagnoses, typical and atypical causes, as well as current and potential treatments.

Phenylketonuria, also known as PKU, is a disorder that affects 1 in 12 000–15 000 births. In most cases, the cause is a deficiency in the enzyme phenylalanine hydroxylase (PH). This genetic condition results in numerous complications due to the incomplete metabolism of the essential amino acid phenylalanine. Clinical manifestations, if the disease is left untreated, include developmental delay and severe intellectual impairment, seizures, autism, eczema, hyperactivity, aggressive behaviour and scleroderma-like skin changes with dilution of hair and skin colour. PKU is implicated in about 0.64% of institutionalized patients worldwide. Early diagnosis after birth is critical and treatment with a phenylalanine-restricted diet will prevent onset of intellectual impairment and neurological abnormalities. Routine paediatric screening for PKU began in the United States in 1961, and the success of screening has spurred the development of similar procedures for other genetic metabolic disorders. See also Amino Acids: Occurrence and Properties, and Amino Acid Metabolism: Genetic Disorders

Clinical Signs and Conditions

  1. Top of page
  2. Introduction
  3. Clinical Signs and Conditions
  4. Classical Phenylketonuria
  5. Nonclassical Phenylketonuria
  6. Future Treatments for Phenylketonuria
  7. Summary
  8. Further Reading

PKU is caused by a deficiency in the enzyme PH or a reduction in the synthesis or regeneration of a critical cosubstrate for the enzyme. In patients afflicted with PKU, this results in a build-up of phenylalanine, as it is not converted to tyrosine. Collectively, the various forms of PKU are also referred to as hyperphenylalaninaemia owing to the increased circulating levels of the amino acid. The excess phenylalanine is metabolized to abnormal breakdown products (the phenylketones), accounting for the common name of the disorder, phenylketonuria. Increased levels of phenylalanine and its metabolites cause irreparable damage to the developing central nervous system unless the condition is diagnosed and treated within the first 3–6 weeks of life. Furthermore, this biochemical defect can result in a variety of cutaneous abnormalities, including diffuse hypopigmentation, eczema and photosensitivity. These cutaneous changes may result from the toxic effects of phenylalanine and its decomposition products in the skin (Figure 1).

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Figure 1. Phenylalanine metabolism. Phenylalanine hydroxylase (PH) catalyses the conversion of phenylalanine to tyrosine. Deficiencies in the activity of this enzyme result in incomplete phenylalanine metabolism and build-up of toxic waste products. BH4, tetrahydrobiopterin; q-BH2, quinonoid dihydrobiopterin; GTP, guanosine triphosphate.

Normally, the essential amino acid phenylalanine is converted to tyrosine by the enzyme PH. The resulting tyrosine can then be converted, via independent pathways, to catecholamines (the neurotransmitters dopamine, noradrenaline (norepinephrine) and adrenaline (epinephrine), melanin or thyroid hormone. In addition, tyrosine can be incorporated, as a fundamental building block, into new proteins. Because tyrosine is itself a nonessential amino acid, PKU is not fatal since a diet high in tyrosine can make up for the lack of PH activity. Reduced synthesis of the cosubstrate tetrahydrobiopterin, or a deficiency in the regenerating enzyme dihydropteridine reductase, may also result in PKU. However, such cases are rarer and are referred to as nonclassical PKU. See also Neurotransmitters

With the aid of routine paediatric screening, a diagnosis of PKU is usually established soon after birth. The newborn screening test that is performed involves the measurement of blood phenylalanine levels by the Guthrie test, a bacterial inhibition assay. Normally, blood phenylalanine levels are below 2 mg dL−1. A blood phenylalanine concentration greater than 20 mg dL−1 with a normal or reduced concentration of tyrosine is diagnostic of classical PKU. For the confirmation of PKU, all amino acid levels must be measured by a quantitative technique, and screening programmes often request repeat specimens in any infant with a blood phenylalanine level greater than normal.

The essential neurological symptoms are intellectual impairment, which may cause corresponding changes in the electroencephalogram, disturbed gait, ataxia, extrapyramidal symptoms, focalized or generalized epileptiform seizures. In general, if treatment of PKU is instituted by 3 weeks of age, prevention of intellectual impairment and neurological abnormalities is seen. However, learning disabilities have been reported in even well-treated patients. Today, it is recommended that patients with PKU continue a phenylalanine-restricted diet indefinitely because of several reports of decreased intellectual ability in children who were taken off the diet at 5 or 6 years of age. Continuation of the diet also aids in prevention of complications that are seen in genetically normal infants of women with PKU (maternal PKU). These complications include microcephaly, congenital heart disease and intrauterine growth delay.

In patients with PKU, cutaneous changes can result from the toxic effects of phenylalanine and its decomposition products in the skin. Changes include reduction of skin, hair and eye colour with increased photosensitivity, although photosensitivity is not universal. Dermatitis is also seen in up to 50% of patients. This dermatitis is virtually indistinguishable from the true atopic dermatitis, a chronic skin condition characterized by red itchy skin. Some are indeed true cases, but others only mimic such lesions. Hair and skin darkening and other skin change reversals may occur with dietary restriction of phenylalanine intake. However, when a diet with normal levels of phenylalanine is resumed, facial eczema may reappear within 24 h.

The current treatment for patients afflicted with PKU is a strict dietary regimen that is restricted in phenylalanine intake yet provides all the other amino acids. The earlier the treatment is started, the more beneficial the effects. While on dietary treatment, patients with PKU need to be wary of products that contain the artificial sweetener aspartame (Nutrasweet). Aspartame, chemically known as N-aspartylphenylalanine methyl ester, is commonly used in sweetened drinks, and the amount in a litre of sweetened drink can be equivalent to the amount of phenylalanine normally obtained from the daily diet. Uninformed patients may therefore be at risk for contradicting their phenylalanine-reduced diet.

Classical Phenylketonuria

  1. Top of page
  2. Introduction
  3. Clinical Signs and Conditions
  4. Classical Phenylketonuria
  5. Nonclassical Phenylketonuria
  6. Future Treatments for Phenylketonuria
  7. Summary
  8. Further Reading

Typically, the term classical PKU refers to hyperphenylalaninaemias resulting from a deficient PH enzyme. If left untreated, classical PKU can lead to severe intellectual impairment. As stated previously, PH is the mixed function oxidase which, in its central reaction mechanism, converts the essential amino acid l-phenylalanine to l-tyrosine in the presence of the cosubstrates tetrahydrobiopterin and molecular oxygen. The recognition that PH was the cause of the hyperphenylalaninaemia was not made until the late 1940s, over 20 years after classical PKU was initially characterized by Asbjörn Fölling in 1934. It is important to note that PKU does not necessarily result from a lack of the entire PH gene. Assays to detect the PH protein show that patients with PKU can produce the enzyme, and kinetic studies provide evidence for enzymatic deficiencies.

The human PH gene was first cloned in 1985; since then many researchers have studied it to determine the molecular basis for PKU. The PH gene comprises 13 exons and spans approximately 90 kilobases on the human chromosomes. Many different defects in the PH gene have been observed and reported for patients with PKU. These polymorphisms include single nucleotide base changes or point mutations, deletions, splice variants, premature stop signals and insertions. Most of the identified polymorphisms are single nucleotide changes that result in a protein with one amino acid substituted by another. These missense mutations constitute approximately 60% of all the known mutations that have been characterized. Splicing mutations and deletions each constitute about 13% of all the mutations. In these cases, a mutation results in defective ribonucleic acid processing (splicing). This can produce an absence of PH or a defective form of the enzyme. Nonsense mutations (inappropriate protein synthesis stop signals) and insertions make up the remainder of the polymorphisms. The majority of the mutations (approximately 80%) lie in the central catalytic domain of the protein between exons 5 and 12, but mutations have been seen in all the different exons. Table 1 lists the various causes of PKU.

Table 1. Genetic defects and substrate deficiencies in phenylketonuria
BH2, dihydrobiopterin; GTP, guanosine triphosphate; PKU, phenylketonuria.
  • Classical PKU (mutations in phenylalanine hydroxylase)

  • Structural mutations

    • Single nucleotide base changes (point mutations)

    • Deletions

  • Insertions

    • Splice variants

    • Premature stop signals

    • Regulatory domain mutations

  • Atypical PKU

  • Decreased enzyme expression

    • Dihydropteridine reductase (DHPR) deficiency

  • No catalytic activity

    • Decreased enzyme production

    • Tetrahydrobiopterin (BH4) synthesis deficiency

    • Block in biosynthesis (GTP cyclohydrolase, BH2 synthetase)

    • Increased degradation

The frequencies at which different populations exhibit distinct polymorphisms vary greatly. Caucasians tend to have a mutation in the splice site of intron 12 where an adenosine is substituted for a guanosine (G [RIGHTWARDS ARROW] A), resulting in a truncated form of PH that is missing the last 52 amino acids. Asians, on the other hand, do not have such a general genotype. Based on these types of geographic localizations, it is clear that PKU has evolved from numerous independent mutations.

Nonclassical Phenylketonuria

  1. Top of page
  2. Introduction
  3. Clinical Signs and Conditions
  4. Classical Phenylketonuria
  5. Nonclassical Phenylketonuria
  6. Future Treatments for Phenylketonuria
  7. Summary
  8. Further Reading

In addition to the classical forms of PKU involving mutations in the PH gene, there are a number of examples of the disease (1–3% of the PKU cases) where the underlying defect is in other systems. Notably, these involve mutations in the dihydropteridine reductase (DHPR) or in the biosynthetic enzymes responsible for biopterin synthesis (see Figure 1). For this reason, neonatal patients exhibiting hyperphenylalaninaemia must also be tested for urinary pteridines, blood DHPR and additional biogenic amines. This last test is necessary because defects in tetrahydrobiopterin (BH4) synthesis or regeneration (from quinonoid dihydrobiopterin; q-BH2) will affect other key biosynthetic enzymes, including tyrosine hydroxylase (rate-limiting enzyme in dopamine, nonadrenaline and adrenaline biosynthesis) and tryptophan hydroxylase (rate-limiting enzyme in serotonin biosynthesis). Unfortunately, these patients are subject to a much more severe prognosis than those with classical PKU. Disruption of these other neurotransmitter systems produces mental health problems and motor problems (similar to Parkinson disease). See also Parkinson Disease

Infants with nonclassical PKU (pterin cosubstrate deficiencies) will require additional therapeutic interventions. First, they require limitations in dietary phenylalanine to prevent the build-up of the amino acid and its phenylketone metabolites. However, this will further exacerbate the deficiency in the catecholamine neurotransmitters (dopamine, noradrenaline and adrenaline). Therefore, patients with biopterin defects are also treated with precursors to the catecholamines that bypass the tyrosine hydroxylase step (l-dopamine; the same compound used as the drug of choice in Parkinson disease). Moreover, they are also treated with the immediate precursor of serotonin, 5-hydroxytryptophan (5-HTP), again bypassing the rate-limiting and pterin-dependent tryptophan hydroxylase step. High-dose BH4 administration in nonclassical (BH4 deficient) PKU has not met with uniform success, presumably due to pharmacokinetic considerations (bioavailability, tissue distribution, etc.), although it is still utilized by some clinicians. See also Neurotransmitters

Future Treatments for Phenylketonuria

  1. Top of page
  2. Introduction
  3. Clinical Signs and Conditions
  4. Classical Phenylketonuria
  5. Nonclassical Phenylketonuria
  6. Future Treatments for Phenylketonuria
  7. Summary
  8. Further Reading

As stated above, the current therapeutic regimen for the majority of the patients with PKU is the dietary restriction of the amino acid, phenylalanine. This can reduce the blood levels of phenylalanine and help prevent the severe intellectual impairment common with this disease. Regardless of successes with this intervention, there are problems with this approach. Dietary restriction is often difficult and lends itself to noncompliance. This is particularly true in cases where dietary phenylalanine restriction is recommended for life. It has been reported, for instance, that a decline in intellectual function and behavioural performance is evident in adults with PKU who have not maintained a low phenylalanine diet.

Dietary management is particularly important in pregnant women with classical PKU who have previously curtailed phenylalanine restriction. Dietary noncompliance during pregnancy can produce PKU-like symptoms in genetically unaffected (heterozygous) offspring. This syndrome is referred to as ‘maternal PKU’. In addition to the intellectual impairments of their offspring, low birthweight and congenital heart disease are often present. To prevent this, dietary treatment must be commenced before the pregnancy even begins, and rigidly maintained for the duration of the pregnancy.

While dietary treatment for PKU is effective and nonhazardous, additional developments in the field are needed in order to find a ‘cure’ for PKU. There are several different mechanisms that are amenable to clinical intervention. For example, drugs could be designed that would prevent phenylalanine absorption from the gut. This would cause ingested phenylalanine to be excreted and may prevent its damaging effects. Enzyme therapies may also be utilized. Insertion of active PH into a ‘fatty’ liposome carrier or administration of regular enzyme injections may serve potential roles in the treatment of PKU.

Additional attention needs to be paid to PH, the primary enzyme in the metabolism of phenylalanine. As stated above, PH hydroxylates phenylalanine in the presence of the necessary substrates to form the amino acid tyrosine. Numerous defects in the gene encoding PH lead to an inactive enzyme and the systemic build-up of phenylalanine in the body. The recent X-ray crystallographic analysis of PH structure has supplied valuable information regarding the structure of the protein. These data may assist researchers in the design of pharmacotherapeutic interventions to treat PKU by accelerating the metabolism of phenylalanine to tyrosine. Further structural and functional studies, such as these, are necessary if we hope to eradicate PKU completely. See also Time-resolved X-ray Crystallography

Recently, researchers using rodent models found that use of oral enzyme therapy, with the enzyme phenylalanine ammonia lyase (PAL), combined with a low protein diet has the potential to replace the traditional phenylalanine-restricted dietary regimen. PAL is an enzyme that does not require a cofactor to convert phenylalanine to trans-cinnamic acid and insignificant amounts of ammonia. Trans-cinnamic acid is a harmless metabolite which is further converted to benzoic acid and then to hippurate, which is excreted in urine. In theory, patients on PAL could break down phenylalanine such that phenylalanine ingestion could be tolerated. In rodent models, it was found that oral administration of PAL attenuated PKU. When humans were used as test subjects, similar results were observed. However, only limited data are available from the studies because of the costs necessary to obtain sufficient amounts of PAL. Further research must be performed before the general population can use PAL, but these studies indicate that oral administration of PAL appears to have great potential as a future treatment for PKU. The costs of acquiring PAL must decrease, however.

Research indicates that PKU is primarily the result of single gene malfunctions. Therefore, a promising alternative to dietary restriction of phenylalanine in the management of PKU is somatic gene therapy. This would involve the insertion of a normal PH gene into a chromosomal location in the nucleus of liver cells (hepatocytes). The normal PH would then be expressed in place of, or in addition to, the mutated PH. Hepatocytes would be the appropriate targets for gene therapy since PH is known to be expressed predominantly within the liver.

In order to get the normal gene to its proper target, the deoxyribonucleic acid (DNA) needs to be inserted into a vector or ‘carrier’ system. To date, the use of three different vector systems has been explored. They include recombinant retroviral vectors, recombinant adenoviral vectors, and DNA–protein complexes. Numerous studies have shown these gene carriers to be successful in lowering blood phenylalanine levels. However, all have serious limitations, such as low transduction efficiencies and the inability to integrate into nondividing cells. Additionally, these vectors may eventually lead to immune responses, or become inactive, which limits long-term efficacy. All of these limitations must first be overcome if genetic therapy for PKU is to become a reality.

Summary

  1. Top of page
  2. Introduction
  3. Clinical Signs and Conditions
  4. Classical Phenylketonuria
  5. Nonclassical Phenylketonuria
  6. Future Treatments for Phenylketonuria
  7. Summary
  8. Further Reading

PKU (hyperphenylalaninaemia) is one of the more common genetic disorders of intermediary metabolism. Disruption of the normal metabolic conversion of phenylalanine to tyrosine results in increased circulating levels of phenylalanine and its metabolic breakdown products. In the developing neonate, these metabolic imbalances produce dramatic clinical problems including severe intellectual impairment. In most cases, the genetic culprit in this problem is a defect in the biosynthetic enzyme PH. In a minority of the cases, there is an alternative problem in the biosynthesis or regeneration of a pivotal cosubstrate of the PH reaction (BH4). Luckily, if properly diagnosed soon after birth, PKU can be managed through dietary interventions that decrease the levels of circulating phenylalanine. Knowing the molecular biology of the disorder provides a long-term hope of ‘curing’ the disease through gene replacement strategies. These approaches may well become routinely available in the future.

Glossary
Dihydropteridine reductase (DHPR)

The enzyme responsible for the conversion of the inactive, oxidized pterin cosubstrate quinonoid dihydrobiopterin (q-BH2) to the active, reduced form, tetrahydrobiopterin (BH4).

Hyperphenylalaninaemia

An error in metabolism whereby dietary phenylalanine is not converted to tyrosine and so its concentrations build up to dangerous levels in the blood.

Phenylalanine hydroxylase (PH)

Enzyme that catalyses the conversion of the essential amino acid phenylalanine to tyrosine.

Phenylketonuria

A genetic inborn error in metabolism in which the activity of phenylalanine hydroxylase is compromised. This error results in raised levels of circulating phenylalanine (hyperphenylalaninaemia) and increased excretion of its ketone metabolites (phenylketonuria).

Polymorphism

A difference in gene sequence that can produce an altered phenotype.

Tetrahydrobiopterin

Essential cosubstrate for phenylalanine hydroxylase, tyrosine hydroxylase and tryptophan hydroxylase. It is also referred to as the pterin cosubstrate or, incorrectly, as the pterin cofactor.

Further Reading

  1. Top of page
  2. Introduction
  3. Clinical Signs and Conditions
  4. Classical Phenylketonuria
  5. Nonclassical Phenylketonuria
  6. Future Treatments for Phenylketonuria
  7. Summary
  8. Further Reading
  • Bremer HJ, Duran D, Kamerling JP, Przyrembel H and Wadman SK (eds) (1981) Disturbances of Amino Acid Metabolism: Clinical Chemistry and Diagnosis, pp. 307327. Baltimore, MD: Urban & Schwarzenberg. [Chapter on clinical chemistry and diagnosis of inherited diseases.]
  • Eisensmith RC and Woo SLC (1995) Molecular genetics of phenylketonuria: from molecular anthropology to gene therapy. Advances in Genetics 32: 199271.
  • Hoang L, Byck S, Prevost L and Scriver CR (1996) PAH Mutation Analysis Consortium Database: a database for disease-producing and other allelic variation at the human PAH locus. Nucleic Acids Research 24(1): 127131.
  • Kaufman S (1997) Tetrahydrobiopterin, pp. 262322. Baltimore, MD: Johns Hopkins University Press. [Chapter on phenylketonuria and its variants.]
  • Nowacki PM, Byck S, Prevost L and Scriver CR (1997) The PAH Mutation Analysis Consortium Database update 1996. Nucleic Acids Research 25(1): 139142.
  • Wurtman R and Ritter-Walker E (1988) Dietary Phenylalanine and Brain Function. Boston, MA: Birthäuser.