Phenylketonuria (PKU)PH Arn, Nemours Children’s Clinic, Jacksonville, FL, USA

r 2014 Elsevier Inc. All rights reserved.

This article is a revision of the previous edition article by Seymour Packma, volume 3, pp 870–873, r 2003, Elsevier Inc.

Introduction

A deficiency in the activity of phenylalanine hydroxylase

(PAH), a hepatic enzyme that converts phenylalanine to tyr-

osine, causes phenylketonuria (PKU). In patients with PKU,

the biochemical block results in the accumulation of phenyl-

alanine, which is converted into phenylketones that are ex-

creted in the urine. PKU is treated by selective restriction of

phenylalanine intake (and tyrosine supplementation) while

providing enough additional protein and nutrients to support

normal growth. Mandatory population newborn screening for

PKU, in combination with postnatal presymptomatic therapy,

begun more than 40 years ago. The presymptomatic insti-

tution of specific dietary therapy prevents mental retardation.

However, therapeutic success has been tempered by the un-

fortunate occurrence of congenital anomalies, secondary to

the teratogenic effects of phenylalanine, in children born to

mothers with PKU who are subject to poor dietary control.

Elevated blood phenylalanine levels may infrequently be

caused by inherited disorders of the biosynthesis of tetra-

hydrobiopterin (BH4), a cofactor in the PAH reaction. The

biopterin disorders are also briefly discussed in this article.

Clinical Features

Mental retardation (IQs often 50 or lower) is the most sig-

nificant clinical finding in untreated or poorly treated PKU.

Patients with PKU appear normal at birth and appear to have

normal early development, even if untreated. Metabolic en-

cephalopathy does not occur in PKU, but neurological

manifestations of the disease appear insidiously and include

changes in muscle tone, reduced rate of growth of head cir-

cumference, and failure of acquisition of milestones. If un-

treated for lengthy periods of time, patients may present with

lighter pigmentation than other family members (reduced

melanin synthesis) and have a musty odor (phenylacetic acid)

and eczema. Patients exposed to chronically elevated phenyl-

alanine levels may ultimately develop microcephaly, seizures,

athetosis, and spasticity. Autistic behavior(s) and hyperactivity

are common. Magnetic resonance imaging (MRI) of the brain

may show dysmyelination, a finding that is potentially re-

versible with the initiation of dietary therapy.

As a rule, treated PKU patients have normal IQs; however,

careful psychological testing has shown degrees of impairment

in abstract reasoning and problem solving, even in some well-

treated individuals. Emotional disorders as well as hyperactive

behavior are more frequently encountered in patients with

PKU than in the general population.

Patients with PKU are classified into clinical subtypes on

the basis of blood phenylalanine levels and phenylalanine

Encyclopedia of the Neurological Sciences, Volume 3 doi:10.1016/B978-0-12-385

tolerance. On an unrestricted diet, a blood phenylalanine level

higher than the normal range (31–110 mM) but lower than

1000 mM is referred to as non-PKU hyperphenylalaninemia

(HPA). Classic PKU is characterized by untreated phenyl-

alanine levels of 41000 mM and a dietary phenylalanine tol-

erance (intake limit) of o500 mg/day. Classification of PKU

into further subtypes based on blood phenylalanine levels and

phenylalanine tolerance has also been proposed and used by

some practitioners. PKU is associated with a high risk of im-

paired cognitive development; however, non-PKU HPA is

generally associated with a lower risk of impaired cognitive

development.

Basic Defect

PKU is an autosomal recessive disorder with a population

prevalence of approximately 1/10 000 in Caucasians of

northern European ancestry. Mutation analysis of the gene

shows that most patients are compound heterozygotes,

carrying a different mutant allele on each chromosome. Pre-

valences of specific mutant alleles differ from population

to population. Variability of the biochemical phenotype

(phenylalanine tolerance) is caused primarily by different

mutations with the PAH gene. The clinical phenotype (cog-

nitive and behavioral) is more complex, however, and many

factors, including other genetic factors, contribute to outcome.

The precise etiology of the mental retardation in untreated

PKU is not understood. Because cerebral protein synthesis is

inhibited by excessive phenylalanine, it is possible that de-

fective brain myelination may be related to decreased bio-

synthesis of myelin proteins. Central nervous system effects

may be ascribed to more global amino acid imbalances. Ele-

vated phenylalanine may affect the central nervous system

concentrations of neutral amino acids by competitive inhib-

ition of a shared amino acid transporter. Brain tyrosine de-

ficiency, with resultant perturbations in brain neurochemistry,

may also contribute to pathogenesis. Variations in phenyl-

alanine transport across the blood–brain barrier may con-

tribute to variability in outcome.

Diagnosis

The Guthrie bacterial inhibition assay was a technical break-

through, allowing newborn screening of large populations.

Fluorometric assays or tandem mass spectrometry are meth-

odologies currently being utilized in screening and moni-

toring. False positives may be seen in neonates with liver

disease or in infants on parenteral alimentation. Confirmation

of the diagnosis is made by analysis of blood phenylalanine

157-4.00077-4 887

888 Phenylketonuria (PKU)

concentration. Molecular analysis of the PAH gene is not re-

quired for confirmation of the diagnosis. All individuals with

confirmed hyperphenylalaninemia must have further screen-

ing for BH4 defects. PKU may be suspected in a child or adult

or it can reasonably be included in the differential diagnosis of

a given patient of any age, presenting with neurodevelop-

mental delay of unknown etiology. In such settings, diagnostic

testing for PKU must be done, even if there is a history or

record of a normal newborn screen.

Treatment

A diet low in phenylalanine is the basis of PKU therapy. If a

neonate has an initial positive screen, and a confirmation of

an elevated blood phenylalanine level (more than 600 mM) is

obtained, dietary restriction of phenylalanine is begun. The

need for treating patients with blood phenylalanine levels

between 400 and 600 mM is debated. If the low-phenylalanine

diet is initiated in the neonatal period (between 7 and 14

days) and maintained throughout life, the underlying bio-

chemical toxicity is ameliorated and mental retardation is

prevented. The goal of dietary therapy is the maintenance

of blood phenylalanine concentrations between 120 and

360 mM. (Normal phenylalanine levels are usually lower than

120 mM.) Significant restriction of dietary phenylalanine is

required for treatment (intake limited to 200–500 mg of

phenylalanine per day), but the exact level of daily phenyl-

alanine intake will vary from patient to patient and will also

vary with age in a given individual patient. Although severe

mutations, in general, require a greater limitation of phenyl-

alanine intake in order to maintain acceptable blood

phenylalanine levels, individual variations of phenylalanine

tolerance may occur, even in patients with identical genotypes.

Therefore, the diet must be adjusted with care for each patient.

The regimen must be initiated and overseen by experts in PKU

at a specialized center, and referral of the patient to such a

specialized center is mandatory.

In earlier therapeutic protocols, treatment was only con-

tinued through the first few years of life, theoretically corres-

ponding to the age at which brain myelination is complete. As

developmental data accumulated, it became evident that

treatment throughout childhood and adolescence was the best

course to preserve intelligence. In recent studies, it has been

shown that brain abnormalities as demonstrated on MRI, and

electrophysiological testing abnormalities referable to the

central nervous system, are observed in adults who are on

unrestricted phenylalanine intake. Accordingly, it is reasonable

to continue treatment into adulthood, and most centers rec-

ommend treatment for life.

Patients who are diagnosed in the neonatal period and

who adhere to the phenylalanine-restricted diet have normal

overall intelligence. However, learning problems can occur in

well-treated patients. Patients may also be more prone to

attention-deficit/hyperactivity disorder (ADHD), decreased

autonomy, and school/social problems. Such potentially ad-

verse and unpredictable manifestations should be brought to

the attention of parents and carefully explained with care and

support during the ongoing and long-term management

process.

Recent reports and clinical experience have documented

the lowering of serum phenylalanine concentrations in re-

sponse to oral administration of pharmacological doses of the

cofactor BH4. Many patients with non-PKU HPA are respon-

sive to BH4 and up to 10% of patients with classical PKU may

respond to the drug. These patients have mutations in the PAH

gene and not in one of the genes encoding enzymes involved

in BH4 biosynthesis. The BH4 response is likely a result of

correction of PAH mutant kinetic effects or of a chaperone

effect of the BH4.

A novel therapeutic approach employs the enzyme

phenylalanine lyase (PAL). PAL converts phenylalanine into

trans-cinnamic acid, a harmless compound, and has been

shown to reduce hyperphenylalaninemia in a PKU mouse

model. Clinical trials of an injectable form are currently

underway in humans.

At the blood–brain barrier, phenylalanine shares a trans-

porter with other large neutral amino acids (LNAA). LNAA

supplementation has decreased phenylalanine levels by com-

petition at the level of this transporter. In noncompliant adults

with PKU, LNAA supplements may help protect the brain

from phenylalanine toxicity.

Somatic gene therapy is being investigated in animal

models.

Maternal Phenylketonuria Syndrome

Elevated maternal blood phenylalanine levels can cross the

placenta and cause fetal birth defects including microcephaly,

dysmorphic features, and congenital heart defects. More than

90% of children born to women with untreated classic PKU

have mental retardation. The risk to the fetus is greatest with

increasing maternal blood phenylalanine levels. For optimal

physical and cognitive fetal outcomes, it is strongly recom-

mended that dietary control be achieved before conception

and that mothers with PKU be monitored carefully by an ex-

perienced center throughout pregnancy. Even at maternal

phenylalanine levels of o360 mM, 6% of infants are born with

microcephaly and 4% with postnatal growth retardation.

Biopterin Disorders

Neonatal hyperphenylalaninemia may rarely be the result of

autosomal recessive defects in the synthesis or recycling of

BH4, an essential cofactor in the PAH reaction. Up to 1% or

2% of patients with hyperphenylalaninemia have a defect in

one of the four enzymes responsible for maintaining BH4

levels. Guanosine triphosphate cyclohydrolase I (GTPCH) and

6-pyruvoyl-tetrahydrobiopterin synthase (PTPS) are essential

enzymes for BH4 biosynthesis, whereas pterin-4a-carbinola-

mine dehydratase (PCD) and dihydropteridine reductase

(DHPR) are responsible for BH4 recycling. An autosomal

dominant form of GTPCH deficiency (dopa-responsive dys-

tonia, Segawa disease, and hereditary progressive dystonia)

presents with dystonia but is not associated with elevated

phenylalanine levels. Because the tyrosine and tryptophan

hydroxylases also require BH4 for proper functioning, these

disorders also result in deficiencies of the neurotransmitters

Phenylketonuria (PKU) 889

L-dopa and 5-hydroxytryptophan (5-HTP). The hyperphenyl-

alaninemia, in association with neurotransmitter deficits,

causes the neurological manifestations associated with the

defects in BH4 synthesis and recycling.

All children with persistent hyperphenylalaninemia must

be screened for biopterin disorders by measuring the levels of

pterin metabolites (neopterin and biopterin). Patients with

GTPCH deficiency have decreased urinary excretion of both

neopterin and biopterin. In PTPS deficiency, neopterin is in-

creased and biopterin decreased, resulting in a greatly elevated

neopterin:biopterin ratio. The neopterin:biopterin ratio in

PCD deficiency is also increased but not to the same extent as

in PTPS deficiency. In PCD deficiency, the characteristic feature

is the presence of primapterin (7-biopterin) in the urine. In

DHPR deficiency, the percentage of biopterin is elevated

(480% in most cases) and the measurement of DHPR activity

in neonatal dried blood spots is employed for diagnosis. Urine

pterin analysis and DHPR activity screening should be per-

formed early in the management of a new patient with per-

sistent hyperphenylalaninemia or these disorders may be

missed.

Untreated patients typically develop neurological manifest-

ations by 4 months, although symptoms can appear in the

neonatal period. Clinical manifestations include progressive

neurological deterioration, microcephaly, movement disorders,

seizures, tone disturbances, oculogyric spasms, swallowing dif-

ficulties, hypersalivation, hyperthermia, and eczema. Transient

forms of both PTPS and PCD deficiencies exist. Importantly,

such symptoms can appear even if the blood phenylalanine

level is maintained in the therapeutic range for classic PAH-

deficiency PKU.

The goals of therapy are to decrease the level of phenyl-

alanine to an acceptable range (120–360 mM) by dietary re-

striction and to correct the neurotransmitter deficiencies with

exogenous supplementation. Accordingly, patients are given

BH4 supplementation, and L-dopa and 5-HTP are adminis-

tered, in doses that are determined for each patient. Adjunctive

agents (e.g., carbidopa and L-deprenyl), which reduce the

catabolism of L-dopa and 5-HTP, may be added to the ther-

apeutic protocol in order to enable the use of lower doses

of these compounds. Measuring levels of cerebrospinal

fluid neurotransmitter metabolites (homovanillic acid and

5-hydroxyindo-lacetic acid) is useful in monitoring the effi-

cacy of treatment.

Side effects of therapy include choreoathetosis and dysto-

nia, which are also features of the underlying disorders.

Tachycardia, diarrhea, and anorexia are associated with 5-HTP

administration. Low cerebrospinal fluid folate concentration is

typically present in DHPR deficiency and is treated by folinic

acid supplementation. Neurological function may improve

with therapy, but the overall prognosis for these disorders is

largely unknown. There are mild forms of DHPR, PTPS, and

PCD deficiencies, and some forms of PTPS and PCD de-

ficiencies may be transient.

See also: Mental Retardation/Intellectual Disability

Further Reading

Bernegger C and Blau N (2002) High frequency of tetrahydrobiopterin-responsiveness among hyperphenylalaninemias: A study of 1919 patientsobserved from 1988 to 2002. Molecular Genetics and Metabolism 77:304–313.

Blau N, Thony B, Cotton RGH, et al. (2001) Disorders of tetrahydrobiopterin andrelated biogenic amines. In: Scriver CR, Beaudet AL, Sly WS, and Valle D (eds.)The Metabolic and Molecular Bases of Inherited Disease, 8th edn., pp. 1725–1778.New York: McGraw-Hill.

Centerwall S and Centerwall W (2000) The discovery of phenylketonuria: The storyof a young couple, two retarded children, and a scientist. Pediatrics 105:89–103.

Enns GM, Martinez DR, Kuzmin AI, et al. (1999) Molecular correlations inphenylketonuria: Mutation patterns and corresponding biochemical and clinicalphenotypes in a heterogeneous California population. Pediatric Research 46:594–602.

Kang TS, Wang L, Sarkissian CN, et al. (2010) Converting an injectable proteintherapeutic into an oral form: Phenylalanine ammonia lyase for phenylketonuria.Molecular Genetics and Metabolism 99: 4–9.

Kayaalp E, Treacy E, Waters P, et al. (1997) Human phenylalanine hydroxylasemutations and hyperphenylalaninemia phenotypes: A meta-analysis ofgenotype–phenotype correlations. American Journal of Human Genetics 61:1309–1317.

Pagon RA, Bird TD, and Dolan CR (eds.) (1993) GeneReviewsTM. Seattle (WA):University of Washington, Seattle.

Scriver CR and Kaufman S (2001) Hyperphenylalaninemia: Phenylalanine hydroxylasedeficiency. In: Scriver CR, Beaudet AL, Sly WS, and Valle D (eds.) The Metabolicand Molecular Bases of Inherited Disease, 8th edn., pp. 1667–1724. New York:McGraw-Hill.

Scriver CR, Levy H and Donlon J Hyperphenylalaninemia: phenylalaninehydroxylase deficiency. In: Scriver CR, Beaudet AL, Sly WS, Valle D, andVogelstein B (eds.) The Metabolic and Molecular Bases of Inherited Disease(OMMBID), Ch. 77. New York, NY: McGraw-Hill.