Phenylketonuria (PKU)

Phenylalanine hydroxylase (PAH) deficiency, better known as PKU, is the most common inborn error of amino acid metabolism. It results from an impaired ability to metabolize the essential amino acid phenylalanine, leading to accumulation in blood and tissues.

Several different classifications have been used in the past to describe PKU severity. Commonly, classic PKU is considered to be present when untreated plasma phenylalanine levels exceed 20 mg/dL (1200 µmol/L) without treatment. Lesser degrees of plasma phenylalanine elevation are often referred to as hyperphenylalaninemia. The American College of Medical Genetics and Genomics has released a practice guideline acknowledging that PAH deficiency represents an unbroken spectrum of disease.[8] While this change in nomenclature is being adopted, providers will likely continue to use previous terminology.

Elevated phenylalanine levels negatively impact cognitive function, and individuals with classic PKU almost always have intellectual disability unless levels are controlled through dietary or pharmacologic treatment.

In the United States and many other countries, PKU is detected by newborn screening, and individuals who are appropriately treated (eg, with a diet low in phenylalanine and/or tetrahydrobiopterin) can have normal intelligence and lead a normal life.

In most patients, the classic type of PKU involves a deficiency of PAH that leads to increased levels of phenylalanine in the plasma (>1200 µmol/L; reference range, 35-90 µmol/L) and to excretion of phenylpyruvic acid (approximately 1 g/d) and phenylacetic acid in the urine. Phenylalanine hydroxylase catalyzes the conversion of L-phenylalanine to L-tyrosine, the rate-limiting step in the oxidative degradation of phenylalanine 

Phenylalanine hydroxylase converts phenylalanine to tyrosine.

The enzyme PAH crystallizes as a tetramer, with each monomer consisting of a catalytic domain and a tetramerization domain. Examination of the mutations causing PKU reveals that some of the most frequent mutations are located at the interface of the catalytic and tetramerization domains.

The mechanism by which elevated phenylalanine levels cause intellectual disability is not known, although restriction of dietary phenylalanine ameliorates this effect if initiated within a few weeks of birth. A strong relation between control of blood phenylalanine levels in childhood and intelligence quotient (IQ) is recognized.

Subtle neuropsychological deficits in children with treated PKU are under investigation. Some investigators have attributed these deficits to small residual neurotransmitter abnormalities (eg, reduced production of neurotransmitters as a result of deficient tyrosine transport across the neuronal cell membranes).

Phenylalanine hydroxylase requires a nonprotein cofactor termed tetrahydrobiopterin (BH4). A small percentage of children with elevated phenylalanine levels exhibit normal PAH levels but have a deficiency in synthesis or recycling of BH4 known as tetrahydrobiopterin deficiency. This condition is sometimes termed malignant PKU and can result from biallelic mutations in the GCH1, PCB1, PTS, or QDPR genes. The BH4 cofactor is also required for hydroxylation of tyrosine (a precursor of dopamine) and tryptophan (a precursor of serotonin). Thus, individuals with BH4 cofactor deficiency can have additional neurologic problems that are not fully corrected by dietary phenylalanine reduction alone, but often require additional treatments that may not be fully effective.

Phenylketonuria (PKU) is an autosomal recessive disorder caused by mutations in the PAH gene, which expresses PAH. This gene is located on 12q23.2, spans about 171 kb, and contains 13 exons. More than 500 different mutations in the PAH gene have been identified.

The PAH gene shows great allelic variation, and pathogenic mutations have been described in all 13 exons of the PAH gene and its flanking region. The mutations can be of various types, including missense mutations (62% of PAH alleles), small or large deletions (13%), splicing defects (11%), silent polymorphisms (6%), nonsense mutations (5%), and insertions (2%).[9]

The 3 genes related to biopterin synthesis defects are located at 11q22.3-23.3, 10q22, and 2p13, and the gene for biopterin recycling defects is located at 4p15.1-16.1.

Phenylketonuria displays a marked genotypic heterogeneity, both within populations and between different populations. There is some broad genotype-phenotype correlation (alleles that tend to be severe and alleles that tend to be mild), but unrelated individuals with identical mutations have some degree of variability in phenylalanine tolerance.

United States statistics

Phenylketonuria (PKU) frequency varies by population. The prevalence in the general US population is approximately 4 cases per 100,000 individuals, and the incidence is 350 cases per million live births. Approximately 0.04% to 1% of the residents in intellectual disability clinics are affected by PKU. A low incidence is reported in African Americans (1:50,000).

International statistics

A high incidence is reported in Turkey (approximately 1 case in 2600 births), the Yemenite Jewish population (1:5300), Scotland (1:5300), Estonia (1:8090),[10] Hungary (1:11,000), Denmark (1:12,000), France (1:13,500), the United Kingdom (1:14,300), Norway (1:14,500), China (1:17,000), Italy (1:17,000), Canada (1:20,000), Minas Gerais State in Brazil (1:20,000),[11] and the former Yugoslavia (1:25,042).[12] A low incidence is reported in Finland (< 1:100,000)[13] and Japan (1:125,000).[9]

Age-, sex-, and race-related demographics

Phenylketonuria is most commonly diagnosed in neonates because of newborn screening programs. Consider PKU at any age in an individual with developmental delay or intellectual disability because infants are missed by newborn screening programs on rare occasions.

No sex predilection is known. Women with PKU must restrict their phenylalanine levels during pregnancy to avoid birth defects and intellectual disability in their infants. Untreated PKU during pregnancy can result in maternal PKU syndrome, which can variably cause congenital heart defects, brain malformations, microcephaly, and intellectual impairment.

In the United States, PKU is most common in whites. Worldwide, PKU is most common in whites and Asians.

The prognosis for normal intelligence is excellent when patients have been put on a diet low in phenylalanine in the first month of life, with careful monitoring. However, school functioning can be mildly impaired in some children, particularly when dietary control is poor.

A quantitative, proportional relation exists between blood phenylalanine levels and intelligence quotient (IQ) for early-treated patients with phenylketonuria (PKU) assessed either during the critical early childhood years (age 0-12 y) or by a lifetime Index of Dietary Control. A 100-μmol/L increase in phenylalanine has resulted in a 1.3- to 4.1-point reduction in IQ.[14]

Patients with PKU who are treated early and continuously can have a normal health-related quality of life and course of life.[15] Well-treated patients should have IQs within approximately 5 to 8 points of their siblings.

Most untreated individuals with PKU have profound intellectual disability. After the discovery of PKU, routine testing of institutionalized patients with intellectual disability identified a 1% incidence of PKU in this group.

Psychiatric disorders have been frequently described among individuals with PKU, including anxiety, depression, personality disorders, and psychosis. Interestingly, psychiatric comorbidities were less common in younger patients than older patients with PKU.[16] Treated patients with PKU can experience subtle performance and attention and behavioral changes, especially when phenylalanine levels exceed 360 µmol/L.

Teach parents how to administer the diet at home, and involve all caregivers as well. Children should begin involvement in their dietary planning as soon as they are developmentally ready. Poor dietary control is often associated with increasing noncompliance by older children, but it could also be due to a more relaxed dietary approach by parents and increasing dietary errors.[17]

Women with phenylketonuria (PKU) should be educated about the risks of untreated pregnancy and the benefits of dietary and, in some cases, pharmacologic, treatment. Patients with PKU should avoid aspartame (an artificial sweetener). Aspartame is widely used in medicines, vitamins, beverages, and other substances.

The phenylalanine-restricted diet with semisynthetic supplementation is not without risk. Patients with PKU under dietary treatment can have low concentrations of trace elements and cholesterol and can have some disturbance to folate metabolism and distortion of their fatty acid profile.[9] Patients may also have decreased intake of calcium, vitamin D, and vitamin B12.

The organization National PKU News is a nonprofit entity dedicated to providing accurate news and information to families and professionals dealing with PKU. This site contains excellent articles and links to other information sources. Information on how to subscribe to a PKU newsletter and on how to contact support groups is available. Numerous other PKU Web sites are available to assist families in search of additional information.

Most individuals with phenylketonuria (PKU) appear normal at birth. If an affected patient does not undergo newborn screening or has false-negative results (rare), progressive developmental delay is the most common presentation. Other findings in untreated children in later infancy and childhood may include vomiting, mousy odor, eczema, seizures, self-mutilation, and severe behavioral disorders.

The cohort of patients identified in the early days of newborn screening were begun on diet modification as neonates and continued on that diet until they were school-aged and were then generally transitioned to a normal diet. While these patients escaped the most severe manifestations of PKU, many described poor focus and deteriorating cognitive skills. Those same individuals who ceased dietary treatment in childhood may also have evidence of white matter changes visible on magnetic resonance imaging (MRI), and those patients may experience an intelligence quotient (IQ) decline of 10 points or more.

The clinical manifestations of PKU are largely of historical interest, because the damaging features of the disease are virtually always prevented through early diagnosis and treatment. Skin findings are as follows:

• Fair skin and hair – This is the most characteristic skin manifestation, resulting from impairment of melanin synthesis; it can be striking in Black and Japanese patients, although not all untreated patients are fair, and treated patients often have typical pigmentation

  Fair skin and hair resulting from impairment of melanin synthesis.

• Eczema (including atopic dermatitis)

• Light sensitivity

• Increased incidence of pyogenic infections

• Increased incidence of keratosis pilaris

• Decreased number of pigmented nevi

• Sclerodermalike plaques

• Hair loss[1]

Other manifestations of untreated PKU are as follows:

• Intellectual disability (the most common finding overall)

• Musty or mousy odor

• Epilepsy (50%)[2]

• Extrapyramidal manifestations (eg, parkinsonism)

• Eye abnormalities (eg, hypopigmentation)

Subtle attention and performance deficits in organization and planning persist in treated patients. These deficits are in some cases related to phenylalanine levels and may interfere with academic achievement.

The incidence of neuropsychiatric disease in PKU appears to be higher than in the general population and includes increased risk of depression, anxiety, and inattention, among others. It has been shown that these symptoms are exacerbated by high phenylalanine levels and improved by lower phenylalanine levels. These findings underscore the need for lifelong diet maintenance.[18]

Older textbooks and protocols occasionally called for phenylalanine-loading studies to help determine if a child with phenylketonuria (PKU) still required phenylalanine restriction after 1 or more years; however, as the treatment range for phenylalanine levels has decreased, these studies generally have been abandoned.

A qualified laboratory should measure plasma phenylalanine and tyrosine. A qualified laboratory should perform blood and urine analysis of biopterin and neopterins in order to rule out defects of biopterin synthesis or recycling. DNA mutation analysis is adjunctive and may be helpful in some cases, although is not required for diagnosis.

Prenatal diagnosis is available but rarely used because the disorder is so treatable. Prenatal diagnosis can be accomplished by DNA mutation analysis following chorionic villous sampling or amniocentesis.

Perform screening on blood samples during the first week of life. Wide variability in phenylalanine concentrations in a 24-hour period in children with PKU may necessitate repeat screening. Screening for PKU involves the following:

• Determination of phenylalanine levels, the standard amino acid analysis done by means of ion exchange chromatography or tandem mass spectrometry
• The Guthrie test as a bacterial inhibition assay; formerly used, now being replaced by tandem mass spectrometry

Further measures may be indicated, depending on the results of screening:

• Abnormal newborn screening results require immediate follow-up in accordance with local regulations.
• Less-prominent elevations of phenylalanine or ambiguous results may require repeat screening.
• More significant elevations still require definitive testing and referral to a metabolic treatment facility experienced with PKU.
• Late diagnoses are usually made during amino acid analysis of individuals who are developmentally abnormal.

Results of urine tests (ie, ferric chloride test) may be negative in the first month of life and are rarely used in current practice. It is important to measure erythrocyte dihydropteridine reductase and urine neopterin and biopterin.

Follow nutritional adequacy on a regular basis because deficiencies of iron, vitamins, selenium, protein, essential fatty acids, and other nutrients have been reported in patient with treated PKU.

Cranial magnetic resonance imaging studies may be indicated in older individuals who have poor dietary control and are experiencing deficits in motor or cognitive function, or when there are behavioral, cognitive, or psychiatric concerns. White matter changes are common and may have some reversibility with improved diet and reduced phenylalanine levels. These areas appear to be related to both higher phenylalanine levels in the blood and brain and to poorer cognitive outcome. In terms of volume loss, the most severely affected brain structures are the cerebrum, the corpus callosum, the hippocampus, and the pons.[4]

Preliminary indications suggest that brain phenylalanine levels can be measured by means of cranial magnetic resonance spectroscopy (MRS) and that these levels may be more predictive of outcome than blood phenylalanine levels are; however, this work is done in only a few centers, and there is some controversy surrounds whether such state-of-the-art technology is useful.

Most patients with phenylketonuria (PKU) are treated in a specialty metabolic disease clinic, and such patients are probably best served by being followed in such a clinic. A psychologist should perform developmental testing at regular intervals. Whenever possible, the patient and parents should work with a nutritionist experienced in PKU, usually as part of a PKU or metabolic disease clinic.

Treatment consists of dietary restriction of phenylalanine often with tyrosine supplementation. Other essential amino acids are supplemented using various medical foods, and vitamin, mineral, and other micronutrients are followed closely. Stringent phenylalanine-restricted diets have been reported to cause deficiencies of iron, zinc, selenium, and other nutrients and essential amino acids in patients with PKU; therefore, the diet requires careful monitoring by a professional trained in PKU management and frequently requires supplementation of required nutrients.

Phenylalanine levels are followed at regular intervals, from 1 to 2 times weekly in neonates to perhaps once per month in older children and adults. Most US facilities recommend that phenylalanine levels be maintained in the range of 2 mg/dL to 6 mg/dL (120-360 µmol/L). This requires expert care and close monitoring.

The diet should not be terminated after adolescence, because strong evidence indicates that hyperphenylalaninemia can have detrimental effects in adult patients. Some adults with untreated PKU who have cognitive decline may show improvement in behavior and physical manifestations when treated with a phenylalanine-restricted diet.

Normal levels of activity should be expected in patients who are adequately treated.

Sapropterin was approved by the US Food and Drug Administration (FDA) as a treatment for PKU. It seems to be effective in a subset of patients. Although patients with classic PKU are less likely to be responders, response has been documented in this group and a trial is not unreasonable.[19] It was estimated that 10% of patients with a classic presentation of PKU and most patients with milder manifestations may respond to sapropterin.[20]

In May 2018, the FDA approved the first enzyme substitute, pegvaliase (Palynziq), to reduce phenylalanine levels in adults with PKU who have uncontrolled phenylalanine levels of more than 600 µmol/L.[6]

Because of the lack of internationally accepted guidelines, the management of PKU varies among countries; however, it is generally agreed that dietary management and/or pharmacologic treatment are essential. The mainstay of the diet consists of phenylalanine restriction and supplementation of other essential amino acids, vitamins, minerals, and energy intake, using medical foods and low-protein foods.[5]

Aspartame must also be avoided. Phenylalanine is one of the primary components of aspartame and is found in many artificially sweetened foods and soft drinks, as well as some vitamins and medicines. A 12-oz can of aspartame-sweetened diet drink contains approximately 105 mg of phenylalanine (ie, 25-50% of the usual daily intake).

Most newborns with PKU require 40 mg/kg/d to 60 mg/kg/d of dietary phenylalanine to maintain normal growth and development. Breastfeeding is usually possible and should not be stopped unless instructed to do so by a local health official or treatment facility. As growth slows, the phenylalanine requirement falls, and most older children and adults tolerate 200 mg/d to 400 mg/d.

Providing some natural phenylalanine is essential in order to prevent deficiency of this essential amino acid. The diet requires virtual elimination of all high-protein foods, such as meat, dairy, nuts, and legumes. Starches, including bread, potatoes, corn, and beans, also must be restricted (a slice of bread or small order of fries contains approximately 120-150 mg of phenylalanine).

Essential amino acids, vitamins, and minerals must be supplemented by using medical foods. Most are consumed as a powder dissolved in liquid (ie, formula). Supplements including capsules, amino acid bars, and amino acids cooked into foods, are becoming available.

Energy and variety are provided by low-protein foods, including fruits and nonstarchy vegetables, as well as specially ordered low-protein foods. Low-protein foods include pastas, breads, imitation cheese, baking mixes, and other foods especially designated for low-protein diets. These foods are covered by medical benefits in some states.

As patients with PKU transition into adolescence, their caregivers have a less direct influence on their diet. It is common to see these patients "cheat" by failing to limit foods such as potatoes, pasta, and bread.

Advice on specific diet recommendations can be found at many Web sites devoted to PKU. The Web site National PKU News has extensive dietary recommendations. Links to a variety of international organizations for PKU can be found under “Related Links.” The material is available in some languages other than English, including German, Danish, Dutch, and Spanish. Food companies distributing products useful for low-protein diets are also listed.

Alternative regimens have been developed in older patients who have difficulty adhering to a strict regimen.[21]

Patients who refuse dietary treatment may benefit to some degree from consuming large neutral amino acids. These may compete with phenylalanine at the blood-brain barrier and block phenylalanine entry into the brain, and they may also result in a modest lowering of plasma phenylalanine levels.

One study of neuropsychological response to large neutral amino acid supplementation found no advantage to consuming large neutral amino acid (LNAA) supplements in patients already on diet and medical food; some benefit in executive functions in some domains was reported, but attention was better on diet and medical food.[22] Large neutral amino acid (LNAA) use in PKU remains somewhat controversial.

Some patients with PKU experience significant lowering of plasma phenylalanine levels after administration of sapropterin, a commercially available, FDA-approved form of the tetrahydrobiopterin (BH4) cofactor.[7] One study found that, although 54% of those with plasma phenylalanine levels lower than 600 µmol/L (10 mg/dL) had a reduction in plasma phenylalanine levels of 30% or more after 10 mg/kg/day of sapropterin, only 10% of those with phenylalanine levels higher than 1200 µmol/L had such a response.[7]

In May 2018, the FDA approved the first enzyme substitute, pegvaliase (Palynziq), to reduce phenylalanine levels in adults with PKU who have uncontrolled phenylalanine levels of more than 600 µmol/L. It is administered as a pegylated SC injection of phenylalanine ammonium lyase, an enzyme capable of substituting for phenylalanine hydroxylase (PAH). 

Approval of pegvaliase was based on 2 phase 3 studies, PRISM-1 and PRISM-2, which evaluated the efficacy and safety of pegvaliase treatment using an induction, titration, and maintenance dosing regimen in adults with PKU. Of 261 participants who received pegvaliase, 72% and 32.6% reached at least 12 months and 24 months of study treatment, respectively, and 65% are still actively receiving treatment. Mean (SD) blood phenylalanine levels were 1232.7 (386.4) μmol/L at baseline, 564.5 (531.2) μmol/L at 12 months, and 311.4 (427) μmol/L at 24 months, a decrease from baseline of 51.1% and 68.7%, respectively.[23]  In 2020, the FDA approved an increased maximum dose.[24]  

Research into gene therapy for the treatment of PKU has been ongoing over the last 2 decades. The focus has been on replacement of the human mutant PAH gene in somatic cells of PKU patients.[25]

Surveys have revealed that maternal phenylalanine blood concentrations higher than 1200 µmol/L are associated with maternal PKU syndrome dysmorphic facies, fetal microcephaly, learning difficulties, congenital heart defects, and intrauterine growth retardation.[26] Accordingly, maternal blood phenylalanine levels should be maintained in the range of 2-6 mg/dL (120-360 µmol/L) by means of dietary control. The diet should provide adequate energy, protein, vitamin, and mineral intake. Dietary needs of phenylalanine vary during pregnancy, so weekly measurement of phenylalanine levels is important.

Treatment at any time during pregnancy may reduce the severity of developmental delay. Women with PKU should start a phenylalanine-restricted diet before conception, and those contemplating pregnancy or who are pregnant should be treated in metabolic or PKU clinics.

Phenylalanine levels are monitored typically twice a week in neonates, weekly in infants, biweekly or every 3 weeks in toddlers, and monthly thereafter, even during adult life.

Attention should be given to variability in blood phenylalanine levels and to maintenance within the recommended range.[27]

During pregnancy, weekly phenylalanine sampling is recommended.

Treatment of phenylketonuria (PKU) is primarily diet-based; however, some patients may benefit from the administration of large neutral amino acids (additional studies are needed). Drugs approved in the United States include an enzyme cofactor (sapropterin) and enzyme substitute (pegvaliase). The goals of pharmacotherapy are to reduce morbidity and to prevent complications.

Large neutral amino acids (NeoPhe-PreKunil, PhenylAde, PreKunil)

Some evidence suggests that consumption of high doses of other large neutral amino acids can increase competition with phenylalanine for crossing the blood-brain barrier into the brain, thus decreasing phenylalanine levels in the brain.

Adults and older teenagers refusing dietary restrictions can be prescribed a preparation of high-dose large neutral amino acids (LNAAs). This therapy may decrease entry of phenylalanine into the CNS. However, it does not protect a developing fetus from the teratogenic effects of phenylalanine.

It should be kept in mind that LNAAs contain high doses of tyrosine and tryptophan. Too much tyrosine can cause headaches, which limits the numbers of tabs that can be consumed. Furthermore, by competitive inhibition, they also counteract uptake of Phe across the blood-brain barrier, thus reducing its impairing effect on neurotransmitter production.

LNAAs may be ideal for young adults, for poorly compliant patients, and for late-diagnosed patients in whom compliance is low and in whom drinking formula can be a burden for the patient and caretakers.

The tablets must be combined with a certain amount of natural protein in order for the diet to contain sufficient protein. PreKunil does not contain lysine, an essential amino acid, and lysine deficiency has been reported. Individuals taking PreKunil continue to require nutritional assessment because teens and adults who are "off diet" often fail to consume sufficient protein to meet essential amino acid and vitamin/mineral requirements.

Class Summary

Clinical trials have shown that a subset of children with classic PKU respond to tetrahydrobiopterin (BH4) therapy, depending on their PAH gene mutation. Synthetic BH4 (sapropterin) is a cofactor for the enzyme phenylalanine hydroxylase (PAH).

Sapropterin (Kuvan)

Sapropterin is a synthetic form of BH4, the cofactor for the enzyme phenylalanine hydroxylase (PAH). PAH hydroxylates phenylalanine (Phe) through an oxidative reaction to form tyrosine. PAH activity is absent or deficient in patients with PKU. Treatment with BH4 can activate residual PAH, improve normal oxidative metabolism of Phe, and decrease Phe levels in some patients. It is to be used in conjunction with a Phe-restricted diet.

Class Summary

PEGylated phenylalanine ammonia lyase (PAL) that substitutes for the deficient phenylalanine hydroxylase (PAH) enzyme activity in patients with PKU and reduces blood phenylalanine concentrations.

Pegvaliase (Palynziq, pegvaliase-pqpz)

PEGylated phenylalanine ammonia lyase (PAL) enzyme that converts phenylalanine (Phe) to ammonia and trans-cinnamic acid. It is indicated to reduce blood Phe concentrations in adults with PKU who have uncontrolled blood Phe concentrations >600 μmol/L on existing management.