Overview of phenylketonuria

INTRODUCTION — Phenylketonuria (PKU; MIM #261600) is a disorder affecting the aromatic amino acid, phenylalanine. It results from a deficiency of phenylalanine hydroxylase (PAH) and, if untreated, results in irreversible intellectual disability among other clinical symptoms [1].

An overview of PKU is presented here. A general discussion of amino acid disorders is presented separately. (See "Inborn errors of metabolism: Classification".)

EPIDEMIOLOGY — The prevalence of PKU is the highest in European and East Asian populations, occurring in approximately 1:10,000 to 1:15,000 live births [1,2]. Some European countries may show a higher prevalence (Italy, Ireland 1:2700 to 1:4500) compared with others (Finland 1:100,000) [2]. Most cases of PKU worldwide are caused by phenylalanine hydroxylase (PAH) deficiency [1,2].

PATHOPHYSIOLOGY — The hepatic enzyme phenylalanine hydroxylase (PAH) catalyzes the conversion of the essential amino acid phenylalanine to tyrosine (figure 1). Tetrahydrobiopterin (BH4) is a cofactor required for PAH activity in addition to molecular oxygen and iron. This pathway accounts for most of the catabolism and is responsible for the disposal of approximately 75 percent of dietary phenylalanine, with the remainder used for protein synthesis [3]. PKU is caused by deficiency of PAH [1]. This results in elevated blood and urine concentrations of phenylalanine and its metabolites, phenylacetate and phenyllactate. Tyrosine concentrations are typically in the normal range, although, occasionally, low concentrations are observed. Defects in BH4 metabolism account for approximately 2 percent of patients with elevated phenylalanine levels. (See 'Tetrahydrobiopterin (BH4) deficiency' below.)

Degree of enzyme activity — Complete enzyme deficiency results in classic PKU, in which serum phenylalanine plasma concentrations in an untreated, newly diagnosed newborn infant exceed 20 mg/dL (1200 micromol/L). Residual enzyme activity causes moderate PKU (phenylalanine concentrations 900 to 1200 micromol/L), mild PKU (phenylalanine concentrations 600 to 900 micromol/L), mild hyperphenylalaninemia (HPA; phenylalanine concentrations 360 to 600 micromol/L), and benign mild HPA that typically does not require treatment (phenylalanine concentrations 120 to 360 micromol/L) [4].

Mechanism of intellectual disability — The mechanism by which the elevated concentration of phenylalanine causes intellectual disability is largely unknown [5]. Excessive phenylalanine is thought to interfere with brain growth, myelination, and neurotransmitter synthesis. Potential mechanisms have been proposed based upon the following studies:

●Abnormal development of neurons, glia, and extracellular matrix of the brain in postnatal rats exposed to high phenylalanine levels was suggested by changes in molecular markers [6]. These included increased hyaluronate-binding activity in extracellular matrix formation, alteration in the content of neural cell adhesion molecule, and increased glial fibrillary acidic protein during cerebellar development.

●High phenylalanine concentrations may disrupt brain development through oxidative stress. In the brains of hyperphenylalaninemic rats, total radical-trapping antioxidant potential was reduced, and chemiluminescence was increased [7]. Phenylalanine inhibited both catalase and glutathione peroxidase in vivo. However, catalase was inhibited only in vitro, and superoxide dismutase was not affected in either condition.

●The phenylalanine metabolites, phenylacetate and phenylpyruvate, may inhibit synthesis of alpha-tocopherolquinone, an essential cofactor for the synthesis of brain arachidonic and docosahexaenoic acid, polyunsaturated fatty acids required for normal brain development [8].

●In cultured rat hippocampal cells, phenylalanine specifically inhibited N-methyl-D-aspartate (NMDA) receptors thought to be involved in the regulation of memory and learning [9].

●The transport of large neutral amino acids (LNAAs) into the brain was inhibited by increased concentration of phenylalanine. Decreased LNAA is thought to inhibit synthesis of protein and neurotransmitters, resulting in deficient concentrations of dopamine and serotonin [1].

PHENYLALANINE HYDROXYLASE DEFICIENCY

Genetics — The mode of inheritance for PKU is autosomal recessive. Nearly all cases are caused by mutations in the gene encoding phenylalanine hydroxylase (PAH), which has been mapped to human chromosome 12q24.1 [3]. More than 1000 mutations, including deletions, insertions, splicing defects, and missense and nonsense mutations, are associated with PAH deficiency [10]. Certain mutations are more common in specific ethnic groups [2]. Most affected patients are compound heterozygotes for two different mutations.

Mutations affect the structure of PAH. PAH is a tetramer, with each monomer consisting of a catalytic and tetramerization domain [2,3]. Most of the mutations that result in PKU are located in the catalytic domain. However, some occur at the interface of the two domains, where they can affect the stability of the enzyme [2].

Clinical features — Because of widespread neonatal screening, overt clinical manifestations of PKU are rare. Newborn infants are asymptomatic prior to the initiation of feeds containing phenylalanine (eg, breast milk or standard infant formula). If undetected during the newborn period, the onset of PKU is insidious and may not cause symptoms until early infancy.

In untreated patients, the hallmark of the disease is irreversible intellectual disability, seizures, behavioral abnormalities, microcephaly, and skin disease (eczematous rash, light pigmentation) due to hyperphenylalaninemia (HPA) [1]. Other findings include abnormalities of gait, sitting posture, and stance. The body and urine may have a "mousy" odor due to the increased concentration of phenylacetic acid. Without dietary restriction, cognitive impairment worsens during myelination in early childhood with increasing dietary exposure to phenylalanine but stabilizes when brain maturation is complete [1].

In a historical series of 51 untreated patients evaluated at 28.8 to 71.8 years of age, the extent of intellectual disability was approximately evenly divided between severe (intelligence quotient [IQ] <35) and mildly to moderately impaired (IQ 36 to 67) [11]. Two had IQ >68. Epilepsy occurred frequently, with 12 (23 percent) adult patients affected. Some patients had loss of motor function over time, although 41 (80 percent) had no further deterioration [11].

Patients treated with continuous dietary intervention after diagnosis on newborn screening may still exhibit some neurologic sequelae, although much less significant than untreated patients. One area of cognition that is particularly affected is executive (frontal lobe) function [12,13]. Mild impairment in this area is seen even in patients with good dietary control of phenylalanine levels [14].

A meta-analysis of 11 studies including 360 patients demonstrated that the reduction in bone mineral density is not clinically significant, and there is no correlation between bone mineral density and phenylalanine concentration, nutrient intake, vitamin D, or parathyroid hormone (PTH) [15].

Brain imaging — Neuroimaging is not performed routinely in patients with PKU but rather is reserved for those individuals with abnormal neurologic presentation.

White matter lesions are apparent on magnetic resonance imaging (MRI) in the majority of patients with PKU, including those who were detected by newborn screening and received early dietary treatment [16,17]. A common finding is a symmetrical increase of T2-weighted signal in the periventricular white matter [17]. The changes are thought to represent increased turnover of myelin caused by increased phenylalanine concentration and may be reversible [18,19].

The severity of the abnormalities appears to be related to dietary status. This was demonstrated by a series of 34 PKU patients aged 8 to 33 years, of whom 25 were detected by newborn screening [19]. In the early diagnosed group, the severity of MRI changes was significantly associated with serum phenylalanine concentration at the time of the study.

Diagnosis — Diagnosis of PKU is based upon the finding of an elevated serum concentration of phenylalanine, followed by molecular testing. Blood elevations may be very high (>20 mg/dL, 1200 micromol/L) in patients with complete deficiency of PAH.

The most useful laboratory method for newborn screening is tandem mass spectrometry [1]. This method also can measure additional amino acids including tyrosine and acylcarnitine ester. A high concentration of phenylalanine together with low to low-normal tyrosine concentration suggests the diagnosis of PKU. In addition, tandem mass spectrometry can identify many other inborn errors of metabolism in a single sample. Elevated phenylalanine levels identified through newborn screening should be confirmed with a second plasma amino acid analysis [1].

Enzyme analysis is not performed to confirm the diagnosis, because PAH activity is expressed only in the liver. Molecular analysis can be used to confirm the diagnosis of PKU through identification of two pathogenic mutations in the PAH gene (homozygous or compound heterozygous mutations). Genetic testing can be used for carrier detection or prenatal diagnosis in families in whom the mutation(s) are known. In some cases, it may be possible to predict enzyme activity and/or tetrahydrobiopterin (BH4) responsiveness from the PAH genotype, although the relationship may be inconsistent [1].

Testing for BH4 defects is reviewed below. (See 'Diagnosis of BH4 deficiency' below.)

Management — Management of patients with HPA or PKU should be provided by an experienced interdisciplinary team of nutritionists, psychologists, social workers, metabolic specialists, and pediatricians (or internists for adult patients). Ideally, adult patients should be seen in an adult clinic with expertise in treating patients with PKU.

Dietary restriction — The mainstay of therapy in PKU is dietary restriction of phenylalanine (table 1) [1]. This requires the use of medical foods including phenylalanine-free protein substitutes (amino acid mixtures) that supply approximately 75 percent of protein requirements (except phenylalanine). Medical foods may contain glycomacropeptides (GMPs) as the protein source. GMP is a natural protein found in cheese whey that contains a small amount of phenylalanine and is supplemented with several large neutral amino acids (LNAAs). Studies have demonstrated the efficacy and palatability of a GMP diet in patients with PKU [20,21]. Breastfeeding infants with PKU is encouraged under the supervision of an experienced metabolic dietitian and is alternated with phenylalanine-free formula feeding. Breast milk is usually limited to approximately 25 percent of feedings, depending upon disease severity. Phenylalanine intake through breast milk must take into account the daily phenylalanine allowance. Breast milk is lower in phenylalanine than standard infant formula (14 mg/ounce versus 19 mg/ounce, respectively). Restriction of the maternal diet has no impact on the amino acid composition of breast milk.

The optimal intake of protein substitute needed to meet requirements, including optimal growth and development, is still uncertain. Uncontrolled studies have conflicting results regarding the benefit of high versus low intake of protein substitute [22,23]. Two weeks of therapy with low (1.2 g/kg per day) and high (2 g/kg per day) protein substitute intake were compared in a randomized crossover study in 25 children with well-controlled PKU whose usual median protein intake was 2.2 g/kg per day [24]. During low-protein substitute intake, median phenylalanine levels increased compared with the control period. During high-protein substitute intake, median phenylalanine levels were unchanged from the control period. However, there was wide variability between subjects, which appeared to be related to the carbohydrate and/or fat content of the protein substitute.

The poor palatability of protein substitutes adversely affects compliance with the diet, especially in older children. The required amount of phenylalanine is provided with small amounts of natural protein [1]. Rare patients may need tyrosine supplementation [25].

Treatment should be initiated as soon as possible, usually before one week of age, in infants with PKU and blood phenylalanine concentration >6 mg/dL (360 micromol/L) [26]. Treatment for elevated but slightly lower levels of phenylalanine is controversial. We recommend treating newborns with persistent phenylalanine levels of 6 to 10 mg/dL (360 to 600 micromol/L). BH4 deficiency should be ruled out as cause of the HPA. (See 'Tetrahydrobiopterin (BH4) deficiency' below.)

Continuation of dietary restriction throughout life appears to be necessary for optimal outcomes [1]. The Agency for Health Research and Quality conducted a meta-analysis of 17 studies (including 432 individuals with PKU) that examined the relationship of blood phenylalanine to IQ. They found an increasing probability of low IQ (<85) at higher blood phenylalanine levels, regardless of whether IQ was measured during childhood or beyond, with a stronger association seen between phenylalanine measured in early childhood and later IQ [27].

Diet for life was also supported by findings in a long-term follow-up study of newborns with PKU who participated in a trial of dietary management [28]. In the initial trial, infants were treated with a phenylalanine-restricted diet until they reached six years of age and then were randomly assigned to continue or discontinue the diet. At approximately 25 years of age, 70 of the initial 211 infants were reevaluated. Children who continued the diet had a decreased rate of eczema (11 versus 28 percent), asthma (0 versus 12 percent), headache (0 versus 31 percent), mental disorders (22 versus 41 percent), hyperactivity (0 versus 14 percent), and hypoactivity (0 versus 19 percent) compared with those who discontinued the diet. Continuation of the diet was associated with better intellectual and achievement test scores and lower blood phenylalanine concentration. MRI abnormalities were associated with higher brain phenylalanine concentrations as assessed by magnetic resonance spectroscopy.

In nonrandomized studies, IQ scores remained stable in older children and adults with PKU who were not on a restricted diet [1]. However, subtle deficits in measures of attention and speed of processing were noted when compared with baseline, continuously treated patients, and/or healthy controls. High phenylalanine levels were shown to affect mood and sustained attention in adults in a randomized, crossover trial [29]. Nine adults were given a supplement containing phenylalanine or placebo for four weeks while on a restricted diet. Scores on the Profile of Mood States (POMS) questionnaire filled out by both the patients and their friend or relative were significantly lower on phenylalanine supplementation compared with placebo.

Pharmacotherapy — The primary pharmacotherapy available for the treatment of mild-to-moderate PKU is a synthetic formulation of BH4 called sapropterin. BH4 is the cofactor for PAH. Pegylated phenylalanine ammonia lyase (pegvaliase, PEG-PAL), an enzyme that degrades phenylalanine, is approved as therapy in adult patients with PKU [30].

Tetrahydrobiopterin/sapropterin — Pharmacologic doses of BH4 is an alternative or adjunct to dietary restriction of phenylalanine in patients with HPA or mild-to-moderate PKU phenotypes [31-34]. The response to BH4 among children with PKU (but not BH4 deficiency) has been evaluated in several studies [33,34]. In one of the larger studies, blood phenylalanine levels were measured in 557 newborns and children with PKU after administration of BH4 (20 mg/kg body weight) [34]. With responsiveness defined by a 30 percent reduction in phenylalanine level, 38 percent of patients responded when phenylalanine was measured eight hours after administration, and 46 percent responded when phenylalanine was measured 24 hours after administration. The prevalence of responsiveness was 79 to 83 percent in patients with HPA (phenylalanine <10 mg/dL, 600 micromol/L), 49 to 60 percent in patients with mild PKU (phenylalanine 10 to 20 mg/dL, 600 to 1200 micromol/L), and 7 to 10 percent in patients with classic PKU (phenylalanine >20 mg/dL, 1200 micromol/L). In a smaller study, responsiveness to BH4 was not consistently predicted by genotype [33].

Sapropterin, a biologically active synthetic form of BH4, was approved by the US Food and Drug Administration (FDA) in December 2007 [35-37]. Sapropterin may be used as an adjunct to dietary restriction in patients with PKU who are responsive to sapropterin [35,38-43]. Patients should be offered a trial of sapropterin therapy to assess responsiveness, except those with two null mutations in trans. Sapropterin responsiveness is commonly determined by obtaining a baseline blood phenylalanine level on the day the medication is initiated and then starting the patient on a single daily dose of sapropterin at 20 mg/kg. Additional blood phenylalanine levels are then obtained at regular intervals, usually at 24 hours, one week, two weeks, and, in some cases, three or four weeks [26].

Responsiveness to sapropterin is determined using an appropriate sapropterin load protocol with an observational period of at least 24 hours (48 hours is better), followed by a one- to four-week trial of sapropterin with subsequent adjustment of sapropterin dose and phenylalanine intake to optimize phenylalanine levels [44].

In children, the suggested starting dose is 10 mg/kg once daily for up to one month. The final dose in children and adults can be adjusted within a range of 5 to 20 mg/kg per day, titrated to blood phenylalanine level.

In a multicenter trial, 89 patients with PKU were randomly assigned to treatment with sapropterin (dose of 10 mg/kg) or placebo for six weeks [45]. Sapropterin therapy was associated with a decrease in mean blood phenylalanine concentration of 3.9 mg/dL (236 micromol/L) compared with a 0.05 mg/dL (3 micromol/L) increase in the placebo group; 44 percent of patients in the treatment group had a reduction of blood phenylalanine concentration from baseline of at least 30 percent compared with 9 percent of controls. Long-term neurologic function in patients with PKU treated with sapropterin has not been assessed [36].

In a follow-up study, 46 patients (aged 4 to 12 years) who responded to sapropterin were randomly assigned (3:1) to sapropterin (dose of 20 mg/kg per day) or placebo for 10 weeks [41]. Subjects continued on a phenylalanine-restricted diet, but, after three weeks, dietary phenylalanine supplements were added or removed every two weeks depending upon phenylalanine control. Phenylalanine tolerance was increased in the sapropterin-treated patients (adjusted mean difference 17.7 mg/kg per day, 95% CI 9.0-27.0 mg/kg per day).

Long-term data on BH4 and sapropterin therapy are limited. In one retrospective study of BH4 or sapropterin treatment with or without dietary restriction in 147 patients, median phenylalanine concentrations were stable and within the desired range in all patients [46]. In addition, the median dietary tolerance for phenylalanine increased. No serious adverse events were reports, and side effects resolved with dose reduction.

In one study, 58 subjects with PKU were given sapropterin 20 mg/kg per day for one month [47]. Responders whose phenylalanine levels decreased at least 15 percent were continued on sapropterin for another year, while nonresponders discontinued the medication. One month of sapropterin supplementation was associated with a significant increase in homovanillic acid, a marker of dopaminergic activity, only in nonresponders. This finding may be explained by the greater availability of sapropterin for tyrosine hydroxylase (TH), a BH4-dependent hydroxylase that catalyzes the synthesis of the dopamine precursor, in nonresponders. After one year of therapy, sapropterin supplementation reduced phenylalanine blood levels and increased phenylalanine dietary tolerance in a subset of patients. However, urinary monoamine metabolite levels showed an inverse relationship with phenylalanine blood levels independent of sapropterin supplementation, suggesting that long-term metabolic control is more important for monoamine metabolism.

Phenylalanine ammonia lyase — PAL is an enzyme derived from the prokaryote Anabaena variabilis (Av) that degrades phenylalanine. A recombinant (r) form conjugated with polyethylene glycol (PEG) to reduce immunogenicity (rAvPAL-PEG, pegvaliase) was approved for adults with PKU by the US FDA following successful clinical trials demonstrating long-term safety and efficacy [48-50]. PRISM-1 and PRISM-2, two phase III clinical trials, evaluated safety and efficacy of pegvaliase following an induction, titration, and maintenance phase. Patients with PKU with blood phenylalanine levels >600 micromol/L were randomly assigned 1:1 to a maintenance dose of 20 mg/day or 40 mg/day of pegvaliase. Within 24 months of treatment, almost 70 percent of patients achieved phenylalanine levels <600 micromol/L, with 60 percent achieving levels <360 micromol/L. Adverse events were more frequent during the first six months of treatment. The most common adverse events included arthralgia, injection-site reaction, and headache. Acute anaphylaxis was rare (12/261 participants) [49]. In a preceding phase I trial, all patients had developed antibodies against PEG by the end of the study (day 42), and less than one-half had developed antibodies to PAL [51].

Other therapies

●Long-chain polyunsaturated fatty acids – Because it is low in animal protein, a phenylalanine-restricted diet results in low blood concentrations of long-chain polyunsaturated fatty acids (LCPUFAs) and docosahexanoic acid (DHA), which may compromise neurodevelopment [52]. In a randomized trial in children with well-controlled HPA, supplementation for 12 months with LCPUFA including DHA increased blood DHA concentration and improved visual function compared with placebo [53]. However, these values returned to baseline after three years [54]. In a nonrandomized trial, supplementation with fish oil (omega-3 LCPUFA) for three months improved the motor skills of children with well-controlled PKU compared with age-matched controls [55].

It is unclear if there is a conditional requirement for dietary DHA to achieve optimal neural and cognitive development. In the absence of dietary recommendations, the provision of 300 mg/100 kcal or 2.7 percent of dietary energy as alpha-linolenic acid, a precursor of DHA, is adequate to prevent essential fatty acid deficiency and supports, in part, the metabolic needs of the body for DHA [56,57].

●Large neutral amino acids – LNAAs (arginine, histidine, isoleucine, leucine, lysine, methionine, threonine, tryptophan, tyrosine, and valine) compete with phenylalanine for the same amino transporter at the blood-brain barrier. Supplementation with LNAAs may therefore significantly reduce the influx of phenylalanine into the brain in patients with PKU [58]. In one study, LNAA supplementation (250 to 500 mg/kg per day) reduced phenylalanine concentrations in plasma due to competitive inhibition of phenylalanine absorption in the small intestine [59]. LNAAs are not recommended for young children or during pregnancy but are an option for adults with PAH deficiency who are not in good metabolic control and do not adhere to other treatment options [60].

Monitoring — Blood concentrations of phenylalanine should be monitored frequently, especially during infancy. The intervals can be extended with increasing age. The National Institutes of Health (NIH) Consensus Development Conference on PKU recommended testing at weekly intervals during the first year, twice monthly from 1 to 12 years of age, and monthly after 12 years of age [1].

The blood phenylalanine concentration associated with optimal neurodevelopmental outcome is uncertain. No consensus exists among treatment centers in the United States or other countries [1]. The NIH Consensus Development Conference on PKU recommended maintaining a blood concentration of 2 to 6 mg/dL (120 to 360 micromol/L) for affected children through 12 years of age and 2 to 15 mg/dL (120 to 900 micromol/L) after 12 years of age [1]. However, although data are limited, higher blood phenylalanine concentrations appear to adversely affect brain function, even in adults [61,62]. Thus, maintenance of lower levels (2 to 10 mg/dL, 120 to 600 micromol/L) is strongly encouraged during adolescence or even beyond. Elevated phenylalanine concentration may be associated with intercurrent illnesses, trauma, high phenylalanine intake, or inadequate intake of amino acid mixture, total energy, and/or protein in relation to metabolic needs.

Tyrosine is an essential amino acid in patients with PKU because of their inability to convert phenylalanine to tyrosine. Consequently, tyrosine concentrations may be low, which may have a negative effect on thyroxine, catecholamine, and melanin synthesis. Patients with PKU need to take particular care that their amino acid mixture is shaken sufficiently before they drink it due to its insolubility [63].

Phenylalanine and tyrosine concentrations and intake of phenylalanine, amino acids, vitamins, minerals, and essential fatty acids (by a three-day record of food intake) should be monitored regularly in patients with PKU, particularly in those individuals who receive insufficient amounts of amino acid mixture.

Outcome — Dietary treatment appears to reverse all signs of PKU except cognitive impairment that has already occurred.

Cognitive outcome — Affected children who are treated by dietary restriction tend to have IQ scores in the average range. However, their IQ scores are, on the average, approximately one-half of a standard deviation lower than unaffected controls. They also score slightly lower than their unaffected parents or siblings [64].

Cognitive outcome appears to be correlated with the extent of control of blood phenylalanine concentration, especially during early childhood. In a review of longitudinal studies of patients with early treatment for PKU, IQ decreased by approximately one-half of a standard deviation for each 300 micromol/L (5 mg/dL) increase in blood phenylalanine concentration [65]. Outcomes were nearly normal when phenylalanine concentration was less than 400 micromol/L (6.7 mg/dL) during early and middle childhood. Another study showed that there was a 1.3 to 3.9 point decline in IQ score for each 100 micromol/L increase in phenylalanine concentration [66].

Behavior problems — Some affected patients have learning disabilities and behavior problems, although evidence is inconsistent. This was examined in a systematic review of four studies that each included more than 20 subjects [67]. Taken together, patients with early-treated PKU appear to have a higher prevalence of behavior problems and psychological disturbance than controls. The range of problems reported included excessive sadness, fear, and anxiety; a sense of isolation and poor self-image; and lack of autonomy and drive. Whether children with PKU are at higher risk for attention deficit hyperactivity disorder is controversial.

Both neurobiologic impairment and the stressful nature of the restrictive diet may contribute to disturbances in behavior [67]. Adequate studies comparing blood phenylalanine levels and behavior disorders are unavailable.

Visual abnormalities — High phenylalanine concentrations appear to cause subclinical visual impairment [68,69]. This was illustrated by a study of pattern reversal visual evoked potentials (VEPs) in patients with PKU [68]. VEPs were abnormal in only one of nine children younger than 14 years old who were on a restricted diet, compared with more than 80 percent of older patients, whether or not they were still on a low-phenylalanine diet.

Resources — The National PKU Alliance and the Cristine M. Trahms Program for Phenylketonuria, among others, provide resources for individuals with PKU, families, caretakers, and professionals who work with children with PKU, including educational material about the PKU diet. A patient self-reporting registry is available through the National PKU Alliance in collaboration with the National Organization for Rare Disorders (NORD).

TETRAHYDROBIOPTERIN (BH4) DEFICIENCY — Tetrahydrobiopterin (BH4) is an essential cofactor for phenylalanine hydroxylase (PAH), tyrosine hydroxylase (TH), and tryptophan hydroxylase [70]. BH4 deficiency can result from decreased regeneration by dihydropteridine reductase (DHPR) and pterin-4-carbinolamine dehydratase (PCD) or from impaired BH4 synthesis. BH4 is synthesized from guanosine triphosphate by guanosine triphosphate cyclohydrolase 1 (GTPCH), 6-pyruvoyl-tetrahydrobiopterin synthase (PTPS), and sepiapterin reductase (SR).

Defects in BH4 metabolism account for approximately 2 percent of patients with elevated phenylalanine levels. However, some variants of these defects, such as GTPCH deficiency, may present with symptoms of neurotransmitter deficiency (eg, dopa-responsive dystonia [DYT5] without hyperphenylalaninemia [HPA]) rather than HPA.

Inheritance — Most disorders of BH4 metabolism are autosomal recessive. Mutations have been detected in the genes encoding DHPR, PCD, GTPCH, PTPS, and SR. One exception to the autosomal recessive mode of inheritance is DYT5 without HPA, which is an autosomal dominant disorder caused by a heterozygous mutation in the gene encoding GTPCH.

Clinical manifestations — Affected patients typically have HPA and/or progressive neurologic deterioration during infancy due to decreased production of the neurotransmitters dopamine, epinephrine, norepinephrine, and serotonin [71]. Untreated patients typically die before reaching one year of age.

Diagnosis of BH4 deficiency — The diagnosis is made by measurement of elevated concentrations of biopterin or neopterin in blood, urine, or cerebrospinal fluid. Neurotransmitter metabolites also are measured in cerebrospinal fluid. Decreased DHPR activity can be demonstrated in red blood cells. BH4 loading test can also be performed [72]. Any newborn infant with elevated phenylalanine concentrations should be tested for BH4 deficiency by measuring DHPR activity in dried blood spots and biopterin/neopterin concentrations in dried urine. These tests should be initiated as soon as possible following the return of a positive newborn test result.

Treatment of BH4 deficiency — BH4 deficiency is treated with a diet low in phenylalanine and supplementation with BH4 (2 to 10 mg/kg per day) [71]. This will lower phenylalanine concentrations but will not improve neurologic status. The neurotransmitter precursors, L-dopa, carbidopa, and serotonin, also are provided [71]. Leucovorin (folinic acid) supplementation is given in DHPR deficiency. It is essential that the treatment is initiated as early as possible in the course of the disease.

PHENYLALANINE EMBRYOPATHY (MATERNAL PKU) — Elevated serum phenylalanine concentration during early pregnancy in a mother with PKU or hyperphenylalaninemia (HPA) with consistent phenylalanine levels of >360 micromol/L can result in phenylalanine embryopathy [73,74]. The risk results from the mother's metabolic control and is independent of whether the fetus has PKU. Females with PKU should be encouraged to receive family planning and preconception services [1,75]. Offspring do not appear to be affected by mild maternal hyperphenylalaninemia (MHPA), blood phenylalanine <360 micromol/L, a condition distinct from PKU.

Pathogenesis — The concentration of phenylalanine is higher in fetal than maternal plasma. In one report, concentrations in fetal blood obtained by cordocentesis were compared with simultaneous maternal venous samples in 14 pregnancies from 19 weeks to term [76]. The overall fetal-maternal concentration ratio was 1.35±0.42 (standard deviation) and decreased gradually over the period of measurement. Because of the concentration gradient, even if maternal phenylalanine levels are in a reasonable range, fetal levels may reach a level that is teratogenic.

Clinical effects — The embryopathic effects of maternal PKU include intrauterine growth restriction, intellectual disability, microcephaly, and cardiac malformations [1,73,74]. The clinical picture is similar to that of alcohol embryopathy.

The risk of abnormalities depends upon the maternal blood phenylalanine concentration and is independent from the fetal genotype (eg, heterozygosity or homozygosity for PKU) [73]. In untreated pregnancies in which the maternal blood phenylalanine concentration was ≥20 mg/dL (1200 micromol/L), microcephaly and intellectual disability occurred in 73 to 92 percent of the offspring, and 12 percent had congenital heart disease [77,78].

Outcome is improved substantially when treatment results in low maternal phenylalanine concentrations ideally before conception or less optimally before 10 weeks gestation [73,74,79-82]. This was illustrated by the Maternal PKU Study, an international collaboration that prospectively evaluated 572 pregnancies in women with PKU and 99 controls [79]. Among the affected mothers who delivered live offspring, metabolic control was achieved before conception or by 10 weeks in 16 and 18 percent, respectively. In these groups, the rates of microcephaly were 3.6 and 5 percent, respectively, substantially lower than rates in untreated pregnancies [77].

The risk of congenital heart disease in offspring depends upon maternal blood phenylalanine concentrations at baseline and during the period of cardiogenesis (4 to 10 weeks gestation). In another report from the Maternal PKU Study cited above, offspring were compared from affected and control pregnancies [80]. Congenital heart disease occurred in 34 of 235 offspring (14 percent) of mothers with a basal phenylalanine level ≥15 mg/dL (≥900 micromol/L) and a persistent level ≥10 mg/dL (≥600 micromol/L) by the eighth week of gestation compared with 1 of 99 controls (1 percent). The rates of coarctation of the aorta (20 percent) and hypoplastic left heart syndrome (11 percent) were greater than would be expected in the general population.

Another report from the Maternal PKU Study examined the effect of microcephaly and congenital heart disease in the offspring on developmental outcome measured by the McCarthy General Cognitive Index at four years of age and the Wechsler Intelligence Scale for Children Revised at six years of age [81]. The overall rates of congenital heart disease and microcephaly were 7.7 and 33 percent, respectively. Infants with both anomalies had higher maternal phenylalanine levels than those with only heart disease. All of the infants with heart disease had maternal phenylalanine levels above 6 mg/dL (360 micromol/L during the first 8 to 10 weeks of gestation). Compared with children without either anomaly, intelligence quotient (IQ) was lower with either microcephaly or congenital heart disease and even lower with both. Decreasing IQ was associated with increasing phenylalanine exposure.

Findings similar to those of the Maternal PKU Study were described in a retrospective report from the United Kingdom PKU Registry between 1978 and 1997 [82]. In this report, infants born to mothers who initiated a phenylalanine-restricted diet before conception had the following outcomes compared with those whose mothers started the diet during pregnancy:

●Greater mean birth weight (3160 versus 2818 grams)

●Greater mean birth head circumference (33.6 versus 32.7 cm)

●Greater mean developmental quotient (DQ) at four years (108.9 versus 96.8)

●Greater mean DQ at eight years (103.4 versus 86.5)

●Lower rates of congenital heart disease (2.4 percent versus 17 percent)

Prevention — Phenylalanine embryopathy can be prevented by dietary restriction of phenylalanine intake in females with PKU or HPA before and during pregnancy. The National Institutes of Health (NIH) Consensus Development statement recommended that plasma phenylalanine levels should be reduced to levels <6 mg/dL (360 micromol/L) at least three months before conception and remain at 2 to 6 mg/dL (120 to 360 micromol/L) during pregnancy [1]. Mothers who conceive while blood levels are greater than recommended should achieve metabolic control as soon as possible. Plasma phenylalanine concentration should be monitored twice weekly or a minimum of once weekly. The amount of phenylalanine tolerated increases during pregnancy. The diet should be adjusted according to plasma levels.

Maternal plasma tyrosine levels should be maintained between 0.9 and 1.8 mg/dL. Tyrosine supplementation may be needed to maintain this range [83].

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Phenylketonuria".)

SUMMARY AND RECOMMENDATIONS

●Forms of phenylketonuria (PKU) – Hepatic phenylalanine hydroxylase (PAH) catalyzes the conversion of phenylalanine to tyrosine (figure 1). PKU is an autosomal recessive disorder. Most cases of PKU are caused by PAH deficiency, which results in elevated blood and urine concentrations of phenylalanine and its metabolites, phenylacetate and phenyllactate. Tyrosine concentration is normal or nearly normal. Approximately 2 percent of patients with hyperphenylalaninemia (HPA) have defects in tetrahydrobiopterin (BH4) metabolism. BH4 is a cofactor required for PAH activity. (See 'Pathophysiology' above.)

•Complete enzyme deficiency results in classic phenylketonuria (PKU; untreated serum phenylalanine concentrations of >1200 micromol/L [20 mg/dL]) and elevated urine phenylketones.

•Residual enzyme activity causes:

-Moderate PKU (phenylalanine concentrations 900 to 1200 micromol/L)

-Mild PKU (600 to 900 micromol/L)

-Mild HPA (360 to 600 micromol/L)

-Benign mild HPA not requiring treatment (120 to 360 micromol/L)

●Clinical features:

•PAH deficiency – Newborn infants are asymptomatic before the initiation of feeds containing phenylalanine (eg, breast milk or standard infant formula). If undetected by newborn screening, PKU onset is insidious and may not cause symptoms until early infancy. In untreated patients, the hallmark of the disease is irreversible intellectual disability, seizures, behavioral abnormalities, microcephaly, and skin disease (eczematous rash, light pigmentation) due to HPA. Other findings include abnormalities of gait, sitting posture, and stance. The body and urine may have a "mousy" odor due to the increased concentration of phenylacetic acid. Without dietary restriction, cognitive impairment worsens during myelination in early childhood with increasing dietary exposure to phenylalanine but stabilizes when brain maturation is complete. (See 'Clinical features' above.)

•Tetrahydrobiopterin (BH4) deficiency – Patients with BH4 deficiency typically have HPA-related manifestations and/or progressive neurologic deterioration during infancy due to decreased production of the neurotransmitters, dopamine, epinephrine, norepinephrine, and serotonin. Untreated patients typically die before reaching one year of age.

●Diagnosis:

•PAH deficiency – Diagnosis of PKU is based upon the finding of an elevated serum concentration of phenylalanine (typically >20 mg/dL [1200 micromol/L] in patients with complete deficiency of PAH). Molecular analysis is used to confirm PKU through identification of PAH mutations and for carrier detection or prenatal diagnosis in families in whom the mutation is known. (See 'Diagnosis' above.)

•Tetrahydrobiopterin (BH4) deficiency – The diagnosis of BH4 deficiency is confirmed by elevated concentrations of biopterin or neopterin in blood, urine, or cerebrospinal fluid, decreased dihydropteridine reductase (DHPR) activity in red blood cells, and/or increased neurotransmitter metabolites in the cerebrospinal fluid. (See 'Diagnosis of BH4 deficiency' above.)

●Management:

•Dietary restriction – The mainstay of therapy in PKU remains dietary restriction of phenylalanine, although enzyme therapy using pegvaliase is approved for adult patients with PKU. Dietary therapy requires the use of medical foods including phenylalanine-free protein substitutes. Dietary restriction should be initiated as soon as possible in infants with PKU and blood phenylalanine concentration >7 to 10 mg/dL (420 to 600 micromol/L). BH4 deficiency should be excluded before initiation of dietary restriction through analysis of DHPR activity in dried blood spots and biopterin/neopterin concentrations in dried urine. Continuation of dietary restriction throughout life appears to be necessary for optimal outcomes. (See 'Dietary restriction' above.)

•Pharmacotherapy – The primary pharmacotherapy available for the treatment of mild-to-moderate PKU is a synthetic formulation of BH4 called sapropterin. BH4 is the cofactor for PAH. Pharmacologic doses of sapropterin are an alternative or adjunct to dietary restriction in patients with HPA or mild-to-moderate PKU phenotypes who demonstrate responsiveness through reduced phenylalanine levels. (See 'Pharmacotherapy' above.)

For patients with BH4 deficiency, the neurotransmitter precursors L-dopa, carbidopa, and serotonin are also provided. (See 'Treatment of BH4 deficiency' above.)

●Monitoring – Blood concentrations of phenylalanine and tyrosine should be monitored frequently, especially during infancy. The intervals can be extended with increasing age. Guidelines recommend testing at weekly intervals during the first year, twice monthly from 1 to 12 years of age, and monthly after 12 years of age. (See 'Monitoring' above.)

●Outcome – Dietary treatment appears to reverse all signs of PKU except cognitive impairment that has already occurred. (See 'Outcome' above.)

●Maternal PKU – Elevated serum phenylalanine concentration during early pregnancy in a mother with PKU or HPA can result in phenylalanine embryopathy (irrespective of whether the fetus has PKU). The embryopathic effects include intrauterine growth restriction, intellectual disability, microcephaly, and cardiac malformations. Phenylalanine embryopathy can be prevented by dietary restriction of phenylalanine in females with PKU or HPA before and during pregnancy. (See 'Phenylalanine embryopathy (maternal PKU)' above.)