Clinical manifestations

Presentation and course

Untreated phenylketonuria produces the gradual onset of profound developmental disability in most, but not all, individuals (35; 39). Intelligence testing reveals that 50% to 70% of individuals with untreated phenylketonuria have IQs below 35, and 88% to 90% below 65, but 2% to 5% of untreated children have intelligence in the normal range. Other clinical features of untreated children include fair pigmentation for ethnicity, eczema, a musty odor, microcephaly, autistic behaviors, and abnormal hand movements. Neurologic features include seizures (26%), spasticity, hyperreflexia, and tremors (35).

Diagnosis of all forms of hyperphenylalaninemia is based on finding elevated whole blood phenylalanine concentrations by newborn screening, with confirmation of hyperphenylalaninemia by quantitative analysis of plasma amino acids. Any children identified with hyperphenylalaninemia should then be tested for a biopterin pathway enzyme deficiency by measuring urinary pterin concentration ratios (biopterin and neopterin) and by measuring enzymes involved in biopterin metabolism in blood.

Thus, it is important that children with seizures born in areas that do not perform newborn screening be assessed for phenylalanine hydroxylase deficiency. The current newborn screening programs are very effective at identifying phenylalanine hydroxylase deficiency, but as with any screening program, clinical suspicion of this disorder should be high for children with a phenotype consistent with untreated disorder. Individuals usually have good outcomes if they are identified early and treated (42). Individuals with phenylketonuria may still have some long-term impairments in some executive functioning (44) and can develop some neurologic symptoms (32).

Prognosis and complications

Prognosis in untreated phenylketonuria is poor, although 2% to 5% of untreated children will have normal intelligence. Even with treatment, children and adults can have mild neurologic issues, such as attention and mental health concerns.

Clinical vignette

A 3.3 kg male infant was born at term. Newborn metabolic screening by tandem mass spectrometry performed at age 24 hours revealed a whole blood phenylalanine concentration of 460 µmol/L (normal is less than190 µmol/L). The ratio of phenylalanine concentration to tyrosine (the product of the phenylalanine hydroxylase reaction) concentration was elevated at 4.1 (normal is less than 3.0), indicating a high likelihood of phenylalanine hydroxylase-deficient hyperphenylalaninemia. Quantitative plasma amino acid analysis at 7 days of age demonstrated a plasma phenylalanine concentration of 1250 µmol/L. Biopterin studies were normal. These results confirmed that the infant had hyperphenylalaninemia due to phenylalanine hydroxylase deficiency and he was classified as having classical phenylketonuria. Dietary therapy based on restriction of dietary phenylalanine intake was initiated.

Some clinicians also perform sequencing of the phenylalanine hydroxylase gene to confirm the diagnosis and to attempt to predict the severity of phenylalanine intolerance and predict response to sapropterin. However, genotype-phenotype correlations are limited.

Biological basis

Etiology and pathogenesis

Phenylketonuria is due to an autosomal recessively inherited deficiency of phenylalanine hydroxylase (located on chromosome 12q). More than 500 different mutations have been reported. Most individuals with phenylketonuria are compound heterozygotes, which provides for great variability in phenotypes.

The enzyme is a homotetramer, and interactions of different mutant gene products differentially affect the final conformation of the enzyme resulting in variability of residual enzyme activity.

Phenylalanine hydroxylase deficiency produces elevated phenylalanine concentrations in plasma, CSF, and urine as a direct result of the inability to convert phenylalanine to tyrosine. Tyrosine concentrations are low relative to the elevated phenylalanine concentrations, but are often within the normal range because of sufficient dietary tyrosine intake. When hydroxylation of phenylalanine is blocked, there is increased transamination of phenylalanine to phenylpyruvic acid, which is readily excreted in urine. Numerous other minor metabolites, such as phenylacetic acid, phenyllactic acid, and phenylethylamine, collectively known as phenylketones, are also excreted.

Neurologic outcomes in untreated patients. The nervous system is the organ primarily affected in untreated phenylketonuria, with developmental delay as its phenotype. Mutation analysis (genotyping) may help predict dietary phenylalanine tolerances, but plasma phenylalanine concentrations (biochemical phenotype) may not predict the presence or degree of developmental delay (clinical phenotype), if untreated. Historically, 2% to 5% of untreated children exhibit normal IQ (35; 39).

It is hypothesized that more than blood phenylalanine concentrations are involved in producing the developmental delay. A saturable carrier, the system L transporter, mediates facilitated diffusion of neutral amino acids, including phenylalanine, tyrosine, and tryptophan, across the blood-brain barrier. In vivo magnetic spectroscopy reveals that the correlation of blood and brain phenylalanine concentrations varies considerably among individuals (55). Thus, significant differences in blood-brain barrier transport may underlie the individual vulnerability to the neurologic effects of phenylketonuria. Further data are needed before spectroscopic measurement of brain phenylalanine concentrations can be used clinically.

At normal physiologic concentrations of neutral amino acids in plasma, the system L neutral amino acid carrier is already nearly saturated and susceptible to competitive inhibition (15). The high concentrations of plasma phenylalanine in phenylketonuria inhibit the uptake of other neutral amino acids, such as tyrosine and tryptophan (34), and oral administration of large neutral amino acids, other than phenylalanine, to individuals with phenylketonuria can inhibit phenylalanine uptake into the brain (37). The characteristics of the neutral amino acid transporter are being utilized as a new treatment for phenylketonuria. Thus, biochemical effects of elevated phenylalanine concentrations in the brain are multiple and include direct inhibition of metabolic processes, generalized deficiency of neutral amino acids in brain leading to impaired protein synthesis, and specific tyrosine and tryptophan deficiencies leading to deficiencies of the neurotransmitters dopamine and serotonin. However, the precise molecular mechanism underlying the neuronal damage associated with hyperphenylalaninemia is not yet proven.

White matter disease is common in untreated phenylketonuria. Poser and van Bogaert were the first to recognize that these lesions are due to demyelination (41). In addition to demyelination, sometimes so severe as to be characterized as status spongiosis, there is gliosis. In gray matter of subjects with untreated phenylketonuria, there is neuronal loss, reduced size of neurons, and decreased dendritic processes on Purkinje cells (24).

Neurologic outcomes in treated patients. White matter changes may also be seen in individuals with phenylketonuria treated from birth. MRIs of the brain in treated children reveal increased signal on T2 images with decreased signal on T1 images, most prominently in parieto-occipital and periventricular white matter, with severe cases also involving more frontal areas (09). Current theories regarding the etiology of these MRI findings have been reviewed (01).

White matter abnormalities do not correlate with IQ, but do correlate with dietary control and plasma phenylalanine concentrations at time of imaging, and the changes may be reversible with improved dietary control (09; 26). Based on imaging characteristics, the white matter changes are suggestive of accumulation of a hydrophilic metabolite within cells without alteration of myelin structure (26). Diffusion tensor imaging (DTI) of 29 individuals with early-treated PKU at ages 6 to 35 years revealed widespread decreased white matter mean diffusivity in comparison to that of non-PKU controls; lower diffusivity was associated with higher blood phenylalanine concentrations and impaired executive function by formal assessments (02). These results suggest that white matter axonal integrity is maintained, but elevated phenylalanine concentrations are associated with impaired water diffusion in white matter.

Neurophysiologic evidence of abnormal brain function is also present. Maturation of EEG patterns is delayed in infants with phenylketonuria (11). Although development of EEG alpha activity is normal, slowing of background rhythms and increased paroxysmal activity is seen in some individuals and the incidence increases with age (38). Prolongation of sensory nerve conduction velocities and somatosensory evoked potentials, as well as prolonged latencies of visual and auditory evoked potentials, is reported in a significant number of adolescents with treated phenylketonuria (30). Supplementation with fish oil improved visual evoked potentials in 36 children with phenylketonuria (04). Early-treated phenylketonuria in adults frequently has nonprogressive brain white matter changes and, less consistently, cortical and subcortical grey matter alterations, which are associated with their degree of metabolic control (and age) (10).

Epidemiology

Phenylketonuria is one of the most common inborn errors of metabolism, affecting 1 in 10,000 live births in Caucasians (42). The incidence frequency varies from 1 in 2600 live births in Turkey to 1 in 119,000 in Japan, with carrier frequencies of 1 in 34 in Turkey to 1 in 173 in Japan. The average carrier frequency in Caucasians is 1 in 50. Biopterin-deficient individuals are rare and account for approximately 1% of individuals with hyperphenylalaninemia, with a higher percentage in some populations.

Prevention

Genetic disease can be prevented by prenatal diagnosis for at-risk couples (previously affected child or in a population with a high carrier rate). Prenatal diagnosis is available for phenylketonuria and biopterin-deficient hyperphenylalaninemia (57). Generally, prevention is aimed at the phenotypic manifestations of the disease: treatment of an affected child to prevent developmental disabilities. Neonatal screening for phenylketonuria is performed in most developed countries so that preventive early treatment can be initiated.

Early treatment of phenylketonuria has resulted in the prevention of birth defects in children born to women with phenylketonuria. Women with phenylketonuria who are not being treated or have poor metabolic control of their phenylalanine concentrations during pregnancy have a significant risk of giving birth to children with microcephaly, intrauterine growth retardation, congenital heart defects, and severe developmental delay. This risk can be avoided by good metabolic control throughout pregnancy, including from the time of conception (40).

Differential diagnosis

Confusing conditions

Phenylalanine hydroxylase deficiency (phenylketonuria) is diagnosed by finding elevated concentrations of whole blood or plasma phenylalanine and/or elevated concentrations of urinary phenylalanine metabolites.

The primary differential diagnosis is between hyperphenylalaninemia caused by phenylalanine hydroxylase deficiency and that caused by biopterin-metabolism enzyme deficiencies. Although both exhibit hyperphenylalaninemia, the latter disorders also exhibit neurotransmitter deficiencies that must be specifically treated.

Associated or underlying disorders

Diagnostic workup

Hyperphenylalaninemia is usually diagnosed by newborn screening. Further diagnosis requires amino acid quantitation to confirm an elevated plasma phenylalanine concentration. In addition, studies of biopterin metabolism should be performed to exclude biopterin-deficient hyperphenylalaninemia. Phenylalanine hydroxylase activity is never assayed because it is only expressed in liver and the diagnosis can be made with certainty in the absence of a liver biopsy. Phenylalanine hydroxylase gene sequencing can be used for diagnostic confirmation and possibly for supplying prognostic information.

Management

Therapy for all of the disorders with elevated phenylalanine concentrations requires a specialized clinic setting with dieticians, social services, psychologists, and metabolic specialists. The management team must be aware of the need for education of individuals with phenylketonuria and their families, of the psychological impact of a chronic disease, and of the financial constraints of expensive, but essential, treatments that may not be fully covered by health insurance.

Dietary treatments. The classic treatment of phenylketonuria is dietary restriction of phenylalanine to a degree that maintains plasma phenylalanine concentrations between 120 to 360 µmol/L yet provides for normal growth. Published recommendations have reiterated the need for lifelong, repetitive monitoring and strict control of blood phenylalanine concentration in order to prevent long-term neurologic complications and allow optimal neurobehavioral function (45; 50). This requires a very restrictive diet and can only be accomplished with the use of a synthetic phenylalanine-deficient formula. Breastfeeding in infancy is possible, but it must be significantly limited. Older versions of synthetic formulas are often poorly tolerated because of taste. This feature and the complexity of the diet lead to high rates of nonadherence to the prescribed restrictions, reaching as high as 75% of adolescents and adults with phenylketonuria in many centers (53). The majority of phenylalanine-deficient formulas and food are supplemented with free amino acids; these are often bitter tasting and may be metabolized less efficiently than whole proteins. Glycomacropeptide (GMP) is an abundant, naturally occurring protein in whey that lacks phenylalanine in its primary amino acid sequence; foodstuffs made with GMP are more palatable than traditional phenylalanine-free foods (28), and their use may be associated with improved bone health and metabolic stress, at least in a murine model of phenylketonuria, in comparison to a diet based on free amino acids (47; 48). The Genetic and Metabolic Dietitians International (GMDI) has published a set of guidelines of dietary recommendations that update in real time (33).

Treatment. The American College of Medical Genetics and Genomics guidelines recommend that every patient with phenylalanine hydroxylase deficiency undergo a trial with sapropterin dihydrochloride (Kuvan™ by BioMarin of Novato, California), a synthetic form of tetrahydrobiopterin (50). These recommendations are based on extensive studies showing the benefit of this treatment on phenylamine concentrations (18). Plasma phenylalanine concentrations decrease to below 600 µmol/L without dietary protein restriction in approximately 10% of adults taking sapropterin (10 mg/kg per day) (08). The percentage of “‘sapropterin-responsive”’ individuals may approach 20% if a higher dose, 20 mg/kg per day, is administered. Long-term efficacy of sapropterin therapy has been demonstrated in randomized, placebo-controlled, crossover trials in adults (27) and children (49) with phenylketonuria. In the latter trial, plasma phenylalanine concentrations decreased by at least 30% in approximately half of children with phenylketonuria although most still required some degree of dietary protein restriction and continued use of phenylalanine-free medical foods. Individuals with ‘milder’ degrees of phenylketonuria who naturally tolerate somewhat greater amounts of dietary protein are more likely to respond to sapropterin treatment, but surprisingly even a few individuals with very severe phenylketonuria exhibit lower plasma phenylalanine concentrations while on sapropterin therapy. Unfortunately, the phenylalanine hydroxylase genotype of the individual patient does not predict sapropterin responsiveness completely (27), and subjects’ response to sapropterin treatment can only be adequately assessed by measuring plasma phenylalanine concentrations repeatedly during a trial of daily sapropterin therapy lasting at least four weeks. Sapropterin is also approved for use during pregnancy in women who are responsive, but experience with this is still limited (23).

Another recommended therapeutic approach is to supply orally all of the large neutral amino acids, except phenylalanine, in order to compete with phenylalanine for transport into the brain, which is mediated by the neutral amino acid carrier at the blood-brain barrier (50). This therapy lowers brain phenylalanine concentrations and leads to improvements in EEG patterns of adults with hyperphenylalaninemia (37). With the addition of arginine and lysine to the neutral amino acid mix, some competition with phenylalanine uptake in the intestinal lumen may also be achieved resulting in decreased plasma phenylalanine concentrations (31). This therapy is not sufficiently robust to be routinely used in young children and is contraindicated in women during pregnancy because it does not block the systemic phenylalanine concentrations. Thus, the primary cohort for this treatment includes adolescents and adults who are unable to adhere to the protein-restricted diet.

In 2018, the FDA approved a phenylalanine ammonia lyase-polyethylene glycol (Pegvaliase ®) that is administered by subcutaneous injection for adults with phenylketonuria. This treatment underwent multiple trials and requires provider and dietitian management to optimize patient outcomes (29). Close management with a physician and dietitian is also needed because this medication has a high risk for anaphylaxis.

Currently, phase 2 clinical trials using PKU gene therapy are recruiting patients. More information can be accessed at the following website:Clinicaltrials.gov.

Outcomes

In treated individuals with phenylketonuria, prognosis is usually good, with normal intelligence. Individuals with phenylketonuria who maintain strict adherence to the diet have the best outcomes; most of those who discontinue the diet will exhibit a decline in IQ scores (19; 43). The severity of decrease in IQ score correlates with the duration off of the diet and the age when the diet was discontinued, with the poorest performance occurring in children who discontinued the diet before six years of age.

However, even children treated early and who successfully maintain their plasma phenylalanine concentrations within the targeted treatment range will likely exhibit mild executive impairment (13; 25). This topic has been the subject of intense research by multiple investigators seeking to define the cause(s) and to find a remedy for these issues; these ongoing efforts have been summarized in a series of review articles (56). Seemingly minor fluctuations in plasma phenylalanine concentrations in well-controlled individuals can interfere with neuropsychological function; younger children are especially sensitive to this effect (20). Even mild elevations of plasma phenylalanine concentrations chronically above the recommended treatment range may have detrimental effects on cognition (12). Some functions (cognitive flexibility, attention, and complex problem solving) have a strong negative correlation with elevated plasma phenylalanine concentrations at the time of testing (43). One theory is that there is dysfunction of prefrontal cortex, possibly due to dopamine depletion (13).

When assessed by their teachers, children with phenylketonuria are more likely to be rated as having more deviant behavior (mannerisms, anxiety, hyperactivity, and less social) than unaffected individuals, but are not rated as more aggressive or disobedient (46). Children with phenylketonuria are more likely to be treated with stimulant medications than unaffected individuals (03). Adolescents and adults with phenylketonuria show more depression, dependency, low frustration tolerance, and low self-esteem, and are prone to social withdrawal (54; 52). The incidence of these abnormalities correlates with duration of poor dietary compliance, but the symptoms often respond to renewed dietary phenylalanine restriction (52). Formal screening for psychiatric symptoms in adults with PKU reveals a high and previously underappreciated incidence of psychosis, paranoid ideation, obsessive-compulsive disorder, depression, and anxiety; both acute and chronic elevations of blood phenylalanine concentration are correlated with the occurrence of psychiatric symptoms (06). Individuals diagnosed and treated late (older than 3 months old), treated adults who have had difficulty with diet adherence, and even untreated adults with cognitive abnormalities who were born prior to the advent of newborn screening may significantly benefit from the favorable behavioral effects of a low-phenylalanine diet (16). It is clear that, even in well-controlled individuals, there are neuropsychological effects of elevated brain phenylalanine concentrations.

Neurologic deterioration, such as spasticity, tremor, and cognitive decline, has been reported in some affected adults (32). Minor neurologic abnormalities, such as tremors and fine motor difficulties, can be seen in fairly well controlled individuals with phenylketonuria; however, significant neurologic deterioration has only been reported in those with probable late treatment or with IQs of less than 80. This has not been seen in individuals treated early, except due to a concomitant vitamin B12 deficiency (07).

Special considerations

Pregnancy

There does not appear to be substantial risk to women with phenylketonuria who are pregnant. However, the fetus is at substantial risk. All babies born to mothers with phenylketonuria will be carriers for phenylketonuria; a few might have phenylketonuria. The real risk, however, is the exposure of the fetus to the elevated maternal phenylalanine levels.

The first report of fetal effects of maternal phenylketonuria was in 1957. Babies born to women with phenylketonuria that are untreated throughout pregnancy have significantly increased risks of exhibiting microcephaly, developmental disability, growth retardation, and congenital heart disease (40). These effects are correlated with maternal plasma phenylalanine concentration and can be avoided with successful dietary therapy preconception or early in pregnancy. However, treatment even late in pregnancy may improve intellectual outcome when compared to untreated pregnancies (51).

Anesthesia

No published information is available, but there is typically no restriction on the use of general anesthesia in individuals with phenylalanine hydroxylase deficiency.