Bernd Christian Schwahn
Hyperphenylalaninemias are an important group of metabolic disorders that present mainly as chronic encephalopathy. Severe hyperphenylalaninemia leading to phenylketonuria (PKU) has a very distinct role in the field of inherited metabolic disorders: PKU is the first genetic disease that could be treated exclusively by dietary manipulation and that could be entirely prevented by universal newborn screening and presymptomatic dietary intervention. This has had a huge impact on pediatric medicine, on the evolution of neonatal mass screening, and on the concept of gene-environment interaction. Genetic defects associated with hyperphenylalaninemia can now be regarded as a strong risk factor for neurodisability, but their outcome is more determined by the degree of metabolic control than by genetic variability.
There is a continuous clinical spectrum of severity that ranges from malformation and mental retardation to asymptomatic mild hyperphenylalaninemia. Symptoms depend on the extent and ontogenetic timing of an organism’s exposure to elevated phenylalanine concentrations. The phenylalanine pool is a function of dietary phenylalanine intake and residual capacity for catabolism.
Intrauterine exposure of an unborn child to elevated phenylalanine concentrations due to maternal hyperphenylalaninemia can disrupt embryo-fetal development. This syndrome, called maternal phenylketonuria (mPKU), has been consistently observed with maternal hyperphenylalaninemia above 20 mg/dL (1200 μmol/L) and includes intrauterine dystrophy; facial dysmorphism resembling fetal alcohol syndrome; microcephaly and mental retardation; and malformations, especially of the heart and great vessels.1,2 The risk for mPKU increases when maternal plasma phenylala-nine concentrations rise above 10 mg/dL (600 μmol/L). The relatively low threshold for embryo-fetal toxicity can be explained by an increased vulnerability of the unborn child and an at least one-and-a-half-fold materno-fetal transplacental concentrative gradient.3
In contrast, children who suffer from severe postnatal hyperphenylalaninemia do not show any symptoms at birth: fetal phenylalanine accumulation is effectively prevented by transplacental clearance. Affected children may become lethargic or appear irritable and have feeding difficulties during the first weeks of life, but this does not usually prompt evaluation and diagnosis. In early infancy, they can develop a peculiar mousy smell due to the excretion of phenylacetic acid, and approximately one third will develop an eczematoid rash or infantile spasms.4,5 A clinical diagnosis of phenylketonuria (PKU) is usually only made in the second half of the first year of life or later, after seizures or delayed psychomotor development lead to further biochemical investigation. At this time, affected infants appear less dark pigmented than their unaffected siblings and present with microcephaly due to decreased brain growth, which is reflected by cortical atrophy on brain imaging. They majority have behavioral disturbances such as restlessness, anxiety, aggression, repetitive behavior, and sleep disturbance.
Approximately 90% of individuals with untreated PKU will have severe mental disability, with intelligence quotients (IQ) under 30 on psychometric assessment.6 Up to 10% of untreated individuals with severe hyperphenylalaninemia escape the phenylketonuria phenotype. It has been hypothesized that their brain may have been protected from high phenylalanine concentrations by alterations of amino acid transport across the blood-brain barrier.
Discontinuation of early dietary treatment before the age of 8 is associated with poorer performance on IQ measures and can lead to behavioral disturbances and mood disorders.7 Exposure to high phenylalanine concentrations beyond puberty causes decreased performance on measures of attention and processing speed but no deterioration of IQ. Many affected adults experience a lack of energy and concentration, mood swings, and tiredness when off diet. Untreated PKU does not seem to shorten the normal life span, apart from complications associated with severe neurodisability.
METABOLIC DERANGEMENT, PATHOPHYSIOLOGY
Hyperphenylalaninemia is caused by impaired hydroxylation of phenylalanine to tyrosine (Fig. 135-1). The enzyme phenylalanine-4-hydroxylase (PAH) is predominantly expressed in liver tissue and needs tetrahydrobiopterin (BH4) as a cofactor. Lack of PAH activity leads to accumulation of phenylalanine, with levels exceeding 20 mg/dL (1200 μmol/L); excretion of its metabolites phenylacetate and phenylpyruvate; and decreased availability of the product tyrosine, which is necessary for synthesis of protein, neurotransmitters, and melatonin.
The damage to the brain is believed to result from direct toxicity of phenylalanine and depletion of tyrosine, tryptophan, and other large neutral amino acids that compete with phenylalanine for uptake into the brain compartment.
Individuals with moderate hyperphenylalaninemia between 10 and 20 mg/dL when untreated (mild PKU) have residual activities of 1% to 5%, and an activity of over 5% may require no or only minor dietary modification.5
Phenylalanine hydroxylase deficiency (Online Mendelian Inheritance in Man [OMIM] No. 261600) is one of the most common inherited metabolic disorders, affecting approximately 1 in 15,000 people in the United States. It has a higher incidence in whites and Native Americans and a lower incidence in blacks, Hispanics, and Asians. In North America, about 75% of individuals with PAH deficiency identified by newborn screening have the severe form and require treatment.8 All forms are transmitted in an autosomal recessive manner. The gene for phenylalanine hydroxylase has been mapped to chromosome 12q24,1and more than 500 mutations have been identified.9-11 Most individuals are compound heterozygotes, and certain PAH alleles are associated with PKU and others with non-PKU hyperphenylalaninemia. However, the relationship between the clinical phenotype and the genotype is not always constant.12 Mutation analysis and genotype determination may be helpful for genetic counseling and allows for antenatal diagnosis in further pregnancies, if required.
FIGURE 135-1. Pathway of phenylalanine and tyrosine degradation. Phenylalanine from dietary protein or endogenous proteolysis is hydroxylated by phenylalanine hydroxylase (PAH) using tetrahydrobiopterin (BH4) as a cofactor. BH4 is thereby oxidized in two steps to dihydrobiopterin (qBH2). BH2 needs to be reduced back to BH4 by dihydropterin reductase (DHPR). In severe PAH deficiency, phenylalanine accumulates and is deaminated to phenylpyruvic acid, which can be further metabolized. Phenylalanine and its alternative metabolites can then be found in urine. Tyrosine stems from dietary protein or endogenous proteolysis or is synthesized from phenylala-nine. The first and rate-limiting step is its deamination to 4-OH-phenylpyruvic acid by the enzyme tyrosine aminotransferase (TAT). This step is reversible. Next, 4-OH phenylpyruvic acid is oxidized via 4-OH phenylpyruvic dioxygenase (HPD) to homogentisic acid. Deficiency of TAT and deficiency or inhibition of HPD lead to accumulation of tyrosine. Homogentisic acid is further oxidized by homogentisate dioxygenase (HGD) to maleylacetoacetic acid (MAA) and fumarylacetoacetic acid (FAA). Fumarylacetoacetate hydrolase (FAH) cleaves FAA to fumaric acid and acetoacetic acid. In FAH deficiency, MAA and FAA accumulate and are converted to succinylacetoacetate, which is decarboxylated to succinylacetone. Succinylacetone inhibits HPD, causing hypertyrosinemia, and porphobilinogen synthase, causing symptoms of acute intermittent porphyria. In addition, 2(2-nitro-4-trifluoromethylbenzoyl)-1,3-cyclohexanedione (NTBC) is a strong inhibitor of HPD and is used to avoid accumulation of MAA and FAA in FAH deficiency.
DIAGNOSTIC TESTS, DIFFERENTIAL DIAGNOSES
Hyperphenylalaninemia is usually identified through routine newborn screening programs between the second and seventh day of life. It is not necessary to wait until milk feeding has started. The endogenous protein catabolism after birth will lead to increased phenylalanine concentrations above 2 to 2.5 mg/dL (120 to 150 μmol/L) and a ratio of phenylalanine over tyrosine above 3 in affected children.8 Diagnostic evaluation of hyperalaninemia in the newborn infant is shown in Fig. 135-2. The screening threshold varies depending on the time of testing. In healthy-term newborns, the screening for PKU using phenylalanine concentrations in dried blood spots is fairly specific, whereas it will often be false positive in premature or sick babies. Abnormal screening results need to be confirmed by quantitative analysis of plasma amino acids to exclude a secondary increase of phenylalanine due to liver dysfunction (eg, caused by immaturity, sepsis, hypoxic multiorgan failure, or galactosemia, where tyrosine and methionine are elevated as well). In babies with PKU, phenylalanine concentrations at screening depend on the timing but are usually in a range of 400 to 900 μmol/L. Due to a rapid postnatal rise, they often are at 1200 to 2000 μmol/L in confirmatory samples, and tyrosine levels are within the normal range.
One to 2% of newborns who screened positive have hyperphenylalaninemia secondary to a deficiency of the PAH cofactor tetrahydrobiopterin (BH4) due to a genetic defect in BH4 synthesis (PTPS, PCD, or GCPDH deficiency) or recycling (DHPR deficiency) (see Table 147-1). These individuals are not clinically distinguishable from those with primary hyperphenylalaninemia but need a completely different treatment. Therefore, every newborn with persistent hyperphenylalaninemia must be tested for total biopterin concentration in blood or urine and for DHPR activity in a dried blood spot. Several countries use a BH4 loading test with a single dose of 20 mg BH4 per kg body weight and monitor phenylalanine concentrations for 24 hours to determine whether a cofactor deficiency is present. Km variants of PAH may also respond to BH4 supplementation.13,14
TREATMENT, PROGNOSIS, AND LONG-TERM OUTCOME
Clinical manifestations of hyperphenylalaninemia can be completely prevented by avoiding excessive phenylalanine accumulation.5 This can be achieved with appropriate dietary restriction of phenylalanine intake and substitution of other amino acids and nutrients. Treatment is periodically monitored using phenylalanine concentrations in plasma or full blood as surrogate marker for phenylalanine accumulation within the body. Target concentrations differ slightly between different countries, but it is generally accepted that during the most vulnerable period of life (the first 5 years), plasma phenylalanine should not fall below one- to twofold the normal mean (1 mg/dL or 60 μmol/L, standard deviation 0.25 mg/dL or 15 μmol/L) and should not exceed four- to sixfold normal (4 to 6 mg/dL or 240 to 360 μmol/L). Most commonly reported blood phenylalanine recommendations in U.S. clinics are 2 to 6 mg/dL for patients until 12 years of age and 2 to 10 mg/dL for those over 12 years.8,15,16
There is much less agreement on treatment targets for older children and adults. It is now recommended to continue diet for life, although treatment targets are usually less ambitious in adolescents and adults and depend on personal objectives and individual susceptibility to adverse effects toward high phenylalanine concentrations. A typical target for adults is to keep plasma phenylalanine below 12 mg/dL or 700 μmol/L.
FIGURE 135-2. Algorithm: Diagnostic evaluation of hyperphenylalaninemia in newborns. qBH2, dihydrobiopterin; BH4, tetrahydrobiopterin; DBS, dried blood spot; DHPR, dihydropterin reductase; LFT, liver function tests; Phe, phenylalanine; Tyr, tyrosine.
Other goals for successful treatment are to avoid malnutrition (especially protein malnutrition due to over-restrictive diet or insufficient phenylalanine-free protein substitution and deficiency of micronutrients such as calcium, phosphate, vitamin B12, long-chain polyunsaturated fatty acids, and selenium) and to promote normal psychosocial development despite the highly artificial and controlled diet.
To meet these targets, it is necessary to frequently monitor phenylalanine levels and to closely monitor growth and psychomotor development and nutritional parameters in blood. Expert dietetic input is important to appropriately advise parents, caregivers, and teachers about the treatment, and early psychosocial intervention should be offered to families who struggle to adhere to dietary recommendations. PKU treatment is done at home and can only be successful with appropriate support from family or caregivers. It requires a multidisciplinary expert team, which is available in regional metabolic centers.
Nondietary Treatment Options
The orally administered plant enzyme phenylalanine ammonia lyase can assist in reducing the dietary phenylalanine pool and therefore increase the dietary allowance of natural protein.18 This principle has not yet been introduced into human therapy.19 A promising new approach is the supplementation with BH4, which improves PAH activity by acting as a chemical chaperone to stabilize the defective enzyme and to protect it against premature proteasomal degradation. BH4 supplementation of 5 to 20 mg/kg body weight per day can improve phenylalanine tolerance in up to 50% of treated individuals with missense mutations affecting the catalytic, regulatory, oligomerization, and BH4-binding domains, and especially in those with moderate hyperphenylalaninemia.17,20