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Phenylketonuria (PKU)

By Lee M. Sanders, MD, MPH, Stanford University

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Phenylketonuria (PKU) is a disorder of amino acid metabolism that causes a clinical syndrome of intellectual disability with cognitive and behavioral abnormalities caused by elevated serum phenylalanine. The primary cause is deficient phenylalanine hydroxylase activity. Diagnosis is by detecting high phenylalanine levels and normal or low tyrosine levels. Treatment is lifelong dietary phenylalanine restriction. Prognosis is excellent with treatment.

Phenylketonuria is most common among all white populations and relatively less common among Ashkenazi Jews, Chinese, and blacks. Inheritance is autosomal recessive; incidence is about 1/10,000 births among whites.

Pathophysiology

Excess dietary phenylalanine (ie, that not used for protein synthesis) is normally converted to tyrosine by phenylalanine hydroxylase; tetrahydrobiopterin (BH4) is an essential cofactor for this reaction. When one of several gene mutations results in deficiency or absence of phenylalanine hydroxylase, dietary phenylalanine accumulates; the brain is the main organ affected, possibly due to disturbance of myelination.

Some of the excess phenylalanine is metabolized to phenylketones, which are excreted in the urine, giving rise to the term phenylketonuria. The degree of enzyme deficiency, and hence severity of hyperphenylalaninemia, varies among patients depending on the specific mutation.

Variant forms

Although nearly all cases (98 to 99%) of PKU result from phenylalanine hydroxylase deficiency, phenylalanine can also accumulate if BH4 is not synthesized because of deficiencies of dihydrobiopterin synthase or not regenerated because of deficiencies of dihydropteridine reductase. Additionally, because BH4 is also a cofactor for tyrosine hydroxylase, which is involved in the synthesis of dopamine and serotonin, BH4 deficiency alters synthesis of neurotransmitters, causing neurologic symptoms independently of phenylalanine accumulation.

Symptoms and Signs

Most children with phenylketonuria are normal at birth but develop symptoms and signs slowly over several months as phenylalanine accumulates. The hallmark of untreated PKU is severe intellectual disability. Children also manifest extreme hyperactivity, gait disturbance, and psychoses and often exhibit an unpleasant, mousy body odor caused by phenylacetic acid (a breakdown product of phenylalanine) in urine and sweat. Children also tend to have a lighter skin, hair, and eye color than unaffected family members, and some may develop a rash similar to infantile eczema.

Diagnosis

  • Routine neonatal screening

  • Phenylalanine levels

In the US and many developed countries, all neonates are screened for phenylketonuria 24 to 48 h after birth with one of several blood tests; abnormal results are confirmed by directly measuring phenylalanine levels. In classic PKU, neonates often have phenylalanine levels > 20 mg/dL (1.2 mM/L). Those with partial deficiencies typically have levels < 8 to 10 mg/dL while on a normal diet (levels > 6 mg/dL require treatment); distinction from classic PKU requires a liver phenylalanine hydroxylase activity assay showing activity between 5% and 15% of normal or a mutation analysis identifying mild mutations in the gene.

BH4 deficiency is distinguished from other forms of PKU by elevated concentrations of biopterin or neopterin in urine, blood, CSF, or all 3; recognition is important, and the urine biopterin profile should be determined routinely at initial diagnosis because standard PKU treatment does not prevent neurologic damage.

Children in families with a positive family history can be diagnosed prenatally by using direct mutation studies after chorionic villus sampling or amniocentesis.

Prognosis

Adequate treatment begun in the first days of life prevents all manifestations of disease. Treatment begun after 2 to 3 yr may be effective only in controlling the extreme hyperactivity and intractable seizures. Children born to mothers with poorly controlled PKU (ie, they have high phenylalanine levels) during pregnancy are at high risk of microcephaly and developmental deficit.

Treatment

  • Dietary phenylalanine restriction

Treatment of phenylketonuria is lifelong dietary phenylalanine restriction. All natural protein contains about 4% phenylalanine. Therefore dietary staples include low-protein natural foods (eg, fruits, vegetables, certain cereals), protein hydrolysates treated to remove phenylalanine, and phenylalanine-free elemental amino acid mixtures. Examples of commercially available phenylalanine-free products include XPhe products (PKU Anamix® for infants, XP Maxamaid® for children 1 to 8 yr, XP Maxamum® for children > 8 yr); Phenex®-1 and Phenex®-2; Phenyl-Free® 1 and Phenyl-Free® 2; pku 1, pku 2, and pku 3; PhenylAde® (varieties); PKU Lophlex LQ®; and Phlexy-10® (multiple formulations). Some phenylalanine is required for growth and metabolism; this requirement is met by measured quantities of natural protein from milk or low-protein foods.

Frequent monitoring of plasma phenylalanine levels is required; recommended targets are between 2 mg/dL and 4 mg/dL (120 to 240μmol/L) for children < 12 yr and between 2 mg/dL and 10 mg/dL (120 to 600 μmol/L) for children > 12 yr. Dietary planning and management need to be initiated in women of childbearing age before pregnancy to ensure a good outcome for the child. Tyrosine supplementation is increasingly used because it is an essential amino acid in patients with PKU. In addition, sapropterin is increasingly being used.

For those with BH4 deficiency, treatment also includes tetrahydrobiopterin 1 to 5 mg/kg po tid; levodopa, carbidopa, and 5-OH tryptophan; and folinic acid 10 to 20 mg po once/day in cases of dihydropteridine reductase deficiency. However, treatment goals and approach are the same as those for PKU.

Key Points

  • PKU is caused by one of several gene mutations that result in deficiency or absence of phenylalanine hydroxylase so that dietary phenylalanine accumulates; the brain is the main organ affected, possibly because of disturbance of myelination.

  • PKU causes a clinical syndrome of intellectual disability with cognitive and behavioral abnormalities; if untreated, the intellectual disability is severe.

  • In the US and many developed countries, all neonates are screened for phenylketonuria 24 to 48 h after birth with one of several blood tests; abnormal results are confirmed by directly measuring phenylalanine levels.

  • Treatment is lifelong dietary phenylalanine restriction; adequate treatment begun in the first days of life prevents all manifestations of disease.

  • Although prognosis is excellent with treatment, frequent monitoring of plasma phenylalanine levels is required; recommended targets are between 2 mg/dL and 4 mg/dL (120 to 240 μmol/L) for children < 12 yr and between 2 mg/dL and 10 mg/dL (120 to 600 μmol/L) for children > 12 yr.

Drugs Mentioned In This Article

  • Drug Name
    Select Trade
  • LODOSYN
  • No US brand name
  • KUVAN

* This is the Professional Version. *