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Frequently asked questions about phenylketonuria

Review common questions about phenylketonuria (PKU)

What causes PKU?

Phenylketonuria (PKU) is an autosomal recessive inborn error of phenylalanine (Phe) metabolism caused by variants in the gene PAH, which encodes the enzyme phenylalanine hydroxylase (PAH). These variants lead to complete or partial deficiency of PAH enzymatic activity, resulting in inability to metabolize the amino acid Phe.

How does PKU affect the brain?

As the PAH enzyme catalyzes the hydroxylation of Phe to tyrosine (Tyr), conversion of Phe to Tyr is impaired. This causes the amino acid Phe to build up in the blood and brain, leading to a range of intellectual disabilities, as well as neurological, neuropsychiatric, and psychosocial consequences.4

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Can PKU cause brain damage?

If not adequately controlled, excess levels of Phe, which are toxic to the brain, disrupt normal neurophysiology. High Phe levels may lead to serious neurological and neuropsychological complications, affecting the way people with PKU think, feel, and act.1,2,6,7

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PKU: how common is it?

In the United States, the estimated prevalence of PKU is 1 case per 15,000 births.4

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How long do you manage PKU?

The American College of Medical Genetics and Genomics released guidelines in 2014 on the diagnosis and management of PKU: management should be started as early as possible and maintained for life; blood Phe levels should be maintained at ≤360 µmol/L throughout life; and any combination of therapies that helps keep blood Phe levels in the recommended range is appropriate.3

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What are common PKU symptoms?

The effects of high Phe are felt throughout a patient’s life.3,7

Young adults and adults with uncontrolled Phe can experience neurologic, psychiatric, and cognitive symptoms, including:

  • Depression
  • Anxiety
  • Impairments in executive functioning
  • Lower IQ than the general adult population

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What comorbidities are associated with PKU?

The burden of PKU extends beyond the brain and into many aspects of adult life. Adults with PKU have an increased risk of several multisystemic comorbidities.8

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How difficult is it to control blood Phe levels?

Phe control typically drops off during adolescence, increasing the severity of neuropsychological symptoms and negatively impacting life trajectory.2,3

Young adults and adult patients can become trapped in a cycle of decline making dietary compliance and PKU management more difficult.6,9

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What is the diet for PKU?

The mainstay of PKU management is lifelong dietary management with a Phe-restricted diet.3,10

A low-Phe diet consists of 3 main parts:3,10

  • Restriction of natural protein
  • Medical food/formula/supplements
  • Modified low-protein foods

While a Phe-restricted diet remains the cornerstone of care for patients with PKU, there are challenges which may make strict adherence difficult for patients.3,11

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What is the benefit of lowering blood Phe levels?

Lowering Phe is recognized as the key strategy in managing PKU.3

Elevated levels of the amino acid Phe correlates with neuropsychological symptom severity, which generally improves with Phe reductions.3

Emerging clinical evidence suggests that additional benefits may be attained when Phe is below recommended levels.12,13

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What are the current clinical guideline recommendations for blood Phe levels?

The American College of Medical Genetics and Genomics (ACMG) recommends starting management as early as possible and maintaining Phe levels ≤360 µmol/L.3

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How is PKU managed?

Globally, the approach for PKU management has been a highly Phe-restricted diet, medical nutritional therapy, and low-protein foods. In combination with a low-Phe diet, prescription medications are also available.3,10

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Learn more about PKU

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Learn the symptoms of uncontrolled PKU

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  1. Ashe K, Kelso W, Farrand S, et al. Psychiatric and cognitive aspects of phenylketonuria: the limitations of diet and promise of new treatments. Front Psychiatry. 2019;10:561. doi:10.3389/fpsyt.2019.00561.
  2. Brown CS, Lichter-Konecki U. Phenylketonuria (PKU): a problem solved? Mol Genet Metab Rep. 2016;6:8-12.
  3. Vockley J, Andersson HC, Antshel KM, et al; American College of Medical Genetics and Genomics Therapeutic Committee. Phenylalanine hydroxylase deficiency: diagnosis and management guidelines. Genet Med. 2014;16(2):188-200.
  4. Blau N, van Spronsen FJ, Levy HL. Phenylketonuria. Lancet. 2010;376(9750):1417-1427.
  5. Rocha JC, MacDonald A. Treatment options and dietary supplements for patients with phenylketonuria. Expert Opin Orphan Drugs. 2018;6(11):667-681.
  6. Bilder DA, Noel JK, Baker ER, et al. Systematic review and meta-analysis of neuropsychiatric symptoms and executive functioning in adults with phenylketonuria. Dev Neuropsychol. 2016;41(4):245-260
  7. van Wegberg AMJ, MacDonald A, Ahring K, et al. The complete European guidelines of phenylketonuria: diagnosis and treatment. Orphanet J Rare Dis. 2017;12(1):162. doi: 10.1186/s13023-017-0685-22.
  8. Burton BK, Jones KB, Cederbaum S, et al. Prevalence of comorbid conditions among adult patients diagnosed with phenylketonuria. Mol Genet Metab. 2018;125(3):228-234.
  9. Thomas J, Nguyen-Driver M, Bausell H, Breck J, Zambrano J, Birardi V. Strategies for successful long-term engagement of adults with phenylalanine hydroxylase deficiency returning to the clinic. J Inborn Errors Metab Screen. 2017;5:1-9.
  10. Rocha JC, MacDonald A. Dietary intervention in the management of phenylketonuria: current perspectives. Pediatric Health Med Ther. 2016;7:155-163.
  11. Ford S, O’Driscoll M, MacDonald A. Living with phenylketonuria: lessons from the PKU community. Mol Genet Metab Rep. 2018;17:57-63.
  12. Evinc SG, Pektas E, Foto-Özdemir D, et al. Cognitive and behavioral impairment in mild hyperphenylalaninemia. Turk J Pediatr. 2018;60(6):617-624.
  13. Jahja R, Huijbregts S, de Sonneville LMJ, van der Meere JJ, van Spronsen FJ. Neurocognitive evidence for revision of treatment targets and guidelines for phenylketonuria. J Pediatr. 2014;164(4):895-899.
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