Phenylketonuria (PKU)

Phenylketonuria (PKU) is a rare genetic condition caused by mutations of the phenylalanine hydroxylase (PAH) gene.¹ If left untreated or if the condition is poorly managed, the disease can lead to elevated blood phenylalanine concentrations and severe mental retardation. Other neurological symptoms, such as developmental delay, epilepsy, and behavioral problems may occur, as well as depression and anxiety disorders.¹ Symptoms are usually progressive and become more apparent as a child grows.² 

What is phenylalanine? 

Phenylalanine is an essential amino acid found in many protein-rich foods and is used in the biosynthesis of other amino acids. It is important in the structure and function of many proteins and enzymes. Loss of PAH activity results in increased concentrations of phenylalanine in the blood and toxic concentrations in the brain.²

Phenylalanine exists as D and L enantiomers. L-Phenylalanine is an essential amino acid required for protein synthesis in humans, and concentrations are regulated with dynamic input and runout flux. The major input sources of phenylalanine are dietary intake, along with endogenous recycling of amino acid stores. Utilization or runout occurs via integration into proteins, oxidation to tyrosine, or conversion to other metabolites.³  

PAH catalyzes the stereospecific hydroxylation of L-Phenylalanine, the committed step in the degradation of this amino acid. PAH activity is predominantly associated with the liver, and the enzyme requires BH₄ (tetrahydrobiopterin), as well as molecular oxygen, as cofactors for its action.³ The enzyme has three key structural domains. The regulatory domain contains a serine residue which is thought to be involved in activation by phosphorylation. The catalytic domain contains a motif of 26 or 27 amino acids responsible for cofactor and ferric iron-binding. The C-terminal domain is thought to be associated with inter-subunit binding.³  

PKU is caused by mutations to the PAH gene, preventing metabolization of phenylalanine, leading to elevated blood concentration levels.⁴  

Hyperphenylalaninaemia (HPA, a milder form of PKU) can be caused by either mutation at the PAH locus, which results in PKU, or from mutations in several loci which affect BH₄ synthesis and regeneration resulting in non-PKU HPA.³ More than 500 disease-causing mutations have been identified in patients with PKU or HPA.³  

Left untreated, PKU is associated with growth failure, microcephaly, seizures, and intellectual impairment caused by the accumulation of toxic by-products of phenylalanine. Decreased or absent PAH activity can also lead to a deficiency of tyrosine and its downstream products, including melanin, L-thyroxine, and the catecholamine neurotransmitters.³

Newborns with PKU initially don’t have any symptoms.⁵  

Untreated children with PKU show impaired brain development. Signs and symptoms include microcephaly, epilepsy, severe intellectual disability, and behavior problems. They may also have a distinct musty body odor due to the excretion of excessive phenylalanine and its metabolites, and skin conditions such as eczema. Decreased skin and hair pigmentation can result from the inhibition of tyrosinase by elevated levels of phenylalanine.⁶  

Additional problems can emerge later in life and include exaggerated deep tendon reflexes, tremors, and paraplegia or hemiplegia. Individuals treated late or never treated may develop severe behavioral or psychiatric problems, such as depression, anxiety, and phobias in their 20s or 30s. In some instances, untreated individuals with PKU who have normal intelligence are diagnosed in adulthood following sudden and severe psychiatric deterioration.⁶ 

Recent figures from neonatal screening studies suggest that the overall worldwide prevalence of PKU is 6.0 per 100,000 neonates.⁷ Other recent estimates suggest that the global prevalence is 1 in 23,930 live births.⁸ It is estimated that there are ~16,500 patients with PKU in the US.⁹   

A study of 16,092 affected patients from 51 countries suggests a large degree of geographic variability in disease prevalence, with the highest prevalence in Italy (1 in every 4,000 births) and the lowest in Thailand (1 in 227,273 births).⁸ The frequency of classic PKU as a proportion of all PKU and HPA cases increases from Europe (56%) via the Middle East (71%) to Australia (80%). A gradient in genotype and phenotype distribution exists across Europe, from classic PKU in the east to mild PKU in the south-west and mild HPA in the south.⁸ In terms of disease severity, it is estimated that 62% of patients have classic PKU, 22% have mild PKU, and 16% have mild HPA.⁸

The variability in geographic prevalence is thought to be explained by migration patterns, rates of reproductive relationships between two closely related individuals (consanguinity), and genetic reserves in different countries.^7,8

Diagnosis is initially undertaken through newborn screening programs in the first weeks of life.¹⁰ This involves taking a small blood sample from the baby within a few days of birth and conducting a blood amino acid analysis to assess phenylalanine levels.¹¹ Patients with increased phenylalanine levels (>120 μmol/L), and therefore a diagnosis of PKU, are further screened for BH₄ responsiveness.² In Europe, this is done via BH₄ loading, where a BH₄ load of 20 mg/kg body weight is given orally, and blood phenylalanine concentration is measured before and at 8, 16, and 24 hours after the BH₄ load is given. A marked normalization (within 8 hours) in phenylalanine concentrations indicates BH₄ deficiency, whereas very little or no reduction in phenylalanine indicates BH₄-non-responsive PKU. In the US, urine and filter-paper-dried blood specimens are obtained for measurements of urinary pterins (neopterin and biopterin) and red blood cell dihydropteridine reductase to assess the possibility of a defect associated with BH₄ deficiency.²  

A diagnosis made after the neonatal period is often referred to as either “late diagnosis” or “untreated PKU”. Late diagnosis refers to children diagnosed between the ages of 3 months and 7 years. Untreated PKU refers to patients untreated by the age of 7. While these thresholds are relatively arbitrary, treatment initiated after the age of 7 rarely results in improved IQ.¹² Diagnosis after the neonatal period usually happens after a patient presents with developmental delay or other PKU-related symptoms and is confirmed following plasma amino acids analysis.¹²

Left untreated, the natural history of PKU involves progressive irreversible neurological impairment during infancy and childhood.¹¹ The most common outcome is severe mental retardation often associated with a “mousy” odor, eczema, and reduced hair, skin, and iris pigmentation. Reduced growth, microcephaly, and neurological signs, such as tremor and epilepsy, may be present. Untreated individuals often have behavioral problems, such as hyperactivity and repetition of certain actions.¹¹ Disease severity positively correlates with blood phenylalanine levels, such that higher levels are associated with greater cognitive impairment.^11,13  

While there is no cure, the effects of PKU can be mitigated by restricting dietary intake of phenylalanine to the minimum level required for normal growth.¹⁰ These dietary restrictions should be adhered to throughout the life of the patient to reduce further irreversible damage. If dietary phenylalanine restriction is implemented promptly after neonatal diagnosis, intellectual disability can be prevented;¹⁰ however, neurophysiological and neuropsychological impairments may persist, even in treated PKU patients.¹⁰ Notably, if the diet is relaxed during childhood, this can adversely affect IQ as the child gets older.¹³ In many adults, dietary management may be ineffective due to long-term non-adherence or inadequate phenylalanine-lowering effects.¹⁴  

In addition to low protein diets aimed at maintaining low levels of phenylalanine, PKU can be managed using phenylalanine-free amino acid supplements.¹⁴ However, these approaches often fail to achieve adequate phenylalanine-lowering effects. 

Given that the primary management approach for PKU is dietary restriction, an important impact of the disease for patients is adhering to the strict dietary regime, which confers a significant patient burden.¹⁵ While dietary compliance in infancy is typically very good, non-compliance increases with age.¹⁶ Reasons associated with poor adherence to the consumption of amino acid mixtures and nutritional plans in adulthood include socializing, palatability, work, and embarrassment.¹⁷ 

Among 12 adherent and 9 non-adherent adults participating in a qualitative study, the non-adherent group appeared unable to accept their disease. While aware of the consequences of non-adherence in children, they felt the management of PKU was still a burden in adulthood, and experienced emotional distress caused by feelings of “diversity” in social contexts.¹⁸ It has also been reported that patients experience difficulties accessing phenylalanine-free protein substitutes and low-protein food, including restrictions imposed by healthcare systems. This contributes to patient and carer stress, and a perception that the disease is not taken seriously by healthcare professionals.¹⁹  

In terms of broader practical, social, and psychological issues of living with PKU, a large UK survey of more than 600 patients found that a proportion of children struggle to maintain focus, experience educational difficulties, anxiety or depression, gastrointestinal symptoms, social exclusion, and relationship issues with friends or family.²⁰ Issues experienced by adults include depression or anxiety, difficulty maintaining focus, low mood, difficulties with relationships, social exclusion, and gastrointestinal issues.²⁰  

PKU can have a particularly marked impact on women before, during, and after pregnancy, given that high blood phenylalanine levels during pregnancy can have a teratogenic effect on the developing fetus.²¹ In a survey of 300 women with PKU in the UK, most expressed concerns, fears, and distress about pregnancy, and two-thirds of patients who had at least one pregnancy reported that having PKU makes pregnancy more stressful and difficult. Patients reported concerns that they may cause harm to the baby, and that they are worried about their ability to manage a strict diet during pregnancy, are anxious about their ability to maintain blood phenylalanine within the target range, and fear having an unplanned pregnancy.²¹  

There are also unique considerations for parents of children with PKU. When a child is diagnosed with PKU, parents must assume immediate responsibility for the management of the disease and prevention of neurological damage.²² This includes supervision of the child’s nutritional intake, ongoing medical appointments, and regular blood tests – and brings with it a significant burden and often psychological stress.²² Parents of children with PKU emphasize the importance of gaining control over the management of the disease and minimizing the impact of PKU on the child to ensure that they can live a normal life.²²

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