What is phenylketonuria (PKU)?
Phenylketonuria, also known as PKU, is an inherited metabolic disorder resulting from a genetic mutation. More specifically, it is a deficiency of the enzyme phenylalanine hydroxylase (PAH). This deficiency increases the levels of phenylalanine (Phe) in the blood. Its signs, symptoms, and severity vary and can affect numerous aspects of an individual's life. These include cognitive impairment, seizures, mental disorders, delayed mental and physical growth, damage to organs, eczema or other skin conditions, potential complications during pregnancy (maternal PKU), and hypopigmentation of the hair, eyes, and skin. A study estimated that PKU occurs in 1 in 23,930 births and collectively affects 0.45 million individuals, with two-thirds requiring treatment for it.1
Due to the severity of the symptoms and phenylketonuria (PKU) being an inherited disease, parents-to-be should be aware of their genetic family history and available screening methods for it. Moreover, the patients and their families must understand the genetics of this disease so they can make informed decisions on treatment plans, lifestyle choices, and support. Such informed decisions ultimately improve the quality of life for individuals living with PKU. This article will explore PKU, its genetic mechanisms, inheritance patterns, symptoms, screening methods, and treatment plans.
The genetics of phenylketonuria (PKU)
Role of genes and mutations in PKU development
Phenylketonuria (PKU) is a disorder caused by mutations in the phenylalanine hydroxylase (PAH) gene on chromosome 12.
The enzyme produced by the PAH gene, bearing the same name, converts phenylalanine (Phe) to tyrosine (Tyr). This reaction requires a co-substrate called tetrahydrobiopterin (BH4). Along with BH4, the protein products of another gene called DNAJC12 can act as a chaperone to facilitate the folding of PAH. The PAH gene is expressed mainly in the liver, kidney, and pancreas. When alterations, scientifically called mutations, occur in a gene, the enzyme it produces becomes inactive or less efficient. Their relationship is that of a recipe and a final dish. In the case of PKU, the PAH enzyme is not produced adequately, resulting in the body's build-up of phenylalanine. This phenomenon is also known as hyperphenylalaninaemia (HPA), which can reach toxic levels.
PKU can be a result of many different alterations in the PAH gene. More than 1,000 of them have been reported in patients worldwide.1,2 These mutations can be null variants such as splicing, nonsense, and frameshift mutations, which cause a complete loss of protein function.
Furthermore, these mutations can be missense variants, like insertions and deletions (indels). This type of mutation results in the production of defective proteins. Furthermore, PKU has been linked to defects in the BH4 metabolism or pathogenetic variants in the DNAJC12 gene.3,4
The inadequate levels of PAH lead to phenylalanine (Phe) accumulation in all body tissues, including blood. It also causes a tyrosine deficiency since the conversion of the former into the latter does not occur as it should. High levels of Phe can reach a toxic level, which damages organs and skin. Moreover, reduced PAH activity results in a high concentration of Phe that converts to phenylpyruvic acid. Phenylpyruvic acid is also known as phenylketone. It can be detected in urine, hence the name phenylketonuria, as the first-ever case was diagnosed via urine examination.5
In addition, tyrosine (Tyr) is a precursor of neurotransmitters in the brain's prefrontal cortex. Tyr deficiency due to the failure to convert Phe into Tyr can cause cognitive impairments. Previously, studies have linked Tyr activity to the improvement of cognitive and neuropsychiatric symptoms like working memory and depression.6,7 Tyr has been widely known to be a key element in melanin production. Melanin decrease leads to hypopigmentation, which affects the skin, hair, and eyes.
Inheritance patterns of PKU
PKU is an autosomal recessive disease. To break it down, autosomal means that the gene in question is located on one of the numbered, non-sex chromosomes. Humans have 22 sets of autosomal chromosomes and 1 set of sex chromosomes, meaning 46 chromosomes in total. Genes are located on chromosomes. Every gene exists in two copies, scientifically called alleles. Each allele is inherited from one parent. Recessive is a variant of a gene whose expression is inferior to another version of it. For PKU, two copies of the mutated gene are required for the disorder to develop, as the non-mutated gene would override the mutated one. Therefore, both parents need to be carriers of the defective gene.
A carrier is someone who carries a recessive genetic mutation. It does not affect their phenotype, so the disease is not expressed. Simply put, they do not suffer from the illness or present symptoms. In the case of PKU, a carrier has one functional copy of the gene to produce the PAH enzyme and metabolise phenylalanine. If both parents are carriers, there is a:
- 25% chance with each pregnancy that their offspring will inherit two copies and suffer from PKU
- 50% chance that the child will be a carrier
- 25% chance that the child will not carry any mutated copies
In contrast, when a disorder is dominant, only one copy of the altered gene from a parent is required to cause the disorder. Therefore, one parent must also be affected by that disorder.
Risk factors for inheriting PKU
The risk factors of PKU are primarily associated with its genetic inheritance pattern. Here are some factors to consider.
Family history
A known family history of PKU patients or carriers would increase the risk of inheriting the condition.
Ethnicity
PKU occurs in all ethnic groups. However, there is a higher frequency in certain populations with a higher prevalence of carriers. Ultimately, this increases the likelihood of inheriting it.
Consanguinity
If there are many carriers in the family, marriages between close relatives (consanguineous marriages) increase the risk of inheriting the mutated gene responsible for PKU.
Genetic testing can identify individuals carrying the mutated gene in all these cases. This allows the families to make informed decisions about family planning and genetic counselling. Furthermore, an early diagnosis in newborns can allow early intervention and treatment.
The symptoms of PKU
The severity of PKU is categorised into different phenotypic profiles:
- Classic PKU - nearly no PAH enzyme activity, significantly high levels of Phe in the blood, patients present many severe symptoms
- Moderate PKU - 10-30% PAH activity, moderately high levels of Phe in blood, less severe symptoms than classical PKU8
- Mild PKU - 30-70% PAH activity, higher than normal levels of Phe in blood, less than in moderate or classic PKU, milder symptoms9
- Mild hyperphenylalaninaemia (HPA) - >70% PAH activity, higher than normal levels of Phe in the blood, but the measurement does not meet the criteria for a PKU diagnosis; mutations in the PAH or other genes related to phenylalanine metabolism, patients may not develop the characteristic symptoms of PKU9
It has been reported that if the mutation in the PAH gene is heterozygous, meaning there are two different alterations in the two copies of the gene, the milder variant is always dominant over the more severe one.2,7
The symptoms of PKU can vary in severity, which is connected to the Phe levels. Symptoms generally include:
- Cognitive impairment
- Epilepsy
- Behavioural disorders
- Microcephaly
- Seizures
- Hyperactivity
- Abnormal body posture
- Motor dysfunction
- Delayed mental and physical development
- Skin conditions like eczema or others
- Mental disorders like anxiety and depression
- Hypopigmentation of skin, hair, and eyes
- Musty smell in breath, skin, sweat, and urine
- Pregnancy complications (maternal PKU)
If the mother has high levels of Phe in her blood during pregnancy, then the baby can display a low birth weight, microcephaly, heart problems, and mental and growth retardation.
Genetic testing methods for PKU
Some cases may not be diagnosed until adulthood. These cases fall in the mild-moderate severity range of symptom development. Generally, early diagnosis of PKU is crucial for optimising early treatments to prevent or minimise symptoms. This is because the brain is more receptive to changes in the early stages of life, allowing the long-term improvement of the health and quality of life of patients with this condition.
Since 1962, phenylketonuria has been diagnosed via newborn screening. These tests are based on screening pH levels in the blood and are done by a heel prick to collect a few drops of blood. These drops are then spotted on filter paper and dried. The sample is usually examined using procedures like the bacterial inhibition assay (Guthrie test), fluorometric microassay (FMA), and tandem mass spectrometry (MS/MS). The latter can measure all amino acids, including Phe-Tyr ratios. In the case of PKU, the ratio would be elevated (more Phe, less Tyr).
Lately, molecular diagnoses of PKU are emerging. An example of this is DNA sequencing. DNA sequencing can be used to look for the mutated gene variants responsible for the disorder.
Treatment options and management strategies for PKU
Early intervention after birth through dietary restrictions or a combination of a certain diet and supplements or medication has been the golden standard for treating PKU for years now. Such treatments are centred on managing and controlling Phe levels for optimal brain development. It is highly advised to consult a healthcare professional for a treatment plan, as risks accompany some methods.
Phenylalanine-restricted diet
Phe is commonly found in protein-rich foods, like eggs and chickpeas. A patient with PKU has to follow a diet low in Phe throughout their lifespan. In addition, diet foods and soft drinks containing aspartame, a Phe-containing synthetic sweetener, should be carefully avoided. Many people discontinue this type of diet as they get older, resulting in the development of symptoms.
Amino acid supplementation
Large neutral amino acids (LNAAs), like tyrosine, can be administered as they inhibit the transport of Phe in the blood plasma. Doing so, reduces the amount that travels between the blood and the brain.
Sapropterin therapy
The BH4 synthetic analogue sapropterin dihydrochloride has been approved by the Food and Drug Administration (FDA). Sapropterin is used for hyperphenylalaninaemia (HPA) treatment in patients with PKU or those with BH4 deficiency. By creating an abundance of BH4, PAH activity is increased. This improves the metabolism of Phe and thereby decreases its concentration in the blood.
Pegvaliase therapy
The new therapeutic Pegvaliase is also FDA-approved. Pegvaliase is an enzyme replacement therapy based on using an enzyme called phenylalanine ammonia-lyase (PAL). It metabolises Phe and brings its blood levels back to normal ranges.
Maternal PKU
Pregnant women with untreated PKU have an increased risk of abnormal foetal development, regardless of the genes of the foetus. Such an effect occurs during the first 8 to 10 weeks of gestation. However, the mother can prevent this by following a controlled diet before and throughout the pregnancy. This keeps the Phe levels in the blood plasma normal.
Summary
Phenylketonuria (PKU) is a complex inherited metabolic disease caused by phenylalanine hydroxylase (PAH) gene mutations. These mutations lead to elevated levels of PAH in tissues and blood and consequential psychoneurological and developmental complications. The severity of the disease ranges from mild to highly severe. Its signs and symptoms include intellectual disabilities, delayed mental and physical growth, skin conditions, organ damage, psychological disorders, and seizures. Its diagnosis is achieved through newborn screening, allowing for early intervention and management strategies. These can consist of dietary restriction of phenylalanine, medications, and amino acid supplementation.
Pregnant women with PKU should strictly maintain their phenylalanine levels to normal ranges to prevent developmental effects on the foetus. Moreover, genetic counselling is necessary to educate PKU patients and their families about the risks of PKU inheritance, screening, and managing options. This facilitates informed decision-making regarding family planning. Understanding PKU's genetic mechanisms, inheritance patterns, and risk factors is critical for early diagnosis, appropriate treatment, and prevention of symptom development. Phenylketonuria is a disease that requires a multidisciplinary approach involving healthcare professionals, researchers, and affected individuals and their families. Do not rely solely on online information; please consult a professional for more appropriate advice.
References
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- Garbade SF, Shen N, Himmelreich N, Haas D, Trefz FK, Hoffmann GF, et al. Allelic phenotype values: a model for genotype-based phenotype prediction in phenylketonuria. Genet Med. 2019 ar;21(3):580–90.
- Anikster Y, Haack TB, Vilboux T, Pode-Shakked B, Thöny B, Shen N, et al. Biallelic mutations in dnajc12 cause hyperphenylalaninemia, dystonia, and intellectual disability. Am J Hum Genet. 2017 Feb 2;100(2):257–66.
- Thöny B, Ding Z, Martínez A. Tetrahydrobiopterin protects phenylalanine hydroxylase activity in vivo: implications for tetrahydrobiopterin-responsive hyperphenylalaninemia. FEBS Lett. 2004 Nov 19;577(3):507–11.
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- Abgottspon S, Muri R, Christ SE, Hochuli M, Radojewski P, Trepp R, et al. Neural correlates of working memory and its association with metabolic parameters in early-treated adults with phenylketonuria. Neuroimage Clin. 2022;34:102974.
- Alabsi A, Khoudary AC, Abdelwahed W. The antidepressant effect of l-tyrosine-loaded nanoparticles: behavioral aspects. Ann Neurosci. 2016 Jul;23(2):89–99.
- Guldberg P, Rey F, Zschocke J, Romano V, François B, Michiels L, et al. A European multicenter study of phenylalanine hydroxylase deficiency: classification of 105 mutations and a general system for genotype-based prediction of metabolic phenotype. Am J Hum Genet. 1998 Jul;63(1):71–9.
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