Phenylketonuria

Abstract

Phenylketonuria (PKU; also known as phenylalanine hydroxylase (PAH) deficiency) is an autosomal recessive disorder of phenylalanine metabolism, in which especially high phenylalanine concentrations cause brain dysfunction. If untreated, this brain dysfunction results in severe intellectual disability, epilepsy and behavioural problems. The prevalence varies worldwide, with an average of about 1:10,000 newborns. Early diagnosis is based on newborn screening, and if treatment is started early and continued, intelligence is within normal limits with, on average, some suboptimal neurocognitive function. Dietary restriction of phenylalanine has been the mainstay of treatment for over 60 years and has been highly successful, although outcomes are still suboptimal and patients can find the treatment difficult to adhere to. Pharmacological treatments are available, such as tetrahydrobiopterin, which is effective in only a minority of patients (usually those with milder PKU), and pegylated phenylalanine ammonia lyase, which requires daily subcutaneous injections and causes adverse immune responses. Given the drawbacks of these approaches, other treatments are in development, such as mRNA and gene therapy. Even though PAH deficiency is the most common defect of amino acid metabolism in humans, brain dysfunction in individuals with PKU is still not well understood and further research is needed to facilitate development of pathophysiology-driven treatments.

Figures

Fig. 1 |. Phenylalanine metabolism and PKU.

Phenylalanine hydroxylase (PAH) catalyses the hydroxylation of l-phenylalanine (Phe) to l-tyrosine (Tyr), a reaction that occurs pre-dominantly in the liver but also in the proximal renal tubules in the kidneys. The reaction requires the reduced pterin tetrahydrobiopterin (BH4), along with ferrous iron and molecular oxygen (not shown) as cofactors. BH4 is oxidized to quinonoid dihydrobiopterin (qBH2) in the course of the hydroxylation reaction; qBH2 is enzymatically recycled back to BH4 in order to support ongoing Phe hydroxylation. In individuals with recessively inherited pathogenetic variants in the PAH gene, PAH enzymatic activity is either entirely lacking or severely diminished. As approximately 90% of the daily dietary intake of Phe must be metabolized through this pathway, PAH deficiency causes the accumulation of Phe in the body, most readily measured as extreme elevations of Phe concentrations in the blood (hyperphenylalaninaemia). Blood Tyr concentration is also diminished relative to that in PAH-sufficient individuals, but hypotyrosinaemia is not typically severe, perhaps because of dietary Tyr intake. In the setting of hyperphenylalaninaemia, deamination of Phe forms phenylpyruvate and other phenylketones, which are readily excreted in the urine and are the source of the colloquial name phenylketonuria (PKU). PLP, pyridoxal 5′-phosphate.

Fig. 2 |. The prevalence of PAH deficiency and different PAH deficiency phenotypes worldwide.

The global prevalence of phenylalanine hydroxylase (PAH) deficiency and that of the different severities of PAH deficiency are depicted. The prevalence of PAH deficiency varies considerably between different geographic regions (coloured map), with the highest prevalence reported in the Republic of Ireland, Germany, Italy, Iran and Jordan. The proportions of cases of classic PKU, mild phenylketonuria (PKU) and mild hyperphenylalaninaemia in representative countries are indicated (pie graphs). Data are from Hillert et al. (2020). As a clear definition of variants of PAH deficiency is lacking, prevalence data for the different PAH severities should be interpreted with care.

Fig. 3 |. Timeline of milestones in the understanding and treatment of PKU.

The initial description and major advances in the understanding of hyperphenylalaninaemia (HPA) and phenylketonuria (PKU) pathogenesis are shown. Furthermore, developments in the treatment of PKU are also highlighted. BH4, tetrahydrobiopterin; HPLC, high-performance liquid chromatography; PAH, phenylalanine hydroxylase.

Fig. 4 |. Pathological manifestations in PKU.

The clinically most important pathological manifestations of phenylalanine hydroxylase (PAH) deficiency are on the brain and are mediated by effects of excessive l-phenylalanine (Phe) concentration. Hypopigmentation (owing to impaired melanin production) is the solitary non-neurological manifestation of PAH deficiency. The large neutral amino acids (LNAAs), including Phe, tyrosine (Tyr) and tryptophan, cross the blood–brain barrier from the circulation by facilitated diffusion down a concentration gradient via the transporter LAT1 (also known as SLCA7A5). Elevated blood Phe competitively inhibits transport of the other LNAAs into the brain, reducing their concentrations; decreased LNAA concentrations may impair cerebral protein synthesis and contribute to monoamine neurotransmitter deficiency. However, the compensatory activity of energy-requiring amino acid exporters (EXPORT) may be able to mitigate amino acid imbalance to some extent and restore homeostasis. Phe competitively inhibits the activities of Tyr hydroxylase (TH) and tryptophan hydroxylase (TPH) in the brain, leading to dopamine and, especially, serotonin deficiencies. This mechanism is probably linked to the high prevalence of anxiety and mood disorders in individuals with hyperphenylalaninaemia. Elevated Phe has also been implicated in epigenetic alterations of gene expression patterns in the brain, and in impaired myelin synthesis, decreased cerebral glucose metabolism (visualized by PET imaging), the formation of amyloid plaque-like fibrils and increased oxidative stress. TYR, tyrosinase.

Fig. 5 |. Proposed algorithm for screening and diagnosis of PKU and monitoring treatment efficacy.

Diagnosis of phenylketonuria (PKU) is made during a neonatal screening programme. Blood obtained with a heel prick from newborns from 12 hours after birth and later is applied to filter paper (that is, a dried blood spot (DBS)), which is used to assess phenylalanine (Phe) concentrations by the Guthrie bacterial inhibition assay, other enzymatic assays or tandem mass spectrometry (TMS). BH4, tetrahydrobiopterin; DHPR, dihydropteridine reductase; HPA, hyperphenylalaninaemia; Tyr, tyrosine.