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Hyperphenylalaninemia (HPA) is a biochemical condition characterized by mildly or strongly elevated concentrations of the amino acid phenylalanine (Phe) in the blood. HPA is commonly diagnosed by newborn screening. The primary cause of HPA is phenylketonuria (PKU), an inborn error of metabolism characterized by persistently elevated plasma concentrations of Phe secondary to a total or partial deficiency of the liver enzyme phenylalanine hydroxylase. The treatment of babies affected with PKU is based on a Phe-restricted diet, aiming to maintain blood Phe concentrations within a range of 120 to 360 μmol/L to prevent the spectrum of neurological disorders associated with PKU, that is, microcephaly, learning disability, epilepsy, pyramidal and extrapyramidal signs, and behavioral changes. This metabolic disorder mainly affects the white matter (e.g., dysmyelination, demyelination, and hypomyelination) and the cortical-subcortical structures. A delay in starting the dietary treatment, or an inadequate Phe-restricted diet, may relentlessly lead to brain damage.

Tetrahydrobiopterin (BH4) is a natural and essential cofactor for the enzymatic hydroxylation of phenylalanine (Phe) and tyrosine (Tyr), and for two tryptophan hydroxylases, three nitric oxide synthases, and glyceryl-ether monooxygenase. Five separate genetic conditions affecting BH4 synthesis or recycling have been identified so far, including deficiency in (1) 6-pyruvoyltetrahydropterin synthase; (2) dihydropteridine reductase; (3) GTP cyclohydrolase I; (4) sepiapterin reductase; and (5) pterin-4α-carbinolamine dehydratase. These disorders cause hyperphenylalaninemia and impaired synthesis of serotonin and dopamine since tyrosine hydroxylase and neuronal tryptophan hydroxylase require BH4 and serotonin/dopamine products (5-hydroxytryptophan and L-dopa). All these five genetic conditions can be identified by newborn screening procedures due to elevated blood levels of Phe (with the sole exception of sepiapterin reductase deficiency). BH4 loading tests and measurement of neurotransmitter metabolites, pterins, and folates in cerebrospinal fluid can add further important information on disease severity. Untreated patients develop a complex neurological phenotype, which includes Parkinson-like features, brain degeneration, and early death. The gold standard treatment of severe disorders of BH4 metabolism is based on replacement therapy with BH4, 5-hydroxytryptophan, L-dopa, and carbidopa, with the addition, in certain cases, of folinic acid supplements and pramipexole. Dopamine agonists can improve L-dopa therapy, making treatment easier, relieving symptoms, stabilizing clinical course, and possibly ameliorating long-term outcomes. The outcome of patients with disorders of biopterin synthesis can be favorable, with either normal or near-normal cognition, and with some residual neurological symptoms usually manifesting diurnal variation, that is, worst when patients become tired or when the dosage or interval for medications is inadequate.

Lysosomal storage disorders (LSDs) are a heterogeneous group of inborn errors of metabolism caused by inherited deficiencies of any of the lysosomal functions, leading to the accumulation of undegraded substrates in multiple tissues and organs. Two-third of LSDs involves the central nervous system, thus representing the most common cause of pediatric neurodegenerative diseases. Substantial progress has been made in our understanding of the pathophysiology of LSDs, leading to newly targeted therapeutic options. Enzyme replacement therapy (ERT) is currently available for seven LSDs including Gaucher disease, Fabry disease, Pompe disease, and mucopolysaccharidosis (MPS) I (Hurler disease), II (Hunter disease), IV A (Morquio A), and VI (Maroteaux–Lamy disease). ERT reduces lysosomal storage, thus slowing or sometimes avoiding progressive visceral damage altogether. However, ERT is unable to cross the blood–brain barrier (BBB), thus lacking efficacy on neurological manifestations. In patients with MPS I (Hurler disease) under 2 years of age and in selected patients with other LSD, hematopoietic stem cell transplantation is indicated. To bypass the BBB, other approaches, using small molecules are currently being tested and include substrate reduction therapy, which decreases the amount of substrate (currently available for type 1 Gaucher disease and for Niemann–Pick type C disease) and pharmacological chaperones, which enhance the residual activity of the mutant enzyme.