Tetrahydrobiopterin (BH4)-deficient hyperphenylalaninemia (HPA) comprises a genetically heterogeneous group of progressive neurologic disorders caused by autosomal recessive mutations in the genes encoding enzymes involved in the synthesis or regeneration of BH4. BH4 is a cofactor for phenylalanine hydroxylase (PAH; 612349), tyrosine hydroxylase (TH; 191290) and tryptophan hydroxylase (TPH1; 191060), the latter 2 of which are involved in neurotransmitter synthesis. The BH4-deficient HPAs are characterized phenotypically by hyperphenylalaninemia, depletion of the neurotransmitters dopamine and serotonin, and progressive cognitive and motor deficits (Dudesek et al., 2001). HPABH4A, caused by mutations in the PTS gene, represents the most common cause of BH4-deficient hyperphenylalaninemia (Dudesek et al., 2001). Other forms of BH4-deficient HPA include HPABH4B (233910), caused by mutation in the GCH1 gene (600225), HPABH4C (261630), caused by mutation in the QDPR gene (612676), and HPABH4D (264070), caused by mutation in the PCBD1 gene (126090). Niederwieser et al. (1982) noted that about 1 to 3% of patients with hyperphenylalaninemia have one of these BH4-deficient forms. These disorders are clinically and genetically distinct from classic phenylketonuria (PKU; 261600), caused by mutation in the PAH gene. Two additional disorders associated with BH4 deficiency and neurologic symptoms do not have overt hyperphenylalaninemia as a feature: dopa-responsive dystonia (612716), caused by mutation in the SPR gene (182125), and autosomal dominant dopa-responsive dystonia (DYT5; 128230), caused by mutation in the GCH1 gene. Patients with these disorders may develop hyperphenylalaninemia when stressed.

Aromatic L-amino acid decarboxylase deficiency (AADCD) is an autosomal recessive inborn error in neurotransmitter metabolism that leads to combined serotonin and catecholamine deficiency (Abeling et al., 2000). The disorder is clinically characterized by vegetative symptoms, oculogyric crises, dystonia, and severe neurologic dysfunction, usually beginning in infancy or childhood (summary by Brun et al., 2010).

Untreated pyridox(am)ine 5'-phosphate oxidase (PNPO) deficiency, characterized by a range of seizure types, is "classic" (i.e., seizure onset in the neonatal period) in about 90% of affected individuals and "late onset" (seizure onset after the neonatal period) in about 10%. In classic PNPO deficiency, seizures (including status epilepticus) often begin on the first day of life and typically before age two weeks. In both classic and late-onset untreated PNPO deficiency, seizure semiology varies from myoclonic to clonic or tonic seizures, and seizures are typically resistant to common anti-seizure medications. Independent of age of onset, seizures respond to life-long treatment with a B6 vitamer: pyridoxal 5'-phosphate (PLP) in about 60% of affected individuals and pyridoxine (PN) in about 40%. About 60% of individuals with PNPO deficiency have developmental impairment, affecting speech, cognition, and behavior; some individuals have neurologic impairment such as muscular hypotonia or dystonia. Severe neurodevelopmental impairment is more likely to occur in individuals with PNPO deficiency who experienced diagnostic delay and prolonged periods of uncontrolled seizures.

Tyrosine hydroxylase (TH) deficiency is associated with a broad phenotypic spectrum. Based on severity of symptoms/signs as well as responsiveness to levodopa therapy, clinical phenotypes caused by pathogenic variants in TH are divided into (1) TH-deficient dopa-responsive dystonia (the mild form of TH deficiency), (2) TH-deficient infantile parkinsonism with motor delay (the severe form), and (3) TH-deficient progressive infantile encephalopathy (the very severe form). In individuals with TH-deficient dopa-responsive dystonia (DYT5b, DYT-TH), onset is between age 12 months and 12 years; initial symptoms are typically lower-limb dystonia and/or difficulty in walking. Diurnal fluctuation of symptoms (worsening of the symptoms toward the evening and their alleviation in the morning after sleep) may be present. In most individuals with TH-deficient infantile parkinsonism with motor delay, onset is between age three and 12 months. In contrast to TH-deficient DRD, motor milestones are overtly delayed in this severe form. Affected infants demonstrate truncal hypotonia and parkinsonian symptoms and signs (hypokinesia, rigidity of extremities, and/or tremor). In individuals with TH-deficient progressive infantile encephalopathy, onset is before age three to six months. Fetal distress is reported in most. Affected individuals have marked delay in motor development, truncal hypotonia, severe hypokinesia, limb hypertonia (rigidity and/or spasticity), hyperreflexia, oculogyric crises, ptosis, intellectual disability, and paroxysmal periods of lethargy (with increased sweating and drooling) alternating with irritability.

Intellectual developmental disorder with language impairment and early-onset dopa-responsive dystonia-parkinsonism (IDLDP) is a neurodevelopmental disorder characterized by global developmental delay affecting motor, cognitive, and speech domains apparent in early childhood or infancy. Some patients may have normal early development in infancy before symptom onset. There is phenotypic heterogeneity and the severity is highly variable; less severely affected individuals have only mild deficits and are able to attend special schools. About half of patients develop various types of seizures that may be refractory or responsive to treatment. Most patients also show movement abnormalities, often hypotonia early in the disease course with later development of dopa-responsive dystonia or parkinsonism (Ramos et al., 2019, Wirth et al., 2020; Singh et al., 2020).