The neurotransmitter deficiency in infants in this group arises as a result of defects in BH4 metabolism. Patients are usually identified by elevated phenylalanine levels on newborn screening, as BH4 is required for phenylalanine hydroxylation in the liver. The accompanying neurotransmitter deficiency results from the lack of BH4, an obligatory cofactor required for the synthesis of catecholamines and serotonin.

Although most academic biochemical genetics clinics that follow children with phenylketonuria (PKU) systematically perform the additional studies required to diagnose this group of disorders, occasionally children are not identified until they have progressive neurologic symptoms or clear evidence of developmental delay despite a phenylalanine-restricted diet. In the past, these patients were referred to as “atypical phenylketonurics”. In some cases such infants are missed because screening was performed before an adequate interval of protein intake, resulting in a falsely negative result on newborn testing.

Approximately 1–3% of patients with elevated blood phenylalanine levels have an associated BH4 deficiency state. Thus, it is critical to identify such children so that BH4 and neurotransmitter precursors can be supplemented as early as possible. The two most commonly identified disorders in children presenting with elevated blood phenylalanine levels in the newborn period are:

  1. 6-pyruvoyltetrahydropterin synthase (PTS) deficiency
  2. dihydropteridine reductase (DHPR) deficiency

PTS deficiency results in inadequate BH4 synthesis, while DHPR deficiency results in decreased regeneration of BH4 from dihydrobiopterin. Both are autosomal recessive disorders in which hyperphenylalaninemia results from a deficiency of BH4. Because of the involvement of BH4 in catecholamine and serotonin synthesis, such infants also have a manifest deficiency of neurotransmitter metabolites in addition to hyperphenylalaninemia. Other conditions in this category include autosomal recessive GTP cyclohydrolase deficiency and primapterinuria.

6-Pyruvoyltetrahydropterin Synthase Deficiency

SMA6-pyruvoyl tetrahydropterin synthase (6-PTPS, 6-PTS or PTS) catalyzes the elimination of inorganic triphosphate from dihydroneopterin triphosphate to form 6-pyruvoyltetrahydropterin. Thus, patients have elevated neopterin to biopterin ratios in urine and plasma. Reduced PTS activity can be documented in red blood cells. In the classic form of the disorder, patients have reduced catecholamine and serotonin metabolites and an increased neopterin to biopterin level in CSF. These patients are usually picked up on newborn screening as phenylketonurics and show progressive signs of neurologic involvement in the first few months of life, including extrapyramidal signs, axial and truncal hypotonia, hypokinesia, feeding difficulties, choreoathetotic or dystonic limb movements, and autonomic symptoms.

Many of these patients, despite early diagnosis and supplementation with BH4 and neurotransmitter precursors, continue to manifest delays in development. A “peripheral” form of the disorder is characterized by normal central neurotransmitter levels and less significant or transient hyperphenylalaninemia. Patients with the peripheral form have an excellent prognosis for normal neurologic development, provided the hyperphenylalaninemia is corrected by diet or BH4 administration.

Dihydropteridine Reductase (DHPR) Deficiency

Dihydropteridine reductase (DHPR) deficiency manifests in a variety of phenotypes, all with hyperphenylalaninemia. The clinical presentation is similar to that observed with PTS deficiency. Without folinic acid to restore methyltetrahydrofolate status in the CNS, these patients can have progressive calcification of the basal ganglia and subcortical regions, despite treatment with BH4 and neurotransmitter precursors. (See references, 3) A juvenile variant has been reported in which siblings were developmentally normal until 6 years of age, at which time they developed progressive encephalopathy, epilepsy, and pyramidal, cerebellar, and extrapyramidal features on clinical examination.

Diagnosis can be confirmed by the pattern of urine pterins and documentation of abnormal DHPR activity in skin fibroblasts. Phenylalanine loading tests are abnormal, and phenylalanine status improves or normalizes with BH4 supplementation. CSF neurotransmitter and pterin analysis reveals reduced HVA, 5-HIAA, decreased or normal BH4 and elevated dihydrobiopterin levels.

Guanosine Triphosphate Cyclohydrolase (GTPCH) Deficiency (Autosomal Recessive)

Although most mutations in guanosine triphosphate (GTP) cyclohydrolase to date have been documented in association with autosomal dominant dopa-responsive dystonia, or Segawa’s disease, these patients do not have hyperphenylalaninemia on routine plasma screening studies. Patients with the autosomal recessive form of GTP cyclohydrolase deficiency, however, present in a similar fashion to patients with DHPR and PTS deficiency. Such patients have severe global developmental impairment, marked hypotonia of the trunk and axial muscles, eye movement abnormalities, limb hypertonia, convulsions and autonomic symptoms including temperature dysregulation, excessive diaphoresis and blood pressure lability caused by the associated catecholamine deficiency. They typically have absent GTP cyclohydrolase activity in blood cells, liver and skin fibroblasts.

By contrast, patients with autosomal dominant dopa-responsive dystonia have preservation of some GTP cyclohydrolase activity in liver because of their heterozygous status, enough to maintain normal phenylalanine levels under usual circumstances. CSF neurotransmitter metabolite analysis reveals low levels of HVA and 5-HIAA, and low neopterin and biopterin levels.

Role of BH4 in the Central Nervous System (CNS)

Because BH4 is required for the hydroxylation of aromatic amino acids, its importance in the central nervous system (CNS) becomes immediately apparent. Tyrosine and tryptophan are required for the synthesis of catecholamines and serotonin. A BH4-dependent process can be strongly suspected when normalization of plasma phenylalanine levels occurs following BH4 supplementation. A dose of 5 mg/kg BH4 is the usual recommended dose for correcting peripheral hyperphenylalaninemia. Because BH4 crosses the blood–brain barrier poorly, however, lifelong supplementation with the neurotransmitter precursors L-dopa and 5-HTP, along with carbidopa to enhance CNS delivery, is necessary in most of the disorders mentioned earlier. A much higher dose of BH4, approximately 20 mg/kg, can normalize CSF BH4 levels, but remains prohibitively expensive, and no studies exist as to the possible additional benefit of such a regimen. The higher requirement of tyrosine hydroxylase for BH4 in comparison with tryptophan hydroxylase may explain the more severe impairment in the catecholaminergic system compared with the serotonergic system in many cases.

Nitric oxide synthase is yet another enzyme with an absolute requirement for BH4 for the oxidation of arginine to nitric oxide. The inability to replete normal levels of BH4 in the CNS with oral administration in its currently used doses may be one reason many children with BH4 deficient disorders develop lifelong cognitive and developmental impairments despite other treatments. Nitric oxide also plays a critical role in CNS neuroprotective mechanisms, and reduced efficiency of this enzyme may result in additional ongoing neuronal injury, cell death and vascular dysregulation and injury.