Overview

The neurotransmitter deficiency in infants in this group arises as a result of defects in BH₄ metabolism (Figure 39-1). 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 co-factor required for the synthesis of catecholamines and serotonin. Although most academic biochemical genetics clinics that monitor children with phenylketonuria systematically perform the additional studies required to diagnose this group of disorders, children occasionally 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 afflicted infants are overlooked because screening is performed before an adequate interval of protein intake, resulting in a false-negative result on newborn testing.

Fig. 39-1 Synthesis and catabolism of catecholamine and indoleamine neurotransmitters.

AADC, aromatic l-amino acid decarboxylase; B₆, pyridoxine; COMT, catechol O-methyltransferase; DBH, dopamine β-hydroxylase; DHPG, dihydrophenyl glycine; DHPR, dihydropteridine reductase; DOPAC, dihydroxyphenylacetic acid; EPI, epinephrine; 5-HIAA, 5-hydroxyindoleacetic acid; 5-HTP, 5-hydroxytryptophan; GTPCH, guanosine triphosphate cyclohydrolase; HVA, homovanillic acid; MAO, monoamine oxidase; MHPG, methoxy 4 hydroxyphenylglycol; NE, neoepinephrine; NOS, nitric oxide synthase; PAH, phenylalanine hydroxylase; PCD, pterin-4α-carbinolamine dehydratase; PTS, 6-pyruvoyltetrahydropterin synthase; qBH2, quinonoid dihydropterin; SPR, sepiapterin reductase; TH, tyrosine hydroxylase; TPH, tryptophan hydroxylase; VMA, vanillylmandelic acid.

Approximately 1–3 percent of patients with hyperphenylalaninemia have an associated BH₄ deficiency state. 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 hyperphenylalaninemia in the newborn period are 6-pyruvoyltetrahydropterin synthase deficiency, accounting for 60 percent of mutations in atypical phenylketonurics, and dihydropteridine reductase deficiency, affecting 30 percent of cases [Longo, 2009]. 6-Pyruvoyltetrahydropterin synthase deficiency results in inadequate BH₄ synthesis; dihydropteridine reductase deficiency results in decreased regeneration of BH₄ from dihydrobiopterin (see Figure 39-1). 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 guanosine triphosphate cyclohydrolase (GTPCH) deficiency and primapterinuria (Table 39-2).

Table 39-2 Metabolite Patterns Observed in Urine, Plasma, and Cerebrospinal Fluid in the Inherited Disorders Affecting Dopamine and Serotonin Metabolism

Role of BH₄ in the Central Nervous System

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 plasma phenylalanine levels return to normal after BH₄ supplementation. A dose of 5–10 mg/kg of BH₄ is the usual recommended dose for correction of peripheral hyperphenylalaninemia, and 10–20 mg/kg per day has been recently advocated in classic phenylketonuria. Because BH₄ crosses the blood–brain barrier poorly, lifelong supplementation with the neurotransmitter precursors l-DOPA and 5-hydroxytryptophan, along with carbidopa to prevent peripheral decarboxylation, is necessary in most of the disorders mentioned earlier. A much higher dose of BH₄, approximately 20 mg/kg, can return BH₄ levels in the cerebrospinal fluid to normal, but it remains prohibitively expensive, and no studies exist as to the possible additional benefit of such a regimen. Dosing can be monitored by assessing clinical response and periodically measuring cerebrospinal fluid levels of neurotransmitter metabolites [Hyland, 2007]. Verifying normalization of serum prolactin levels has been advocated, but sensitivity is limited, as is detection of overadministration of precursors [Concolino et al., 2008]. The greater 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.

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 that many children with BH₄-deficient disorders develop lifelong cognitive and developmental impairments despite other treatments. Nitric oxide also plays a critical role in CNS injury and neuroprotective mechanisms, and reduced efficiency of this enzyme may result in an imbalance of neuronal injury and vascular dysregulation and repair.

6-Pyruvoyltetrahydropterin Synthase Deficiency

6-Pyruvoyltetrahydropterin synthase catalyzes the elimination of inorganic triphosphate from dihydroneopterin triphosphate to form 6-pyruvoyltetrahydropterin. Patients have elevated neopterin to biopterin ratios in urine and plasma. Reduced 6-pyruvoyltetrahydropterin synthase 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 cerebrospinal fluid. Patients are usually detected by newborn screening, as with phenylketonurics. They exhibit 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 delay in development [Dudesek et al., 2001]. A “peripheral” form of the disorder is associated with nearly one-third of known mutations with the human PTS gene [Thony and Blau, 2006], and is characterized by normal central neurotransmitter levels and less significant or transient hyperphenylalaninemia [Niederwieser et al., 1987]. 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 Deficiency

Dihydropteridine reductase deficiency manifests in a variety of phenotypes, all with hyperphenylalaninemia. The clinical presentation is similar to that of central 6-pyruvoyltetrahydropterin synthase 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 [Woody et al., 1989]. 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 [Larnaout et al., 1998]. Diagnosis can be confirmed by the pattern of urine pterins and documentation of abnormal dihydropteridine reductase activity in skin fibroblasts [Milstien et al., 1980]. Results of phenylalanine loading tests are abnormal, and phenylalanine status improves or returns to normal with BH₄ supplementation. Cerebrospinal fluid neurotransmitter and pterin analysis reveals reduced concentrations of homovanillic acid and 5-hydroxyindoleacetic acid, decreased or normal BH₄ levels, and elevated dihydrobiopterin levels.

Autosomal-Recessive Guanosine Triphosphate Cyclohydrolase Deficiency

Although most mutations in GTPCH 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 GTPCH deficiency, however, present in a fashion similar to patients with dihydropteridine reductase deficiency and 6-pyruvoyltetrahydropterin synthase deficiency. Such patients have severe global developmental impairment, marked hypotonia of the trunk and axial muscles, eye movement abnormalities, limb hypertonia, and convulsions; autonomic symptoms include temperature dysregulation, excessive diaphoresis, and blood pressure lability caused by the associated catecholamine deficiency. They typically have absent GTPCH activity in blood cells, liver, and skin fibroblasts. By contrast, patients with autosomal-dominant dopa-responsive dystonia have preservation of some GTPCH activity in liver because of their heterozygous status, enough to maintain normal phenylalanine levels under usual circumstances. Cerebrospinal fluid neurotransmitter metabolite analysis reveals low concentrations of homovanillic acid and 5-hydroxyindoleacetic acid, and low neopterin and biopterin levels.

Pterin-4α-Carbinolamine Dehydratase Deficiency (Primapterinuria)

Pterin-4α-carbinolamine dehydratase deficiency, or primapterinuria, is a cause of mild hyperphenylalaninemia [Blau et al., 1988; Adler et al., 1992]. These infants are usually identified on newborn screening but generally have a benign course with normal development. Phenylalanine hydroxylase catalyzes the conversion of phenylalanine to tyrosine, during which BH₄ is converted to the unstable carbinolamine 4α-hydroxytetrahydrobiopterin [Citron et al., 1993]. Carbinolamine dehydratase catalyzes the dehydration of carbinolamine to quinonoid dihydropterin (qBH2). The decreased rate of dehydration is responsible for the production of 7-biopterin in some mildly hyperphenylalaninemic individuals [Thony et al., 1998]. Urine studies reveal an excess of 7-substituted pterins, reduced biopterin levels, and an increased neopterin to biopterin ratio.

Overview

Neurotransmitter deficiency disorders not associated with hyperphenylalaninemia span an increasingly complex spectrum of clinical phenotypes, ranging from ataxia and mental retardation to spastic diplegia to exercise-induced dystonia. The lack of ascertainment by way of newborn screening and increasingly broad phenotypic spectrum make these disorders as a group much more challenging to recognize. Other than autosomal-dominant dopa-responsive dystonia, the remaining disorders in this category are all inherited in an autosomal-recessive fashion with the exception of monoamine oxidase A deficiency, a rare X-linked recessive disorder. In general, heterozygous carriers for mutations in this group do not have a discernible phenotype, with rare exceptions in tyrosine hydroxylase deficiency and combined monoamine oxidase A and B deficiency.

Segawa’s Disease or Autosomal-Dominant Dopa-Responsive Dystonia

The best-described and most widely identified entity among this group of disorders is autosomal-dominant dopa-responsive dystonia caused by GTPCH deficiency, or Segawa’s disease [Segawa et al., 1976]. Identification and treatment of this disorder can be extremely rewarding because patients often benefit greatly from directed treatment of the associated dopamine deficiency state [Harwood et al., 1994]. Patients with a classic presentation of exercise-induced dystonia are not difficult to recognize. This diagnosis should also be considered in patients with spastic diplegia, especially when significant fluctuation in gait or worsening gait at the end of the day is noted, and in patients with more atypical presentations, including writer’s cramp, asymmetric limb dystonia, tremor, or restless leg-type symptoms. In patients with a classic presentation, many clinicians can make the diagnosis on a presumptive basis, after observing remission of symptoms with a trial of l-DOPA/carbidopa.

As of 2009 there are more than 85 known mutations of the GCH-1 gene, and in a large review of dopa-responsive dystonia, it was found that 60 percent are point mutations [Clot et al., 2009]. Although inheritance is autosomal-dominant, penetrance is incomplete, and variable expressivity among family members with the same mutation is well documented [Steinberger et al., 1998]. For instance, one might see spastic diplegia, writer’s cramp, restless leg syndrome, and more typical exercise-induced dystonia phenotypes among different members of the same family. The female to male ratio in sporadic cases is 4:1, and investigators have confirmed increased penetrance of GTPCH-I gene mutations in females [Ichinose et al., 1994; Furukawa et al., 1998]. Of note, a review of 47 patients with GCH-1 mutations or deletions found 40 patients manifested dystonic symptoms in the first decade of life [Clot et al., 2009].

Not surprisingly, mood and sleep disorders appear to be unusually prevalent in families with the disorder. Recent study of the neurologic and psychiatric phenotype of affected individuals within three extended families showed a higher frequency of major depressive disorder and obsessive-compulsive disorder, as well as disrupted sleep in 55 percent of patients [Van Hove et al., 2006]. These data help to support the idea that the disease carries an increased burden of attentional difficulties, anxiety, dysphoria, depression, and sleep disorders.

Cerebrospinal fluid neurotransmitter metabolite and pterin studies are helpful in confirming the diagnosis in these patients and can help characterize the degree of associated dopamine and serotonin deficiency. Phenylalanine loading is also valuable in confirming a suspected diagnosis but is not specific to the disorder, as phenylketonuria heterozygotes also manifest delayed phenylalanine clearance. GTPCH activity can be measured directly in skin fibroblasts. Urine pterin analysis serves as the first step in evaluation, with urine biopterin levels being low in autosomal-dominant GTPCH deficiency. Cerebrospinal fluid analysis should be performed before institution of a treatment trial of l-DOPA/carbidopa because treatment results in increased levels of homovanillic acid and 3-O-methyldopa. The typical pattern in cerebrospinal fluid in dopa-responsive dystonia resulting from GTPCH deficiency is a low homovanillic acid level, normal or low 5-hydroxyindoleacetic acid level, and reduced BH₄ level. Patients who are heterozygous for a GTPCH mutation, despite their normal blood phenylalanine levels on routine screening, have abnormal phenylalanine metabolism if they are stressed by administration of an oral phenylalanine load. If cytokine-stimulated fibroblast enzyme analysis of biopterin metabolism is not feasible or patients decline a cerebrospinal fluid examination and have an otherwise typical presentation, an oral phenylalanine loading test may be helpful in supporting the diagnosis. Phenylalanine is administered orally to the fasting patient at a dose of 100 mg/kg. Serum phenylalanine and tyrosine levels are obtained at baseline and 1, 2, and 4 hours after loading. An elevated phenylalanine to tyrosine ratio and delayed clearance of phenylalanine are typical in patients with GTPCH deficiency because BH₄ levels are typically reduced. Gene sequencing is available for confirmation of diagnosis, but has not yet supplanted biochemical studies as the diagnostic standard.

Treatment with l-DOPA/carbidopa leads to significant benefit or resolution of motor symptoms in the majority of patients with Segawa’s disease within a few weeks. However, compound heterozygotes or patients with long-standing or more severe motor manifestations, such as parkinsonism or long-standing spastic paraparesis, may require more careful titration of dosing, with gradual adjustment during a period of several months. Mood manifestations, such as depression and anxiety, often respond to l-DOPA treatment to some degree, but some patients have additional benefit from directed treatment of their associated serotonin deficiency, either with the serotonin precursor 5-hydroxytryptophan or with a serotonin reuptake inhibitor. Anecdotal evidence suggests that anticholinergic agents may also help ameliorate postural dystonia and tremor [Segawa et al., 1976].

Aromatic l-Amino Acid Decarboxylase or Dopa-Decarboxylase Deficiency

Aromatic l-amino acid decarboxylase is a pyridoxine-dependent enzyme that decarboxylates l-DOPA and 5-hydroxytryptophan to make dopamine and serotonin, respectively. Patients with this disorder typically present in the first few months of life with dystonia or intermittent limb spasticity, axial and truncal hypotonia, oculogyric crises, autonomic symptoms, and ptosis [Hyland et al., 1992]. Neonatal symptoms, including poor suck and feeding difficulties, ptosis, lethargy, and hypothermia, are common. Neurologic signs and symptoms are clearly evident within the first few months of life in all patients described to date. These patients demonstrate multisystemic involvement with a wide array of neurologic difficulties, including problems with sleep, attention, emotional regulation, and cognitive function that extend well beyond their motor difficulties. As they get older, gross motor delays with fluctuating tone, ataxia, and expressive speech impairment are prominent features, even in the patients with the best outcomes.

The phenomenology of the movement disorder is remarkably similar among the cases and, not surprisingly, shares a number of features in common with children with BH₄ deficiency disorders, such as 6-pyruvoyltetrahydropterin synthase deficiency and dihydropteridine reductase deficiency, and the autosomal-recessive form of GTPCH deficiency. Intermittent oculogyric crises and limb dystonia, generalized athetosis, and an overall paucity of voluntary movement become evident between 1 and 6 months of age. Tongue thrusting, ocular convergence spasm (Figure 39-2), myoclonic jerks, and episodes of sudden loss of head control or episodes resembling flexor spasms are common and frequently lead to a clinical diagnosis of epilepsy. Oculogyric crises, orofacial dystonia, torticollis, limb tremor with attempted voluntary movement, and blepharospasm are often the most compelling evidence supporting a defect in dopaminergic transmission. Breath-holding or apneic spells, paroxysmal sweating, nasal congestion, sudden respiratory or cardiorespiratory arrest, unresponsiveness associated with hypoglycemia, intermittent hypothermia, and feeding and gastrointestinal issues are manifestations of the often profound autonomic dysfunction these patients demonstrate [Swoboda et al., 2003]. First- and second-degree relatives of affected individuals have been shown to have a high incidence of psychiatric disorders, including depression, psychosis, suicide, or suicide attempts [Manegold et al., 2009].

Fig. 39-2 Episodic neurologic manifestations in aromatic l-amino acid decarboxylase deficiency.

A, Ocular convergence spasm, ptosis, and orofacial dystonia. B, Torticollis and limb dystonia. C, Torticollis and limb rigidity.

Cerebrospinal fluid neurotransmitter metabolites demonstrate a characteristic pattern: low homovanillic acid and 5-hydroxyindoleacetic acid levels; markedly elevated 3-O-methyldopa, 5-hydroxytryptophan, and l-DOPA; and normal biopterin and neopterin levels. Plasma l-DOPA is markedly elevated. Urine catecholamines may be reduced or elevated, specifically with vanillactic acid, despite normal preliminary organic acid results [Manegold et al., 2009; Abeling et al., 1998, 2000].

Although some children benefit in terms of the underlying movement disorder, treatment is complex, and these patients are vulnerable to an array of medication-related side effects. Instead of replacement of neurotransmitter precursors, as in the BH₄ deficiency-related disorders, the use of neurotransmitter receptor agonists or strategies to hinder reuptake or metabolism of endogenously produced neurotransmitters is necessary. Reported benefit has been noted in a subset of patients with monoamine oxidase inhibitors, dopamine receptor agonists, anticholinergic agents, pyridoxine, and, in rare cases associated with a defect in the gene encoding aromatic l-amino acid decarboxylase at the dopa-binding site, l-DOPA [Swoboda et al., 1999]. Dopamine receptor antagonists, such as clozapine, have been proposed as a new therapeutic option, given their increase in aromatic l-amino acid decarboxylase activity in a mouse model [Allen et al., 2009].

However, in spite of a variety of treatment interventions directed at ameliorating the effects of the associated neurotransmitter deficiency state, overall clinical outcomes in aromatic l-amino acid decarboxylase deficiency remain poor. All patients have had some degree of cognitive impairment, and there is increasing evidence of a gender-dependent discrepancy in severity, with females demonstrating the most severe phenotypes [Pons et al., 2004].