|
PWS Articles PWS Research
Other |
[ Printable Page | Edit ]
Research Notes: Tetrahydrobiopterin DeficiencySee also: Phenylketonuria (PKU) Tetrahydrobiopterin is a cofactor in the -
Links
From eMedicine Background Tetrahydrobiopterin (BH4) deficiencies are disorders that affect phenylalanine (Phe or F) homeostasis, as well as brain biosynthesis of catecholamines, serotonin, and (occasionally) nitric oxide. BH4 deficiencies are grouped with phenylketonuria (PKU), which is an inborn error of protein metabolism that results from an impaired ability to metabolize the essential amino acid Phe. Similar to PKU, BH4 deficiencies negatively affect developmental function; however, BH4 deficiencies also affect neurologic functioning, depending on the variant. Some, but not all, BH4 deficiencies may be detected with PKU screening tests used in Western countries, depending on the variant. BH4 deficiencies are heterogeneous. They range from mild forms that do not require treatment to severe cases that are difficult to ameliorate even with therapy. Pathophysiology Enzymatic reactions and defects BH4 deficiencies fall into 4 main categories, depending on the enzymatic defect that leads to a lack of BH4. Through September 2006, 193 different mutant alleles or molecular lesions identified in the guanosine triphosphate cyclohydrolase I (GCH1), 6-pyruvoyl-tetrahydropterin synthase (PTPS), sepiapterin reductase (SPR), carbinolamine-4a-dehydratase (PCD), or dihydropteridine reductase (DHPR) genes had been identified (Blau, 2006). The most well-established human function of BH4 is as the cofactor for Phe-4-hydroxylase (PAH), tyrosine-3-hydroxylase, and tryptophan-5-hydroxylase. The last 2 are key enzymes in biogenic amine biosynthesis, that is, aromatic amino acid synthesis. In addition to hydroxylating aromatic amino acids, BH4 serves as the cofactor for nitric oxide synthase and glyceryl-ether mono-oxygenase. These reactions are based on the ability of BH4 to react with molecular oxygen to form an active oxygen intermediate that can hydroxylate substrates. Although BH4 is absolutely essential for nitric oxide synthase activity, its exact function with different forms of the enzyme and its mechanism of action remain to be defined. BH4 is synthesized from guanosine triphosphate (GTP) in at least 4 enzymatic steps by the action of 3 enzymes. GTP cyclohydrolase I (GTPCH), the first enzyme in BH4 biosynthesis, catalyzes the formation of 7,8-dihydroneopterin triphosphate from GTP in a single reaction step. GTPCH is subject to feedback inhibition by BH4. A single-copy gene, GCH1, located on chromosome band 14q22.1-q22.2 encodes GTPCH. In the next step, 6-pyruvoyl-tetrahydropterin synthase (PTPS) catalyzes the conversion of 7,8-dihydroneopterin triphosphate to 6-pyruvoyl-tetrahydropterin. The PTS gene on chromosome band 11q22.3-q23.3 encodes PTPS. Sepiapterin reductase (SR) is a nicotinamide adenine dinucleotide phosphate (NADP), reduced form, (NADPH) oxidoreductase. It is required for the final 2-step reduction of the dike to intermediate 6-pyruvoyl-tetrahydropterin to BH4. The SPR gene on chromosome band 2p14-p12 encodes SR. During the enzymatic hydroxylation of aromatic amino acids, molecular oxygen is consumed and BH4 is peroxidated and oxidized. The pterin intermediate is subsequently reduced back to BH4 by 2 enzymes and a reduced pyridine nucleotide (ie, NADH) in a complex recycling reaction. Molecular oxygen is first bound to BH4 to form an unstable 4-alpha-peroxy-BH4. The mono-oxygenation of aromatic amino acids is thus concomitant with oxidation of BH4 to 4-alpha-hydroxy-BH4 (pterin-4-alpha-carbinolamine). Pterin-4-alpha-carbinolamine is subsequently dehydrated to quinonoid-dihydrobiopterin (q-dihydrobiopterin) and water by the specific and highly efficient pterin-4-alpha-carbinolamine dehydratase (PCD). The PCBD gene on chromosome band 10q22 encodes PCD. In the last step of BH4 recycling, q-dihydrobiopterin is reduced back to BH4 by the NADH-dependent dihydropteridine reductase (DHPR). Folate inhibitors, such as methotrexate, inhibit the activity of the enzyme both in vivo and in vitro. The QHPR gene on chromosome band 4p15.3 encodes DHPR. Genetic factors BH4 deficiency comprises heterogeneous autosomal recessive disorders caused by mutations in the PTS (most common), SPR, QHPR, and GCH1 genes. Defects in the SPR gene cause neurotransmitter deficiency without hyperphenylalaninemia (HPA); defects in the GTPCH gene (ie, GCH1) may also cause autosomal dominant dopa-responsive dystonia (DRD). BH4 deficiency without HPA occurs in 6,10-methylenetetrahydrofolate reductase deficiency and vitiligo or DRD. Fiori et al (2005) noted that HPA is an inherited metabolic disorder due to deficiency of the enzyme PAH or its cofactor BH4. BH4-responsiveness in PAH-deficient HPA is a recently described characteristic of most mild phenotypes. BH4-responsive patients have reduced plasma Phe levels after the oral administration of BH4. The investigators determined the incidence of BH4-responsiveness among a nonselected cohort of patients with PAH-deficient hyperphenylalaninemia and evaluated the phenotype-genotype correlations. They evaluated 11 patients born in Lombardy, Italy, between January 2000 and December 2004. HPA (107 patients) was classified after BH4-loading test, an analysis of urinary pterin levels, and a determination of DHPR activity in the blood. The researchers assessed the patients for BH4-responsiveness. Patients were given 6R-BH4 20mg/kg orally as a single dose, and plasma samples were obtained at 0, 4, 8, and 24 hours after administration. In patients with basal plasma Phe levels of 360 mmol/L, a combined Phe (100 mg Phe/kg) and BH4 (20 mg/kg) loading test was performed. Patients were considered responsive to BH4 when their plasma Phe levels decreased by 30% at 8 hours after oral administration of BH4. The investigators found that BH4 significantly lowered blood Phe levels in 91 (85%) of 107 patients with PAH-deficient HPA. Most of the responsive patients had mild HPA (77%), though some had mild (7%) or moderate (7%) PKU. One patient with classical PKU was responsive to BH BH4. Eighteen mutations were associated to the BH4-responsive phenotype. The authors concluded that a consistent number of patients with PAH-deficient hyperphenylalaninemia responded to BH4 and that BH4 responsiveness seemed to be common in mild phenotypes. Genotype was not the only factor that determined BH BH4 responsiveness. Frequency United States - The incidence of classic PKU is approximately 1 case in 15,000 births. The incidence of BH4 deficiency is approximately 1 case per 1 million births, or 1.5-2% of cases of PKU. International - In Taiwan, 2-30% of cases of PKU are attributed to BH4 deficiency. In Turkey, which has the highest incidence of PKU in the world with approximately 1 case per 2600 births, 15% of cases are due to BH4 deficiency. In Saudi Arabia, 66% of PKU cases are due to BH4 deficiency. The incidence also appears to be increased in southern Brazil. Such increased incidences are thought to be related to consanguinity. Pangkanon et al reported the first 2 cases of PTPS deficiency in Thailand. Both cases were males with phenylalanine levels of 25.23 mg/dL and 23.4 mg/dL, respectively. The urinary pterins analysis showed low biopterin levels, low percentages of urinary biopterin, and high neopterin levels. The mutation analysis of the patient with a phenylalanine level of 25.23 mg/dL revealed a point mutation in exon 4 and a homozygous C-to-T transition at nucleotide 200 in codon 67 (T67M). The other patient demonstrated a compound heterozygous in exon 4, C-to-A transition at nucleotide 200 and exon 5, and C-to-T transition at nucleotide 259 of the PTS gene, confirming PTPS deficiency. Mortality/Morbidity Patients with severe of BH4 deficiency present with mental retardation and neurologic impairment. Early death may result. Patients with mild cases can have mild degrees of mental retardation and neurologic impairment. Race - Children of Chinese, Turkish, and Saudi Arabian descent are most often affected. Sex - No sex predilection is reported. The mode of inheritance is autosomal recessive. Age BH4 deficiency is most commonly diagnosed in newborns by means of newborn screening programs. Consider BH4 deficiency in patients of any age who have PKU and developmental delay or mental retardation with neurologic impairment. Newborn screening does not always detect the disease. Patients who are symptomatic usually present by the age of 4 months. Clinical History Most neonates with tetrahydrobiopterin (BH4) deficiencies appear healthy at birth. In severe PTPS deficiency, the incidence of prematurity and low birth weight is increased. In severe PTPS deficiency, lead-pipe or cogwheel rigidity and stiff-baby syndrome have been reported. BH4 deficiency converts neuronal nitric oxide synthases (NOSs) into an efficient peroxynitrite synthase, which is responsible for the increase in neuronal vulnerability to hypoxia-induced mitochondrial damage and necrosis. Endothelial BH4 availability is essential for maintaining pulmonary vascular homeostasis, and it is a critical mediator in the pathogenesis of pulmonary hypertension. It is also a novel therapeutic target. BH4 restores utilization of the flow reserve in the coronary microcirculation in subjects with hypercholesterolemia. This finding suggests that BH4 deficiency may contribute to dysfunction of the coronary microcirculatory n in hypercholesterolemia. Physical At birth, neonates with BH4 deficiencies often appear healthy. Pigmentary dilution can be noted. Physical findings manifest as the neonate matures. Patients may have red hair. They may have poor suckling, decreased spontaneous movements, and a floppy-baby appearance. If the disease is not detected on newborn screening, affected children develop progressive developmental delay and neurologic impairment that manifests as psychomotor retardation, progressive neurologic deterioration, convulsions, abnormal movements, hypersalivation, and swallowing difficulties. ... Workup Lab Studies Pterins (eg, neopterin, monapterin, isoxanthopterin, biopterin, primapterin, pterin) are measured in urine. Typical urinary pterin profiles are as follows: In GTPCH deficiency, neopterin and biopterin levels are low. In PTPS deficiency, the neopterin level is high and the biopterin level is low. In DHPR deficiency, the neopterin level is in the reference range or slightly increased, and the biopterin level is high. In PCD deficiency, the neopterin level is initially high, the biopterin level is in the subnormal range, and a primapterin level (7-substituted biopterin) is present. DHPR activity in RBCs can be measured on Guthrie card. In a loading test with tetrahydrobiopterin (BH4), the blood Phe level is lowered to the reference range value (e2 mg/dL) 4-8 hours after an oral loading dose of BH4 is given. When the preload blood Phe level is >20 mg/dL, the test result is positive if the level decreases <10 mg/dL for 4 hours, even if it does not decrease to the reference range at 4-8 hours after loading. In classic PKU (due to PAH deficiency), the change in blood Phe is minimal. Combined Phe and BH4 loading is performed. Determine levels of neurotransmitter metabolites (eg, 5-hydroxyindoleacetic acid [5HIAA], homovanillic acid [HVA]) and pterins in CSF. Determine levels of folates (eg, 5-methyltetrahydrofolate [5MTHF]) in the CSF. Enzyme activity (ie, PTPS, GTPCH, DHPR, SR) in RBCs, WBCs, or fibroblasts (FBs) can be measured. A Phe-loading test can be used in patients with DRD (Segawa disease). DNA analysis can be used to look for mutations in the affected genes. In DHPR, prolactin levels may be elevated, and they can be evaluated to monitor therapy. Consider investigating the presence of deficiencies in iron, vitamins, selenium, protein, essential fatty acids, and other nutrients that have been reported in treated PKU. However, investigating these deficiencies is not part of the standard evaluation of BH4 deficiencies. When dopamine levels are monitor to assess the treatment and disease, the measurement of serum prolactin levels instead of CSF HVA levels is recommended. Because dopamine inhibits the secretion of prolactin, the serum prolactin concentration reflects the cerebral production of dopamine and functions as a useful indicator of dopamine creation and content in the hypothalamus. Hyperprolactinemia has been documented in a number of patients with BH4 deficiencies. Continued monitoring of serotonin and folate metabolism is performed by assessing 5HIAA and 5MTHF levels in the CSF. Imaging Studies In 1 study from Taiwan, MRI showed fewer white-matter changes but magnetic resonance (MR) spectroscopy showed more in white-matter changes patients with BH4 deficiency than in patients with classic PKU. MR spectroscopy may be useful for monitoring dosages of supplements used to treat this disorder. In addition, MR spectroscopy may be helpful in understanding the neurophysiologic changes that occur in association with this disease. In a study from Turkey, cranial CT in 2 patients with DHPR demonstrated severe cortical and subcortical atrophy and bilateral corticomedullary and basal ganglial calcifications. These findings indicate that CT has a role in monitoring such patients. Procedures In some cases, gene therapy has been used, with a possible effect (Mikami, 1990; Thony, 1996; Laufs, 1998; Laufs, 2000). Gene therapy is not widely used, and its use is purely experimental. Treatment Medical Care Most patients are treated in a specialty metabolic clinic, usually under the direction of a geneticist or a pediatric endocrinologist. Treatment of tetrahydrobiopterin (BH4) deficiencies consists of BH4 supplementation or dietary changes to control blood Phe concentration and replacement therapy with neurotransmitter precursors (eg, levodopa [LD] and carbidopa, 5-hydroxytryptophan [5HT]). In DHPR deficiency, folinic acid is supplemented. Depending on the variant, levels of the relevant enzymes are checked. In DHPR, some positive reports have documented the use of monoamine oxidase (MAO) B inhibitor. Treatment is determined on the basis of enzyme-defect phenotype, as follows: GTPCH (severe) - LD, 5HT, BH4 PTPS (severe) - LD, 5HT, BH4 PTPS (mild) - BH4 PTPS (transient) - BH4 in the neonatal period DHPR (severe) - LD, 5HT, low-Phe diet, folinic acid DHPR (mild) - Low-Phe diet PCD (transient) - BH4 in the neonatal period Consultations A psychologist should perform developmental testing at regular intervals. Whenever possible, the patient and his or her parents should work with a nutritionist and a geneticist experienced in BH4 deficiency. Diet - Treatment of BH4 deficiencies consists of BH4 supplementation (2-20 mg/kg/d) or diet to control blood Phe and, in DHPR deficiency, supplements of folinic acid (10-20 mg/d). Activity - BH4 deficiencies are heterogeneous. They range from mild forms that require only marginal, if any, treatment to severe forms that are sometimes difficult to treat. In many cases, normal activity can be expected if the patient adheres to treatment. Medication Treatment of tetrahydrobiopterin (BH4) deficiencies consists of BH4 supplementation or diet to control blood Phe and supplements of folinic acid (10-20 mg/d) in DHPR deficiency. Drug Category: Pteridines These replace the missing essential cofactor in the enzymatic hydroxylation of the 3 aromatic amino acids. For information on how to obtain BH4, consult the Tetrahydrobiopterin Web site. Drug Name Tetrahydrobiopterin (BH4) These are used to supply necessary catecholamine replacement in the neurotransmitter pathway. Drug Name Levodopa and carbidopa (Sinemet) Drug Name 5-Hydroxytryptophan (5-HTP) Drug Category: Vitamins These increase levels of factors necessary in the amino acid pathways. Drug Name Leucovorin (Wellcovorin) When high doses of neurotransmitters are necessary, the concurrent use of selective MAO B inhibitors is recommended because such use reduces the required dosage of administered precursors. Drug Name Selegiline (Eldepryl) Follow-up Deterrence/Prevention: Avoid substances containing aspartame. Avoid drugs that effect folate metabolism such as methotrexate and trimethoprim-sulfamethoxazole. Prognosis: The prognosis for normal intelligence is good with dietary and medical treatment. Nontreatment and treatment failure are associated with neurologic and cognitive dysfunction. Treatment is not always successful. Patient Education: Teach parents how to administer the diet, medications, and supplements at home, and involve all caregivers. Children should begin involvement in their dietary and medical planning as soon as they are developmentally ready. Medical/Legal Pitfalls
Special Concerns During pregnancy, levels of pterins can be evaluated in amniotic fluid and in other maternal material to determine if the fetus has a tetrahydrobiopterin (BH4) deficiency. Such tests are usually performed only in women who have had children with BH4 deficiency. |