Tetrahydrobiopterin Deficiency Workup
- Author: Noah S Scheinfeld, JD, MD, FAAD; Chief Editor: Luis O Rohena, MD more...
Pterins (eg, neopterin, monapterin, isoxanthopterin, biopterin, primapterin, pterin) are measured in urine. Typical urinary pterin profiles are detailed below.
In guanosine triphosphate (GTP) cyclohydrolase I (GTPCH) deficiency, neopterin and biopterin levels are low. In 6-pyruvoyl-tetrahydropterin synthase (PTPS) deficiency, the neopterin level is high and the biopterin level is low. In dihydropteridine reductase (DHPR) deficiency, the neopterin level is in the reference range or slightly increased, and the biopterin level is high. In carbinolamine-4a-dehydratase (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 more than 20 mg/dL, the test result is positive if the level decreases less than 10 mg/dL for 4 hours, even if it does not decrease to the reference range at 4-8 hours after loading. In classic phenylketonuria (PKU) due to Phe-4-hydroxylase (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 cerebrospinal fluid (CSF). Determine levels of folates (eg, 5-methyltetrahydrofolate [5MTHF]) in the CSF. Enzyme activity (ie, PTPS, GTPCH, DHPR, sepiapterin reductase [SR]) in RBCs, WBCs, or fibroblasts (FBs) can be measured. A Phe-loading test can be used in patients with dopa-responsive dystonia (DRD), which is also termed 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 monitored to assess the treatment and disease, the measurement of serum prolactin levels instead of CSF homovanillic acid (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 numerous patients with BH4 deficiencies.
Continued monitoring of serotonin and folate metabolism is performed by assessing 5HIAA and 5MTHF levels in the CSF.
Dhondt noted that since 1980, BH4 metabolism had been screened in 2,186 babies with hyperphenylalaninemia, using measurement of pteridines in urine to recognize BH4 synthesis deficiency (GTPCH and PTPS deficiency) and direct DHPR assay in dried blood samples to recognize DHPR deficiency. Seventy three babies with BH4 deficiency were identified. This screening demonstrated that tests on blood and urine collected on filter paper cards were convenient and simple to assemble and evaluate. In half of the babies with BH4 deficiency, the blood phenylalanine level was less than 10 mg/dL (0.6 mmol/L). A paper on a 5-year experience of diagnosis of tetrahydrobiopterin deficiency using filter paper blood spots examination has been published, showing it is a valuable technique in skilled hands.
In one study from Taiwan, MRI showed fewer white-matter changes but 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 scanning in 2 patients with DHPR demonstrated severe cortical and subcortical atrophy and bilateral corticomedullary and basal ganglial calcifications. These findings indicate that CT scanning has a role in monitoring such patients.
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