Tetrahydrobiopterin biosynthesis defects examined in cytokine‐stimulated fibroblasts

Incubation of primary skin fibroblast cultures with the cytokines interferon-gamma and tumour necrosis factor-alpha stimulates the de novo pathway of tetrahydrobiopterin biosynthesis. Fibroblasts from patients with the two most common types of genetic defects of tetrahydrobiopterin metabolism that cause hyperphenylalaninaemia show characteristic pterin responses predicted by the nature of the defect. Cells from a child with 6-pyruvoyl-tetrahydropterin synthase deficiency produce higher than normal levels of neopterin and no biopterin. Fibroblasts from dihydropteridine reductase-deficient children produce normal levels of tetrahydrobiopterin, which gradually becomes partially oxidized, and higher than normal levels of neopterin. As a model for cells with the rarest form of tetrahydrobiopterin deficiency, lack of GTP cyclohydrolase activity, normal fibroblasts were treated with 2,4-diamino-6-hydroxypyrimidine to inhibit GTP cyclohydrolase activity by > 90%, the level expected in patients with a GTP cyclohydrolase deficiency. Neopterin and biopterin synthesis rates of < 10% of normal levels were readily detectable. Therefore, analysis of the patterns of the pterins synthesized by fibroblasts can aid in the diagnosis of the hyperphenylalaninaemias caused by disorders of tetrahydrobiopterin metabolism.