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Abstract

Recycling of the cofactor tetrahydrobiopterin (BH4) via dihydropteridine reductase (DHPR) is required for the synthesis of tyrosine from phenylalanine, as well as dopamine and serotonin precursors, L-Dopa and 5-HT, respectively. Patients with DHPR deficiency have severe neurological symptoms, including brain atrophy, dystonia and epilepsy. The pathophysiology remains mostly unknown. In order to model the disorder to better understand its pathophysiology we characterized all three homologs of the zebrafish (Danio rerio), Qdpra, Qdprb1 and Qdprb2 in the developing embryo. We analyzed the genes’ temporal expression during development using whole mount in situ hybridization and qRT-PCR and investigated their functional relevance using morpholino mediated knockdown. We further used a diagnostic approach to examine amino acid levels in both wildtype and morphant embryos. We were able to identify the homolog Qdpra to be involved in the production of melanin in the early embryo, likely due to its conserved BH4 recycling function. Morphants showed hyperphenylalaninemia and significantly depleted melanin levels. We could show that qdpra is co-expressed with the BH4 dependent enzyme Phenylalanine hydroxylase and is active in the liver post 3 days of development. Qdprb2 was found to be not required, nor expressed in the early embryo. qdprb1 was detected in the early proliferative regions of the eye and midbrain. We could show that Qdprb1 influences glial development, while neuronal development was unaffected. Intriguingly, Qdprb1 inhibits glutamine production. We discovered that similar to Qdprb1 morphant embryos exposed to high levels of glutamine, exhibit a depletion of astroglial markers in eye and midbrain. We could not detect any involvement in BH4 and therefore conclude glia differentiation by regulating glutamine as a novel function. Although the underlying mechanisms remain to be explored, we were able to observe high glutamine levels in a severe DHPR patient, just as we observed in Qdprb1 morphants. Our study is the first to give insights into the pathophysiology of DHPR deficiency, by modeling the disorder in zebrafish and unraveling a novel role for the homolog Qdprb1 in regulating gliogenesis.