Characterisation of Parkinson’s Disease risk gene, GTP Cyclohydrolase I, in zebrafish

GTP Cyclohydrolase I (GCH1) is a known Parkinson’s Disease (PD) risk gene, identified in sporadic and familial PD cases. Additionally, mutations in GCH1 are a causative factor for the childhood-onset disorder dopa-responsive dystonia (DRD). GCH1 catalyses the rate-limiting step in the synthesis of tetrahydrobiopterin (BH4), which is an essential cofactor for the synthesis of dopamine and other catecholaminergic neurotransmitters. We hypothesised that GCH1 deficiency may lead to dopaminergic cell degeneration, and thus development of PD, as a direct result of metabolic dopamine deficiency. We generated a gch1 mutant zebrafish line (gch1-/-) to investigate this hypothesis, and to explore additional pathomechanisms by which GCH1 deficiency may contribute to PD risk. gch1-/- zebrafish develop neurotransmitter deficits by 5 dpf, a hypoactive motor phenotype by 8 dpf, and mortality by 12 dpf. Despite striking neurotransmitter deficits, dopaminergic neurons show no evidence of degeneration, however, protein levels of the dopamine-producing enzyme, tyrosine hydroxylase, are depleted. Gene expression analysis from larval brains revealed metabolic dysregulation, induction of matrix metalloproteinases, and inflammatory activation in gch1-/-. Morphological analysis and functional activity assays of microglial activation identified a heightened activation state of microglia in gch1-/-, indicative of a neuroinflammatory phenotype. we reject our hypothesis that dopamine deficiency directly predisposes to degeneration, as despite severe biochemical deficits we do not observe a reduction in the number of dopaminergic neurons in Gch1-deficient zebrafish; however, this project identifies 2 mechanisms – tyrosine hydroxylase (TH) depletion, and neuroinflammation - in Gch1-deficient larvae, which both represent potential pathological mechanisms which may lead to neurodegeneration. We propose that in cases of GCH1-deficiency, PD symptoms appear at an early subclinical stage of nigral degeneration as a result of dopamine deficiency, and this is exacerbated by depletion of TH protein.