Tetrahydrobiopterin (BH4) treatment stabilizes tyrosine hydroxylase: Rescue of tyrosine hydroxylase deficiency phenotypes in human neurons and in a knock-in mouse model

Proteostatic regulation of tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine biosynthesis, is crucial for maintaining proper brain neurotransmitter homeostasis. Variants of the TH gene are associated with tyrosine hydroxylase deficiency (THD), a rare disorder with a wide phenotypic spectrum and variable response to treatment, which affects protein stability and may lead to accelerated degradation, loss of TH function and catecholamine deficiency. In this study, we investigated the effects of the TH cofactor tetrahydrobiopterin (BH₄) on the stability of TH in isolated protein and in DAn- differentiated from iPSCs from a human healthy subject, as well as from THD patients with the R233H variant in homozygosity (THDA) and R328W and T399M variants in heterozygosity (THDB). We report an increase in TH and dopamine levels, and an increase in the number of TH+ cells in control and THDA cells. To translate this in vitro effect, we treated with BH₄ a knock-in THD mouse model with Th variant corresponding to R233H in patients. Importantly, treatment with BH₄ significantly improved motor function in these mice, as demonstrated by increased latency on the rotarod test and improved horizontal activity (catalepsy). In conclusion, our study demonstrates the stabilizing effects of BH₄ on TH protein levels and function in THD neurons and mice, rescuing disease phenotypes and improving motor outcomes. These findings highlight the therapeutic potential of BH₄ as a treatment option for THDA patients with specific variants and provide insights into the modulation of TH stability and its implications for THD management.