Objective: Characterize a new cellular model of Tyrosine Hydroxylase deficiency (THD) based on induced pluripotent stem cells (iPSCs) to better understand the pathophysiology and test different therapeutic approaches in vitro.

Background: THD is an inherited metabolic disorder caused by a defect in the TH enzyme, which catalyses the rate-limiting step in the biosynthesis of dopamine (DA). Two clinical phenotypes have been described: i) “Type A” which refers to a progressive hypokinetic-rigid syndrome and dystonia with an onset in infancy or childhood and L-Dopa responsiveness; ii)  “Type B” which produces a severe early-onset encephalopathy, mental retardation, oculogyric crises and parkinsonism with sub-optimal L-Dopa response.

Method: We established lines of induced pluripotent stem cells (iPSCs) from fibroblasts derived from one THD -Type A patient , one THD -Type B patient, two healthy young individuals and one isogenic control obtained by CRISPR/Cas9 technology.

Results: Upon DAn differentiation, neurons harbouring TH mutations (Types A and B) exhibited THD-related phenotypes such as: decreased tyrosine hydroxylase immunoreactivity, decreased TH protein expression, reduced levels of DA metabolites and altered expression levels of DA-related genes compared to control iPSC- derived neurons.
In addition, in this spontaneous THD human model, both type A and B cultures presented a reduction in the total neurite length. Moreover Type B showed an impairment in TH neuronal arborization whereas Type A TH+ expressing neurons had an impaired neuronal transport of TH.
L-Dopa + Carbidopa treatment in THD A derived neurons, normalized TH protein expression, DA metabolites levels and neuronal phenotypes. However, the treatment did not rescue neuronal deficits in THD B derived neurons, thus suggesting that early pathological events in THD B mutant neural cells may be crucial for the pathogenesis of the disease.

Conclusion: This human iPSC-based model mimics not only the phenotype observed in THD patients but also the response to the existent treatment, highlighting new possible molecular mechanisms of the disease that can disclose new opportunities for future preclinical studies.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/cellular-modeling-of-tyrosine-hydroxylase-deficiency-recapitulates-patient-phenotypes-and-response-to-treatment/