iPSC‐based modeling of THD recapitulates disease phenotypes and reveals neuronal malformation

Tyrosine hydroxylase deficiency (THD) is a rare genetic disorder leading to dopaminergic depletion and early‐onset Parkinsonism. Affected children present with either a severe form that does not respond to L‐Dopa treatment (THD‐B) or a milder L‐Dopa responsive form (THD‐A). We generated induced pluripotent stem cells (iPSCs) from THD patients that were differentiated into dopaminergic neurons (DAn) and compared with control‐DAn from healthy individuals and gene‐corrected isogenic controls. Consistent with patients, THD iPSC‐DAn displayed lower levels of DA metabolites and reduced TH expression, when compared to controls. Moreover, THD iPSC‐DAn showed abnormal morphology, including reduced total neurite length and neurite arborization defects, which were not evident in DAn differentiated from control‐iPSC. Treatment of THD‐iPSC‐DAn with L‐Dopa rescued the neuronal defects and disease phenotype only in THDA‐DAn. Interestingly, L‐Dopa treatment at the stage of neuronal precursors could prevent the alterations in THDB‐iPSC‐DAn, thus suggesting the existence of a critical developmental window in THD. Our iPSC‐based model recapitulates THD disease phenotypes and response to treatment, representing a promising tool for investigating pathogenic mechanisms, drug screening, and personalized management. Synopsis This is the first proof of principle that iPSC‐derived dopaminergic (DA) neurons from mild (THDA) and severe (THDB) patients reproduce distinct disease‐related features and reveal abnormal morphologies. The study highlights the importance of prenatal treatment with L‐Dopa in cases of THDB. THDA and THDB‐iPSCs produce fewer DA neurons than control‐iPSCs, as judged by reduced numbers of TH+ cell and TH protein expression. THDA and THDB‐iPSC‐derived DA neurons exhibit low intracellular DA levels and its metabolites and reduced expression of other DA markers, indicating impaired DA neuron function. DA neurons generated from both THD patient‐specific iPSCs show altered neuronal morphology (fewer and shorter neurites) compared to healthy controls. THDA neuronal defects can be recovered with L‐Dopa treatment. L‐Dopa added at an early stage of neural differentiation leads to prevention of neuronal defects in THDB cells. Graphical Abstract This is the first proof of principle that iPSC‐derived dopaminergic (DA) neurons from mild (THDA) and severe (THDB) patients reproduce distinct disease‐related features and reveal abnormal morphologies. The study highlights the impo