SYNAPTIC MECHANISMS UNDERLYING RIMBP1-LINKED DYSTONIA

Dystonia is a disabling hyperkinetic movement disorder characterized by involuntary muscle contractions that lead to abnormal postures and repetitive movements. Although extensively studied, its pathophysiological mechanisms remain poorly understood. Given the absence of structural brain lesions in most patients, disrupted synaptic function has long been considered a key contributor to dystonia pathogenesis. We previously identified bi-allelic frameshift, nonsense, and missense variants in TSPOAP1, which encodes the presynaptic active zone RIM-binding protein 1 (RIMBP1), as a novel genetic cause of autosomal recessive dystonia (Mencacci et al., JCI, 2021). Here, we systematically investigate how these pathogenic variants disrupt synaptic transmission to cause dystonia and explore whether the resulting defects can be corrected using FDA-approved drugs. We reprogrammed fibroblasts from patients carrying bi-allelic TSPOAP1 variants into induced pluripotent stem cells (iPSCs) and used CRISPR/Cas9 homology-directed repair to generate isogenic control lines. We then differentiated isogenic mutant and control iPSCs into functional induced human neurons and assessed their morphology and function using confocal and stimulated emission depletion (STED) microscopy, electron microscopy, micro-electrode array technology, and patch-clamp electrophysiology. Our data show that pathogenic variants in RIMBP1 impair neural network activity and alter the abundance and distribution of synaptic puncta along dendrites. Overall, we uncover convergent presynaptic abnormalities shared across dystonia-causing TSPOAP1 variants, which may be corrected through genetic or pharmacological approaches currently under investigation.