A hypofunctional PREX1 variant (p.Y191C) leads to neurodevelopmental abnormalities and epilepsy by attenuating RAC1...

The RAC subfamily of Rho family small GTPases plays a crucial role in neurodevelopment by regulating cellular signaling pathways and the actin cytoskeleton. The activity of RAC is controlled by guanine nucleotide exchange factors (GEFs), which facilitate the transition from an inactive GDP-bound form to an active GTP-bound form. A novel de novo variant, NM_020820.4:c.572 A > G p.(Y191C), was identified in PREX1, which encodes a RAC-specific GEF, by whole-exome sequencing of a patient with epilepsy. Biochemical analysis using recombinant proteins demonstrated that the p.Y191C variant reduced the GDP/GTP exchange activity of PREX1 toward RAC1 and attenuated RAC1-PAK1 signaling compared with wild-type PREX1, suggesting that the p.Y191C variant is hypofunctional. To explore its neurodevelopmental consequences in vivo, we performed in utero electroporation-mediated RNA interference targeting PREX1 in cerebrocortical progenitor cells in mice at embryonic day 14 (E14). While no significant effects on radial migration or morphological development were observed at E17, PREX1-knockdown neurons were located more apically within layer II/III than controls at postnatal day 0 (P0). By P7, these neurons showed aberrant dendritic arborization with a significant increase in apical dendritic branching. Likewise, dendritic overgrowth was observed in granule cells of the hippocampal dentate gyrus following PREX1 knockdown at P0, with minimal effects on dendritic spine morphology. Functionally, PREX1 knockdown enhanced spontaneous Ca²⁺ activity in cultured hippocampal neurons and depolarization-evoked Ca²⁺ responses in layer II/III cortical neurons in acute brain slices. These findings indicate that reduced PREX1-RAC1 signaling leads to mislocalization, dendritic overgrowth, and neuronal hyperexcitability, which may underlie epilepsy in the human case.