miR-199a functions downstream of MeCP2 in neurons of MECP2 duplication syndrome models.

Duplication of the methyl-CpG-binding protein 2 (MECP2) gene causes MECP2 duplication syndrome (MDS), a severe neurodevelopmental disorder with an unclear pathology. We previously showed that MeCP2 promotes the processing of specific microRNAs (miRNAs), including miR-199a, to regulate neuronal functions. Here, we demonstrate that neurons derived from MDS model mice and patient-induced pluripotent stem cells (iPSCs) exhibit morphological abnormalities, such as abnormal dendrite outgrowth, enlarged soma size, increased glutamatergic synapse density, and hyperactivation of the mechanistic target of rapamycin (mTOR) signaling. MeCP2 overexpression increased miR-199a production in both models. Blocking miR-199a-5p improved soma size and mTOR activity, while inhibiting miR-199a-3p normalized dendritic outgrowth. Crossing MDS model mice with miR-199a-2 knockout mice ameliorated synaptic and mTOR abnormalities. Human MDS cortical organoids exhibited reduced neuronal activity, which was reversed by suppressing miR-199a-5p. These findings identify miR-199a as a key downstream mediator of MeCP2 in MDS, providing new insights into its molecular pathology.