Nfix Phase Separation Underlies Potential Mechanisms Of Craniofacial Abnormalities

This study explores alternative regulatory mechanisms of NFIX in Marshall-Smith syndrome (characterized by midface hypoplasia and micrognathia) and Malan syndrome (marked by macrocephaly and frontal bossing), hypothesizing that their distinct phenotypes arise from non-canonical pathways beyond transcriptional regulation. The intrinsically disordered regions (IDRs) associated with weak interactions required for biomolecular condensate formation were analyzed using Predictor of Natural Disordered Regions (PONDR) website. EGFP-tagged plasmids encoding either full-length NFIX or an IDR-truncated mutant were subsequently constructed and transfected into osteogenic U2OS cells. Biomolecular condensate dynamics were analyzed via live-cell imaging, and differential protein interaction networks were mapped by IP-MS. GraphPad Prism 9 was used for graphical data visualized. A threshold of p < 0.05 was applied to define statistically significant differences. Full-length EGFP-NFIX formed dynamic nuclear biomolecular condensates in U2OS cells, characterized by FRAP recovery and fusion events, consistent with membraneless organelle properties. PONDR analysis predicted a critical intrinsically disordered region (IDR: aa245-494), whose truncation abolished condensate formation and led to diffuse nuclear distribution IP-MS revealed multiple proteins with IDR-dependent binding specificity, showing significant enrichment in bone homeostasis-related pathways, including MAPK/ERK and Wnt signaling. Our findings demonstrate that NFIX regulates craniofacial homeostasis not only through canonical transcriptional activity but also via biomolecular condensate formation. This IDR-dependent phase separation mechanism provides a novel framework for understanding the pathogenesis of NFIX-related craniofacial dysmorphogenesis syndromes.