Molecular and Cellular Basis of Hyper-Assembly, Protein Aggregation and Impaired Neurodevelopment Driven by a Rare Pathogenic Mutation in DDX3X

Current studies estimate that 1-3% of females with unexplained Intellectual Disability (ID) present de novo splice site, nonsense, frameshift, or missense mutations in the DDX3X protein (DEAD-Box Helicase 3 X-Linked). However, the cellular and molecular mechanisms by which DDX3X mutations impair brain development are not fully comprehended. Here, we show that the ID-linked missense mutation L556S renders DDX3X prone to aggregation. By using a combination of biophysical assays, in vitro and in vivo imaging approaches, we demonstrate that this mutant assemble solid-like condensates and amyloid-like fibrils. Although we observed greatly reduced expression of the mutant allele in the patient who exhibits skewed X inactivation, this appears to be enough to sequestrate healthy protein into solid-like ectopic granules, compromising cell function. Reduced viability of SH-SY5Y cells and impaired development of patient-derived brain organoids unfold with aberrant granules formation. Therefore, our data suggests ID-linked DDX3X L556S mutation as a disorder arising from protein misfolding and aggregation.