DDX3X-mediated translation of structured cardiac mRNAs is essential for female heart development

Sex differences influence congenital heart disease (CHD) development, yet underlying molecular mechanisms remain largely unclear. We demonstrate that the X-linked RNA helicase DDX3X associates in the heart with ribosomal subunit proteins, and eCLIP mapping reveals its preferential binding to cardiac mRNAs with long, structured 5' untranslated regions (UTRs) that can hinder translation. Using a cardiomyocyte-specific mouse Ddx3x knockout model, we show that female embryos lacking Ddx3x die at midgestation from heart failure due to impaired translation of key cardiac regulators, whereas male littermates survive. Ribosome profiling and proteomics demonstrate that DDX3X is required for efficient translation of female differential cardiac mRNAs. Reporter assays confirm that translation of essential cardiac genes such as Srf and Rcor2 depends on their 5' UTRs and requires DDX3X. These findings uncover a sex-specific posttranscriptional mechanism by which DDX3X safeguards female heart development through selective mRNA translation, providing insight into how X-linked dosage-sensitive regulators contribute to CHD.