Dominant variants in major spliceosome U4 and U5 small nuclear RNA genes cause neurodevelopmental disorders through splicing disruption

Variants in RNU4-2, encoding the small nuclear RNA (snRNA) U4, were recently identified as a major cause of neurodevelopmental disorders (ReNU syndrome). Here, we investigated de novo variants in 50 snRNAs in a French cohort of 23,649 individuals with rare disorders and collected data of additional patients through an international collaboration. Altogether, we identified 133 probands with pathogenic or likely pathogenic variants in RNU4-2 and 15 individuals with de novo and/or recurrent variants in constrained regions of RNU5B-1, one of five genes encoding U5. These variants cluster in evolutionarily conserved regions of U4 and U5 essential for splicing. RNU4-2 variants affecting stem III are associated with milder phenotypes than those in the T-loop (quasi-pseudoknot). Phaseable variants associated with severe phenotypes occurred on the maternal allele. Individuals with RNU4-2 variants show specific defects in alternative 5 splice site usage, correlating with variant location and clinical severity. Additionally, we report an episignature associated with severe ReNU syndrome. This study further highlights the importance of de novo variants in snRNAs and establishes RNU5B-1 as a new neurodevelopmental disorder gene.