Compound heterozygous mutations in the noncoding RNU4ATAC gene affect the homeostasis and function of the Integrator complex

Various genetic diseases associated with microcephaly and developmental defects are due to pathogenic variants in the U4atac snRNA, a component of the minor spliceosome essential for the removal of U12-type introns from eukaryotic messenger RNAs. While it has been shown that a few RNU4ATAC mutations result in impaired binding of essential protein components, the molecular defects of the vast majority are still unknown. Here, we used cells derived from two RNU4ATAC compound heterozygous (g.108_126del;g.111G>A) twin patients with MOPD1 phenotypes to analyze the molecular consequences of the mutations on snRNPs formation and splicing. We show that the U4atac108_126del mutant is unstable and that the 111G>A mutation located in the apical 3’ stem-loop of U4atac induces defects in 3’-end maturation giving rise to decreased levels of mature U4atacG111A snRNA as well as of minor di- and tri-snRNPs. We also found that the mutant cells have alterations in splicing of minor introns and contain lower levels of INTS7 and INTS10 proteins. Altogether, our results demonstrate that the compound heterozygous g.108_126del;g.111G>A U4atac mutations lead to defects in 3’-end processing of U4atac transcripts and affect the homeostasis and function of the Integrator complex. RNA-seq on mRNA from 1 control and 2 MOPD1 patients (2 replicates of each)