The minor spliceosome component U4atac regulates JAK/STAT signaling to modulate hematopoiesis and immune responses in Drosophila melanogaster

The small nuclear RNA U4atac is a core component of the minor spliceosome. In humans, homozygous or compound heterozygous point mutations in U4atac cause rare developmental disorders, such as Roifman syndrome, characterized by growth restriction, brain anomalies, and immune deficiency. To better define the pathophysiological role of U4atac mutations, we here establish a model of minor spliceosome dysfunction by generating a Drosophila melanogaster CRISPR/Cas9-induced U4atac mutant in the highly conserved stem II region. U4atac homozygous mutants exhibit growth and neurodevelopmental defects, immunodeficiency, and gastrointestinal symptoms. Using bulk RNA-sequencing and functional assays, we reveal that mutations in U4atac affect the splicing of a large set of transcripts involved in innate immunity, hematopoiesis, and intestinal cell functions, including the Drosophila Janus kinase (JAK) homolog hopscotch (hop). Importantly, U4atac deficiency reduces Hop expression and causes Hop-related hematopoietic defects at the embryonic and larval stages. Notably, we also observe reduced expression of Jak1 and attenuated activation of downstream signaling in patients with Roifman syndrome. Thus, our work identifies alterations of Jak signaling as part of the pathogenesis of RNU4atac-opathy. The minor spliceosome is responsible for splicing 0.5% of all introns. However, the critical function is underscored by several pathologies linked to deficiencies in core minor spliceosome components. Here, the authors generate a CRISPR-Cas9 Drosophila mutant of the minor spliceosome component U4atac and show that mutations in U4atac affect the splicing and expression levels of Jak homolog hopscotch, resulting in reduced JAK/STAT signaling activation and defective blood cell development and immune responses.