Loss of Asrij/ OCIAD1 in Mice Promotes Hematopoietic Stem and Progenitor Cell Expansion Due to Increased CSN5-Mediated P53 Degradation

Asrij/OCIAD1 is implicated in several human hematological malignancies and plays essential roles in Drosophila hematopoiesis and immunity. However, its function in mammalian hematopoiesis remains unknown. We report generation and characterization of the first asrij null (knockout, KO) mice. Asrij KO mice are viable and fertile with no gross abnormalities. However, by six months of age they exhibit increased peripheral blood cell counts, splenomegaly and an expansion of bone marrow hematopoietic stem and progenitor cells (HSPCs: Lin- Sca1+ c-Kit+) with more myeloid output. KO HSPCs show increased proliferation upon co-culture with OP9 stromal cells in vitro and accelerated differentiation towards the myeloid lineage in a methylcellulose colony-forming assay. Asrij depletion also confers greater repopulation ability to bone marrow cells in primary and secondary transplants, indicating loss of quiescence and a transplantable leukemia. Molecular and biochemical analysis showed reduced p53 levels in KO HSPCs with a concomitant increase in p53 ubiquitination. Further, CSN5 (COP9 signalosome subunit 5) levels were higher in KO HSPC, leading to more p53 degradation. In agreement with this, Nutlin-3 treatment could restore the increased KO HSPC counts to control levels. Expectedly, mutant mice had greater DNA damage, with reduced survival upon genotoxic or chemical stress, indicating stem cell exhaustion. CSN5 interacts with Asrij via the conserved OCIA domain, known to be essential for stem cell maintenance. Thus, Asrij is essential to sustain p53 levels critical for maintaining bone marrow HSPC quiescence.