Adenosine to inosine (A-to-I) RNA editing is mediated by ADAR1 and ADAR2, with ADAR1 the predominant ADAR expressed in hematopoiesis. ADAR1 has two isoforms, a nuclear p110 protein and a predominantly cytoplasmic p150 form. Loss of ADAR1 protein, or its editing activity, results in sensing of under-/un-edited cellular dsRNA by MDA5, triggering a fatal type I interferon response. ADAR1 is essential for HSC maintenance and the erythroid, B- and T-lymphoid lineages. Whilst editing is the primary function of ADAR1, protein dependent, editing independent functions have been suggested. We now have identified a protein dependent, RNA editing independent role for ADAR1p150 in T cell homeostasis and HSC repopulating activity in vivo.
We recently established viable adult ADAR1 protein deficient (A1-/-), editing deficient (A1EA/EA) and ADAR1p150 (A1p150-/-) deficient mice by co-deleting MDA5 and PKR (PMID:37797622). Peripheral blood (PB) analysis of adult A1+/+, A1-/-, A1EA/EA and A1p150-/- mice demonstrated that the A1-/- and A1p150-/- had severely reduced PB T cells. Thymic T cell development was largely normal, indicating reduced survival of the PB T cells as the likely cause. Bone marrow (BM) transplantation demonstrated that the A1-/- and A1p150-/- BM failed to regenerate T cells in the recipients, however, unexpectedly, they also had a profound defect in HSC reconstitution. A1EA/EA had normal reconstitution, demonstrating that the phenotypes are independent of RNA editing by ADAR1. The estimated HSC competitive repopulating units in the A1-/- and A1p150-/- were 200-600 fold reduced compared to A1EA/EA and A1+/+, respectively.
These data demonstrate ADAR1p150 protein, independent of both A-to-I editing and MDA5/PKR signalling, regulates T cell homeostasis and HSC repopulating capacity. Mechanistic studies are investigating the basis of this defect using different ADAR1p150 mutants.