ADAR1 Is Essential For Erythroid Development

Understanding the mechanisms controlling erythroid differentiation may allow therapeutic regulation of erythropoiesis. Focus so far has concentrated on the roles of transcription factors in determining erythroid fate, however one mechanism of gene regulation that has been poorly explored is that of direct modification of RNA. It is now becoming apparent that RNA sequences can be widely modified by RNA editing. Adenosine-to-Inosine (A-to-I) RNA editing is a posttranscriptional process resulting in sequence alterations to RNA transcripts, which is catalyzed by members of the adenosine deaminase acting on RNA (ADAR) family of enzymes. We have previously reported that targeted disruption of ADAR1 in hematopoietic stem cells (HSC) resulted in a global upregulation of type I and II interferon-inducible transcripts and rapid apoptosis (Hartner et al., Nature Immunology 2009). This study identified that ADAR1 is essential for maintenance of both the fetal and adult HSC compartment in a cell-autonomous fashion. Although ADAR1 is dispensable for myeloid development and B-lymphopoiesis, investigation of its role in erythropoiesis is granted owing to the severe loss erythrocytes in ADAR1 null mice. To determine the role of ADAR1 in erythropoiesis, we generated mice with an erythroid-restricted deletion of ADAR1 (EpoR-Cre Adar1^fl/-). These animals were found to die in utero at ∼E14.5. The fetal liver (FL) was small and had significantly lower cellularity than in control littermates. Analysis of FL erythropoiesis demonstrated increased apoptosis and a loss of cells after the phenotypic polychromatic erythroblast stage of erythroid differentiation. When transplantation studies were done with EpoR-Cre Adar1^fl/- FL and control EpoR-Cre Adar1^fl/+ FL, recipients of EpoR-Cre Adar1^fl/- FL could not restore erythropoiesis and were anemic in contrast to the controls. They also developed splenomegaly suggestive of extramedullary hemopoiesis occurring as a compensatory mechanism. To understand the mechanism through which ADAR1 regulates erythroid development, gene expression arrays and miRNA profiling were performed. As with HSCs, loss of ADAR1 caused a significant upregulation of IFN signatures in EpoR-Cre Adar1^fl/- FL. Interestingly, despite evidence related to regulation of miRNA function by ADARs, there were only subtle changes to the expression pattern and levels of miRNAs in Adar1-deficient erythroid cells. These results demonstrate that ADAR1 is an essential in vivo mediator of erythroid cell fate and a critical negative regulator of the IFN response in cells specifically committed to erythropoiesis.