Cell-extrinsic emergency myelopoiesis regulated by ABCB7

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Abstract Dysregulation of myelopoiesis can be either cell-intrinsic or cell-extrinsic. Cell-intrinsic mutations can result in myelodysplastic syndromes (MDSs) or myeloproliferative disorders (MPNs), while cell-extrinsic systemic inflammation promotes emergency myelopoiesis. However, the cause of the enhanced myelopoiesis is not always clear, and these two mechanisms are not necessarily mutually exclusive. Patients with chronic inflammation or autoimmune disease are at an increased risk for MDS, and chronic inflammation is often observed in MDS patients. Provocatively, our preliminary data demonstrates that a cell-extrinsic emergency myelopoiesis progressively develops when the inner mitochondrial membrane transporter ABCB7 is conditionally deleted in differentiating B cells in the bone marrow. ABCB7 is intrinsically required for B cell development beyond the pro-B cell stage. In 4-5 month old mice engineered to lack ABCB7 in B-lineage cells, bone marrow hematopoiesis is disrupted. Bone marrow erythropoiesis is dramatically reduced, and bone marrow myelopoiesis is greatly enhanced. To compensate for the loss of bone marrow marrow erythropoiesis, stress erythropoiesis is initiated in the spleen, however, the mice remain anemic. Importantly, the enhanced numbers of myeloid cells being produced in the bone marrow were not labelled with a cre-dependent reporter, establishing that the myeloid expansion is cell-extrinsic. ABCB7-deficient pro-B cells exhibit iron overload and DNA damage, which could provide the stimuli for initiation of chronic inflammation leading to progressive cell- extrinsic emergency myelopoiesis. Intriguingly, ABCB7 expression is decreased in myelodysplastic disorders in which patients have a mutation in the RNA splicing gene SF3B1. ABCB7 RNA levels are decreased by 60- 70% in SF3B1-mutated CD34+ cells from MDS-RS patients due to aberrant splicing and nonsense-mediated decay. We hypothesize that myelodysplasia in patients with SF3B1-mutated myelodysplastic disorders is not strictly cell-intrinsic, but instead there is a cell-extrinsic contribution mediated by decreased expression of ABCB7 in B-lineage precursor cells. To test this hypothesis, we have created a novel dox-regulated ABCB7 mouse model that allows us to address the hematopoietic consequences of decreased ABCB7 expression. These studies will establish that reduced expression of ABCB7, in both mouse and human hematopoiesis, leads to dysfunctional myelopoiesis due to B-lineage cells. Our studies will have important therapeutic implications for treating SF3B1-mutated myeloid disorders, as elimination of the B-lineage cells could contribute to amelioration of disease.