Bok Promotes Erythropoiesis in a Mouse Model of Myelodysplastic Syndrome

BCL-2 Ovarian Killer (BOK) is a pro-apoptotic member of the BCL-2 family of proteins best characterized for its putative ability to induce apoptosis in response to Endoplasmic Reticulum (ER) stress, when stabilized from ER-associated degradation (ERAD). Although ER stress appropriately activates the unfolded protein response (UPR) in BOK-disrupted cells, as measured by PERK and eIF2-alpha phosphorylation, downstream effector signaling, including ATF4 and CHOP, is defective. A functional role for BOK as a tumor suppressor is suggested by its genetic location in one of the 20 most frequent, focally deleted chromosomal regions across all human cancers. To evaluate the consequences of BOK loss in the pathogenesis of myelodysplasia (MDS) and Acute Myeloid Leukemia (AML), we used the Nup98-HoxD13 (NHD13) transgenic mouse model of MDS/AML. In this model, both overexpression of anti-apoptotic BCL-2 and deletion of pro-apoptotic PUMA rescue cytopenias, but surprisingly delay progression to AML. In contrast, AML developed in 36.7% of NHD13 mice lacking BOK between the age of 8 and 13 months with a similar overall survival to the NHD13 mice. However, the loss of BOK exacerbated the anemia of the NHD13 mice, which raised a potential connection between BOK and the regulation of erythropoiesis in cells experiencing stress from the NHD13 translocation. NHD13 mice deficient for BOK exhibited significantly lower hemoglobin (Hb), lower mean cell hemoglobin concentration (MCHC) and higher mean cell volume (MCV) than NHD13 mice, whereas other lineages were unaffected. Mouse colony forming unit assays revealed there is a decreased amount of erythroid progenitor stem cells (BFU-E) in the bone marrow of NHD13-transgenic/BOK-deficient mice, which hinted at a diminished ability to produce RBCs in the absence of BOK. Isolation of various stages of erythroid progenitors in the bone marrow by CD44/TER119 FACS sorting revealed that both NHD13 and NHD13-transgenic/BOK-deficient mice have an increase in proerythroblasts relative to more mature red blood cells. Preliminary RT-QPCR analysis shows decreased expression of UPR components in the RBC progenitors of both BOK-deficient and NHD13-transgenic/BOK-deficient mice. Interestingly, CHOP is not only a component of the UPR, but also an erythropoietin target gene necessary for erythroid differentiation. These results suggest that in addition to its pro-apoptotic function, BOK may have other regulatory roles within the cell, and specifically a role in regulating erythropoiesis when certain RBC progenitors experience ER stress.