Multiple Roles for the Transcription Factors Sox6 and Jumonji in Mouse Hematopoiesis

Abstract

Sox6, a member of the Sox transcription factor family, is essential for the silencing of epsilon-y-globin gene expression in definitive erythropoiesis of mice and humans. Homozygous Sox6 null mice are neonatal lethal, precluding analysis at later stages. We created adult mice that are deficient in Sox6 specifically in hematopoietic tissues, by transplanting embryonic liver stem cells from Sox6-deficient mice into lethally-irradiated congenic wild-type adult mice. The mice receiving mutant stem cells (mutant-engrafted) showed high expression levels of epsilon-y in bone marrow, spleen and circulating blood compared to mice receiving wild-type and heterozygous stem cells (control-engrafted). The level of expression of epsilon-y in circulating blood was directly correlated with the percentage of successful mutant donor cell engraftment. Additionally, the mutant-engrafted adult mice showed an increase in erythroid precursor cells in bone marrow, spleen and blood. Thus, Sox6 continues to function as a major regulator of epsilon-y in adult definitive erythropoiesis and is required for normal erythrocyte maturation. Moreover, Sox6 may provide a novel therapeutic target by reactivating epsilon-y in patients with hemoglobinopathies such as sickle cell anemia and beta-thalassemia.We have also identified another transcription factor, jumonji, as a downstream target of Sox6. Jumonji is a crtitical transcription factor in neural, cardiac and erythroid development. We report here that jumonji is over-expressed in the fetal liver of Sox6-deficient mice (p100H/p100H). Transfection assays in H2.35 cells reveal that a ~1.6-kb genomic fragment, including the 5' UTR of jumonji, contains both promoter activity and Sox6-mediated repression. Chromatin immunoprecipitation and electromobility shift assays demonstrate that Sox6 binds to a region within the second exon of jumonji. Further transfection analyses confirm that one of five putative binding sites for Sox6 in this region is required for the majority of Sox6-mediated transcriptional repression. In irradiated mice engrafted with Sox6-deficient hematopoietic stem cells, jumonji expression levels are significantly elevated in blood and bone marrow. These results demonstrate that Sox6 plays a major role in the direct repression of jumonji transcription, and it is likely that jumonji plays a cell-autonomous role in the subsequent hematopoietic cell phenotype seen in Sox6-deficient mice.