The transcription factor Sox6 induces differentiation and cell cycle withdrawal in BCR-ABL+ and JAK2 V617F+ cellular model systems of leukemia

SOX6 is a transcription factor (TF) belonging to the Sry-related HMG-box TFs family. It controls terminal differentiation and lineage specification of many cell types, by mediating cell cycle withdrawal and activation of lineage specific genes. In hematopoiesis, Sox6 sustains cell survival, proliferation and terminal maturation of murine erythroid cells. To explore the role of SOX6 in human erythropoiesis, we first overexpressed it in erythroleukemic BCR-ABL+ K562 cell line. In these cells, Sox6 overexpression induces a strong erythroid differentiation coupled with growth arrest. This last effect may be mediated by SOCS3 (Suppressor Of Cytokines Signalling 3), that is a direct transcriptional target of SOX6. SOCS3 represses two important pathways in erythropoiesis: the IGF1/IGF1R pathway, in response to insulin-like growth factor 1 (IGF1) and the Epo-Jak-STAT pathway, in response to erythropoietin. In particular, in K562 cells, proliferation depends on autocrine IGF1 signalling, induced by BCR-ABL. Cells overexpressing Sox6 or SOCS3 display a reduction in IGF1 expression, suggesting that SOX6 plays a pivotal role in blocking cell proliferation through SOCS3 upregulation and by inhibiting the BCR-ABL-dependent IGF1 signalling. Leukemias and myelodisplasias are characterized by a block in differentiation leading to an excess of proliferating immature cells. Since SOX6 is a potent inducer of growth arrest and differentiation, I investigated the effect of its ectopic expression in different model systems of leukemia, to enforce these cells to overcome their pathological conditions. Therefore, I used leukemic model systems that varies for their degrees of differentiation/maturation and to the presence/absence of specific genetic lesions (i.e. BCR-ABL fusion oncogene, JAK2V617F mutation). Sox6 overexpression blocks proliferation in all the tested BCR-ABL+ cell lines (erythroleukemic K562, megakaryoblastic MEG.01) and surprisingly even in the lymphoblastic SUPB15 and B-ALL BCR-ABL GFP+. The JAK2V617F mutation, typical of myelodysplastic syndromes, makes cells unresponsive to SOCS3. Remarkably, in JAK2V617F+ cell lines (thrombocythemic UKE1 and SET2; erythroleukemic HEL), Sox6 overexpression induces a block in cellular proliferation strictly dependent on the copy number of the JAK2V617F alleles, further confirming SOCS3 as a key effector of SOX6. A second aspect controlled by SOX6 is the activation of a set of specific erythroid genes that lead erythroblastic progenitors to undergo terminal differentiation. Indeed, Sox6 overexpression enhances erythroid differentiation in bipotent megakaryoblastic and erythroleukemic cells (K562 and HEL) and enforces megakaryoblastic cells towards an erythroid fate (MEG.01, SET2 and UKE1). Surprisingly, Sox6 overexpression is capable of downregulating genes essential for B-cells identity (i.e. PAX5), whereas it is not sufficient to activate an erythroid program in lymphoblastic cells (SUPB15 and B-ALL BCR-ABL GFP+). Taken together, these findings suggest that SOX6 plays a major role in activating cell cycle withdrawal, acting on SOCS3, and interferes with cell fate decision by downregulating specific cellular programs and enforcing, where possible, an erythroid lineage choice in leukemic and myelodysplastic cell lines. Finally, I explored the impact of SOX6 ectopic expression in B-ALL BCR-ABL GFP+ capability to engraft and generate leukemia in C57BL/6J mice. B-ALL BCR-ABL GFP+ SOX6+ cells are not capable to engraft recipient mice, suggesting that SOX6 overexpression in such cellular model interferes with the onset of leukemia. Taken together, these results strongly point to a pivotal role for SOX6 in suppressing cells proliferation both in vitro and in vivo, and pave the way for the identification of potential druggable targets to develop new therapeutical approaches for leukemic and myelodysplastic syndromes.