SALL4 Is a Key Survival Factor in Acute B-Cell Lymphoblastic Leukemia

Abstract 2428

SALL4 is a zinc-finger transcriptional factor and a member of the SALL gene family. It plays an essential role in the maintenance of ESC pluripotent and self-renewal properties by interacting with other two key regulators in ESCs, Nanog and Oct4. We previously have shown that stem cell factor SALL4 is aberrantly expressed in 75% of acute B-cell lymphoblastic leukemia (B-ALL). We have also shown that SALL4 is aberrantly expressed in AML, and down-regulation of SALL4 in AML leads to significant cell death.

In this study, we focused on investigating the functional role of SALL4 in human B-ALL leukemogenesis. We first assessed the SALL4 mRNA level in four B-ALL cell lines (REH, Nalm6, 697, Blin-1) and five primary patient samples by qRT-PCR. We observed SALL4 mRNA in these four cell lines increased when compared to normal human CD34 negative BM cells. Moreover 4 of 5 primary samples showed high level expression of SALL4, suggesting that SALL4 might play a role in B-ALL pathogenesis.

Then, we selected a SALL4 expressing B-ALL cell line (REH and Nalm6) and attenuated SALL4 expression through GFP-labeled shRNA approach in these cell lines. We monitored the growth of SALL4 knockdown and control REH and Nalm6 cells through MTS assay. SALL4 knockdown cells had a decreased growth rate compared to that of the control cells. We also stained SALL4 knockdown and control cells with Annexin V and 7-AAD by flow cytometric quantitation of apoptotic cells. The percentages of apopotic cells in SALL4 knockdown cells were much higher than these in controls. These data demonstrated that inhibition of SALL4 in REH cells and Nalm6 cells led to reduced proliferation and increased apoptosis.

We then examined the oncogenesis ability of SALL4 knockdown REH cells in a mouse xenotransplantation model. SALL4 knockdown or control REH cells were injected intravenously into immunodeficient mice. All the recipients succumbed to fatal leukemia within 4 to 6 weeks post transplantation. In both BM and spleen of SALL4 knockdown recipients the engrafted proportion of GFP⁺ cells was significantly decreased compared to the initial donor cells. Whereas, in both BM and spleen control recipients the percentage of GFP⁺ REH cells engrafted was similar to that of initial donor cells. This suggests that down-regulation of SALL4 is essential for B-ALL engraftment.

To rule out the observed engraftment defect was due to homing, we next performed homing assay. SALL4 knockdown or control cells were injected intravenously into immunodeficient mice as well. Three hours of the injection, mice were sacrificed and analyzed the percentage of GFP⁺ cells in BM and spleen by flow cytometry. There was no difference among SALL4 knockdown and the control.

Furthermore, we performed gene expression profiling on apoptosis-related genes in SALL4 knockdown and control REH cells. The result showed that in SALL4-knockdown REH, TNF mediated cell apoptosis pathways was up-regulated as well as multiple caspase members. The expression of Caspase 3, Caspase 8, FADD was up-regulated in both SALL4 knockdown REH and Nalm6 when compared to these controls, and was verified by real time RT-PCR. This suggests that SALL4 could repress apoptosis through the TNF signal pathway. In summary, we report a novel SALL4/TNF pathway in maintaining cell survival in B-ALL.