Spi-1 transgenic mice develop a clonal erythroleukemia which does not depend on p53 mutation

Abstract

Spi-1 transcriptional activation and wild-type p53 extinction are two oncogenic alterations involved in the malignant transformation of erythroblasts during the Friend acute erythroleukemia. To dissect the contribution of these alterations in the deregulation of the di erentiation and proliferation of erythroblasts, we generated spi-1 transgenic mice. Analysis of these animals revealed that Spi-1 overexpression was directly involved in the block of proerythroblast di erentiation. However, the erythroleukemia that develops in these animals evolved in two steps. During the early step (HS1 step), non tumorigenic proerythroblasts remained strictly dependent upon erythropoietin (Epo) for their survival and proliferation. Later on, Epo-independent and tumorigenic proerythroblasts emerged (HS2 step) suggesting that other oncogenes cooperate with Spi-1 to lead to a fully malignant phenotype. By provirus tagging, we demonstrate that the HS1 step was clonal indicating that a cell selection must occur in vivo. Analysis of the nature of p53 in both the in vivo HS1 and HS2 proerythroblasts and in cultured erythroblastic cell lines showed that ± p53 was normal in the HS1 primary tissues but was mutated in the HS1 cultured cell lines ± p53 was frequently altered in HS2 primary tissues but was found normal in some mice. These data indicate that (i) the blockage of the erythroblast di erentiation by Spi-1 occurs independently of p53 alteration (ii) p53 alteration is not necessary to confer Epo independence and tumorigenicity to spi-1 transgenic proerythroblasts.

Alterations of the phosphoinositide 3-kinase and mitogen-activated protein kinase signaling pathways in the erythropoietin-independent Spi-1/PU.1 transgenic proerythroblasts

“…We already reported that the Jak2/STAT5 pathway was at rest in HS2 cells as a direct consequence of an inactive EpoR, 26 excluding that anomalies in this pathway could lead to a proliferative signal in the HS2 cells. PI3K is involved in the regulation of many signaling events involved in mitogenesis and apoptosis.…”

“…26 Then, we focused on other signaling pathways regulated by Epo in erythroid cells. First, we examined the contribution of PI3K to the survival and proliferation of HS1 and HS2 cells by analyzing the effect of the PI3K inhibitor LY294002 27 on cell growth ( Figure 2A).…”

Retroviral insertional activation of the Fli-3 locus in erythroleukemias encoding a cluster of microRNAs that convert Epo-induced differentiation to proliferation

“…In agreement with this result, a previous study has demonstrated that overexpression of Spi-1 in a transgenic mouse model blocks differentiation and induces Epo-dependent proliferation of erythroblasts. 40 Thus, high Spi-1 expression and low p27 expression in HB60-5 cells may be responsible for the block in differentiation in these cells. Since Spi-1 up-regulation was observed only in HB60-5 cells, perhaps the function of the mir-17-92 Posttranscriptional regulation of p27 by mir-17-92 cluster through a specific binding to the p27 3ЈUTR was examined using luciferase assay.…”

Functional Cross-Antagonism between Transcription Factors FLI-1 and EKLF

“…Direct evidence for deregulated expression of SPI-1/PU.1 in leukemogenesis was provided by the spontaneous development of multistep erythroleukemia in SPI-1 transgenic mice (4,5,44). SPI-1/PU.1 overexpression is involved in both the survival and the inhibition of differentiation of erythroid cells (47).…”