Functional cooperation between the transcription factor Spi1/PU.1 and the signaling factor PTPN11 in the development of erythroleukemia

Erythroleukemias are a subgroup of acute myeloid leukemias characterized by an accumulation of erythroid progenitors impaired in their ability to differentiate and resulting from acquired mutations targeting proteins involved in signaling molecules and epigenetic factors. Additionally, they show aberrant expression of key transcriptional regulators, including SPI1/PU.1. The SPI1 transcription factor is a key regulator required for many steps of haematopoiesis. Moreover, its expression must be reduced for terminal erythroid differentiation and, if not, SPI1 causes an erythroid differentiation arrest at the stage of CFU-E (Colony Forming Unit-Erythroid) and inhibits apoptosis, contributing to erythroleukemia. In leukemic cells of a murine transgenic Spi1 (TgSpi1) model, the overexpression of SPI1 was shown to be accompanied in 90% of sick mice by activating mutations of PTPN11, a growth factor-activated protein tyrosine phosphatase, involved in signaling pathways crucial for proliferation, survival, and differentiation. The goal of the project investigated during my Master internship is to understand how the relationship between SPI1 and PTPN11 contributes to leukemic development. To this aim, we investigated the functions of both factors at the cellular and molecular level, using in vitro and in vivo models of erythroleukemia. First, using blast cells derived from leukemic TgSpi1 mice that highly express SPI1 and treating them to reduce SPI1 expression and/or with pharmacological inhibition of PTPN11, we showed that both SPI1 and PTPN11 activation are essential to promote proliferation, survival and blockage of erythroid differentiation at CFU-E stage. Forcing expression of both factors in normal erythroid progenitors also clearly shows that SPI1 and PTPN11 activating mutations, identified in TgSpi1 erythroleukemic mice, are required to inhibit the differentiation of the progenitors. Searching which signaling pathways were altered by activated PTPN11 in the erythroid blasts, we revealed that it activates the RAS/MAPK pathway and inhibit the JAK/STAT5 pathways. At this step, our results do not indicate that the two factors act on each other. Interestingly, SPI1 and PTPN11, both, act on common targets. In particular, they both repress expression of the pro-apoptotic gene, BcL2l11 and the pro-differentiating St3gal6 and Alas2 genes; some of these targets being synergistically altered by SPI1 and PTPN11. Altogether, our results clearly demonstrate that the two factors cooperate in leukemic development. Our data are consistent with both, synergistic effects of SPI1 and PTPN11 by acting on different molecular pathways or hierarchical activity of both in the same molecular pathway. Additional works will be required to distinguish these two possibilities.