Spi-1/PU.1 Transgenic Mice Develop Multistep Erythroleukemias

Abstract

Insertional mutagenesis of the spi-1 gene is associated with the emergence of malignant proerythroblasts during Friend virus-induced acute erythroleukemia. To determine the role of spi-1/PU.1 in the genesis of leukemia, we generated spi-1 transgenic mice. In one founder line the transgene was overexpressed as an unexpected-size transcript in various mouse tissues. Homozygous transgenic animals gave rise to live-born offspring, but 50% of the animals developed a multistep erythroleukemia within 1.5 to 6 months of birth whereas the remainder survived without evidence of disease. At the onset of the disease, mice became severely anemic. Their hematopoietic tissues were massively invaded with nontumorigenic proerythroblasts that express a high level of Spi-1 protein. These transgenic proerythroblasts are partially blocked in differentiation and strictly dependent on erythropoietin for their proliferation both in vivo and in vitro. A complete but transient regression of the disease was observed after erythrocyte transfusion, suggesting that the constitutive expression of spi-1 is related to the block of the differentiation of erythroid precursors. At relapse, erythropoietin-independent malignant proerythroblasts arose. Growth factor autonomy could be partially explained by the autocrine secretion of erythropoietin; however, other genetic events appear to be necessary to confer the full malignant phenotype. These results reveal that overexpression of spi-1 is essential for malignant erythropoiesis and does not alter other hematopoietic lineages.

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

“…In the HS2 step, proerythroblast proliferation was growth factor-independent and these cells were also tumorigenic in vivo (Moreau-Gachelin et al, 1996). F-MuLV (B3) infection did not change the timing of the disease progression or the properties of the proliferating proerythroblasts which were Epo-independent in vitro and tumorigenic in vivo.…”

“…It was previously reported that B3 had a low virulence and induced various types of leukemias only after more than 10 months (Sitbon et al, 1986). Among the 58 B3-infected spi-1 transgenic mice, 28 developed the hepatosplenomegaly (HS1) and anemia characteristics of the transgenic disease (Moreau-Gachelin et al, 1996). The incidence of the disease (48%) was similar to that observed in uninfected transgenic mice, whereas the mean latency of outgrowth (3.0+1.5 months) appeared slightly shorter than in uninfected animals (3.6+1.5 months).…”

“…Transgenic mice overexpressing the spi-1 gene in erythroid cells develop an erythroleukemic process that has been instrumental to de®ne the role of Spi-1 in the blockage of proerythroblast di erentiation (Moreau-Gachelin et al, 1996). In addition the multistep evolution of the disease suggests that activation of another(other) putative oncogene(s) might be necessary for the proerythroblast to reach full malignancy.…”

“…It can also be determined genetically by activation of regulatory factors that interact with the transcriptional enhancers that drive the transgene transcription. Indeed, our previous observations have shown that the transgene was silent during the embryonic and neonatal life and was not expressed in the transgenic mice that remained healthy (Moreau-Gachelin et al, 1996). However, the clonality of the disease can also re¯ect a cellular selection process with regard to the occurrence of a biochemical event necessary to the Spi-1 function.…”

“…It regulates the transcription of many myeloid and B lymphoid genes (for review Moreau-Gachelin, 1994) and its inactivation in mice results in a multilineage defect characterized by an absence of B lymphocytes and macrophages (McKercher et al, 1996;Scott et al, 1997). Although the down regulation of spi-1 during the chemicallyinduced di erentiation of Friend cells suggested that Spi-1 was involved in the di erentiation arrest of the proerythroblast (Schuetze et al, 1992), the role of Spi-1/PU 1 in the transformation of erythroid cells was deduced from the pathology observed in mice overexpressing a spi-1 transgene (Moreau-Gachelin et al, 1996). These Spi-1 transgenic mice develop a two step erythroleukemic process.…”