ABSTRACT
The myeloproliferative leukemia virus (MPLV) is an acute leukemogenic murine replication-defective retrovirus. By sequencing the envelope gene of a biologically active MPLV clone, we found that this region comprises a novel oncogene named v-mpl in phase with two parts of the Friend murine leukemia virus envelope gene. The MPLV env region could encode an env-mpl fusion polypeptide that presents the characteristics of a transmembrane protein. We show that in vitro infection of bone marrow cells with helper-free MPLV readily yields immortalized factor-independent hematopoietic cell lines of different lineages. In mice, the c-mpl proto-oncogene is expressed in hematopoietic tissues as a 3 kb mRNA. Since v-mpl shares strong structural analogies with the hematopoietin receptor superfamily, it is likely that MPLV has transduced a truncated form of an as yet unidentified hematopoietic growth factor receptor.
Subject(s)
Cell Transformation, Neoplastic , Oncogenes , Receptors, Cell Surface/genetics , Retroviridae/genetics , Amino Acid Sequence , Animals , Bone Marrow Cells , Cell Line , Colony-Forming Units Assay , DNA, Viral/genetics , DNA, Viral/isolation & purification , Gene Library , Genes, Viral , Hematopoietic Stem Cells/cytology , Mice , Mice, Inbred C57BL , Mice, Inbred DBA , Mice, Nude , Molecular Sequence Data , Muridae , RNA, Viral/genetics , RNA, Viral/isolation & purification , Viral Envelope Proteins/geneticsABSTRACT
The myeloproliferative leukemia virus (MPLV), a novel murine retroviral complex that does not transform fibroblasts, has been shown to cause an acute leukemia in adult mice accompanied by a progressive polycythemia. The present study demonstrates that, on in vivo inoculation, MPLV induces a rapid suppression of growth factor requirement for in vitro colony formation by both the late and the primitive erythroid progenitor cells. CFU-e-derived erythrocytic colonies developed and differentiated in semi-solid medium without the addition of erythropoietin (Epo). In addition, the formation of CFU-e colonies was not altered by the presence of specific neutralizing Epo antibodies. In the spleen, the CFU-e pool size increased rapidly up to 30-fold. By day 6 postinfection, 100% of these progenitor cells were Epo-independent. The in vivo effects of MPLV-infection on early erythroid progenitor cell compartments were examined in cultures grown for seven days. The concentration of erythroid progenitor cells was twofold elevated in spleen from MPLV-infected mice. As early as day 4 postinfection, 50% of these progenitors produced fully hemoglobinized colonies in serum-free cultures without the addition of interleukin-3 (IL-3) and Epo. Most spontaneous colonies were large and contained up to 10(5) cells per colony. They were composed of either erythroblasts only (16%) or erythroblasts and megakaryocytes (70%); few of them were multipotential (14%). In the marrow, the total number of BFU-e was reduced and only few factor-independent bursts were observed, suggesting a rapid migration of infected progenitors from marrow to spleen. Furthermore, the data show that abnormal erythropoiesis was due to the replication defective MPLV information and was not influenced by the Fv-2 locus.
Subject(s)
Erythropoiesis , Erythropoietin/physiology , Hematopoietic Stem Cells/pathology , Leukemia Virus, Murine , Leukemia, Experimental/pathology , Animals , Bone Marrow/pathology , Erythropoietin/immunology , Helper Viruses/physiology , Immune Sera/pharmacology , Leukemia Virus, Murine/physiology , Leukemia, Experimental/microbiology , Mice , Mice, Inbred C57BL , Mice, Inbred CBA , Neutralization Tests , Spleen/pathology , Tumor Stem Cell AssayABSTRACT
The myeloproliferative leukemia virus (MPLV) is a new acute leukemogenic, nonsarcomatogenic retroviral complex that is generated during the in vivo passage of a molecularly cloned Friend ecotropic helper virus. Examination of viral RNA expression in MPLV-producing cells revealed the presence of two distinct molecular species that hybridized with a long terminal repeat or an ecotropic env-specific probe but not with a xenotropic mink cell focus-forming virus env-specific probe derived from a spleen focus-forming virus: an 8.2-kilobase species corresponding to a full-length Friend murine leukemia virus (F-MuLV) and a deleted species with a genomic size of 7.4 kilobases. This deleted virus was biologically cloned by limiting dilutions and single cell cloning in Mus dunni fibroblasts. Three nonproducer clones with normal morphologies and containing one single integrated copy of the deleted virus were superinfected with F-MuLV, Moloney murine leukemia virus, Gross murine leukemia virus, mink cell focus-forming virus (HIX), or the amphotropic 1504 murine leukemia virus. All pseudotypes caused macroscopic and microscopic abnormalities in mice that were similar to those seen in the parental stock. A comparison of the physical maps of F-MuLV and MPLV, which was deduced from the restriction enzyme digests of unintegrated proviral DNAs, indicated that the MPLV-defective genome (i) is probably derived from F-MuLV, (ii) has conserved the F-MuLV gag and pol regions, and (iii) is deleted and rearranged in the env region in a manner that is clearly distinct from that of Friend or Rauscher spleen focus-forming viruses.
