Effects of 5' and 3' truncations of the myb gene on the transforming ability of avian myeloblastosis virus (AMV)

Characterization of a rearrangement in the c-MYB promoter in the acute lymphoblastic leukemia cell line CCRF-CEM

Despite the frequent description of 6q- structural abnormalities in human leukemias and lymphomas, rearrangements of the c-MYB locus have not been detected. We have detected a rearrangement in the c-MYB proto-oncogene in the cell line CCRF-CEM, an immature T-cell leukemia cell line which is not 6q-. Due to this rearrangement, a large portion of the c-MYB promoter conserved between the human and murine c-MYB genes is lost. The rearranged locus, which we have designated MRR (MYB rearranged region), has been cloned and mapped to chromosome 6. Field inversion gel electrophoresis (FIGE) studies reveal that the MRR sequence is linked to the c-MYB locus, suggesting that the rearrangement is due to a submicroscopic deletion. The rearrangement appears to have no effect on c-MYB promoter activity as analyzed in CCRF-CEM cells. The normal locus of the MRR sequence has been cloned from a human placental genomic library. Partial sequence analysis of this clone reveals that a portion of the DNA lost in the rearrangement shows a high degree of homology to a member of the myc family of oncogenes. Thus the characterization of this rearrangement has yielded a new set of probes for the study of chromosome 6q abnormalities in human leukemias and lymphomas and provides the first evidence for potential involvement of the c-MYB locus itself in submicroscopic deletions within chromosome 6.

Overexpression of C-terminally but not N-terminally truncated Myb induces fibrosarcomas: a novel nonhematopoietic target cell for the myb oncogene

The myb oncogene encodes a DNA-binding transcriptional transactivator which can become a hematopoietic cell-transforming protein following the deletion of amino acid sequences from either its amino or carboxyl terminus. Although a number of hematopoietic tumors express terminally deleted variants of Myb, the involvement of truncated Myb in nonhematopoietic tumors has not been adequately investigated. To assess the full spectrum of Myb's oncogenic capability, a replication-competent retroviral vector (RCAMV) was used to express a full-length protein (C-Myb), an amino-terminally truncated protein (VCC- or delta N-Myb), a carboxyl-terminally truncated protein (T-Myb), or a doubly truncated protein (VCT-Myb) in vivo. These viruses were injected intravenously into 10-day chicken embryos, and the infected chicks were monitored for tumors. Approximately 4 to 8 weeks after hatching, the majority (30 of 39 [77%]) of animals infected with the T-Myb retrovirus (without 214 carboxyl-terminal residues) developed nodular muscle tumors which could be identified by both morphologic and immunohistochemical criteria as fibrosarcomas. Identically appearing tumors could also be found in the kidney of some T-Myb-infected animals. The T-Myb-induced fibrosarcomas expressed the appropriately sized T-Myb protein, contained an unaltered proviral T-myb gene, and showed clonal proviral integration sites. In comparison, no sarcomas were observed in any of the animals infected with the amino-terminally truncated (VCC- and delta N-Myb) or doubly truncated (VCT-Myb) viruses. A loss of carboxyl-terminal but not amino-terminal sequences can thus convert Myb into a potent in vivo transforming protein for nonhematopoietic mesenchymal cells. In comparison, a truncation of either or both ends of the protein can activate Myb into a hematopoietic cell-transforming protein.

Overexpression of C-terminally but not N-terminally truncated Myb induces fibrosarcomas: a novel nonhematopoietic target cell for the myb oncogene

The myb oncogene encodes a DNA-binding transcriptional transactivator which can become a hematopoietic cell-transforming protein following the deletion of amino acid sequences from either its amino or carboxyl terminus. Although a number of hematopoietic tumors express terminally deleted variants of Myb, the involvement of truncated Myb in nonhematopoietic tumors has not been adequately investigated. To assess the full spectrum of Myb's oncogenic capability, a replication-competent retroviral vector (RCAMV) was used to express a full-length protein (C-Myb), an amino-terminally truncated protein (VCC- or delta N-Myb), a carboxyl-terminally truncated protein (T-Myb), or a doubly truncated protein (VCT-Myb) in vivo. These viruses were injected intravenously into 10-day chicken embryos, and the infected chicks were monitored for tumors. Approximately 4 to 8 weeks after hatching, the majority (30 of 39 [77%]) of animals infected with the T-Myb retrovirus (without 214 carboxyl-terminal residues) developed nodular muscle tumors which could be identified by both morphologic and immunohistochemical criteria as fibrosarcomas. Identically appearing tumors could also be found in the kidney of some T-Myb-infected animals. The T-Myb-induced fibrosarcomas expressed the appropriately sized T-Myb protein, contained an unaltered proviral T-myb gene, and showed clonal proviral integration sites. In comparison, no sarcomas were observed in any of the animals infected with the amino-terminally truncated (VCC- and delta N-Myb) or doubly truncated (VCT-Myb) viruses. A loss of carboxyl-terminal but not amino-terminal sequences can thus convert Myb into a potent in vivo transforming protein for nonhematopoietic mesenchymal cells. In comparison, a truncation of either or both ends of the protein can activate Myb into a hematopoietic cell-transforming protein.

Transformation of chicken myelomonocytic cells by a retrovirus expressing the v-myb oncogene from the long terminal repeats of avian myeloblastosis virus but not Rous sarcoma virus

To test the effect of long terminal repeat (LTR) regulatory sequences on the transforming capability of the v-myb oncogene from avian myeloblastosis virus (AMV), we have constructed replication-competent avian retroviral vectors with nearly identical structural genes that express v-myb from either AMV or Rous sarcoma virus (RSV) LTRs. After transfection into chicken embryo fibroblasts, virus-containing cell supernatants were used to infect chicken myelomonocytic target cells from preparations of 16-day-old embryonic spleen cells. Both wild-type AMV and the virus expressing v-myb from AMV LTRs (RCAMV-v-myb) were able to transform the splenocyte cultures into a population of immature myelomonocytic cells. The transformed cells expressed the p48v-Myb oncoprotein and formed compact foci when grown in soft agar. In contrast, the virus expressing v-myb from RSV LTRs (RCAS-v-myb) was repeatedly unable to transform the same splenocyte cells, despite being able to infect fibroblasts with high efficiency. This difference in the transforming activities of v-myb-expressing viruses with different LTRs most likely results from the presence of a factor (or factors) within the appropriate myelomonocytic target cell that promotes specific expression from the AMV but not from the RSV LTR.

Protein truncation is required for the activation of the c-myb proto-oncogene

The protein product of the v-myb oncogene of avian myeloblastosis virus, v-Myb, differs from its normal cellular counterpart, c-Myb, by (i) expression under the control of a strong viral long terminal repeat, (ii) truncation of both its amino and carboxyl termini, (iii) replacement of these termini by virally encoded residues, and (iv) substitution of 11 amino acid residues. We had previously shown that neither the virally encoded termini nor the amino acid substitutions are required for transformation by v-Myb. We have now constructed avian retroviruses that express full-length or singly truncated forms of c-Myb and have tested them for the transformation of chicken bone marrow cells. We conclude that truncation of either the amino or carboxyl terminus of c-Myb is sufficient for transformation. In contrast, the overexpression of full-length c-Myb does not result in transformation. We have also shown that the amino acid substitutions of v-Myb by themselves are not sufficient for the activation of c-Myb. Rather, the presence of either the normal amino or carboxyl terminus of c-Myb can suppress transformation when fused to v-Myb. Cells transformed by c-Myb proteins truncated at either their amino or carboxyl terminus appear to be granulated promyelocytes that express the Mim-1 protein. Cells transformed by a doubly truncated c-Myb protein are not granulated but do express the Mim-1 protein, in contrast to monoblasts transformed by v-Myb that neither contain granules nor express Mim-1. These results suggest that various alterations of c-Myb itself may determine the lineage of differentiating hematopoietic cells.

Protein truncation is required for the activation of the c-myb proto-oncogene

The protein product of the v-myb oncogene of avian myeloblastosis virus, v-Myb, differs from its normal cellular counterpart, c-Myb, by (i) expression under the control of a strong viral long terminal repeat, (ii) truncation of both its amino and carboxyl termini, (iii) replacement of these termini by virally encoded residues, and (iv) substitution of 11 amino acid residues. We had previously shown that neither the virally encoded termini nor the amino acid substitutions are required for transformation by v-Myb. We have now constructed avian retroviruses that express full-length or singly truncated forms of c-Myb and have tested them for the transformation of chicken bone marrow cells. We conclude that truncation of either the amino or carboxyl terminus of c-Myb is sufficient for transformation. In contrast, the overexpression of full-length c-Myb does not result in transformation. We have also shown that the amino acid substitutions of v-Myb by themselves are not sufficient for the activation of c-Myb. Rather, the presence of either the normal amino or carboxyl terminus of c-Myb can suppress transformation when fused to v-Myb. Cells transformed by c-Myb proteins truncated at either their amino or carboxyl terminus appear to be granulated promyelocytes that express the Mim-1 protein. Cells transformed by a doubly truncated c-Myb protein are not granulated but do express the Mim-1 protein, in contrast to monoblasts transformed by v-Myb that neither contain granules nor express Mim-1. These results suggest that various alterations of c-Myb itself may determine the lineage of differentiating hematopoietic cells.