Initially, lymphoid cells transformed by v-abl or BCR/ABL oncogenes are poorly oncogenic but progress to full transformation over time. Although expression of the oncogene is necessary to initiate and maintain transformation, other molecular mechanisms are thought to be required for full transformation. To determine whether tumor progression in ABL oncogene-transformed lymphoid cells has a genetic basis, we examined whether progression of the malignant phenotype of transformed clones correlates with particular cytogenetic abnormalities. A modified in vitro bone marrow transformation model was used to obtain clonal Abelson murine leukemia virus-transformed B lymphoid cells that were poorly oncogenic. Multiple subclones were then derived from each clone and maintained over a marrow-derived stromal cell line for several weeks. Over time, clonally related Abelson murine leukemia virus-transformed subclones progressed asynchronously to full transformation. The data show that tumor progression can occur in the absence of detectable cytogenetic changes but, more importantly, that certain cytogenetic abnormalities appear reproducibly in highly malignant subclones. Therefore, three independent subclones showed deletion in a common region of chromosome 13. Other highly malignant cells carried a common breakpoint in the X chromosome, and, finally, two subclones carried an additional chromosome 5. These results are consistent with the hypothesis that ABL oncogenes are sufficient for the initial transformation of cells but that additional genetic events can drive oncogenic progression. These observations further suggest that diverse genetic mechanisms may be able to drive tumor progression in cells transformed with ABL oncogenes.
Transformed lymphocytes from Abelson-diseased mice express levels of a B lineage transformation-associated antigen elevated from that found on normal lymphocytes.
