HEMATOPOIETIC TRANSFORMING POTENTIAL OF ACTIVATED RAS IN CHIMERIC MICE

Although activating mutations in ras genes are the most common genetic abnormality in human hematologic malignancies, the role of ras mutati ons as an initiating event in leukemogenesis remains unclear, To asses s the consequences of ectopic expression of an activated ras gene in n ormal hematopoietic cells in vivo, lethally irradiated mice were recon stituted with bone marrow cells infected with a mutant ras-containing retrovirus [murine stem cell virus (MSCV)-v-H-ras] based on the MSCV r etroviral vector which efficiently transduces functional genes into he matopoietic stem/progenitor cells, Despite a marked myeloid leukocytos is detectable in the peripheral blood within 4 weeks of engraftment, n one of 22 primary or secondary transplant recipients studied for longe r periods of time presented with myeloid neoplasms, Instead, 18 of the MSCV-v-H-ras mice developed pre-T-celI thymic lymphomas and/or pre-B- cell lymphoblastic leukemia/lymphomas between 7 and 12 weeks post-tran splantation. The pre-B and pre-T lymphoid tumors that arose in one ani mal were shown to harbor a; common MSCV-v-H-ras provirus, indicating t hat the target cell for transformation was a bipotential lymphoid prec ursor. To more precisely examine the effects of activated ras expressi on on the behavior of hematopoietic progenitors, infected bone marrow cells were assayed in methylcellulose cultures under conditions favora ble for growth of multilineage myeloid colonies or were passaged as bu lk suspension cultures in the presence of various hematopoietic growth factors, including interleukin (IL)-3, IL-4, IL-6 and IL-7. MSCV-dire cted expression of v-H-ras selectively promoted the formation of large dense colonies comprised of monocyte-macrophages in methylcellulose c ultures. When transferred to liquid cultures, the vast majority of the cells underwent terminal macrophage differentiation, By comparison, t umorigenic B-lymphoid and mixed lymphoid/myeloid cell lines were routi nely established from the bulk suspension cultures, with cell lines of predominantly myeloid phenotype emerging only in IL-6-supplemented cu ltures/ These results, considered together with previous findings, sug gest that activating ras mutations could be an initiating genetic alte ration in human acute lymphoblastic leukemia but are more Likely to be a postinitiation change in human acute myeloid leukemia.