Oncogene co-operation in leukaemogenesis.

The multistep development of haematopoietic malignancies, like other neoplasms, reflects sequential mutations that either activate proto-oncogenes or disrupt tumour suppressor genes. In a few spontaneous leukaemias or lymphomas, more than one mutation has now been identified, and the experimental analysis of oncogene co-operation is advancing rapidly via retroviral gene delivery and characterization of transgenic mice bearing oncogenes. In transgenic models, tumorigenesis can be accelerated by introducing another oncogene or by using a retrovirus as an insertional mutagen to identify cellular genes that collaborate with the transgene. Leukaemogenesis can be promoted by some ten pairs of oncogenes. The myc nuclear oncoprotein, for example, can collaborate with cytoplasmic oncoproteins such as ras, raf, bcl-2, pim-1 and v-abl, as well as with nuclear products such as bmi-1 or the tumour suppressor p53. The genes in such partnerships seem to provide complementary functions. For example, myc seems to prevent cells from becoming quiescent, whereas bcl-2 blocks programmed cell death; and others, for example ras, may diminish growth factor requirements. The products of genes that collaborate may lie on separate signal transduction pathways, leading to distinct nuclear targets. Key targets are postulated to be regulators of the cell cycle, especially the cyclins and associated kinases that govern progression in the G1 phase.