Avian erythroblastosis: a model system to study oncogene co-operation in leukemia.

Avian erythroblastosis virus, AEV-ES4, provides an ideal in vitro system to study oncogene co-operation in the development of erythroid leukaemias in chickens. Two oncogenes that use distinct signal transduction pathways have been identified; these act to produce a highly malignant phenotype and are an oncogenic version of a plasma membrane growth factor receptor that regulates haematopoietic progenitor self renewal and a mutated version of a transcription factor that suppresses specific gene expression. These two diverse types of oncogenes act together to generate a more malignant phenotype than would be expected merely from a summation of the individual oncogene effects. The aspects of normal growth control and the regulation of differentiation, which are altered in this system to generate the leukaemic phenotype, include changes in the balance between proliferation versus maturation, altered expression of differentiation genes and changes in growth factor responsiveness. These are exactly those features that are altered in human leukaemias. Thus, the avian leukaemia virus model has taught us important lessons that will help us to understand the basic molecular mechanisms that give rise to human leukaemia.