A TEMPERATURE-SENSITIVE P210 BCR-ABL MUTANT DEFINES THE PRIMARY CONSEQUENCES OF BCR-ABL TYROSINE KINASE EXPRESSION IN GROWTH-FACTOR DEPENDENT CELLS

The Philadelphia translocation commonly observed in chronic myeloid le ukaemia (CML) and a proportion of cases of acute leukaemia results in the creation of a chimeric fusion protein, BCR-ABL. The fusion protein exhibits an elevated tyrosine kinase activity as compared to normal A BL. Using a temperature sensitive mutant of p210 BCR-ABL (ts-p210) we find that the primary effect of BCR-ABL expression in an IL-3 dependen t cell line is to prolong survival following growth factor withdrawal; only a small proportion of cells remain viable and rapidly evolve to complete growth factor independence. During passage in the presence of IL-3 at the temperature permissive for kinase activity, ts-p210 expre ssing cultures become dominated by completely growth factor independen t cells within 10-30 days. There is also a significant difference betw een BCR-ABL and IL-3 mediated signalling with respect to the MAP kinas e pathway; in contrast to IL-3 stimulation or v-ABL expression, BCR-AB L does not signal ERK 2 (MAP 2 kinase) activation, underlining the app arent inability of BCR-ABL to deliver an immediate proliferative signa l in Ba/F3 cells. Our data suggest that growth factor independence doe s not simply reflect the convergence of BCR-ABL and IL-3 mediated sign alling pathways and its development, at least in Ba/F3 cells, requires prolonged exposure to BCR-ABL kinase activity. We suggest that the my eloid expansion characteristic of CML may result from the prolongation of survival of myeloid progenitor cells under conditions of limiting growth factor rather than their uncontrolled proliferation.