Novel Role of Activated Ras in Leukaemogenesis: Induction of Angiogenesis.

Mutations in Ras genes are among the most frequent genetic alterations found in leukaemia. Our previous results have indicated that mutant N-ras (N-ras^m) expression in the murine haematopoietic progenitor cells results in the development of myeloproliferative disorders that resemble human leukaemia. Our group has recently established a ‘humanised’ NOD/SCID mouse model of Ras-induced leukaemogenesis. N-ras^m was transduced into normal human primitive haematopoietic progenitor cells (HPC) using recombinant GFP-expressing retrovirus to initiate the leukaemogenic transformation. When injected in NOD/SCID mice, N-ras^m/GFP-expressing HPCs induce a pre-leukaemic condition characterised by the increased expansion of human N-ras^m/GFP-positive cells with concomitant myeloid lineage expansion in the bone marrow of the recipient mice. In long-term culture N-ras^m induces expansion of primitive CD34+/CD117+ and CD34+CXCR4+ progenitor cells enriched with NOD/SCID repopulating cells as well as myelomonocytic CD34−/CD14+ cells within the expanse of erythroid and lymphoid lineages. Microarray expression analysis revealed the induction of a number of the angiogenic factors and extracellular matrix proteins including VEGF, SDF-1, IL3, PDGF 1 and 2, metalloproteinases 9, 11, 12 and 14 that promote angiogenesis. The expansion of the primitive endothelial (CD34+VEGFR2+) and lymphatic (CD34+VEGFR3+) progenitor cells was consistently observed in N-ras^m-transduced cultures. In addition, myelomonocytic CD34−CD14+ cells expanded by N-ras^m also appear to contribute to neo-angiogenesis. N-ras^m-induced pro-angiogenic factors also act as mitogenic and survival factors for pre-leukaemia HPCs by inhibiting apoptosis and promoting leukaemia-inititating-cell engraftment into the bone marrow of the recipient mice. N-ras^m acts to modulate the interaction of leukaemia-initiating-cells with the bone marrow stroma by the induction of extracellular matrix degradation. We propose that the induction of pro-angiogenic factors by N-ras^m acts to promote leukaemogenic transformation of leukaermia-inititating stem cells and needs to be investigated as a new candidate therapeutic target for leukaemia.