Cell Death and Survival Hematopoietic Expression of Oncogenic BRAF Promotes Aberrant Growth of Monocyte-Lineage Cells Resistant to PLX4720

Mutational activation of BRAF leading to expression of the BRAF oncoprotein was recently identified in a high percentage of specific hematopoietic neoplasms in monocyte/histiocyte and mature B-cell lineages. Although BRAF is a driver oncoprotein and pharmacologic target in solid tumors such as melanoma, lung, and thyroid cancer, it remains unknown whether BRAF is an appropriate therapeutic target in hematopoietic neoplasms. To address this critical question, we generated a mouse model expressing inducible BRAF in the hematopoietic system, and evaluated the efficacy of pathway-targeted therapeutics against primary hematopoietic cells. In this model, BRAF expression conferred cytokine-independent growth to monocyte/macrophage-lineage progenitors leading to aberrant in vivo and in vitromonocyte/macrophage expansion. Furthermore, transplantation of BRAF-expressing bone marrow cells promoted an in vivo pathology most notable for monocytosis in hematopoietic tissues and visceral organs. In vitro analysis revealed thatMAP–ERK kinase inhibition, but not RAF inhibition, effectively suppressed cytokine-independent clonal growth of monocyte/macrophage-lineage progenitors. However, combined RAF and phosphoinositide 3-kinase (PI3K) inhibition effectively inhibited cytokineindependent colony formation, suggesting autocrine PI3K pathway activation. Taken together, these results provide evidence that constitutively activated BRAF drives aberrant proliferation of monocyte-lineage cells. Implications: This study supports the development of pathway-targeted therapeutics in the treatment of BRAF-expressing hematopoietic neoplasms in the monocyte/histiocyte lineage. Mol Cancer Res; 11(12); 1530–41. 2013 AACR.