Polyploidization can precede the development of aneuploidy in cancer. Polyploidization in megakaryocytes (Mks), in contrast, is a highly controlled developmental process critical for efficient platelet production via unknown mechanisms. Using primary cells, we demonstrate that the guanine exchange factors GEF-H1 and ECT2, which are often overexpressed in cancer and are essential for RhoA activation during cytokinesis, must be downregulated for Mk polyploidization. The first (2N-4N) endomitotic cycle requires GEF-H1 downregulation, whereas subsequent cycles (>4N) require ECT2 downregulation. Exogenous expression of both GEF-H1 and ECT2 prevents endomitosis, resulting in proliferation of 2N Mks. Furthermore, we have shown that the mechanism by which polyploidization is prevented in Mks lacking Mkl1, which is mutated in megakaryocytic leukemia, is via elevated GEF-H1 expression; shRNA-mediated GEF-H1 knockdown alone rescues this ploidy defect. These mechanistic insights enhance our understanding of normal versus malignant megakaryocytopoiesis, as well as aberrant mitosis in aneuploid cancers. Video Abstract: Megakaryocyte polyploidization is critical for efficient platelet production. Gao et al. find that this process depends on downregulation of two RhoA-directed guanine nucleotide exchange factors (RhoGEFs) that control distinct stages of cytokinesis and are often overexpressed in cancer. Defects in RhoGEF downregulation may contribute to certain megakaryocytic leukemias. © 2012 Elsevier Inc.
Gao, Y., Smith, E., Ker, E., Campbell, P., Cheng, E. chun, Zou, S., … Krause, D. S. (2012). Role of RhoA-Specific Guanine Exchange Factors in Regulation of Endomitosis in Megakaryocytes. Developmental Cell, 22(3), 573–584. https://doi.org/10.1016/j.devcel.2011.12.019