The RASSF1A Isoform of RASSF1 Promotes Microtubule Stability and Suppresses Tumorigenesis

Abstract

The RASSF1A isoform of RASSF1 is frequently inactivated by epigenetic alterations in human cancers, but it remains unclear if and how it acts as a tumor suppressor. RASSF1A overexpression reduces in vitro colony formation and the tumorigenicity of cancer cell lines in vivo. Conversely, RASSF1A knockdown causes multiple mitotic defects that may promote genomic instability. Here, we have used a genetic approach to address the function of RASSF1A as a tumor suppressor in vivo by targeted deletion of Rassf1A in the mouse. Rassf1A null mice were viable and fertile and displayed no pathological abnormalities. Rassf1A null embryonic fibroblasts displayed an increased sensitivity to microtubule depolymerizing agents. No overtly altered cell cycle parameters or aberrations in centrosome number were detected in Rassf1A null fibroblasts. Rassf1A null fibroblasts did not show increased sensitivity to microtubule poisons or DNA-damaging agents and showed no evidence of gross genomic instability, suggesting that cellular responses to genotoxins were unaffected. Rassf1A null mice showed an increased incidence of spontaneous tumorigenesis and decreased survival rate compared with wild-type mice. Irradiated Rassf1A null mice also showed increased tumor susceptibility, particularly to tumors associated with the gastrointestinal tract, compared with wild-type mice. Thus, our results demonstrate that Rassf1A acts as a tumor suppressor gene.

ACKNOWLEDGMENTS

L.V.D.W. and D.J.A. are recipients of Australian NHMRC postdoctoral research fellowships (CJ Martin/RG Menzies Fellowship and CJ Martin Fellowship, respectively). K.K.T. is supported by an MRC predoctoral fellowship, M.A.G. is supported by an MRC clinical research training fellowship, and M.J.A. is supported by Cancer Research UK. This research was supported by the Wellcome Trust.

We thank Ron Laskey for helpful discussions, Ian Roberts for assistance with cytogenetic analysis, Graham Bignell for assistance with quantitative RT-PCR experiments, Evelyn Grau for microinjection of the targeted ES cell lines, and the personnel of Team 83 for care and monitoring of the animals.