A role for the RASSF1A tumor suppressor in the regulation of tubulin polymerization and genomic stability

The high frequency with which the novel tumor suppressor RASSFIA is inactivated by promoter methylation suggests that it plays a key role in the development of many primary human tumors. Yet the mechanism of RASSFIA action remains unknown. We now show that RASSFIA associates with microtubules and that this association is essential for RASSFIA to mediate its growth inhibitory effects. Overexpression of RASSFIA promotes the formation of stable microtubules, whereas a dominant-negative fragment of RASSFIA destabilizes microtubule networks. The RASSF1 protein is expressed as two main isoforms, 1A and 1C. The smaller 1C isoform also associates with microtubules but is less effective at stabilizing them. Because RASSFIA and RASSF1C localize to the mitotic spindle, we examined their effects upon genomic instability. RASSFIA and RASSF1C block activated Ras-induced genomic instability. However, a point mutant of RASSF1C, identified in human tumors, was severely defective for stabilizing tubulin and was unable to block the genomic destabilizing effects of Ras. Thus, we identify a role for RASSF1A/C in the control of microtubule polymerization and potentially in the maintenance of genomic stability.