Checkpoint-Independent Stabilization of Kinetochore-Microtubule Attachments by Mad2 in Human Cells

Faithful chromosome segregation is required for cell and organism viability and relies on both the mitotic checkpoint and the machinery that corrects kinetochore-microtubule (k-MT) attachment errors [1-3]. Most solid tumors have aneuploid karyotypes and many missegregate chromosomes at high rates in a phenomenon called chromosomal instability (CIN) [4-6]. Mad2 is essential for mitotic checkpoint function and is frequently overexpressed in human tumors that are CIN [1, 7-13]. For unknown reasons, cells overexpressing Mad2 display high rates of lagging chromosomes [14, 15]. Here, we explore this phenomenon and show that k-MT attachments are hyperstabilized by Mad2 overexpression and that this undermines the efficiency of correction of k-MT attachment errors. Mad2 affects k-MT attachment stability independently of the mitotic checkpoint because k-MT attachments are unaltered upon Mad1 depletion and Mad2 overexpression hyperstabilizes k-MT attachments in Mad1-deficient cells. Mad2 mediates these effects with Cdc20 by altering the centromeric localization and activity of Aurora B kinase, a known regulator of k-MT attachment stability. These data reveal a new function for Mad2 to stabilize k-MT attachments independent of the checkpoint and explain why Mad2 overexpression increases chromosome missegregation to cause chromosomal instability in human tumors.