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Chromosome Segregation
Upregulation of the mitotic checkpoint gene Mad1

The mitotic checkpoint (also known as the spindle assembly checkpoint) is the major cell cycle checkpoint acting during mitosis to prevent chromosome missegregation. Previous work has demonstrated that reduction of mitotic checkpoint proteins, including Mad1, weakens checkpoint signaling, resulting in chromosome missegregation and aneuploidy. We found that elevating levels of Mad1 in cells that were previously chromosomally stable (Movie 1) also weakens mitotic checkpoint signaling, leading to chromosome missegregation (Movie 2). Mad1 plays an essential role in mitotic checkpoint signaling by recruiting its binding partner Mad2 to unattached kinetochores where it is converted into an active inhibitor of the Anaphase Promoting Complex/Cyclosome. Elevated levels of Mad1 saturate available binding sites and sequester Mad2 away from kinetochores, weakening mitotic checkpoint signaling.

These findings are clinically relevant, as Mad1 expression at both the mRNA and protein level is frequently elevated in human cancers. Moreover, patients with tumors expressing heightened levels of Mad1 mRNA exhibit decreased survival compared to those with lower levels of Mad1. These data suggest that upregulation of Mad1 is a clinically relevant mechanism of inducing aneuploidy in human tumors. (Ryan et al, PNAS, 2012)

 
 
Roles of cancer genes in chromosome segregation

Loss or mutation of multiple tumor suppressors, including ARF, Rb, PTEN, and APC results in chromosome missegregation and aneuploidy. We are currently determining the mechanism by which loss of the ARF tumor suppressor causes mitotic defects (Figure 1).

Histone H2B-RFP

Movie 1. Normal mitosis in a DLD1 human colon cancer cell.

Movie 2. Upregulation of Mad1 causes chromosome missegregation in DLD1 cells.

 
 

Figure 1. Arrow indicates a lagging chromosome in an ARF null cell.

 
  Cell & Regenerative Biology                 School of Medicine and Public Health                 University of Wisconsin - Madison