Kinetochore and Cancer

Errors during cell division may result in unequal distribution of DNA between the daughter cells. Gain or loss in the number of chromosomes of the genome is a known cause for miscarriages and birth defects in human, and a hallmark of cancer. We are investigating a signaling pathway that regulates integrity of cell division. This pathway termed the spindle checkpoint (also known as mitotic checkpoint) monitors interactions between spindle microtubules and kinetochores, the microtubule binding platforms of chromosomes.

If mistakes occur in the microtubule-kinetochore connections, the spindle checkpoint becomes active and prevents separation of sister chromatids until errors in the chromosome alignment are corrected. Although the main principles of the spindle checkpoint are well documented many molecular details remain to be explored.

The main function of mitotic checkpoint is to control the activity of a large mitotic E3 ubiquitin ligase called the Anaphase Promoting Complex/Cyclosome (APC/C). At the onset of anaphase, mitotic cohesins that hold sister chromatids together are subjected to proteolytic cleavage in an APC/C regulated manner. The activity of APC/C itself is regulated by two possibly independent pathways that rely on members of Mad, Bub, and Cdc20 protein families. According to a current model, these evolutionarily conserved molecules interact with each other to form checkpoint protein sub-complexes at the kinetochores. The assembly of these protein complexes is regulated by an unknown mechanism that is dependent on microtubule occupancy and/or physical tension applied at the kinetochores. Presence of even one misaligned chromosome causes a cell-wide inhibition of APC/C.
 
Specific Aims

1. We are investigating the function of two protein groups that locate at the interface between the microtubules and the outer kinetochores; the microtubule plus-end proteins and the Ndc80 complex proteins. The specific question we wish to answer is how the members of these protein groups interact with each other and how they contribute to spindle checkpoint signaling.
    
2. We are also studying the formation of spindle checkpoint protein sub-assemblies that form at the kinetochores in a microtubule occupancy/tension dependent manner. We wish to elucidate how the inhibitory signals are created at the kinetochores and how they are broadcasted throughout the cell.

Significance

Although spindle checkpoint protein deficiency is unlikely to be observed in live humans, haploinsufficiency (loss of an allele) of a mitotic checkpoint protein can contribute to cancer formation. Haploinsufficiency of Mad2 protein has been shown to result in a defective mitotic checkpoint in human cancer cells and primary mouse embryonic fibroblasts, and to increase susceptibility to lung cancer in mice. Understanding spindle checkpoint malfunction may help to explain origin of genetic instability and identify novel therapeutic possibilities for treatment of cancer.

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