N-terminus-modified Hec1 suppresses tumour growth by interfering with kinetochore-microtubule dynamics

Mitotic proteins are attractive targets to develop molecular cancer therapeutics due to the intimate interdependence between cell proliferation and mitosis. In this work, we have explored the therapeutic potential of the kinetochore (KT) protein Hec1 (Highly Expressed in Cancer protein 1) as a molecular target to produce massive chromosome missegregation and cell death in cancer cells. Hec1 is a constituent of the Ndc80 complex, which mediates KT-microtubule (MT) attachments at mitosis and is upregulated in various cancer types. We expressed Hec1 fused with enhanced green fluorescent protein (EGFP) at its N-terminus MT-interaction domain in HeLa cells and showed that expression of this modified Hec1, which localized at KTs, blocked cell proliferation and promoted apoptosis in tumour cells. EGFP-Hec1 was extremely potent in tumour cell killing and more efficient than siRNA-induced Hec1 depletion. In striking contrast, normal cells showed no apparent cell proliferation defects or cell death following EGFP-Hec1 expression. Live-cell imaging demonstrated that cancer cell death was associated with massive chromosome missegregation within multipolar spindles after a prolonged mitotic arrest. Moreover, EGFP-Hec1 expression was found to increase KT-MT attachment stability, providing a molecular explanation for the abnormal spindle architecture and the cytotoxic activity of this modified protein. Consistent with cell culture data, EGFP-Hec1 expression was found to strongly inhibit tumour growth in a mouse xenograft model by disrupting mitosis and inducing multipolar spindles. Taken together, these findings demonstrate that stimulation of massive chromosome segregation defects can be used as an anti-cancer strategy through the activation of mitotic catastrophe after a multipolar mitosis. Importantly, this study represents a clear proof of concept that targeting KT proteins required for proper KT-MT attachment dynamics constitutes a powerful approach in cancer therapy.

Mast, a conserved microtubule-associated protein required for bipolar mitotic spindle organization

Through mutational analysis in Drosopjila we have identified the gene multiple asters (mast), which encodes a new 165 kDa protein. mast mutant neuroblasts are highly polyploid and show severe mitotic abnormalities including the formation of mono- and multi-polar spindles organized by an irregular number of microtubule-organizing centres of abnormal size and shape. The mast gene product is evolutionarily conserved since homologues were identified from yeast to man, revealing a novel protein family. Antibodies against Mast and analysis of tissue culture cells expressing an enhanced green fluorescent protein-Mast fusion protein show that during mitosis, this protein localizes to centrosomes, the mitotic spindle, centromeres and spindle midzone. Microtubule-binding assays indicate that Mast is a microtubule-associated protein displaying strong affinity for polymerized microtubules. The defects observed in the mutant alleles and the intracellular localization of the protein suggest that Mast plays an essential role in centrosome separation and organization of the bipolar mitotic spindle.