HCDC4 Mutation Leads to Deregulation of Cyclin E Relative to the Cell Cycle

Analysis of cyclin E in the breast tumor-derived cell line found to be mutated for hCDC4 (SUM149PT) indicated that the protein was overexpressed relative to non-transformed human mammary epithelial cells and most other BC derived cell lines (13). However, more interestingly, when the SUM149PT cell line was analyzed by immunofluorescence microscopy all cells in an asynchronous population were cyclin E-positive. This is in contrast to other tumor-derived cell lines and non-transformed cells where only 30-60% of cells stain positively for cyclin E. Therefore, loss ofCdc4 function, presumably leading to loss of SCFCdc4 protein-ubiquitin ligase activity, results in deregulation of cyclin E relative to cell cycle progression.

Cyclin E status was also determined in endometrial tumors. Similarly to the SUM149PT cell line, endometrial tumors that had hCDC4 mutations showed a deregulation of cyclin E relative to the cell cycle. Sections from hCDC4-mutated tumors typically resulted in well over 60% cyclin E positive nuclei, whereas similar analysis of non-mutated tumors always resulted in levels significantly lower than 60%. Even tumors that had extremely high cyclin E levels, but were wild-type at the hCDC4 locus, exhibited a normal distribution of cyclin E positive and negative nuclei. Surprisingly, cyclin E protein levels in endometrial tumors did not correlate well with hCDC4 mutational status (23). Most tumors that had high cyclin E levels, based on western blotting, did not have hCDC4 mutations. Conversely, most tumors that had HCDC4 mutations had only moderate levels of cyclin E. These results suggest that when the sCFhCDC4 pathway is inactivated, other modes of cyclin E turnover can compensate. However, these alternative pathways cannot restore cell cycle regulation of cyclin E.

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