Cell-cycle progression is regulated by the cyclin-dependent kinase (Cdk) family of protein kinases, so named because their activation depends on association with regulatory subunits known as cyclins. Cyclin E normally accumulates at the G1/S boundary, where it promotes S phase entry and progression by activating Cdk2. In normal cells, cyclin E/Cdk2 activity is associated with DNA replication-related functions. However, deregulation of cyclin E leads to inefficient assembly of pre-replication complexes, replication stress, and chromosome instability. In malignant cells, cyclin E is frequently overexpressed, correlating with decreased survival in breast cancer patients. Transgenic mice deregulated for cyclin E in the mammary epithelia develop carcinoma, confirming that cyclin E is an oncoprotein. However, it remains unknown how cyclin E-mediated replication stress promotes genomic instability during carcinogenesis. Here, we show that deregulation of cyclin E causes human mammary epithelial cells to enter into mitosis with short unreplicated genomic segments at a small number of specific loci, leading to anaphase anomalies and ultimately deletions. Incompletely replicated regions are preferentially located at late-replicating domains, fragile sites, and breakpoints, including the mixed-lineage leukemia breakpoint cluster region (MLL BCR). Furthermore, these regions are characterized by a paucity of replication origins or unusual DNA structures. Analysis of a large set of breast tumors shows a significant correlation between cyclin E amplification and deletions at a number of the genomic loci identified in our study. Our results demonstrate how oncogene-induced replication stress contributes to genomic instability in human cancer.
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