Investigating the Role of Cyclin D1 in the Promotion of Genomic Instability and Breast Cancer

摘要

Cyclin D1 deregulation is implicated in the genesis of breast cancer, and elevated cyclin D1 protein expression occurs in the absence of gene amplification, suggesting that post-translational regulation is disrupted during the neoplastic process. Cyclin D1 protein is tightly regulated following the G1/S transition via threonine-286 (T286) phosphorylation and cytoplasmic degradation directed by the SCFFbx4 E3 ubiquitin ligase. Disruption of this regulation is deleterious to cell homeostasis, as nuclear cyclin D1 accumulation promotes DNA re-replication and genomic instability mediated by CUL4 repression and subsequent stabilization of the replication factor CDT1. Consistently, our recent work utilizing Fbx4-/- primary murine embryonic fibroblasts (MEFs) revealed that loss of Fbx4 accelerates growth, facilitates cyclin D1 protein stabilization in S-phase, and promotes nuclear cyclin D1 accumulation. However, increased proliferation in early passage Fbx4-/- MEFs is antagonized by DNA damage checkpoint activation with marked accumulation of DNA double strand break (DSB)-associated nuclear foci, consistent with nuclear cyclin D1 driving genomic instability. Additionally, further molecular dissection of nuclear cyclin D1-dependent genomic instability identified the protein arginine methyltransferase 5/methylosome protein 50 (PRMT5/MEP50) complex as a substrate and effector of cyclin D1-dependent neoplastic transformation, as nuclear cyclin D1 retention during S-phase increases PRMT5- dependent histone methylation at key target promoters, including CUL4A/B, providing a direct correlation among aberrant cyclin D1/CDK4 activity, transcriptional regulation, and perturbation of DNA replication fidelity. Collectively, these findings reveal an intricate relationship wherein nuclear cyclin D1/CDK4 activity modulates genetic alterations necessary for perturbed DNA replication, genomic instability, and ultimately neoplasia.