Résumé
Breast cancer is the most frequently diagnosed cancer in women. This cancer is the leading cause of death in women aged from 35 to 65 years old. Different treatments are now available depending on tumor subtypes. However, some patients are still refractory to these therapies and are at risk of disease relapse. Cancer research has long focused on aberrant cancer cell division but today it is evident that the resistance to programmed cell death is also a major characteristic of the disease.D-type cyclins regulate cell cycle by allowing the transition from the G1-phase to the S-phase. These regulatory subunits activate the Cyclin-Dependent Kinases 4/6 (CDK4/6) that phosphorylate the retinoblastoma proteins which then release the E2F transcription factors. Nuclear Cyclin D1 (CycD1) is therefore central in the control of division. The Ccnd1 gene is amplified in human cancers and half of breast cancer patients bare an overexpression of CycD1. CycD1 is required for mammary carcinoma onset and progression in a CDK4 kinase-dependent manner. Hence, specific CDK4/6 inhibitors have been developed and authorized in the clinics against breast cancer. Unfortunately, some patients remain insensitive to this treatment. In this frame, the specific targeting of CycD1 could represent a strategic alternative in clinics to overcome these pitfalls. Indeed, in addition to cell cycle regulation with CDK4, CycD1 is also involved in CDK4-independent features of cancer cells like cell survival. However, to date, no clear mechanism for the impact of CycD1 in tumor maintenance is established to demonstrate the therapeutic value of its targeting.Moreover, recent studies have demonstrated the participation of CycD1 in adult organs to regulate glucose metabolism and hematopoiesis. As a consequence, to avoid any undesirable side effects, we decided to gauge the potential CycD1 implication in post-mitotic organs body-wide. We set up a new hypersensitive technology named Tandem-HTRF based on the energy transfer between two antibodies to reveal the unexpected dynamics of CycD1 expression in adult organ. Then, we discovered that alterations of CycD1 expression induced dramatic functional consequences on the survival capacities of healthy adult post-mitotic cells.Based on these limitations, we developed a novel RNAi approach specific to cancer cells named TAG-RNAi. This technology allows the silencing of CycD1 in cancer cells only to spare healthy cells. This innovative approach consists in the targeting of a mRNA tag only present on CycD1 from cancer cells. Using this technique, we found that the specific silencing of CycD1 induces a rapid and spontaneous regression of tumors driven by the RAS or ERBB2 oncogenes. Then, thanks to a proteomics screening in vivo, I discovered that under pro-apoptotic stresses the cytoplasmic CycD1 interacts with the procaspase-3 protein and blocks its activation to prevent cancer cell apoptosis. Altogether, my work demonstrates the clinical value of the specific targeting of CycD1 in cancers to increase the efficacy of chemotherapeutic treatments.Hence, it remained to be determined how to apply in patients RNAi against CycD1 only in cancer cells. Because the exotic tagging of its gene was instrumental in mice cancer models, we reasoned that human cancer mutations could represent such a specific tag. We have extended the concept of TAG-RNAi to somatic mutations characteristic of human cancers to successfully target the expression of KRAS-G12V or BRAF-V600E mutants as examples. The idea is therefore to identify Ccnd1 mutations in cancer patients in order to apply TAG-RNAi as a custom therapeutic approach that will manage side effects. More unanticipated, CycD1 expression represents a new biomarker for both cancer and age-related disorders: low CycD1 levels predispose to degenerative complications while high CycD1 levels indicate increased susceptibility to cancer and resistance to treatment.