Poster de conférence
Résumé : - Introduction:The cyclin-dependent kinases CDK4/6 are key regulators of the cell cycle entry, by phosphorylating the onco-suppressor retinoblastoma protein (pRb). Their deregulated activity is a major oncogenic event in most types of tumours, including breast cancer. Although the assembly of CDK4 with a D-type cyclin (CCND1-3) has been considered to be the main level of activity control, our group has shown that the activating T172-phosphorylation of CDK4 is actually the central rate-limiting event that initiates the cell cycle decision and signals the presence of active CDK4 in various cell systems (1,2,3). CDK4/6 inhibitory compounds are being tested in a growing number of phase II/III clinical trials in almost all cancers. Due to unprecedented improvement of the progression-free survival (4), the use of palbociclib (PD0332991) - the first specific inhibitor of CDK4/6 - was recently approved by the FDA to treat advanced estrogen receptor-positive breast tumors in combination with letrozole. Yet, no reliable sensitivity prediction tool exists to indicate which tumors might be potentially sensitive to CDK4/6 inhibitors.- Aims:To investigate the post-translational modifications of CDK4 in the different subtypes of breast tumors and apply them as a biomarker to predict the tumor sensitivity to CDK4/6 inhibitors. - Methods and results:Palbociclib sensitivity was first compared in a subset of 20 breast cancer cell lines representative of different molecular subtypes. Using the BrDU proliferation method, to assess their rate of DNA synthesis, we observed that the majority of tested cell lines are sensitive to the drug with IC50 around 15 nM with either full or partial inhibition of DNA synthesis. Sensitivity to the drug was correlated to the CDK4 modified forms, resolved by 2D-gel electrophoresis. As previous observed (1), CDK4 is resolved by its charge in three forms: the native CDK4, the Thr172-phosphorylated form and a not yet defined, unphosphorylated, form. We found that CDK4 T172-phosphorylation was present in all sensitive cell lines (mainly luminal and HER2-amplified cell lines, but also some triple-neaative cell lines). On the opposite, this phosphorylation, which signals the presence of active CDK4, was absent in all the completely insensitive cell lines.The CDK4 forms were further analyzed in 56 fully-profiled breast tumors and three distinct modification profiles were identified. The proportions of these profiles differed among breast tumors according to their clinic-pathological characteristics, molecular subtypes and risk. In the first profile, the phosphorylated CDK4 was undetectable despite a high KI67 index. This profile, identical to the one seen in the insensitive breast cancer cell lines, was also detected in normal breast samples and was more frequent in basal-like and triple-negative tumors. In the second and third profiles, the CDK4 phosphorylation was detectable and its intensity was either above or below the intensity of the unidentified CDK4 form, respectively. This second profile, seen in the sensitive cell lines, was most frequent among HER2+ tumors and luminal B tumors but it was also present in 6 of 16 basal-like and triple-negative tumors. The third profile was most frequent in luminal A tumors.Finally, we identified a 11-gene expression signature that faithfully predicted the CDK4 modification profiles of breast tumors and cell lines (concordance rates of 84% and 100%, respectively). This signature was evaluated in a merged independent dataset of 4034 published gene expression profiles. In this dataset, 70% of triple-negative tumors, 18% of HER2-positive tumors and 5% of ER-positive tumors were predicted to have the first CDK4 profile and thus to be completely unresponsive to CDK4/6 inhibitors. The phosphorylated CDK4 was predicted to be the major modified form in 26% of triple-negative tumors, 48% of HER2- positive tumors and 56% of ER-positive tumors. These patients are expected to benefit the most from treatment with CDK4 inhibitors. As tumors with the third CDK4 modification profile generally present low grade and risks, the added value of including CDK4/6 inhibitors in their treatment compared to surgery and hormone therapy alone might be more questionable.- Conclusions : We identified CDK4 phosphorylation as the most direct biomarker of CDK4/6 inhibitors sensitivity in breast cancer. The developed 11-gene based surrogate marker uncovers the possibility to extend the palbociclib treatment to presently ineligible patients.- References : 1. Bockstaele, L. et al. (2006). Mol. Cell. Biol. 26, 5070-5085.2. Paternot, S. et al. (2010). Cell Cycle 9, 689-699.3. Bisteau, X. et al. (2013) PLoS Genet. 9(5):e1003546. doi: 10.1371/journal.pgen.1003546.4. Finn, R.S. et al. (2015). Lancet Oncol.16, 25-35.

Documents en relation

DI-fusion