Le rôle de la cycline D1 dans la lymphopoïèse. – Cyclin D1

D type cyclins are fundamental to initiate cell division in response to exogenous stimuli. The individual members of this family (D1, D2, D3) are believed to have redundant roles, but arguing against a mutual redundancy, only D1 is very frequently involved in transformation in general and lymphoid neoplasias in particular. We studied mice where the D1 cyclin box was removed by homologous recombination. Surprisingly, we found that these mice had two opposite phenotypes, that correlated with the expression of a truncated cyclin D1 protein comprising the C' domains, described to be involved in transcription regulation. When this truncated protein was not expressed, other D2, D3 cyclins were also absent, the bone-marrow precursors failed to divide and to differentiate into lymphoid lineage cells. In contrast, the when a truncated D1 regulatory fragment was expressed, it was present in abnormal high amounts, D2, D3 were expressed at abnormal high levels and lymphoid lineage cells showed increased proliferation and could develop lymphoid hyperplasia. These results suggest that the D1 cyclin could have a fundamental role as a master cyclin in cell cycle entry and progression and, moreover, that this role is mediated by D1 C' regulatory domains. This project aims: 1) To prove that a complete absence of D1 leads to a block in lymphoid differentiation, by removing D1 using RNA interference. 2) To determine if D1 is directly involved in the expression of other D type cyclins by Chip analysis. 3) To quantify by con-focal microscopy the distribution of D1 truncated protein in 'hyper-proliferation' mice, and to evaluate if this truncated protein can migrate to the nucleus upon cell activation. 4) To force the expression of a D1 truncated protein using lentivirus translation and to determine how this over-expression influences division rates and lymphoid differentiation. 5) To determine if this truncated protein is differentially expressed in normal and transformed cell lines. In spite of a massive effort, the understanding of the mechanisms by which D1 mediates transformation is far from understanding. We believe that this project can be a major breakthrough in this domain since it opens a totally new perspective on the putative role of D1 in general, and its C' regulatory domain in particular in controlling cell division in the response to environmental signals.