RB depletion is required for the continuous growth of tumors initiated by loss of RB

Abstract

The retinoblastoma (RB) tumor suppressor is functionally inactivated in a wide range of human tumors where this inactivation promotes tumorigenesis in part by allowing uncontrolled proliferation. RB has been extensively studied, but its mechanisms of action in normal and cancer cells remain only partly understood. Here, we describe a new mouse model to investigate the consequences of RB depletion and its re-activation in vivo. In these mice, induction of shRNA molecules targeting RB for knock-down results in the development of phenotypes similar to Rb knock-out mice, including the development of pituitary and thyroid tumors. Re-expression of RB leads to cell cycle arrest in cancer cells and repression of transcriptional programs driven by E2F activity. Thus, continuous RB loss is required for the maintenance of tumor phenotypes initiated by loss of RB, and this new mouse model will provide a new platform to investigate RB function in vivo. Author summary The retinoblastoma protein (RB) is a central regulator of the cell cycle. Functional inactivation of RB leads to unchecked proliferation and is a frequent occurrence in human cancer. Experiments with pre-clinical models have clearly demonstrated that loss of RB is a strong promoter of cancer development. However, the mechanisms through which RB carries out its tumor suppressor function remain only partly understood. Here, we describe a new genetically engineered mouse model in which RB expression can be turned on or off. Using this controllable RB knock-down system, we investigated both the consequences of RB loss in normal cells and its re-expression in cancer cells. We found that long-term RB knock-down in mice leads to the development of pituitary and thyroid tumors, recapitulating previously reported phenotypes of Rb knockout mice, thus validating this new mouse model. Notably, re-expression of RB in pituitary tumors initiated by loss of RB was sufficient to block tumor growth, indicating that these tumors are still dependent on RB loss for their expansion.