Role of the retinoblastoma tumor suppressor in the maintenance of genome integrity.

The retinoblastoma tumor suppressor (RB) is functionally inactivated at high frequency in human cancers. Based on the role of RB as a negative regulator of cell cycle this event would be expected to contribute to deregulated proliferation. However, evidence suggests that loss of RB not only mediates aberrant proliferation, but compromises the fidelity of cell cycle transitions leading to a breakdown in genome integrity. This review is focused on the mechanisms underlying this facet of RB function and the contribution of this process to tumorigenesis.

RB loss abrogates cell cycle control and genome integrity to promote liver tumorigenesis.

BACKGROUND & AIMS: The retinoblastoma (RB) tumor suppressor is functionally inactivated in most hepatocellular carcinomas (HCC), although the mechanisms by which RB suppresses liver tumorigenesis are poorly defined. We investigated the impact of RB loss on carcinogen-induced liver tumorigenesis. METHODS: Mice harboring liver-specific RB ablation and normal littermates were exposed to the hepatocarcinogen diethylnitrosamine (DEN). The influence of RB loss on liver tumorigenesis was assessed by evaluating tumor multiplicity, proliferation, and genome integrity within tumors arising in RB-deficient and wild-type livers. In silico analyses were used to probe the association between gene expression signatures for RB loss and chromosomal instability and the ability of genes up-regulated by RB loss to predict the survival of human HCC patients. RESULTS: RB deficiency significantly increased tumor multiplicity in livers exposed to DEN. Although hepatocytes in nontumor regions of DEN-exposed livers were quiescent regardless of RB status, tumors arising in RB-deficient livers were significantly more proliferative than those in normal livers and expressed high levels of RB/E2F target genes. Analysis of genes up-regulated by RB loss demonstrated significant overlap with a gene expression signature associated with chromosomal instability. Correspondingly, tumors arising in RB-deficient livers were significantly more likely to harbor hepatocytes exhibiting altered ploidy. Finally, gene expression analysis of human HCCs demonstrated that elevated expression of RB-regulated genes independently predicts poor survival. CONCLUSIONS: RB deletion in the mouse liver enhances DEN-induced tumorigenesis, associated with increased hepatocyte proliferation and compromised genome integrity. Evaluation of RB status may be a useful prognostic factor in human HCC.