Tailoring to RB: tumour suppressor status and therapeutic response

Abstract

The retinoblastoma tumour suppressor (RB) is a crucial regulator of cell-cycle progression that is invoked in response to a myriad of anti-mitogenic signals. It has been hypothesized that perturbations of the RB pathway confer a synonymous proliferative advantage to tumour cells; however, recent findings demonstrate context-specific outcomes associated with such lesions. Particularly, loss of RB function is associated with differential response to wide-ranging therapeutic agents. Thus, the status of this tumour suppressor may be particularly informative in directing treatment regimens.

Figure 1. Schematic of RB in cell cycle control

Mitogenic signals stimulate the expression of D-type cyclins (Cyc) and a concomitant increase in cyclin-dependent kinase 4 (CDK4) and CDK6 activity. These factors initiate RB phosphorylation, which is augmented by the activity of CDK2 complexes with cyclins A and E. The phosphorylation of RB disrupts its association with E2F. This inactivation of RB allows for the expression of a transcriptional programme that enables progression through S-phase and mitosis. At the transition from mitosis to G1, RB is dephosphorylated through the action of phosphatases. Importantly, a large number of anti-mitogenic signals function to prevent RB phosphorylation either by limiting the activity of CDK4, CDK6 and CDK2 complexes or by inducing the activity of CDK inhibitors.

Figure 2. Influence of RB loss: context dependence

a | RB loss leads to diverse outcomes, such as hyperplasia or genome instability, in different cellular and tissue contexts. b | A diverse class of signals leads to the dephosphorylation and activation of RB. Activated RB can mediate cell-cycle inhibition leading to quiescence and/or dormancy or senescence. With the loss of RB, it is possible to escape cell-cycle inhibition and progress to either proliferation or apoptotic cell death.

Figure 3. Mutiple markers for RB dysfunction

a | Direct analyses of the status of RB1 are used in genetic tests to establish the presence of heritable retinoblastoma. RB levels can be directly interrogated by immunohistochemical analyses and the use of phospho-specific RB antibodies provides additional information as to the status of RB. However, as RB can be inactivated through multiple mechanisms, indirect markers for RB function can also be particularly informative. In this context, gene expression signatures reveal the downstream consequence of RB loss. p16^INK4A levels are increased in specific RB-negative tumours, and these cells can be discerned from pockets of senescent pre-neo-plastic cells by the inclusion of a proliferative marker such as Ki67. b | Gene-signature analyses have shown the functional groups of genes that are deregulated by the loss of RB1 through either deletion or expression of viral oncoproteins. This gene expression signature overlaps with the gene expression grade index and the Oncotype Dx signatures by 65% and 80% respectively.

Figure 4. Exploiting RB deficiency therapeutically

a | RB limits the pro-apoptotic activity of E2F1, so RB-deficient cells are more prone to apoptosis. Cyclin-dependent kinase 2 (CDK2) activity further limits E2F1 activity. Thus, CDK2 inhibitors can further increase E2F1 activity and drive RB-deficient cells to apoptosis. b | In Drosophila melanogaster models, E2F activity can induce apoptosis; however, this is limited by the action of epidermal growth factor receptor (EGFR) signalling and the apoptosis inhibitor 5 (API5, also known as AAC11). c | In mouse models of retinoblastoma, p53 inactivation is required for tumour development and the survival of RB-deficient cells. In tumorigenesis, this inactivation occurs owing to an upregulation of MDM2 or MDM4. The activity of these oncogenes can be targeted by the nutlins, a group of drugs that disrupt the interaction between MDM2 and p53.