RB1: a prototype tumor suppressor and an enigma

Abstract

The retinoblastoma susceptibility gene (RB1) was the first tumor suppressor gene to be molecularly defined. RB1 mutations occur in almost all familial and sporadic forms of retinoblastoma, and this gene is mutated at variable frequencies in a variety of other human cancers. Because of its early discovery, the recessive nature of RB1 mutations, and its frequency of inactivation, RB1 is often described as a prototype for the class of tumor suppressor genes. Its gene product (pRB) regulates transcription and is a negative regulator of cell proliferation. Although these general features are well established, a precise description of pRB's mechanism of action has remained elusive. Indeed, in many regards, pRB remains an enigma. This review summarizes some recent developments in pRB research and focuses on progress toward answers for the three fundamental questions that sit at the heart of the pRB literature: What does pRB do? How does the inactivation of RB change the cell? How can our knowledge of RB function be exploited to provide better treatment for cancer patients?

Keywords: E2F; cell proliferation; pRB; tumor suppressor.

Figure 1.

pRB and E2F provide cell cycle regulation of promoter activity. An interaction between pRB and E2F/DP heterodimeric complexes represses transcription of E2F-regulated promoters. This interaction can be detected in quiescent cells, differentiated cells, and cells arrested in G1 by activation of checkpoint pathways. When cells enter a cell division cycle, CDKs phosphorylate RB (depicted by yellow circles), leading to the disruption of E2F repressor complexes and the accumulation of activator E2F complexes that drive transcription.

Figure 2.

pRB has multiple mechanisms of action. (A) Shortly after the discovery of the interaction between RB and E2F, the model for pRB's mechanism action was relatively simple: pRB acts in the nucleus, where it associates with E2F complexes and represses promoters. Initially, the mechanism of repression was not known, and E2F targets were thought to be regulated in much the same way. (B) An updated model illustrating several of the layers of complexity that have been added to pRB's mechanism of action over the past two decades. Note that pRB recruits several different types of corepressors to E2F targets (depicted in red and pink), and, under certain conditions, E2F/RB complexes associate with coactivator complexes (green) and increase transcription of some targets. pRB does not act solely at E2F-binding sites but also associates with several transcription factors in addition to E2F. pRB has transcription-independent activities in the nucleus (illustrated here by its association with Skp2) and in the cytosol, where it associates with mitochondria.

Figure 3.

Multiple forms of pRB. (A) During G1 and in several types of arrested cells, pRB is hypophosphorylated. A recent study (Narasimha et al. 2014) revealed that this form of pRB is monophosphorylated on any one of 14 Cdk phosphorylation sites (denoted by the different yellow shapes) and converted to a fully inactive, hyperphosphorylated protein by Cyclin E/Cdk2 (or Cyclin A/Cdk2). Evidence that individual phosphorylation sites can selectively affect pRB's interaction with binding proteins leads to the speculation in B that specific monophosphorylation events may determine the localization and function of pRB. Note that the modified forms of pRB can coexist and that the post-translation regulatory code modulating pRB function need not be limited to phosphorylation but may also involve other types of protein modification. The specific functional properties of the monophosphorylated forms of pRB are as yet unknown.

Figure 4.

The consequences of RB1 inactivation. The ablation of RB1 impacts many cellular processes. These effects are highly interconnected, and it remains to be determined which specific changes are essential for tumorigenesis. A key goal in the immediate future will be to identify the consequences of RB1 inactivation that can be best exploited therapeutically to target RB1 mutant tumors.