Significance of RB Loss in Unlocking Phenotypic Plasticity in Advanced Cancers

Abstract

Cancer cells can undergo plasticity in response to environmental stimuli or under selective therapeutic pressures that result in changes in phenotype. This complex phenomenon of phenotypic plasticity is now recognized as a hallmark of cancer. Lineage plasticity is often associated with loss of dependence on the original oncogenic driver and is facilitated, in part, by underlying genomic and epigenetic alterations. Understanding the molecular drivers of cancer plasticity is critical for the development of novel therapeutic strategies. The retinoblastoma gene RB1 (encoding RB) is the first tumor suppressor gene to be discovered and has a well-described role in cell-cycle regulation. RB is also involved in diverse cellular functions beyond cell cycle including differentiation. Here, we describe the emerging role of RB loss in unlocking cancer phenotypic plasticity and driving therapy resistance across cancer types. We highlight parallels in cancer with the noncanonical role of RB that is critical for normal development and lineage specification, and the downstream consequences of RB loss including epigenetic reprogramming and chromatin reorganization that can lead to changes in lineage program. Finally, we discuss potential therapeutic approaches geared toward RB loss cancers undergoing lineage reprogramming.

Figure 1:. Flattening of Waddington’s landscape upon RB1 loss.

An adaptation of Waddington’s landscape of differentiation (from Waddington 1957 (175)). Modeling the transition from a progenitor cell (blue) to a differentiated state (orange, green, purple), like a ball rolling down a contoured landscape. Left. Cross-section of the schematic showing ‘peaks’ supported by roman columns facilitating normal differentiation. One of columns represents Chromosome 13 harboring the RB1 locus highlighted in red. Right. In the case of RB1 loss, the roman column is damaged leading to flattening of the landscape and free movement of the balls from one state to another depicting cancer plasticity.

Figure 2:. Mechanisms of RB loss and diverse biological consequences.

Left. RB loss can occur via diverse mechanisms including homozygous loss, heterozygous loss, frameshift/point mutations, constitutive hyper-phosphorylation, and promoter DNA methylation. Right. Loss of RB has two major biological consequences including cell cycle deregulation and lineage plasticity. Cell cycle deregulation is mainly attributable to deregulated E2F activity, while phenotypic plasticity may be contributed by effects on epigenetic reprogramming, chromatin restructuring, transcription, metabolism, DNA repair, and DNA replication.

Figure 3:. RB loss in normal development.

Rb1 loss is embryonically lethal in mice, and mouse embryos die within 16 days of gestation with neural developmental defects. Tissue specific loss of Rb has been studied in different tissue types (highlighted in blue in mouse embryo). Downstream effects of Rb loss in specific tissues including epidermis, retina, intestine, adipose, and bone are depicted and described on the right.

Figure 4:. Role of RB in transcription, epigenetic regulation, and chromatin organization.

A schematic of the chromatin organization representing the diverse roles of RB (1) RB binds to E2F and recruits histone deacetylases (HDAC) and DNA methyltransferases (DNMT) to repress transcriptional activity of E2F. (2) During cellular senescence, RB recruits HP1 and SUV39H1 to form facultative heterochromatin marks (H3K9me3) to repress gene expression. (3) RB recruits EZH2 to deposit H3K27me3 at repetitive sequences at telomeric and pericentric regions facilitating chromatin condensation. (4) RB binds cohesion mediating chromatin condensation and also binds to CTCF at topologically associated domain (TAD) boundaries (insulators).

Figure 5:. Strategies to target cancers with RB-loss.

Left. Schematic of therapeutic targets in RB-loss mediated cell cycle deregulation and checkpoint proteins. Right. Illustration of three strategies to target lineage plasticity upon RB-loss.