pRB is a multifunctional protein. In addition to its role as a negative regulator of the G1 to S transition, pRB has also been shown to promote cellular differentiation, modulate cell fate decisions, be important for oncogene-induced senescence, and affect cellular sensitivity to apoptosis (reviewed in [38
]). Currently it is unclear which of these activities are most important for tumor suppression and, depending on the context, their relative importance is likely to vary. There is increasing evidence that CIN and aneuploidy have causative roles in tumorigenesis and in the evolution of cancer cells. Given the extensive changes seen in pRB-deficient cells it seems likely that pRB’s role in maintaining genome stability also contributes to its tumor suppressive activity [43
Mutation of RB1 is a rate-limiting event in the development of most retinoblastomas. Recent studies suggest that homozygous mutation of RB1 leads to the appearance of benign retinomas that subsequently progress to retinoblastoma [40
]. The role of pRB in E2F-regulated promotion of cell cycle progression, as well as promotion of differentiation and senescence explain why loss of pRB activity would be beneficial at the initial stages of tumor development. It is generally thought that there is a temporal aspect of tumor evolution in which cells gradually acquire numerous mutations [82
]. The presence of chromosomal instability would promote such evolution and, indeed, the malignant progression from retinoma to retinoblastoma has been correlated with greatly increased levels of aneuploidy and genomic instability [40
In an alternative view, recent work by several groups highlights the idea that not all tumor progression is gradual and that occasional isolated events can occur that greatly advance tumor evolution in a single step (punctuated equilibrium). One example of this is cytokinesis failure and the generation of a tetraploid cell. In the context of p53 mutations, tetraploidy has been shown to be initiating for tumor formation, and the subsequent presence of extra centrosomes promotes CIN through the formation of merotelic attachments [28
]. A second example of such a disastrous event is the recently described chromothripsis, in which a single chromosome is shattered and then haphazardly pieced back together, resulting in massive rearrangements, deletions and amplifications along a single chromosome [83
], potentially leading to oncogene amplifications or tumor suppressor deletions. That usually only one chromosome is involved suggests that the affected chromosome is spatially separated. This may occur by resolution of chromosome bridges following cytokinesis, as proposed by the authors, or alternatively by the formation of micronuclei, which occasionally result following merotelic attachment. Interestingly, work by David Pellman and colleagues shows that chromatin located in micronuclei accumulate damage (personal communication). Merotely can also give rise to lagging chromosomes during anaphase that are positioned under the cytokinetic furrow. Subsequent furrow ingression can cause chromosome breakage [84
]. In support of this idea, recent work shows that DNA double strand breaks are apparent following merotelic attachments [86
] and exciting new work by Medema and colleagues show that following cytokinesis, cells predisposed to forming merotelic attachments exhibit increased levels of DNA damage and subsequent chromosomal abnormalities (personal communication). This suggests that the increase in merotelic attachments that occur when pRB is lost may lead not only to the missegregation of whole chromosomes, but may also predispose afflicted chromosomes to catastrophic damage, increasing the chance of tumorigenic mutations.
Although increased severity of chromosomal changes and aneuploidy correlate with tumor progression, the generation of aneuploidy by increasing chromosome missegregation rates alone is growth inhibitory in culture [8
], and results in few tumors in only a subset of tissues, and late in life, when examined in mouse models [10
]. The fact that many other tumors are able to tolerate such ploidy changes and continue to propagate with an ever-changing genome indicates that they have acquired specific adaptive mechanisms and these can perhaps be targeted to halt tumor growth [89
]. Recent work has linked growth arrest in newly aneuploid cells to metabolic abnormalities [90
] and activation of the p53 pathway[59
]. A series of studies have shown that pRB loss is sufficient for cells to acquire ploidy changes and to remain competent to proliferate (see ). This suggests that corruption of pRB pathway activity is likely to be a significant factor contributing to tolerance of aneuploidy. Understanding how pRB activity contributes to genome stability, ensuring accurate chromosome segregation and the intolerance of ploidy changes, could prove useful in devising therapeutic approaches that target aneuploid cancers.
These observations illustrate the point that the inactivation of pRB has the potential to cause multiple types of changes (). For example, the links to condensin and cohesin connect pRB to the organization of chromosome structure, gene regulation, and DNA damage responses and repair. Potentially, each of these roles may have a variety of consequences for tumor cells. The centromeric dysfunction and merotelic attachment seen when pRB is lost can result in aneuploidy, chromosome instability, and genomic instability, and each of these phenomena presents a different set of risks for additional copy gains or losses and/or mutation of oncogenes and tumor suppressor genes.
Complex role of pRB in maintenance of genomic stability
Taken together, the current information shows that pRB loss promotes defects in mechanisms of chromosome segregation, instigating changes in whole chromosome copy number as well as more complex subchromosomal changes. The loss of pRB causes a consistent, low level of chromosome missegregation (CIN) and this effect is likely to involve both E2F dependent and E2F-independent pathways. pRB pathway lesions also impair the DNA damage response pathway. Through these changes, disregulation of the pRB pathway promotes cell cycle progression and mitotic failure, resulting in genomic instability and aneuploidy (). Data showing the prevalence of merotelic attachments in CIN tumor cells, together with new findings suggesting that such erroneous attachments can lead to both whole chromosome segregation defects and the accumulation of DNA damage, highlight the potential importance of pRB’s influence on mitotic fidelity.