Most cancers arise through a two-step process in which an initiating mutation requires further tumour-promoting mutations to instigate the full-blown disease. Retinoblastoma is a childhood cancer of the retina that is one of the few tumour types for which the initiating genetic lesion is known — both copies of the retinoblastoma (Rb) tumour-suppressor gene are inactivated. However, few of the additional tumour-promoting lesions have been identified. On page 61 of this issue, Laurie et al.1 report that amplification of the numbers of MDMX and MDM2 genes occurs frequently in human retinoblastoma. They show that increased expression of MDMX can promote tumour development in genetic models of retinoblastoma in mice. They also show that a drug called nutlin-3, which blocks some actions of MDM proteins, can stop tumour progression in the eye, opening up a promising avenue for the potential treatment of these tumours.
MDMX and MDM2 are structurally related proteins that act as antagonists of a cell-signalling pathway named after its most famous member — the tumour suppressor p53 (ref. 2). The p53 protein is a gene regulatory factor that normally inhibits cell proliferation and induces cell death in response to cellular stress. The Rb tumour-suppressor pathway also inhibits cell proliferation, in part by blocking the expression of genes required for the cell to divide. Normally, loss of Rb leads to induction of p14ARF, a key activator of p53 (ref. 3; Fig. 1). But in many cancers, p53 activity is blocked by mutations in the p53 gene or alterations to other genes in the pathway, allowing cells to escape death and leading to uncontrolled division. In human retinoblastoma, however, the p53 pathway is intact4, and acute Rb loss in the human retina induces p14ARF expression, which should unleash the lethal potential of p53 in response to this mutation1. So how do retinoblastoma cells subvert the intact p53 pathway to prevent it from killing them?
Figure 1: Role of MDM proteins in retinoblastoma.
a, Under normal conditions, levels of p53 protein are kept low, partly through negative regulation by MDM proteins. MDM2 is an enzyme that tags p53 with a ubiquitin molecule (Ub), thereby promoting p53 degradation. MDMX also interacts physically with p53 and inhibits its gene-regulatory activity. b, Mutation of the retinoblastoma gene (Rb) results in increased production of the p14ARF protein, which in turn leads to inactivation of MDM2, thus promoting p53 pathway activation. This leads to cell-cycle arrest or cell death, or facilitates DNA repair. c, Increased expression of MDM proteins in the absence of Rb blocks activation of p53, leading to survival of the abnormal cells and tumour progression. Independently of p53, interactions of MDM with other proteins that regulate cell division, survival and differentiation could also promote tumour progression13.
High resolution image and legend (38K)Laurie et al. provide evidence that the p53 pathway is circumvented in retinoblastoma cells by increased expression of MDMX or MDM2. These proteins interact with p53, blocking its gene-regulatory activity and promoting its degradation2 (Fig. 1). Analysis of human retinoblastoma reveals that the number of MDMX and MDM2 genes is amplified in 65% and 10% of the tumours, respectively, and that this correlates negatively with p53 levels. Moreover, the levels of MDMX RNA and protein were increased in several recently removed human tumours, and the authors confirm that MDMX antagonizes p53-mediated activation of certain target genes, cell-cycle arrest and programmed cell death in retinoblastoma cell lines.
To investigate the functional significance of the MDMX protein in the development of retinoblastoma, the authors turned to mouse models and cultured human fetal retina tissue (retinal explants). In the mouse, the development of retinoblastoma requires the elimination of Rb and one additional Rb family member, either p130 or p107 (ref. 5). However, as occurs in humans, the tumours in these models can develop with intact p53. Laurie et al. show that MDMX expression inhibits cell death, promotes proliferation and exacerbates retinoblastoma progression in mice lacking Rb and p107. MDMX expression also blocks cell death and promotes proliferation in human fetal retinal explants where Rb expression was reduced experimentally. This effect was not observed with an MDMX mutant that does not bind to p53. Furthermore, in both the mouse and the human retinal models, MDMX induced a rapid reduction in the expression of cell markers of differentiation — the cellular process of maturation and specialization. This suggests that MDMX may be promoting tumours with a less differentiated and more aggressive character.
On the basis of their findings, Laurie et al.1 conclude that MDMX promotes retinoblastoma by blocking p53 activity. However, this interpretation is inconsistent with previous reports6, 7 showing that cell death in the retinas of mice lacking either Rb or both Rb and p107 is independent of p53. Moreover, inhibition of programmed cell death does not seem to be an essential feature of retinoblastoma in the mouse8. Given these issues, is it possible that the anti-differentiation effects of MDMX are more significant in tumour development than its anti-death effects? Clearly, MDMX in this context can promote tumour development, but as suggested elsewhere9, this could be mediated in a p53-independent fashion through its interaction with other key regulatory proteins10. Interestingly, the growth-promoting and anti-differentiation interactions of MDM2 with other proteins, such as Numb, also require the p53-binding site of MDM2 (Fig. 1)11.
Finally, Laurie et al.1 show that nutlin-3, a drug that blocks the interaction between MDM2 and p53, can unleash p53-dependent death in MDMX-expressing retinoblastoma cells; this has also been shown by Elison et al.12. Moreover, combined direct delivery to the eye of nutlin-3 and topotecan, a drug that promotes p53 activation, synergistically inhibits tumour growth of intraocularly injected retinoblastoma cells in mice, without being toxic to the animals. However, these studies will need to be extended to models of spontaneous MDMX-driven tumour development. So, although the mechanisms by which MDM proteins facilitate the progression of retinoblastoma require further analysis, their functional relevance in the cancer provides an exciting potential target for its treatment.
