By Sophia Hu
Cancer is one of the leading causes of death across the globe and we humans have been developing battling strategies for many years, yet we still struggle. It is therefore intriguing to find out that cancer mortality in elephants is less than 5% while the human counterpart is 25% (1). However, if all cells are equally susceptible to oncogenic mutations, animals with bigger body mass (number of cells) and longer lifespans (number of cell divisions) should have a higher risk of developing cancer (2). Indeed, for certain cancer types, the cancer risk could be proportional to around the sixth power of age within species (2). Surprisingly, despite there being a positive correlation between body mass/lifespan and cancer risk within species, there is no such correlation between species (3). This is referred to as “Peto’s Paradox”. If we take into account the fold difference in both body mass and lifespan when comparing cancer risks between humans and mice, human tissue is approximately 3 trillion less susceptible to cancer than mouse tissue gram for gram. Certain cancer-proof mechanisms must have evolved to account for this phenomenon but mechanistic explanations have been elusive thus far (4). Nonetheless, there are several attempts to try and resolve Peto’s Paradox.
Sulak et al. identified that elephants have retro-replicated tumour suppressor gene TP53, which encodes for the protein p53 involved in cell cycle arrest and is the most frequently mutated gene in cancer. p53 also plays a role in DNA repair and apoptosis in response to stress (5,6). More specifically, p53 acts as a zinc-containing transcription factor that inhibits transcription by transactivating CDK inhibitor p21 which impedes phosphorylation of RBL1 and RBL2, which then stabilises the DREAM complex, a master regulator of the cell cycle (5,6). The complex then represses genes involved in DNA repair, among others (7). Hence, under normal physiological conditions, the p53 level must be kept low (8). This is achieved by the rapid proteasome-mediated degradation, which can either be ubiquitin-dependent or -independent although the former is more important, and the E3 ligase MDM2 is the primary negative regulator of p53 in this mechanism (8). MDM2 binds to the BOX-I motif on p53 and transfers poly-ubiquitin to multiple lysine residues of p53, targeting it for degradation (1).
Using evolutionary genomics and comparative cell biology, Sulak et al. found that elephants have a single TP53 gene plus 19 TP53 retrogenes. The TP53 copy number increased simultaneously with the evolution of large body sizes by repeated segmental duplications. It appears that elephants have hyperactive TP53 signalling in response to DNA damage, hence inducing apoptosis at lower thresholds of genotoxic stress (4). This augmentation is found to be dependent on the TP53 retrogenes (TP53RTG) and currently two models of action have been proposed: the ‘decoy’ and the ‘guardian’ models. In the ‘decoy’ model, TP53RTG proteins act as a decoy for the MDM2 complex, allowing p53 to escape negative regulation (9). Though contrary to Abegglen et al.’s finding, Sulak et al. failed to show a physical interaction between TP53RTG proteins and MDM2 using both in silico prediction and immunoprecipitation (4). Unlike the decoy model, the guardian model suggests a physical interaction between TP53RTG and p53. TP53RTG proteins may protect p53 from MDM2 degradation by dimerizing with p53, hence preventing the tetramerization of p53 that is required for efficient MDM2-mediated ubiquitination (4). This is supported by both in silico prediction and immunoprecipitation (4). It could be argued that the western blot results from Abegglen et al.seem a bit ambiguous as the 90 KDa (MDM2) band was rather dim. Furthermore, the ‘decoy’ model implies a hyperactive TP53 signalling pathway even without DNA damage, which necessitates a mechanistic explanation for how elephants tolerate an enhanced basal TP53 level. Though the answer to whether p53 interacts with MDM2 may not be so clear-cut. A recent study attempted to investigate the p53-MDM2 interaction by focusing on the BOX-I sequences (MDM2 binding motif) of p53 isoforms (1). They discovered that the elephant p53 isoforms display a spectrum of MDM2 docking capacity, implying a fine-tuning regulative mechanism. The FxxxWxxL motif on BOX-I correlates with a higher binding capacity to MDM2 and variations in this motif (e.g. FxxxGxxL) potentially alter the structure and positioning of the BOX-I at the MDM2 binding interphase (1). All in all, the ‘decoy’ and the ‘guardian’ models are likely not mutually exclusive. Further studies with quantitative data are therefore essential to uncover the true mechanisms of TP53RTG proteins.
Apart from having extra copies of TP53, the upregulation of LIF6 (leukemia inhibitory factor 6), a multifunctional interleukin-6 class cytokine, in response to DNA damage could also partly resolve Peto’s paradox in elephants (3). Vazquez et al. found that elephants harbour multiple copies of LIF genes and they are structurally similar to LIF-T, an isoform of LIF protein that induces caspase-dependent apoptosis. Based on this finding, they further investigated these LIF copies and identified that the LIF6 gene is transcribed and they are transcriptionally upregulated by p53 in response to DNA damage to induce mitochondrial dysfunction and apoptosis.
A thorough understanding of the cancer resistance mechanisms and the underlying signalling pathways in elephants could aid us to develop personalized cancer therapy, which takes into account the molecular profiles of individual patients and tailors the therapeutic approaches according to the separation and clustering of cancer types (10). For example, the p53–MDM2 axis discussed above can be druggable targets for cancer therapies. By inhibiting MDM2, p53 signalling could be enhanced and induce apoptosis in cancerous cells. There are already some promising MDM2 inhibitors such as Nutlins, Benzodiazepinediones, Spiro-oxindoles, and RITA (6). They act either by mimicking p53 and blocking the p53-binding pocket on MDM2 or by inducing conformational change on p53 thus preventing it from binding to MDM2 (6). The challenge in developing better MDM2 antagonists is that they often have high potency in vitro but few of them have suitable pharmacokinetic properties and tolerable toxicity profiles in vivo (6).
Possible mechanisms underlying cancer resistance in elephants have been discussed. Extra copies of tumour suppressor genes such as TP53 and LIF6 could partially resolve Peto’s paradox. However, the caveat is that body mass and lifespan do not correlate with the number of tumour suppressor genes (11). The relationship between the amplification of certain tumour suppressor genes and carcinogenesis suppression would require further studies to confirm. An understanding of these mechanisms would provide us with insights into cancer therapy development.
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