Complicating the role of p53 in aging

Abstract

Recently, we described a p53 mutant mouse model (p53+/m mouse model) that presented a paradox, in which tumor free survival was coupled to reduced lifespan instead of extended lifespan (Tyner et al ., 2002). The p53+/m mice expressed a truncated p53 transcript attached to a leader sequence and showed a moderate increase in p53 activity. Based on this data we hypothesized that enhanced levels of p53 caused the early aging and cancer resistance phenotypes. This hypothesis was attractive as it indirectly implied a 'fine tuning' mechanism of tumor suppressor gene expression to maintain homeostasis at the organismal level (Campisi, 2002;Kirkwood, 2002). Our results also lent support to the evolutionary theory of antagonistic pleiotropy that predicts the deleterious effects of tumor-suppressor genes later in life (Williams, 1957). These observations notwithstanding, the p53+/m mouse model presented a caveat, in which tumor free survival was coupled to reduced lifespan instead of extended lifespan. Studies in model organisms have shown that enhanced resistance to oxidative stress correlates with increased longevity (Johnson et al ., 2002;Murakami et al ., 2003). The tumor suppressor functions of p53 overwhelmingly categorize it as a longevity assurance gene and suggest a synergistic effect on lifespan via tumor free survival of the organism. We show that there are 24 genes deleted in the p53+/m mouse model, complicating the role of p53 in aging and raising the possibility that the phenotypes while resembling 'premature aging-like' symptoms, may not be related to the physiological changes seen during normal aging processes.Along with the partial deletion of one of the p53 alleles, the p53+/m mouse had an upstream deletion that was originally not well defined due to the lack of mouse genome sequence data. This undefined deletion was a major drawback for our hypothesis, leaving open the possibility that the loss of other unidentified genes affected the p53+/m phenotypes. The importance of the upstream deletion was further underlined by the fact that we were unable to generate a p53 m/m mouse from p53+/m crosses even though p53 nullizygosity does not lead to embryonic lethality in mice (Donehower et al ., 1992). To determine the extent of the deletion, we blasted the leader sequence (initially cloned along with the truncated p53 transcript) to the Ensembl mouse genome sequence database and found it to be within the Rho guanine exchange factor 15 ( Arhgef15 ) gene approximately 0.6 Mb upstream of p53.To ascertain the fusion of the Argef15 and p53 alleles, we performed Southern blot assays with multiple restriction enzymes that were common to both the alleles. A schematic representation of the wild-type alleles of p53 and Argef15 is shown in Fig. 1 along with the restriction enzyme sites used for the characterization of the breakpoint present in the p53+/m mouse model. As shown in Fig. 2(a,b), the fusion of the two alleles leads to changes in the restriction fragment length polymorphism (RFLP) patterns of the wt and 'm' alle...

Aging-associated truncated form of p53 interacts with wild-type p53 and alters p53 stability, localization, and activity

“…While we hypothesize that most of the cancer resistance and accelerated aging phenotypes can be explained primarily by effects of the M protein on wild-type p53 in the p53+/m mouse, the possibility remains that some or all of the phenotypes could be attributed to haploinsufficiency resulting from the 600 kb deletion upstream of the truncated p53 gene. In fact, there has been some controversy in the literature regarding this issue (Gentry and Venkatachalam, 2005;Vijg and Hasty, 2005). Several lines of evidence support our hypothesis that the cancer resistance and aging phenotypes are driven by chronic p53 hyperactivity in the p53+/m mouse.…”

“…Unfortunately, the nature of the serendipitously generated m allele mutation precluded a straightforward mechanistic interpretation. Exons 1−6 of the m allele are deleted, resulting in expression of a chimeric RNA message in which a leader sequence from a gene (Arhgef15) 600 kb upstream of p53 is fused to exons 7−11 of the p53 gene (Tyner et al, 2002;Gentry and Venkatachalam, 2005). This m message produces a truncated C-terminal p53 protein translationally initiated from an ATG codon in p53 exon 7.…”

“…In these mice, the m allele message is initiated not from the p53 promoter, but from an upstream region of the Rho guanine exchange factor 15 (Arhgef15) and contains a short leader sequence derived from that gene prior to splicing into exon 7 of the p53 gene (which contains a functional translational initiation codon) (Gentry and Venkatachalam, 2005). The chimeric transcript that results is translated into a truncated 24 kDa C-terminal p53 variant and is the result of a 600 kb deletion event between the Arhgef15 gene and the p53 gene.…”

Replicative stress, stem cells and aging

“…More recently, HSC number and proliferative potential have been shown to be decreased in p53 +/m mice and increased in late age haploinsufficient p53 +/− mice (Dumble et al, 2007). Unfortunately, the deletion leading to this truncation also causes haploinsufficiency in 23 nearby genes (Gentry and Venkatachalam, 2005). Although the potential synergistic effects of such a large deletion significantly complicates interpretation of these findings, the phenotypes observed in p53 +/m , p53 m/− , p53 +/− , and p53 −/− mice suggest that the premature aging phenotypes in p53 +/m mice are most likely related to the p53 truncation (Vijg and Hasty, 2005).…”

Relationships between stem cell exhaustion, tumour suppression and ageing

“…p53 hyperactivation in these mice is accompanied by a B20% shorter median lifespan and the premature appearance of several age-related phenotypes, including osteoporosis, kyphosis, generalised organ atrophy, impaired wound healing, hair re-growth defects and slowed recovery following myeloablation. While it has been shown that the deletion leading to this truncation also causes haploinsufficient loss of 23 nearby genes (Gentry and Venkatachalam, 2005), a careful analysis of the phenotypes observed in p53 þ /m , p53 m/À p53 þ /À and p53 À/À mice suggests that the premature ageing phenotypes in p53 þ /m mice are most likely related to the p53 truncation (Vijg and Hasty, 2005). In addition, phenotypes consistent with those observed in p53 þ /m mice have been obtained by expressing a naturally occurring N-terminally truncated isoform of p53 called p44 (Maier et al, 2004).…”