Search

The tight control of wild-type p53 by mainly MDM2 in normal cells is permanently lost in tumors harboring mutant p53, which exhibit dramatic constitutive p53 hyperstabilization that far exceeds that of wild-type p53 tumors. Importantly, mutant p53 hyperstabilization is critical for oncogenic gain of function of mutant p53 in vivo. Current insight into the mechanism of this dysregulation is fragmentary and largely derived from ectopically constructed cell systems. Importantly, mutant p53 knock-in mice established that normal mutant p53 tissues have sufficient enzymatic reserves in MDM2 and other E3 ligases to maintain full control of mutant p53. We find that in human cancer cells, endogenous mutant p53, despite its ability to interact with MDM2, suffers from a profound lack of ubiquitination as the root of its degradation defect. In contrast to wild-type p53, the many mutant p53 proteins which are conformationally aberrant are engaged in complexes with the HSP90 chaperone machinery to prevent its aggregation. In contrast to wild-type p53 cancer cells, we show that in mutant p53 cancer cells, this HSP90 interaction blocks the endogenous MDM2 and CHIP (carboxy-terminus of Hsp70-interacting protein) E3 ligase activity. Interference with HSP90 either by RNA interference against HSF1, the transcriptional regulator of the HSP90 pathway, or by direct knockdown of Hsp90 protein or by pharmacologic inhibition of Hsp90 activity with 17AAG (17-allylamino-17-demethoxygeldanamycin) destroys the complex, liberates mutant p53, and reactivates endogenous MDM2 and CHIP to degrade mutant p53. Of note, 17AAG induces a stronger viability loss in mutant p53 than in wild-type p53 cancer cells. Our data support the rationale that suppression of mutant p53 levels in vivo in established cancers might achieve clinically significant effects. Mol Cancer Res; 9(5); 577–88. ©2011 AACR.

Supplementary Figures S1-S3 from Functional Inactivation of Endogenous MDM2 and CHIP by HSP90 Causes Aberrant Stabilization of Mutant p53 in Human Cancer Cells

Supplementary Figure Legends from Mitochondrially Targeted p53 Has Tumor Suppressor Activities In vivo

Supplementary Figure 4A from Mitochondrially Targeted p53 Has Tumor Suppressor Activities In vivo

Supplementary Figure 7 from Mitochondrially Targeted p53 Has Tumor Suppressor Activities In vivo

Supplementary Figure 8 from Mitochondrially Targeted p53 Has Tumor Suppressor Activities In vivo

Supplementary Table 4C from Mitochondrially Targeted p53 Has Tumor Suppressor Activities In vivo

Supplementary Figure 3 from Mitochondrially Targeted p53 Has Tumor Suppressor Activities In vivo

Supplementary Figure 5 from Mitochondrially Targeted p53 Has Tumor Suppressor Activities In vivo

Complex proapoptotic functions are essential for the tumor suppressor activity of p53. We recently described a novel transcription-independent mechanism that involves a rapid proapoptotic action of p53 at the mitochondria and executes the shortest known circuitry of p53 death signaling. Here, we examine if this p53-dependent mitochondrial program could be exploited for tumor suppression in vivo. To test this, we engage Eμ-Myc transgenic mice, a well-established model of p53-dependent lymphomagenesis. We show that exclusive delivery of p53 to the outer mitochondrial membrane confers a significant growth disadvantage on Eμ-Myc–transformed B-cells of p53-deficient or alternate reading frame–deficient genotypes, resulting in efficient induction of apoptosis and impinged proliferation. Conversely, normal cells from thymus, spleen, and bone marrow showed poor infectivity with these viruses. This proof-of-principle experiment shows that exclusive reliance on the direct mitochondrial program exerts a significant tumor suppressor activity in vivo. Our in vivo data on the direct mitochondrial apoptotic p53 program lays the groundwork to further investigate its efficacy and safety and to address its possible therapeutic value in the future.

Supplementary Figure 2 from Mitochondrially Targeted p53 Has Tumor Suppressor Activities In vivo

Supplementary Figure 1 from Mitochondrially Targeted p53 Has Tumor Suppressor Activities In vivo

Supplementary Tables 1 & 2 from Mitochondrially Targeted p53 Has Tumor Suppressor Activities In vivo

Supplementary Figure 6 from Mitochondrially Targeted p53 Has Tumor Suppressor Activities In vivo

Motivation Gene expression and regulation, a key molecular mechanism driving human disease development, remains elusive, especially at early stages. Integrating the increasing amount of population-level genomic data and understanding gene regulatory mechanisms in disease development are still challenging. Machine learning has emerged to solve this, but many machine learning methods were typically limited to building an accurate prediction model as a ‘black box’, barely providing biological and clinical interpretability from the box. Results To address these challenges, we developed an interpretable and scalable machine learning model, ECMarker, to predict gene expression biomarkers for disease phenotypes and simultaneously reveal underlying regulatory mechanisms. Particularly, ECMarker is built on the integration of semi- and discriminative-restricted Boltzmann machines, a neural network model for classification allowing lateral connections at the input gene layer. This interpretable model is scalable without needing any prior feature selection and enables directly modeling and prioritizing genes and revealing potential gene networks (from lateral connections) for the phenotypes. With application to the gene expression data of non-small-cell lung cancer patients, we found that ECMarker not only achieved a relatively high accuracy for predicting cancer stages but also identified the biomarker genes and gene networks implying the regulatory mechanisms in the lung cancer development. In addition, ECMarker demonstrates clinical interpretability as its prioritized biomarker genes can predict survival rates of early lung cancer patients (P-value < 0.005). Finally, we identified a number of drugs currently in clinical use for late stages or other cancers with effects on these early lung cancer biomarkers, suggesting potential novel candidates on early cancer medicine. Availabilityand implementation ECMarker is open source as a general-purpose tool at https://github.com/daifengwanglab/ECMarker. Supplementary information Supplementary data are available at Bioinformatics online.

Age-related prostate diseases are among the most frequently encountered conditions in the male population and age remains a major risk for prostate cancer (PCa) development. Understanding the cellular origins of age-related prostate hyperplasia and dysplasia and finding novel ways to halt progression towards malignancy remain fundamental challenges of PCa treatment, prevention, and patient stratification. Elucidating how aging alters the barriers posed by normal homeostasis to tumor cells expansions is of paramount importance for understanding PCa initiation. To date, it remains elusive how prostate clonal activity maintains the constant prostate size and what homeostatic barriers need to be disrupted for the age-related hyperproliferations to occur and lead to cancer. Moreover, altered aging prostate epithelial cell types and mechanisms that facilitate clonal expansion of tumor cells in aging are not well understood. These unresolved questions hamper the development of anti-proliferative therapies designed to block the deregulated activity of aged prostate epithelial cells. We used in vivo lineage tracing with multicolor reporters, whole organ mapping and single cell transcriptomics to study clonal dynamics of the luminal epithelial layer at single cell resolution. We report here the molecular characterization of a newly identified “intermediate” hybrid state in the luminal compartment with increased basal features. These luminal intermediate cells are associated with increased clonal activity and Wnt signaling, including the atypical and understudied Wnt4 ligand. This cell state becomes dominant in aging mouse and human prostate tissue. Within a set of adult/middle age and advanced age normal human prostate samples, we have identified increased levels of Wnt4 expression in aging luminal cells. Moreover, this “pro-proliferative” state has exacerbated growth in oncogenic set-ups leading to clonal expansions in early stages of prostate cancer. We further delineated the master regulators of these luminal intermediate state and singled out p63 as a putative critical activator. In sum, our results suggest that the luminal lineage is maintained by a balancing act of luminal cells activating a “basal-like” pro-proliferative program in response to cellular loss due to normal turnover. These cells are less dependent on androgens, and thus survive better in the low androgen milieu of aging prostate. This active pro-proliferative program is susceptible to hyperproliferation alterations and potentially contributes to the onset of early prostate cancer and might serve as a cell of origin. Our ongoing studies address the targetable vulnerabilities in the Wnt4-p63 luminal signaling in order to provide new venues for therapeutic interventions against age-related prostate hyperproliferations. Citation Format: Fu Luo, Lara F. Tshering, Karis Tutuska, Mariola Szenk, Diana Rubel, James Rail, Savanah Russ, Gábor Balázsi, Flaminia Talos. Clonal dynamics and single cell transcriptomics of prostate luminal epithelial cells in aging and cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3076.

Prostate cancer (PCa) is one of the most common cancers in men worldwide and a leading cause of cancer-related mortality. Though PCa is known to be substantially heterogeneous at the molecular and histological level, there is still little understanding of the tumor subpopulations (“clones”) generating this intratumor heterogeneity (ITH) and their underlying clonal dynamics and interactions. The impact of ITH on disease outcome remains unknown and is thus a significant hurdle in treatment of PCa. This barrier is especially evident in progression to castration resistance: after initial androgen dependence, subpopulations that are castration resistant and less dependent on androgens emerge to populate the tumor, contributing to disease progression. The introduction of second generation androgen deprivation therapies has shown some success in treating castration resistant prostate cancer (CRPC), but eventually drug resistant clones emerge and the disease recurs. Understanding the fundamental differences–both phenotypic and genomic–between clones may hold the key to improving therapies, preventing disease progression, and prolonging patient survival. Using a multi-omics approach, we are exploring the dynamic changes in these subpopulations during progression to castration and drug resistance, focusing on the differences between clones at the transcriptomic and genomic level. Thus far, we have developed a mouse model of ITH in CRPC with the multicolor Confetti reporter, allowing for spatial distinction between different clones. Using single cell sequencing and RNAseq, we have mapped the evolution of early-stage clones post-castration and identified pathways and gene regulatory networks that potentially grant specific clones their castration resistant phenotype. We have extended our system to in vitro 3D organoid cultures to study the evolution of these clones longitudinally during treatment with the antiandrogens enzalutamide and abiraterone, developing a drug resistant in vitro model of CRPC. We are currently using our organoid system to identify novel drug resistance mechanisms in order to target advanced CRPC, which until now has been considered untreatable and lethal. By characterizing clonal dynamics during disease progression, our studies provide new mechanistic insights for identifying pre-existing high-risk clones with drug resistant phenotypes in PCa and uncovering novel therapeutic targets in CRPC. Our work has implications for designing new therapies aimed to control the emergence of advanced disease, preventing CRPC and drug resistance, with the ultimate goal of reducing mortality and improving prognosis. Citation Format: Lara F Tshering, Fu Luo, Nam D. Nguyen, James G. Rail, Ting Jin, Daifeng Wang, Flaminia Talos. Intratumor heterogeneity and clonal dynamics underlying treatment resistance in prostate cancer [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2021; 2021 Apr 10-15 and May 17-21. Philadelphia (PA): AACR; Cancer Res 2021;81(13_Suppl):Abstract nr 3126.

Understanding the molecular mechanisms underlying early cancer development is still a challenge. To address this, we developed an interpretable, data-driven machine learning approach to identify the gene biomarkers that predict the clinical outcomes of early cancer patients. As a demonstration, we applied this approach into large-scale pan-cancer datasets including TCGA to find out how effective it would be at identifying the developmental gene expression biomarkers across tumor stages for various cancer types. Results confirmed that artificial neural network prediction embedding nonlinear feature selection outperforms other classifiers. Moreover, and more relevant to the goal of machine learning interpretable classifiers, we found that early cancer patient groups clustered by the biomarkers selected have significantly more survival differences than ones by early TNM stages, suggesting that this method identified novel early cancer molecular biomarkers. Furthermore, using lung cancer as a study case, we leveraged the hierarchical architectures of neural network to identify the developmental regulatory networks controlling the expression of early cancer biomarkers, providing mechanistic insights of functional genomics driving the onset of cancer development. Finally, we reported the drugs targeting early cancer biomarkers, revealing potential genomic medicine affecting the early cancer development.

SummaryIdentification of driver mutations in human diseases is often limited by cohort size and availability of appropriate statistical models. We propose a framework for the systematic discovery of genetic alterations that are causal determinants of disease, by prioritizing genes upstream of functional disease drivers, within regulatory networks inferred de novo from experimental data. We tested this framework by identifying the genetic determinants of the mesenchymal subtype of glioblastoma. Our analysis uncovered KLHL9 deletions as upstream activators of two previously established master regulators of the subtype, C/EBPβ and C/EBPδ. Rescue of KLHL9 expression induced proteasomal degradation of C/EBP proteins, abrogated the mesenchymal signature, and reduced tumor viability in vitro and in vivo. Deletions of KLHL9 were confirmed in > 50% of mesenchymal cases in an independent cohort, thus representing the most frequent genetic determinant of the subtype. The method generalized to study other human diseases, including breast cancer and Alzheimer’s disease.

Trp73, a member of the p53 gene family, plays a crucial role in neural development. We describe two main phenotypic variants of p73 deficiency in the brain, a severe one characterized by massive apoptosis in the cortex leading to early postnatal death and a milder, non-/low-apoptosis one in which 50% of pups may reach adulthood using an intensive-care breeding protocol. Both variants display the core triad of p73 deficiency: cortical hypoplasia, hippocampal malformations, and ventriculomegaly. We studied the development of the neocortex in p73 KO mice from early embryonic life into advanced age (25 months). Already at E14.5, the incipient cortical plate of the p73 KO brains showed a reduced width. Examination of adult neocortex revealed a generalized, nonprogressive reduction by 10–20%. Area-specific architectonic landmarks and lamination were preserved in all cortical areas. The surviving adult animals had moderate ventricular distension, whereas pups of the early lethal phenotypic variant showed severe ventriculomegaly. Ependymal cells of wild-type ventricles strongly express p73 and are particularly vulnerable to p73 deficiency. Ependymal denudation by apoptosis and reduction of ependymal cilia were already evident in young mice, with complete absence of cilia in older animals. Loss of p73 function in the ependyma may thus be one determining factor for chronic hydrocephalus, which leads to atrophy of subcortical structures (striatum, septum, amygdala). p73 Is thus involved in a variety of CNS activities ranging from embryonic regulation of brain size to the control of cerebrospinal fluid homeostasis in the adult brain via maintenance of the ependyma. J. Comp. Neurol. 522:2663–2679, 2014. © 2014 Wiley Periodicals, Inc.

In embryogenesis, p63 is essential to develop mammary glands. In the adult mammary gland, p63 is highly expressed in the basal cell layer that comprises myoepithelial and interspersed stem/progenitor cells, and has limited expression in luminal epithelial cells. In adult skin, p63 has a crucial role in the maintenance of epithelial stem cells. However, it is unclear whether p63 also has an equivalent role as a stem/progenitor cell factor in adult mammary epithelium. We show that p63 is essential in vivo for the survival and maintenance of parity-identified mammary epithelial cells (PI-MECs), a pregnancy-induced heterogeneous population that survives post-lactational involution and contain multipotent progenitors that give rise to alveoli and ducts in subsequent pregnancies. p63+/- glands are normal in virgin, pregnant and lactating states. Importantly, however, during the apoptotic phase of post-lactational involution p63+/- glands show a threefold increase in epithelial cell death, concomitant with increased activation of the oncostatin M/Stat3 and p53 pro-apoptotic pathways, which are responsible for this phase. Thus, p63 is a physiologic antagonist of these pathways specifically in this regressive stage. After the restructuring phase when involution is complete, mammary glands of p63+/- mice again exhibit normal epithelial architecture by conventional histology. However, using Rosa(LSL-LacZ);WAP-Cre transgenics (LSL-LacZ, lox-stop-lox β-galactosidase), a genetic in vivo labeling system for PI-MECs, we find that p63+/- glands have a 30% reduction in the number of PI-MEC progenitors and their derivatives. Importantly, PI-MECs are also cellular targets of pregnancy-promoted ErbB2 tumorigenesis. Consistent with their PI-MEC pool reduction, one-time pregnant p63+/- ErbB2 mice are partially protected from breast tumorigenesis, exhibiting extended tumor-free and overall survival, and reduced tumor multiplicity compared with their p63+/+ ErbB2 littermates. Conversely, in virgin ErbB2 mice p63 heterozygosity provides no survival advantage. In sum, our data establish that p63 is an important survival factor for pregnancy-identified PI-MEC progenitors in breast tissue in vivo.

The tight control of wild-type p53 by mainly MDM2 in normal cells is permanently lost in tumors harboring mutant p53, which exhibit dramatic constitutive p53 hyperstabilization that far exceeds that of wild-type p53 tumors. Importantly, mutant p53 hyperstabilization is critical for oncogenic gain of function of mutant p53 in vivo. Current insight into the mechanism of this dysregulation is fragmentary and largely derived from ectopically constructed cell systems. Importantly, mutant p53 knock-in mice established that normal mutant p53 tissues have sufficient enzymatic reserves in MDM2 and other E3 ligases to maintain full control of mutant p53. We find that in human cancer cells, endogenous mutant p53, despite its ability to interact with MDM2, suffers from a profound lack of ubiquitination as the root of its degradation defect. In contrast to wild-type p53, the many mutant p53 proteins which are conformationally aberrant are engaged in complexes with the HSP90 chaperone machinery to prevent its aggregation. In contrast to wild-type p53 cancer cells, we show that in mutant p53 cancer cells, this HSP90 interaction blocks the endogenous MDM2 and CHIP (carboxy-terminus of Hsp70-interacting protein) E3 ligase activity. Interference with HSP90 either by RNA interference against HSF1, the transcriptional regulator of the HSP90 pathway, or by direct knockdown of Hsp90 protein or by pharmacologic inhibition of Hsp90 activity with 17AAG (17-allylamino-17-demethoxygeldanamycin) destroys the complex, liberates mutant p53, and reactivates endogenous MDM2 and CHIP to degrade mutant p53. Of note, 17AAG induces a stronger viability loss in mutant p53 than in wild-type p53 cancer cells. Our data support the rationale that suppression of mutant p53 levels in vivo in established cancers might achieve clinically significant effects. Mol Cancer Res; 9(5); 577–88. ©2011 AACR.

The p53 family member p73 is essential for brain development, but its precise role and scope remain unclear. Global p73 deficiency determines an overt and highly penetrant brain phenotype marked by cortical hypoplasia with ensuing hydrocephalus and hippocampal dysgenesis. The ΔNp73 isoform is known to function as a prosurvival factor of mature postmitotic neurons. In this study, we define a novel essential role of p73 in the regulation of the neural stem cell compartment. In both embryonic and adult neurogenesis, p73 has a critical role in maintaining an adequate neurogenic pool by promoting self-renewal and proliferation and inhibiting premature senescence of neural stem and early progenitor cells. Thus, products of the p73 gene locus are essential maintenance factors in the central nervous system, whose broad action stretches across the entire differentiation arch from stem cells to mature postmitotic neurons.

Udgivelsesdato: 2009-Aug p63, an ancestral transcription factor of the p53 family, has three C-terminal isoforms whose relative in vivo functions are elusive. The p63 gene is essential for skin and limb development, as vividly shown by two independent global knockout mouse models. Both strains, although constructed differently, have identical and severe phenotypes, characterized by absent epidermis and hindlimbs and only rudimentary forelimbs at birth. Here we show that mice from one model, Brdm2, express normal levels of truncated p63 proteins that contain the DNA binding and oligomerization domain but lack the long carboxy-terminal SAM (sterile alpha-motif) and post-SAM domains that are specific for the alpha and beta isoforms. As such, transcriptionally active p63 proteins from Brdm2 mice resemble the naturally occurring p63gamma isoforms, which of all the p63 isoforms most closely resemble p53. Thus, Brdm2 mice are p63alpha/beta isoform-specific knockout mice, gaining unexpected new importance. Our studies identify that p63alpha/beta but not p63gamma are absolutely required for proper skin and limb development.

Macrophage migration inhibitory factor (MIF) is a potent regulator of inflammation and cell growth. Using the Emu-Myc lymphoma mouse model, we demonstrate that loss of MIF markedly delays the onset of B-cell lymphoma development in vivo. The molecular basis for this MIF-loss-induced phenotype is the perturbed DNA-binding activity of E2F factors and the concomitantly enhanced tumor suppressor activity of the p53 pathway. Accordingly, premalignant MIF-null Emu-Myc B-cells are predisposed to delayed S-phase progression and increased apoptosis. MIF-deficient lymphomas that do arise under these conditions contain frequent ARF deletions and p53 inactivating mutations. Conversely, MIF expression is retained in tumors developed by wild-type Emu-Myc animals, and the presence of one or both MIF alleles is sufficient to accelerate the development of Myc-induced lymphomas. Collectively, these results indicate that MIF promotes Myc-mediated tumorigenesis, at least in the B-lymphoid compartment, and implicate MIF as a mediator of malignant cell growth in vivo.

In response to our recent report on the Brdm2 mouse model for p63 function1, Mikkola et al.2 raise concerns on three of our statements: 1) They maintain their initial claim that no functional p63 protein of any kind is synthesized in mice homozygous for the Brdm2 allele; 2) they report the occurrence of spontaneous wild-type allelic reversions in these mice, leading to patches of completely normal epidermis, and propose that these normal reverted skin patches are in fact the multilayered epithelia we described; 3) they insist that the Brdm2 allele is phenotypically indistinguishable from a global p63 knockout.

Trp73, a member of the p53 gene family, plays a crucial role in neural development. We describe two main phenotypic variants of p73 deficiency in the brain, a severe one characterized by massive apoptosis in the cortex leading to early postnatal death and a milder, non-/low-apoptosis one in which 50% of pups may reach adulthood using an intensive-care breeding protocol. Both variants display the core triad of p73 deficiency: cortical hypoplasia, hippocampal malformations, and ventriculomegaly. We studied the development of the neocortex in p73 KO mice from early embryonic life into advanced age (25 months). Already at E14.5, the incipient cortical plate of the p73 KO brains showed a reduced width. Examination of adult neocortex revealed a generalized, nonprogressive reduction by 10-20%. Area-specific architectonic landmarks and lamination were preserved in all cortical areas. The surviving adult animals had moderate ventricular distension, whereas pups of the early lethal phenotypic variant showed severe ventriculomegaly. Ependymal cells of wild-type ventricles strongly express p73 and are particularly vulnerable to p73 deficiency. Ependymal denudation by apoptosis and reduction of ependymal cilia were already evident in young mice, with complete absence of cilia in older animals. Loss of p73 function in the ependyma may thus be one determining factor for chronic hydrocephalus, which leads to atrophy of subcortical structures (striatum, septum, amygdala). p73 Is thus involved in a variety of CNS activities ranging from embryonic regulation of brain size to the control of cerebrospinal fluid homeostasis in the adult brain via maintenance of the ependyma.

p73 is dispensable for commitment to neural stem cell fate, but is essential for neural stem cell maintenance and for blocking premature differentiation

The ancient p53 paralogs p63 and p73 regulate specific tissue formation, cell survival and cell death via their TA and ΔN isoforms. Targeted disruption of the p73 locus leads to severe defects in the development of the central nervous system (CNS), and p73 has recently been shown to be an essential regulator of neural stem cell maintenance and differentiation in both embryonal and adult neurogenesis. In contrast, global p63-/- mice lack skin and limbs. Moreover, p63 is detectable in embryonic cortex. It has previously been proposed to also play critical pro-death and pro-survival roles in neural precursors of the developing sympathetic and central nervous system, respectively, based on experimental overexpression and siRNA-mediated knockdown of p63. Here we perform an extensive analysis of the developing central nervous system in global p63-/- mice and their wildtype littermates. Brain and spinal cord of embryos and newborn mice were assessed in vivo for neuroanatomy, histology, apoptosis, proliferation, stemness and differentiation, and in vitro for self-renewal and maturation in neurosphere assays. None of these analyses revealed a detectable phenotype in p63-/- mice. Hence, despite the profound impact of p63 on the development of stratified epithelia and limbs, p63 is completely dispensable for proper development of the central nervous system. Thus, despite their strong homology, the non-overlapping tissue specificity of p63 and p73 functions appears more pronounced than previously anticipated.

Cellular defects resulting in chromosomal instability and aneuploidy are the most common features of human cancers. As a major tumor suppressor and intrinsic part of several cellular checkpoints, p53 contributes to maintenance of the stability of the genetic material, both in quality (ensures faithful replication) and quantity (preservation of diploidy). Although the exact trigger of p53 in case of numerical chromosomal aberrations is unknown, the absence of p53 allows polyploid cells to proliferate and generate unstable aneuploid progeny. A more recent addition to the p53 family, p73, emerged as an important contributor to genomic integrity when p53 is inactivated. p73 loss in p53-null background leads to a rapid increase in polyploidy and aneuploidy, markedly exceeding that caused by p53 loss alone. Constitutive deregulation of Cyclin-Cdk and p27/Kip1 activities and excess failure of the G2/M DNA damage checkpoint are important deficiencies associated with p73 loss.

Cellular defects resulting in chromosomal instability and aneuploidy are the most common features of human cancers. As a major tumor suppressor and intrinsic part of several cellular checkpoints, p53 contributes to maintenance of the stability of the genetic material, both in quality (ensures faithful replication) and quantity (preservation of diploidy). Although the exact trigger of p53 in case of numerical chromosomal aberrations is unknown, the absence of p53 allows polyploid cells to proliferate and generate unstable aneuploid progeny. A more recent addition to the p53 family, p73, emerged as an important contributor to genomic integrity when p53 is inactivated. p73 loss in p53-null background leads to a rapid increase in polyploidy and aneuploidy, markedly exceeding that caused by p53 loss alone. Constitutive deregulation of Cyclin-Cdk and p27/Kip1 activities and excess failure of the G2/M DNA damage checkpoint are important deficiencies associated with p73 loss.

To better understand the role of E2F1 in tumor formation, we analyzed spontaneous tumorigenesis in p53(-/-)E2F1(+/+) and p53(-/-)E2F1(-/-) mice. We show that the combined loss of p53 and E2F1 leads to an increased incidence of sarcomas and carcinomas compared to the loss of p53 alone. E2F1-deficient tumors show wide chromosomal variation, indicative of genomic instability. Consistent with this, p53(-/-)E2F1(-/-) primary fibroblasts have a reduced capacity to maintain genomic stability when exposed to S-phase inhibitors or genotoxic drugs. A major mechanism of E2F1's contribution to genomic integrity lies in mediating stabilization and engagement of the Rb protein.

Previous studies showed that p53 plays a central role in G1 and DNA damage checkpoints, thus contributing to genomic stability. We show here that p73 also plays a role in genomic integrity but this mechanism is manifest only when p53 is lost. Isolated p73 loss in primary cells does not induce genomic instability. Instead, it results in impaired proliferation and premature senescence due to compensatory activation of p53. Combined loss of p73 and p53 rescues these defects, but at the expense of exacerbated genomic instability. This leads to rapid increase in polyploidy and aneuploidy, markedly exceeding that of p53 loss alone. Constitutive deregulation of cyclin-Cdk activities and excess failure of the G2/M DNA damage checkpoint appear to fuel increased ploidy abnormalities upon p53/p73 loss, while primary mitotic defects do not play a causal role. These data indicate that p73 is essential for suppressing polyploidy and aneuploidy when p53 is inactivated.

Complex proapoptotic functions are essential for the tumor suppressor activity of p53. We recently described a novel transcription-independent mechanism that involves a rapid proapoptotic action of p53 at the mitochondria and executes the shortest known circuitry of p53 death signaling. Here, we examine if this p53-dependent mitochondrial program could be exploited for tumor suppression in vivo. To test this, we engage Eμ-Myc transgenic mice, a well-established model of p53-dependent lymphomagenesis. We show that exclusive delivery of p53 to the outer mitochondrial membrane confers a significant growth disadvantage on Eμ-Myc–transformed B-cells of p53-deficient or alternate reading frame–deficient genotypes, resulting in efficient induction of apoptosis and impinged proliferation. Conversely, normal cells from thymus, spleen, and bone marrow showed poor infectivity with these viruses. This proof-of-principle experiment shows that exclusive reliance on the direct mitochondrial program exerts a significant tumor suppressor activity in vivo. Our in vivo data on the direct mitochondrial apoptotic p53 program lays the groundwork to further investigate its efficacy and safety and to address its possible therapeutic value in the future.