Abstract

Aberrations in the p53 tumor suppressor pathway are associated with hematologic malignancies. p53-dependent cell cycle control, senescence, and apoptosis functions are actively involved in maintaining hematopoietic homeostasis under normal and stress conditions. Whereas loss of p53 function promotes leukemia and lymphoma development in humans and mice, increased p53 activity inhibits hematopoietic stem cell function and results in myelodysplasia. Thus, exquisite regulation of p53 activity is critical for homeostasis. Most of our understanding of p53 function in hematopoiesis is derived from genetically engineered mice. Here we summarize some of these models, the various mechanisms that disrupt the regulation of p53 activity, and their relevance to human disease.

Introduction

The tumor suppressor p53 is a ubiquitously expressed transcription factor maintained in an inactive, latent form in most cell types. p53 is stabilized and activated in response to cellular stressors, such as DNA damage, hypoxia, and oncogene activation.1  Activated p53 triggers a multifaceted transcriptional program, which, depending on the cell type and damage signal, initiates cell cycle arrest, senescence, or apoptotic pathways.1-3  As p53 activation determines the fate of the cell, it is very tightly regulated in vivo. Multiple biologic inhibitors have been identified that regulate p53 stability and activity.4  Mdm2 and Mdm4 (also known as Mdmx) are homologous proteins and the primary negative regulators of p53 activity and stability under both normal and stress conditions.5  Both these proteins bind p53 and inactivate it by masking its transactivation domain.6-8  In addition, Mdm2 also functions as an E3-ubiquitin ligase that targets p53 and promotes its degradation by the 26S proteosomal machinery.9-11 

The mouse is commonly used in conjunction with ionizing radiation (IR) to probe p53 activation in vivo. IR acts as a prototypical DNA damage insult causing double-strand DNA breaks that induce p53 activity.12  IR induced p53-dependent acute pathologies can be observed in different tissues such as the CNS, hair follicles, hematopoietic system (HP), gastrointestinal tract, and germ cells.13  However, long-term effects of increased p53 activity are most obvious in the HP and gastrointestinal systems, both of which are highly regenerative systems that typically collapse in a p53- and IR dose-dependent manner. Whole-body radiation, typically around 8 Gy, obliterates the hematopoietic system, whereas higher dosages are required to collapse the gastrointestinal system in a wild-type mouse.13  Loss of cellularity of these tissues is the primary cause of IR-mediated demise in these animals.

The HP system is most vulnerable to DNA damage because of its highly proliferative nature.14  It is composed of multiple hematopoietic organs (fetal liver, bone marrow, thymus, and spleen), which provide the microenvironment required for self-renewal and multilineage differentiation of hematopoietic stem cells (HSCs).15-17  HSCs are multipotent cells that have the potential to self-renew and also give rise to progenitor cells, namely, common myeloid progenitors and common lymphoid progenitors, which subsequently differentiate to mature hematopoietic cells of myeloid and lymphoid lineages (Figure 1).18  These abilities of HSCs are critical for maintaining the lifelong supply of hematopoietic cell types in the bone marrow and peripheral blood. Under normal conditions, HSCs are maintained in a quiescent state, and only a small fraction of cells enter the cell cycle to give rise to lineage-specific progenitors, necessary for replenishing the daily need of circulating blood cells.19  Under stress conditions, HSCs readily enter the cell cycle and maintain their cell pool by cell-renewal. Mouse HSCs lack the expression of cell surface molecule for lineage-specific mature cells (Lin) but express stem cell antigen 1 (Sca1+) and c-Kit (Kit+, CD117), a transmembrane tyrosine kinase that serves as a receptor for stem cell factor. These HSCs can be identified by flow cytometry as the LinSca1+Kit+ (LSK) population.20,21  Whereas LSK defines the basic marker set used to identify the HSCs, additional markers, such as CD34, CD38, CD48, CD150, and Flt3, are also used in conjunction to further refine HSCs.22-26  Some investigators also use rhodamine staining or Hoechst dye efflux side population to identify the stem cell population.26 

Figure 1

Role of p53 in HSCs. A simplistic model depicting changes in p53 levels during differentiation of HSCs into the lineage components (left). p53 activity levels influence HSC functionality (right).

Figure 1

Role of p53 in HSCs. A simplistic model depicting changes in p53 levels during differentiation of HSCs into the lineage components (left). p53 activity levels influence HSC functionality (right).

Most of our understanding about p53 regulation and stress response comes from a series of genetically engineered mouse models generated during the past 2 decades. In this review, we discuss models that disrupt the p53 pathway that expand our understanding of p53-mediated stress and tissue-dependent cell fate decisions in hematopoiesis.

p53 deregulation in hematopoietic tumors

p53 regulates a range of physiologic functions, including ageing, development, cell metabolism, differentiation, fertility, skin pigmentation, stem cell function, and tissue homeostasis.27  Given its pleiotropic activity, it comes as no surprise that disruption of the p53 pathway is a common denominator in many malignancies. Indeed, > 50% of human tumors carry a mutation in TP53, and many other tumors carrying wild-type TP53 alleles exhibit attenuated p53 activity by various mechanisms.28,29  Germline mutations of TP53 cause Li-Fraumeni syndrome, a genetic disorder characterized by a dramatically increased incidence of many kinds of tumors that include sarcomas, breast cancers, leukemia, and lymphomas.30,31  Genetically engineered mice that lack p53 or express a mutant form of p53 (p53R172H, p53R172P, p53R270H) are prone to early onset of tumor development.32-36  A high percentage of these tumors are lymphomas, a tumor of hematopoietic T- or B-cell origin.33-36  Furthermore, p53 gene dosage impacts tumor latency as mice heterozygous for these mutations develop tumors slightly later than homozygous mutant mice.33-35 

Surprisingly, in contrast to solid tumors, hematologic malignancies present a rather low incidence of genetic alterations in TP53 (10%-20%).37,38 TP53 mutations/deletions have been reported in chronic lymphocytic leukemia (CLL), marginal zone lymphoma, follicular lymphoma, and diffuse large B-cell lymphoma.39  Nonetheless, aberrations in TP53 correlate with an inferior clinical outcome in hematologic cancers.40,41  For instance, in patients with CLL, deletion of the short arm of chromosome 17 (TP53 maps to band 17p13.1), is remarkably predictive of poor outcome with standard chemo-immunotherapeutic regimens.42  Loss of TP53 also acts as a prognostic marker for acute myeloid leukemia.43 

These data suggest that other mechanisms besides mutations could be involved in deregulation of the p53 pathway. Indeed, attenuation of p53 function by increased levels of its inhibitors Mdm2 and Mdm4 results in tumors of hematologic origin in mice.44,45  In humans, overexpression of MDM2 has been associated with CLL, follicular lymphoma, diffuse large B-cell lymphoma, marginal zone lymphoma, acute myeloid leukemia, mantle cell lymphoma, Burkitt lymphoma, acute lymphocytic leukemia, multiple myeloma, and plasma cell leukemia.39,46  A single nucleotide polymorphism (SNP) in MDM2 (SNP309), which increases the transcription of MDM2 from the P2 promoter, has been linked to many different hematologic malignancies, including CLL and other B-cell hematologic tumors.47 

The genomic region at chromosome 1q harboring the p53− regulator MDM4 (also known as HDMX) is also amplified in several types of cancer.48  MDM4 amplifications have been recently described in a high percentage of patients with myeloproliferative neoplasm during their transformation to acute myeloid leukemia.49  In this context, 1q gains (ie, MDM4 amplifications) are mutually exclusive with TP53 mutations, a phenomenon also observed in solid tumors.50  Overexpression of MDM4 has also been reported in CLL, which responds poorly to the MDM2 inhibitor Nutlin, implicating MDM4 as an important p53 inhibitor in CLL.51  Altogether, these data highlight the importance of dampening p53 levels in transformation of the hematopoietic system.

p53 loss impairs HSC function

In the hematopoietic system, p53 is primarily expressed in HSCs and regulates their quiescence and self-renewal.52  This is critical for preserving the lifelong pool of HSCs that sustain the highly regenerative hematopoietic system.52  Genetically engineered mice that lack p53 have a 2- to 3-fold increase in their HSC pool.52-55  This is probably the result of the higher rate of cellular proliferation of p53-deficient HSCs.52  Interestingly, though, these highly proliferative p53-deficient HSCs lack the equivalent repopulation capability compared with their wild-type counterparts (Figure 1).52-55  This implies that the absence of p53 compromises the functional fitness of HSCs. The engraftment potential of HSCs, a mark of their fitness, is also reduced with age and correlates with decreasing p53 activity in mice.56,57  On the other hand, total bone marrow cells from p53-null mice easily outgrow wild-type bone marrow cells in competitive bone marrow transplantation assays,52-55  indicating that either the progenitor cells or external factors originating in the bone marrow modulate HSC function. Of note, the short-term engraftment advantage of p53−/− bone marrow cells comes at a price as the recipient mice typically die much earlier because of hematologic tumors. Unregulated proliferation of HSCs in the absence of p53 also makes them prone to accumulate mutations. The early tumorigenesis and death of p53−/− mice or mice implanted with p53−/− bone marrow cells precludes any long-term analyses of HSC composition on life span of the animal.58 

Under nonstress conditions, p53 functions as an antioxidant and lowers reactive oxygen species (ROS) levels, thereby protecting HSCs from DNA damage and mutations.59  This function of p53 could explain the increased incidences of hematologic tumors in p53+/− and p53−/− mice.

These data suggest that, under certain conditions, transient inhibition of p53 function could potentially be used to promote proliferation of HSCs and accelerate the recovery of bone marrow without impairing the “stemness” of these cells. Indeed, inhibition of p53 by pifithrins (which specifically inhibit p53 activity) stimulates HSC proliferation in both in vitro and in vivo settings.60  This also correlates with the finding that suppression of p53 function facilitates hematopoietic reconstitution after cytotoxic therapy.61  Taken together, these data indicate that p53 plays a cytoprotective role in HSCs by preventing their entry into the cell-cycle and maintaining their quiescence.

The role of p53 is not limited to HSCs as p53 also negatively regulates self-renewal in neural62  and mammary stem cells.63  Furthermore, depletion of p53 promotes induced pluripotent stem cell generation.64-66 

Damage-induced p53 activity is detrimental for hematopoiesis

Induction of p53 activity in response to DNA damage also impairs hematopoiesis. DNA damage results in phosphorylation of p53 by ATM and Chk2 kinases and subsequent activation.67-69  Histopathologic analyses and bone marrow transplantation assays confirm that mice with enhanced p53 activity (via various genetic modifications) die of sublethal doses of IR exclusively because of hematopoietic failure.70  Furthermore, unirradiated HSCs outcompete HSCs that are treated with low-dose IR in transplantation assays.71  This is probably the result of the increased p53 activity, impairing the cellular fitness of irradiated stem and progenitor cells of the hematopoietic system. In agreement, loss of p53 protects HSCs and progenitor cells from IR damage in p53-null mice,72  possibly because radiation confers a selective advantage for long-term expansion of p53-deficient HSCs and progenitor cells.71,73  Treatment of irradiated mice with pifithrin-α, a drug that inhibits p53 activity, also nullifies the effects of radiation on the hematopoietic system.60 

Damage-induced p53 activates downstream targets involved in both apoptosis and cell cycle arrest, Nonetheless, the apoptotic pathway is perceived as predominant in hematopoietic organs.52  Meticulous studies show that the spleen, thymus, and bone marrow (major hematopoietic organs) in p53-null mice are refractory to p53 dose-dependent DNA damage.74  Thymocytes from wild-type mice undergo apoptosis after IR, whereas p53-null thymocytes remain completely resistant and p53 heterozygous thymocytes show an intermediate apoptotic response to DNA damage (Table 1).75,76  Irradiation also accelerates lymphomagenesis in p53-deficient mice. Remarkably, the most dramatic increase in lymphoma incidence is seen in p53+/− mice.58,77,78  The prevailing view that acute p53 apoptotic response to IR protects against tumorigenesis by eliminating cells with excessive DNA damage was challenged by 2 recent studies. Concurrent restoration of p53 activity in a mouse model with radiation did not protect against tumor onset, whereas restoration 8 days after IR was effective in tumor suppression. Similarly, conditional deletion of p53 4 weeks after IR resulted in lymphoma rates comparable with disrupting p53 before IR.79,80  These data suggest that p53 activity after IR is more important for tumor protection.

Table 1

Summary of mouse models with distinct hematopoietic phenotypes

Mice/allelesp53 activity, %Mdm2 dosage, %Mdm4 dosage, %PhenotypeReferences
Wt 100 100 100 Normal — 
p53+/− 50 100 100 Tumor prone after IR 58  
p53−/− 100 100 Tumor prone, increases in the number and proliferation of HSCs, radioresistant 33,34,53,58  
p53R172P ∼ 50 100 100 Tumor prone, undergo cell cycle arrest but not apoptosis, radioresistant 32  
p53R172H 100 100 Transcriptionaly null, tumor prone, radioresistant 35,36  
p53S18A < 100 100 100 Compromised apoptosis in thymocytes 139  
p53S23A < 100 100 100 Homozygous mice develop B-cell lineage tumors, decreased apoptosis in thymocytes 140  
p53S18AS23A < 100 100 100 Tumor prone, no apoptosis after IR in thymocytes 141  
p53T21AT23A > 100 100 100 Small, lymphopenic, low HSCs, aging phenotype, radiosensitive 81  
p537KR > 100 100 100 Higher p53 activity after IR, low HSCs at baseline, radiosensitive 82  
Super p53 > 100 100 100 Radiosensitive 83  
Mdm2+/− > 100 50 100 Radiosensitive, delayed lymphomagenesis in Eμ-Myc background 70,91  
Mdm2−/− > 100 100 Embryo lethal because of enhanced apoptosis 87,88  
Mdm2Puro7-12 > 100 ∼30 100 Small, lymphopenic, hypocellular spleen, thymus and bone marrow; radiosensitive 84  
Mdm2−/−p53P/R172P/R172P ∼ 50 100 Died at weaning of hematopoietic defects 110,112  
Mdm2Tg < 100 > 100 100 Tumor prone, more sarcomas 44  
Mdm2SNP309G < 100 > 100 100 Early-onset tumors 94  
Mdmx−/− > 100 100 Embryo lethal because of proliferation defects 95,96  
Mdmx+/− 100 100 50 Radiosensitive, delayed lymphomagenesis in Eμ-Myc background 70  
Mdm4+/ΔRING 100 100 100 Homozygous mice lethal, radioresistant 98  
Mdmx3SA 100 100 100 Radioresistant at 10 Gy 97  
Mdm2+/−Mdm4+/− > 100 50 50 Lethal by weaning, hematopoietic defects 70  
Mdm2+/−Mdm4+/−p53+/− 50 50 50 developmentally normal 70  
p21−/− 100 100 100 Normal, late tumors, radioresistant in C57BL/6 background, sensitive in mixed strain background 103,104  
Puma−/− 100 100 100 Normal, radioresistant 142,143  
Mice/allelesp53 activity, %Mdm2 dosage, %Mdm4 dosage, %PhenotypeReferences
Wt 100 100 100 Normal — 
p53+/− 50 100 100 Tumor prone after IR 58  
p53−/− 100 100 Tumor prone, increases in the number and proliferation of HSCs, radioresistant 33,34,53,58  
p53R172P ∼ 50 100 100 Tumor prone, undergo cell cycle arrest but not apoptosis, radioresistant 32  
p53R172H 100 100 Transcriptionaly null, tumor prone, radioresistant 35,36  
p53S18A < 100 100 100 Compromised apoptosis in thymocytes 139  
p53S23A < 100 100 100 Homozygous mice develop B-cell lineage tumors, decreased apoptosis in thymocytes 140  
p53S18AS23A < 100 100 100 Tumor prone, no apoptosis after IR in thymocytes 141  
p53T21AT23A > 100 100 100 Small, lymphopenic, low HSCs, aging phenotype, radiosensitive 81  
p537KR > 100 100 100 Higher p53 activity after IR, low HSCs at baseline, radiosensitive 82  
Super p53 > 100 100 100 Radiosensitive 83  
Mdm2+/− > 100 50 100 Radiosensitive, delayed lymphomagenesis in Eμ-Myc background 70,91  
Mdm2−/− > 100 100 Embryo lethal because of enhanced apoptosis 87,88  
Mdm2Puro7-12 > 100 ∼30 100 Small, lymphopenic, hypocellular spleen, thymus and bone marrow; radiosensitive 84  
Mdm2−/−p53P/R172P/R172P ∼ 50 100 Died at weaning of hematopoietic defects 110,112  
Mdm2Tg < 100 > 100 100 Tumor prone, more sarcomas 44  
Mdm2SNP309G < 100 > 100 100 Early-onset tumors 94  
Mdmx−/− > 100 100 Embryo lethal because of proliferation defects 95,96  
Mdmx+/− 100 100 50 Radiosensitive, delayed lymphomagenesis in Eμ-Myc background 70  
Mdm4+/ΔRING 100 100 100 Homozygous mice lethal, radioresistant 98  
Mdmx3SA 100 100 100 Radioresistant at 10 Gy 97  
Mdm2+/−Mdm4+/− > 100 50 50 Lethal by weaning, hematopoietic defects 70  
Mdm2+/−Mdm4+/−p53+/− 50 50 50 developmentally normal 70  
p21−/− 100 100 100 Normal, late tumors, radioresistant in C57BL/6 background, sensitive in mixed strain background 103,104  
Puma−/− 100 100 100 Normal, radioresistant 142,143  

— indicates not applicable.

Overall, p53 response to genotoxic stressors perturbs hematopoiesis by promoting apoptosis and altering the dynamics of HSCs and progenitor cell proliferation.

Constitutive p53 activity impairs hematopoietic homeostasis

Increase in p53 activity above basal levels is detrimental to long-term HSC fitness (Figure 1). p53 activity is characterized by the phosphorylation of amino terminal serine and threonine residues. Transgenic mice in which the p53 phosphorylation sites at threonine-21 and serine-23 are mutated to aspartic acid, a phospho-mimicking amino acid, have constitutive p53 activation and exhibit an ageing phenotype that correlates with depletion of adult stem cells in multiple tissues, including the bone marrow (Table 1).81  Similarly, another knock-in mouse model (p53-7KR) in which 7 C-terminal lysine residues, targets of Mdm2 ubiquitination, are mutated has constitutive p53 activity accompanied by a drastic depletion of the HSC population.82  HSCs from these mice also do not measure up to the repopulation potential of wild-type cells in transplantation assays. Other mouse models of high basal p53 activity, such as Super p53 mice (which express additional copies of p53), and mice haploinsufficient for p53 inhibitors Mdm2 and Mdm4 or mice with even less Mdm2 levels (see below), are overtly radiosensitive and exhibit hematopoietic defects in response to low levels of IR exposure (Table 1).70,82-84  Thus, an increase in basal p53 level impairs the stemness of HSCs, thereby sensitizing mice to lower doses of IR and results in HP failure.

Enhanced p53 function is also implicated in HSC ageing. p53+/m mice that express a truncated p53 protein, which forms constitutive active p53 complexes, exhibit an ageing phenotype and reduced number of proliferating HSCs with lower repopulation potential compared with wild-type mice.56,85 

Aberrations in p53 regulators have major implications for hematopoiesis

Mouse models show that aberrations in the expression level of Mdm2 and Mdm4 modulate p53 activity and impact the hematopoietic system. Decreased expression of either of these 2 inhibitors leads to activation of p53 and associated anomalies in the hematopoietic system (Table 1).70,84 

Mdm2 is required during primitive erythropoiesis to inhibit p53-mediated apoptosis.86  Genetically engineered mice with homozygous deletion of Mdm2 die prematurely at E3.5 because of lethal activation of p53.87,88  Recently Evan's group89  used a novel strategy to bypass this limitation and probed Mdm2 role in p53 regulation during hematopoiesis. They used a mouse model in which p53 activity could be reversibly toggled between inactive and active stages.89  Restoration of p53 activity in an Mdm2-null adult mouse led to an almost complete ablation of radio-sensitive tissues that included the bone marrow, thymus, spleen, small intestine, and colon via apoptosis and resulted in lethality. Consistent with the induction of bone marrow aplasia, analyses of peripheral blood also revealed a dramatic reduction in all hematopoietic lineages. Thus, inhibition of p53 activity by Mdm2 is required for HP viability.

Mice heterozygous for Mdm2 are normal and do not exhibit any apparent phenotypic abnormalities.87,88  They do, however, exhibit hyperpigmentation of foot pads, a characteristic of enhanced p53 activity.90  This enhancement of p53 activity is probably the reason that unirradiated BM cells from Mdm2+/− mice fail to compete with wild-type BM cells in competitive transplantation assays.71  In addition, Mdm2+/− mice are radiosensitive and die of sublethal IR dosage predominantly because of p53-mediated disruption of the hematopoietic system.70  Haploinsufficiency of Mdm2 also inhibits lymphomagenesis probably through up-regulation of p53 function as Eμ-myc Mdm2+/− mice exhibit a delayed lymphoma onset compared with Eμ-myc mice.91  These data suggest that a small increase in p53 activity is tolerated but compromises the ability to cope with stress.

Reduction of Mdm2 to < 50% of normal levels results in even more distinctive hematopoietic abnormalities in mice. Mice expressing hypomorphic and null alleles of Mdm2 encoding only 30% of normal Mdm2 levels (Mdm2Puro7-12) are small and have abnormally low levels of lymphocytes, red cells, neutrophils, and platelets in peripheral blood (Table 1).84,92,93  The spleen, thymus, and bone marrow of these mice also show a decrease in the number of lymphoid cells and are hypocellular. This correlates with increased p53 activity and subsequent apoptosis of hematopoietic cells in these organs. This HP defect is completely dependent on p53 as it disappears in a p53-null background.84 

Even more subtle fluctuations in Mdm2 levels impact p53 activity. Data from human studies suggest a correlation of a single nucleotide polymorphism in the MDM2 promoter, SNP309G, with early onset of lymphoma and leukemogenesis.47  Our laboratory has generated mouse models for the corresponding human MDM2 SNP309 polymorphism. Mice homozygous for the Mdm2SNP309G allele express slightly elevated Mdm2 levels probably because of a more efficient SP1 binding site at the P2-MDM2 promoter.94 Mdm2SNP309G/G mice exhibit lower levels of apoptosis in thymus than Mdm2SNP309T/T mice in response to 1 Gy IR.94 Mdm2SNP309G/G mice also die of early-onset tumors, most of which are lymphomas. The impact of this polymorphism on HSCs function remains to be investigated.

Altogether, these data suggest that Mdm2 dosage manifests p53-mediated hematologic anomalies (Table 1).84,92  Mdm2 levels inversely correlate with p53 levels/activity. Decreased Mdm2 levels recapitulate conditions of high p53 activation, which over time impairs the cellular fitness of stem cells and leads to hematologic aberrations. Down-modulation of Mdm2 levels or disruption of Mdm2-p53 interaction could be used therapeutically to activate p53 in hematologic tumors with wild-type p53. Given that p53 is very quickly activated to lethal limits in the absence of Mdm2, such strategies have to be transient and reversible to preclude any side effects. Of note, the effect of transient activation of p53 in the HSC population has not been examined.

Mdm4, the other critical but relatively weaker inhibitor of p53 compared with Mdm2, is required during the rapidly growing phase of definitive erythropoiesis.86 Mdm4-null mice exhibit p53-dependent proliferation defects and lethality during embryogenesis (Table 1).95,96  Mice heterozygous for Mdm4 are normal but overtly radiosensitive.70  Similar to Mdm2+/− mice, irradiated Mdm4+/− mice die exclusively because of bone marrow aplasia.70  Importantly, p53 loss rescues this radiolethal phenotype. In addition, similar to Mdm2, haploinsufficiency of Mdm4 delays lymphoma onset in Eμ-myc mice.70 

Conversely, mice encoding a stable form of Mdm4 (named Mdmx-3SA), which is not targeted by Mdm2 for degradation, are radioresistant and can survive 10 Gy IR (Table 1).97  This is probably because of irreversible p53 binding and, thus, inhibition of p53 activity. We also observe a similar radioresistant phenotype in mice with an Mdm4 allele that lacks the sequence encoding the RING domain necessary for Mdm4-Mdm2 interaction.98  RING domain-deficient Mdm4ΔRING is more stable than Mdm4, resulting in less p53 activity. Thus, heterozygous Mdm4+/ΔRING mice survive 6 Gy IR, unlike Mdm4+/− mice, and do not die of hematopoietic defects. These data validate the role of Mdm4 in dampening p53 activity during hematopoietic homeostasis and in response to DNA damage.

Mdm4 has also been tested as a therapeutic target for p53 activation.99  Restoration of p53 in Mdm4−/− adult tissues induces p53-target genes and causes widespread apoptosis in radiosensitive tissues. Most of the damage is limited to the lymphoid organs and the bone marrow.99  Interestingly, unlike Mdm2, the p53-dependent pathologies induced in Mdm4-deficient tissues are milder and fully reversible, implying that short-term inhibition of Mdm4 for p53 activation could be a more beneficial strategy with less toxicity in hematologic malignancies.

Compound haploinsufficiency of both Mdm2 and Mdm4 in a double-heterozygous mouse results in a much more severe phenotype.70  At mid-gestation, the Mdm2+/−Mdm4+/− embryos show characteristics of aplastic anemia and are smaller than wild-type embryos. Some of the embryos also have exencephaly associated with cleft palate or other types of neural tube closure defects. At birth, some double-heterozygous pups survive and are smaller than their wild-type littermates. Histologic examination of tissue sections from these pups show marked hypoplasia and atrophy in the bone marrow, spleen, and thymus along with decreased extramedullary hematopoiesis in the liver. Complete peripheral blood count and bone marrow pathology indicate systemic and severe depletion of all blood cells in these double-heterozygous animals. Importantly, deletion of a single p53 allele reverts all these defects, suggesting a dose-dependent equilibrium between p53 and its inhibitors.70  These experiments highlight the exquisite balance between p53 and Mdm proteins in maintaining hematopoietic homeostasis. The hematopoietic compartment contains highly proliferative cells that are exquisitely sensitive to Mdm2 and Mdm4 dosage and the resultant p53 activity.

In summary, loss of p53 function provides radioprotection and promotes HSC proliferation together with an increase in hematologic tumors. On the other hand, enhanced p53 activity inhibits HSC proliferation and imparts radiosensitivity (Figure 1). Altogether, these results indicate that p53 plays a critical role in regulating hematopoietic cell functions under both normal and stressed conditions.

Role of p53 downstream targets in hematopoiesis

p53-target genes involved in both apoptotic (eg, Puma) and cell cycle arrest (eg, p21) pathways are significantly induced in the bone marrow after IR (Figure 2). The role of p53 target proapoptotic genes Puma and Bax in modulating the function of hematopoietic stem and progenitor cells in response to high-dose irradiation is already established.100-102  Mice lacking either of these genes do not exhibit radiation-induced lethality (Table 1). Indeed, deletion of Puma or Bax rescues the lethality caused by p53 hyper-activity in sensitive mouse models.81,102  HSCs from Puma-null mice are highly quiescent and along with the progenitor cells are also more efficient in DNA-damage repair.100,101  This provides further evidence in support of dominant role of apoptotic genes in hematopoietic degeneration. In agreement, mice with compromised p53 apoptotic activity (p53R172P retain activation of cell cycle genes) do not undergo HP failure after DNA damage (V.P., unpublished observations, November 2011). These data suggest that inhibition of p53 apoptotic activity or down modulation of specific targets could be used as a potential strategy to protect HSCs from the cytotoxic and myelosuppressive effects of radio/chemotherapy.

Figure 2

p53 regulates HSC quiescence. (A) Under homeostatic conditions, p53 regulates HSC quiescence mediated by Gfi-1 and Necdin and maintains HSC self-renewal by an unknown mechanism. (B) Under stress conditions, enhanced p53 activity promotes transcription of downstream targets, which trigger cell cycle arrest, senescence, or apoptosis. p53 also transactivates Pig genes, which induce ROS levels and promote p53 stability.

Figure 2

p53 regulates HSC quiescence. (A) Under homeostatic conditions, p53 regulates HSC quiescence mediated by Gfi-1 and Necdin and maintains HSC self-renewal by an unknown mechanism. (B) Under stress conditions, enhanced p53 activity promotes transcription of downstream targets, which trigger cell cycle arrest, senescence, or apoptosis. p53 also transactivates Pig genes, which induce ROS levels and promote p53 stability.

Still unclear is the role of p21-mediated cell cycle arrest in response to radiation. In a mixed strain background, loss of p21 affected hematopoietic recovery after IR as the HSC pool was completely depleted because of uncontrolled cell cycling, leading to lethality (Table 1).103  However, in the C57Bl/6 background, no such fatality or HSC exhaustion was observed.104  In our hands, we also did not observe an increased radio-susceptibility in p21 null mice on a C57Bl/6 background after 6 Gy IR (V.P., unpublished observations, October 2010). In addition, it was recently shown that deletion of a single p21 allele could partially rescue the radio-lethality of p53-7KR mice after 6 Gy IR.82 p53-7KR mice display a differential transcriptional activity with higher induction of p21 in the bone marrow over apoptotic genes. These data further emphasize that, under certain conditions, p21 contributes to hematopoietic sensitivity by controlling transition through the G1/S boundary. However, as mentioned in the previous paragraph, blocking apoptosis completely rescues the HSC defects in wild-type and p53 hyperactive mice, supporting it as the predominant pathway involved.

Studies are ongoing to identify p53 downstream targets that are specifically involved in maintaining HSC quiescence. Gfi-1 (growth factor independent-1) and Necdin are 2 direct p53 targets that have been implicated in HSC quiescence under steady-state conditions (Figure 2).52  Gfi-1 is a SNAG-domain containing zinc-finger transcriptional repressor that restricts HSC proliferation and preserves their functional integrity.105  In in vivo competitive assays, Gfi-1-null HSCs demonstrate excessive cycling and impaired self-renewal.106,107  The other target, Necdin, is a growth-suppressing protein that is highly expressed in HSCs.52,108  Down-regulation of Necdin diminishes HSC quiescence, whereas its up-regulation increases HSC quiescence.52  Recent mouse studies have also confirmed that Necdin-null adult HSCs are less quiescent and more proliferative than normal.109  Other targets modulated by p53 during differentiation of HSCs remain to be identified.

p53515C/515C mice as a model for apoptosis-independent activities of p53 in hematopoiesis

Previously, our laboratory has generated a knock-in mouse (p53515C) that expresses a mutant form of p53 protein, p53R172P, which is defective for apoptosis but retains partial cell cycle arrest and senescence functions.32  Homozygosity of the p53515C allele partially rescues the early embryonic lethality of Mdm2-null mice.110 Mdm2−/−p53515C/515C mice are born at expected Mendelian ratio with normal cellularity of all major hematopoietic organs at birth. However, soon after, these organs undergo progressive degradation, which culminates in complete arrest of hematopoiesis. By day P10-P14, the spleens and thymi lose all lymphoid cells, and the bone marrow becomes completely aplastic, resulting in death of these young pups. These observations indicate that, in the absence of Mdm2, increase in p53R172P activity causes rapid and progressive loss of hematopoietic stem and progenitor cells postpartum. These results are in congruence with a report that p53 is maintained at low levels during embryonic development but increases a few days after birth.111  Overall, these data highlight the importance of p53-dependent cell cycle arrest in hematopoiesis.

To gain further insights into the mechanism causing the HP defect, we investigated the stress signals that instigate increase in p53 levels and activity during prenatal and postnatal development.112  We detected higher ROS levels in the bone marrow of Mdm2−/−p53515C/515C mice compared with controls. Previous studies have shown that p53 transactivates a set of ROS-inducing genes dubbed p53-induced genes (PIG) that lead to cell death.113  In agreement, we observed higher mRNA levels of some of the Pig genes in BM cells of Mdm2−/−p53515C/515C mice. Thus, a positive feedback loop is established wherein p53R172P transactivates Pig genes, which induce ROS levels and in turn further stabilize p53. Importantly, antioxidant treatment of Mdm2−/−p53515C/515C postnatal pups with N-acetyl cysteine in vivo rescued the hematopoietic defect. These data uphold the pro-oxidant role of p53 under stress conditions, perhaps as a possible signal to stabilize p53, in contrast to its antioxidant role in nonstress conditions (Figure 2).59  Stress because of the absence of Mdm2 induces ROS and stabilizes/activates p53R172P, leading to an increase in cell cycle arrest and senescence functions.112  This is further corroborated by elevated mRNA levels of p53 targets p21 and Dcr2 and an independent senescence marker p15 in the BM cells.114  Furthermore, senescence-associated βgal activity is also higher in the postnatal BMs. The hematopoietic defect in these mice is HSC-specific as bone marrow cells from these mice fail to rescue lethally irradiated wild-type mice in BM transplantation assays. Importantly, these defects are p53-dependent.

Extrapolation of these data implicate Mdm2 as a critical regulator of p53 activity in hematopoietic tissues when ROS levels increase. This regulation is critical for stem cell survival and differentiation. The increased ROS levels observed in patients with myeloid malignancies115  may also be p53-dependent. As such, antioxidant treatment could be a therapeutic strategy for these patients. Furthermore, the data also show that, in addition to apoptosis, other p53 downstream pathways, such as cell cycle arrest and senescence, are also involved in depletion of HSCs and progenitors cells.

TP53 in ribosomopathies: Diamond Blackfan anemia and 5q− syndrome

Ribosomal stress resulting from impaired nascent ribosome biogenesis or defects in ribosome function is another mechanism by which the p53 pathway is activated and commonly associates with the hematopoietic phenotype observed in several bone marrow failure syndromes.116  Ribosomal proteins function primarily as structural components of the 40S and 60S ribosomal subunits. Mutations in individual ribosomal proteins disrupt the assembly of the protein into the ribosomal subunits.117-121  As a result, unengaged ribosomal proteins remain free to bind and block Mdm2, leading to p53 accumulation and activation of a cell cycle arrest response in the erythroid lineage.122-124  This ultimately results in an erythroid failure phenotype that characterizes several genetic syndromes, such as Diamond-Blackfan anemia (DBA).117,125  DBA is an autosomal dominant syndrome characterized by aplasia of the erythroid lineage that is associated with growth retardation and limb, cardiac and craniofacial malformations with a very wide range of severity in all its phenotypic manifestations.126  Patients with DBA exhibit severe macrocytic anemia with reticulocytopenia and aplasia of red blood cell precursors in the context of an otherwise normocellular bone marrow.127  The absence of red blood cell precursors is the result of impaired proliferation and increased apoptosis at the erythrocyte burst-forming unit stage. Knockdown of RPS19 (the ribosomal protein most frequently involved in DBA) in mouse fetal liver cells induces increased levels of p53 and p21 with decreased proliferation and delay of the G1/S phase of the cell cycle.128  A similar erythroid phenotype in RPS19-deficient zebrafish is alleviated on suppression of p53.129  In human bone marrow–derived CD34+ cells in which the expression of RPS19 or RPS14 is down-modulated, the subsequent erythroid phenotype is completely rescued on treatment with the p53 inhibitor pifithrin-α. Conversely, nutlin-3, a compound that activates p53 through inhibition of Mdm2, selectively impairs erythropoiesis.130  Mice homozygous for RPS19 loss are not viable, whereas heterozygous RPS19 mice are born but exhibit no DBA phenotype.131,132  Interestingly, an inducible mouse model of mutated RPS19 (R62W mutation) completely recapitulates a DBA phenotype (growth retardation, anemia, and reduced erythroid progenitors and circulating red blood cells), suggesting a dominant negative role for RPS19 mutations.133  Induction of the mutant allele reduces the expression levels of RPS19 to 10% of normal endogenous levels. Ubiquitous expression of the mutant transgene results in lethality in early embryogenesis, but inducible expression at later time points induces ribosomal RNA-processing defects in bone marrow cells that phenocopy DBA. McGowan and Mason also identified a knock-in allele of RPS19 in a forward genetic screen in mice for defects in skin color.134  The RPS19DSK/+ mice are characterized by dark tail, ears, and feet, a white belly spot, growth retardation, and mild macrocytic anemia. Intriguingly, deletion of p53 from the bone marrow mitigated the skin and anemia phenotypes. Incidentally, dark skin phenotype are also observed in mice haploinsufficient for Mdm2 (Mdm2 heterozygous or Mdm2 hypomorphic alleles) or Mdm4 (Mdm4 heterozygosity).70,90  These data indicate that p53 function is critical for the phenotypic manifestations of human ribosomapathies.

Since the initial description of RPS19 mutations as the cause of DBA,117  mutations in RPS24, RPS17, and RPL35A have also been implicated in DBA.120,121,135  Recent studies have determined that mutations in ribosomal genes RPS19, RPS24, RPS17, RPS10, RPS26, RPS7, RPL5, RPL11, and RPL35A account for ∼ 50% of the known genetic alterations in affected persons.116,126  Defective ribosome biogenesis has been linked to many other congenital syndromes, including Schwachman-Diamond syndrome, X-linked dyskeratosis congenita, cartilage hair hypoplasia, Treacher Collins syndrome, and the 5q− syndrome.116  Importantly, all these clinical phenotypes, when induced in genetically engineered mouse models, can be rescued by crossing such models to p53-null mice, indicating their dependence on hyperactivation of the p53 pathway.134 

A prime example is the 5q− syndrome, a clonal disorder characterized by macrocytic anemia with erythroid hypoplasia and increased platelet counts with dysplastic hypolobulated megakaryocytes, typically affecting women.136  The striking overlapping features with DBA lead to the identification of haploinsufficiency of RPS14 as responsible for this phenotype. RPS14 was identified in an RNA interference screen of each gene within the commonly deleted region as responsible for the 5q− syndrome.137  A mouse model in which coordinate deletion of loci syntenic with the common deleted region of the 5q− syndrome (including RPS14 haploinsufficiency) recapitulates the erythroid phenotype characteristic of 5q− syndrome.138  Intercrossing this mouse with p53-deficient mice completely rescues the progenitor cell defect, restoring all hematopoietic bone marrow populations, again implicating a p53-dependent mechanism in the pathophysiology of this disorder induced by RPS14 alterations.138  Thus, increased p53 activity caused by ribosomal dysfunction is critical in the evolution of myelodysplastic syndromes, again emphasizing the need to squelch p53 activity.

Perspectives and conclusions

Studies using genetically modified mice have provided invaluable insights regarding the regulation of the p53-Mdm axis under physiologic conditions as well as under stress (oncogenic or otherwise) in a variety of human diseases. This information is critical from a translational viewpoint as pharmacologic agents that impinge on the p53 pathway are starting to be tested in clinical trials. For instance, p53-reactivating therapies are being tested for treatment of leukemia and other hematologic diseases with wild-type but deregulated p53 function. One such strategy involves the disruption of the p53-Mdm2 interaction using synthetic inhibitors, such as Nutlin. However, based on the results from the mouse models, caution is warranted when treating patients with such drugs as too much p53 activity could also impart toxicity to healthy hematopoietic cells as well as other normal tissues and cause depletion of HSCs and other toxicities. Furthermore, caution should be exercised when agents of that kind are used in patients whose malignancies express mutant forms of p53 (such as the p53R172H or p53R172P model) as their use in such context might lead to stabilization of mutant p53 with potentially catastrophic consequences. On the other hand, a transient disruption of the p53-Mdm2 interaction could be used to achieve myelosuppression before bone marrow transplantation while minimizing the use of chemotherapeutic agents in conditioning regimens, thus avoiding the toxicity associated with them. Inhibitors of Mdm4 are currently under development as well and could also be used for the same indications.

The knowledge gained regarding p53 regulation in hematopoiesis can be translated to the treatment of leukemia. Leukemogenesis is the result of uncontrolled proliferation and abnormal differentiation of leukemia stem cells. These cells share multiple features with normal HSCs, such as quiescence, and are also regulated by p53. It is plausible that alterations in the p53 pathway that sensitize normal HSCs to apoptosis or regulate their quiescence may produce similar effects in leukemia stem cells. Although the therapeutic window may be too narrow, in patients where the p53 pathway is still intact, disrupting the p53-mdm2 interaction could be conducive to apoptosis and eradication of leukemia stem cells.

The availability of mouse models that recapitulate human disease further our mechanistic insight into disease manifestation and provide a unique opportunity to test strategies targeting p53 activity in a preclinical setting. However, mouse models do have their own set of limitations. Most of the models used in laboratory carry a germline deletion/mutation of p53 pathway component, which differs from the subtle somatic mutations observed in human tumors. The genetic background of mouse models also influences the tumor spectrum. Differences in the mouse and human stroma could affect the tumor initiation, progression, and treatment profiles. Moreover, the common strategy of using whole-body radiation response for testing the fitness of the hematopoietic system presents a scenario unlikely ever to be recapitulated in human patients. Nonetheless, the benefits of using these models to test hypotheses far outweigh the negatives.

In conclusion, mouse models with alterations in the p53 pathway have been invaluable in shedding light as to the role of p53 in hematopoiesis. During steady state, basal p53 activity regulates HSC quiescence and self-renewal. Lack of p53 activity promotes HSC proliferation, which eventually leads to accumulation of DNA damage and tumor development. Excessive p53 activation, on the other hand, adversely affects cellularity of key organs and promotes loss of hematopoietic progenitors. Therefore, fine-tuned regulation of p53 activity is critical to maintain homeostasis and recovery after tissue damage, particularly in tissues with high cellular turnover, such as the bone marrow. A better understanding of the p53 regulatory mechanisms is critical to harness p53 activity for therapeutic purposes in cancer and other diseases.

Acknowledgments

The authors thank Dr James Jackson, MD Anderson Cancer Center, for critical reading of the manuscript.

This work was supported by the National Institutes of Health (grant CA47296, G.L.). V.P. is supported by the Brown Foundation.

National Institutes of Health

Authorship

Contribution: V.P., A.Q.-C., and G.L. wrote different sections of the manuscript.

Conflict-of-interest disclosure: The authors declare no competing financial interests.

Correspondence: Guillermina Lozano, Department of Genetics, University of Texas MD Anderson Cancer Center, 1515 Holcombe Blvd, Houston, TX 77030; e-mail: gglozano@mdanderson.org.

References

References
1
Vousden
 
KH
Lu
 
X
Live or let die: the cell's response to p53.
Nat Rev Cancer
2002
, vol. 
2
 
8
(pg. 
594
-
604
)
2
Vousden
 
KH
Prives
 
C
Blinded by the light: the growing complexity of p53.
Cell
2009
, vol. 
137
 
3
(pg. 
413
-
431
)
3
Prives
 
C
Hall
 
PA
The p53 pathway.
J Pathol
1999
, vol. 
187
 
1
(pg. 
112
-
126
)
4
Collavin
 
L
Lunardi
 
A
Del Sal
 
G
p53-family proteins and their regulators: hubs and spokes in tumor suppression.
Cell Death Differ
2010
, vol. 
17
 
6
(pg. 
901
-
911
)
5
Perry
 
ME
The regulation of the p53-mediated stress response by MDM2 and MDM4.
Cold Spring Harb Perspect Biol
2010
, vol. 
2
 
1
pg. 
a000968
 
6
Momand
 
J
Zambetti
 
GP
Olson
 
DC
George
 
D
Levine
 
AJ
The mdm-2 oncogene product forms a complex with the p53 protein and inhibits p53-mediated transactivation.
Cell
1992
, vol. 
69
 
7
(pg. 
1237
-
1245
)
7
Oliner
 
JD
Kinzler
 
KW
Meltzer
 
PS
George
 
DL
Vogelstein
 
B
Amplification of a gene encoding a p53-associated protein in human sarcomas.
Nature
1992
, vol. 
358
 
6381
(pg. 
80
-
83
)
8
Shvarts
 
A
Steegenga
 
WT
Riteco
 
N
, et al. 
MDMX: a novel p53-binding protein with some functional properties of MDM2.
EMBO J
1996
, vol. 
15
 
19
(pg. 
5349
-
5357
)
9
Kubbutat
 
MH
Jones
 
SN
Vousden
 
KH
Regulation of p53 stability by Mdm2.
Nature
1997
, vol. 
387
 
6630
(pg. 
299
-
303
)
10
Haupt
 
Y
Maya
 
R
Kazaz
 
A
Oren
 
M
Mdm2 promotes the rapid degradation of p53.
Nature
1997
, vol. 
387
 
6630
(pg. 
296
-
299
)
11
Honda
 
R
Tanaka
 
H
Yasuda
 
H
Oncoprotein MDM2 is a ubiquitin ligase E3 for tumor suppressor p53.
FEBS Lett
1997
, vol. 
420
 
1
(pg. 
25
-
27
)
12
Fei
 
P
El-Deiry
 
WS
p53 and radiation responses.
Oncogene
2003
, vol. 
22
 
37
(pg. 
5774
-
5783
)
13
Gudkov
 
AV
Komarova
 
EA
Pathologies associated with the p53 response.
Cold Spring Harb Perspect Biol
2010
, vol. 
2
 
7
pg. 
a001180
 
14
Gudkov
 
AV
Komarova
 
EA
The role of p53 in determining sensitivity to radiotherapy.
Nat Rev Cancer
2003
, vol. 
3
 
2
(pg. 
117
-
129
)
15
Chen
 
J
Astle
 
CM
Harrison
 
DE
Development and aging of primitive hematopoietic stem cells in BALB/cBy mice.
Exp Hematol
1999
, vol. 
27
 
5
(pg. 
928
-
935
)
16
Morrison
 
SJ
Uchida
 
N
Weissman
 
IL
The biology of hematopoietic stem cells.
Annu Rev Cell Dev Biol
1995
, vol. 
11
 (pg. 
35
-
71
)
17
Cumano
 
A
Godin
 
I
Pluripotent hematopoietic stem cell development during embryogenesis.
Curr Opin Immunol
2001
, vol. 
13
 
2
(pg. 
166
-
171
)
18
Chao
 
MP
Seita
 
J
Weissman
 
IL
Establishment of a normal hematopoietic and leukemia stem cell hierarchy.
Cold Spring Harb Symp Quant Biol
2008
, vol. 
73
 (pg. 
439
-
449
)
19
Attar
 
EC
Scadden
 
DT
Regulation of hematopoietic stem cell growth.
Leukemia
2004
, vol. 
18
 
11
(pg. 
1760
-
1768
)
20
Zhao
 
Y
Lin
 
YG
Zhan
 
YX
, et al. 
Murine hematopoietic stem cell characterization and its regulation in BM transplantation.
Blood
2000
, vol. 
96
 
9
(pg. 
3016
-
3022
)
21
van de Rijn
 
M
Heimfeld
 
S
Spangrude
 
GJ
Weissman
 
IL
Mouse hematopoietic stem-cell antigen Sca-1 is a member of the Ly-6 antigen family.
Proc Natl Acad Sci U S A
1989
, vol. 
86
 
12
(pg. 
4634
-
4638
)
22
Warr
 
MR
Pietras
 
EM
Passegue
 
E
Mechanisms controlling hematopoietic stem cell functions during normal hematopoiesis and hematological malignancies.
Wiley Interdiscip Rev Syst Biol Med
2011
, vol. 
3
 
6
(pg. 
681
-
701
)
23
Osawa
 
M
Hanada
 
K
Hamada
 
H
Nakauchi
 
H
Long-term lymphohematopoietic reconstitution by a single CD34-low/negative hematopoietic stem cell.
Science
1996
, vol. 
273
 
5272
(pg. 
242
-
245
)
24
Kiel
 
MJ
Yilmaz
 
OH
Iwashita
 
T
Terhorst
 
C
Morrison
 
SJ
SLAM family receptors distinguish hematopoietic stem and progenitor cells and reveal endothelial niches for stem cells.
Cell
2005
, vol. 
121
 
7
(pg. 
1109
-
1121
)
25
Miller
 
JS
McCullar
 
V
Punzel
 
M
Lemischka
 
IR
Moore
 
KA
Single adult human CD34(+)/Lin-/CD38(−) progenitors give rise to natural killer cells, B-lineage cells, dendritic cells, and myeloid cells.
Blood
1999
, vol. 
93
 
1
(pg. 
96
-
106
)
26
Mayle
 
A
Luo
 
M
Jeong
 
M
Goodell
 
MA
Flow cytometry analysis of murine hematopoietic stem cells [published online ahead of print June 26, 2012].
Cytometry A
 
27
Brady
 
CA
Attardi
 
LD
p53 at a glance.
J Cell Sci
2010
, vol. 
123
 
15
(pg. 
2527
-
2532
)
28
Vogelstein
 
B
Lane
 
D
Levine
 
AJ
Surfing the p53 network.
Nature
2000
, vol. 
408
 
6810
(pg. 
307
-
310
)
29
Hainaut
 
P
Hernandez
 
T
Robinson
 
A
, et al. 
IARC database of p53 gene mutations in human tumors and cell lines: updated compilation, revised formats and new visualisation tools.
Nucleic Acids Res
1998
, vol. 
26
 
1
(pg. 
205
-
213
)
30
Malkin
 
D
Li
 
FP
Strong
 
LC
, et al. 
Germ line p53 mutations in a familial syndrome of breast cancer, sarcomas, and other neoplasms.
Science
1990
, vol. 
250
 
4985
(pg. 
1233
-
1238
)
31
Srivastava
 
S
Zou
 
ZQ
Pirollo
 
K
Blattner
 
W
Chang
 
EH
Germ-line transmission of a mutated p53 gene in a cancer-prone family with Li-Fraumeni syndrome.
Nature
1990
, vol. 
348
 
6303
(pg. 
747
-
749
)
32
Liu
 
G
Parant
 
JM
Lang
 
G
, et al. 
Chromosome stability, in the absence of apoptosis, is critical for suppression of tumorigenesis in Trp53 mutant mice.
Nat Genet
2004
, vol. 
36
 
1
(pg. 
63
-
68
)
33
Donehower
 
LA
Harvey
 
M
Slagle
 
BL
, et al. 
Mice deficient for p53 are developmentally normal but susceptible to spontaneous tumours.
Nature
1992
, vol. 
356
 
6366
(pg. 
215
-
221
)
34
Jacks
 
T
Remington
 
L
Williams
 
BO
, et al. 
Tumor spectrum analysis in p53-mutant mice.
Curr Biol
1994
, vol. 
4
 
1
(pg. 
1
-
7
)
35
Lang
 
GA
Iwakuma
 
T
Suh
 
YA
, et al. 
Gain of function of a p53 hot spot mutation in a mouse model of Li-Fraumeni syndrome.
Cell
2004
, vol. 
119
 
6
(pg. 
861
-
872
)
36
Olive
 
KP
Tuveson
 
DA
Ruhe
 
ZC
, et al. 
Mutant p53 gain of function in two mouse models of Li-Fraumeni syndrome.
Cell
2004
, vol. 
119
 
6
(pg. 
847
-
860
)
37
Nahi
 
H
Selivanova
 
G
Lehmann
 
S
, et al. 
Mutated and non-mutated Tp53 as targets in the treatment of leukaemia.
Br J Haematol
2008
, vol. 
141
 
4
(pg. 
445
-
453
)
38
Newcomb
 
EW
p53 gene mutations in lymphoid diseases and their possible relevance to drug resistance.
Leuk Lymphoma
1995
, vol. 
17
 
3
(pg. 
211
-
221
)
39
Xu-Monette
 
ZY
Medeiros
 
LJ
Li
 
Y
, et al. 
Dysfunction of the Tp53 tumor suppressor gene in lymphoid malignancies.
Blood
2012
, vol. 
119
 
16
(pg. 
3668
-
3683
)
40
Preudhomme
 
C
Fenaux
 
P
The clinical significance of mutations of the p53 tumour suppressor gene in haematological malignancies.
Br J Haematol
1997
, vol. 
98
 
3
(pg. 
502
-
511
)
41
Krug
 
U
Ganser
 
A
Koeffler
 
HP
Tumor suppressor genes in normal and malignant hematopoiesis.
Oncogene
2002
, vol. 
21
 
21
(pg. 
3475
-
3495
)
42
Oscier
 
D
Wade
 
R
Davis
 
Z
, et al. 
Prognostic factors identified three risk groups in the LRF CLL4 trial, independent of treatment allocation.
Haematologica
2010
, vol. 
95
 
10
(pg. 
1705
-
1712
)
43
Seifert
 
H
Mohr
 
B
Thiede
 
C
, et al. 
The prognostic impact of 17p (p53) deletion in 2272 adults with acute myeloid leukemia.
Leukemia
2009
, vol. 
23
 
4
(pg. 
656
-
663
)
44
Jones
 
SN
Hancock
 
AR
Vogel
 
H
Donehower
 
LA
Bradley
 
A
Overexpression of Mdm2 in mice reveals a p53-independent role for Mdm2 in tumorigenesis.
Proc Natl Acad Sci U S A
1998
, vol. 
95
 
26
(pg. 
15608
-
15612
)
45
Xiong
 
S
Pant
 
V
Suh
 
YA
, et al. 
Spontaneous tumorigenesis in mice overexpressing the p53-negative regulator Mdm4.
Cancer Res
2010
, vol. 
70
 
18
(pg. 
7148
-
7154
)
46
Toledo
 
F
Wahl
 
GM
Regulating the p53 pathway: in vitro hypotheses, in vivo veritas.
Nat Rev Cancer
2006
, vol. 
6
 
12
(pg. 
909
-
923
)
47
Post
 
SM
Pant
 
V
Abbas
 
H
Quintas-Cardama
 
A
Prognostic impact of the MDM2SNP309 allele in leukemia and lymphoma.
Oncotarget
2010
, vol. 
1
 
3
(pg. 
168
-
174
)
48
Laurie
 
NA
Donovan
 
SL
Shih
 
CS
, et al. 
Inactivation of the p53 pathway in retinoblastoma.
Nature
2006
, vol. 
444
 
7115
(pg. 
61
-
66
)
49
Harutyunyan
 
A
Klampfl
 
T
Cazzola
 
M
Kralovics
 
R
p53 lesions in leukemic transformation.
N Engl J Med
2011
, vol. 
364
 
5
(pg. 
488
-
490
)
50
Veerakumarasivam
 
A
Scott
 
HE
Chin
 
SF
, et al. 
High-resolution array-based comparative genomic hybridization of bladder cancers identifies mouse double minute 4 (MDM4) as an amplification target exclusive of MDM2 and Tp53.
Clin Cancer Res
2008
, vol. 
14
 
9
(pg. 
2527
-
2534
)
51
Bo
 
MD
Secchiero
 
P
Degan
 
M
, et al. 
MDM4 (MDMX) is overexpressed in chronic lymphocytic leukaemia (CLL) and marks a subset of p53 wild-type CLL with a poor cytotoxic response to Nutlin-3.
Br J Haematol
2010
, vol. 
150
 
2
(pg. 
237
-
239
)
52
Liu
 
Y
Elf
 
SE
Miyata
 
Y
, et al. 
p53 regulates hematopoietic stem cell quiescence.
Cell Stem Cell
2009
, vol. 
4
 
1
(pg. 
37
-
48
)
53
TeKippe
 
M
Harrison
 
DE
Chen
 
J
Expansion of hematopoietic stem cell phenotype and activity in Trp53-null mice.
Exp Hematol
2003
, vol. 
31
 
6
(pg. 
521
-
527
)
54
Akala
 
OO
Park
 
IK
Qian
 
DL
Pihalja
 
M
Becker
 
MW
Clarke
 
MF
Long-term haematopoietic reconstitution by Trp53(−/−)p16(Ink4a−/−)p19(Arf−/−) multipotent progenitors.
Nature
2008
, vol. 
453
 
7192
(pg. 
228
-
232
)
55
Chen
 
J
Ellison
 
FM
Keyvanfar
 
K
, et al. 
Enrichment of hematopoietic stem cells with SLAM and LSK markers for the detection of hematopoietic stem cell function in normal and Trp53 null mice.
Exp Hematol
2008
, vol. 
36
 
10
(pg. 
1236
-
1243
)
56
Dumble
 
M
Moore
 
L
Chambers
 
SM
, et al. 
The impact of altered p53 dosage on hematopoietic stem cell dynamics during aging.
Blood
2007
, vol. 
109
 
4
(pg. 
1736
-
1742
)
57
Feng
 
Z
Hu
 
W
Teresky
 
AK
Hernando
 
E
Cordon-Cardo
 
C
Levine
 
AJ
Declining p53 function in the aging process: a possible mechanism for the increased tumor incidence in older populations.
Proc Natl Acad Sci U S A
2007
, vol. 
104
 
42
(pg. 
16633
-
16638
)
58
Kemp
 
CJ
Wheldon
 
T
Balmain
 
A
p53-deficient mice are extremely susceptible to radiation-induced tumorigenesis.
Nat Genet
1994
, vol. 
8
 
1
(pg. 
66
-
69
)
59
Sablina
 
AA
Budanov
 
AV
Ilyinskaya
 
GV
Agapova
 
LS
Kravchenko
 
JE
Chumakov
 
PM
The antioxidant function of the p53 tumor suppressor.
Nat Med
2005
, vol. 
11
 
12
(pg. 
1306
-
1313
)
60
Leonova
 
KI
Shneyder
 
J
Antoch
 
MP
, et al. 
A small molecule inhibitor of p53 stimulates amplification of hematopoietic stem cells but does not promote tumor development in mice.
Cell Cycle
2010
, vol. 
9
 
7
(pg. 
1434
-
1443
)
61
Wlodarski
 
P
Wasik
 
M
Ratajczak
 
MZ
, et al. 
Role of p53 in hematopoietic recovery after cytotoxic treatment.
Blood
1998
, vol. 
91
 
8
(pg. 
2998
-
3006
)
62
Meletis
 
K
Wirta
 
V
Hede
 
SM
Nister
 
M
Lundeberg
 
J
Frisen
 
J
p53 suppresses the self-renewal of adult neural stem cells.
Development
2006
, vol. 
133
 
2
(pg. 
363
-
369
)
63
Cicalese
 
A
Bonizzi
 
G
Pasi
 
CE
, et al. 
The tumor suppressor p53 regulates polarity of self-renewing divisions in mammary stem cells.
Cell
2009
, vol. 
138
 
6
(pg. 
1083
-
1095
)
64
Hong
 
H
Takahashi
 
K
Ichisaka
 
T
, et al. 
Suppression of induced pluripotent stem cell generation by the p53-p21 pathway.
Nature
2009
, vol. 
460
 
7259
(pg. 
1132
-
1135
)
65
Kawamura
 
T
Suzuki
 
J
Wang
 
YV
, et al. 
Linking the p53 tumour suppressor pathway to somatic cell reprogramming.
Nature
2009
, vol. 
460
 
7259
(pg. 
1140
-
1144
)
66
Krizhanovsky
 
V
Lowe
 
SW
Stem cells: the promises and perils of p53.
Nature
2009
, vol. 
460
 
7259
(pg. 
1085
-
1086
)
67
Banin
 
S
Moyal
 
L
Shieh
 
S
, et al. 
Enhanced phosphorylation of p53 by ATM in response to DNA damage.
Science
1998
, vol. 
281
 
5383
(pg. 
1674
-
1677
)
68
Hirao
 
A
Kong
 
YY
Matsuoka
 
S
, et al. 
DNA damage-induced activation of p53 by the checkpoint kinase Chk2.
Science
2000
, vol. 
287
 
5459
(pg. 
1824
-
1827
)
69
Canman
 
CE
Lim
 
DS
Cimprich
 
KA
, et al. 
Activation of the ATM kinase by ionizing radiation and phosphorylation of p53.
Science
1998
, vol. 
281
 
5383
(pg. 
1677
-
1679
)
70
Terzian
 
T
Wang
 
Y
Van Pelt
 
CS
Box
 
NF
Travis
 
EL
Lozano
 
G
Haploinsufficiency of Mdm2 and Mdm4 in tumorigenesis and development.
Mol Cell Biol
2007
, vol. 
27
 
15
(pg. 
5479
-
5485
)
71
Bondar
 
T
Medzhitov
 
R
p53-mediated hematopoietic stem and progenitor cell competition.
Cell Stem Cell
2010
, vol. 
6
 
4
(pg. 
309
-
322
)
72
Lotem
 
J
Sachs
 
L
Hematopoietic cells from mice deficient in wild-type p53 are more resistant to induction of apoptosis by some agents.
Blood
1993
, vol. 
82
 
4
(pg. 
1092
-
1096
)
73
Marusyk
 
A
Porter
 
CC
Zaberezhnyy
 
V
DeGregori
 
J
Irradiation selects for p53-deficient hematopoietic progenitors.
PLoS Biol
2010
, vol. 
8
 
3
pg. 
e1000324
 
74
Cui
 
YF
Zhou
 
PK
Woolford
 
LB
Lord
 
BI
Hendry
 
JH
Wang
 
DW
Apoptosis in bone marrow cells of mice with different p53 genotypes after gamma-rays irradiation in vitro.
J Environ Pathol Toxicol Oncol
1995
, vol. 
14
 
3
(pg. 
159
-
163
)
75
Lowe
 
SW
Schmitt
 
EM
Smith
 
SW
Osborne
 
BA
Jacks
 
T
p53 is required for radiation-induced apoptosis in mouse thymocytes.
Nature
1993
, vol. 
362
 
6423
(pg. 
847
-
849
)
76
Clarke
 
AR
Purdie
 
CA
Harrison
 
DJ
, et al. 
Thymocyte apoptosis induced by p53-dependent and independent pathways.
Nature
1993
, vol. 
362
 
6423
(pg. 
849
-
852
)
77
French
 
JE
Lacks
 
GD
Trempus
 
C
, et al. 
Loss of heterozygosity frequency at the Trp53 locus in p53-deficient (+/−) mouse tumors is carcinogen- and tissue-dependent.
Carcinogenesis
2001
, vol. 
22
 
1
(pg. 
99
-
106
)
78
Mitchel
 
RE
Jackson
 
JS
Morrison
 
DP
Carlisle
 
SM
Low doses of radiation increase the latency of spontaneous lymphomas and spinal osteosarcomas in cancer-prone, radiation-sensitive Trp53 heterozygous mice.
Radiat Res
2003
, vol. 
159
 
3
(pg. 
320
-
327
)
79
Christophorou
 
MA
Ringshausen
 
I
Finch
 
AJ
Swigart
 
LB
Evan
 
GI
The pathological response to DNA damage does not contribute to p53-mediated tumour suppression.
Nature
2006
, vol. 
443
 
7108
(pg. 
214
-
217
)
80
Hinkal
 
G
Parikh
 
N
Donehower
 
LA
Timed somatic deletion of p53 in mice reveals age-associated differences in tumor progression.
PLoS One
2009
, vol. 
4
 
8
pg. 
e6654
 
81
Liu
 
D
Ou
 
L
Clemenson
 
GD
, et al. 
Puma is required for p53-induced depletion of adult stem cells.
Nat Cell Biol
2010
, vol. 
12
 
10
(pg. 
993
-
998
)
82
Wang
 
YV
Leblanc
 
M
Fox
 
N
, et al. 
Fine-tuning p53 activity through C-terminal modification significantly contributes to HSC homeostasis and mouse radiosensitivity.
Genes Dev
2011
, vol. 
25
 
13
(pg. 
1426
-
1438
)
83
Garcia-Cao
 
I
Garcia-Cao
 
M
Martin-Caballero
 
J
, et al. 
“Super p53” mice exhibit enhanced DNA damage response, are tumor resistant and age normally.
EMBO J
2002
, vol. 
21
 
22
(pg. 
6225
-
6235
)
84
Mendrysa
 
SM
McElwee
 
MK
Michalowski
 
J
O'Leary
 
KA
Young
 
KM
Perry
 
ME
mdm2 is critical for inhibition of p53 during lymphopoiesis and the response to ionizing irradiation.
Mol Cell Biol
2003
, vol. 
23
 
2
(pg. 
462
-
472
)
85
Tyner
 
SD
Venkatachalam
 
S
Choi
 
J
, et al. 
p53 mutant mice that display early ageing-associated phenotypes.
Nature
2002
, vol. 
415
 
6867
(pg. 
45
-
53
)
86
Maetens
 
M
Doumont
 
G
Clercq
 
SD
, et al. 
Distinct roles of Mdm2 and Mdm4 in red cell production.
Blood
2007
, vol. 
109
 
6
(pg. 
2630
-
2633
)
87
Montes de Oca Luna
 
R
Wagner
 
DS
Lozano
 
G
Rescue of early embryonic lethality in mdm2-deficient mice by deletion of p53.
Nature
1995
, vol. 
378
 
6553
(pg. 
203
-
206
)
88
Jones
 
SN
Roe
 
AE
Donehower
 
LA
Bradley
 
A
Rescue of embryonic lethality in Mdm2-deficient mice by absence of p53.
Nature
1995
, vol. 
378
 
6553
(pg. 
206
-
208
)
89
Ringshausen
 
I
O'Shea
 
CC
Finch
 
AJ
Swigart
 
LB
Evan
 
GI
Mdm2 is critically and continuously required to suppress lethal p53 activity in vivo.
Cancer Cell
2006
, vol. 
10
 
6
(pg. 
501
-
514
)
90
Terzian
 
T
Dumble
 
M
Arbab
 
F
, et al. 
Rpl27a mutation in the sooty foot ataxia mouse phenocopies high p53 mouse models.
J Pathol
2011
, vol. 
224
 
4
(pg. 
540
-
552
)
91
Alt
 
JR
Greiner
 
TC
Cleveland
 
JL
Eischen
 
CM
Mdm2 haplo-insufficiency profoundly inhibits Myc-induced lymphomagenesis.
EMBO J
2003
, vol. 
22
 
6
(pg. 
1442
-
1450
)
92
Mendrysa
 
SM
O'Leary
 
KA
McElwee
 
MK
, et al. 
Tumor suppression and normal aging in mice with constitutively high p53 activity.
Genes Dev
2006
, vol. 
20
 
1
(pg. 
16
-
21
)
93
Mendrysa
 
SM
Perry
 
ME
The p53 tumor suppressor protein does not regulate expression of its own inhibitor, MDM2, except under conditions of stress.
Mol Cell Biol
2000
, vol. 
20
 
6
(pg. 
2023
-
2030
)
94
Post
 
SM
Quintas-Cardama
 
A
Pant
 
V
, et al. 
A high-frequency regulatory polymorphism in the p53 pathway accelerates tumor development.
Cancer Cell
2010
, vol. 
18
 
3
(pg. 
220
-
230
)
95
Parant
 
J
Chavez-Reyes
 
A
Little
 
NA
, et al. 
Rescue of embryonic lethality in Mdm4-null mice by loss of Trp53 suggests a nonoverlapping pathway with MDM2 to regulate p53.
Nat Genet
2001
, vol. 
29
 
1
(pg. 
92
-
95
)
96
Migliorini
 
D
Lazzerini Denchi
 
E
Danovi
 
D
, et al. 
Mdm4 (Mdmx) regulates p53-induced growth arrest and neuronal cell death during early embryonic mouse development.
Mol Cell Biol
2002
, vol. 
22
 
15
(pg. 
5527
-
5538
)
97
Wang
 
YV
Leblanc
 
M
Wade
 
M
Jochemsen
 
AG
Wahl
 
GM
Increased radioresistance and accelerated B cell lymphomas in mice with Mdmx mutations that prevent modifications by DNA-damage-activated kinases.
Cancer Cell
2009
, vol. 
16
 
1
(pg. 
33
-
43
)
98
Pant
 
V
Xiong
 
S
Iwakuma
 
T
Quintas-Cardama
 
A
Lozano
 
G
Heterodimerization of Mdm2 and Mdm4 is critical for regulating p53 activity during embryogenesis but dispensable for p53 and Mdm2 stability.
Proc Natl Acad Sci U S A
2011
, vol. 
108
 
29
(pg. 
11995
-
12000
)
99
Garcia
 
D
Warr
 
MR
Martins
 
CP
Brown Swigart
 
L
Passegue
 
E
Evan
 
GI
Validation of MdmX as a therapeutic target for reactivating p53 in tumors.
Genes Dev
2011
, vol. 
25
 
16
(pg. 
1746
-
1757
)
100
Shao
 
L
Sun
 
Y
Zhang
 
Z
, et al. 
Deletion of proapoptotic Puma selectively protects hematopoietic stem and progenitor cells against high-dose radiation.
Blood
2010
, vol. 
115
 
23
(pg. 
4707
-
4714
)
101
Yu
 
H
Shen
 
H
Yuan
 
Y
, et al. 
Deletion of Puma protects hematopoietic stem cells and confers long-term survival in response to high-dose gamma-irradiation.
Blood
2010
, vol. 
115
 
17
(pg. 
3472
-
3480
)
102
Kirsch
 
DG
Santiago
 
PM
di Tomaso
 
E
, et al. 
p53 controls radiation-induced gastrointestinal syndrome in mice independent of apoptosis.
Science
2010
, vol. 
327
 
5965
(pg. 
593
-
596
)
103
Cheng
 
T
Rodrigues
 
N
Shen
 
H
, et al. 
Hematopoietic stem cell quiescence maintained by p21cip1/waf1.
Science
2000
, vol. 
287
 
5459
(pg. 
1804
-
1808
)
104
van Os
 
R
Kamminga
 
LM
Ausema
 
A
, et al. 
A Limited role for p21Cip1/Waf1 in maintaining normal hematopoietic stem cell functioning.
Stem Cells
2007
, vol. 
25
 
4
(pg. 
836
-
843
)
105
Gilks
 
CB
Bear
 
SE
Grimes
 
HL
Tsichlis
 
PN
Progression of interleukin-2 (IL-2)-dependent rat T cell lymphoma lines to IL-2-independent growth following activation of a gene (Gfi-1) encoding a novel zinc finger protein.
Mol Cell Biol
1993
, vol. 
13
 
3
(pg. 
1759
-
1768
)
106
Hock
 
H
Hamblen
 
MJ
Rooke
 
HM
, et al. 
Gfi-1 restricts proliferation and preserves functional integrity of haematopoietic stem cells.
Nature
2004
, vol. 
431
 
7011
(pg. 
1002
-
1007
)
107
Zeng
 
H
Yucel
 
R
Kosan
 
C
Klein-Hitpass
 
L
Moroy
 
T
Transcription factor Gfi1 regulates self-renewal and engraftment of hematopoietic stem cells.
EMBO J
2004
, vol. 
23
 
20
(pg. 
4116
-
4125
)
108
Forsberg
 
EC
Prohaska
 
SS
Katzman
 
S
Heffner
 
GC
Stuart
 
JM
Weissman
 
IL
Differential expression of novel potential regulators in hematopoietic stem cells.
PLoS Genet
2005
, vol. 
1
 
3
pg. 
e28
 
109
Asai
 
T
Liu
 
Y
Di Giandomenico
 
S
, et al. 
Necdin, a p53 target gene, regulates the quiescence and response to genotoxic stress of hematopoietic stem/progenitor cells.
Blood
2012
, vol. 
120
 
8
(pg. 
1601
-
1612
)
110
Liu
 
G
Terzian
 
T
Xiong
 
S
, et al. 
The p53-Mdm2 network in progenitor cell expansion during mouse postnatal development.
J Pathol
2007
, vol. 
213
 
4
(pg. 
360
-
368
)
111
Komarova
 
EA
Chernov
 
MV
Franks
 
R
, et al. 
Transgenic mice with p53-responsive lacZ: p53 activity varies dramatically during normal development and determines radiation and drug sensitivity in vivo.
EMBO J
1997
, vol. 
16
 
6
(pg. 
1391
-
1400
)
112
Abbas
 
HA
Maccio
 
DR
Coskun
 
S
, et al. 
Mdm2 is required for survival of hematopoietic stem cells/progenitors via dampening of ROS-induced p53 activity.
Cell Stem Cell
2010
, vol. 
7
 
5
(pg. 
606
-
617
)
113
Polyak
 
K
Xia
 
Y
Zweier
 
JL
Kinzler
 
KW
Vogelstein
 
B
A model for p53-induced apoptosis.
Nature
1997
, vol. 
389
 
6648
(pg. 
300
-
305
)
114
Collado
 
M
Gil
 
J
Efeyan
 
A
, et al. 
Tumour biology: senescence in premalignant tumours.
Nature
2005
, vol. 
436
 
7051
pg. 
642
 
115
Hole
 
PS
Darley
 
RL
Tonks
 
A
Do reactive oxygen species play a role in myeloid leukemias?
Blood
2011
, vol. 
117
 
22
(pg. 
5816
-
5826
)
116
Narla
 
A
Ebert
 
BL
Ribosomopathies: human disorders of ribosome dysfunction.
Blood
2010
, vol. 
115
 
16
(pg. 
3196
-
3205
)
117
Draptchinskaia
 
N
Gustavsson
 
P
Andersson
 
B
, et al. 
The gene encoding ribosomal protein S19 is mutated in Diamond-Blackfan anaemia.
Nat Genet
1999
, vol. 
21
 
2
(pg. 
169
-
175
)
118
Choesmel
 
V
Fribourg
 
S
Aguissa-Toure
 
AH
, et al. 
Mutation of ribosomal protein RPS24 in Diamond-Blackfan anemia results in a ribosome biogenesis disorder.
Hum Mol Genet
2008
, vol. 
17
 
9
(pg. 
1253
-
1263
)
119
Flygare
 
J
Aspesi
 
A
Bailey
 
JC
, et al. 
Human RPS19, the gene mutated in Diamond-Blackfan anemia, encodes a ribosomal protein required for the maturation of 40S ribosomal subunits.
Blood
2007
, vol. 
109
 
3
(pg. 
980
-
986
)
120
Gazda
 
HT
Grabowska
 
A
Merida-Long
 
LB
, et al. 
Ribosomal protein S24 gene is mutated in Diamond-Blackfan anemia.
Am J Hum Genet
2006
, vol. 
79
 
6
(pg. 
1110
-
1118
)
121
Cmejla
 
R
Cmejlova
 
J
Handrkova
 
H
Petrak
 
J
Pospisilova
 
D
Ribosomal protein S17 gene (RPS17) is mutated in Diamond-Blackfan anemia.
Hum Mutat
2007
, vol. 
28
 
12
(pg. 
1178
-
1182
)
122
Zhu
 
Y
Poyurovsky
 
MV
Li
 
Y
, et al. 
Ribosomal protein S7 is both a regulator and a substrate of MDM2.
Mol Cell
2009
, vol. 
35
 
3
(pg. 
316
-
326
)
123
Ofir-Rosenfeld
 
Y
Boggs
 
K
Michael
 
D
Kastan
 
MB
Oren
 
M
Mdm2 regulates p53 mRNA translation through inhibitory interactions with ribosomal protein L26.
Mol Cell
2008
, vol. 
32
 
2
(pg. 
180
-
189
)
124
Yadavilli
 
S
Mayo
 
LD
Higgins
 
M
Lain
 
S
Hegde
 
V
Deutsch
 
WA
Ribosomal protein S3: a multi-functional protein that interacts with both p53 and MDM2 through its KH domain.
DNA Repair (Amst)
2009
, vol. 
8
 
10
(pg. 
1215
-
1224
)
125
Ellis
 
SR
Lipton
 
JM
Diamond Blackfan anemia: a disorder of red blood cell development.
Curr Top Dev Biol
2008
, vol. 
82
 (pg. 
217
-
241
)
126
Ball
 
S
Diamond Blackfan anemia.
Hematology Am Soc Hematol Educ Program
2011
, vol. 
2011
 (pg. 
487
-
491
)
127
Lipton
 
JM
Ellis
 
SR
Diamond Blackfan anemia 2008-2009: broadening the scope of ribosome biogenesis disorders.
Curr Opin Pediatr
2010
, vol. 
22
 
1
(pg. 
12
-
19
)
128
Sieff
 
CA
Yang
 
J
Merida-Long
 
LB
Lodish
 
HF
Pathogenesis of the erythroid failure in Diamond Blackfan anaemia.
Br J Haematol
2010
, vol. 
148
 
4
(pg. 
611
-
622
)
129
Danilova
 
N
Sakamoto
 
KM
Lin
 
S
Ribosomal protein L11 mutation in zebrafish leads to haematopoietic and metabolic defects.
Br J Haematol
2011
, vol. 
152
 
2
(pg. 
217
-
228
)
130
Dutt
 
S
Narla
 
A
Lin
 
K
, et al. 
Haploinsufficiency for ribosomal protein genes causes selective activation of p53 in human erythroid progenitor cells.
Blood
2011
, vol. 
117
 
9
(pg. 
2567
-
2576
)
131
Matsson
 
H
Davey
 
EJ
Frojmark
 
AS
, et al. 
Erythropoiesis in the Rps19 disrupted mouse: analysis of erythropoietin response and biochemical markers for Diamond-Blackfan anemia.
Blood Cells Mol Dis
2006
, vol. 
36
 
2
(pg. 
259
-
264
)
132
Matsson
 
H
Davey
 
EJ
Draptchinskaia
 
N
, et al. 
Targeted disruption of the ribosomal protein S19 gene is lethal prior to implantation.
Mol Cell Biol
2004
, vol. 
24
 
9
(pg. 
4032
-
4037
)
133
Devlin
 
EE
Dacosta
 
L
Mohandas
 
N
Elliott
 
G
Bodine
 
DM
A transgenic mouse model demonstrates a dominant negative effect of a point mutation in the RPS19 gene associated with Diamond-Blackfan anemia.
Blood
2010
, vol. 
116
 
15
(pg. 
2826
-
2835
)
134
McGowan
 
KA
Mason
 
PJ
Animal models of Diamond Blackfan anemia.
Semin Hematol
2011
, vol. 
48
 
2
(pg. 
106
-
116
)
135
Farrar
 
JE
Nater
 
M
Caywood
 
E
, et al. 
Abnormalities of the large ribosomal subunit protein, Rpl35a, in Diamond-Blackfan anemia.
Blood
2008
, vol. 
112
 
5
(pg. 
1582
-
1592
)
136
Van den Berghe
 
H
Cassiman
 
JJ
David
 
G
Fryns
 
JP
Michaux
 
JL
Sokal
 
G
Distinct haematological disorder with deletion of long arm of no. 5 chromosome.
Nature
1974
, vol. 
251
 
5474
(pg. 
437
-
438
)
137
Ebert
 
BL
Pretz
 
J
Bosco
 
J
, et al. 
Identification of RPS14 as a 5q− syndrome gene by RNA interference screen.
Nature
2008
, vol. 
451
 
7176
(pg. 
335
-
339
)
138
Barlow
 
JL
Drynan
 
LF
Hewett
 
DR
, et al. 
A p53-dependent mechanism underlies macrocytic anemia in a mouse model of human 5q− syndrome.
Nat Med
2010
, vol. 
16
 
1
(pg. 
59
-
66
)
139
Sluss
 
HK
Armata
 
H
Gallant
 
J
Jones
 
SN
Phosphorylation of serine 18 regulates distinct p53 functions in mice.
Mol Cell Biol
2004
, vol. 
24
 
3
(pg. 
976
-
984
)
140
MacPherson
 
D
Kim
 
J
Kim
 
T
, et al. 
Defective apoptosis and B-cell lymphomas in mice with p53 point mutation at Ser 23.
EMBO J
2004
, vol. 
23
 
18
(pg. 
3689
-
3699
)
141
Chao
 
C
Herr
 
D
Chun
 
J
Xu
 
Y
Ser18 and 23 phosphorylation is required for p53-dependent apoptosis and tumor suppression.
EMBO J
2006
, vol. 
25
 
11
(pg. 
2615
-
2622
)
142
Jeffers
 
JR
Parganas
 
E
Lee
 
Y
, et al. 
Puma is an essential mediator of p53-dependent and -independent apoptotic pathways.
Cancer Cell
2003
, vol. 
4
 
4
(pg. 
321
-
328
)
143
Villunger
 
A
Michalak
 
EM
Coultas
 
L
, et al. 
p53- and drug-induced apoptotic responses mediated by BH3-only proteins puma and noxa.
Science
2003
, vol. 
302
 
5647
(pg. 
1036
-
1038
)