Transcription activity is required for p53-dependent tumor suppression

Molecular Subtypes and the Role of TP53 in Diffuse Large B-Cell Lymphoma and Richter Syndrome

Diffuse large B-cell lymphoma (DLBCL) is the most common lymphoid malignancy and a heterogeneous entity comprised of several biologically distinct subtypes. Recently, novel genetic classifications of DLBCL have been resolved based on common mutational patterns indicative of distinct pathways of transformation. However, the complicated and costly nature of the novel classifiers has precluded their inclusion into routine practice. In view of this, the status of the TP53 gene, which is mutated or deleted in 20-30% of the cases, has emerged as an important prognostic factor for DLBCL patients, setting itself apart from other predictors. TP53 genetic lesions are particularly enriched in a genetic subtype of DLBCL that shares genomic features with Richter Syndrome, highlighting the possibility of a subset of DLBCL arising from the transformation of an occult chronic lymphocytic leukemia-like malignancy, such as monoclonal B-cell lymphocytosis. Patients with TP53-mutated DLBCL, including those with Richter Syndrome, have a particularly poor prognosis and display inferior responses to standard chemoimmunotherapy regimens. The data presented in this manuscript argue for the need for improved and more practical risk-stratification models for patients with DLBCL and show the potential for the use of TP53 mutational status for prognostication and, in prospect, treatment stratification in DLBCL.

Expression and role of FKBPL in lung adenocarcinoma

Dysregulated expression of FK506-binding protein like (FKBPL) has been demonstrated to play crucial roles in tumour development. However, the role of FKBPL in lung adenocarcinoma (ADC) remains unclear. Using immunohistochemical staining, we showed that FKBPL expression was significantly lower in lung ADC than the normal tissues (P < 0.0001). Patients with well or moderately differentiated tumours have higher FKBPL expression compared with patients with poor differentiated tumours (P = 0.037). However, no significant associations were found between FKBPL expression and other clinicopathological variables (P > 0.05 for all). Cox univariate analysis showed that high FKBPL expression was correlated with prolonged overall survival (OS) (P = 0.010). Kaplan-Meier analysis further confirmed that the FKBPL-low group showed a significantly shorter OS than the FKBPL-high group (P = 0.0081). FKBPL expression was not shown as an independent prognostic factor for OS in the multivariate analysis (P = 0.063). Moreover, our study demonstrated that FKBPL could suppress the proliferation of lung ADC cells by delaying cell cycle G1/S phase transition. In addition, FKBPL resulted in increased apoptosis in lung ADC cells. Using the Human Apoptosis Array Kit, we observed that overexpression of FKBPL in lung ADC A549 cells significantly decreased the anti-apoptotic proteins, including heat shock protein 32 (HSP32), heat shock protein 27 (HSP27), and paraoxonase-2 (PON2). FKBPL depletion significantly attenuated the pro-apoptotic protein, phospho-p53 (S46), in lung ADC H1975 cells. These new findings provide an experimental basis for further theoretical investigation of lung ADC.

Oncogenic Runx1-Myc axis in p53-deficient thymic lymphoma

p53 deficiency and Myc dysregulation are frequently associated with cancer. However, the molecular mechanisms linking these two major oncogenic events are poorly understood. Using an osteosarcoma model caused by p53 loss, we have recently shown that Runx3 aberrantly upregulates Myc via mR1, a Runx consensus site in the Myc promoter. Here, we focus on thymic lymphoma, a major tumour type caused by germline p53 deletion in mice, and examine whether the oncogenic Runx-Myc axis plays a notable role in the development of p53-deficient lymphoma. Mice lacking p53 specifically in thymocytes (LP mice) mostly succumbed to thymic lymphoma. Runx1 and Myc were upregulated in LP mouse lymphoma compared with the normal thymus. Depletion of Runx1 or Myc prolonged the lifespan of LP mice and suppressed lymphoma development. In lymphoma cells isolated from LP mice, knockdown of Runx1 led to Myc suppression, weakening their tumour forming ability in immunocompromised mice. The mR1 locus was enriched by both Runx1 and H3K27ac, an active chromatin marker. LP mice with mutated mR1 had a longer lifespan and a lower incidence of lymphoma. Treatment with AI-10-104, a Runx inhibitor, improved the survival of LP mice. These results suggest that Myc upregulation by Runx1 is a key event in p53-deficient thymic lymphoma development and provide a clinical rationale for targeting the Runx family in p53-deficient malignancies.

Altered pathways and targeted therapy in double hit lymphoma

High-grade B-cell lymphoma with translocations involving MYC and BCL2 or BCL6, usually referred to as double hit lymphoma (DHL), is an aggressive hematological malignance with distinct genetic features and poor clinical prognosis. Current standard chemoimmunotherapy fails to confer satisfying outcomes and few targeted therapeutics are available for the treatment against DHL. Recently, the delineating of the genetic landscape in tumors has provided insight into both biology and targeted therapies. Therefore, it is essential to understand the altered signaling pathways of DHL to develop treatment strategies with better clinical benefits. Herein, we summarized the genetic alterations in the two DHL subtypes (DHL-BCL2 and DHL-BCL6). We further elucidate their implications on cellular processes, including anti-apoptosis, epigenetic regulations, B-cell receptor signaling, and immune escape. Ongoing and potential therapeutic strategies and targeted drugs steered by these alterations were reviewed accordingly. Based on these findings, we also discuss the therapeutic vulnerabilities that coincide with these genetic changes. We believe that the understanding of the DHL studies will provide insight into this disease and capacitate the finding of more effective treatment strategies.

Interferon-stimulated gene 15 enters posttranslational modifications of p53

The tumor suppressor protein p53 is a central governor of various cellular signals. It is well accepted that ubiquitination as well as ubiquitin-like (UBL) modifications of p53 protein is critical in the control of its activity. Interferon-stimulated gene 15 (ISG15) is a well-known UBL protein with pleiotropic functions, serving both as a free intracellular molecule and as a modifier by conjugating to target proteins. Initially, attentions have historically focused on the antiviral effects of ISG15 pathway. Remarkably, a significant role in the processes of autophagy, DNA repair, and protein translation provided considerable insight into the new functions of ISG15 pathway. Despite the deterministic revelation of the relation between ISG15 and p53, the functional consequence of p53 ISGylation appears somewhat confused. More important, more recent studies have hinted p53 ubiquitination or other UBL modifications that might interconnect with its ISGylation. Here, we aim to summarize the current knowledge of p53 ISGylation and the differences in other significant modifications, which would be beneficial for the development of p53-based cancer therapy.

An Lck-cre transgene accelerates autoantibody production and lupus development in (NZB × NZW)F1 mice

Lupus is an autoimmune disease characterized by the development of antinuclear autoantibodies and immune complex-mediated tissue damage. T cells in lupus patients appear to undergo apoptosis at an increased rate, and this enhanced T cell apoptosis has been postulated to contribute to lupus pathogenesis by increasing autoantigen load. However, there is no direct evidence to support this hypothesis. In this study, we show that an Lck-cre transgene, which increases T cell apoptosis as a result of T cell-specific expression of cre recombinase, accelerates the development of autoantibodies and nephritis in lupus-prone (NZB × NZW)F1 mice. Although the enhanced T cell apoptosis in Lck-cre transgenic mice resulted in an overall decrease in the relative abundance of splenic CD4(+) and CD8(+) T cells, the proportion of activated CD4(+) T cells was increased and no significant change was observed in the relative abundance of suppressive T cells. We postulate that the Lck-cre transgene promoted lupus by enhancing T cell apoptosis, which, in conjunction with the impaired clearance of apoptotic cells in lupus-prone mice, increased the nuclear antigen load and accelerated the development of anti-nuclear autoantibodies. Furthermore, our results also underscore the importance of including cre-only controls in studies using the cre-lox system.

The p53 Mutation/Deletion Profile in a Small Cohort of the Omani Population with Diffuse Large B-Cell Lymphoma

OBJECTIVES

Mutations/deletions affecting the TP53 gene are considered an independent marker predicting a poor prognosis for patients with diffuse large B-cell lymphoma (DLBCL). A cohort within a genetically isolated population was investigated for p53 mutation/deletion status.

METHODS

Deoxyribonucleic acid (DNA) samples were extracted from 23 paraffin-embedded blocks obtained from DLBCL patients, and subjected to polymerase chain reaction (PCR) amplification and sequencing of exons 4-9 of the p53 gene.

RESULTS

While 35% of patients analysed displayed allelic deletions (P <0.01), immunohistochemical analysis revealed a mutation rate of 69.5%. It is noteworthy that the rate of p53 mutations/deletions in this small cohort was found to be higher than that previously reported in the literature. Interestingly, patients with p53 mutations displayed a better overall survival when compared to those without. The survival of patients treated with rituximab-containing combination chemotherapy was significantly better than those who did not receive rituximab (P <0.05). Furthermore, a modelling analysis of the deleted form of p53 revealed a huge structural change affecting the DNA-binding domain.

CONCLUSION

The TP53 mutation/deletion status plays a role in mechanism(s) ruling the pathogenesis of DLBCL and may be useful for stratifying patients into distinct prognostic subsets.

Illuminating p53 function in cancer with genetically engineered mouse models

The key role of the p53 protein in tumor suppression is highlighted by its frequent mutation in human cancers and by the completely penetrant cancer predisposition of p53 null mice. Beyond providing definitive evidence for the critical function of p53 in tumor suppression, genetically engineered mouse models have offered numerous additional insights into p53 function. p53 knock-in mice expressing tumor-derived p53 mutants have revealed that these mutants display gain-of-function activities that actively promote carcinogenesis. The generation of p53 knock-in mutants with alterations in different domains of p53 has helped further elucidate the cellular and biochemical activities of p53 that are most fundamental for tumor suppression. In addition, modulation of p53 post-translational modification (PTM) status by generating p53 knock-in mouse strains with mutations in p53 PTM sites has revealed a subtlety and complexity to p53 regulation. Analyses of mouse models perturbing upstream regulators of p53 have solidified the notion that the p53 pathway can be compromised by means other than direct p53 mutation. Finally, switchable p53 models that allow p53 reactivation in tumors have helped evaluate the potential of p53 restoration therapy for cancer treatment. Collectively, mouse models have greatly enhanced our understanding of physiological p53 function and will continue to provide new biological and clinical insights in future investigations.

Karyotyping of diffuse large B-cell lymphomas: loss of 17p is associated with poor patient outcome

BACKGROUND

Cytogenetic studies of patients with diffuse large B-cell lymphoma (DLBCL) have revealed a large spectrum of chromosomal abnormalities, some of which may be clinically relevant. We wanted to evaluate possible associations between commonly acquired chromosome aberrations and prognosis in a large cohort of patients.

METHODS

All patients with DLBCL treated at our center during 1999-2010 with an abnormal G-banding karyotype determined on cells short-term cultured from diagnostic biopsies were included. Detailed information on staging, treatment, and outcome was available for all patients.

RESULTS

Of the 110 patients available for analysis, there were 48 deaths and 27 relapses after a median follow-up of 4.5 yr. Eleven different chromosomal abnormalities were detected in more than ten percent of patients. Of those, only loss of 17p, including the TP53 tumor suppressor gene, was significantly associated with inferior long-term prognosis. Five year overall and progression-free survival frequencies were 32% and 27% for patients with loss of 17p and 67% and 59% in patients without this abnormality.

CONCLUSION

In a relatively large cohort of patients with DLBCL analyzed by chromosome banding, loss of 17p was the only chromosomal abnormality associated with inferior survival in uni- and multivariate analysis.

Mutational profile and prognostic significance of TP53 in diffuse large B-cell lymphoma patients treated with R-CHOP: report from an International DLBCL Rituximab-CHOP Consortium Program Study

TP53 mutation is an independent marker of poor prognosis in patients with diffuse large B-cell lymphoma (DLBCL) treated with cyclophosphamide, hydroxydaunorubicin, vincristine, and prednisone (CHOP) therapy. However, its prognostic value in the rituximab immunochemotherapy era remains undefined. In the present study of a large cohort of DLBCL patients treated with rituximab plus CHOP (R-CHOP), we show that those with TP53 mutations had worse overall and progression-free survival compared with those without. Unlike earlier studies of patients treated with CHOP, TP53 mutation has predictive value for R-CHOP-treated patients with either the germinal center B-cell or activated B-cell DLBCL subtypes. Furthermore, we identified the loop-sheet-helix and L3 motifs in the DNA-binding domain to be the most critical structures for maintaining p53 function. In contrast, TP53 deletion and loss of heterozygosity did not confer worse survival. If gene mutation data are not available, immunohistochemical analysis showing > 50% cells expressing p53 protein is a useful surrogate and was able to stratify patients with significantly different prognoses. We conclude that assessment of TP53 mutation status is important for stratifying R-CHOP-treated patients into distinct prognostic subsets and has significant value in the design of future therapeutic strategies.

p53 N-terminal phosphorylation: a defining layer of complex regulation

The p53 tumor suppressor is a critical component of the cellular response to stress. As it can inhibit cell growth, p53 is mutated or functionally inactivated in most tumors. A multitude of protein-protein interactions with transcriptional cofactors are central to p53-dependent responses. In its activated state, p53 is extensively modified in both the N- and C-terminal regions of the protein. These modifications, especially phosphorylation of serine and threonine residues in the N-terminal transactivation domain, affect p53 stability and activity by modulating the affinity of protein-protein interactions. Here, we review recent findings from in vitro and in vivo studies on the role of p53 N-terminal phosphorylation. These modifications can either positively or negatively affect p53 and add a second layer of complex regulation to the divergent interactions of the p53 transactivation domain.

Dysfunction of the TP53 tumor suppressor gene in lymphoid malignancies

Mutations of the TP53 gene and dysregulation of the TP53 pathway are important in the pathogenesis of many human cancers, including lymphomas. Tumor suppression by p53 occurs via both transcription-dependent activities in the nucleus by which p53 regulates transcription of genes involved in cell cycle, DNA repair, apoptosis, signaling, transcription, and metabolism; and transcription-independent activities that induces apoptosis and autophagy in the cytoplasm. In lymphoid malignancies, the frequency of TP53 deletions and mutations is lower than in other types of cancer. Nonetheless, the status of TP53 is an independent prognostic factor in most lymphoma types. Dysfunction of TP53 with wild-type coding sequence can result from deregulated gene expression, stability, and activity of p53. To overcome TP53 pathway inactivation, therapeutic delivery of wild-type p53, activation of mutant p53, inhibition of MDM2-mediated degradation of p53, and activation of p53-dependent and -independent apoptotic pathways have been explored experimentally and in clinical trials. We review the mechanisms of TP53 dysfunction, recent advances implicated in lymphomagenesis, and therapeutic approaches to overcoming p53 inactivation.

Full p53 transcriptional activation potential is dispensable for tumor suppression in diverse lineages

Over half of all human cancers, of a wide variety of types, sustain mutations in the p53 tumor suppressor gene. Although p53 limits tumorigenesis through the induction of apoptosis or cell cycle arrest, its molecular mechanism of action in tumor suppression has been elusive. The best-characterized p53 activity in vitro is as a transcriptional activator, but the identification of numerous additional p53 biochemical activities in vitro has made it unclear which mechanism accounts for tumor suppression. Here, we assess the importance of transcriptional activation for p53 tumor suppression function in vivo in several tissues, using a knock-in mouse strain expressing a p53 mutant compromised for transcriptional activation, p53(25,26). p53(25,26) is severely impaired for the transactivation of numerous classical p53 target genes, including p21, Noxa, and Puma, but it retains the ability to activate a small subset of p53 target genes, including Bax. Surprisingly, p53(25,26) can nonetheless suppress tumor growth in cancers derived from the epithelial, mesenchymal, central nervous system, and lymphoid lineages. Therefore, full transactivation of most p53 target genes is dispensable for p53 tumor suppressor function in a range of tissue types. In contrast, a transcriptional activation mutant that is completely defective for transactivation, p53(25,26,53,54), fails to suppress tumor development. These findings demonstrate that transcriptional activation is indeed broadly critical for p53 tumor suppressor function, although this requirement reflects the limited transcriptional activity observed with p53(25,26) rather than robust transactivation of a full complement of p53 target genes.

Distinct p53 transcriptional programs dictate acute DNA-damage responses and tumor suppression

The molecular basis for p53-mediated tumor suppression remains unclear. Here, to elucidate mechanisms of p53 tumor suppression, we use knockin mice expressing an allelic series of p53 transcriptional activation mutants. Microarray analysis reveals that one mutant, p53(25,26), is severely compromised for transactivation of most p53 target genes, and, moreover, p53(25,26) cannot induce G(1)-arrest or apoptosis in response to acute DNA damage. Surprisingly, p53(25,26) retains robust activity in senescence and tumor suppression, indicating that efficient transactivation of the majority of known p53 targets is dispensable for these pathways. In contrast, the transactivation-dead p53(25,26,53,54) mutant cannot induce senescence or inhibit tumorigenesis, like p53 nullizygosity. Thus, p53 transactivation is essential for tumor suppression but, intriguingly, in association with a small set of novel p53 target genes. Together, our studies distinguish the p53 transcriptional programs involved in acute DNA-damage responses and tumor suppression-a critical goal for designing therapeutics that block p53-dependent side effects of chemotherapy without compromising p53 tumor suppression.

Characterization of functional domains necessary for mutant p53 gain of function

Tumor cells, including SW480 carcinoma cells that carry a mutant p53, are addicted to the mutant for their survival and resistance to growth suppression by chemotherapeutic agents. Here, we investigated whether various classes of p53 mutants share a common property and functional domains necessary for mutant p53 gain of function. To test this, we generated SW480 cell lines in which endogenous mutant R273H/P309S can be inducibly or stably knocked down, whereas a small interfering RNA-resistant mutant p53 along with a mutated functional domain can be inducibly or stably expressed. We found that both contact-site (R248W and R273H) and conformation (G245S and R249S) mutants are able to maintain the transformed phenotypes of SW480 cells conferred by endogenous mutant p53. We also found that activation domains 1-2 and the proline-rich domain are required for mutant p53 gain of function. Interestingly, we showed that the C-terminal basic domain, which is required for wild-type p53 activity, is an inhibitory domain for mutant p53. Furthermore, we showed that deletion of the basic domain enhances, whereas a mutation in activation domains 1-2 and deletion of the proline-rich domain abolish mutant p53 to regulate Gro1 and Id2, both of which are regulated by and mediate endogenous mutant p53 gain of function. These results indicate that both conformation and contact-site mutants share a property for cell transformation, and the domains critical for wild-type p53 tumor suppression are also required for mutant p53 tumor promotion. Thus, the inhibitory basic domain and the common property for p53 mutants can be explored for targeting tumors with mutant p53.

p53 and stem cells: new developments and new concerns

As the guardian of the genome, the tumor suppressor p53 prevents the accumulation of genetic mutations by inducing cell cycle arrest, apoptosis or senescence of somatic cells after genotoxic and oncogenic stresses. Recent studies have identified the roles of p53 in suppressing pluripotency and cellular dedifferentiation. In this context, p53 suppresses the self-renewal of embryonic stem cells after DNA damage and blocks the reprogramming of somatic cells into induced pluripotent stem cells (iPSCs). If the inactivation of p53 pathway is a prerequisite for successful reprogramming, these findings raise concerns for the genomic stability and tumorigenecity of iPSCs and their derivatives. Elucidation of the roles of p53 as a barrier to pluripotency and cellular dedifferentiation might also reveal the mechanisms by which p53 coordinates tumor suppression and aging.