The C terminus of p53 regulates gene expression by multiple mechanisms in a target- and tissue-specific manner in vivo.

The p53 tumor suppressor is a transcription factor that mediates varied cellular responses. The C terminus of p53 is subjected to multiple and diverse post-translational modifications. An attractive hypothesis is that differing sets of combinatorial modifications therein determine distinct cellular outcomes. To address this in vivo, a Trp53(ΔCTD/ΔCTD) mouse was generated in which the endogenous p53 is targeted and replaced with a truncated mutant lacking the C-terminal 24 amino acids. These Trp53(ΔCTD/ΔCTD) mice die within 2 wk post-partum with hematopoietic failure and impaired cerebellar development. Intriguingly, the C terminus acts via three distinct mechanisms to control p53-dependent gene expression depending on the tissue. First, in the bone marrow and thymus, the C terminus dampens p53 activity. Increased senescence in the Trp53(ΔCTD/ΔCTD) bone marrow is accompanied by up-regulation of Cdkn1 (p21). In the thymus, the C-terminal domain negatively regulates p53-dependent gene expression by inhibiting promoter occupancy. Here, the hyperactive p53(ΔCTD) induces apoptosis via enhanced expression of the proapoptotic Bbc3 (Puma) and Pmaip1 (Noxa). In the liver, a second mechanism prevails, since p53(ΔCTD) has wild-type DNA binding but impaired gene expression. Thus, the C terminus of p53 is needed in liver cells at a step subsequent to DNA binding. Finally, in the spleen, the C terminus controls p53 protein levels, with the overexpressed p53(ΔCTD) showing hyperactivity for gene expression. Thus, the C terminus of p53 regulates gene expression via multiple mechanisms depending on the tissue and target, and this leads to specific phenotypic effects in vivo.

p53 and cell cycle effects after DNA damage.

Flow cytometry, a valuable technique that employs the principles of light scattering, light excitation, and emission of fluorochrome molecules, can be used to assess the cell cycle position of individual cells based on DNA content. After the permeabilization of cells, the DNA can be stained with a fluorescent dye. Cells which have a 2N amount of DNA can be distinguished from cells with a 4N amount of DNA, making flow cytometry a very useful tool for the analysis of cell cycle checkpoints following DNA damage. A critical feature of the cellular response to DNA damage is the ability to pause and repair the damage so that consequential mutations are not passed along to daughter generations of cells. If cells arrest prior to DNA replication, they will contain a 2N amount of DNA, whereas arrest after replication but before mitosis will result in a 4N amount of DNA. Using this technique, the role that p53 plays in cell cycle checkpoints -following DNA damage can be evaluated based on changes in the profile of the G1, S, and G2/M phases of the cell cycle.

Determine the effect of p53 on chemosensitivity.

The p53 tumor suppressor protein plays a central role in mediating the cellular response to a variety of stresses. Activation of p53 signaling will trigger cell cycle arrest or apoptosis in normal cells, depending on such factors as cell type and genetic context. The ability of a cell to circumvent either of these p53-directed outcomes leads to inappropriate proliferation, thereby contributing to the development of cancer. As such, tumors frequently escape the apoptotic pathway in response to cell stress. DNA-damaging agents, however, achieve significant tumor cytotoxicity in spite of this hallmark characteristic. Tumors treated with DNA-damaging drugs often undergo alternate forms of cell death, such as senescence or mitotic catastrophe, in addition to apoptosis that may ultimately lead to regression. Although not a predictor of chemotherapy response in patients per se, p53 status in tumor-derived cells is frequently a determinant of the death pathway promoted by these agents. The cytotoxic effects of DNA-damaging agents can be readily appreciated using such tools as cell cycle analysis, phopsho-H3(Ser10) immunoblotting, and annexin V detection.

Mdm2 and tumorigenesis: evolving theories and unsolved mysteries.

Since the discovery of Mdm2, the contribution of this RING E3 ubiquitin ligase to the pathobiology of cancer has focused almost exclusively on its role as a negative regulator of the p53 tumor suppressor. Under normal conditions, Mdm2 promotes the ubiquitin- and proteasome-dependent degradation of p53. Levels of p53 are thus kept sufficiently low to allow for cell survival and cell cycle progression. In the context of such insults as DNA damage or ribosomal stress, however, the Mdm2-p53 interaction is disrupted and p53 is stabilized. The myriad intracellular outcomes of p53 activation together comprise a robust program of tumor suppression that is short-circuited in cancer. Over half of all human malignancies are known to have lost p53 expression or sustained p53 mutation, whereas many of the remaining tumors harbor other alterations in key mediators of p53 activity that include overexpression of Mdm2. Therapies targeting the interaction between Mdm2 and p53 represent a possible means of pharmacologically reactivating the p53 pathway in this latter setting. The degree of overlap across the biological consequences of either p53 loss or Mdm2 overexpression, however, has not been thoroughly explored. Indeed, a body of evidence for several p53-independent functions of Mdm2 suggests that disrupting the Mdm2-p53 interaction may fail to address the full spectrum of oncogenic effects specific to tumors that overexpress Mdm2.

IGFBP-4 tumor and serum levels are increased across all stages of epithelial ovarian cancer.

BACKGROUND: We sought to identify candidate serum biomarkers for the detection and surveillance of EOC. Based on RNA-Seq transcriptome analysis of patient-derived tumors, highly expressed secreted proteins were identified using a bioinformatic approach. METHODS: RNA-Seq was used to quantify papillary serous ovarian cancer transcriptomes. Paired end sequencing of 22 flash frozen tumors was performed. Sequence alignments were processed with the program ELAND, expression levels with ERANGE and then bioinformatically screened for secreted protein signatures. Serum samples from women with benign and malignant pelvic masses and serial samples from women during chemotherapy regimens were measured for IGFBP-4 by ELISA. Student's t Test, ANOVA, and ROC curves were used for statistical analysis. RESULTS: Insulin-like growth factor binding protein (IGFBP-4) was consistently present in the top 7.5% of all expressed genes in all tumor samples. We then screened serum samples to determine if increased tumor expression correlated with serum expression. In an initial discovery set of 21 samples, IGFBP-4 levels were found to be elevated in patients, including those with early stage disease and normal CA125 levels. In a larger and independent validation set (82 controls, 78 cases), IGFBP-4 levels were significantly increased (p < 5 × 10-5). IGFBP-4 levels were ~3× greater in women with malignant pelvic masses compared to women with benign masses. ROC sensitivity was 73% at 93% specificity (AUC 0.816). In women receiving chemotherapy, average IGFBP-4 levels were below the ROC-determined threshold and lower in NED patients compared to AWD patients. CONCLUSIONS: This study, the first to our knowledge to use RNA-Seq for biomarker discovery, identified IGFBP-4 as overexpressed in ovarian cancer patients. Beyond this, these studies identified two additional intriguing findings. First, IGFBP-4 can be elevated in early stage disease without elevated CA125. Second, IGFBP-4 levels are significantly elevated with malignant versus benign disease. These findings provide the rationale for future validation studies.

Application of RNA-Seq transcriptome analysis: CD151 is an Invasion/Migration target in all stages of epithelial ovarian cancer.

BACKGROUND: RNA-Seq allows a theoretically unbiased analysis of both genome-wide transcription levels and mutation status of a tumor. Using this technique we sought to identify novel candidate therapeutic targets expressed in epithelial ovarian cancer (EOC). METHODS: Specifically, we sought candidate invasion/migration targets based on expression levels across all tumors, novelty of expression in EOC, and known function. RNA-Seq analysis revealed the high expression of CD151, a transmembrane protein, across all stages of EOC. Expression was confirmed at both the mRNA and protein levels using RT-PCR and immunohistochemical staining, respectively. RESULTS: In both EOC tumors and normal ovarian surface epithelial cells we demonstrated CD151 to be localized to the membrane and cell-cell junctions in patient-derived and established EOC cell lines. We next evaluated its role in EOC dissemination using two ovarian cancer-derived cell lines with differential levels of CD151 expression. Targeted antibody-mediated and siRNA inhibition or loss of CD151 in SKOV3 and OVCAR5 cell lines effectively inhibited their migration and invasion. CONCLUSION: Taken together, these findings provide the first proof-of-principle demonstration for a next generation sequencing approach to identifying candidate therapeutic targets and reveal CD151 to play a role in EOC dissemination.

A critical re-appraisal of BRCA1 methylation studies in ovarian cancer.

A central challenge facing gynecologic oncology is achieving personalized care in ovarian cancer treatment. The current ovarian cancer classification scheme distinguishes tumors based on histopathologic subtype, grade, and surgical stage. Recent molecular investigations have highlighted distinguishing genetic features of certain tumors within a given category, and given the rapid pace of technologic advancement combined with plummeting costs for complete genomic sequencing this classification will markedly improve. Clinical studies have begun to explore the influence of currently known distinctions on the natural history of the disease, most recently with particular attention to the BRCA1 status of tumors. Mutations in the BRCA1 gene have long been known to increase a woman's risk of developing ovarian cancer. As has been shown, BRCA1-associated ovarian cancers may be associated with characteristic differences in therapeutic response and overall survival, and further defining these subsets may become instrumental in clinical decision-making. Therefore, given the eightfold difference (5-40%) in reported frequency of BRCA1 inactivation by methylation in the pioneering studies in the field, a critical re-appraisal of the literature, techniques, samples used, and interpretations of BRCA1 inactivation is warranted along with a review of the more recent and comprehensive molecular studies.

A phase I trial of SJG-136 (NSC#694501) in advanced solid tumors.

PURPOSE: SJG-136 is a pyrrolobenzodiazepine dimer that forms DNA crosslinks and has demonstrated broad antitumor activity. We undertook this trial to determine the maximum-tolerated dose (MTD), toxicities and pharmacokinetic (PK) profile of SJG-136 in patients with an advanced solid tumor. PATIENTS AND METHODS: In this phase I study, patients were treated with SJG-136 on days 1, 8 and 15 of a 28-day cycle. Dose levels studied were 10, 20, 40 and 60 microg/m2. PK parameters of SJG-136 were assessed following the intravenous administration of SJG-136 on days 1 and 15 of cycle 1. RESULTS: Twenty-one patients with advanced solid tumors were treated. Patients had a median of two prior chemotherapy regimens. Fatigue was dose-limiting with SJG-136 60 microg/m2/day administered on days 1, 8 and 15 of a 28-day cycle. Grade 3 thrombocytopenia and delayed onset liver toxicity were seen in one patient each. PK parameters of SJG-136 indicated dose-proportional increases in systemic exposure with increasing doses. No objective responses were seen. CONCLUSION: For patients with advanced solid tumors, the MTD of SJG-136 is 40 microg/m2/day administered on days 1, 8 and 15 of a 28-day cycle. The major dose limiting toxicity was fatigue. Alternative dosing strategies are now being evaluated.

Randomized phase II study of pulse erlotinib before or after carboplatin and paclitaxel in current or former smokers with advanced non-small-cell lung cancer.

PURPOSE: A prior study demonstrated that addition of continuous daily erlotinib fails to improve response rate or survival in non-small-cell lung cancer (NSCLC) patients treated with carboplatin and paclitaxel. However, preclinical data support the hypothesis that intermittent administration of erlotinib before or after chemotherapy may improve efficacy. We tested this hypothesis in patients with advanced NSCLC. PATIENTS AND METHODS: Eligible patients were former or current smokers with chemotherapy-naive stage IIIB or IV NSCLC. All patients received up to six cycles of carboplatin (area under the curve = 6) and paclitaxel (200 mg/m(2)), with random assignment to one of the following three erlotinib treatments: erlotinib 150 mg on days 1 and 2 with chemotherapy on day 3 (150 PRE); erlotinib 1,500 mg on days 1 and 2 with chemotherapy on day 3 (1,500 PRE); or chemotherapy on day 1 with erlotinib 1,500 mg on days 2 and 3 (1,500 POST). The primary end point was response rate. RESULTS: Eighty-six patients received treatment. The response rates for the 150 PRE, 1,500 PRE, and 1,500 POST arms were 18% (five of 28 patients), 34% (10 of 29 patients), and 28% (eight of 29 patients), respectively. The median overall survival times were 10, 15, and 10 months for the 150 PRE, 1,500 PRE, and 1,500 POST arms, respectively. The most common grade 3 and 4 toxicities were neutropenia (39%), fatigue (15%), and anemia (12%). Grade 3 and 4 rash and diarrhea were uncommon. CONCLUSION: Patients treated on the 1,500 PRE arm had the highest response rate and longest survival, with ranges similar to those reported for carboplatin, paclitaxel, and bevacizumab in a more restricted population. Further evaluation of this strategy in a phase III trial is proposed.

Human midsized neurofilament subunit induces motor neuron disease in transgenic mice.

Aberrant accumulation of neurofilaments is a feature of human motor neuron diseases. Experimentally motor neuron disease can be induced in transgenic mice by overexpressing the mouse neurofilament light subunit (NF-L), the human heavy subunit (NF-H), or mouse peripherin. Here we describe that mice harboring a bacterial artificial chromosome (BAC) transgene containing the human midsized neurofilament subunit (NF-M) gene develop a progressive hind limb paralysis associated with neurofilamentous accumulations in ventral horn motor neurons and axonal loss in ventral motor roots. Biochemical studies revealed that all three mouse neurofilament subunits along with the human NF-M contributed to filament formation, although filaments contained less peripherin. In addition the endogenous mouse NF-M became less phosphorylated in the presence of the human protein and accumulated in the cell bodies of affected neurons even though phosphorylated human NF-M did not. Remaining motor axons contained an increased density of neurofilaments and morphometric studies showed that principally small myelinated axons were lost in the transgenic animals. Removing half of the mouse NF-M by breeding the transgene onto the mouse NF-M heterozygous null background offered no protection against the development of disease, arguing that the effect is not simply due to elevation of total NF-M. Collectively these studies argue that the human and mouse NF-M proteins exhibit distinct biochemical properties and within mouse neurons are not interchangeable and that indeed the human protein may be toxic to some mouse neurons. These studies have implications for the use of human neurofilament transgenic mice as models of amyotrophic lateral sclerosis.