Cell Death and Cancer Genetics
Yibin Deng, M.D., Ph.D.
The tumor-suppressor TP53 gene encodes the p53 protein that maintains genomic integrity and prevents tumorigenesis in response to a variety of genotoxic stresses. The importance of p53 in tumor suppression is highlighted by mutations that lead to the loss of wild-type p53 function and/or oncogenic gain of function (GOF) identified in more than half of human cancers. The comprehensive genomic/whole exons sequencing analyses sponsored by The Cancer Genome Atlas (TCGA) consortium confirmed the high frequency of TP53 mutations in all of the sequenced human cancers. TCGA studies, for example, revealed 96 percent of ovarian cancers; 37 percent of breast cancers; 54 percent of colorectal cancers; and 81 percent of lung squamous cell carcinomas display TP53 mutations. Mouse genetic studies provide compelling evidence that TP53 mutations play a causal role in tumorigenesis. The mechanisms that underlie wild-type, p53-mediated tumor suppression and mutant p53- driven tumor development, however, remain incompletely understood. Our laboratory, therefore, focuses on understanding how the wild-type p53 suppresses tumorigenesis and why the oncogenic GOF of mutant p53 found in cancer patients promotes tumor development. To translate our bench work to bedside, we have been utilizing genomic and proteomic approaches, bioinformatics, computational modeling, RNAi-based screening, and genetically engineered mouse models (GEMMs) that recapitulate the salient characteristics of human cancers to discover the crucial, .druggable. targets for cancer cells. Our ultimate goal is to find the Achilles. heel of cancer cells to selectively and efficiently kill them while leaving the normal cells unharmed. In the past year, our laboratory has made progress in the following three major areas:
1. Understanding wild-type, p53-mediated signaling pathways in tumor suppression in vivo
While many studies have focused on the role of apoptosis and/or senescence in p53-mediated tumor suppression, recent findings suggest that p53 induces DRAM (Damage-Regulated Autophagy Modulator)-dependent autophagy. To study the role of DRMA-dependent autophagy in tumorigenesis, we generated conditional Dram knockout mice. Our findings suggest that Dram potentially functions as a tumor suppressor because deletion of Dram promotes spontaneous tumor development in mouse models. We currently are trying to dissect the molecular basis underlying Dram-deficiencydriven tumorigenesis in vivo. We also are exploring whether and how the crosstalk between p53-initiated autophagy and p53-mediated cell metabolism leads to tumor initiation, progression, and metastasis.
To answer the critical function of p53-mediated autophagy, apoptosis and senescence in suppressing tumor development in vivo, we have generated .triple. mutant mice utilizing the conditional Dram knockout mice to breed with mice deficient in p53-mediated apoptosis (p53R172P knockin or Puma knockout) and senescence-deficient mice (p21 knockout). We expect that by utilizing these complex, genetically engineered mouse models, we will be able to address the critical question about how the p53- regulated signaling axis contributes to its tumor suppressive function in vivo.
2. Gain-of-function of mutant p53 in telomere uncapping-driven breast tumorigenesis
Human sporadic breast carcinomas are characterized by the presence of complex cytogenetic aberrations. One of the foremost challenges for breast cancer researchers is to develop experimental model systems that identify pathogenetic events driving breast tumor development. Our long-term goal in this project is to establish .chromosomal instability. mouse breast cancer models and discover the .causal. genomic events driving breast tumorigenesis in vivo. One important mechanism that can give rise to the unstable breast cancer genome is the dysfunction of telomeres. Telomeres are nucleoprotein caps that protect chromosomal ends from being recognized as damaged DNA and prevent chromosome end-to-end fusions.
“Finding effective and selective means of killing prostate
cancer cells carrying Pten/p53 deficiency is critical to
successfully treat currently incurable CRPC.”
Dr. Yibin Deng
Telomeres that no longer can exert end-protective functions are said to be dysfunctional, and these telomeres could arise either from progressive telomere attrition (telomere shortening) or when components of the telomeric DNA-binding proteins . termed .shelterin complex. . are perturbed (telomere uncapping). In human breast carcinomas, chromosomal instability fueled by dysfunctional telomeres is associated with the transition from benign ductal hyperplasia to malignant ductal carcinoma in situ. This strongly supports the notion that telomere dysfunction-induced chromosome instability initiates the development of breast cancers. Our laboratory has been engineering a novel mouse breast cancer model harboring telomere uncapping-induced chromosomal instability without affecting the activity of telomerase. Importantly, the mouse model also expresses .hot spot. mutant p53 protein found in cancer patients in breast epithelium. We believe that this mouse model will faithfully recapitulate the genetic abnormality commonly observed in human sporadic breast carcinomas. We are establishing and utilizing this novel mouse breast cancer model to identify the key genetic pathways perturbed in chromosomal instability driven mammary tumorigenesis and target these pathways with novel therapeutics that potentially could suppress human breast cancer.
Our studies currently suggest that endogenous expression of .hot spot. mutant p53 promotes breast tumor development in comparison with the loss of p53 in breast epithelial cells. To understand the molecular mechanism underlying mutant p53-mediated GOF, we found that .hot spot. mutant p53 protein cooperates with CCCTC-binding factor (CTCF) to transcriptionally downregulate PHLPP2 resulting in activation of AKT1- mTORC1 signaling to exert its oncogenic GOF in multiple independent, yet complementary, mouse models. Surprisingly, the activated AKT1-mTORC1 signaling generates a positive feedback loop to sustain mutant p53 protein expression through 4EBP1-eIF4E translation axis. Accordingly, genetic and pharmacologic targeting of the AKT1-mTORC1 signaling axis not only strikingly blocks mutant p53-driven GOF properties but also dramatically diminishes mutant p53 protein. Given that mutant p53 is essential for its GOF but targeting the mutant p53 protein for cancer therapy has proved challenging, our novel findings provide a potential and effective therapeutic strategy for human cancers carrying mutant p53.
3. Exploring the molecular targets involved in selective killing of cancer cells
Our laboratory has a long-standing interest in understanding genetic pathways that allow for selective targeting of cancer cells while leaving normal cells untouched.
We recently made progress in our study on prostate cancers. Prostate cancer strikes one in six men and is the second-leading cause of cancer-related deaths in men after lung cancer in the United States. Prostate cancer arises mainly from prostatic intraepithelial neoplasia (PIN), a precursor lesion that ultimately progresses to adenocarcinoma and systemic metastasis. Conventional androgen deprivation therapy (ADT) by surgical and/ or chemical castration remains the gold standard-of-care therapy for metastatic prostate cancer. Unfortunately, these prostate cancers invariably develop resistance to conventional ADT and progress to a more aggressive, castration-resistant prostate cancer (CRPC) within 18 to 24 months.
The discovery that persistent androgen receptor (AR) signaling plays a crucial role in the progression of CRPC leads to .second generation. ADT treatments, such as androgen synthesis blocker abiraterone recently approved by the Food and Drug Administration (2011, FDA); and the second generation of AR signaling inhibitor enzalutamide (formerly MDV3100) (2012, FDA), which has demonstrated efficacy against chemotherapy resistant CRPC with median increase in survival of four to five months. Nearly all CRPC patients, however, inevitably develop acquired resistance to the .second generation,. anti-AR signaling axis treatments within about six to 12 months. No therapeutic options currently exist for CRPC patients who have developed resistance to the second generation of anti-androgen receptor (AR) signaling axis therapy. We found that co-deletion of Pten and p53 in prostate epithelium . often observed in human lethal CRPC . leads to AR-independent CRPC and, thus, confers de novo resistance to .second generation. androgen deprivation therapy (ADT) in multiple independent, yet complementary, preclinical mouse models. In striking contrast, mechanism-driven, co-targeting hexokinase 2 (HK2)-mediated Warburg effect with 2-deoxyglucose (2-DG) and ULK1- dependent autophagy with chloroquine (CQ) selectively kill cancer cells through intrinsic apoptosis to cause tumor regression in xenograft and lead to near-complete tumor suppression in Pten-/p53-deficiency-driven CRPC mouse model. Mechanistically, 2-DG causes AMPK phosphorylation, which, in turn, inhibits mTORC1-S6K1 translation signaling to preferentially block anti-apoptotic protein MCL-l synthesis to prime mitochondriadependent apoptosis while simultaneously ULK1-driven autophagy for cell survival to counteract the apoptotic action of anti-Warburg effect. Inhibition of autophagy with CQ, accordingly, sensitizes cancer cells to apoptosis upon 2-DG challenge. Given that 2-DG is recommended for phase II clinical trials for prostate cancer and that CQ has been used clinically as an anti-malaria drug for many decades, the preclinical results from our .proof-of principle. studies in vivo are imminently translatable to clinical trials to evaluate the therapeutic efficacy by the combination modality for a subset of currently incurable CRPC patients.
Our laboratory also is utilizing multiple genetic and pharmacological approaches to identify targets that can be targeted selectively in human lung and colon cancers. Our ongoing projects involve collaborations with researchers from Texas Tech University Health Sciences Center School of Pharmacy in Amarillo, Texas; The University of Texas M.D. Anderson Cancer Center in Houston, Texas; Roswell Park Cancer Institute in Buffalo, N.Y.; and Mayo Clinic College of Medicine in Rochester, Minn. Our research projects are supported by grants from the National Cancer Institute of the National Institutes of Health and The Hormel Foundation.
Other Professional Activities:
Grant Reviewer, National Cancer Institute