Alexander M Ishov, PhD

RESEARCH INTEREST

Functions of histone variants in castration-resistant prostate cancer

Chemoresistance in breast cancer

Nuclear Structure and Function

Epigenetic Regulation of Gene Expression

Tumor Suppression

RESEARCH PROJECTS

Functions of histone variants in castration-resistant prostate cancer – Prostate cancer (PCa) is the second leading cause of cancer-related mortality in American men. Androgen ablation therapies are initially effective in ~90% of PCa patients. Unfortunately, these therapies offer only a temporary relief and the disease eventually recurs with a lethal outcome. PCa that relapsed after hormonal therapies is the major cause of disease lethality and is referred to as castration-resistant PC (CRPC). To make progress in CRPC treatment, we must develop evidence-based strategies for choosing therapy interventions. A better understanding of mechanisms that control androgen receptor (AR) activation and allow CRPC to circumvent hormonal ablation therapies is critically important for improving disease outcome. Advantages in the PCa field have clearly demonstrated that transcriptional control of AR-induced genes is regulated by epigenetic modifications of chromatin. The main chromatin unit, nucleosome, can be modulated by post-translational histones modifications, and also by the incorporation of histone variants, that, in combination, determines epigenetics properties and transcription activity of chromatin. The “histone variants barcode” hypothesis is based on differences between histone variants, and postulates that incorporation of transcription-associated H3.3 and H2A.Z variants creates active territories of chromatin. Specifically, we aim to answer two questions: First, do histone variants contribute to CRPC etiology, and second, is interaction between histone chaperones and AR play a role in this process. Altogether, this work supposes to rationally identify novel targeted and combinatorial therapeutic regimens for treatment of CRPC.

Chemoresistance in breast cancer – Taxanes are among the most powerful anticancer agents in breast cancer chemotherapy. A large number of patients are resistant to taxanes. Therefore it is essential to develop prognostic tools and predictive markers for appropriate chemotherapy selection. Deficiency of protein Daxx (see review Lindsay et al., 2008, Frontiers in Bioscience) can determine resistance to taxane-induced mitotic catastrophe by reversibly blocking cells in mitosis (Lindsay et al., Cell Cycle, 2007). Current project is to examine Daxx as a paclitaxel sensitivity factor that can be used in selection of breast cancer patients to receive taxane therapy. Specifically, we will: 1. examine the role of Daxx in paclitaxel induced cell death, and 2. elucidate the function of Daxx in mitotic progression as a mechanism of Daxx-dependent resistance to paclitaxel treatment.

Tumor supression function of Daxx – A predominant reason for high mortality rates in breast malignancies is that most patients cannot be effectively diagnosed (and properly treated) for metastasis probabilities. Thus, it is important to develop novel and early markers for metastasis potential of tumors. Over-expression of proto-oncogene c-met results in increased mobility/invasion and correlates with elevated metastatic potential. We described Daxx as a repressor of c-met transcription and cell mobility/invasion (Morozov et al., Oncogene 2008). Daxx is down regulated upon hypoxia, which is accompanied by increased accumulation of c-Met. Thus, Daxx is a potential metastasis gatekeeper. The main goal of this project is to evaluate protein Daxx as an early predictive marker for increased probability of metastasis development. We will determine: 1. the mechanism of Daxx mediated repression of human c-met promoter upon hypoxic conditions; 2. the physiological consequences of Daxx reduction upon hypoxic condition; 3. validate Daxx as an early predictive marker for metastasis potential in breast cancer.

ND10: a nuclear sensor – Another scientific interest of our lab is the spatial and temporal regulation of nuclear functions in the context of dynamic changes in protein distribution. The intra-nuclear target of this research, dynamic structure called ND10 or PML body, accumulates several proteins (including Daxx, PML, SP100, SUMO, p53, ATRX) that are involved in a wide range of cellular activities, including regulation of transcription, cell cycle progression, senescence, and apoptosis (Ishov et al., J Cell Biol 1999). ND10 function is a temporary storage site of proteins that can be rapidly released from this domain to perform their activity at alternative locations upon changes in the cell cycle or application of stress conditions, including viral invection (Ishov et al., J Cell Biol 1996, 1997). Using a combination of cell biological, genetic, molecular and biochemical strategies, we intend to identify the ND10-associated mechanisms that control nuclear homeostatic balance of specific limiting factors during normal and pathological cellular activities, including carcinogenesis and viral infection.

Function of Daxx as an epigenetic marker at heterochromatin – Heterochromatin generally represents transcriptionaly inactive part of genome that is replicated during the end of S-phase and requires the re-establishment of silent epigenetic markers such as deacetylated histones and methylated DNA to remain repressed after successive cell divisions. Proteins that accumulate at these sites during and after replication are therefore potentially involved in transcription repression. At the end of S-phase Daxx is released from ND10, and is transiently accumulated at heterochromatin where it forms a complex with ATP-dependent chromatin remodeling SWI/SNF protein ATRX (Ishov et al., J Cell Sci 2004). To seek for Daxx function at heterochromatin, we performed a search for heterochromatin-associated partners of Daxx using biochemichal purification and genetic screens. We intend to identify specific targets of Daxx mediated repression using the microarray approach, study the mechanisms and investigate the physiological effect of this repression on cellular and organism level.