Phase I/II Study of EBV-LMP1 Targeted DNAzyme in Nasopharyngeal Carcinoma

INTRODUCTION Nasopharyngeal carcinoma (NPC) is a serious health problems worldwide, particularly in the southern Chinese population, with an incidence rate ranging from 15 to 50 per 100 000. NPC is an epithelial malignancy with a striking racial and geographic distribution differences. High incidence rates are observed in the southeast Chinese population, and similar rates have been reported in these people wherever they have migrated, including Singapore, Taiwan, and Hong Kong. This incidence is almost 100 fold higher than in white populations. The most unique feature of NPC is its almost universal association with the infection of Epstein-Barr virus (EBV), which is the first human virus identified to be involved in the pathogenesis of several malignancies and has a particularly close association with NPC, as EBV genome can be detected in virtually all NPC cells. While radiotherapy has been the first-line treatments for NPC, radio-resistance remains a significant clinical issue for the NPC radiotherapy. Thus, there is unmet medical needs to discover and develop novel radiosensitizers for NPC therapy.

EBV infection in NPC is classified as type II latent infection in which only EBV nuclear antigen-1(EBNA-1), latent membrane protein-1(LMP1), LMP2, and EBV early RNA (EBER) expressions can be detected. Among these proteins, LMP1 is thought to play a key role in the pathogenesis of NPC. As a 60kD integral membrane protein, LMP1 functions as a constitutively active tumor necrosis factor receptor (TNFR), and contributes to multiple aspects of NPC through activating a number of signaling pathways including nuclear factor NF-κB, activator protein-1(AP-1), and Janus kinase/signal transducer and activator of transcription(JAK/STAT). Activation of NF-kB or AP-1 by LMP1 has been linked to the upregulation of some cellular proteins and inhibition of apoptosis. Depending on the cell types, expression of LMP1 has been shown to play different roles in response to biological and physiological stimulus. It acts as a primary oncoprotein for human cell immortalization and is also shown as the only EBV-coded product that can transform rodent fibroblast cell line, human epithelial cells and keratinocytes.

Given the critical role of viral oncoproteins in transformation and apoptosis, suppression of some viral oncoproteins would provide a sensible strategy to genetically treat NPC. Indeed, antisense oligonucleotides against LMP1 or EBNA1 have been shown to inhibit viral oncoprotein expression, induce apoptosis, and sensitize the EBV-positive cells to cytotoxic agents. Recently, some experimental studies indicated that the RNA interference against LMP1 exhibited an anti-proliferative and anti-metastasis effect in LMP1 expressing NPCs. These results suggested that EBV-encoded LMP1 may present a potential molecular target for treatment of EBV-associated carcinomas.

DNAzymes are synthetic, single-stranded DNA oligonucleotides that can be engineered to bind to their complementary sequence in a target messenger RNA (mRNA) through Watson-Crick base pairing and cleave the mRNA at predetermined phosphodiester linkages. A general model for the DNAzyme has been proposed, and is known as the ''10-23'' model. A ''10-23'' DNAzyme has a catalytic domain of 15 deoxyribonucleotides, flanked by two substrate-recognition domains of seven to nine deoxyribonucleotides at each arm. In vitro analyses showed that this type of DNAzyme could effectively cleave its substrate RNA at purine: pyrimidine junctions under physiological conditions. These agents have been used in a number of in vitro and in vivo applications to inhibit the expression of their target genes and the dependent genes. Their capacity to block development of a diverse range of pathologies in animal models suggests that DNAzymes can be used as therapeutic agents.

To develop EBV-LMP1 targeted DNAzymes for NPC treatment, we showed that the phosphorothioate-modified ''10-23'' DNAzymes specifically targeted at the LMP1 mRNA could significantly down-regulate the expression of LMP1 in a nasopharyngeal carcinoma cell (NPC) and affected the down-stream pathways activated by LMP1, including the NF-κB pathway. It was also demonstrated that suppression of the LMP1 expression by the LMP1-targeted DNAzyme DZ1 could enhance radiosensitivity both in vivo and vitro. Radio-resistance has been one of the impediments in clinical settings for effective cancer therapy, which is thought to be associated with multiple signaling pathways in different cancer types. ATM (ataxia telangiectasia mutated) is a nuclear 350-kDa protein kinase with a carboxylterminal phosphatidylinositol 3-kinase-like kinase domain[1]. It functions as a member of a coordinated system that detects DNA breaks; arrests the cells temporarily at G1, S, or G2 checkpoints; and activates DNA repair. Cells lacking functional ATM protein show increased sensitivity to ionizing radiation (IR) and other genotoxic events. NF-κB (nuclear factor kappa B) can activate a great number of genes involved in stress responses, inflammation, and programmed cell death (apoptosis). P50 homodimers or p50/p65 or p50/c-Rel heterodimers bind to the NF-κB DNA binding sites in the promoter regions of many stress-responsive genes, suggesting a complex gene and physiological regulation network controlled by NF-κB in stress response. The elevated basal NF-κB activity in certain cancers has been linked to tumor resistance to chemotherapy and radiation. Inhibition of NF-κB blocked the adaptive radioresistance. Our studies for the molecular mechanism of the LMP1-DNAzyme mediated radiosensitization revealed that LMP1 activated the ATM expression through the NF-κB pathway and inhibition of LMP1 expression by the DNAzyme attenuated the binding of NF-κB transcription factor to the ATM promoter. Further evidence showed that the radiosensitivity was recovered when the ATM expression was knocked down by siRNA in NPCs. Together, all our data support our hypothesis and provide solid experimental basis for the use of LMP1-targeted DNAzymes as potential radiosensitizers for treatment of the EBV-associated carcinomas.