ONYX-015, an E1B gene-attenuated adenovirus, causes tumor-specific cytolysis and antitumoral efficacy that can be augmented by standard chemotherapeutic agents

Recurrent squamous-cell carcinoma of the head and neck: overview of current therapy and future prospects

Locoregional recurrence is the most common cause of failure after head and neck cancer surgery. It is a disease which causes significant morbidity especially on speech and swallowing. There are many different treatments available including surgery, reirradiation and chemotherapy. However, none of these have produced any significant survival benefit. Because of this, there has been considerable interest in the development of new biological therapies such as gene therapy and immunotherapy for this disease. The objectives of this article are to provide an overview of the currently available therapies for recurrent head and neck cancer including gene therapy and immunotherapy. Prevention of recurrent disease by the detection and treatment of minimal residual disease is also discussed.

p53 alterations in recurrent squamous cell cancer of the head and neck refractory to radiotherapy

The aim of the study was to determine the incidence of p53 alterations by mutation, deletion or inactivation by mdm2 or human papillomavirus (HPV) infection in recurrent squamous cell cancer of the head and neck (SCCHN) refractory to radiotherapy. Twenty-two tumours were studied. The p53 status of each tumour was analysed by sequencing of exons 4-9 and by immunohistochemistry. Mdm2 expression was assessed by immunohistochemistry and HPV infection was assessed by polymerase chain reaction of tumour DNA for HPV 16, 18 and 33. Fifteen (68%) of the 22 tumours studied had p53 mutations, while seven had wild-type p53 sequence. p53 immunohistochemistry correlated with the type of mutation. HPV DNA was detected in 8 (36%) tumours and all were of serotype HPV 16. Of these, five were in tumours with mutant p53 and three were in tumours with wild-type p53. Mdm2 overexpression was detected in 11 (50%) tumours. Of these, seven were in tumours with mutant p53 and four were in tumours with wild-type p53. Overall, 21 of the 22 tumours had p53 alterations either by mutation, deletion or inactivation by mdm2 or HPV. In this study, the overall incidence of p53 inactivation in recurrent head and neck cancer was very high at 95%. The main mechanism of inactivation was gene mutation or deletion which occurred in 15 of the 22 tumours studied. In addition, six of the seven tumours with wild-type p53 sequence had either HPV 16 DNA, overexpression of mdm2 or both which suggested that these tumours had p53 inactivation by these mechanisms. This high incidence of p53 dysfunction is one factor which could account for the poor response of these tumours to radiotherapy and chemotherapy. Therefore, new therapies for recurrent SCCHN which either act in a p53 independent pathway, or which restore p53 function may be beneficial in this disease.

Gene therapy for carcinoma of the breast: Pro-apoptotic gene therapy

The dysregulation of apoptosis contributes in a variety of ways to the malignant phenotype. It is increasingly recognized that the alteration of pro-apoptotic and anti-apoptotic molecules determines not only escape from mechanisms that control cell cycle and DNA damage, but also endows the cancer cells with the capacity to survive in the presence of a metabolically adverse milieu, to resist the attack of the immune system, to locally invade and survive despite a lack of tissue anchorage, and to evade the otherwise lethal insults induced by drugs and radiotherapy. A multitude of apoptosis mediators has been identified in the past decade, and the roles of several of them in breast cancer have been delineated by studying the clinical correlates of pathologically documented abnormalities. Using this information, attempts are being made to correct the fundamental anomalies at the genetic level. Fundamental to this end are the design of more efficient and selective gene transfer systems, and the employment of complex interventions that are tailored to breast cancer and that are aimed concomitantly towards different components of the redundant regulatory pathways. The combination of such genetic modifications is most likely to be effective when combined with conventional treatments, thus robustly activating several pro-apoptotic pathways.

The nuclear export signal within the E4orf6 protein of adenovirus type 5 supports virus replication and cytoplasmic accumulation of viral mRNA

During the late phase of adenovirus infection, viral mRNA is efficiently transported from the nucleus to the cytoplasm while most cellular mRNA species are retained in the nucleus. Two viral proteins, E1B-55 kDa and E4orf6, are both necessary for these effects. The E4orf6 protein of adenovirus type 5 binds and relocalizes E1B-55 kDa, and the complex of the two proteins was previously shown to shuttle continuously between the nucleus and cytoplasm. Nucleocytoplasmic transport of the complex is achieved by a nuclear export signal (NES) within E4orf6. Mutation of this signal sequence severely reduces the ability of the E1B-55 kDa-E4orf6 complex to leave the nucleus. Here, we examined the role of functional domains within E4orf6 during virus infection. E4orf6 or mutants derived from it were transiently expressed, followed by infection with recombinant adenovirus lacking the E4 region and determination of virus yield. An arginine-rich putative alpha helix near the carboxy terminus of E4orf6 contributes to E1B-55 kDa binding and relocalization as well as to the synthesis of viral DNA, mRNA, and proteins. Further mutational analysis revealed that mutation of the NES within E4orf6 considerably reduces its ability to support virus production. The same effect was observed when nuclear export was blocked with a competitor. Further, a functional NES within E4orf6 contributed to the efficiency of late virus protein synthesis and viral DNA replication, as well as total and cytoplasmic accumulation of viral late mRNA. Our data support the view that NES-mediated nucleocytoplasmic shuttling strongly enhances most, if not all, intracellular activities of E4orf6 during the late phase of adenovirus infection.

Adenovirus in cancer therapy: a gastrointestinal case study

Approved clinical trials involving gene therapy have been taking place for less than 10 years. As more knowledge is gained about genetics and disease, nurses must have a fundamental understanding of what genes do, how gene therapy is administered, and the risks and potential benefits of treatment. New methods for the administration of adenoviral vectors for cancer treatment in the endoscopy lab are currently being developed. By staying abreast of new treatment methods using gene or viral therapy, the gastrointestinal nurse can actively participate in clinical trials. A basic background of genetics and gene therapy in cancer is discussed. Safety issues and nursing implications regarding the administration and recovery of patients following gene therapy in the GI endoscopy lab are also addressed. An actual case study of a patient who participated in a study utilizing an attenuated adenovirus as investigational therapy for pancreatic adenocarcinoma is presented.

New cytotoxics and non-cytotoxics in epithelial ovarian cancer

Several new cytotoxic agents with activity in relapsed ovarian cancer are being combined with paclitaxel plus platinum as the first step to assess their impact in randomized trials against the standard treatment. These include topotecan, gemcitabine, epirubicin, and liposomal doxorubicin. Because of overlapping toxicities, there have been challenges in combining some of these agents in full dose with combination paclitaxel plus platinum. These have been overcome by use of sequenced administration. In addition to these new agents, novel non-cytotoxic drugs targeting specific signaling molecules or the tumor microenvironment provide additional avenues for clinical investigation. Many of these agents are rational to assess in ovarian cancer where laboratory research has pinpointed a number of alterations in molecules involved in cell signaling and cell cycle control. Examples include the antibody to HER 2/neu, agents targeting protein kinase C alpha, the p53 gene, and matrix metalloproteinase inhibitors. The challenges facing their assessment include how to determine adequate dosing when toxic effects are minimal and how to assess evidence of antitumor activity, short of conducting randomized studies. Finally, how best to use such agents together with conventional chemotherapy, in combination or in sequence, is unknown. Large clinical studies with some of these agents will provide some answers to their impact and how best to use them in the first-line management of advanced epithelial ovarian cancer.

Combined therapy with chemotherapeutic agents and herpes simplex virus type 1 ICP34.5 mutant (HSV-1716) in human non-small cell lung cancer

A replication-selective herpes simplex virus type 1 ICP34.5 mutant (HSV-1716) has shown efficacy both in vitro and in vivo against human non-small cell lung cancer (NSCLC) cell lines but complete eradication of tumor has not been accomplished with a single viral treatment in our murine xenograft models. Therefore, strategies to enhance the efficacy of this treatment were investigated. We determined the oncolytic activity of HSV-1716 in NCI-H460 cells in combination with each of four chemotherapeutic agents: mitomycin C (MMC), cis-platinum II (cis-DDP), methotrexate (MTX), or doxorubicin (ADR). Isobologram analysis was performed to evaluate the interaction between the viral and chemotherapeutic agents. The oncolytic effect of HSV-1716 in combination with MMC was synergistic in two of five NSCLC cell lines. In the other three cell lines, the combined effect appeared additive. No antagonism was observed. The in vivo effect of this combination was then examined in a murine xenograft model. NCI-H460 flank tumors were directly injected with HSV-1716 (4 x 106 PFU) followed by intravenous MMC administration (0.17 mg/kg) 24 hr later. After 3 weeks, the mean tumor weight in the combined treatment group was significantly less than either individual treatment in an additive manner. The synergistic dose of MMC neither augmented nor inhibited viral replication in vitro and HSV-1716 infection did not upregulate DT-diaphorase, which is the primary enzyme responsible for MMC activation. In summary, the combination of HSV-1716 with common chemotherapeutic agents may augment the effect of HSV-based therapy in the treatment of NSCLC.

Strategies for enhancing viral-based gene therapy using ionizing radiation

Many gene-therapy strategies under investigation aim to increase the efficacy of current cancer-treatment regimens. Promising results have been obtained in the laboratory and early clinical trials using viral-based motifs specifically designed to enhance the efficacy of ionizing radiation or chemotherapy. These strategies fall into two general categories: replication-incompetent viral shuttle vectors for the delivery of specific genes encoding a chemo/radiation modulator and attenuated replication-competent viruses with proposed replicative advantages in tumor cells. In this review, we discuss the rational, molecular mechanisms, and clinical application of these strategies with particular focus on recent research applying these viral-based strategies to improve the therapeutic index of ionizing radiation.

Viruses and apoptosis

Successful viral replication requires not only the efficient production and spread of progeny, but also evasion of host defense mechanisms that limit replication by killing infected cells. In addition to inducing immune and inflammatory responses, infection by most viruses triggers apoptosis or programmed cell death of the infected cell. This cell response often results as a compulsory or unavoidable by-product of the action of critical viral replicative functions. In addition, some viruses seem to use apoptosis as a mechanism of cell killing and virus spread. In both cases, successful replication relies on the ability of certain viral products to block or delay apoptosis until sufficient progeny have been produced. Such proteins target a variety of strategic points in the apoptotic pathway. In this review we summarize the great amount of recent information on viruses and apoptosis and offer insights into how this knowledge may be used for future research and novel therapies.

Retrovirus-mediated IL-4 gene therapy in spontaneous adenocarcinomas from MMTV-neu transgenic mice

Gene therapy approaches to the treatment of experimental cancer are usually based on established neoplastic cell lines which are manipulated in vitro and subsequently transplanted in host animals. However, the relevance of these artificial models to the biology and therapy of human tumors is uncertain. We have previously validated an experimental model based on MMTV-neu transgenic mice in which breast tumors arise spontaneously in 100% of animals and have many features in common with their human counterpart, including the involvement of the neu oncogene and the ability to metastatize. In this article we report the effect of intratumoral, retrovirus-mediated, IL-4 expression on the growth of breast tumors arising in these mice. The size of IL-4 inoculated tumors on the right side was significantly smaller than that of controlateral untreated tumors, suggesting a local effect of IL-4. In addition, the non-injected tumors on the left side of treated animals were significantly smaller than those arising in control transgenic mice, suggesting that IL-4 can also inhibit tumor growth systemically. These findings suggest that IL-4 gene transfer can significantly reduce the growth rate of spontaneously arising breast tumors and that immune-based gene therapy could efficiently complement other approaches based on different mechanisms, such as suicide gene transfer or antisense technology.

Gene therapy for gliomas: molecular targets, adenoviral vectors, and oncolytic adenoviruses

Currently, most of the approved clinical gene therapy protocols involve cancer patients and several of the therapies are designed to treat brain tumors. Two factors promoting the use of gene therapy for gliomas are the failure and toxicity of conventional therapies and the identification of the genetic abnormalities that contribute to the malignancy of gliomas. During the malignant progression of astrocitic tumors several tumor suppressor genes are inactivated, and numerous growth factors and oncogenes are overexpressed progressively. Thus, theoretically, brain tumors could be treated by targeting their fundamental molecular defects, provided the gene-drug can be delivered to a sufficient number of malignant cells. However, gene therapy strategies have not been abundantly successful clinically, in part because the delivery systems are still imperfect. In the first part of this brief review we will discuss the most common targets for gene therapy in brain tumors. In the second part, we will review the evolution of adenoviruses as gene vehicles. In addition, we will examine the role of recombinant mutant oncolytic adenoviruses as anticancer tools. From the results to date it is clear that gene therapy strategies for brain tumors are quite promising but more critical research is required, mainly in the vector field, if the strategies are to achieve their true potential in ameliorating patients with gliomas.

The cell cycle and drug discovery: the promise and the hope

In recent years, there have been major developments in the understanding of the cell cycle. It is now known that normal cellular proliferation is tightly regulated by the activation and deactivation of a series of proteins that constitute the cell cycle machinery. The expression and activity of components of the cell cycle can be altered during the development of a variety of diseases where aberrant proliferation contributes to the pathology of the illness. Apart from yielding a new source of untapped therapeutic targets, it is likely that manipulating the activity of such proteins in diseased states will provide an important route for treating proliferative disorders, and the opportunity to develop a novel class of future medicines.

Experimental drugs and drug combinations in pancreatic cancer

The current role of chemotherapy in pancreatic cancer is limited. Chemotherapy usually consists of 5-fluorouracil (5FU) and gemcitabine either as a single agent or in combinations. However, response rates are below 15% with minor effects on overall survival. Due to the aggressive behavior of the disease, current emphasis of new experimental chemotherapy is also focusing on clinical benefit: improvement of pain, performance status or weight. The results with gemcitabine indicated that evaluation of new chemotherapeutic agents in pancreatic cancer should not be limited to the evaluation of response rates; single agent gemcitabine not only showed higher response rates than 5FU, but also resulted in clinical benefit for the patients. Several new agents have been introduced into the clinic for treatment of various gastro-intestinal malignancies, whereas novel agents with different types of targets, such as marimastat deserve further attention. Several oral formulations of 5FU, such as capecitabine, UFT, and eniluracil with 5FU, aim to simulate long-term continuous infusion. Response rates of these formulations are comparable to those of 5FU continuous infusion and 5FU bolus injections. However, the convenience of oral administration with reliable plasma drug concentrations makes these agents very attractive as a replacement of traditional 5FU administration. Since 5FU acts by inhibition of thymidylate synthase (TS), resulting in inhibition of DNA synthesis, several new antifolates, directed towards TS, have been developed. However, these agents, such as ZD1694 (Tomudex, Raltitrexed) and LY231514 (MTA, multitargetted antifolate) showed only limited efficacy. Other new agents active in colorectal cancer, e.g. the topoisomerase I inhibitors topotecan and CPT-11, showed only minor activity. The same was observed for the taxanes. Combinations of gemcitabine (cisplatin, 5FU, epirubicin, marimastat) show promising activities, not only regarding response but also with respect to clinical benefit. The effects were better than that for each agent separately. Thus, despite limited activity of single agents, novel combinations especially with gemcitabine are promising, with emphasis on improvement of the clinical benefit of patients.

Infectivity and expression of the early adenovirus proteins are important regulators of wild-type and DeltaE1B adenovirus replication in human cells

An adenovirus mutant lacking the expression of the large E1B protein (DeltaE1B) has been reported to replicate selectively in cells lacking the expression of functionally wild-type (wt) p53. Based on these results the DeltaE1B or ONYX-015 virus has been proposed to be an oncolytic virus which might be useful to treat p53-deficient tumors. Recently however, contradictory results have been published indicating that p53-dependent cell death is required for productive adenovirus infection. Since there is an urgent need for new methods to treat aggressive, mutant p53-expressing primary tumors and their metastases we carefully examined adenovirus replication in human cells to determine whether or not the DeltaE1B virus can be used for tumor therapy. The results we present here show that not all human tumor cell lines take up adenovirus efficiently. In addition, we observed inhibition of the expression of adenovirus early proteins in tumor cells. We present evidence that these two factors rather than the p53 status of the cell determine whether adenovirus infection results in lytic cell death. Furthermore, the results we obtained by infecting a panel of different tumor cell lines show that viral spread of the DeltaE1B is strongly inhibited in almost all p53-proficient and -deficient cell lines compared to the wt virus. We conclude that the efficiency of the DeltaE1B virus to replicate efficiently in tumor cells is determined by the ability to infect cells and to express the early adenovirus proteins rather than the status of p53.

Roles for the E4 orf6, orf3, and E1B 55-kilodalton proteins in cell cycle-independent adenovirus replication

Adenoviruses bearing lesions in the E1B 55-kDa protein (E1B 55-kDa) gene are restricted by the cell cycle such that mutant virus growth is most impaired in cells infected during G(1) and least restricted in cells infected during S phase (F. D. Goodrum and D. A. Ornelles, J. Virol. 71:548-561, 1997). A similar defect is reported here for E4 orf6-mutant viruses. An E4 orf3-mutant virus was not restricted for growth by the cell cycle. However, orf3 was required for enhanced growth of an E4 orf6-mutant virus in cells infected during S phase. The cell cycle restriction may be linked to virus-mediated mRNA transport because both E1B 55-kDa- and E4 orf6-mutant viruses are defective at regulating mRNA transport at late times of infection. Accordingly, the cytoplasmic-to-nuclear ratio of late viral mRNA was reduced in G(1) cells infected with the mutant viruses compared to that in G(1) cells infected with the wild-type virus. By contrast, this ratio was equivalent among cells infected during S phase with the wild-type or mutant viruses. Furthermore, cells infected during S phase with the E1B 55-kDa- or E4 orf6-mutant viruses synthesized more late viral protein than did cells infected during G(1). However, the total amount of cytoplasmic late viral mRNA was greater in cells infected during G(1) than in cells infected during S phase with either the wild-type or mutant viruses, indicating that enhanced transport of viral mRNA in cells infected during S phase cannot account for the difference in yields in cells infected during S phase and in cells infected during G(1). Thus, additional factors affect the cell cycle restriction. These results indicate that the E4 orf6 and orf3 proteins, in addition to the E1B 55-kDa protein, may cooperate to promote cell cycle-independent adenovirus growth.

Coxsackie and adenovirus receptor (CAR)-dependent and major histocompatibility complex (MHC) class I-independent uptake of recombinant adenoviruses into human tumour cells

The role of two receptors, previously proposed to mediate the entry of adenoviruses into human cells, the coxsackie and adenovirus receptor (CAR) and the major histocompatibility complex (MHC) class I heavy chain has been investigated. The expression of MHC class I in many tumours is reduced or absent, therefore if this were a means by which adenoviruses gained entry into cells, it would have important implications for their application in cancer treatment. In order to determine if MHC class I heavy chain is involved in adenovirus type 5 (Ad5) uptake, the binding of recombinant Ad5 fibre knob domain (which mediates viral attachment) to human cell lines that had greatly different levels of surface MHC class I was studied. We also created derivatives of a non-permissive Chinese hamster ovary (CHO) cell line that expressed human class I (HLA-A2) and found that these cells did not bind fibre or take up virus. In addition, the extracellular domain of CAR was expressed in E. coli and used to generate a polyclonal anti-CAR antibody. This antibody blocked both 125I labelled fibre knob binding and virus uptake. Thus CAR, and not MHC class I, is a receptor for human adenoviruses in cultured tumour cells. Tissue CAR levels may therefore be an important factor in the efficiency of adenovirus-mediated gene therapy.

Injectable modalities as local and regional strategies for head and neck cancer

New approaches to the treatment of head and neck cancer involve delivery of the therapeutic agent by direct injection into the tumor lesions. This locoregional therapy allows adaptation of conventional chemotherapy agents, such as bleomycin and cisplatin, to achieve higher drug concentrations in the tumor while avoiding severe systemic toxicities. Novel therapies administered through tumor injection, such as gene transfer, photosensitization, and biologic response modification, have been investigated in preclinical studies and are currently in different phases of clinical trial.

Selective killing of cancer cells based on loss of heterozygosity and normal variation in the human genome: a new paradigm for anticancer drug therapy

Most drugs for cancer therapy are targeted to relative differences in the biological characteristics of cancer cells and normal cells. The therapeutic index of such drugs is theoretically limited by the magnitude of such differences, and most anticancer drugs have considerable toxicity to normal cells. Here we describe a new approach for developing anticancer drugs. This approach, termed variagenic targeting, exploits the absolute difference in the genotype of normal cells and cancer cells arising from normal gene sequence variation in essential genes and loss of heterozygosity (LOH) occurring during oncogenesis. The technology involves identifying genes that are: 1) essential for cell survival; 2) are expressed as multiple alleles in the normal population because of the presence of one or more nucleotide polymorphisms; and 3) are frequently subject to LOH in several common cancers. An allele-specific drug inhibiting the essential gene remaining in cancer cells would be lethal to the malignant cell and would have minimal toxicity to the normal heterozygous cell that retains the drug-insensitive allele. With antisense oligonucleotides designed to target two alternative alleles of replication protein A, 70-kDa subunit (RPA70) we demonstrate in vitro selective killing of cancer cells that contain only the sensitive allele of the target gene without killing cells expressing the alternative RPA70 allele. Additionally, we identify several other candidate genes for variagenic targeting. This technology represents a new approach for the discovery of agents with high therapeutics indices for treating cancer and other proliferative disorders.

Therapeutic efficacy of an adenovirus-mediated anti-H-ras ribozyme in experimental bladder cancer

Ras oncogenes are thought to play a critical role in cellular proliferation and tumorigenesis. Reversal of the malignant phenotype, inhibition of tumor growth, and decreased tumorgenicity have been demonstrated with the use of anti-H-ras ribozymes. In this study, the therapeutic efficacy of a hammerhead ribozyme targeting the mutated H-ras oncogene was investigated in an experimental bladder cancer model using a recombinant adenovirus as delivery vehicle. Tumors were established in nude mice by subcutaneous injection of EJ human bladder carcinoma cells harboring a point mutation of the H-ras gene. The tumors were treated with intralesional injections of an adenovirus expressing an anti-H-ras ribozyme (rAd-Hras Rz) by different schedules at serial titers, and the tumor inhibition efficacy was analyzed. The viral infection efficacy and kinetics of ribozyme expression were also evaluated. Intralesional injection of rAd-Hras Rz resulted in significant antineoplastic effects in a dose-dependent fashion. Complete regression of the tumor was achieved by rAd-Hras Rz in several cases without recurrence during the 50-day observation period. Although there was moderate vector-associated cytotoxicity in this cell line, complete regressions were not observed in the cases treated with control adenovirus vectors or vectors expressing an inactive anti-H-ras ribozyme or anti-H-ras antisense oligonucleotides. These results suggest the efficacy of a ribozyme-encoding adenovirus in the experimental gene therapy of human bladder cancer.

p53-Independent and -dependent requirements for E1B-55K in adenovirus type 5 replication

The adenovirus type 5 mutant dl1520 was engineered previously to be completely defective for E1B-55K functions. Recently, this mutant (also known as ONYX-015) has been suggested to replicate preferentially in p53(-) and some p53(+) tumor cell lines but to be attenuated in primary cultured cells (C. Heise, A. Sampson-Johannes, A. Williams, F. McCormick, D. D. F. Hoff, and D. H. Kirn, Nat. Med. 3:639-645, 1997). It has been suggested that dl1520 might be used as a "magic bullet" that could selectively lyse tumor cells without harm to normal tissues. However, we report here that dl1520 replication is independent of p53 genotype and occurs efficiently in some primary cultured human cells, indicating that the mutant virus does not possess a tumor selectivity. Although it was not the sole host range determinant, p53 function did reduce dl1520 replication when analyzed in a cell line expressing temperature-sensitive p53 (H1299-tsp53) (K. L. Fries, W. E. Miller, and N. Raab-Traub, J. Virol. 70:8653-8659, 1996). As found earlier for other E1B-55K mutants in HeLa cells (Y. Ho, R. Galos, and J. Williams, Virology 122:109-124, 1982), dl1520 replication was temperature dependent in H1299 cells. When p53 function was restored at low temperature in H1299-tsp53 cells, it imposed a modest defect in viral DNA replication and accumulation of late viral cytoplasmic mRNA. However, in both H1299 and H1299-tsp53 cells, the defect in late viral protein synthesis appeared to be much greater than could be accounted for by the modest defects in late viral mRNA levels. We therefore propose that in addition to countering p53 function and modulating viral and cellular mRNA nuclear transport, E1B-55K also stimulates late viral mRNA translation.

'Autoreplication' of the vector genome in recombinant adenoviral vectors with different E1 region deletions and transgenes

High transgene stabilities of 1 year and more have been reported in immunodeficient hosts after adenovirus-mediated gene transfer. Transgene persistence of this duration could be due to inherently high stability of the episomal viral vector DNA. An alternative explanation would be limited 'autoreplication' of transgenic vector DNA, just sufficient to counteract slow but continuous degradation within the host cells. Autoreplication could occur in the absence of any production of infectious virus particles, based on residual activity of the adenoviral DNA replication system only. To test this hypothesis, a series of DNA metabolic labeling studies in non-permissive cells cultures transfected with different vectors was conducted. Due to extensive E1 region deletions none of the vectors was able to produce viral progeny in non-permissive cells. Vectors fell into two categories, however, with respect to their autoreplication potential. Neosynthesis of vector DNA in non-permissive vector-transfected cells was readily detectable in 'type A', but not in 'type B' vectors. In addition to their different transgene expression cassettes, vector DNA sequencing showed a less extensive E1 deletion in type A (nucleotides 453-3333 of wild-type virus) as compared to type B vectors (nucleotides 325-3523). Autoreplication was also associated with high transcriptional activity of several viral genes (E1B-14k, adenoviral DNA polymerase, single-strand DNA-binding protein, E4-25k), in contrast to type B vectors. In addition to these 'wild-type' transcripts, 'irregular' recombinant transcripts were detected in autoreplication vectors which contained the transgenic cDNA in conjunction with adenoviral vector sequences. Exogenous or cryptic promotors may (under certain conditions) enhance the transcriptional activity of a vector in such a way that autoreplication occurs. Conditions determining the level of transcriptional enhancement (extent of E1 deletion, type of promoter and transgene, etc) need to be further defined before rational design of adenovectors with high autoreplication capacity becomes possible. In summary, we have shown autoreplication to be a novel feature of certain E1-deleted adenovectors with likely relevance for their stability in vivo, but also with possibly adverse consequences for target cell function or vector immunogenicity. Full characterization of adenoviral vector systems should therefore include a description of their autoreplication capacity.

Functional coexpression of HSV-1 thymidine kinase and green fluorescent protein: implications for noninvasive imaging of transgene expression

Current gene therapy technology is limited by the paucity of methodology for determining the location and magnitude of therapeutic transgene expression in vivo. We describe and validate a paradigm for monitoring therapeutic transgene expression by noninvasive imaging of the herpes simplex virus type 1 thymidine kinase (HSV-1-tk) marker gene expression. To test proportional coexpression of therapeutic and marker genes, a model fusion gene comprising green fluorescent protein (gfp) and HSV-1-tk genes was generated (tkgfp gene) and assessed for the functional coexpression of the gene product, TKGFP fusion protein, in rat 9L gliosarcoma, RG2 glioma, and W256 carcinoma cells. Analysis of the TKGFP protein demonstrated that it can serve as a therapeutic gene by rendering tkgfp transduced cells sensitive to ganciclovir or as a screening marker useful for identifying transduced cells by fluorescence microscopy or fluorescence-activated cell sorting (FACS). TK and GFP activities in the TKGFP fusion protein were similar to corresponding wild-type proteins and accumulation of the HSV-1-tk-specific radiolabeled substrate, 2'-fluoro-2'-deoxy-1beta-D-arabinofuranosyl-5-iodo-uracil (FIAU), in stability transduced clones correlated with gfp-fluorescence intensity over a wide range of expression levels. The tkgfp fusion gene itself may be useful in developing novel cancer gene therapy approaches. Valuable information about the efficiency of gene transfer and expression could be obtained by non-invasive imaging of tkgfp expression with FIAU and clinical imaging devices (gamma camera, positron-emission tomography [PET], single photon emission computed tomography [SPECT]), and/or direct visualization of gfp expression in situ by fluorescence microscopy or endoscopy.

Replication-competent herpes simplex virus vector G207 and cisplatin combination therapy for head and neck squamous cell carcinoma

Replication-competent virus vectors are attractive therapeutic agents for cancer. G207, a second-generation, multimutated herpes simplex virus type 1 (HSV-1), is one such vector that is safe in primates and efficacious against human tumors in athymic mice. Squamous cell carcinoma is the most frequently encountered malignancy of the head and neck, and the chemotherapeutic agent cisplatin is a standard treatment for recurrent head and neck cancer. In this study we examine the therapeutic potential of G207, alone and in combination with cisplatin, against squamous cell carcinoma. Human squamous cell carcinoma cell lines are sensitive to G207 replication and cytotoxicity in vitro at a multiplicity of infection of 0.01, including cisplatin sensitive (UMSCC-22A), moderately sensitive (UMSCC-38), and weakly sensitive (SQ20B) cell lines. Cisplatin did not inhibit the cytopathic effect of G207. G207 inhibited the growth of established subcutaneous head and neck tumors in athymic mice. The therapeutic effects of cisplatin and G207 in vivo were independent. However, in cisplatin-sensitive tumors (UMSCC-38), combination therapy resulted in 100% cures in contrast to 42% with G207 or 14% with cisplatin alone. We conclude that G207 should be considered for the treatment of head and neck cancer and that combination with chemotherapeutic agents may improve efficacy.

Corepressor required for adenovirus E1B 55,000-molecular-weight protein repression of basal transcription

Adenovirus E1B 55,000-molecular-weight protein (55K) binds to host cell p53, stabilizing it, greatly increasing its affinity for its cognate DNA-binding site, and converting it from a regulated activator to a constitutive repressor. Here we analyzed the mechanism of repression by the p53-E1B 55K complex. E1B 55K repression requires that 55K be tethered to the promoter by binding directly to DNA-bound p53. Transcription from an assembled, p53-activated preinitiation complex was not repressed by the subsequent addition of E1B 55K, suggesting that either sites of 55K interaction with p53 or targets of 55K in the preinitiation complex are blocked. Specific E1B 55K repression was observed in reactions lacking TFIIA and with recombinant TATA-binding protein in place of TFIID, conditions under which p53 does not activate transcription. Thus, E1B 55K does not simply inhibit a p53-specific activation mechanism but rather blocks basal transcription. As a consequence, E1B 55K may repress transcription from any promoter with an associated p53-binding site, no matter what other activators associate with the promoter. E1B 55K did not repress basal transcription in reactions with recombinant and highly purified general transcription factors and RNA polymerase II but rather required a corepressor that copurifies with the polymerase.

Cytochrome P450-based cancer gene therapy: recent advances and future prospects

Cytochrome P450-based cancer gene therapy is a novel prodrug activation strategy for cancer treatment that has substantial potential for improving the safety and efficacy of cancer chemotherapeutics. The primary goal of this strategy is to selectively increase tumor cell exposure to cytotoxic drug metabolites generated locally by a prodrug-activating P450 enzyme. This strategy has been exemplified for the alkylating agents cyclophosphamide and ifosfamide, which are bioactivated by select P450 enzymes whose expression is generally high in liver and deficient in tumor cells. Transduction of tumors with a prodrug-activating P450 gene, followed by prodrug treatment, greatly increases intratumoral formation of activated drug metabolites. This leads to more efficient killing of the transduced tumor cells without a significant increase in host toxicity. P450 gene therapy is accompanied by substantial bystander cytotoxicity which greatly enhances the therapeutic effect by extending it to nearby tumor cells not transduced with the therapeutic P450 gene. Although endogenous P450 reductase is not expected to be a limiting factor in prodrug activation in tumor cells that express moderate levels of an exogenous P450 gene, P450 reductase transduction has recently been found to substantially enhance intratumoral prodrug activation and its associated therapeutic effects. Using this gene combination, an overall 50- to 100-fold increase in tumor cell kill in vivo over that provided by hepatic drug activation alone has been observed. Striking improvements in therapeutic effects can thus be achieved using an established anticancer drug in an intratumoral prodrug activation strategy based on the combination of a cytochrome P450 gene with the gene encoding NADPH-P450 reductase. This strategy is readily extendable to several other widely used P450-activated cancer chemotherapeutic prodrugs, as well as to prodrugs that undergo P450 reductase-dependent bioreductive activation and which may exhibit synergy when combined with P450-activated prodrugs in a P450/P450 reductase-based cancer gene therapeutic regimen.

Control of inducible chemoresistance: enhanced anti-tumor therapy through increased apoptosis by inhibition of NF-kappaB

Programmed cell death (apoptosis) seems to be the principal mechanism whereby anti-oncogenic therapies such as chemotherapy and radiation effect their responses. Resistance to apoptosis, therefore, is probably a principal mechanism whereby tumors are able to overcome these cancer therapies. The transcription factor NF-kappaB is activated by chemotherapy and by irradiation in some cancer cell lines. Furthermore, inhibition of NF-kappaB in vitro leads to enhanced apoptosis in response to a variety of different stimuli. We show here that inhibition of NF-kappaB through the adenoviral delivery of a modified form of IkappaBalpha, the inhibitor of NF-kappaB, sensitizes chemoresistant tumors to the apoptotic potential of TNFalpha and of the chemotherapeutic compound CPT-11, resulting in tumor regression. These results demonstrate that the activation of NF-kappaB in response to chemotherapy is a principal mechanism of inducible tumor chemoresistance, and establish the inhibition of NF-kappaB as a new approach to adjuvant therapy in cancer treatment.

Gene therapy in veterinary medicine

Gene therapy in simple terms is the introduction of a gene into a cell, in vivo, in order to ameliorate a disease process. Human clinical trials have focused on the correction of monogenic deficiency diseases, cancer and AIDS. This paper summarises the technology of gene therapy, gives a brief synopsis of the current applications of gene therapy to veterinary medicine and discusses some of the problems which need to be overcome so that gene therapy can become accepted clinical practice.

Control of inducible chemoresistance: enhanced anti-tumor therapy through increased apoptosis by inhibition of NF-kappaB

Programmed cell death (apoptosis) seems to be the principal mechanism whereby anti-oncogenic therapies such as chemotherapy and radiation effect their responses. Resistance to apoptosis, therefore, is probably a principal mechanism whereby tumors are able to overcome these cancer therapies. The transcription factor NF-kappaB is activated by chemotherapy and by irradiation in some cancer cell lines. Furthermore, inhibition of NF-kappaB in vitro leads to enhanced apoptosis in response to a variety of different stimuli. We show here that inhibition of NF-kappaB through the adenoviral delivery of a modified form of IkappaBalpha, the inhibitor of NF-kappaB, sensitizes chemoresistant tumors to the apoptotic potential of TNFalpha and of the chemotherapeutic compound CPT-11, resulting in tumor regression. These results demonstrate that the activation of NF-kappaB in response to chemotherapy is a principal mechanism of inducible tumor chemoresistance, and establish the inhibition of NF-kappaB as a new approach to adjuvant therapy in cancer treatment.

The replicative capacities of large E1B-null group A and group C adenoviruses are independent of host cell p53 status

Recent reports suggest that an early region 1B (E1B) 55, 000-molecular-weight polypeptide (55K)-null adenovirus type 5 (Ad5) mutant (dl1520) can replicate to the same extent as wild-type (wt) Ad5 in cells either deficient or mutated in p53, implicating p53 in limiting viral replication in vivo. In contrast, we show here that the replicative capacity of Ad5 dl1520 is wholly independent of host cell p53 status, as is the replicative capacity of comparable Ad12 E1B 54K-null adenoviruses (Ad12 dl620 and Ad12 hr703). Furthermore, we show that there is no requirement for complex formation between p53 and Ad5 E1B 55K or Ad12 E1B 54K for a productive infection, such that wt Ad5 and wt Ad12 will both replicate in cells which are null for p53. In addition, we find that these Ad5 and Ad12 mutant viruses induce S phase irrespective of the p53 status of the cell and that, therefore, S-phase induction does not correlate with the replicative capacity of the virus. Interestingly, the replicative capacities of the large E1B-null adenoviruses correlated positively with the ability to express E1B 19K and were related to the ability to repress premature adenovirus-induced apoptosis. Infection of primary human cells indicated that Ad5 dl1520, wt Ad5, and wt Ad12 replicated better in cycling normal human skin fibroblasts (HSFs) than in quiescent HSFs. Thus, the cell cycle status of the host cell, upon infection, also influences viral yield.

Targeting the replication of adenoviral gene therapy vectors to lung cancer cells: the importance of the adenoviral E1b-55kD gene

It has been proposed that an adenovirus with the E1b-55kD gene deleted has a selective advantage in replicating in cancer cells that have mutations in the p53 gene (Bischoff et al., 1996). We have explored this hypothesis in several lung cancer cell lines, and evaluated potential mechanisms that might regulate the replication of Ad338, an E1b-55kD-deleted virus, with the objective of developing a rational approach for targeting gene therapy to lung tumors. Our data show that Ad338 replicates poorly in three lung cancer cell lines with various p53 mutations (H441, H446, and Calu1), yet this virus replicates to a high level in a lung cancer cell line with wild-type p53 (A549) and in a normal lung fibroblast line (IMR90). Viral DNA replication, expression of viral proteins, and shutoff of host cell proteins were not important variables in limiting the replication of the E1b-55kD-deleted virus. However, the cell lines resistant to host cell protein shutoff were also the most resistant to the cytopathic effect induced by mutant and wild-type virus and the only cells to survive for 8 days following infection. The E1b-55kD protein clearly has an important role in viral replication beyond its interaction with p53. Thus, an E1b-55kD-deleted virus cannot be used to specifically target viral replication to p53-mutated lung cancer cells.

Is primary chemotherapy useful for all patients with primary invasive breast cancer?

Chemotherapy dose intensification in breast tumours is being evaluated in many multicentre trials, its indication being based on a clinical response in high-risk patients, thus selecting for tumours with rapid proliferation and low resistance. However, results from randomized trials are still pending. Clinical and pathological responses to therapy are valuable surrogate endpoints following primary chemotherapy. They will make it possible to distinguish at an early stage between patients who still retain an apoptotic response to chemotherapy and those patients whose disease will progress rapidly due to resistance mechanisms. For practical purposes, patients at risk and capable of responding represent the population of choice for primary systemic chemotherapy. Thus, by investigating mechanisms of response and resistance during the first courses of treatment we may target chemotherapy at those patients likely to benefit most from this treatment. A number of immunotherapy and vaccination trials are being conducted in many different centres. There is a lot of anecdotal evidence that cancer vaccines could help patients, but little yet in the way of solid, reproducible clinical data. Best responses to clinical testing would ideally be expected in early-stage disease because there is less tumour bulk and the patient's immune system is still able to respond. Patients with early breast cancer who are at high risk of recurrence and who have failed to respond to primary chemotherapy might be given the option of participating in adjuvant vaccination trials following the completion of local therapy.

p53-oriented cancer therapies: current progress

Nearly twenty years after the initial discovery of p53, we are now in an ideal position to exploit our vast knowledge of p53 biology in the creation of novel cancer therapies. Disruption of p53 function through mutation, or other means, occurs very frequently in human cancer. Loss of p53 function has been linked with unfavourable prognosis in a large number of tumour types, as indicated by more aggressive tumours, early metastasis and decreased survival rates. Many different avenues of research have converged upon p53 to highlight this protein as being one of the foremost cellular responders to stress, in particular to DNA damage. Huge advances have been made in understanding the complex role p53 plays in the regulation of apoptosis and cell cycle arrest. This review is not meant to be a comprehensive description of p53 biology, but rather serves to highlight current progress in the development of p53-oriented cancer therapies. These may be categorised into three basic strategies: gene replacement therapy using wild-type p53, restoration of p53 function by other means and, finally, targeting of the p53 dysfunction itself. Rapid progress is expected to be made regarding the identification of conventional pharmaceutical agents which either work in a p53-independent manner or act preferentially in p53 defective cells. Gene replacement therapy with wild-type p53 also holds considerable potential for obtaining clinically relevant results quickly. The other forms of cancer therapies based around p53 are much further behind in the developmental process, but may prove to more efficacious in the long run, especially in terms of specificity. As with many other fields, the innovation of successful p53-oriented cancer therapies is only limited by our understanding of p53 biology and the creative use of such knowledge.

The adenovirus E4orf6 protein contributes to malignant transformation by antagonizing E1A-induced accumulation of the tumor suppressor protein p53

The adenovirus type 5 (Ad5) E4orf6 protein promotes focus formation of primary baby rat kidney (BRK) cells in cooperation with Ad5 E1 proteins. This activity is most likely related to the ability of the E4orf6 protein to bind to p53 and modulate its tumor suppressor functions. In this study we report that transformed BRK cells that stably express E4orf6 in addition to E1A and E1B (ABS cells) displayed multiple additional properties commonly associated with a high grade of oncogenic transformation compared to cells expressing only E1A and E1B (AB cells). These properties included morphological alterations, markedly enhanced growth rates and growth to much higher saturation densities. Following injection into nude mice ABS-derived tumors exhibited accelerated growth and, based on histopathological criteria, proofed to be much more malignant compared to tumors generated by AB cells. Interestingly, these highly transformed properties of ABS cells correlated with a dramatic reduction of p53 steady-state levels which inversely correlated with E4orf6 expression. From these results we conclude that expression of the Ad5 E4orf6 protein (i) confers additional transformed in vitro properties to primary rat cells expressing the Ad5 E1 proteins, and (ii) increases the tumorigenic and malignant potential of these cells in vivo. Our data suggest that the Ad5 E4orf6 protein enhances the intrinsic ability of E1-transformed rat cells to grow in a neoplastic state by completely inactivating p53 tumor suppressor function in combination with the E1A and E1B proteins.

p53 status does not determine outcome of E1B 55-kilodalton mutant adenovirus lytic infection

The ability of the adenovirus type 5 E1B 55-kDa mutants dl1520 and dl338 to replicate efficiently and independently of the cell cycle, to synthesis viral DNA, and to lyse infected cells did not correlate with the status of p53 in seven cell lines examined. Rather, cell cycle-independent replication and virus-induced cell killing correlated with permissivity to viral replication. This correlation extended to S-phase HeLa cells, which were more susceptible to virus-induced cell killing by the E1B 55-kDa mutant virus than HeLa cells infected during G1. Wild-type p53 had only a modest effect on E1B mutant virus yields in H1299 cells expressing a temperature-sensitive p53 allele. The defect in E1B 55-kDa mutant virus replication resulting from reduced temperature was as much as 10-fold greater than the defect due to p53 function. At 39 degreesC, the E1B 55-kDa mutant viruses produced wild-type yields of virus and replicated independently of the cell cycle. In addition, the E1B 55-kDa mutant viruses directed the synthesis of late viral proteins to levels equivalent to the wild-type virus level at 39 degreesC. We have previously shown that the defect in mutant virus replication can also be overcome by infecting HeLa cells during S phase. Taken together, these results indicate that the capacity of the E1B 55-kDa mutant virus to replicate independently of the cell cycle does not correlate with the status of p53 but is determined by yet unidentified mechanisms. The cold-sensitive nature of the defect of the E1B 55-kDa mutant virus in both late gene expression and cell cycle-independent replication leads us to speculate that these functions of the E1B 55-kDa protein may be linked.

New p53-based anti-cancer therapeutic strategies

The p53 gene is frequently mutated in human tumours and therefore an important target for therapeutic intervention. Several p53-based strategies for treatment of cancer are currently under development. p53 gene therapy has resulted in tumour regression in patients with lung cancer. A mutant adenovirus can obliterate tumour cells carrying mutant p53 or lacking p53, but is unable to replicate in normal cells. Furthermore, current studies suggest that reactivation of mutant p53 proteins in tumours using small p53-activating molecules may initiate p53-dependent apoptosis and thus eliminate the tumour.

Replication of ONYX-015, a potential anticancer adenovirus, is independent of p53 status in tumor cells

The 55-kDa E1B protein of adenovirus, which binds to and inactivates the tumor suppressor protein p53, is not expressed in the adenoviral mutant termed ONYX-015 (i.e., dl1520). It was reported that the mutant virus due to a deletion in E1B is able to replicate only in cells deficient for wild-type p53. Accordingly, dl1520 is currently being evaluated as a potential tool in the therapy of p53 deficient cancers. In contrast, we report here that dl1520 replicates independently of the p53 status in various tumor cell lines (U87, RKO, A549, H1299, and U373). In addition, the inhibition of p53-mediated transcriptional activation in wild-type p53 containing U2OS cells, by overexpression of a transdominant negative p53 mutant, did not render the cells permissive for dl1520 replication. Finally, we show that, depending on the multiplicity of infection, the deleted virus is able to replicate in and to kill primary human cells. Thus, the molecular basis for the growth differences of dl1520 within different cell types remains to be determined.

Intratumoral infusion of fluid: estimation of hydraulic conductivity and implications for the delivery of therapeutic agents

We have developed a new technique to measure in vivo tumour tissue fluid transport parameters (hydraulic conductivity and compliance) that influence the systemic and intratumoral delivery of therapeutic agents. An infusion needle approximating a point source was constructed to produce a radially symmetrical fluid source in the centre of human tumours in immunodeficient mice. At constant flow, the pressure gradient generated in the tumour by the infusion of fluid (Evans blue-albumin in saline) was measured as a function of the radial position with micropipettes connected to a servo-null system. To evaluate whether the fluid infused was reabsorbed by blood vessels, infusions were also performed after circulatory arrest. In the colon adenocarcinoma LS174T with a spherically symmetrical distribution of Evans blue-albumin, the median hydraulic conductivity in vivo and after circulatory arrest at a flow rate of 0.1 microl min(-1) was, respectively, 1.7x10(-7) and 2.3x10(-7) cm2 mmHg(-1) s. Compliance estimates were 35 microl mmHg(-1) in vivo, and 100 microl mmHg(-1) after circulatory arrest. In the sarcoma HSTS 26T, hydraulic conductivity and compliance were not calculated because of the asymmetric distribution of the fluid infused. The technique will be helpful in identifying strategies to improve the intratumoral and systemic delivery of gene targeting vectors and other therapeutic agents.

Generation of fiber-mutant recombinant adenoviruses for gene therapy of malignant glioma

Recombinant adenovirus (Adv)-mediated gene transduction is a powerful technology for cancer gene therapy. In this article, we report the generation of a fiber-mutant Adv vector, using the Adv genomic DNA-terminal protein complex (DNA-TPC) cotransfection method. First, a fiber-mutant construct in a plasmid carrying the right-side two-thirds of the human adenovirus type 5 (Ad5) genome (pTR) was cotransfected with Ad5 DNA-TPC, yielding the recombinant Adv with the desired fiber mutation. The DNA-TPC from the mutant Adv was then utilized to produce a second-step recombinant Adv with an expression cassette in the place of E1. By this procedure, we generated a fiber mutant, F/K20, that has a linker and a stretch of 20 lysine residues added at the C terminus of the fiber. By using Adv carrying a reporter lacZ gene (AxCAZ2) with either F/K20 or wild-type fiber (F/wt), we examined the transduction efficiency of F/K20-Adv. No significant difference in the transduction efficiency between F/K20 and F/wt-Adv was observed for a human fibroblast line, WI-38, or various tumor cell lines, including melanoma, prostate, esophageal, and pancreatic cancer lines. In clear contrast, F/K20-Adv showed a remarkably enhanced efficiency in genetic transduction of human glioma cells. In all four human glioma lines tested, the multiplicities of infection (MOIs) for transduction of 50% of the population (ED50) were decreased with F/K20-Adv compared with F/wt-Adv: 7-fold for T98G, 14-fold for U251, 9-fold for U373, and 42-fold for U87 cells. Therefore, we attempted to apply F/K20-Adv for gene therapy of malignant glioma. Glioma cells infected with F/K20-Adv carrying genes for interleukin 2 or interleukin 12 produced a high level of each cytokine at a much lower MOI than did cells infected with F/wt-Adv. Infection with F/K20-Adv carrying the wild-type p53 tumor suppressor gene resulted in an enhanced level of p53 protein expression and an increased incidence of F/K20-Adv in transduction efficiency for malignant glioma, providing promising tools for gene therapy.

Reovirus therapy of tumors with activated Ras pathway

Human reovirus requires an activated Ras signaling pathway for infection of cultured cells. To investigate whether this property can be exploited for cancer therapy, severe combined immune deficient mice bearing tumors established from v-erbB-transformed murine NIH 3T3 cells or human U87 glioblastoma cells were treated with the virus. A single intratumoral injection of virus resulted in regression of tumors in 65 to 80 percent of the mice. Treatment of immune-competent C3H mice bearing tumors established from ras-transformed C3H-10T1/2 cells also resulted in tumor regression, although a series of injections were required. These results suggest that, with further work, reovirus may have applicability in the treatment of cancer.

The p53 tumour suppressor gene

BACKGROUND

Abnormalities of the p53 tumour suppressor gene are thought to be central to the development of a high proportion of human tumours. This article reviews current understanding of its function and potential clinical significance.

METHODS

Material was identified from previous review articles, references cited in original papers, a Medline search of the literature over the 12 months to January 1998, and by scanning the latest issues of relevant journals.

RESULTS AND CONCLUSION

p53 is considered to be a stress response gene, its product (the p53 protein) acting to induce cell cycle arrest or apoptosis in response to DNA damage, thereby maintaining genetic stability in the organism. These functions are executed by a complex and incompletely understood series of steps known as the 'p53 pathway', part of which involves induction of the expression of a number of other genes. As p53 is the most commonly mutated gene in human cancer, it has attracted a great deal of interest as a prognostic factor, diagnostic tool and therapeutic target. However, despite many promising studies, its potential in practical cancer management has still to be realized.