Oncolytic viruses: programmable tumour hunters

Despite significant improvements in early detection and refinements of therapeutic protocols over the last several decades, cancer remains one of the leading causes of death in North America. In particular, treatment of metastatic cancers is a highly desirable and yet still elusive goal of the oncologist. One strategy which holds promise is the use of self replicating viral strains with the ability to specifically kill tumour but not normal cells. These so-called "oncolytic viruses" are in general, attenuated for growth in normal cells but are able to exploit tumour specific, genetic defects to gain a growth advantage. In this review, we will discuss the virus:host cell interactions which help form the niche occupied by oncolytic viruses. The current and potential clinical applications/limitations will be discussed for oncolytic viruses from the herpesvirus, adenoviruses, picornavirus, rhabdovirus, and paramyxovirus families.

The murine double-stranded RNA-dependent protein kinase PKR is required for resistance to vesicular stomatitis virus

Interferon (IFN)-induced antiviral responses are mediated through a variety of proteins, including the double-stranded RNA-dependent protein kinase PKR. Here we show that fibroblasts derived from PKR(-/-) mice are more permissive for vesicular stomatitis virus (VSV) infection than are wild-type fibroblasts and demonstrate a deficiency in alpha/beta-IFN-mediated protection. We further show that mice lacking PKR are extremely susceptible to intranasal VSV infection, succumbing within days after instillation with as few as 50 infectious viral particles. Again, alpha/beta-IFN was unable to rescue PKR(-/-) mice from VSV infection. Surprisingly, intranasally infected PKR(-/-) mice died not from pathology of the central nervous system but rather from acute infection of the respiratory tract, demonstrating high virus titers in the lungs compared to similarly infected wild-type animals. These results confirm the role of PKR as the major component of IFN-mediated resistance to VSV infection. Since previous reports have shown PKR to be nonessential for survival in animals challenged with encephalomyocarditis virus, influenza virus, and vaccinia virus (N. Abraham et al., J. Biol. Chem. 274:5953-5962, 1999; Y. Yang et al., EMBO J. 14:6095-6106, 1995), our findings serve to highlight the premise that host dependence on the various mediators of IFN-induced antiviral defenses is pathogen specific.

Exon-skipping in BCR/ABL is induced by ABL exon 2

The BCR/ABL fusion gene is pathognomonic for chronic myelogenous leukaemia (CML). We have previously reported alternative splicing of BCR/ABL, as indicated by the detection of both p190- and p210-encoding transcripts, in about 60% of CML patient samples. These exon-skipping events involved the joining of ABL exon 2 to variable upstream BCR exons. Similarly, ABL exon 2 is alternatively spliced to either of two upstream ABL exons (1a or 1b) in c-ABL. We have constructed BCR and BCR/ABL minigenes to study this phenomenon in more detail. These constructs were transfected into various cell types and splicing was assessed by reverse transcriptase PCR. Whereas the basic BCR minigene expressed exon-inclusive transcripts only, insertion of genomic DNA spanning ABL exon 2 induced exon-skipping but only when expressed in the CML cell lines K562 and EM3. In this study we localized the required sequence element to ABL exon 2 itself. These results mimic the splicing phenotype displayed by most CML patients. We propose a model where a trans-factor present in some CML cells interacts with ABL exon 2 pre-mRNA to promote skipping of upstream BCR exons.

Expression of p210 and p190 BCR-ABL due to alternative splicing in chronic myelogenous leukaemia

The hallmark of chronic myelogenous leukaemia (CML) is the presence of the Philadelphia chromosome and its resultant fusion message, BCR-ABL, and fusion protein, p210. Patients with CML in blast crisis, or with Philadelphia positive acute lymphoblastic leukaemia (ALL), can have a smaller BCR-ABL fusion transcript possessing only the first exon of BCR fused to ABL. This smaller transcript encodes a 190 kD protein which is more strongly transforming than the p210 protein derived from the larger CML-associated transcript. We performed RT-PCR on samples from CML patients in chronic phase to determine the frequency and mechanism of p190 and p210 co-expression and to see if this correlated with clinical indices. We examined the peripheral blood or marrow of 67 patients with CML and found that 35 of them expressed both transcripts whereas the remainder expressed the p210-encoding transcript exclusively. Additional PCR products of an intermediate size were also frequently detected and have been isolated and sequenced. Data from two of these products indicate that they are the result of alternative splicing and include variable combinations of BCR exons. We believe that the expression of the p190-encoding transcript in the chronic phase of CML is also due to alternative splicing. A comparison of patients co-expressing the p190- and p210-encoding transcripts with those patients who expressed only the p210-encoding transcript detected significantly higher white blood cell (WBC) counts and blast cell counts at time of testing as well as significantly higher white blood cell counts at diagnosis.

Engraftment of human chronic lymphocytic leukemia cells in SCID mice: in vivo and in vitro studies

Chronic lymphocytic leukemia (CLL) is a heterogeneous disease of the elderly which can present in one of three stages; benign, intermediate or advanced. The molecular events governing the progression of CLL are poorly understood. In order to develop model systems for predicting the aggressiveness of leukemic clones in CLL, in vivo transplantation of SCID mice with CLL cells, and the in vitro growth of CLL cells on mouse and human stromal layers, were investigated. Bone marrow or peripheral blood cells from 40 patients at different stages of CLL were transplanted into 172 immune-deficient SCID mice. Thirty-five percent of SCID mice injected with CLL cells were positive for the presence of human DNA by Southern blot or PCR analysis. The most frequently involved sites were the spleen, lung, kidney and bone marrow, at levels corresponding from 0.1 to 10 percent human DNA. Thrice-weekly intraperitoneal injections of IL-2, alone or in combination with IL-7, did not increase the level of human cell engraftment. SCID mice developed endogenous thymic lymphomas at an incidence of 10-33 percent, a rate that was not increased by CLL cell transplantation. In vitro, CLL cells were able to proliferate for 9 weeks on human stromal layers supplemented with CM (conditioned media from a culture of the human bladder carcinoma cell line 5637), but failed to thrive on the murine stromal cell line MTE cultured either in CM or autologous serum. FACS analysis revealed that 81 percent of proliferating cells on human stromal layers carried the CD5 cell surface marker, identifying them as CLL cells. Previously EBV-negative CLL cells became EBV-positive after 9 to 12 weeks in culture. The results of this study provide a firm foundation for the development of in vivo and in vitro model systems for the study of human CLL.

Dysregulation of HOX11 by chromosome translocations in T-cell acute lymphoblastic leukemia: a paradigm for homeobox gene involvement in human cancer

The translocation t(10;14)(q24;q11) is observed in the course of routine cancer cytogenetic studies in 5-10% of patients with T-cell acute lymphoblastic leukemia (ALL). Recent molecular dissections of t(10;14) translocations support the hypothesis that these relatively gross chromosomal mutations represent key genetic steps in neoplastic transformation. The genes consistently involved are the T-cell receptor (TCR) delta-chain gene in 14q11 and a human homeobox-containing gene in 10q24, HOX11, initially identified through cloning of t(10;14) translocations. Like other homeoproteins, HOX11 binds DNA with sequence specificity and is likely to be a transcription factor, controlling the expression of developmentally important genes. The t(10;14) translocations arise as a result of aberrant physiological recombinational events that occur at early stages of T-cell development, probably during failed attempts at TCR gene rearrangement. The net result of the aberrant genetic recombinations is inappropriate expression of HOX11 in individual T-cells that acquire the mutation. Tlx-1, the murine homolog of HOX11, is expressed embryologically in the developing spleen and in structures derived from cranial neural crest cells and migratory paraxial mesoderm. Mice homozygously deleted for Tlx-1 are asplenic. Thus, HOX11 may be one of the first examples in mammals of a "master gene" acting as a regulatory switch controlling a downstream program of organ-specific cell growth and proliferation. Preliminary tumorigenicity assays suggest that HOX11 expression in hematopoietic cells most likely plays an immortalization role in neoplastic transformation.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of the Shope fibroma virus DNA ligase gene

The Shope fibroma virus (SFV) DNA ligase gene has been cloned and sequenced, and the biochemical requirements of the gene product have been determined in vitro. The SFV ligase gene maps to the BamHI L1/L2 boundary and spans 1.7 kb. The gene is predicted to encode a 559-amino-acid protein of M(r) = 63,139 which shares 45% amino acid identity with Orthopoxvirus ligases. The C-terminal two-thirds of the protein appears to encode the catalytic domain and shares distant homology with many ligases. The N-terminal homology is shared between only Orthopoxviruses and Leporipoxviruses and suggests that DNA ligases may be composite structures consisting of two independently evolved protein domains. Although the the gene encodes features characteristic of both early and late poxviral genes, Northern analysis showed that SFV ligase is expressed as a late gene product. In order to prove the identity of the protein it was expressed as a glutathione S-transferase fusion in Escherichia coli, affinity purified, and shown to be a Mg2+.ATP-dependent ligase in vitro. The recombinant protein can also form a covalent ligase.AMP complex characteristic of ATP-dependent DNA ligases. The SFV ligase gene can be disrupted and is thus not essential for viral growth in culture. This was shown by recombining a PCR product, encoding a P7.5 promoter and E. coli guanine phosphoribosyltransferase gene (gpt) into the open reading frame, and selecting for gpt+ viruses. This work provides insights into the evolution of Orthopoxviruses and Leporipoxviruses and strains suitable for a detailed analysis of the role DNA ligases play in poxviral recombination.