Transforming potential of a myc-containing variant of feline leukemia virus in vitro in early-passage feline cells

Exploring the link between viruses and cancer in companion animals: a comprehensive and comparative analysis

Currently, it is estimated that 15% of human neoplasms globally are caused by infectious agents, with new evidence emerging continuously. Multiple agents have been implicated in various forms of neoplasia, with viruses as the most frequent. In recent years, investigation on viral mechanisms underlying tumoral transformation in cancer development and progression are in the spotlight, both in human and veterinary oncology. Oncogenic viruses in veterinary medicine are of primary importance not only as original pathogens of pets, but also in the view of pets as models of human malignancies. Hence, this work will provide an overview of the main oncogenic viruses of companion animals, with brief notes of comparative medicine.

Infectious Causes of Neoplasia in the Domestic Cat

Simple Summary

Increasingly, cancers are being linked to infections with viruses, bacteria, and parasites in human medicine. This review summarises the current literature regarding neoplasia occurring in association with infectious diseases in domestic cats. To date, most studies have focused on the role of viruses, especially feline leukaemia virus and feline immunodeficiency virus in association with lymphoma, or the role of papillomavirus in cutaneous and oral neoplasms in cats. Recently, there has been a focus on a potential role of mouse mammary tumour virus in feline mammary carcinoma and lymphoma and studies assessing the potential role of gammaherpes virus and hepadnaviruses in feline neoplasia. Additionally, there has been some focus on potential bacterial and parasitic associations with neoplasia; including reports assessing potential associations between Helicobacter species and gastrointestinal neoplasms, and case reports of neoplasia in association with Platynosomum fastosum and Opisthorchis viverrini.

Abstract

In recent years, growing attention has been paid to the influence/role of infectious diseases in tumour development and progression. Investigations have demonstrated that some infectious organisms can have a direct role in the development of neoplasia, whereas others can predispose to neoplasia by alterations in the immune response, or by creating a pro-inflammatory environment. Feline leukaemia virus was one of the first infectious agents recognised as an oncogenic organism, and along with feline immunodeficiency virus has received the most attention. Since the discovery of this retrovirus, several other organisms have been associated with neoplastic processes in cats, these include gammaherpes virus, mouse mammary tumour virus, papillomaviruses, hepadnavirus, Helicobacter species, and the parasitic infections Platynosomum fastosum and Opisthorchis viverrini. This review summarises the findings to date.

The FeLV-945 LTR confers a replicative advantage dependent on the presence of a tandem triplication

Feline leukemia virus (FeLV), like other naturally occurring retroviruses, is characterized by a high degree of genetic diversity. FeLV-945 is a natural isolate derived from non-B-cell non-T-cell lymphomas classified anatomically as multicentric. FeLV-945 exhibits a unique structural motif in the LTR composed of a 21-bp tandem triplication downstream of a single copy of enhancer. The unique FeLV-945 LTR is precisely conserved among eight independent multicentric lymphomas collected in a geographic cluster. Previous studies using reporter gene constructs predict that the FeLV-945 LTR would confer a replicative advantage on the virus that contains it, particularly in primitive hematopoietic cells. Such an advantage may account for the precise conservation of the unique LTR sequence. To test that prediction, a set of recombinant, infectious FeLVs was developed that are isogenic other than the presence of the FeLV-945 LTR or mutations of it. Replication assays show that the FeLV-945 LTR confers a distinct growth advantage in K-562, FEA, and 3201 cells and implicate the 21-bp triplication in that function. Replacement of two copies of the triplicated element with random sequence greatly diminished the replicative capacity, thus implicating the triplicated sequence itself in LTR function. The 21-bp triplication was shown to contain specific nuclear protein binding sites, which may account for the selective pressure to conserve the sequence.

Tumorigenic potential of a recombinant retrovirus containing sequences from Moloney murine leukemia virus and feline leukemia virus

A recombinant retrovirus, termed MoFe2-MuLV, was constructed in which the U3 region of T-lymphomagenic Moloney murine leukemia virus (Mo-MuLV) was replaced by that of FeLV-945, a provirus of unique long terminal repeat (LTR) structure identified only in non-T-cell, non-B-cell lymphomas of the domestic cat. The LTR of FeLV-945 is unusual in that it contains only a single copy of the transcriptional enhancer followed 25 bp downstream by a 21-bp sequence in triplicate in tandem. Infectivity of MoFe2-MuLV was demonstrated in vitro in SC-1 cells and in vivo in neonatal NIH-Swiss mice. Tumors occurred in MoFe2-MuLV-infected animals following a latency period of 4 to 10 months (average, 6 months). The results of Southern blot analysis of the T-cell receptor beta locus demonstrated that all tumors were lymphomas of T-cell origin. MoFe2-MuLV LTRs were amplified by PCR from tumor DNA and were characterized by nucleotide sequence analysis. LTRs from the tumors that occurred with relatively shorter latency predominantly retained the original MoFe2-MuLV sequence intact and unaltered. Tumors that occurred with relatively longer latency contained LTRs that also retained the 21-bp sequence triplication characteristic of the original virus but had acquired various duplications of enhancer sequences. The repeated identification of enhancer duplications in late-appearing tumors suggests that the duplication affords a selective advantage, although apparently not in the efficient induction of T-cell lymphoma. Proto-oncogenes known to be targets of insertional mutagenesis in the majority of Mo-MuLV-induced tumors or in feline non-T-cell, non-B-cell lymphomas were shown not to be rearranged in any tumor examined. Mink cell focus-inducing (MCF) proviral DNA was readily detectable in some, but not all, tumors. The presence or absence of MCF did not correlate with the kinetics of tumor induction. These studies indicate that the single-enhancer, triplication-containing FeLV LTR, typical of non-T-cell, non-B-cell lymphomas in cats, is competent in the induction of T-cell lymphoma in mice. The findings suggest that the mechanism of MoFe2-MuLV-mediated lymphomagenesis may differ from that of Mo-MuLV-mediated disease, considering the possible involvement of novel oncogenes and the variable presence of MCF recombinants.

Apparent uncoupling of oncogenicity from fibroblast transformation and apoptosis in a mutant myc gene transduced by feline leukemia virus

The T17 v-myc oncogene was transduced by feline leukemia virus in a spontaneous feline T-cell lymphosarcoma. Molecular cloning and sequencing of the v-myc gene revealed several unique mutations, including a large deletion affecting amino acids 49 to 124 and a 3-bp insertion within the basic DNA binding domain which converts Leu-362 to Phe-Arg. The T17 lymphoma cell line was found to express a truncated 50-kDa Myc protein at exceptionally high levels, while the endogenous c-myc gene was not detectably expressed. These observations suggest that the mutant Myc product expresses an oncogenic function in T cells. Further evidence that the T17 mutant gene retains oncogenic potential was provided by its detection in clonally integrated proviruses in secondary tumors induced by feline leukemia virus T17, where no reversion mutations were found in any of three tumors examined. However, the mutant T17 v-myc gene did not induce transformation in a chicken embryo fibroblast assay, in contrast to wild-type feline c-myc, which conferred higher growth rates on the chicken fibroblasts, along with altered morphology and the ability to form foci in soft agar. Chicken cells over-expressing feline c-myc died by apoptosis when cultured with low serum concentrations, while the T17 mutant had no discernible effect. These results suggest that the leukemogenic potential of Myc can be uncoupled from its ability to cause transformation in fibroblasts. A possible explanation for this apparent paradox is that developing T cells are acutely sensitive to a subset of Myc functions which are insufficient for fibroblast transformation.

Function of a unique sequence motif in the long terminal repeat of feline leukemia virus isolated from an unusual set of naturally occurring tumors

Feline leukemia virus (FeLV) proviruses have been characterized from naturally occurring non-B-cell, non-T-cell tumors occurring in the spleens of infected cats. These proviruses exhibit a unique sequence motif in the long terminal repeat (LTR), namely, a 21-bp tandem triplication beginning 25 bp downstream of the enhancer. The repeated finding of the triplication-containing LTR in non-B-cell, non-T-cell lymphomas of the spleen suggests that the unique LTR is an essential participant in the development of tumors of this particular phenotype. The nucleotide sequence of the triplication-containing LTR most closely resembles that of FeLV subgroup C. Studies performed to measure the ability of the triplication-containing LTR to modulate gene expression indicate that the 21-bp triplication provides transcriptional enhancer function to the LTR that contains it and that it substitutes at least in part for the duplication of the enhancer. The 21-bp triplication confers a bona fide enhancer function upon LTR-directed reporter gene expression; however, the possibility of a spacer function was not eliminated. The studies demonstrate further that the triplication-containing LTR acts preferentially in a cell-type-specific manner, i.e., it is 12-fold more active in K-562 cells than is an LTR lacking the triplication. A recombinant, infectious FeLV bearing the 21-bp triplication in U3 was constructed. Cells infected with the recombinant were shown to accumulate higher levels of viral RNA transcripts and virus particles in culture supernatants than did cells infected with the parental type. The triplication-containing LTR is implicated in the induction of tumors of a particular phenotype, perhaps through transcriptional regulation of the virus and/or adjacent cellular genes, in the appropriate target cell.

Haematological disorders associated with feline retrovirus infections

Feline oncornavirus and lentivirus infections have provided useful models to characterize the virus and host cell factors involved in a variety of marrow suppressive disorders and haematological malignancies. Exciting recent progress has been made in the characterization of the viral genotypic features involved in FeLV-associated diseases. Molecular studies have clearly defined the causal role of variant FeLV env gene determinants in two disorders: the T-lymphocyte cytopathicity and the clinical acute immunosuppression induced by the FeLV-FAIDS variant and the pure red cell aplasia induced by FeLV-C/Sarma. Variant or enFeLV env sequences also appear to play a role in FeLV-associated lymphomas. Additional studies are required to determine the host cell processes that are perturbed by these variant env gene products. In the case of the FeLV-FAIDS variant, the aberrant env gene products appear to impair superinfection interference, resulting in accumulation of unintegrated viral DNA and cell death. In other cases it is likely that the viral env proteins interact with host products that are important in cell viability and/or proliferation. Understanding of these mechanisms will therefore provide insights to factors involved in normal lymphohaematopoiesis. Similarly, studies of FeLV-induced haematological neoplasms should reveal recombination or rearrangement events involving as yet unidentified host gene sequences that encode products involved in normal cell growth regulation. These sequences may include novel protoncogenes or sequences homologous to genes implicated in human haematological malignancies. The haematological consequences of FIV are quite similar to those associated with HIV. As with HIV, FIV does not appear to directly infect myeloid or erythroid precursors, and the mechanisms of marrow suppression likely involve virus, viral antigen, and/or infected accessory cells in the marrow microenvironment. Studies using in vitro experimental models are required to define the effects of each of these microenvironmental elements on haematopoietic progenitors. As little is known about the molecular mechanisms of FIV pathogenesis, additional studies of disease-inducing FIV strains are needed to identify the genotypic features that correlate with virulent phenotypic features. Finally, experimental FIV infection in cats provides the opportunity to correlate in vivo virological and haematological changes with in vitro observations in a large animal model that closely mimics HIV infection in man.

Control of IL-6 expression and response in fibroblasts from patients with systemic sclerosis

Systemic sclerosis (SSc) is an autoimmune connective tissue disease of unknown etiology in which aberrant fibroblast function results in fibrosis of the skin and internal organs. A distinguishing feature of dermal fibroblasts cultured from SSc lesions is that they produce constitutively, i.e., without exogenous stimulation, as much as 30-fold more interleukin-6 (IL-6) than do normal fibroblasts. The present study indicates that the mechanism of constitutive IL-6 secretion involves the accumulation of IL-6 mRNA in affected SSc fibroblasts, mediated by the constitutive binding of nuclear factors to the IL-6 promoter. DNA-protein complexes formed using nuclear extracts of constitutively expressing cells are distinct from those using extracts of normal cells, with or without exogenous stimulation of IL-6; thus, the mechanisms which regulate constitutive and inducible IL-6 gene expression are apparently distinct. The data also demonstrate that dermal fibroblasts respond very rapidly to IL-6 by increasing expression of the IL-6 gene, thus suggesting a mechanism for the establishment and/or persistence of constitutive expression. The constitutive secretion of IL-6 may play an important role in the perpetuation of the local immune dysregulation and fibroblast activation in the SSc lesion.

Expression of c-myc, c-myb, and c-sis in fibroblasts from affected and unaffected skin of patients with systemic sclerosis

We examined c-sis, c-myc, and c-myb proto-oncogene expression in fibroblasts cultured from affected and unaffected skin of patients with systemic sclerosis (SSc), and from healthy donor skin. Total cellular RNA from cultured dermal fibroblasts was used in slot blot analysis and scanning densitometry or phosphorimaging to quantify steady-state levels of proto-oncogene mRNAs. PDGF B-chain levels in culture supernatants of fibroblasts were determined by ELISA. Our results demonstrate that steady-state levels of c-myc and c-myb mRNA were elevated 1.5- to 5.6-fold in intralesional fibroblasts from SSc patients as compared to other cells examined. Levels of c-sis mRNA and PDGF-B protein were comparable regardless of source. Elevated c-myc and c-myb expression may be indicative of, and may contribute to, fibroblast activation in SSc.

Human recombinant interleukin-4 induces proliferation and interleukin-6 production by cultured human skin fibroblasts

The effect of human recombinant interleukin-4 (hrIL-4) on normal human adult dermal fibroblasts in terms of proliferation and IL-6 production was studied. Fibroblasts were exposed to different concentrations of IL-4 for various periods of time. Proliferation was measured using a [3H]thymidine incorporation assay. IL-6 production was measured at the transcriptional, protein, and functional levels by Northern blot analysis, radioimmunoassay, and B9 bioassay, respectively. Our results show that hrIL-4 significantly stimulated (two- to fivefold) fibroblasts to increase the incorporation of [3H]thymidine in a dose- and time-dependent manner. However, hrIL-1, hrIL-2, hrIL-5, or hrTNF alpha, at the same concentration (100 U/ml) and for the same time period (4 days), did not. In addition, IL-4 significantly induced (four- to eightfold) the production of immunoreactive and biologically functional IL-6. However, IL-4 was not as potent an inducer of IL-6 as IL-1. The IL-4-induced IL-6 production was dose and time dependent and was due, at least in part, to a dramatic increase in the steady-state levels of IL-6 mRNA. This is the first report describing the ability of IL-4 to activate human dermal fibroblasts in terms of proliferation and IL-6 production.

Insertional mutagenesis of flvi-2 in tumors induced by infection with LC-FeLV, a myc-containing strain of feline leukemia virus

LC-FeLV is a myc-containing strain of feline leukemia virus (FeLV) which exhibits only partial transforming activity in vitro and in vivo. LC-FeLV infection in kittens may induce, but does not necessarily induce, thymic lymphosarcoma in viremic animals after a short latency. These observations suggest that infection with LC-FeLV is not sufficient to induce complete transformation and that another genetic event(s) is required. One possibility for such an event is that the integrating provirus acts as an insertional mutagen and thereby disrupts the structure or function of another proto-oncogene. Using a strategy of transposon tagging, this possibility was examined in eight feline T-cell lymphosarcomas, including four induced by experimental infection with LC-FeLV, three induced by natural infection with FeLV, and one FeLV-negative tumor. The analysis demonstrated one locus, termed flvi-2, to be structurally altered in six of the tumors examined, including three induced by LC-FeLV and three in which no activated myc oncogene is apparent. Inverse polymerase chain reaction was used to demonstrate the presence and transcriptional orientation of proviruses integrated at flvi-2 in five of these tumors. The flvi-2 locus does not hybridize to cloned probes representing 21 previously identified proto-oncogenes or common domains of retroviral integration. Thus, the data suggest that interruption of the flvi-2 locus cooperates with the myc oncogene in the induction of T-cell lymphomas by LC-FeLV; indeed, the observations indicate that the insertional mutagenesis of flvi-2 plays a role in T-cell lymphomagenesis even in the absence of feline v-myc.

Receptor-mediated leukaemogenesis: hypothesis revisited

The discovery of the first example of retroviral transduction of an immunological effector molecule has led us to reconsider the possible importance of cell surface receptors of the immune system in leukaemia development. Antigen receptors on lymphoid cells not only bind external ligands but are crucial in the control of cellular proliferation. The concept of autocrine stimulation in oncogenesis is already well established and we see no reason to exclude the possibility of analogous mechanism operating through antigen receptors. At present, we are investigating the oncogenic function of the retrovirus (FeLV-T17) carrying a T-cell receptor gene (v-tcr). In addressing the general concept of oncogenesis by ligand/receptor interactions in the immune system we face the problem of the diversity and, for T-cell antigen receptors, the complex nature of receptor-ligand interaction. Nevertheless, the implications of the model encourage us to continue to search for new experimental tools and approaches to the question.

Tumorigenic potential of a myc-containing strain of feline leukemia virus in vivo in domestic cats

The oncogenic capacity of a myc-containing strain of feline leukemia virus (FeLV), termed LC-FeLV, has been examined after inoculation of the virus into neonatal kittens. Like other myc-containing strains of FeLV, LC-FeLV may induce with relatively short latency, but does not necessarily induce, thymic lymphosarcoma in viremic animals. Naturally occurring and experimentally induced tumors are T-cell lymphomas which contain clonally integrated LC-FeLV proviral DNA and which cannot readily be cultivated in vitro in the presence or absence of exogenously supplied interleukin-2. Acquisition of myc by FeLV decreases the period of latency before the appearance of tumors but does not expand the spectrum of tumors induced by FeLV alone.

A simple method of examining retroviral species produced by infected cells

A simple and rapid method to examine the retroviral species present in the culture supernatant of productively infected cells is described. The method involves purification of viral genomic RNA directly from the culture supernatant and examination of that RNA by Northern or dot blot analysis. The method provides qualitative and quantitative information about the RNA species present and is particularly valuable for the detection of genetic variants in the population.