Experimental transmission of feline fibrosarcoma to cats and dogs

Feline Pit2 functions as a receptor for subgroup B feline leukemia viruses

Different subgroups of feline leukemia virus (FeLV) use different host cell receptors for entry. Subgroup A FeLV (FeLV-A) is the virus that is transmitted from cat to cat, suggesting that cells expressing the FeLV-A receptor are important targets at the earliest stages of infection. FeLV-B evolves from FeLV-A in the infected cat through acquisition of cellular sequences that are related to the FeLV envelope gene. FeLV-Bs have been shown to infect cells using the Pit1 receptor, and some variants can infect cells at a lower efficiency using Pit2. Because these observations were made using receptor proteins of human or rodent origin, the role that Pit1 and Pit2 may play in FeLV-B replication in the cat is unclear. In this study, the feline Pit receptors were cloned and tested for their ability to act as receptors for different FeLV-Bs. Some FeLV-Bs infected cells expressing feline Pit2 and feline Pit1 with equal high efficiency. Variable region A (VRA) in the putative receptor-binding domain (RBD) was a critical determinant for both feline Pit1 and feline Pit2 binding, although other domains in the RBD appear to influence how efficiently the FeLV-B surface unit can bind to feline Pit2 and promote entry via this receptor. An arginine residue at position 73 in VRA was found to be important for envelope binding to feline Pit2 but not feline Pit1. Interestingly, this arginine is not found in endogenous FeLV sequences or in recombinant viruses recovered from feline cells infected with FeLV-A. Thus, while FeLV-Bs that are able to use feline Pit2 can evolve by recombination with endogenous sequences, a subsequent point mutation during reverse transcription may be needed to generate a virus that can efficiently enter the cells using the feline Pit2 as its receptor. These studies suggest that cells expressing the feline Pit2 protein are likely to be targets for FeLV-B infection in the cat.

Feline vaccine-associated fibrosarcoma: an ultrastructural study of 20 tumors (1996-1999)

Twenty feline vaccine-associated sarcomas were examined by transmission electron microscopy. Tumors contained pleomorphic spindle cells, histiocytoid cells, and giant cells. Most tumors contained myofibroblasts, which had morphologic features similar to those of fibroblasts. These cells were further distinguished by subplasmalemmal dense plaques and thin cytoplasmic actin myofilaments organized as elongated bundles concentrated at irregular intervals forming characteristic dense bodies. Intracellular crystalline particulate material was found in 5 of the 20 tumors. Energy dispersive X-ray spectroscopy was used to identify the crystalline material within one tumor as aluminum-based. One tumor from a feline leukemia virus-infected cat contained budding and immature retroviral particles.

Possible retroviral origin of prion disease: could prion disease be reconsidered as a preleukemia syndrome?

A retroviral etiology might explain why amyloid plaque and/or spongiosis are or are not associated with neuronal death in prion diseases. While retroviral genes themselves may be responsible for neuronal death, a retrovirus may also cause mutations in cellular genes. Hence, the prion gene may be altered by a retrovirus in the same way as a cellular proto-oncogene is altered to produce an oncogene, either by transduction or by integration of the provirus in its vicinity. In both cases, the resulting abnormal prion protein, acting as a catalyst, may induce the formation of amyloid plaques. In addition, a wild type retrovirus may recombine to the vesicular stomatitis virus (VSV) to give rise to a pseudotyped retrovirus able to induce spongiosis. It is reported here that in scrapie, a blood monocytoid cell proliferates in vitro. If confirmed in other species, this raises the question of the potential link between prion disease and leukemia. Indeed neurovirulent strains of murine leukemia virus, a slow acting retrovirus, are known to induce spongiform encephalopathies. A preliminary attempt to purify reverse transcriptase by chromatography, using the classical protocol, failed because of the presence of a prion-like protein secreted by the blood mononuclear cells which stuck to the phosphocellulose column. Therefore, if a retrovirus is present in prion diseases, it would be evidenced only in animals developing the disease in the absence of prion protein. From this point of view, mice obtained in 1997 by the group of D. Dormont in France, offer a unique opportunity to test the retroviral hypothesis.

Possible retroviral origin of prion diseases

The proposed hypothesis that a retrovirus might be involved in the etiology of spongiform encephalopathies, integrates experimental results obtained from different fields of research. While retroviral genes themselves may be responsible for neuronal death, a retrovirus may also cause mutations in cellular genes. Hence, the prion gene may be altered by a retrovirus in the same way as a cellular proto-oncogene is altered to give an oncogene, either by transduction or by integration of the provirus in its vicinity. In both cases, the resulting abnormal prion protein, acting as a catalyst, may induce the formation of amyloid plaques. In addition, a wild type retrovirus may recombine to the vesicular stomatitis virus (VSV) to give rise to a pseudotyped retrovirus carrying the VSV G gene, known to induce spongiosis. Therefore a retroviral etiology might explain why amyloid plaque and/or spongiosis are or are not associated with neuronal death in prion diseases.

Transduction of Notch2 in feline leukemia virus-induced thymic lymphoma

Feline leukemia virus (FeLV) is thought to induce neoplastic diseases in infected cats by a variety of mechanisms, including the transduction of host proto-oncogenes. While FeLV recombinants that encode cellular sequences have been isolated from tumors of naturally infected animals, the acquisition of an unrelated host gene has never been documented in an experimental FeLV infection. We isolated recombinant FeLV proviruses encoding feline Notch2 sequences from thymic lymphoma DNA of two cats inoculated with the molecularly cloned virus FeLV-61E. Four recombinant genomes were identified, three in one cat and one in the other. Each had similar but distinct transduction junctions, and in all cases, the insertions replaced most of the envelope gene with a region of Notch2 that included the intracellular ankyrin repeat functional domain. The product of the FeLV/Notch2 recombinant provirus was a novel, truncated 65- to 70-kD Notch2 protein that was targeted to the cell nucleus. This virally encoded Notch2 protein, which resembles previously constructed, constitutively activated forms of Notch, was apparently expressed from a subgenomic transcript spliced at the normal envelope donor and acceptor sequences. The data reported here implicate a nuclear, activated Notch2 protein in FeLV-induced leukemogenesis.

Retroviruses and bone diseases

In 1980, retroviruses were shown to be pathogenic to humans, and experimentation on animals involving retroviruses as causal agents of tumors and degenerative diseases of bone, brain, and lung gained interest. Osteopetrosis, which can be either inherited in rodents or retrovirally induced in cats, is exemplary. Because of replication cycle, retroviruses can be propagated not only as infectious agents but also as cellular genes. If a retroviral infection occurs in germ line cells, the viral genes, which must integrate in the host's DNA, can be passed on to the progeny and inherited as Mendelian characteristics. Therefore, a retroviral etiology could account for diseases that present either as sporadic (infectious) or familial (inherited), although they may be similar in their clinical manifestations. This approach led to the finding of 2 new human retroviruses: 1 in a patient who had sporadic benign osteopetrosis, and the other in a patient who had sporadic paraarticular osteoma. In both patients, the retrovirus was isolated from mononuclear blood cells, not from bone cells, because of the links between bone and the immune system. A systematic search for retroviruses in patients who have sporadic bone disease, which also may appear as inherited disease, has yet to be performed. Patients with sporadic disease could be managed by antiretroviral agents such as Zidovudin.

Malignant transformation of human fibroblast strain MSU-1.1 by v-fes requires an additional genetic change

To determine whether the v-fes oncogene can malignantly transform human fibroblasts that have acquired an infinite life span and are partially growth-factor-independent, we transfected cell strain MSU-1.1 with a plasmid containing the v-fes oncogene and a bacterial histidinol dehydrogenase gene. Of the 60 independent histidinol-resistant clones isolated and assayed for v-fes expression using a fes-specific monoclonal antibody, 6 were found to express the v-fes protein at a detectable level. When progeny cells from these 6 clones were further characterized, 3 of the 6 clonal populations exhibited a significant increase in the ability to form medium-sized colonies in agarose, but none were tumorigenic in athymic mice. However, when the 6 populations were propagated for many generations, the same 3 populations acquired the ability to form very large colonies in agarose ( > or = 100 microM in diameter) at a frequency of 2% to 17%, and formed malignant tumors in athymic mice. This suggests that an additional genetic change required for malignant transformation had been spontaneously acquired in 3 of the v-fes -transformed cell strains. To determine whether the change or changes were the equivalent of an activated sis or ras proto-oncogene, we transfected the v-fes oncogene into derivative strains of MSU-1.1 that express a transfected v-sis, c-H-ras or c-N-ras oncogene, but that do not form tumors, and assayed the v-fes-expressing transfectants for tumorigenicity. The results showed that when complemented either by a ras oncogene expressed at a somewhat enhanced level or by the v-sis oncogene, v-fes can supply the additional change required for malignant transformation.

Interference with superinfection and with cell killing and determination of host range and growth kinetics mediated by feline leukemia virus surface glycoproteins

The functions of the surface glycoproteins (SU) of feline leukemia viruses (FeLVs) are of interest since these proteins mediate virus infection and interference and are critical determinants of disease specificity. In this study, we examined the biochemical and genetic determinants of SU important to virus entry and cell killing. In particular, we developed and used vesicular stomatitis virus (VSV)/FeLV pseudotype virus interference assays to determine interference subgroupings and assess mechanisms of host cell restriction. We also assessed roles of SU in virus growth kinetics and in the inhibition of cell killing caused by superinfection with cytopathic virus. Subgroup classification by VSV/FeLV pseudotype assay was in agreement with that defined previously by focus interference assay and was found to be determined by changes near the N terminus of SU for FeLV subgroups A (FeLV-A) and C. Virus host range restriction was found to be mediated at the level of virus entry in most cases, although postentry events mediated restriction in the failure of a subgroup A-like, T-cell cytopathic and immunodeficiency-inducing clone (FeLV-FAIDS-EECC) to replicate in feline fibroblasts. FeLV-FAIDS-EECC-induced cell killing was also inhibited by prior infection with one of two FeLV-A isolates. This inhibition could be conveyed by as few as four amino acid changes near the N terminus of the FeLV-A SU and also appeared to be mediated at a postentry level. Lastly, the SU-coding sequence was also found to determine differences in growth kinetics of viruses within the same subgroup. These studies demonstrate that subtle alterations in the FeLV SU, particularly in the N-terminal region, impart multiple significant functional differences which distinguish virus variants.

Colonization of a squamous cell carcinoma in the bovine horn core by Aspergillus terreus

Aspergillus terreus, an opportunistic pathogen, was found to be associated with cancerous tissue of the horn core in a weak and old debilitated cow. The organism was isolated in pure and luxurient growth from a surgically operated specimen of the horn core, and was also demonstrated in the fresh mounts and PAS stained sections of the infected tissues. The isolate showed resistance to nystatin (100 micrograms) when tested by paper disk diffusion technique. This appears to be the first report on the occurrence of A. terreus in bovine squamous cell carcinoma of the horn core.

NRC's shot in the arm for biotech

Feline leukaemia viruses (FeLV) are exogenous retroviruses that can be detected in most cats with leukaemia, aplastic anaemia, myeloproliferative diseases and fatal immunosuppression. FeLV isolates have been divided into three subgroups, based on the viral envelope-determined properties of interference and host range in vitro. FeLV-A is present in all natural isolates and is generally minimally pathogenic. FeLV-B is found with FeLV-A in isolates from approximately 40% of natural infections and in a higher percentage of cats with lymphoma. Following the fundamental observations of genetic reassortment of avian retroviruses with endogenous viral genes and the origination of lymphomagenic viruses during the ontogeny of AKR mice, we show here that transfection of feline cells with FeLV-A DNA results in its recombination with endogenous FeLV-related sequences to produce viruses with the structural and host range properties of FeLV-B. Thus in vitro propagation of a retrovirus may result in the generation of variants with very different properties.

An overview of retrovirus replication and classification

This introductory chapter has presented an overview of how retroviruses replicate and how they are classified within the family Retroviridae. The genomic structure of retroviruses, so reminiscent of bacterial transposons and other similar genetic elements, and reverse transcriptase, which leads to the reverse flow of genetic information from RNA to DNA, are responsible for many of the properties of these viruses which make them both fascinating and important as causes of cancer and other diseases. The requirement for integration shared by most retroviruses leads directly to most of the phenomena resulting from their interaction with target cells. Certainly latency, at the level of the organism, is one such property relevant to how we think of vaccines and therapeutic reagents. The ability of retroviruses to acquire oncogenes from cellular DNA has greatly facilitated our understanding of the genetics of neoplasia. Additionally, the use of retroviral vectors to introduce new genes into genetically defective animals is a consequence of the genetic organization of retroviruses. Classification of viruses at the species level is difficult for several reasons. In particular, viruses do not sexually reproduce in any conventional sense, and it is difficult to identify a population of virions which make up a genetically distinct pool. Thus, the definition of individual species is often controversial and is not necessarily aided by the criteria used to define larger phylogenetic groups. In the latter case, retroviruses have distinctive morphological and biochemical features which allow their classification at the family, subfamily, genus, and subgenus levels. Additional classification occurs by accounting for factors such as host range, cross neutralization, ability to compete in interspecies radioimmunoassays, and genetic homology detected by hybridization under conditions of relaxed stringency. Direct comparison of nucleotide sequences offers the hope that mathematical criteria will be developed that can define the level of differences characteristic of individual species, genuses, and subfamilies.

Structural analysis of a variant clone of Snyder-Theilen feline sarcoma virus

A variant clone of Snyder-Theilen feline sarcoma virus (ST-FeSV) encoding a polyprotein with a molecular weight of approximately 104 kDa (P104) was compared to the P85 encoding prototype clone of ST-FeSV. Analysis of chimeric genes constructed with the viral oncogenes of the two clones indicated that the variant clone coded for a larger polyprotein than the prototype clone because of genetic differences in its 3' portion. Comparative DNA sequence analysis revealed that one nucleotide just upstream of the termination condon TGA in the prototype proviral DNA was deleted from the variant clone resulting in a 468-bp larger open reading frame. Furthermore, it appeared that the U3 regions of the long terminal repeats (LTRs) of the variant clone contained an insertion of 71 bp as compared to the LTRs of the prototype clone. In addition, both clones differed also from each other with respect to genetic sequences deleted from their env gene regions.

Structure of the feline c-fes/fps proto-oncogene: genesis of a retroviral oncogene

The nucleotide sequence of the feline c-fes/fps proto-oncogene was analyzed. Comparison with v-fes and v-fps revealed that all v-fes/fps homologous sequences were dispersed over 11 kilobase pairs in 19 interspersed segments. All segments, numbered exon 1 to exon 19 as in the chicken and human loci, were flanked by consensus splice junctions. The putative promoter region contained a CATT sequence and three CCGCCC motifs which were also found in the human locus at similar positions. About 200 nucleotides downstream of a translational stop codon in exon 19, a putative poly(A) addition signal was identified. Using the putative translation initiation codon in exon 2, a 93,000-molecular-weight protein could be deduced. This protein resembled very well the putative protein of the human c-fes/fps proto-oncogene (94% overall homology) and, although less well, the putative protein of the chicken c-fes/fps proto-oncogene (70% overall homology). As far as the feline c-fes/fps proto-oncogene sequences transduced to the Gardner-Arnstein (GA) and Snyder-Theilen (ST) strains of feline sarcoma virus (FeSV) are concerned, homology in deduced amino acid sequences between the GA- and ST-v-fes viral oncogenes and the proto-oncogene was 99%. Analysis of the recombination junctions between feline leukemia virus and v-fes sequences in GA- and ST-FeSV proviral DNA revealed for the left-hand junction the involvement of homologous recombination, presumably at the DNA level. The right-hand junction, which appeared identical in the GA-FeSV and ST-FeSV genomes, could have been the result of a site-specific recombination at the RNA level.

Monoclonal antibodies to the v-fes product and to feline leukemia: virus P27 interspecies-specific determinants encoded by feline sarcoma viruses

Monoclonal antibodies to p27 gag and v-fes specific determinants on the gag-onc poly-protein encoded by Snyder-Theilen feline sarcoma virus (ST-FeSV) were prepared. In order to obtain hybridoma clones specific to the antigenic determinants encoded by the FeSV genome, Lou rats were immunized with ST-FeSV-transformed, virus-nonproducing syngeneic cells, and boosted with either the same cells or affinity-purified feline leukemia virus (FeLV) p27. Three distinct clones reactive to both FeLV p27 and p85gag-fes, and one clone specific for a p85fes determinant were established. The anti-p27 monoclonal antibodies also reacted with the polyproteins p95gag-fes and p83gag-fgr, from Gardner-Arnstein (GA) and Theilen-Pedersen (TP1) FeSV, respectively. The anti-p27 monoclonal antibodies reacted with MuLV p30 and RD114 p28 but not with RSV, MMTV, or BLV. These results indicated that the part of the p27 gag gene that is preserved in ST-, GA, and TP1-FeSV encodes interspecies-specific p27 determinants.

A new acute transforming feline retrovirus with fms homology specifies a C-terminally truncated version of the c-fms protein that is different from SM-feline sarcoma virus v-fms protein

The HZ5-feline sarcoma virus (FeSV) is a new acute transforming feline retrovirus which was isolated from a multicentric fibrosarcoma of a domestic cat. The HZ5-FeSV transforms fibroblasts in vitro and is replication defective. A biologically active integrated HZ5-FeSV provirus was molecularly cloned from cellular DNA of HZ5-FeSV-infected FRE-3A rat cells. The HZ5-FeSV has oncogene homology with the fms sequences of the SM-FeSV. The genome organization of the 8.6-kilobase HZ5-FeSV provirus is 5' delta gag-fms-delta pol-delta env 3'. The HZ5-and SM-FeSVs display indistinguishable in vitro transformation characteristics, and the structures of the gag-fms transforming genes in the two viruses are very similar. In the HZ5-FeSV and the SM-FeSV, identical c-fms and feline leukemia virus p10 sequences form the 5' gag-fms junction. With regard to v-fms the two viruses are homologous up to 11 amino acids before the C terminus of the SM-FeSV v-fms protein. In HZ5-FeSV a segment of 362 nucleotides then follows before the 3' recombination site with feline leukemia virus pol. The new 3' v-fms sequence encodes 27 amino acids before reaching a TGA termination signal. The relationship of this sequence with the recently characterized human c-fms sequence has been examined. The 3' HZ5-FeSV v-fms sequence is homologous with 3' c-fms sequences. A frameshift mutation (11-base-pair deletion) was found in the C-terminal fms coding sequence of the HZ5-FeSV. As a result, the HZ5-FeSV v-fms protein is predicted to be a C-terminally truncated version of c-fms. This frameshift mutation may determine the oncogenic properties of v-fms in the HZ5-FeSV.

Characterization of human c-fes/fps reveals a new transcription unit (fur) in the immediately upstream region of the proto-oncogene

Comparison of nucleotide sequence data of the 5' region of a fes/fps viral oncogene with those of the v-fes/fps homologous regions of man and cat revealed the position of the 3' portion of an as yet unidentified c-fes/fps exon. Comparative Southern blot and heteroduplex analysis of human and feline DNA immediately upstream of the v-fes/fps homologous regions showed extensive but discontinuous homology over a 9 kbp DNA stretch, which we have designated as fur. Northern blot analysis of mRNA from KG-1 myeloid cells with fes/fps- or fur-specific probes revealed a 3.0 kb fes/fps and a 4.5 kb fur transcript. Analysis of a number of tissues of an adult Wistar Lewis rat for the presence of fur transcripts revealed its differential expression pattern. An 0.95 kbp fes/fps-related and a 2.2 kbp fur-related cDNA recombinant clone were isolated from an oligo(dT)-primed KG-1 cDNA library. Comparative nucleotide sequence analysis of the fes/fps cDNA and its human genomic counterpart indicated that the cDNA contained genetic sequences that were identical to and colinear with exon 15-19 and, furthermore, that the poly(A) addition signal near the 3' end of exon 19 was functional. Similar analysis of the 2.2 kbp fur cDNA indicated that the poly(A) addition signal of the fur transcript was in close proximity of the newly discovered fes/fps exon. The region in between contained a CATT sequence but no 'TATA' box. The fur transcript was characterized by a long noncoding region at its 3' end.

A new acute transforming feline retrovirus and relationship of its oncogene v-kit with the protein kinase gene family

A new acute transforming feline retrovirus, the Hardy-Zuckerman 4 feline sarcoma virus (HZ4-FeSV), has been isolated from a feline fibrosarcoma. The viral genome of HZ4-FeSV contains a new oncogene designated v-kit, has the structure 5' delta gag-kit-delta pol-delta env 3' and specifies a gag-kit polyprotein of relative molecular mass 80,000. The predicted kit amino-acid sequence displays partial homology with tyrosine-specific protein kinase oncogenes. HZ4-FeSV appears to have been generated by transduction of feline c-kit sequences with feline leukaemia virus.

Molecular cloning of the feline c-fes proto-oncogene and construction of a chimeric transforming gene

The feline c-fes proto-oncogene, different parts of which were captured in feline leukemia virus (FeLV) to generate the transforming genes (v-fes) of the Gardner-Arnstein (GA) strain of feline sarcoma virus (FeSV) and the Snyder-Theilen strain (ST) of FeSV, was cloned and its genetic organization determined. Southern blot analysis revealed that the c-fes genetic sequences were distributed discontinuously and colinearly with the v-fes transforming gene over a DNA region of around 12.0 kb. Using cloned c-fes sequences, complementation of GA-FeSV transforming activity was studied. Upon replacement of the 3' half of v-fesGA with homologous feline c-fes sequences and transfection of the chimeric gene, morphological transformation was observed. Immunoprecipitation analysis of these transformed cells revealed expression of high Mr fusion proteins. Phosphorylation of these proteins was observed in an in vitro protein kinase assay, and tyrosine residues appeared to be involved as acceptor amino acid.

Transcription and expression of the herpes simplex virus tk gene inserted into proviral sequences of feline leukemia virus

Recombinant DNA molecules containing the herpesvirus tk gene inserted near the middle of a cloned feline leukemia virus proviral genome, in the same transcriptional orientation as the long terminal redundancies (LTRs), were used to transform human tk- cells. Analysis of RNA from cloned lines indicates that the 5' LTR promotes a high level of transcription which, as a result of differing RNA splicing and polyadenylation pathways, results in three large, abundant RNAs, two of which contain the entire tk coding region. The tk promoter itself initiates transcription of a smaller, relatively rare tk mRNA, of the same length and abundance as found in cells transformed with the tk gene alone. Assays indicate that there is little if any thymidine kinase (TK) enzymatic activity contributed by the abundant LTR-promoted transcripts. This is presumably due to inefficient initiation of tk translation from the longer LTR-initiated transcripts because of upstream AUG codons in the viral sequences. RNA blots indicate that the viral LTR is stronger as a promoter than the tk promoter. The results also indicate that about one-third of the LTR-initiated transcripts are polyadenylated at the tk poly(A) site, while the rest use the poly(A) site of the 3' LTR.

Cytotoxic immune response of puppies to feline sarcoma virus induced tumors

The cytotoxic immune response of puppies to feline sarcoma virus induced tumors was studied. Neonatal puppies were compared with adolescent dogs. Three different types of cytotoxicity were investigated: complement dependent cytotoxicity, T-cell-mediated cytotoxicity and antibody dependent cellular cytotoxicity. The relationship between the spontaneous regression of the sarcoma and the development of the immune system of the puppies is discussed.

Nucleotide sequence of the gag gene and gag-pol junction of feline leukemia virus

The nucleotide sequence of the gag gene of feline leukemia virus and its flanking sequences were determined and compared with the corresponding sequences of two strains of feline sarcoma virus and with that of the Moloney strain of murine leukemia virus. A high degree of nucleotide sequence homology between the feline leukemia virus and murine leukemia virus gag genes was observed, suggesting that retroviruses of domestic cats and laboratory mice have a common, proximal evolutionary progenitor. The predicted structure of the complete feline leukemia virus gag gene precursor suggests that the translation of nonglycosylated and glycosylated gag gene polypeptides is initiated at two different AUG codons. These initiator codons fall in the same reading frame and are separated by a 222-base-pair segment which encodes an amino terminal signal peptide. The nucleotide sequence predicts the order of amino acids in each of the individual gag-coded proteins (p15, p12, p30, p10), all of which derive from the gag gene precursor. Stable stem-and-loop secondary structures are proposed for two regions of viral RNA. The first falls within sequences at the 5' end of the viral genome, together with adjacent palindromic sequences which may play a role in dimer linkage of RNA subunits. The second includes coding sequences at the gag-pol junction and is proposed to be involved in translation of the pol gene product. Sequence analysis of the latter region shows that the gag and pol genes are translated in different reading frames. Classical consensus splice donor and acceptor sequences could not be localized to regions which would permit synthesis of the expected gag-pol precursor protein. Alternatively, we suggest that the pol gene product (RNA-dependent DNA polymerase) could be translated by a frameshift suppressing mechanism which could involve cleavage modification of stems and loops in a manner similar to that observed in tRNA processing.

Isolation of a feline leukaemia provirus containing the oncogene myc from a feline lymphosarcoma

The molecular mechanism by which feline leukaemia virus (FeLV) induces lymphoid malignancy in domestic cats is largely unknown. Using the insertional activation model of c-myc by avian leukosis virus in the induction of bursal lymphoma in chickens, we have now characterized the c-myc locus in feline tissues and investigated the possibility that FeLV may also insert within feline c-myc. We used the 1.5 kilobase (kb) PstI fragment of MC29 v-myc in Southern blot analysis to characterize the structure of the c-myc locus in the DNA of 31 naturally occurring feline lymphomas. Analysis of a cloned c-myc gene from one lymphoma demonstrated that sequences homologous to v-myc occupy 2.6 kb of feline DNA in which a putative intron of 0.5 kb separates sequences homologous to the 5' and 3' exons represented in avian v-myc. We also observed in the DNA of this lymphoma tumour-specific fragments homologous to v-myc. Characterization of these molecularly cloned myc-hybridizing fragments revealed the presence of at least two identical FeLV proviruses 5.5 kb in length, each containing long terminal repeats enclosing a spliced version of the feline myc gene.

Myristylation of gag-onc fusion proteins in mammalian transforming retroviruses

Four cell lines producing transforming proteins encoded by three mammalian oncogenes (fes, abl, and ras) were investigated for incorporation of [3H]myristate into gag-onc fusion proteins. Using 5-min pulse-labelings, fusion proteins of Abelson murine leukemia virus, Gardner-Arnstein strain of feline sarcoma virus (FeSV), and Snyder-Theilen strain of FeSV were shown to be myristylated. In a 4-hr pulse, p29gag-ras of rat sarcoma virus (RaSV) was also shown to incorporate radiolabel. The fatty acid was recovered from this labeled protein by acid hydrolysis, and identified by reverse-phase thin-layer chromatography to be [3H]myristic acid. The results indicate that substitution of viral gag sequences by cellular oncogene sequences does not abolish their ability to become post-translationally modified by this long chain fatty acid (A. Schultz and S. Oroszlan, J. Virol. 46, 355-361). It is assumed that in the fusion proteins the myristyl moiety is linked through an amide linkage to the amino-terminal glycine as previously found for several retroviral gag precursor polyproteins (L. E. Henderson, H. C. Krutzsch, and S. Oroszlan, Proc. Natl. Acad. Sci. USA 80, 339-343). The possible role of myristylation of transforming proteins is discussed.

Isolation of a new feline sarcoma virus (HZ1-FeSV): biochemical and immunological characterization of its translation product

A new strain of feline sarcoma virus, designated HZ1-FeSV, was isolated from a 4-year-old domestic cat with multicentric fibrosarcoma. A primary tumor cell line was established and virus produced from that line was found to induce foci in feline embryonic lung fibroblasts (FLF3) and mink lung fibroblasts (CCL64) in tissue culture and fibrosarcomas in inoculated 10-week-old kittens. The derivation of transformed nonproducer clones of FLF3 and CCL64 cells containing helper virus-rescuable, focus-forming activity indicated that HZ1-FeSV was defective for replication. The only discernible translation product of the HZ1-FeSV genome in cultured cells was a 100,000-Da polyprotein (P100) which contained amino-terminal sequences of the FeLV gag gene precursor protein covalently linked to a sarcoma virus-specific domain. Immunoprecipitates containing P100 exhibited a protein kinase activity capable of phosphorylating tyrosine residues of P100. Immunologically, P100 was highly cross-reactive with gag-fes polyproteins encoded by two previously characterized strains of FeSV, the GA- and the ST-FeSV. By comparison of methionine-containing tryptic peptides, the HZ1-FeSV protein was shown to be more closely related to the GA-FeSV protein than to the ST-FeSV protein, but to be distinguishable from both other proteins.

Nucleotide sequence of the feline retroviral oncogene v-fms shows unexpected homology with oncogenes encoding tyrosine-specific protein kinases

The nucleotide sequence encoding the transforming polyprotein of the McDonough strain of feline sarcoma virus was determined. This sequence includes 231 nucleotides specifying a leader peptide, 1,377 nucleotides encoding most of the feline leukemia virus-derived gag gene, and 2,969 nucleotides representing the viral transforming gene v-fms. A single open reading frame was predicted to encode a fusion polyprotein of 160,000 daltons (P160gag-fms). Fourteen potential sites for glycosylation were predicted within the v-fms-encoded portion of the protein, consistent with previous observations that the primary translation product is rapidly glycosylated. The presence of hydrophobic signal peptides within the amino-terminal leader sequence and in the middle of the v-fms-encoded moiety suggests that the transforming glycoprotein becomes oriented with its amino terminus within the lumen of the rough endoplasmic reticulum and its carboxyl terminus protruding across the membrane of the rough endoplasmic reticulum into the cytoplasm. The latter portion of the protein shows unexpected homology to tyrosine-specific protein kinases encoded by several of the known retroviral oncogenes.

Feline cytotoxic immune mechanisms against virus-associated leukemia and fibrosarcoma

Humoral and cellular cytotoxic immune mechanisms of cats were compared against feline leukemia virus (FeLV)- and feline sarcoma virus (FeSV)-transformed cells. The groups of animals studied were nonexposed control cats; FeLV-infected immune or viremic tumor-bearing cats; FeSV-inoculated tumor progressor or regressor cats, and cats immunized with FeSV-transformed autochthonous fibroblasts (ATF). Sera containing complement-dependent antibodies (CDA), which lysed FeLV-producer lymphoma lines, had no cytotoxic effects when tested against FeLV-producer FeSV-transformed fibroblasts. Sera with lytic CDA activity were also tested for antibody-dependent cellular cytotoxic (ADCC) effects with peripheral blood lymphocytes (PBL) from nonimmune cats. No ADCC activity was detected against either lymphoid or fibroblast target lines. To demonstrate that cat PBL contained ADCC effector cells, antibody-coated murine target cells were employed and positive results obtained. Natural killer (NK) assays were performed using PBL from normal and tumor-bearing cats. Cytotoxic effects were only detectable to FeLV-producer lymphomas, and comparable levels of NK activity were found in normal and lymphoid tumor-bearing animals. In cats immunized with ATF, a population of effector cells was found in peripheral blood which had functional characteristics of cytotoxic T lymphocytes (CTL). The killing of ATF by CTL-like cells was not inhibited by FeLV/FeSV immune sera or by sera from autochthonous immune cats. The comparative importance of humoral and cellular cytotoxic mechanisms against FeLV- and FeSV-induced tumors is discussed.

The Hardy-Zuckerman 2-FeSV, a new feline retrovirus with oncogene homology to Abelson-MuLV

There is substantial evidence that oncogenes (v-onc) of acute transforming retroviruses have been acquired by transduction of cellular genes (c-onc) with retroviruses. Feline leukaemia virus (FeLV)-associated feline fibrosarcomas have proven to be extremely useful for the isolation of acute transforming retroviruses of a mammalian species. Three different v-onc genes have been identified in five acute transforming feline retroviruses. The Susan McDonough feline sarcoma virus (SMFeSV) contains the oncogene fms (ref. 4). The Snyder-Theilen (ST) and Gardner-Arnstein (GA) FeSVs contain the oncogene fes (ref. 4), which is homologous to the oncogene fps of the avian sarcoma viruses FSV, RRCII, PRCIV and 16L (refs 7, 8). The v-onc sequences of the Parodi-Irgens (PI) FeSV have recently been found to be homologous with the v-sis sequences of the simian sarcoma virus. We report here the isolation of another acute transforming feline retrovirus from a naturally occurring feline fibrosarcoma, designated the Hardy-Zuckerman 2 feline sarcoma virus (HZ2-FeSV) and demonstrate that the HZ2-FeSV and Abelson murine leukaemia virus (A-MuLV) have homologous oncogenes.

Mutant feline sarcoma proviruses containing the viral oncogene (v-fes) and either feline or murine control elements

The sequences required for transformation by the Gardner-Arnstein (GA) strain of feline sarcoma virus (GA-FeSV) were defined by site-directed, in vitro mutagenesis of molecularly cloned proviral DNA. Portions of the Ga-FeSV provirus, subcloned in the plasmid pBR322, were mutagenized by deletion or frameshift at XhoI restriction sites flanking the nucleotide sequences presumed to encode the GA-FeSV transforming polyprotein (P108(gag-fes)). The biological activity of subgenomic and reconstructed full-genome-length molecules was assayed by transfection and focus induction in NIH 3T3 cells. Both mutant and wild-type molecules containing the intact P108(gag-fes) coding region induced foci of transformed cells at efficiencies between 10(4) and 10(5) focus-forming units per pmol of DNA; a deletion mutant lacking 3'-terminal v-fes sequences was completely nontransforming in parallel assays. Representative subcloned foci of transformed NIH 3T3 cells synthesized P108(gag-fes) with associated in vitro protein kinase activity. Focus-forming viruses could be rescued from transformed subclones induced by full-length proviral DNA, but not from cells transformed by subgenomic DNA lacking a 3' long terminal repeat (LTR). It was concluded that: (i) nucleotide sequences encoding P108(gag-fes) and its associated kinase activity are responsible for transformation, (ii) the GA-FeSV 3' env and LTR sequences are not required for focus induction, and (iii) the 3' LTR is necessary for rescue of infectious FeSV RNA. A chimeric DNA containing the 5' LTR and P108(gag-fes) coding region of GA-FeSV joined to the 3' LTR of Moloney murine sarcoma virus was both transforming and rescuable at high efficiency. Restriction analysis showed that passaged stocks of rescued transforming virus contained Moloney murine sarcoma virus U3 sequences at both proviral DNA termini, consistent with generally accepted models for LTR formation during reverse transcription. Wild-type GA-FeSV and the chimeric virus (here designated as GAHT), each rescued from NIH 3T3 cells with the same amphotropic murine leukemia virus, yielded approximately equal numbers of foci when titrated on CCL 64 mink cells. By contrast, on mouse NIH 3T3 cells, the focus-forming titer of GAHT was 1 to 2 log higher than that of FeSV. The foci induced on NIH 3T3 cells by GAHT appeared earlier and were reproducibly larger than those induced by GA-FeSV. Differences in transforming activity on NIH 3T3 cells were also found using colony formation in agar, showing that the more rapid appearance and larger size of foci formed in liquid media were not due to virus spread. These data suggest that transcriptional control signals within the viral LTR regulate the levels of the transforming gene product in a species-specific manner.

Analysis of the primary translational product and integrated DNA of a new feline sarcoma virus, GR-FeSV

The Gardner-Rasheed strain of feline sarcoma virus (GR-FeSV), is a recent isolate of a naturally occurring cat sarcoma. The primary translational product of GR-FeSV (GR P70) was shown to be a phosphoprotein with associated tyrosine-specific protein kinase activity. The relationship between the GR-FeSV provirus and once genes of other transforming retroviruses known to code for tyrosine kinases was examined by molecular hybridization. Probes repesenting onc genes of Snyder-Theilen and McDonough strains of feline sarcoma virus, Rous sarcoma virus, and Abelson murine leukemia virus did not detectably hybridize integrated GR-FeSV. These findings suggest that GR-FeSV contains a distinct tyrosine kinase-coding onc gene.

Mink type C virus: biochemical characterization of the structural polypeptides

Mink type C virions contained six major protein species of approximate M.W. of 90,000, 70,000, 30,000, 15,000, 12,000 and 10,000. The two largest polypeptides were glycosylated and the 12,000 M.W. polypeptide was the major phosphoprotein of the virion. Two-dimensional tryptic peptide map of the 30,000 M.W. major structural protein of MiLV showed a pattern distinct from those of analogous proteins from mouse and endogenous cat type C viruses. Significant peptide homology of this protein was, however, found with the corresponding protein of infectious feline type C virus (FeLV).

Active immunization with feline leukemia virus envelope glycoprotein suppresses growth of virus-induced feline sarcoma

The efficiency was examined of immunization with feline leukemia virus glycoprotein complexes (gp85 rosettes) to protect cats against tumors induced by feline sarcoma virus (FeSV). The glycoprotein was isolated from feline leukemia virus (FeLV). Young cats were vaccinated with the purified viral glycoprotein and challenged with FeSV (FeLV). FeLV gp85 antibody levels were measured by enzyme-linked immunosorbent assay and tumor volumes were determined. In immunized animals tumor development was reduced. Gp85 antibody levels before challenge were correlated inversely with tumor size (r2 = 0.79). This method appears to be suitable for fast and efficient testing of future FeLV vaccines.

Feline sarcoma virus-specific transformation-related proteins and protein kinase activity in tumor cells

Polyproteins (gag-fes) encoded by the Synder-Theilen (ST) and the Gardner-Arnstein (GA) strains of feline sarcoma virus (FeSV) were previously shown to be associated with mink or rat cells that were nonproductively transformed in vitro. In the present study we demonstrated that the same gag-fes proteins were found in cat cells transformed in vitro. Of greater importance, these transformation-related proteins were also in cells taken from fresh biopsies of FeSV-induced tumors. Cells from fibrosarcomas induced with ST-FeSV had gag-fes proteins that were characteristic of this strain. Fibrosarcomas and melanomas were induced with GA-FeSV and both types of tumors contained the protein that is characteristic of cells transformed in vitro with this virus. Expression of these proteins in cultured tumor cells appeared to be independent of the passage level. Based on two-dimensional tryptic peptide analysis, the gag-fes proteins of cat tumor cells appeared to be indistinguishable from those found in cells transformed in vitro. The polyproteins of the cat tumor cells have a closely associated protein kinase activity, as demonstrated in the in vitro assay, and phosphorylated tyrosine residues. Gag-fes proteins of either the ST or GA class were not present in cell cultures initiated from five spontaneous cat tumors.

Similar transforming growth factors (TGFs) produced by cells transformed by different isolates of feline sarcoma virus

Fisher rat embryo cells transformed by each of three independent isolates of feline sarcoma virus (FeSV) are shown to release transforming growth factors (TGFs) into cell culture medium. These acid- and heat-stable peptides compete for binding to, and stimulate phosphorylation of, EGF membrane receptors and promote anchorage-independent cell growth. Cells transformed by the Gardner and Synder-Theilen strains of FeSV produce high titers of TGF (60-200 ng eq EGF/liter) while cells transformed by McDonough FeSV produce TGF at only low levels (less than 10 ng eq EGF/liter). Growth factors produced by cells transformed by each of the three FeSV isolates functionally and biochemically resemble each other, mouse sarcoma growth factor (SGF), and TGFs produced by human tumor cells.

Naturally occurring retroviruses (RNA tumor viruses). II

This is the second of two papers describing the biology of the naturally occurring RNA tumor viruses (oncoviruses). It will appear in two parts. In the first paper [Cancer Investigation 1(1):67-83 (1983)] the general properties of this class of viruses and the biology of the retroviruses of the "lower" vertebrates was discussed. In this paper the oncoviruses of the "higher" animals are described. Part one deals with cat retroviruses.

Nucleotide sequences of feline sarcoma virus long terminal repeats and 5' leaders show extensive homology to those of other mammalian retroviruses

The nucleotide sequences of the Gardner-Arnstein feline sarcoma virus (FeSV) long terminal repeat and the adjacent leader sequences 5' to the viral gag gene were determined. These were compared with homologous portions of Synder-Theilen FeSV and with previously published sequences for Moloney murine sarcoma virus and simian sarcoma virus proviral DNA. More than 75% of the residues in the FeSV R and U5 regions were homologous to sequences within the same regions of the other viral long terminal repeats. Unexpectedly, alignment of the FeSV sequences with those of the Moloney murine sarcoma and simian sarcoma viruses showed similar extents of homology within U3. The homologous U3 regions included the inverted repeats, a single set of putative enhancer sequences, corresponding to a "72-base-pair" repeat, and sequences, including the CAT and TATA boxes, characteristic of eucaryotic promotors. The 5' leader sequences of both FeSV strains included a binding site for prolyl tRNA and a putative splice donor sequence. In addition, the FeSV leader contained a long open reading frame which was adjacent to and in phase with the ATG codon at the 5' end of the FeSV gag gene. The open reading frame could code for a signal peptide of about 7.4 kilodaltons. Our results support the concept that the virogenic portions of both FeSV and simian sarcoma virus were ancestrally derived from viruses of rodent origin, with conservation of regulatory sequences as well as the viral structural genes.

Monoclonal antibodies to feline sarcoma virus gag and fes gene translational products

A series of hybridomas have been isolated which produce monoclonal antibodies directed against polyprotein gene products of the Gardner, Snyder-Theilen, and McDonough strains of FeSV. Within these are representatives of several immunoglobulin classes including IgG1, IgG2a, IgG2b, and IgM. Antibody produced by one hybridoma recognizes immunologic determinants localized within an FeLV gag gene structural component (p15) common to polyproteins encoded by all three FeSV isolates whereas antibody produced by a second is specific for p30 determinants unique to P170gag-fms. Additional hybridomas secrete antibody directed against v-fes-encoded determinants common to the Gardner and Snyder-Theilen FeSV-encoded polyproteins. GA P110gag-fes and ST P85gag-fes immuno-precipitated by antibody directed against p15 exhibit tyrosine-specific protein kinase activity but lack such activity when precipitated by antibody specific for their acquired sequence (v-fes) components.

Association of the transforming proteins of the ST and GA strains of feline sarcoma virus and their in vitro associated protein kinase activities with cellular membranes

The translation products of the Snyder-Theilen (ST) and Gardner-Arnstein (GA) strains of feline sarcoma virus (FeSV), termed gag-fes proteins, are high molecular weight polyproteins containing different amounts of the amino terminus of the feline leukemia virus (FeLV) gag gene-coded precursor protein linked to a similar sarcoma virus-specific polypeptide. Both polyproteins are phosphoproteins with indistinguishable in vitro associated tyrosine-specific protein kinase activities. The polyproteins are extremely hydrophobic proteins which are intimately associated with the plasma membrane fraction of transformed cells. Approximately 10% of the proteins are modified by glycosylation and expressed on the cell surface where they are accessible to lactoperoxidase-mediated radio-iodination and trypsinization. Cell surface localization of the polyproteins does not appear to be necessary for transformation. However, preliminary evidence suggests that the amount of FeLV p30 sequences at the amino end of the proteins may have some effect on the intracellular distribution of the gag-fes polyproteins and on the phenotype of the transformed cell.

Human gene (c-fes) related to the onc sequences of Snyder-Theilen feline sarcoma virus

The onc gene (v-fes) of the acutely transforming feline sarcoma virus (Snyder-Theilen strain) has homologous cellular sequences (c-fes) in all vertebrate species, including humans. We isolated from a human DNA library recombinant phages containing overlapping c-fes sequences. The human c-fes locus spans a region of 3.4 kilobases and contains 1.4 kilobases of DNA homologous to the viral onc sequence interspersed with three intervening sequences.

Nucleotide sequences of feline retroviral oncogenes (v-fes) provide evidence for a family of tyrosine-specific protein kinase genes

The nucleotide sequences encoding the transforming polyproteins of the Snyder-Theilen and Gardner-Arnstein strains of feline sarcoma virus (FeSV) have been determined. These sequences include a viral transforming gene (v-fes), derived from cellular proto-oncogene sequences (c-fes) of domestic cats by recombination with feline leukemia virus (FeLV). The v-fes sequences are predicted to encode a polypeptide domain strikingly similar to that specified by the transforming gene (v-fps) of the avian Fujinami sarcoma virus. In addition, the 3' 0.8 kilobase pairs of v-fes encode amino acid sequences homologous to the carboxy-terminal portion of pp60src, the transforming protein encoded by the avian Rous sarcoma virus src gene. Thus different feline and avian retroviral transforming genes, all of which encode functionally related proteins with associated tyrosine-specific kinase activities, must be derived from divergent members of the same proto-oncogene family.

Human gene (c-fes) related to the onc sequences of Snyder-Theilen feline sarcoma virus

The onc gene (v-fes) of the acutely transforming feline sarcoma virus (Snyder-Theilen strain) has homologous cellular sequences (c-fes) in all vertebrate species, including humans. We isolated from a human DNA library recombinant phages containing overlapping c-fes sequences. The human c-fes locus spans a region of 3.4 kilobases and contains 1.4 kilobases of DNA homologous to the viral onc sequence interspersed with three intervening sequences.

Isolation of 16L virus: a rapidly transforming sarcoma virus from an avian leukosis virus-induced sarcoma

We have isolated a replication-defective rapidly transforming sarcoma virus (designated 16L virus) from a fibro-sarcoma in a chicken infected with td107A, a transformation-defective deletion mutant of subgroup A Schmidt-Ruppin Rous sarcoma virus. 16L virus transforms fibroblasts and causes sarcomas in infected chickens within 2 wk. Its genomic RNA is 6.0 kilobases and contains sequences homologous to the transforming gene (fps) of Fujinami sarcoma virus (FSV). RNase T1 oligonucleotide analysis shows that the 5' and 3' terminal sequences of 16L virus are indistinguishable from (and presumably derived from) td107A RNA. The central part of 16L viral RNA consists of fps-related sequences. These oligonucleotides fall into four classes: (i) oligonucleotides common to the putative transforming regions of FSV and another fps-containing avian sarcoma virus, UR1; (ii) an oligonucleotide also present in FSV but not in UR1; (iii) an oligonucleotide also present in UR1 but not in FSV; and (iv) an oligonucleotide not present in either FSV, UR1, or td107A. Cells infected with 16L virus synthesize a protein of Mr 142,000 that is immunoprecipitated with anti-gag antiserum. This protein has protein kinase activity. These results suggest that 16L virus arose by recombination between td107A and the cellular fps gene.

Chromosomal assignment of the human homologues of feline sarcoma virus and avian myeloblastosis virus onc genes

Retroviral transforming genes, v-onc genes, are derived from normal cellular sequences that are called cellular onc (c-onc) genes. DNA from mouse-human somatic cell hybrids that have selectively lost human chromosomes was used in Southern blots to map the chromosomal location of two human onc genes. Cloned human homologues of retroviral onc genes were used as probes. Because the human c-fes gene, which is homologous to feline sarcoma virus, segregates concordantly with human chromosome 15, and the human c-myb gene, which is homologous to avian myeloblastosis virus onc genes, segregates concordantly with human chromosome 6, we have assigned the c-fes and the c-myb genes to human chromosomes 15 and 6, respectively. Nonrandom chromosomal defects involving these human chromosomes have been observed in neoplasms. These studies should be valuable in determining whether specific rearrangements involving these chromosomes result in the abnormal expression of these onc genes in human malignancies.

Molecular cloning of the Fujinami sarcoma virus genome and its comparison with sequences of other related transforming viruses

Full-length proviral DNA of Fujinami sarcoma virus (FSV) of chickens was molecularly cloned and characterized. An analysis of FSV DNA integrated in mammalian cells showed that restriction endonuclease SacI has a single cleavage site on FSV DNA. Unintegrated closed circular FSV DNA obtained from newly infected cells was linearized by digestion with SacI and cloned into lambdagtWES.lambdaB. The following three different molecules were isolated: FSV-1 (4.4 kilobases [kb]) and FSV-2 (4.7 kb), which appeared to be full-length FSV DNA molecules containing either one or two copies of the long terminal repeat structure, and FSV-3 (6 kb), which consisted of part FSV DNA and part DNA of unknown origin. An analysis of the structure of cloned FSV-1 and FSV-2 DNA molecules by restriction endonuclease mapping and hybridization with appropriate probes showed that about 2.6 kb of the FSV-unique sequence called FSV-fps is located in the middle of the FSV genome and is flanked by helper virus-derived sequences of about 1.3 kb at the 5' end and 0.5 kb at the 3' end. The long terminal repeats of FSV were found to have no cleavage site for either EcoRI or PvuI. Upon transfection, both FSV-1 DNA and FSV-2 DNA were able to transform mammalian fibroblasts. Four (32)P-labeled DNA fragments derived from different portions of the FSV-fps sequence were used for hybridization to viral RNAs. We found that sequences within the 3' half of the FSV-fps gene are homologous to RNAs of PRCII avian sarcoma virus and the Snyder-Theilen strain of feline sarcoma virus, both of which were previously shown to contain transforming genes related to FSV-fps. These results suggest that the 3' portion of the FSV-fps sequence may be crucial for the transforming activity of fps-related oncogenic sequences.