Long terminal repeat regions from exogenous but not endogenous feline leukemia viruses transactivate cellular gene expression

We have previously reported that the long terminal repeat (LTR) region of feline leukemia viruses (FeLVs) can enhance expression of certain cellular genes such as the collagenase IV gene and MCP-1 in trans (S. K. Ghosh and D. V. Faller, J. Virol. 73:4931-4940, 1999). Genomic DNA of all healthy feline species also contains LTR-like sequences that are related to exogenous FeLV LTRs. In this study, we evaluated the cellular gene transactivational potential of these endogenous FeLV LTR sequences. Unlike their exogenous FeLV counterparts, neither nearly full-length endogenous FeLV molecular clones (CFE-6 and CFE-16) nor their isolated LTRs were able to activate collagenase IV gene or MCP-1 expression in transient transfection assays. We had also demonstrated previously that production of an RNA transcript from exogenous FeLV LTRs correlates with their transactivational activity. In the present study, we demonstrate that the endogenous FeLV LTRs do not generate LTR-specific RNA transcripts in the feline embryo fibroblast cell line AH927. Furthermore, infection of AH927 cells by an exogenous FeLV subgroup A virus did not induce production of such LTR-specific transcripts from the endogenous proviral genomes, although the LTR-specific transcripts from the exogenous virus were readily detected. Finally, LTR-specific transcripts were not generated in BALB/3T3 cells transiently transfected with isolated CFE-6 LTR, in contrast to transfections with LTRs from exogenous viruses. Our data thus suggest that the inability of endogenous FeLV LTRs in gene transactivation is not due to cell line specificity or presence of any upstream inhibitory cis-acting element. Endogenous, nonleukemogenic FeLV LTRs, therefore, do not transactivate cellular gene expression, and this property appears to be specific to exogenous, leukemogenic FeLVs.

Differential pathogenicity of two feline leukemia virus subgroup A molecular clones, pFRA and pF6A

F6A, a molecular clone of subgroup A feline leukemia virus (FeLV) is considered to be highly infectious but weakly pathogenic. In recent studies with a closely related subgroup A molecular clone, FRA, we demonstrated high pathogenicity and a strong propensity to undergo recombination with endogenous FeLV (enFeLV), leading to a high frequency of transition from subgroup A to A/B. The present study was undertaken to identify mechanisms of FeLV pathogenesis that might become evident by comparing the two closely related molecular clones. F6A was shown to have an infectivity similar to that of FRA when delivered as a provirus. Virus load and antibody responses were also similar, although F6A-infected cats consistently carried higher virus loads than FRA-infected cats. However, F6A-infected cats were slower to undergo de novo recombination with enFeLV and showed slower progression to disease than FRA-infected cats. Tumors collected from nine pF6A- or pFRA-inoculated cats expressed lymphocyte markers for T cells (seven tumors) and B cells (one tumor), and non-T/B cells (one tumor). One cat with an A-to-A/C conversion developed erythrocyte hypoplasia. Genomic mapping of recombinants from pF6A- and pFRA-inoculated cats revealed similar crossover sites, suggesting that the genomic makeup of the recombinants did not contribute to increased progression to neoplastic disease. From these studies, the mechanism most likely to account for the pathologic differences between F6A and FRA is the lower propensity for F6A to undergo de novo recombination with enFeLV in vivo. A lower recombination rate is predicted to slow the transition from subgroup A to A/B and slow the progression to disease.

A novel association between the human heat shock transcription factor 1 (HSF1) and prostate adenocarcinoma

A search for differentially expressed genes in a pair of nonmetastatic (PC-3) versus metastatic variant (PC-3M) human prostate carcinoma cell lines led to identification of the human heat shock factor (HSF1) as an overexpressed gene product in PC-3M cells. Analysis of primary prostate cancer specimens indicated that HSF1 is generally up-regulated in most of the malignant prostate epithelial cells relative to the normal prostate cells. Among the known effectors of HSF1 action, constitutive levels of HSP70 and HSP90 are not significantly altered by the naturally elevated expression of HSF1 as in PC-3M cells or by transduced overexpression of HSF1 in PC-3 cells. The basal levels of HSP27 in both cases are, however, consistently increased by two- to threefold. With respect to response to heat shock, high basal concentration of HSP90 is not further enhanced in these cells, and HSP70 is up-regulated irrespective of HSF1 level. Heat shock, however, causes an increase in HSP27 when HSF1 is up-regulated, except when the expression of HSF1 is already too high. These results document for the first time that HSF1 is overexpressed in human prostate cancer cells, at least one consequence of which in the prostate cancer cell lines tested is stimulation of both basal and stress-induced expression of HSP27, an important factor in cell growth, differentiation, or apoptosis.

A novel truncated env gene isolated from a feline leukemia virus-induced thymic lymphosarcoma

We PCR amplified the exogenous feline leukemia virus (FeLV)-related env gene species from lymphosarcomas induced by intradermally administered plasmid DNA of either the prototype FeLV, subgroup A molecular clone, F6A, or a new molecular clone, FeLV-A, Rickard strain (FRA). Of the nine tumors examined, six showed the presence of deleted env species of variable sizes in the tumor DNA. One env mutant, which was detected in a FRA-induced thymic lymphosarcoma, had a large internal deletion beginning from almost the N-terminal surface glycoprotein (SU) up to the middle region of the transmembrane (TM) protein of the env gene. The deduced polypeptide of this truncated env (tenv) retained the complete signal peptide and seven amino acids of the N-terminal mature SU of FRA env gene, followed by eight amino acids from the frameshift in the TM region. To study the biological function of tenv, we used a murine retrovirus vector to produce amphotropic virions. Infection of feline fibroblasts (H927), human fibrosarcoma cells (HT1080), or human B-lymphoma cells (Raji) led to pronounced cytotoxicity, while the tenv virus did not induce significant cytotoxicity to feline T-lymphoma cells (3201B) or human T-lymphoma cells (CEM). Together, these results convincingly demonstrated that the genetic events that led to truncation in the env gene occurred de novo in FeLV lymphomagenesis and that such a product, tenv could induce cytotoxicity to fibroblastic and B-lymphoid cells but not to T-lymphoid tumor cells. This type of selective toxicity might be potentially important in the development of the neoplastic disease.

Suppression of prostate carcinoma cell invasion by expression of antisense L-plastin gene

Based on the finding that gene expression for the actin-bundling protein L-plastin is inducible by androgen and that L-plastin is overexpressed in malignant epithelium of the prostate, we examined the functional consequences of L-plastin down-regulation in prostate carcinoma cell lines by both transfection and retroviral infection. We constructed retroviral vectors to express two different regions of the L-plastin gene, a 1713-bp 3'-coding portion and a 163-bp 5'-untranslated region, both in antisense orientation. Introduction of either constructs into prostate carcinoma cell lines, PC-3 and its isogenic but metastatic variant PC-3M cells, reduced the growth rates of both cell lines. In vitro invasion and motility of PC-3 and PC-3M cells were drastically suppressed (approximately 10-fold) by the expression of the antisense constructs. Evidence was obtained to indicate that L-plastin protein levels were indeed decreased by the antisense expression. The antisense construct for the 5'-untranslated region with the most unique sequence for the L-plastin gene was more effective in down-regulation efficiency compared with the larger antisense construct in the coding region, which maintains homology to other members of the plastin gene family. Cells infected with the 163-bp antisense virus, which were also tested in a nude mouse diaphragm invasion model, showed suppression of in vivo invasion of both PC-3 and PC-3M cells. These results suggested that overexpression of L-plastin might be functionally involved in prostate cancer invasion and metastasis, and raised the possibility that L-plastin gene-specific antisense delivery could potentially be a useful approach to interfere with prostate cancer progression in vivo.

Recombinant feline leukemia virus (FeLV) variants establish a limited infection with altered cell tropism in specific-pathogen-free cats in the absence of FeLV subgroup A helper virus

Feline leukemia virus subgroup B (FeLV-B) is commonly associated with feline lymphosarcoma and arises through recombination between endogenous retroviral elements inherited in the cat genome and corresponding regions of the envelope (env) gene from FeLV subgroup A (FeLV-A). In vivo infectivity for FeLV-B is thought to be inefficient in the absence of FeLV-A. Proposed FeLV-A helper functions include enhanced replication efficiency, immune evasion, and replication rescue for defective FeLV-B virions. In vitro analysis of the recombinant FeLV-B-like viruses (rFeLVs) employed in this study confirmed these viruses were replication competent prior to their use in an in vivo study without FeLV-A helper virus. Eight specific-pathogen-free kittens were inoculated with the rFeLVs alone. Subsequent hematology and histology results were within normal limits, however, in the absence of detectable viremia, virus expression, or significant seroconversion, rFeLV proviral DNA was detected in bone marrow tissue of 4/4 (100%) cats at 45 weeks postinoculation (pi), indicating these rFeLVs established a limited but persistent infection in the absence of FeLV-A. Altered cell tropism was also noted. Focal infection was seen in T-cell areas of the splenic follicles in 3/4 (75%) rFeLV-infected cats analyzed, while an FeLV-A-infected cat showed focal infection in B-cell areas of the splenic follicles. Nucleotide sequence analysis of the surface glycoprotein portion of the rFeLV env gene amplified from bone marrow tissue collected at 45 weeks pi showed no sequence alterations from the original rFeLV inocula.

Retrovirus receptor PiT-1 of the Felis catus

We isolated a cDNA encoding a feline homolog of human PiT-1, a sodium-dependent phosphate symporter which is utilized by gibbon ape leukemia virus (GALV) as a receptor for entry into host cells. The overall homology between the human and feline receptors is 92 and 93% at the nucleotide and deduced amino acid levels, respectively. Hydropathy analyses implied ten potential membrane spanning regions and, in analogy to human and murine homologs, five extracellular and four intracellular loops. Strikingly, the amino acid sequence of the fourth extracellular loop, which is critical for GALV surface glycoprotein binding, has complete identity between the human and feline PiT-1s, while the mouse PiT-1, non-functional for GALV entry, is quite divergent. Ectopic expression of the feline PiT-1 in guinea pig cells, which are non-permissive to feline leukemia virus (FeLV), subgroup B virus, conferred susceptibility to FeLV-B infection confirming the functional ability of the cloned product to serve as a receptor for a natural retrovirus of the homologous species.

Pathogenicity induced by feline leukemia virus, Rickard strain, subgroup A plasmid DNA (pFRA)

A new provirus clone of feline leukemia virus (FeLV), which we named FeLV-A (Rickard) or FRA, was characterized with respect to viral interference group, host range, complete genome sequence, and in vivo pathogenicity in specific-pathogen-free newborn cats. The in vitro studies indicated the virus to be an ecotropic subgroup A FeLV with 98% nucleotide sequence homology to another FeLV-A clone (F6A/61E), which had also been fully sequenced previously. Since subgroup B polytropic FeLVs (FeLV-B) are known to arise via recombination between ecotropic FeLV-A and endogenous FeLV (enFeLV) env elements, the in vivo studies were conducted by direct intradermal inoculation of the FRA plasmid DNA so as to eliminate the possibility of coinoculation of any FeLV-B which may be present in the inoculum prepared by propagating FeLV-A in feline cell cultures. The following observations were made from the in vivo experiments: (i) subgroup conversion from FeLV-A to FeLV-A and FeLV-B, as determined by the interference assay, appeared to occur in plasma between 10 and 16 weeks postinoculation (p.i.); (ii) FeLV-B-like recombinants (rFeLVs), however, could be detected in DNA isolated from buffy coats and bone marrow by PCR as early as 1 to 2 weeks p.i.; (iii) while a mixture of rFeLV species containing various amounts of N-terminal substitution of the endogenous FeLV-derived env sequences were detected at 8 weeks p.i., rFeLV species harboring relatively greater amounts of such substitution appeared to predominate at later infection time points; (iv) the deduced amino acid sequence of rFeLV clones manifested striking similarity to natural FeLV-B isolates, within the mid-SU region of the env sequenced in this work; and (v) four of the five cats, which were kept for determination of tumor incidence, developed thymic lymphosarcomas within 28 to 55 weeks p.i., with all tumor DNAs harboring both FeLV-A and rFeLV proviruses. These results provide direct evidence for how FeLV-B species evolve in vivo from FeLV-A and present a new experimental approach for efficient induction of thymic tumors in cats, which should be useful for the study of retroviral lymphomagenesis in this outbred species.

Downregulation of human FGF8 activity by antisense constructs in murine fibroblastic and human prostatic carcinoma cell systems

Previously, we described cloning of three alternatively spliced mRNA forms of human FGF8, a, b, and e, of which the b form is the major expressed species in both normal and tumor prostatic epithelial cells. In this report, we describe construction and overexpression of sense and antisense sequences of either the full length FGF8b coding region (215-amino acids or 215aa), 103aa N-terminal part or a smaller N-terminal region (34aa), each including the 23aa putative signal peptide domain, via a retrovirus system. While the morphologic transforming activities of the sense 215aa and 103aa constructs were similar in NIH3T3 cells, 103aa displayed reduced soft agar clonogenic activity. The 34aa construct was practically inert in these assays, although its expression could mimic the ability of 215aa or 103aa in conferring cell growth under reduced serum condition. Overexpression of any of the three constructs in antisense orientation, however, was similarly effective in reversing the morphology and anchorage-independent growth property of FGF8b-transfected NIH3T3 cells. The expression of the antisense 215aa construct significantly reduced the growth rate of the human prostatic carcinoma DU145 cells and inhibited their soft agar clonogenic activity and in vivo tumorigenicity in nude mice. Taken together, these results identify N-terminal portions of FGF8 protein isoform for having the domains necessary for one or more of the biologic effects examined, and suggest that low levels of FGF8 expressed in prostatic epithelial cells may contribute significantly to their growth and tumorigenic properties.

In vivo evolution and selection of recombinant feline leukemia virus species

Ecotropic feline leukemia viruses subgroup A (FeLV-A) is known to recombine with endogenous FeLV (enFeLV) env elements yielding polytropic FeLV-B viruses. However, scattered nucleotide differences exist between enFeLV env elements and corresponding sequences of exogenous FeLV-B isolates. To address this disparity, we examined recombinant FeLV (rFeLV) viruses obtained from three experimentally-induced feline thymic tumors, along with rFeLVs derived from one naturally-occurring thymic tumor. Two of the three experimental cats were challenged with a FeLV-A/Rickard preparation, while one cat received this FeLV-A along with a mixture of in vitro-generated rFeLVs. The FeLV-A/Rickard preparation employed in this study was shown to be free of detectable rFeLVs since no recombinant products were observed in this preparation following nested PCR analyses. For each of the four tumor DNAs, nucleotide sequence analysis was performed on multiple clones of rFeLV-specific PCR products derived from the surface glycoprotein (SU) portion of the recombinant proviral env gene. Relative to the parental enFeLV sequence used to generate the rFeLVs, a total of 19 nucleotide differences were found scattered within the SU region of the env gene in these in vivo-derived rFeLV clones. Most interestingly, this set of 19 differences led to complete sequence identity with natural FeLV-B isolates. Our results indicate these differences are present early in the in vivo evolution of recombinant viruses, suggesting that rFeLVs harboring these differences are strongly selected. We also present evidence indicating an in vivo selection pattern exists for specific recombinant species containing relatively greater amounts of enFeLV-derived SU sequence. This in vivo selection process appears to be gradual, occurring over the infection timecourse, yielding rFeLV species which have recombination structural motifs similar to those seen in natural FeLV-B isolates.

Molecular heterogeneity in prostate cancer: can TP53 mutation unravel tumorigenesis?

While prostate cancer is the most common malignant visceral neoplasm of men, its etiology remains largely unknown and its clinical course unpredictable. Molecular genetics of prostate cancer has become a fruitful area of investigation and might provide clues to understanding these phenomena. Mutation of the TP53 tumor suppressor gene (encoding the p53 protein) has been commonly reported as a critical event in human carcinogenesis, but recent findings in prostate cancer research call into question the correlation between TP53 mutation and prognosis for patients with this tumor. Whole-mount analysis has begun to address the histologic significance of the focal evolution of TP53 mutation in a pre-existing cancer and to reveal its role throughout the process of tumor progression. This model might also apply to other tumors.

p53 protein and gene alterations in pathological stage C prostate carcinoma

PURPOSE

We determined the extent of p53 immunoreactivity in pathological stage C prostate cancer as well as its correlation to tumor grade, substage, recurrence and proliferation rate. To define better the temporal relationship of p53 nuclear reactivity in prostate cancer p53 immunoreactivity was evaluated in all associated prostatic intraepithelial neoplasia lesions.

MATERIALS AND METHODS

Using immunohistochemistry p53 status and proliferation rate were determined in 96 tumors from patients with pathological stage C prostate cancer. Single strand conformational polymorphism in exons 5 to 8 was used in a subset of specimens to assess the association of p53 nuclear accumulation with mutations in the p53 gene.

RESULTS

p53 Nuclear reactivity was demonstrated in 10 tumors (10.4%), including 6 with high and 4 with low level nuclear reactivity. Of the tumors 86 (89.6%) had no evidence of p53 immunoreactivity. Each of the 6 tumors with high level p53 reactivity had associated areas of prostatic intraepithelial neoplasia that also showed p53 nuclear reactivity. Furthermore, pathological stage C substage (C1, 2 or 3) was significantly associated with p53 nuclear reactivity (p = 0.04). Proliferation rates were correlated with p53 nuclear reactivity (p = 0.09), while there was no association with tumor grade or recurrence. p53 Gene alterations were noted in 2 of the 3 p53 positive tumors versus no alterations in the p53 gene of 3 p53 negative tumors.

CONCLUSIONS

p53 Nuclear accumulation is uncommon in pathological stage C prostate cancer and its presence in premalignant prostatic intraepithelial neoplasia lesions suggests that it may be an early event in a subset of prostate cancers.

Steroid hormone induction and expression patterns of L-plastin in normal and carcinomatous prostate tissues

Application of differential display to the comparison of androgen-stimulated and unstimulated human prostate carcinoma cell line LNCaP identified androgen induction of the L-plastin gene, which encodes an actin-binding protein isoform. Further investigation demonstrated that L-plastin expression in LNCaP cells is up-regulated by both dihydrotestosterone and estradiol. This induction of expression is detected as early as 2 hours after addition of steroids to the cell culture. L-plastin expression is also detected in other prostate carcinoma cell lines by reverse transcriptase polymerase chain reaction and immunohistochemistry but not in the single normal adult prostate epithelial cell line that is available to us. Analysis of multiple primary prostatic tumor tissues as well as normal and tumor tissues of the same prostate gland showed that tumor tissues exhibit a higher level of expression as compared with the normal tissues. Immunohistochemical study using anti-L-plastin antiserum on normal and carcinomatous prostate tissues showed a very striking difference in the staining patterns. Positive staining was seen in the fibromuscular stroma in normal prostates but not in the glandular epithelial cells. In contrast, strong staining was seen predominantly within the carcinomatous glandular epithelial cells. Taken together, these results suggest that the expression of L-plastin in prostatic epithelial cells is linked to the malignant state and that, once expressed in carcinomas, its expression is regulated by steroid hormone receptors.

Molecular cloning and characterization of human FGF8 alternative messenger RNA forms

Three alternatively spliced mRNA isoforms of the human fibroblast growth factor-8 (FGF8) gene, expressed in a prostatic carcinoma cell line, have been isolated as cDNA clones and characterized by DNA sequencing. The clones, designated FGF8a, FGF8b, and FGF8e, differ from each other at the NH2-terminal region of the mature proteins and share extensive nucleotide sequence homology in the protein coding region to the corresponding mouse cDNA isoforms that were previously reported. FGF8a and FGF8b exhibit identical amino acid sequences to those of their murine counterparts. FGF8e displays partial sequence variation from the corresponding mouse clone only in the extra exon sequence found in this isoform in both species. There is extensive sequence diversity between FGF8 (human) and Fgf8 (murine) genes in the 3'-untranslated region of the mRNAs. Northern blot analyses revealed FGF8 mRNA expression only in fetal kidney tissue among the various fetal and adult human tissues tested. The reverse transcription-PCR amplification method, however, detected FGF8 mRNA expression in adult prostate, kidney, and testes (the tissues that were tested) and in all normal and tumor prostatic epithelial cell lines examined; although expression of both FGF8a and FGF8b was seen in kidney and testes, FGF8b appeared to be the predominantly expressed species in the prostatic tissue and cell lines analyzed by reverse transcription-PCR. To address the biological effect of specific isoform expression, NIH3T3 cells were transfected with a eukaryotic expression vector containing cDNA for FGF8a, FGF8b, or FGF8e. Consistent with previous reports on differences in the transforming potential of mouse FGF8 isoforms, human FGF8b was found to induce marked morphological transformation to NIH3T3 cells and strong tumorigenicity of the transfected cells in nude mice. Human FGF8a and FGF8e were moderately transforming in NIH3T3 cells, and the transfected cells were moderately tumorigenic in vivo. These results document the production of three alternatively spliced FGF8 mRNAs in human tissues and the transforming and tumorigenic potential of their protein products. Moreover, these data, combined with the tissue-specific expression of these isoforms, suggest that they may have different biological functions.

Feline leukemia virus variants in experimentally induced thymic lymphosarcomas

This study was initiated to evaluate the in vivo infectivity and pathogenicity of a group of recombinant feline leukemia viruses (rFeLVs) previously generated by in vitro forced recombination between a FeLV subgroup A virus (FeLV-A) and an endogenous FeLV (enFeLV) envelope (env) element (Sheets et al., 1992, Virology 190, 849-855). To determine infectivity of rFeLVs, neonatal cats were inoculated with rFeLVs alone or in combination with FeLV-A. The recombinant viruses were able to replicate efficiently in vivo only when administered along with FeLV-A. Of six co-infected cats, three developed thymic lymphosarcomas, one severe aplastic anemia, and two cachexia and depression; all were viremic and seroconverted shortly after inoculation. While both virus types were detected in virtually all tissues examined from these tumor-bearing cats, there was a particularly noteworthy sequence reversion in the rFeLVs. It is known that exogenous FeLV isolates carry a conserved neutralizing MGPNL epitope in the middle of the surface glycoprotein domain of the env gene. In contrast, the parental recombinant viruses used to inoculate these cats harbored the enFeLV-derived MGPNP sequence at this position. However, all in vivo-propagated recombinants displayed the MGPNL sequence, while the env-encoded backbone flanking the MGPNL sequence was that of the parental recombinant virus. These results suggest that viruses with the MGPNL epitope have an in vivo proliferative advantage. The data also provide an explanation for the conservation of this epitope in exogenous FeLVs despite the existence of variant forms in enFeLV proviral elements with which they can recombine.

Heterogeneity in intratumor distribution of p53 mutations in human prostate cancer

Prostatic carcinoma from 65 patients have been examined for the occurrence of point mutations in the p53 tumor suppressor gene locus within the region of exons 5 to 8. Overall, only a small fraction of tumors (12.3%) was found to contain p53 mutations. No significant correlation was detected between the presence of the mutant gene and either tumor volume or histopathological grade. However, metastatic prostatic tumors are found to display a higher percentage (21.4%) of p53 mutations compared with primary adenocarcinomas (9.8%). Analysis of the topographical distribution of the p53 mutant genotype revealed two remarkable findings. First, multifocal tumors within a prostate appear to differ in harboring the mutant gene, and second, evidence is obtained for intratumor heterogeneity in the distribution of the mutant p53 allele. Together these findings appear to explain, at least in part, why there has been a wide discrepancy in the reported detection frequency of p53 mutations in prostate cancer specimens. It appears that the outcome of mutation analysis would depend not only on which tumors but also which regions of the tumors are included in the study. Furthermore, the observed heterogeneous topographical distribution of the mutation, if confirmed to be unique to prostate cancer, may have important implications in the understanding of the biology of prostate carcinogenesis.

Endogenous env elements: partners in generation of pathogenic feline leukemia viruses

Feline leukemia viruses (FeLVs), which are replication-competent oncoretroviruses of the domestic cat species, are contagiously transmitted in natural environments. They are capable of inducing either acute antiproliferative disease or, after prolonged latency, lymphoid malignancies in this animal population. Current knowledge of the recombinational events between infectious FeLV and noninfectious endogenously inherited FeLV-like elements is reviewed, and the potential role of the derived recombinant viruses in pathogenesis is discussed. Major observations made are as follows: (1) Up to three fourths of the exogenous FeLV envelope glycoprotein (SU), beginning from the N-terminal end, can be replaced by sequences from an endogenous FeLV to produce biologically active chimeric FeLVs. The in vitro replication efficiency or cell tropism of the recombinants appears to be influenced by the amount of SU sequences replaced by the endogenous partner, as well as by the locus of origin of the endogenous sequences. (2) Generation of FeLV recombinants in tissue culture cells corresponds closely to the findings from natural tumors. There is direct evidence, based on molecular genetic analysis, for the prevalence of recombinant proviruses in naturally arising FeLV-induced lymphomas. (3) Certain recombinants harboring an altered primary neutralizing epitope in the middle of SU corresponding to the endogenous FeLV sequence can evade immunity developed against common FeLV infection. In several other recombinants, the epitope sequence is found to be frequently mutated during the process of recombination. (4) FeLV variants with altered epitope, although they may not be efficient in replication in vivo, apparently are capable of causing focal infection in target organs. Evidence is also presented that when coinfected with an exogenous FeLV, the epitope sequence in the variants is reverted to the exogenous type, providing an explanation why this sequence is found to be conserved in all natural isolates of FeLV. (5) A prototype chimeric polyprotein containing most of the SU from the endogenous source is abnormally processed and becomes trapped in the endoplasmic reticulum. A functional consequence of such trapping is interference with specific FeLV infection. (6) Some recombinants, while only poorly replicating in the host, may have the ability to infect target erythroid progenitor cells for the induction of strong cytopathic effect. (7) Some other recombinants appear to potentiate lymphomagenesis by exogenous FeLV and others to acquire properties to infect CNS endothelial cells, an event that could potentially be related to FeLV-induced neuropathogenicity. (8) Of multiple recombinant viruses, a specific recombinant species was found to occur in each of the three cats examined in which lymphoma was experimentally induced, and it was exclusively seen in one of these cats. This recombinant FeLV may potentially be a candidate for strong leukemogenic function. In addition to commonly encountered virus envelope changes, another prominent viral factor involved in tumorigenesis is mutated FeLV transcription regulatory sequences, most frequently with enhancer duplication or triplication. Although only a limited amount of information is available in the area of insertional mutagenesis in FeLV neoplastic disease, activation of certain key nuclear transcription factor genes has been documented.

Abnormal processing of a recombinant feline leukemia virus envelope polyprotein and its interference with subgroup C virus infection

Processing of the env polyprotein of a noninfectious feline leukemia virus (FeLV) recombinant, named r6gp, was examined in human-transfected cells. The r6gp provirus was previously generated in the frame of FeLV, subgroup B, GA clone with substitution of all but 40 C-terminal amino acid sequences of the surface glycoprotein (SU) from an endogenous FeLV provirus element (CFE-6). Although r6gp produced a normal size (85 kDa) env glycoprotein precursor, the product, unlike the precursor of the parental virus, was neither additionally glycosylated nor further processed into mature env proteins. Biochemical observations were consistent with the idea that the chimeric env polyprotein was trapped in the endoplasmic reticulum (ER) and were directly supported by immunofluorescence microscopy analyses. Interestingly, the residence of the chimeric protein in the ER specifically interfered with FeLV, subgroup C (Sarma) virus infection but not the parental FeLV-B virus infection. Since FeLV-C provirus sequences could be readily detected in the infected cells, it appeared that r6gp env expression did not block entry of the challenge virus. While FeLV-B and CFE-6 env genes share an extensive overall sequence homology, a variable region (region VI) of CFE-6 near the C-terminus of SU, which was retained in the r6gp construct, exhibits a considerably higher degree of homology to FeLV-C than FeLV-B. Thus, we propose that region VI is involved in conferring specificity for the env polyprotein oligomerization in the ER, and that co-oligomerization of the trapped r6gp env with FeLV-C is the reason for specific interference with FeLV-C infection. The results also demonstrate for the first time a functional abnormality of a recombinant FeLV env gene which is structurally similar to those commonly detected in FeLV-induced feline lymphosarcomas.

Recombination between feline exogenous and endogenous retroviral sequences generates tropism for cerebral endothelial cells

Brain tissues of domestic cats that died of aplastic anemia from infection with either parental feline leukemia virus (FeLV), subgroup C, or a mixture of FeLV-C and recombinants between FeLV-C and an endogenous FeLV provirus were examined by the immunoperoxidase staining technique using a monoclonal antibody (C11D8) directed against an epitope of the viral surface glycoprotein (SU). Positive staining of the central nervous system (CNS) capillary endothelial cells with no labeling on neuronal or glial cells was observed in cats that were inoculated with the virus mixture. This was in contrast to brain tissue of cats infected with FeLV-C alone, which showed no such staining. While non-CNS endothelial cells derived from human umbilical vein (HUVEC) could be readily infected in culture by FeLV-C, endothelial cells derived from human retina (REC) or brain (BEC) were resistant to infection by this parental virus. These latter cells in culture, however, could be infected by the viral mixture. The data suggested that at least one or more of the presumptive recombinant viruses could specifically infect CNS-derived endothelial cells. Using polymerase chain reaction and DNA sequencing strategies to amplify and analyze DNA fragments of the proviral SU region from cells infected with REC-selected viruses, we found the occurrence of a single recombinant in which two-thirds of the SU gene from the N-terminus of FeLV-C was replaced by the endogenous FeLV element. This recombinant virus, when molecularly cloned, should be useful in determining its potential in vivo neuropathogenicity.

Pathogenicity of a subgroup C feline leukemia virus (FeLV) is augmented when administered in association with certain FeLV recombinants

There is evidence to suggest that infectious feline leukemia viruses (FeLVs) may be altered biologically because of homologous recombination with non-infectious endogenous FeLV (enFeLV) sequences in the infected cells. To evaluate the role of such recombination events in FeLV pathogenesis, a molecular clone of subgroup C FeLV, Sarma strain (FSC), was tested for induction of aplastic anemia in the absence or presence of mixtures of recombinants between FSC and an enFeLV element. In the recombinants, FSC sequences in the viral surface glycoprotein (SU) protein were variably replaced by the corresponding sequences of the enFeLV. The results showed that the virus mixtures varied in their infectivity to neonatal specific pathogen-free cats. One group of mixtures, although exhibiting relatively reduced infectivity, represented the most acute disease-inducing agents. The presence of recombinants in this mixture significantly accelerated the development of erythrocyte aplasia compared to cats infected with FSC alone. In addition, infected cells appeared to be distributed differently in various hematopoietic organs with respect to infection with FSC versus viral mixture. Viral recombinants which were present in this inoculum mixture, however, could not be detected in the plasma or infected tissues of the cats at the end stage of the disease, although their presence in the plasma at the early stages could be detected. Clearly, parental FSC outgrew the recombinants in the infected animals, since its detection was prominent at all stages of the progression of the disease. Therefore, we hypothesize that recombinants initially present in the infected animals, while only poorly replicated compared to FSC in the host, might have had the opportunity to infect certain target cells (potentially erythroid progenitor cells) and then disappeared with the associated cytopathic effect.

Fusion activity dissociated from replication ability in feline immunodeficiency virus (FIV) in human cells

Multinucleated-giant-cell formation followed by cell killing was observed after cocultivation of the feline immunodeficiency virus (FIV)-producing feline T-cell line 3201/FIV with various human cells, including T-cell lines carrying human T-cell lymphotropic virus type I (HTLV-I). The susceptibility to giant cell formation varied with the cell lines tested. Cocultivation of irradiated 3201/FIV cells with MT-2 cells resulted in giant cell formation as early as 2 h in culture, with striking resemblance to that induced by human immunodeficiency virus (HIV). MT-4 cells (HTLV-I positive) and H9 cells (HTLV-I negative) were less susceptible than MT-2 to the induction of syncytia. MOLT-4 cells (HTLV-I negative) had intermediate sensitivity to syncytia formation. No syncytia were observed in the monocytic cell line U-937 (HTLV-I negative). Syncytia formation between 3201/FIV and MT-2 cells was inhibited by polyclonal cat anti-FIV antisera but not polyclonal cat anti-feline leukemia virus (FeLV) antisera, goat anti-FeLV, uninfected specific-pathogen-free cat serum, human anti-HTLV-I antisera, or normal human and goat serum. Concentrated cell-free FIV supernatant from 3201/FIV also induced giant cells of MT-2 cells that were indistinguishable from those induced by cocultivation. Giant cells and extensive cell killing associated with giant cell formation declined and disappeared within 10 days. Surviving cells appeared to be of normal size and grew continuously without expressing FIV antigen or releasing infective virus. Although Southern blot analysis using probes specific for FIV could not detect proviral DNA in any of the five human cell lines cocultured with irradiated 3201/FIV cells, the polymerase chain reaction (PCR) technique detected FIV-specific DNA in MOLT-4 cells. DNA from the FIV-PCR positive MOLT-4 cells was PCR negative for endogenous FeLV-specific sequences, indicating that the MOLT-4 cell DNA was not contaminated with DNA from feline cells (i.e., 3201 cells). The FIV-MOLT-4 cells remained PCR positive for FIV after 40 passages, suggesting stable integration in the human cell line. These findings indicate that FIV is capable of forming proviral DNA in human T-lymphoid cells by cocultivation, although this FIV-carrying human cell line failed to produce replication-competent viruses.

Coincident involvement of flvi-2, c-myc, and novel env genes in natural and experimental lymphosarcomas induced by feline leukemia virus

The flvi-2 locus is a target of insertional mutagenesis in thymic lymphosarcomas induced by feline leukemia virus (FeLV). flvi-2 encodes the gene bmi-1, whose product is implicated as a myc-collaborator in the induction of B- and T-cell lymphoma. We have examined the involvement of flvi-2 and myc in natural and experimentally induced FeLV-positive feline lymphosarcomas which are heterogeneous in anatomical origin, geographic origin, and strain of FeLV involved. We further compared these findings with previous reports of novel FeLV env genes in the same tumors. The results show that proviral insertion at flvi-2 occurs commonly in natural and experimental feline thymic lymphosarcomas of diverse origins [52% overall], and that alterations in c-myc commonly accompany insertional mutagenesis of flvi-2 [54% overall]. However, 46% of tumors with flvi-2 insertions apparently lack involvement of c-myc. These observations support the hypothesis that interruption of flvi-2 may be an early event in a multistep cascade, one possibility for completion of which is activation of c-myc. Interruption of flvi-2 was not observed in nonthymic lymphosarcomas of alimentary or multicentric origin, although c-myc may be involved. A proportion of both thymic and nonthymic tumors have been shown previously to contain FeLV proviruses with recombinant or mutant env genes. Our findings strongly implicate the insertional mutagenesis of flvi-2, the activation of c-myc, and the emergence of novel env genes in FeLV-mediated lymphomagenesis, particularly in the induction of thymic lymphosarcoma. The data show that these events may overlap, but do not necessarily occur concurrently.

North-American twins with IDDM. Genetic, etiological, and clinical significance of disease concordance according to age, zygosity, and the interval after diagnosis in first twin

In 224 twin pairs (132 monozygotic, 86 dizygotic, and 6 of uncertain zygosity) in whom the index twin had developed IDDM before 30 yr of age, 51 of the co-twins (38 monozygotic, 10 dizygotic, and 3 of uncertain zygosity) subsequently became diabetic. On the basis of concordance ratios, which were significantly discrepant (P < 0.01) between monozygotic and dizygotic twins, the substantial genetic role in IDDM etiology is confirmed. For the monozygotic co-twin of an IDDM case, the relative risk is significantly related to an early age at proband diagnosis (P < 0.01 for 0-4 vs. 5-9 yr of age). However, among monozygotic co-twins at any age, IDDM risk decreases as time passes after the proband diagnosis (P < 0.01 for 0-23 vs. > or = 24 mo after a proband diagnosis at 5-9 yr of age). Moreover, a structural-equation analysis suggests a profound contribution to liability (as much as 79%) from the twins' shared environment. Risk to like-sex male dizygotic co-twins is as high as that to monozygotic co-twins, significantly higher than that to like-sex female dizygotic co-twins (P < 0.005), and even higher than that to male co-twins in unlike-sex dizygotic pairs (P < 0.05). Overall, the risk to the dizygotic co-twin of a case is significantly higher (P < 0.001) than that to a non-twin sibling, as reported in the literature. The observed male excess is consistent with reported patterns of IDDM in experimental animals, and in certain circumstances in humans. Taken together, these observations suggest an important early acquired determinant of IDDM, independent of genetic determinants. On the basis of Kaplan-Meier IDDM-free survival curves, if the proband is diagnosed before 15 yr of age, the long-term risk to the co-twin is estimated at 44% (monozygotic) and 19% (dizygotic); it reaches 65% for the co-twin of a monozygotic proband diagnosed before 5 yr of age. An IDDM discordant period of no more than 3 yr was observed in 60% of the pairs destined to become concordant, offering a very brief window for intervention following the recognition of high risk.

Recombinant feline leukemia virus genes detected in naturally occurring feline lymphosarcomas

Using a polymerase chain reaction strategy aimed at detecting recombinant feline leukemia virus (FeLV) genomes with 5' env sequences originating from an endogenous source and 3' env sequences resulting from FeLV subgroup A (FeLV-A), we detected recombinant proviruses in approximately three-fourths of naturally occurring thymic and alimentary feline lymphosarcomas (LSAs) and one-third of the multicentric LSAs from cats determined to be FeLV capsid antigen positive by immunofluorescence assay. In contrast, only 1 of 22 naturally arising FeLV-negative feline LSAs contained recombinant proviruses, and no recombinant env gene was detected in seven samples from normal tissues or tissues from FeLV-positive animals that died from other diseases. Four preferred structural motifs were identified in the recombinants; one is FeLV-B like (recognizing that FeLV-B itself is a product of recombination between FeLV-A and endogenous env genes), and three contain variable amounts of endogenous-like env gene before crossing over to FeLV-A-related sequences: (i) a combination of full-length and deleted env genes with recombination at sites in the middle of the surface glycoprotein (SU), (ii) the entire SU encoded by endogenous-like sequences, and (iii) the entire SU and approximately half of the transmembrane protein encoded by endogenous-like sequences. Additionally, three of the thymic tumors contained recombinant proviruses with mutations in the vicinity of the major neutralizing determinant for the SU protein. These molecular genetic analyses of the LSA DNAs correspond to our previous results in vitro and support the occurrence and association of viral recombinants and mutants in vivo in FeLV-induced leukemogenesis.