Comparison of an avian osteopetrosis virus with an avian lymphomatosis virus by RNA-DNA hybridization

Induction of nephroblastoma by myeloblastosis-associated virus type 1: state of proviral DNAs in tumor cells

Myeloblastosis-associated virus type 1 (MAV1) derived from a molecular clone of infectious proviral DNA (B. Perbal, J. S. Lipsick, J. Svoboda, R. F. Silva, and M. A. Baluda, J. Virol. 56:240-244, 1985) was shown to specifically induce nephroblastoma in chickens and therefore belongs to the MAV-N class. We show that nephroblastomas are polyclonal tumors containing rearranged proviral genomes. Rearrangements occur preferentially in the gag-pol region of the MAV1 proviral genome, and similar rearrangements can be detected in well-developed independent tumors.

MAV-2-O replicates at a reduced rate in cells from the osteopetrosis resistant G-B1 chicken

The replication of the avian osteopetrosis virus MAV-2-O was compared in chick embryo fibroblasts from two strains of chicken. These were G-B1 which is relatively resistant to MAV-2-O and CB which is susceptible. The production of MAV-2-O was delayed in G-B1 cells (compared with CB cells). The same result was observed after infection with Rous sarcoma viruses of subgroups B, C, and D. In addition, the transforming viruses induced foci on G-B1 fibroblasts 24 to 48 hours later than on CB fibroblasts. In G-B1 cells there was also a delayed kinetics of intracellular viral RNA production. Integrated and linear unintegrated MAV-2-O DNA species were also present in lower amounts in G-B1 than in CB fibroblasts at 3 days postinfection. In vivo studies confirmed the in vitro situation. There was a marked difference in the amount of virus present in the osteoid bone matrix and the osteocytic lacunae of osteopetrotic bones from susceptible and G-B1 chickens. In contrast to the bone lesions from susceptible animals, budding virus particles were not detectable in lesions from G-B1 chickens. There was no difference in the amount of virus in osteopetrotic and non-osteopetrotic bone of susceptible chickens suggesting that virus replication alone is not sufficient for induction of osteopetrosis and that an additional specific virus-cell interaction is required. The relative resistance of strain G-B1 may therefore, be a consequence of a reduced frequency of this interaction. Its basis may be the lower amount of integrated, as well as unintegrated, viral DNA.

Chicken strain G-B1 exhibits a relative resistance to avian osteopetrosis

The disease induced by the avian myeloblastosis associated virus MAV-2-O in the susceptible chicken strains Brown Leghorn (BLH) and Prague CB (CB) was compared with that induced in the resistant G-B1 strain. Osteopetrosis, stunting and lymphoid organ atrophy were more severe in BLH than in CB chickens. G-B1 animals remained superficially normal until the end of the experiment. In contrast to the other two strains, the histopathological changes were very mild and there was no sign of immunosuppression. After 4 months, however, nephroblastomas could be detected in more than 50 per cent of the infected G-B1 chickens. Similar tumors were also found in CB birds kept for up to 5 months. Antibodies against MAV-2-O specific viral proteins were detected in plasma from infected G-B1 chickens but the titers were less than in plasma of convalescent birds. Virus could be demonstrated in peripheral blood until the end of the experiment (at 8 weeks). Therefore the resistance of the G-B1 strain is due neither to a restriction at the receptor level nor the result of a humoral immune reaction, but represents a new type of relative resistance at the cellular level. From (CC X G-B1)F1 and (CC X G-B1)F2 crosses the resistant phenotype is determined by a single genetic factor. This gene is not linked to the major histocompatibility complex. There is also a sex-dependent factor, possibly hormonal, involved in the resistant phenotype.

Transformation of Brown Leghorn chicken embryo fibroblasts by avian myeloblastosis virus proviral DNA

Brown Leghorn chicken embryo fibroblasts were transfected with a mixture of avian myeloblastosis virus (AMV) and myeloblastosis-associated virus type 1 (MAV1) proviral DNA purified from lambda-Charon 4A recombinant clones. A transformed cell line (T1AM) able to grow without anchorage in semisolid medium was obtained. The presence of both proviral AMV and MAV sequences was detected in T1AM DNA by hybridization with v-myb- and MAV1-specific probes. Altered AMV and MAV1 proviral genomes were found in T1AM genome. Characterization of the RNA species expressed in transformed cells showed that in addition to a 2.5-kilobase (kb) putative subgenomic v-myb-specific RNA, three other myb-containing RNAs (9.4, 8.4, and 7.0 kb) were present in T1AM cells. No AMV genomic RNA was detected. Also, a new 5.0-kb MAV1-specific RNA species was expressed in transformed cells in addition to MAV1 genomic RNA species (7.8 kb). No infectious AMV virions are released by T1AM cells. Chicken embryo fibroblasts infected by T1AM-released virions contained and expressed all MAV1 sequences detected in T1AM transformed cells but did not express any transformation parameter. These results indicated that the presence of AMV proviral sequences in T1AM cells is responsible for their transformed phenotype.

Avian leukosis virus-induced osteopetrosis is associated with the persistent synthesis of viral DNA

DNAs from 19 cases of avian leukosis virus-induced osteopetrosis have been analyzed for viral sequences. Among these were instances of rapid, intermediate, and slow onset osteopetrosis. The DNAs from osteopetrotic bone contained no evidence for osteopetrosis being caused by proviral insertions into or viral transductions of a host protooncogene. Instead, DNAs from osteopetrotic bone displayed evidence for osteopetrosis being associated with the persistent synthesis of viral DNA. Each of the 19 DNAs contained unintegrated as well as integrated viral DNA. Rapid onset osteopetrosis contained about 3X more viral and proviral DNA than intermediate or late onset osteopetrosis. Unintegrated viral DNA could not be detected in DNAs extracted from the bursa bone marrow of osteopetrotic chickens or in DNA extracted from the normal bones of an avian leukosis virus-infected chicken. Thus, the persistent synthesis of unintegrated viral DNA was observed in osteopetrotic but not normal tissues of avian leukosis virus-infected chickens.

Pathogenesis of osteopetrosis induced by rapid and slow onset plaque isolates of an avian osteopetrosis virus

Examination of bone from chickens infected as 10-day-old embryos with isolates of an avian osteopetrosis virus revealed that MAV-2(O) plaque isolate 32/2/4 caused rapid bone growth, while MAV-2(O) plaque isolate 13 caused a mild form of bone growth. MAV-2(O) plaque isolate 32/2/4 caused anemia when injected into the 8-day-old hatched chick and bone growth in ovo when injected into the 4-day-old embryo. Passive administration of neutralizing antibody protected against MAV-2(O)-induced bone growth when antibody was given to the embryo 1 day after virus. Neutralizing antibody also protected against an acute anemia observed when normal and bursectomized chickens were given MAV-2(O) 32/2/4, but antibody did not prevent the appearance of a chronic anemia or osteopetrosis in bursectomized chickens. Repeated animal passage of a slow onset plaque isolate of MAV-2(O) caused the virus to progressively induce more severe bone growth and anemia.

Induction of angiosarcomas by ring-necked pheasant virus

Ring-necked pheasant virus, an avian leukosis virus, when injected into 10-day old chick embryos, caused angiosarcomas in the lungs of infected chickens within a short time. Angiosarcomas appeared as localized foci of proliferating cells in the lungs as early as 2 weeks posthatch, and by 6 weeks, the lungs of the infected chickens were frequently filled with tumor cells. Between 3 and 10 weeks of age, 80% of infected chickens died of the angiosarcomas; the 20% which lived 8 weeks or longer had small lung lesions and also developed fibrosarcomas, osteopetrosis, nephroblastoma, and lymphoid leukosis. Chickens with lung tumors were cyanotic, had breathing difficulty, and had packed cell volumes in excess of 50%. Other changes not necessarily correlated with lung tumor mass were stunting, lymphoid organ involution, and profuse diarrhea. Ring-necked pheasant virus has a genome RNA of 8.2 kb. This observation, together with its replication and disease induction after repeated plaque purification, suggests that ring-necked pheasant virus is a replication-competent avian retrovirus. Therefore, our results suggest that ring-necked pheasant virus is an avian leukosis virus which causes angiosarcomas rapidly at high incidence and which, therefore, may induce this type of tumor by a mechanism different from the induction of sarcomas by avian sarcoma viruses.

Rous-associated virus type 7 induces a syndrome in chickens characterized by stunting and obesity

Infection of 10-day-old chicken embryos with an avian retrovirus. Rous-associated virus type 7, resulted in a disease characterized by stunting and hyperlipidemia. By 20 days after hatch, infected chickens were smaller than hatchmates and developed ataxia and obesity over the next 30 days. Histological examinations of livers from infected chickens revealed a diffuse panlobular fatty infiltrate involving an accumulation of fat in microdroplets. Electron microscopic examinations of livers from infected chickens revealed hepatocytes with swollen mitochondria that lacked cristae. The thyroid and pancreas were infiltrated with lymphoblastoid cells by 1 week after hatch. An examination of the blood revealed a mild anemia, a frank lipemia, and high levels of uric acid. This syndrome induced by Rous-associated virus type 7 in chickens may be useful for elucidating the nature of several diseases, including that found in the fatty liver and kidney syndrome of chickens and that observed in a strain of obese chickens.

In vitro translation of avian myeloblastosis virus RNA

Avian myeloblastosis virus (AMV)-infected cells contain two viral mRNA's, a genome-sized 34S (7.5-kilobase) mRNA and a 21S (2.5-kilobase) subgenomic mRNA, which contains the AMV-specific sequences (myb sequences). We found that AMV virions packaged both the 7.5-kilobase full-length genomic RNA and the 2.5-kilobase subgenomic RNA. In vitro translation of AMV virion RNA sized by sucrose density gradient centrifugation yielded 76,000-, 56,000-, 48,500-, 47,000-, and 32,000-dalton products. The 76,000-dalton protein was coded for by RNA throughout the gradient, but the peak of activity was at 34S to 35S. The 56,000-, and 48,500-, and 32,000-dalton proteins were encoded in a 21S RNA, and 47,000-dalton protein was encoded in an RNA of approximately 24S. The 76,000-dalton protein was identified as Pr76gag, based upon immunoprecipitation with specific antiserum and the presence of the 19* dipeptide. 7-Methylguanosine triphosphate inhibited the syntheses of Pr76gag and the 56,000-, 48,500-, and 32,000-dalton proteins, but not the synthesis of the 47,000-dalton protein. The 56,000-, 48,500-, 47,000-, and 32,000-dalton proteins were not immunoprecipitated by anti-gag, anti-reverse transcriptase, or anti-gp85 antiserum. Two-dimensional peptide maps of the 56,000- and 48,500-dalton proteins indicated that they were unique. In vitro translational products of myeloblastosis-associated virus 1 were also analyzed to aid in the identification of the AMV myb gene product(s); the translational products analyzed included Pr76gag, p60env, and a 56,000-dalton polypeptide which apparently was not identical to the 56,000-dalton AMV translational product, as determined by two-dimensional peptide mapping. Our data indicated that one of these proteins (56,000, 48,500, or 32,000 daltons) may represent the product of the AMV myb gene and, therefore, the putative transforming protein(s) of AMV.

Independent recombination between avian leukosis virus terminal sequences and host DNA in virus-induced proliferative disease

A cDNA transcript of Rous sarcoma virus, which contained the long terminal repeat (LTR) and some additional 3'-terminal sequences, was inserted into the plasmid pBR322. This recombinant plasmid, p53, was then used as a hybridization probe to detect viral terminal sequences in DNA from a number of tissues of birds with a variety of avian leukosis virus (ALV)-induced proliferative diseases. Using restriction endonuclease digestion and blot hybridization analysis, we detected, in addition to standard ALV genomes, viral terminal sequences linked to host DNA and not to viral genes. In DNA from bursal lymphomas and nephroblastomas, we observed small numbers of integration sites occupied by sequences in p53 and lacking most or all of the remainder of the viral genome. In DNA from osteopetrosis, we observed apparently multiple copies of molecules containing host DNA linked to viral LTR sequences. Some of these structures were contained in discrete, probably unintegrated, DNA molecules. We concluded that viral LTR sequences can be inserted as independent elements during recombination with host DNA in some forms of interaction between exogenous retroviruses and host cells. Because the LTRs have been implicated in integration and transcription of viral genes, the possibility that translocation or activation, or both, of host genes may occur as a consequence of viral infection is reinforced by these observations.

Correlation of RNA binding affinity of avian oncornavirus p19 proteins with the extent of processing of virus genome RNA in cells

We purified the p19 proteins from the Prague C strain of Rous sarcoma virus, avian myeloblastosis virus, B77 sarcoma virus, myeloblastosis-associated virus-2(0), and PR-E 95-C virus and measured their binding affinities for 60S viral RNA by the nitrocellulose filter binding technique. The apparent association constants of the p19 proteins from Rous sarcoma virus Prague C, avian myeloblastosis virus, and B77 sarcoma virus for homologous and heterologous 60S RNAs were similar (1.5 x 10(11) to 2.6 x 10(11) liters/mol), whereas those of myeloblastosis-associated virus-2(0) and PR-E 95-C virus were 10-fold lower. The sizes and relative amounts of the virus-specific polyadenylic acid-containing RNAs in the cytoplasms of cells infected with Rous sarcoma virus Prague C, myeloblastosis-associated virus-2(0), and PR-E 95-C virus were determined by fractionating the RNAs on agarose gels containing methylmercury hydroxide, transferring them to diazobenzyloxymethyl paper and hybridizing them to a 70-nucleotide complementary DNA probe. In cells infected with Rous sarcoma virus Prague C we detected 3.4 x 10(6)-, 1.9 x 10(6)-, and 1.1 x 10(6)-dalton RNAs, in PR-E 95-C virus-infected cells we detected 3.4 x 10(6)-, 1.9 x 10(6)- and 0.7 x 10(6)-dalton RNAs, and in cells infected with myeloblastosis-associated virus-2(0) we detected 3 x 10(6)- and 1.3 x 10(6)-dalton RNAs. Each of these RNA species contained RNA sequences derived from the 5' terminus of genome-length RNA, as evidenced by hybridization with the 5' 70-nucleotide complementary DNA. The ratios of subgenomic mRNA's to genome-length RNAs in cells infected with myeloblastosis-associated virus-2(0) and PR-E 95-C virus were three- to five-fold higher than the ratio in cells infected with Rous sarcoma virus Prague C. These results suggest that more processing of viral RNA in infected cells is correlated with lower binding affinities of the p19 protein for viral RNA, and they are consistent with the hypothesis that the p19 protein controls processing of viral RNA in cells.

Pathology of chickens infected with avian nephoblastoma virus MAV-2(N)

A neophroblastoma-inducing myeloblastosis-associated virus, MAV-2(N), derived from avian myeloblastosis virus was characterized with respect to biochemical composition and avian pathogenesis. Purified fibroblast-grown virus contained the same size 35S ribonucleic acid and the same relative amounts of viral polypeptides as another myeloblastosis-associated virus inducing predominantly osteopetrosis MAV-2(O). Plaque-purified MAV-2(N) induced a 76 to 93% incidence of nephroblastoma and a 3 to 50% incidence of osteopetrosis in SPAFAS and line 15 x 7 chickens: the oncogenic spectrum and the onset of nephroblastoma varied with the line of chicken and the route of injection. Renal neoplasms were manifest in chickens older than 2 months and grew to a massive size. Furthermore, 29% of control chickens housed with MAV-2(N)-infected chickens demonstrated nephroblastoma. MAV-2(N)-infected chickens had growth rates and blood packed cell volumes comparable to those of uninfected chickens. Infected chickens 2 months of age had increased kidney, liver, and spleen weights; tumor-bearing chickens 3 to 4 months of age had increased liver, lung, brain, pancreas, and bone weights. The concentration of albumin was decreased and the concentration of gamma globulin was increased in the serum of MAV-2(N)-INFECTED CHICKENS. Analysis of the sera of nephroblastoma-bearing chickens for virus and antibody showed that three states existed: (i) high levels of neutralizing antibody, (ii) high levels of virus, and (iii) simultaneous presence of both at low levels. The pathological and virological features of MAV-2(N) which distinguish it from MAV-2(O) are discussed.

Characterization of anemia induced by avian osteopetrosis virus

Chickens infected intravenously at 8 days after hatching with an avian osteopetrosis virus developed a severe, progressive anemia in the absence of osteopetrosis. The anemia was characterized as a pancytopenia, in which erythrocytes, granulocytes, and thrombocytes decreased concomitantly. Serum bilirubin levels were normal, whereas erythrocytes from infected chickens demonstrated a slightly elevated osmotic fragility. A negative Coombs test indicated that there was no evidence for erythrocyte-bound antibody. Erythrocytes from infected animals had slightly decreased 51Cr-labeled erythrocyte survival time when compared with normal. Examination of marrow histological preparations, together with ferrokinetic studies with 59Fe, indicated that marrow failure occurred during the acute phase of the anemia. Circulating virus was present during the development and acute phases of the anemia, but disappeared during the recovery phase of the disease. Neutralizing antibody appeared after the disappearance of circulating virus. It is concluded that virus infection induced both marrow failure (aplastic crisis) and decreased erythrocyte survival.

Characterization of the immunosuppression accompanying virus-induced avian osteopetrosis

Infection of chickens with a myeloblastosis-associated virus which induced a high incidence of osteopetrosis was accompanied by immunosuppression. The immunosuppression was manifested in the following ways. The weight of the bursa, spleen, and thymus was depressed in infected chickens. Infected animals had a diminished capacity to form hemolytic plaques in a direct assay. Spleen cells from osteopetrotic animals did not respond to phytohemagglutinin, and the spleen and bursa had a decreased proportion of cells possessing surface immunoglobulin. Osteopetrotic animals failed to show an age-dependent increase in the proportion of cells demonstrating surface immunoglobulin that was observed in normal animals. However, several individual chickens with heavy osteopetrosis responded to antigenic stimulation in a normal fashion, indicating that although immunosuppression usually accompanies avian osteopetrosis, it may not contribute directly to abnormal bone proliferation.

Biological characterization of avian osteopetrosis

Chicks infected as 12-day-old embryos with an end-point purified derivative of avian myeloblastosis virus developed a rapidly progressive osteopetrosis that manifested within 1 week of hatching. A detailed comparison of osteopetrotic chicks and normal hatchmates revealed the following. (i) Osteopetrotic chicks exhibited a stunting syndrome, growing at a mean rate that was 26% of the control rats. (ii) At autopsy, the mass of the lymphoid organs was reduced, whereas the mass of the heart, pancreas, kidneys, lungs, brain, liver, and bones of osteopetrotic chicks was increased. Edema was likely responsible for most of the increase in organ weight. (iii) Infected chicks exhibited a normochromic, normocytic anemia that was virus dose dependent and was not required for the development of osteopetrosis. (iv) Bone collagen content was normal. (v) Osteopetrotic bone was initially hypomineralized, but later became more fully mineralized. (vi) The concentrations of alpha, beta, and gamma globulins in the plasma were elevated in osteopetrotic chicks, whereas albumin concentration was decreased. (vii) The level of plasma alkaline phosphatase was elevated in osteopetrotic chicks, yet the level of acid phosphatase was unchanged. (viii) Body and bone temperatures were unchanged.

RNA viruses and cancer: Lucy Wortham James Lecture (Basic Science)

Some animal viruses that contain RNA replicate through a DNA intermediate. The molecular details of the replication of these viruses, which are called ribodenoxyviruses, are starting to be known. The ribodenoxyviruses belonging to a single species may either cause sarcomas, leukemia or no disease. The viruses belonging to a single species differ only in whether or not they contain genes for disease formation. In the case of Rous sarcoma virus, the virus causes sarcomas by adding a gene for sarcoma formation to the genome of infected cells. Ribodeoxyviruses appear to undergo different kinds of genetic changes at extraordinarily high rates. In addition, nucleotide sequences related to ribodeoxyvirus RNA are present in the DNA of many uninfected cells. These nucleotide sequences may represent a virus precursor, and ribodeoxyviruses are hypothesized to have evolved from these nucleotide sequences in uninfected cells. These data have led us to hypothesis that non-viral carcinogens act to mutate a cellular gene(s) that is involved in the same types of information transfer and genetic variation as ribodeoxyviruses and thus give rise to the formation of cancer gene(s).