Rapid induction of osteopetrosis by subgroup E recombinant viruses

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.

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.