Induction of leukemia in chicken by bovine leukemia virus due to insertional mutagenesis

Bovine leukemia virus (BLV) was inoculated into one-day-old chickens. In a small part of inoculated chickens leukemia developed during observation period of one year. Out of 88 birds inoculated, only 4 developed histopathologically verified leukemia. The induced leukemia was characterized by enlarged liver and spleen. The organs were infiltrated with leukemic cells. The DNAs of body organs of inoculated chickens were analysed by Southern blot hybridization for the presence of BLV specific sequences. Out of 9 suspicious chickens tested in 6 birds the BLV was found to be integrated into host DNA either as a complete viral genome or as a part corresponding to its 3'-end. The leukemic cells were monoclonal as regard to the integration site of the BLV provirus. Neither the expression of BLV provirus in chicken leukemic cells nor the antibody response to BLV antigens in inoculated birds was detected. The rearrangements and amplification of erb-B and myb loci of protooncogenes in leukemic cells was detected. There were no changes in loci of following protooncogenes: myc, sis, fes, fps, erb A, src and yes. All obtained data taken together suggest that the BLV induced leukemia in chickens is caused by insertional mutagenesis.