Nonrandom chromosomal abnormalities in bovine lymphoma

Despite detailed knowledge of the genetic map of the bovine leukemia virus (BLV), the mechanism whereby BLV infection results in transformation and B-lineage restriction of tumors is poorly understood. The aim of this study was to gain new insight into pathogenetic mechanisms of BLV-induced tumorigenesis by determining the karyotypes of BLV-associated lymphomas in cattle. Metaphases in cells from lymphoid tumors from 20 mature dairy cows were banded and analyzed after short-term, unstimulated culture. Nineteen out of twenty cases exhibited clonal abnormalities, 17 cases were hyperdiploid, and 16 cases had extremely complex chromosomal changes. Recurrent chromosomal anomalies were identified and there was clear evidence for the evolution of increasing chromosomal instability in 12 cases. The most common abnormalities were the acquisition of additional small chromosomes (23-29); trisomy of chromosomes 5 and 7, and Robertsonian translocations and isochromosome rearrangements involving chromosomes 10, 12, 23, and 26. Monosomy X, trisomy X, and translocations involving the X chromosome were also detected. Chromosomes 2, 3, 4, 6, 8, 9, 11, 13, 14, 19, and 21 were infrequently involved in either structural or numerical changes. Structural rearrangements of chromosomes 10, 12, 23, and 26 may reflect primary abnormalities occurring relatively early in transformation, whereas trisomy 5 may be an extremely common secondary abnormality. While comparison of these findings with the current bovine gene map raises intriguing possibilities for pathogenetic mechanisms, further studies are needed before hypothetical mechanisms linking chromosomal abnormalities with BLV-induced transformation can be made.