LACK OF LTR AND ENV GENETIC-VARIATION DURING BOVINE LEUKEMIA VIRUS-INDUCED LEUKEMOGENESIS

Genetic variation of the Bovine Leukemia Virus (BLV) appears to be lim ited in vitro and during the latent phase of the disease. However, cel ls in tumors often harbor deleted proviruses that are defective for ex pression. In order to gain insight into the involvement of viral genet ic variation during pathogenesis, the BLV LTR and the env proviral seq uences were analyzed in tumor tissues. A sheep (M230) was injected wit h the cloned BLV provirus 344 and became persistently infected with ci rculating lymphocytes reaching 345,000/mm(3). After 11 months, this in fected sheep developed leukemia-lymphoma. DNA was extracted from perip heral blood leukocytes al the time of tumor development and the LTR an d the env gene were amplified, using the polymerase chain reaction pro cedure, cloned, and sequenced. Twenty independent LTR and twenty env c lones were analyzed. It appeared that the in vivo mutation rate in the env gene was 0.043% (eight mutations including seven transitions out of 18,300 bp). Five point mutations (all transitions) were identified in the LTR, corresponding to 0.041% modifications (four mutations out of 9740 bp). These mutation rate values (0.043 and 0.041) were close t o those due to the Tag DNA polymerase errors (0.030%). Altogether, the se data demonstrate the lack of genetic variation in the LTR and the e nv gene during this case of BLV-induced pathogenesis in vivo. They con firm that the defectiveness of some BLV proviruses in vivo, thus, is n ot a mandatory step in the leukemogenic process. (C) 1995 Academic Pre ss, Inc.