Retrovirus insertion and transcriptional activation of the multidrug-resistance gene in leukemias treated by a chemotherapeutic agent in vivo

To understand the molecular basis for multidrug-resistant (MDR) cancer cell s in vivo, this study analyzed molecular changes of the mdr1a gene region i n leukemia cells in mice during continuous treatment with vincristine, An i nverse insertion of murine leukemia retrovirus (MuLV) into the 5'-flanking region of the mdr1a gene was found. This insertion was concomitantly accomp anied by up-regulation of the mdr1a gene and the loss of chemosensitivity. Deletion of long-terminal repeat(LTR) sequences dramatically decreased the mdr1a promoter-driven reporter activity. The MuLV LTR insertion appears to exert its enhancer activity on mdr1a transcription du ring the appearance o f MDR leukemia cells. Two mechanisms were postulated to explain the mdr1a g ene activation by retrovirus insertion during in vivo chemotreatment: de no vo insertion of MuLV induced by vincristine treatment and selection of a sm all fraction of pre existing cells carrying MuLV insertion during vincristi ne treatment. No rearranged sequence was detected by polymerase chain react ion in parental cells. This result argued for the first mechanism. The rand omly altered distribution of MuLV during repetitive chemotreatment might al so be consistent with this hypothesis. On the other hand, the retrovirus in sertion was detected at the same site of the mdr1a promoter region in 2 ind ependent experiments, which suggests the second mechanism. It should be not ed that in vivo chemotreatment using vincristine could generate the mdr1a-o verexpressing cells through retrovirus insertion and the enhancer effect of the LTR. (C) 2001 by The American Society of Hematology.