Poor Expression of MDR1 Transgene in HeLa Cells by Bicistronic Moloney Murine Leukemia Virus-Based Vector

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Abstract
Cotransfer of a therapeutic gene together with the human MDR1 gene provides an opportunity to increase the number of transduced marrow cells, expressing the therapeutic gene, by in vivo selection for MDR1. We have used an Lg-MDR1-IRES-neo (LgMIN) retroviral vector, containing MDR1 and neo genes, separated by the EMCV IRES. Human HeLa or canine CTAC cells, transduced with GALV env pseudotyped LgMIN at an MOI of less than 0.01 to ensure 1 proviral copy/genome, were selected with either G418 for neo expression or colchicine for MDR1 expression. The titer determined on HeLa cells with G418 selection was eightfold higher than that with colchicine selection. In contrast, the same viral supernatant exhibited only a 1.4-fold difference between neo- and MDR1-based viral titer values for CTAC cells. The transduced HeLa cells, with one intact proviral copy per genome, exhibited a 55-fold higher resistance to G418 but only a 4-fold higher resistance to colchicine and a 2-fold higher resistance to Taxol compared with nontransduced cells. About 23% of the transduced cell population did not express vector-derived P-glycoprotein (P-gp) as detected by anti-human P-gp MAb MRK-16. This could explain the difference in viral titers obtained on CTAC cells but not that obtained on HeLa cells. The vector-mediated increase in expression of P-gp was about 20-fold higher in CTAC cells as compared with HeLa cells. These results indicated suppression of expression of vector-derived MDR1 in HeLa cells, in contrast with CTAC cells. To investigate further the possible reasons for this difference, genomic DNA was isolated from the G418-resistant individual colonies of infected cells and analyzed by PCR for full-length proviral MDR1. For transduced CTAC and HeLa cells, selected at a G418 concentration of 1 mg/ml, PCR detected aberrant forms of MDR1 in 17 to 25% of colonies tested. The aberrant forms consisted of MDR1 genes with 2- and 0.7-kb deletions. DNA sequencing across the 2-kb and the 0.7-kb deletion junction suggests cryptic splicing in the producer cell line as the origin of these deletions. The 2-kb deletion corresponds to MDR1 mRNA cryptic splicing via donor (codon 113) and acceptor (codon 773). The 0.7-kb deletion corresponds to splicing via the same donor and a different acceptor (codon 344). When transduced HeLa cells were selected at a higher concentration of G418 (3 mg/ml), the aberrant forms were detected at an increased frequency of about 50% of colonies tested. These results indicate that vector-derived MDR1 is a poor selective marker in HeLa cells but not in CTAC cells and that deletions, which inactivated the MDR1 gene in a bicistronic Mo-MuLV vector, may provide an advantage for expression of the second transgene in HeLa cells. Our studies of Mo-MuLV-based bicistronic MDR1-containing vector expression in human HeLa and canine CTAC cells showed that three factors contributed to poor MDR1 transgene expression in human HeLa cells. The two major factors were the weak nature of MDR1 as a selective marker and the reduced expression of transgenes by MoMLV vector backbone in HeLa cells; the third, minor factor was a rearrangement of the vector-derived MDR1 transgene. In addition, we found suggestive evidence that coexpression of MDR1 in a bicistronic vector may downregulate the expression of the second transgene.