Reduction of mdr1 gene amplification in human multidrug-resistant LoVo DX cell line is promoted by triple helix-forming oligonucleotides

We have demonstrated previously that the GT tripler-forming oligodeoxyribonucleotide (TFO) d(TGTGTTTTTGTTTTGTTGGTTTTGTTT), named TFO ID, targeted to a polypyrimidine-polypurine coding sequence located within human multidrug-resistance mdr1 gene, specifically and significantly reduced mdr1 mRNA levels in the drug-resistant T-leukemic CEM-VLB100,, cell line. In this article, we demonstrate that TFO 1D is effective at inhibiting not only transcription but also replication of mdr1 genes, leading to a loss of amplified gene copies in the drug-resistant colon adenocarcinoma LoVo DX cell line, In contrast, TFO ID does not alter replication of the constitutive mdr1 gene copy in the corresponding parental sensitive LoVo 109 cell line. A specific reduction in mdr1 gene amplification levels was also obtained with the pyrimidine TFO d(CTTTTTCTTTTCTTCCTTTTCTTT), named TFO 24TC, directed against the same polypyrimidine-polypurine sequence of the mdr1 gene. We suggest that triple helix-forming oligonucleotides might affect the replication of unstable chromosomal elements as amplicons in actively replicating cells by causing a local impairment of DNA polymerase activity. This study lends support to the notion that TFO may be used to reduce gene amplification aiming to control neoplastic progression in cancer cells bearing amplified oncogenes.