Drug resistance, both primary and acquired, is a major obstacle to advances in cancer chemotherapy. In vitro, multidrug resistance can be mediated by P-glycoprotein (PGYI), a cell surface phosphoglycoprotein that acts to efflux natural products from cells. PGYI is encoded by the MDRI gene located at 7q21.1. Overexpression of MDRI has been demonstrated in many cancers, both in patient tumors and in cell lines selected with a variety of chemotherapeutic agents. Recent studies in drug-selected cell lines and patients samples have identified hybrid mRNAs comprised of an active, but apparently random, gene fused 5' to MDRI. This observation indicates that random chromosomal rearrangements, such as translocations and inversions, leading to 'capture' of MDRI by constitutively expressed genes may be a mechanism for activation of this gene following drug exposure. In this study, fluorescence in situ hybridization (FISH) using whole chromosome paints (WCP) and bacterial artificial chromosome (BAC)-derived probes showed structural rearrangements involving 7q in metaphase and interphase cells, and comparative genomic hybridization (CGH) revealed high levels of amplification at chromosomal breakpoints. In an adriamycin-selected resistant colon cancer line (S48-3s/Adr), WCP4/WCP7 revealed t(4;7)(q31;q21) and BAC-derived probes demonstrated that the breakpoint lay between MDRI and sequences 500-1000 KB telomeric to it. Similarly, in a subline isolated following exposure to actinomycin D (S48-3s/ActD), a hybrid MDRI gene composed of heme oxygenase-2 sequences (at 16p13) fused to MDRI was identified and a rearrangement confirmed with WCP7 and a subtelomeric 16p probe. Likewise, in a paclitaxel- selected MCF-7 subline where CASP sequences (at 7q22) were shown to be fused to MDRI, WCP7 showed an elongated chromosome 7 with a homogeneously staining regions (hsr); BAC-derived probes demonstrated that the hsr was composed of highly amplified MDRI and CASP sequences. In all three selected cell lines, CGH demonstrated amplification at breakpoints involving MDRI (at 7q21) and genes fused to MDRI at 4q31, 7q22, and 16p13.3. Finally, in samples obtained from two patients with drug refractory ALL, BAC-derived probes applied to archived marrow cells demonstrated that a breakpoint occurred between MDRI and sequences 500-1000 KB telomeric to MDRI, consistent with a random chromosomal rearrangement. These results support the proposal that random chromosomal rearrangement leading to capture and activation of MDRI is a mechanism of acquired drug resistance.
|Original language||English (US)|
|Number of pages||11|
|Journal||Genes Chromosomes and Cancer|
|State||Published - Sep 1998|
ASJC Scopus subject areas
- Cancer Research