Preferential expression of a mutant allele of the amplified MDR1 (ABCB1) gene in drug-resistant variants of a human sarcoma.
Activation of the MDR1 (ABCB1) gene is a common event conferring multidrug resistance (MDR) in human cancers. We investigated MDR1 activation in MDR variants of a human sarcoma line, some of which express a mutant MDR1, which facilitated the study of allelic gene expression. Structural alterations of MDR1, gene copy numbers, and allelic expression were analyzed by cytogenetic karyotyping, oligonucleotide hybridization, Southern blotting, polymerase chain reaction, and DNA heteroduplex assays. Both chromosome 7 alterations and several cytogenetic changes involving the 7q21 locus are associated with the development of MDR in these sarcoma cells. Multistep-selected cells and their revertants contain three- to six-fold MDR1 gene amplification compared with that of the drug-sensitive parental cell line MES-SA and single-step doxorubicin-selected mutants. MDR1 gene amplification precedes the emergence of a mutant allele in cells that were coselected with doxorubicin and a cyclosporin inhibitor of P-glycoprotein (P-gp). Allele-specific oligonucleotide hybridization showed that the endogenous mutant allele was present as a single copy, with multiple copies of the normal allele. Reselection of revertant cells with doxorubicin in either the presence or the absence of the P-gp inhibitor resulted in exclusive reexpression of the mutant MDR1 allele, regardless of the presence of multiple wild-type MDR1 alleles. These data provide new insights into how multiple alleles are regulated in the amplicon of drug-resistant cancer cells and indicate that increased expression of an amplified gene can result from selective transcription of a single mutant allele of the gene.
Chen
Kevin G
KG
Oncology Division, Department of Medicine, Stanford University School of Medicine, 269 Campus Drive, Stanford, CA 94305-5151, USA.
Lacayo
Norman J
NJ
Durán
George E
GE
Wang
Yan
Y
Bangs
C Dana
CD
Rea
Susan
S
Kovacs
Mary
M
Cherry
Athena M
AM
Brown
J Martin
JM
Sikic
Branimir I
BI
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1 / Q000096:biosynthesis; D000483:Alleles; D000970:Antineoplastic Agents / Q000494:pharmacology; D020223:Chromosome Painting / Q000379:methods; D020732:Cytogenetic Analysis / Q000379:methods; D004252:DNA Mutational Analysis; D004273:DNA, Neoplasm / Q000032:analysis / Q000235:genetics; D004317:Doxorubicin / Q000378:metabolism / Q000494:pharmacology; D019008:Drug Resistance, Neoplasm / Q000235:genetics*; D005784:Gene Amplification / Q000235:genetics*; D018628:Gene Dosage; D015972:Gene Expression Regulation, Neoplastic / Q000235:genetics*; D019450:Genes, MDR / Q000235:genetics*; D014644:Genetic Variation / Q000235:genetics; D006801:Humans; D007621:Karyotyping; D027425:ATP-Binding Cassette, Sub-Family C Proteins / Q000096:biosynthesis / Q000235:genetics*; D009154:Mutation / Q000235:genetics*; D012333:RNA, Messenger / Q000096:biosynthesis; D012509:Sarcoma / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism / Q000473:pathology; D014316:Tritium / Q000378:metabolism; D014407:Tumor Cells, Cultured
D020168:ATP Binding Cassette Transporter, Subfamily B, Member 1; D000970:Antineoplastic Agents; D004273:DNA, Neoplasm; D004317:Doxorubicin; D027425:ATP-Binding Cassette, Sub-Family C Proteins; D012333:RNA, Messenger; D014316:Tritium