Genetic Basis of Multidrug Resistance 
Philippe Gros, Ph.D. — International Research Scholar 
Dr. Gros is Associate Professor of Biochemistry at McGill University, Montreal, and a member of the McGill 
Cancer Center and the McGill Center for the Study of Host Resistance. He received his Ph.D. degree from 
McGill University and pursued postdoctoral training in molecular biology and cancer research at 
Massachusetts General Hospital with Joel Habener and at the Massachusetts Institute of Technology with 
David Housman. 
TUMOR cells in vivo and in vitro can develop 
simultaneous resistance to a wide range of 
structurally and functionally unrelated cytotoxic 
drugs. Such multidrug resistance (MDR) severely 
impedes the chemotherapeutic treatment of 
many types of tumors. Structural or functional 
characteristics common to drugs of the MDR 
spectrum are few. In general these drugs are 
small, hydrophobic natural products that often 
contain a basic nitrogen atom and penetrate 
the cell by passive diffusion across the mem- 
brane. MDR is associated with a decreased in- 
tracellular drug accumulation and concomitant 
increased drug efflux from resistant cells, both 
ATP-dependent . 
MDR is caused by the overexpression of a high- 
molecular-weight membrane phosphoglycopro- 
tein called P-glycoprotein (P-gp) . P-gp has been 
found capable of binding photoactivatable ana- 
logues of ATP and cytotoxic drugs, suggesting 
that it functions as an ATP-driven efflux pump 
that reduces the intracellular accumulation of 
drugs in resistant cancer cells. Recent studies 
have shown that increased P-gp expression in neu- 
roblastomas and soft-tissue sarcomas causes lack 
of response to chemotherapy and is associated 
with very poor prognosis and outcome of these 
diseases. 
P-gp is encoded by a small family of closely 
related genes, termed mdr or pgp, that share con- 
siderable sequence homology and common an- 
cestral origins. This gene family has three 
members in rodents (mdrl, mdr 2, and mdr 3) 
and two in humans {MDRl and MDR2) .We have 
isolated and characterized full-length cDNA 
clones corresponding to the three mouse genes 
and have deduced the amino acid sequences of 
the three predicted polypeptides. P-gps share 
considerable sequence homology (80-85 per- 
cent identity) and common structural features, 
including 1 2 predicted transmembrane domains 
and two nucleotide binding sites. 
Each P-gp is formed by two homologous halves 
that show sequence conservation with a large 
group of bacterial transport proteins participat- 
ing in the import and export of specific substrates 
in Escherichia coli. This evolutionary conserva- 
tion is in keeping with P-gp's proposed drug ef- 
flux function. 
The normal physiological function of P-gps has 
yet to be elucidated. Each P-gp isoform is ex- 
pressed in a tissue-specific fashion, generally on 
the apical surface of secretory epithelial cells 
such as those of the bile canalicular, the brush 
border of the intestine, and the proximal tubule 
of the kidney. It has also been found in endothe- 
lial cells of the blood-brain barrier and in early 
pluripotent stem cells of the hematopoietic sys- 
tem. From these findings it appears that P-gp ei- 
ther plays a normal detoxifying role against envi- 
ronmental xenobiotics or transports normal 
physiological substrates yet to be identified. 
Recently it was shown that the mrfr gene family 
is itself part of a larger family of sequence-related 
genes shown to play key physiological functions 
in normal cells and tissues. These include the 
STE6 gene of the yeast Saccharomyces cerevi- 
siae, responsible for the transmembrane trans- 
port of the "a" mating pheromone; the pfmdrl 
gene of the malarial parasite Plasmodium falci- 
parum, associated with chloroquine efflux from 
resistant isolates of this parasite; and in humans, 
the CFTR chloride channel gene, mutations of 
which cause cystic fibrosis, and the RING family 
genes, which code for peptide pumps implicated 
in antigen presentation by T lymphocytes. There- 
fore it appears that the mdr supergene family 
codes for membrane-associated transport pro- 
teins that may transport different types of sub- 
strates by the same mechanism. 
We have carried out functional analyses of indi- 
vidual members of the mouse mdr gene family. 
For this, we have transfected and overexpressed 
cDNAs that correspond to each member of the 
family. We observed that mdrl and mdr3, but 
not mdr2, could directly confer MDR to other- 
wise drug-sensitive cells. In addition, the profile 
of drug resistance conferred by mdrl and mdr3 
appeared distinct. 
One of the key unanswered questions about P- 
gp and MDR is how a single transport protein can 
apparently recognize and transport a large group 
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