Typically, a first course of chemotherapy may prove highly beneficial, nearly annihilating a tumor. But a few resistant cancer cells often survive and proliferate. Too often, despite more aggressive second and third courses of chemotherapy, the remaining drug-defiant cells thrive, displaying increasing resistance to drug therapy and eventually displaying virtual invulnerability to chemotherapy. After the drug's effectiveness fades, the patient relapses.
Research is under way on many fronts to better understand multidrug resistance in cancer and to find drugs that may block or reverse the development of drug resistance in cancer cells. Studies have found that P-glycoprotein, called the multidrug transporter, is expressed in increased amounts as a result of genetic alterations in cells to build their resistance to many anticancer drugs. Intriguingly, P-glycoprotein is also intrinsically involved in the functions of the blood-brain barrier, which protectively prevents toxic substances–and many therapeutic drugs–from entering the brain.
Genetic and molecular studies have shown that, in the human, these multidrug resistant cells contain amplified MDR1 genes, resulting in increased gene expression. The MDR1a gene, which encodes P-glycoprotein, is expressed at significant levels in about half of all human cancers. The multidrug transporter is one of two major mechanisms identified in the efflux, or extraction, of drugs from animal cells. The other mechanism for drug efflux involves expression of a gene known as the multidrug resistance associated protein (MRP). Both the MDR1 and the MRP genes are members of a superfamily of ATP-dependent transporters, as noted in a review article in the 1995 Annual Review of Genetics.1 There are probably other, unidentified members of this superfamily that are involved in other forms of drug resistance.
It was the role of the MDR1 gene that inspired Dr. Alfred H. Schinkel and Dr. Piet Borst of the Netherlands Cancer Institute in Amsterdam to take an interest in comparable animal genes with physiological functions very similar to the human MDR1 gene–the murine MDR1a gene.
"We were interested in work that might help in finding a reversal agent to block the development of multidrug resistance in cancer cells," he explains. "The goal is to overcome or reduce drug resistance in tumors so that you can get higher levels of chemotherapy agents into the tumor without having to increase the plasma level of the agents, which are extremely toxic and virulent."
Schinkel developed a knockout mouse, in which the mouse's MDR1a gene was disrupted, and his work demonstrated similarities between the drug-transport influences of the mouse MDR1a and those of the human MDR1 gene. Subsequently, an MDR1b knockout mouse was developed. Schinkel and others believe the two mouse genes have largely overlapping functions, but with varying influences on transporting different drugs.
Other research, beginning several years ago, showed that steroid hormones enhance multidrug resistance gene expression. This work was verified in 1993 with a targeted disruption study of the murine MDR1b gene. Another current line of research seeks to develop gene therapy using human MDR1 vectors to carry a passenger gene to modify cells genetically to protect bone marrow of cancer patients from the cytotoxic effects of potent anticancer drugs. Clinical trials are under way involving MDR1 retrovirus transduction of autologous bone marrow transplants in patients with cancer of the brain, breast, and ovary.
In research with mice, a large number of studies utilize MDR genes and the MDR1a knockout mouse. Earlier studies with various mouse and human MDR genes have shown that two mouse genes, the MDR1a and the MDR1b, and the human MDR1 gene can confer the multidrug resistance phenotype to drug-sensitive cells, but the MDR2 and MDR2 genes cannot.
Dr. Schinkel and collaborators are currently investigating a double-knockout MDR mouse model as a further step in their quest to understand the regulation of the MDR gene family and the factors that control differential expression of its gene variants in defining future cancer treatment options and addressing the enigma of multidrug resistance in cancer cells.
"Since MDR1a knockout mice show increased expression of the MDR1b gene in liver, it is possible that other functions of MDR1a may be partially compensated for by MDR1b expression. Consequently, MDR1a/MDR1b double-knockout mice will need to be analyzed to determine whether other hypothesized functions of P-glycoproteins in handling of endogenous steroids and in excretion of other xenobiotics are correct."3
Thus, scientists' hopes for a double-knockout MDR mouse illustrate how advances made possible by transgene technology quickly lead to a perceived need for still more highly specific transgenic models for newly opened avenues of research.
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