UF researchers discover how leukemia cells develop resistance to treatment

Despite the onslaught of anticancer drugs bent on killing them, certain leukemia cells thrive by generating an enzyme critical to their survival, University of Florida researchers report in the current issue of The Biochemical Journal.

The findings could someday lead to new strategies aimed at conquering drug resistance in cancer patients.

Since the 1960s, doctors have used a powerful combination of chemotherapy drugs to treat acute lymphoblastic leukemia, or A.L.L., one of the disease’s four main forms and the most common childhood cancer. The concoction is so effective that in more than 98 percent of children the disease goes into remission. Yet a quarter eventually relapse and no longer respond to the treatment.

Experiments in cell cultures demonstrated why this might happen: Within 12 hours of being bathed with chemotherapy drugs, some leukemic cells in culture ramped up production of the protective enzyme and continued producing it for months. The result? The number of cancer cells killed by treatment dwindled, and drug-resistant cells multiplied.

UF scientists, led by Michael Kilberg, Ph.D., a UF professor of biochemistry and molecular biology, also recently identified the DNA sequence that’s responsible for switching on the gene that increases the amount of the enzyme the resistant cells produce.

“Now we are developing gene therapy techniques to use that piece of DNA as a therapeutic tool,” said Kilberg, who is affiliated with the UF Shands Cancer Center. “Ideally we’d like to be able to interrupt the activation of that gene in the leukemic cell. Therefore, the child who has become resistant to treatment would be sensitive again.”

Currently, patients whose cancer has recurred and is resistant to treatment frequently resort to bone marrow transplantation, their only remaining option and one that is not always successful.

“The outcome for children who relapse within the first few years after diagnosis is particularly poor, with only 5 to 10 percent being long-term survivors,” said Stephen Hunger, M.D., chief of the division of pediatric hematology/oncology at UF’s College of Medicine. “Bone marrow transplantation is an important treatment option for children with relapsed A.L.L., but many patients still die post-transplant.”

One of the drugs doctors give leukemia patients is an enzyme that breaks down one of 20 common amino acids that all cells need for growth and cell division, Kilberg said.

“Most cells have an enzyme inside them that’s the opposite of the one you’re giving as a drug -- it makes the amino acid,” he said.

“This particular kind of leukemia, however, has very low levels of that enzyme, so it usually can’t manufacture the amino acid very well. When you give the enzyme that breaks down the amino acid, those cells can’t divide and grow, so they undergo programmed cell death.”

When leukemia recurs, however, the drug-resistant cancer cells have somehow evolved the ability to sense that the amino acid essential to life is in short supply. They then activate the gene responsible for producing the enzyme that makes it, Kilberg said.

“This happens in all cells, and it helps nonleukemic cells survive the drug treatment,” Kilberg said. “But it’s particularly bad when it happens in the leukemic cell, because now that cell isn’t killed by the enzyme we give – now it can make the amino acid it needs and the drug is totally ineffective.”

In recent years, scientists have favored two main theories regarding the development of drug resistance, said Bruce M. Camitta, M.D., a professor of pediatrics at the Medical College of Wisconsin in Milwaukee.

“(One theory is that) resistant cells are present at the time of initial diagnosis and are ‘selected’ as chemotherapy eliminates nonresistant cells. (The other is that) resistant cells arise as a result of mutations during the course of treatment,” Camitta said.

“Currently, the best approach to either of these scenarios is early intensive multidrug chemotherapy to eliminate resistant cells or to decrease chances for mutation to resistance.

“UF researchers have meticulously documented what was previously only partially shown,” he added. “It should now be possible to prevent or to treat resistance by designing inhibitors specific for the (amino acid-producing enzyme).”

Kilberg said UF scientists are seeking to show that in principle any tumor cell can be made sensitive to the enzyme doctors give by interfering with the cell’s ability to produce the crucial amino acid.

“The really exciting part of this is if we can show this works in the leukemic cell -- and we’re nowhere near that yet -- then in principle the same approach could work for any tumor cell,” Kilberg said.

Kilberg’s research could pave the way for new ways to combat drug resistance, Hunger said.

“We need to identify new drugs that are active against this disease and learn how to use existing drugs more effectively,” Hunger said. “Dr. Kilberg’s work provides information that may be useful in developing strategies to overcome this resistance. More effective ways to use (the enzyme) will also be useful for adults with A.L.L., and both children and adults with non-Hodgkin’s lymphoma.”

Kilberg collaborated with Bradley S. Fletcher, M.D., Ph.D., an instructor in pharmacology and experimental therapeutics; former UF graduate student Ara M. Aslanian; and Randy McClellan, a postdoctoral fellow in pediatrics.

“We’re still in the early stages of understanding the basic science,” Kilberg said. “I always worry about people getting false hopes about therapy being just around the corner. We’ve got a long way to go. Like many basic science studies you have to walk before you can run. We’re really trying to provide a strong biochemical basis for this therapy.”

The UF Shands Cancer Center is an interdisciplinary initiative connecting clinical and basic researchers at the UF Health Science Center’s Gainesville and Jacksonville campuses, Shands at UF and Shands Jacksonville who perform original scientific research and enhance clinical strategies for the diagnosis, treatment and prevention of cancer.