For more than a decade, Dr. Susan Rosenberg, professor of molecular and human genetics at Baylor College of Medicine, has solidified her premise that when cells are under stress, the rate of gene changes called mutations goes up, a finding that has implications for a wide variety of biological phenomena – from evolution to antibiotic resistance and cancer.
In a report that appears online today in the Proceedings of the National Academy of Sciences, she and her colleagues not only show that this stress-induced mutagenesis occurs in an entirely new assay system, it also is a significant contributor to spontaneous mutation – those changes in the genetic code that are not purposefully caused in the laboratory.
Rate of mutation
Real world cares about the reasons why bacteria are resistant to antibiotics or why anti-cancer drugs do not work, said Rosenberg. Her works provides an explanation for some of that. Rosenberg is a professor in the departments of molecular virology and microbiology and biochemistry and molecular biology at BCM. She is also a member of the faculty of the NCI-designated Dan L Duncan Cancer Center at BCM.
Her new assay in which she measures the rate of mutation that occurs in bacteria called Escherichia coli or E. coli eliminates the possibility of laboratory artifacts. The new system does not rely on a gene in an extra chunk of DNA called a plasmid that exists alongside E. coli's only chromosome. Instead, the new assay system uses only bacterial cells without the plasmid.
Previously, the cells she used had an inactive gene for the metabolism of a sugar called lactose. When she placed these bacterial cells on a bed of lactose that they could not use for food, they starved. This stressor increased the mutation rate.
This time, she used bacteria that had a defective gene for resistance to an antibiotic called tetracycline. If the gene had worked, the cells would be resistant to the antibiotic. Instead, these cells were susceptible. Again, she starved the cells. In response to the starvation, the cells increased their rate of mutation – including mutation of the defective gene.
"What we see is that they have a dose-dependent response relative to starvation that gives them a mutation that confers antibiotic resistance," she said.
In another set of experiments, she and her colleagues attempted to find out how much stress-induced mutagenesis contributed to spontaneous mutation. To do this, they studied starved cells that were not being stimulated with a second stressor, a DNA break.
One by one, they eliminated the stress-response pathways within the cells that they knew contributed to stress-induced mutagenesis.
"When we did that, half of the mutagenesis went away," said Rosenberg. "That means we can say half of spontaneous mutation is stress-inducible."
Tackling antibiotic resistant bacteria
Her findings challenge many assumptions about mutation and evolution, but they also explain some inconsistencies. While mutation can be random, it may not always occur at a consistent rate. Instead, an organism may increase its rate of mutation when it is in an inhospitable environment that provides a stressor. Her theory does not say that increases in the rate of mutation means that the organism is trying to accomplish a mutation that will help it survive, but organisms that do this will have a higher chance of survival.
Not only are the findings applicable to understanding how genetic diversity occurs, they also can help tackle problems such as antibiotic resistant bacteria or chemotherapy-resistant cancer cells.
"This could be a big change in perspective," she said. Both antibiotics and anti-cancer drugs can be considered anti-proliferative – they stop cells from growing or kill them.
However, as physicians have learned, killing all the cells is nearly impossible.
"You kill some and aggravate or stress the rest," she said. "These drugs are stressors. When you put these cells in a stressing environment, you might turn up the mutation rate." Some of the mutations will be lethal to the cells, but others might make them resistant to the drugs. As the rate of mutation goes up, that likelihood increases.
Stress turn up mutation rate
In fact, she said, a few laboratories have already shown that when human cells are stressed, they turn up the mutation rate as well.
As one cancer specialist told her, "Here, in the ‘first world,' we don't die of cancer. We die of therapy-resistant cancer. The problem we have is that the therapies we have are inadequate."
Others who took part in this research include Dr. Chandan Shee, Dr. Janet L. Gibson, Michele C. Darrow and Dr. Caleb Gonzalez, all of BCM.
Funding for this work came from the U.S. Public Health Service.