Scott Keeney, a researcher at Memorial Sloan-Kettering Cancer Center, was riding on a New York subway in 2001 when he felt a sharp pain in his chest. His doctor thought it might be an infection and ordered an x-ray, and then a CT scan. Keeney, who was 35 at the time, had a cancerous tumor growing beneath his sternum.
"I was lucky," Keeney says. Memorial Sloan-Kettering is known as a world leader in the treatment of solid tumors like the germ cell tumor Keeney had been diagnosed with. Less than a month after he first felt the pain in his chest, Keeney began chemotherapy. Some of the drugs he took, such as bleomycin and etoposide, work by attacking the DNA in rapidly dividing cells, such as those in a growing tumor. Keeney knew these drugs well: He was already using them in his research to investigate how organisms repair DNA damage.
Keeney's chemotherapy was successful, and he has been cancer-free for more than five years. Yet he has a slightly higher risk for future cancers because of the damage done to his DNA by the chemotherapeutic compounds. This has given him a personal stake in figuring out how that damage occurs.
Keeney studies a particular phase of meiosis, which occurs in germ cells, the cellular precursors of sperm and eggs. Germ cells, like almost all of the cells in our bodies, contain two versions of each chromosome—one inherited from each parent. When a germ cell is preparing to divide into sperm or egg cells, which will contain only a single copy of each chromosome, the corresponding chromosomes line up next to each other and exchange pieces. The genetic exchange is such that if one chromosome were originally stained blue and the other stained red, the resulting chromosomes would be a mosaic of blue and red. This exchange of genetic material is known as recombination.
When Keeney was a postdoctoral fellow in the laboratory of Nancy Kleckner at Harvard University, he identified a protein known as SPO11, which plays a critical role in recombination. SPO11 slices through a DNA molecule, creating double-strand breaks. "The cell deliberately damages its DNA every time a reproductive cell is made," Keeney says. Other proteins, many of which have yet to be identified, then repair the breaks. These proteins usually stitch the severed DNA molecules together just as they were, so that the original DNA molecule is unchanged. But in some cases, the proteins instead splice the adjoining chromosomes together, creating the crossovers that result in mosaic chromosomes.
Recombination is an essential part of reproduction for most organisms that reproduce sexually. In humans, when recombination malfunctions during meiosis, the wrong number of chromosomes can end up in a sperm or egg cell, causing genetic disorders such as Down syndrome.
Keeney studies recombination in yeast, and he collaborates with Memorial Sloan-Kettering researcher Maria Jasin to study recombination in mice. He and his colleagues have created organisms in which SPO11 is defective or missing altogether. They use these mutant cells to probe how recombination occurs and why it is more likely to occur in some parts of the genome than others.
One of Keeney's major discoveries helped researchers understand the number of times adjoining chromosomes are spliced together during recombination. SPO11 creates many double-strand breaks, but crossovers occur with just a few of those breaks. How does a cell know when and where to exchange the breaks in the chromosome pairs? "These are complicated problems," he says. "But SPO11 has given us a unique avenue to get at this biology." Keeney and his colleagues created a mutant version of SPO11 that produces far fewer double-strand breaks than normal. Yet crossovers occurred at the same frequency as before. Some mechanism in the cell, which Keeney is now investigating, regulates the number of crossovers.
Keeney also has been researching topoisomerases, a family of proteins related to SPO11 that alter the physical structure of DNA. Many man-made drugs and natural compounds affect the activity of cells by targeting topoisomerases. For example, drugs used to treat cancer, like the etoposide Keeney received during his own cancer treatment, prevent DNA from being repaired after it is cut by topoisomerases. Antibiotics like ciprofloxacin act in a similar way on the topoisomerases of pathogenic bacteria. "Knowing how cells repair DNA is important for understanding drug resistance and for designing new chemotherapeutic agents," Keeney says.