Jun 10 2005
If a dividing cell's activity is a pop song, then the same process in an immune cell is an extended-play dance remix. The basics of cell division are the same in both, but there's a heck of a lot more going on in immune cells, Johns Hopkins scientists have discovered.
All dividing cells have to faithfully copy their DNA so that both new cells get the same information, and immune cells are no exception. But only immune cells must do some genetic rearranging - a genetic "jam session" - so they can make the endless variety of antibodies needed to fight infections and foreign proteins in general. If this recombination happens at the wrong time or interrupts the wrong genes, lymphoma, a cancer of tissues that make immune cells, may result.
Although the jam session itself - the actual rearrangement of particular genes - is well- studied and has an official name, V(D)J recombination, no one had ever tied its beginning or end to the process of cell division.
Writing in the June 10 issue of Molecular Cell, researchers from Johns Hopkins report that the band leader that normally launches the DNA-copying machinery to start cell division also brings the jam session to a close, intricately connecting the two processes.
"V(D)J recombination essentially breaks, cuts out and reattaches DNA, so it uses cell's normal DNA repairing machinery, which just happens to be already working overtime as the cell prepares to copy its DNA," says Stephen Desiderio, M.D., Ph.D., professor of molecular biology and genetics in Johns Hopkins' Institute for Basic Biomedical Sciences, which he directs. "Our guess is that the cell uses the same protein to start DNA copying and end recombination in order to prevent breaks from happening at a time when a repair can't be made."
Desiderio's team a decade earlier discovered that a protein called RAG2, which helps break genes for recombination, ebbs and flows with the steps of cell division. It reaches peak levels just before DNA copying starts, then drops quickly and stays low until the cell completely divides.
"It was clear that destruction of RAG2 signals the end of active recombination, but whether its decline just happened to coincide with the beginning of DNA copying or whether the two processes were somehow linked was unclear," he says. "Now we have the first biochemical evidence that it's no mere coincidence."
The evidence was uncovered in experiments by Hao Jiang, Ph.D., who developed a system in which he could sort out the proteins and other players involved in the destruction of RAG2 without the complexity of working in living cells.
Using his "cell-free" system, Jiang and his colleagues discovered that a dizzying loop of activity - featuring a band leader called Skp2 - ends the recombination of immune genes.
If the jam session is going well, the immune genes are being broken and recombined with help from RAG2. Suddenly, levels of a protein called Skp2 rise. Skp2 binds to RAG2, then recruits other proteins that tag RAG2 for destruction, halting the jam session. At the same time, other copies of Skp2 are causing the destruction of another protein in order to take the brakes off the DNA copying machinery.
"This is the first evidence showing that a protein that controls regular cell division also controls an exclusive kind of recombination that happens only in immune cells," says Desiderio. "This form of recombination is essentially a step in cell division in these immune cells."
Broken DNA, whether cut for recombination or other reasons, can cause the break to be repaired, the cell to "commit suicide," or the cell to divide more slowly than normal. If the cell stalls, the break might eventually be repaired improperly.
"This can lead to frank genomic rearrangement that is no longer limited to the place where recombination is supposed to take place, and that in turn can eventually lead to certain lymphomas," says Desiderio.
Jiang, now a postdoctoral fellow at The Rockefeller University, received the 2004 David Israel Macht Research Award for aspects of this work during the 27th Annual Young Investigators' Day at the Johns Hopkins School of Medicine. He conducted the research as a graduate student in the Graduate Program in Immunology, working in Desiderio's laboratory.