Volume 5, Issue 39 November 2008
Doing the DNA Shuffle
Mark Schlissel is Dean of Biological Sciences in the College of Letters and Science and a professor of immunology and pathogenesis. Image credit: UC Berkeley
In the battle against infection, the odds look stacked against us. We rely on antibodies to recognize and sound the alarm on potential invaders. Yet our cells come programmed with less than 30,000 genes, far fewer than the billions of foreign structures we might encounter.
Even so, says Mark Schlissel, a Berkeley professor of immunology and pathogenesis, "the immune system is capable of recognizing literally hundreds of millions of foreign chemical structures."
This phenomenal flexibility comes courtesy of a remarkable DNA shuffling system called V(D)J recombination. Just as riffling a deck of cards can produce an endless variety of poker hands, shuffling specialized DNA segments in developing immune cells can produce a different antibody structure nearly every time. During the reaction, enzymes select one of many available genetic versions for each variable antibody segment, snip out the unused portions, and stitch the chosen pieces back together. The resulting antibody travels to the surface of the immune cell, or lymphocyte, where it can recognize bacteria, viruses and toxins in the bloodstream.
The immune system produces a staggering variety of different antibodies using a process called V(D)J recombination. VH, DH, and JH gene segments are rearranged during lymphocyte development to generate new DNA blueprints for each antibody. Image credit: Mark Schlissel
"All of us have developing lymphocytes in our bone marrow shuffling these antibody genes around continuously, from the time we are in the womb," Schlissel says. Schlissel studies V(D)J recombination and its place in lymphocyte development. Understanding when this reaction occurs and how it is regulated will help scientists learn to treat leukemia, lymphoma, immune deficiencies and a wide array of autoimmune diseases.
V(D)J recombination occurs in two types of lymphocytes, B cells and T cells. The genes modified in each cell type are different, but the enzymes involved are the same. How does the recombinase determine which genes to work on in each cell? In a pivotal experiment, Schlissel isolated nuclei from B cells and T cells, and added recombinase enzymes to each. The enzymes rearranged T cell genes when given T cell nuclei, and B cell genes when given B cell nuclei. This indicated that the enzymes were responding to differences in the accessibility of various genes within the cells' DNA. Subsequent experiments have implicated structural variations in how the DNA is packaged into a structure called chromatin in different types of cells. Schlissel is fleshing out how these modifications affect recombinase behavior and influence the fate of a nascent lymphocyte.
The AMuLV virus infects early B cell precursors, arresting their development and causing leukemia. Inset: Treating leukemic cells with the chemotherapeutic agent Gleevec turns B cell recombinase genes back on, allowing the cells to mature. Image credit: Mark Schlissel
Gene shuffling may be an elegant solution to a thorny biological problem, but it also carries risks. "Many leukemias are caused by errors in this shuffling mechanism," Schlissel says. "Instead of shuffling two antibody genes together, they'll mistakenly take a potential cancer gene and shuffle it next to a piece of antibody gene."
To understand what can go wrong, Schlissel employs a virus that causes leukemia in mice. The virus has a gene that turns recombinase enzyme genes off, at the same time making lymphocytes malignant. Schlissel has not only identified the lymphocyte genes affected by the virus, but discovered that this mechanism contributes to how a chemotherapy drug for leukemia called Gleevec operates. "More than 90 percent of people with chronic myelogenous leukemia have a chromosomal translocation that involves this same gene," Schlissel says.
Every shuffle of DNA segments also carries the danger of producing an antibody that recognizes the body's own tissues. To avoid such potential autoimmune reactions, every new antibody undergoes self-tolerance testing in the bone marrow. If the antibody flunks, the recombinase returns to the nucleus for another round of gene rearrangement. Schlissel is tracking down the cellular signals that regulate this process. "There's a signal that notifies the nucleus to keep making recombination enzymes because a bad antibody has been made, as well as a signal that says you did a good job, shut off the system. We're tracking down both," Schlissel says.
Schlissel demonstrated that recombination enzymes recognize antibody genes in B cell nuclei and T cell receptor genes in T cell nuclei but both types of genes in purified DNA. Arrows indicate evidence of gene rearrangement where B=B cell nuclei, T = T cell nuclei, N = skin cell nuclei, and gen. DNA = purified DNA. Image credit: Mark Schlissel
Schlissel is now poised to begin a very different avenue in his career. This fall, he began a five-year appointment as Dean of Biological Sciences in the College of Letters and Science. As dean, he says, "I feel I can contribute to scientific success much more broadly, to help recruit and retain faculty across the entire landscape of modern biology. I plan to try to promote good science, make sure that our educational mission is given adequate attention and resources, and spend our limited financial resources wisely and responsibly."
Serving as dean is a full-time job in and of itself, but Schlissel is determined to retain his own research program as well. Here, Schlissel is following in the footsteps of his mentor Don Brown, who ran a prolific laboratory and served as director of the Carnegie Institute of Washington for many years. "He said his job was to make it so he was the only guy in the building that had to worry about anything other than science. I'd like to do the same."