By Sally Pobojewski
News and Information Services
A U-M researcher has discovered that antibodies produced by individuals with the autoimmune disorder systemic lupus erythematosus (SLE) actually change the shape of the patients’ DNA molecules to produce a tighter bond between antibodies and DNA.
Binding between lupus antibodies and DNA molecules is the first step in a series of immunological reactions that can cause serious tissue damage and sometimes death in lupus patients, according to Gary D. Glick, assistant professor of chemistry.
Glick’s discovery is significant because it is the only known example of DNA being physically changed by an antibody and because it may lead to the development of new drugs that can block the binding process and prevent lupus-related tissue damage.
The first disclosure of Glick’s evidence for this “induced fit” phenomenon was published in the Feb. 24 issue of the Journal of the American Chemical Society.
The human body produces millions of different types of protein molecules called antibodies, each customized to bind to one specific invading bacteria or virus and hold it prisoner until it is engulfed and destroyed by white blood cells.
“Some individuals with lupus produce a unique type of antibody that sees DNA as the ‘enemy,’” Glick explained. A component of all living organisms, DNA is a complex molecule containing the genetic code that determines every cell’s structure and function.
“While everyone has small amounts of antibodies that bind with DNA, these anti-DNA antibodies are different—both in the extreme level of activity directed against DNA molecules and in the active binding mechanism itself,” Glick said.
Researchers estimate there are more than 500,000 persons with lupus in the United States. Ninety percent are women and the disease is much more common in Blacks and Hispanics.
“Lupus symptoms range from mild to life-threatening, and the disease can attack any organ in the body,” said David A. Fox, chief of rheumatology in the Department of Internal Medicine and director of the U-M Multipurpose Arthritis and Musculoskeletal Diseases Center. “In severe cases, these abnormal antibodies bind to sections of DNA and become lodged in the patient’s kidneys where they trigger inflammatory reactions by white blood cells that can lead to kidney failure.”
Glick believes the “induced fit” mechanism used by anti-DNA antibodies may be much more common than scientists believe. “Most researchers visualize protein binding as a passive process like a key fitting in a lock,” Glick said. “They believe the key either fits or it doesn’t.
“I believe protein binding is an active process and that antibodies frequently force structural changes in target molecules to produce a binding site or strengthen an existing bond,” Glick said. “Induced fit will eventually be seen as a common mechanism in biochemical recognition.”
In his experiments, Glick used electrophoretic assays to monitor the interaction between anti-DNA antibodies and segments of synthesized DNA with a common “hairpin loop” structure. In this type of structure, a DNA double helix is connected at one end with a single strand of DNA.
Glick discovered that lupus antibodies bind to the single strand of DNA and then somehow “melt” the adjacent double-strand segment, producing a larger single-strand bubble capable of sustaining a much stronger bond with the antibody.
“We don’t believe the hairpin structure is specifically targeted by the antibody,” Glick explained. “It’s the close proximity of DNA double-strands to single-strands that is important. The antibody recognizes this particular structure, which could be found in several different patterns. Hairpins are just a convenient model to study.”
In future research, Glick hopes to identify the exact structural element or feature of DNA that is targeted by lupus antibodies, and generate a three-dimensional image of the binding site itself.
“Once we know precisely what and where the binding site is, organic chemists may be able to synthesize a new drug that can block anti-DNA antibodies,” Glick said. “It’s not a cure for lupus, but it could be the beginning of an effective new treatment for the disease.”
“Prof. Glick’s research is a good example of how interdisciplinary collaboration can benefit biomedical research,” Fox said. “His background in organic chemistry gave him the expertise to tackle this problem from a different perspective.”
Researchers at the U-M Multipurpose Arthritis and Musculoskeletal Diseases Center contributed technical expertise on lupus, provided anti-DNA antibodies from lupus patients, and produced new anti-DNA monoclonal antibodies from mice for use in Glick’s research, says Fox.
The research is funded by the National Arthritis Foundation, the National Institutes of Health and the U-M Multipurpose Arthritis and Musculoskeletal Diseases Center.
Others assisting with the research include Edward W. Voss Jr., professor of microbiology and immunology at the University of Illinois at Champaign-Urbana; and U-M graduate students Shawn Y. Stevens and Patrick C. Swanson.
