FMF occurs when a gene called MEFV, which encodes the protein pyrin, is mutated. The mutation, in turn, enhances rather than diminishes the immune signaling activity of pyrin. Pyrin signaling plays a key role in regulating the body's immune system, particularly inflammation.
FMF primarily affects populations that emerged from the regions around the Mediterranean Basin. Beginning in childhood, patients experience recurrent 24-hour to three-day episodes of symptoms that include fever, severe abdominal pain, painful swollen joints, skin rashes, or sharp, stabbing chest pain. Some FMF patients also develop a buildup of an inflammatory protein in the kidneys and other vital organs, which can lead to kidney failure and death.
As many as 20 to 30 percent of the Jewish, Arab, Armenian and Turkish populations are carriers of a single copy of the FMF mutation, although a smaller percentage harbor the two copies usually needed to develop symptoms. The attacks of FMF can often be prevented by the daily use of colchicine, a medication that is also sometimes used for gout. However, colchicine does not always work in FMF, and it can cause serious side effects.
In 1997, a team led by National Human Genome Research Institute Scientific Director Dan Kastner, M.D., Ph.D., identified pyrin as the protein mutated in FMF. Identification of pyrin led to the development of genetic tests for FMF and the subsequent discovery of other genetically distinct hereditary fevers. But two important mysteries remained, according to Dr. Kastner. "We did not know how mutations in pyrin cause inflammation, or why the carrier frequency is so high in Mediterranean populations," he said.
Dr. Kastner and collaborators subsequently showed that pyrin regulates inflammation through interleukin 1 (IL-1), a cell-signaling protein, or cytokine, produced by white blood cells. The IL-1 cytokine is a key component of the innate immune system . It's release by monocytes and macrophages, two types of white blood cells, unleashes a variety of immune cells that rally to the body's defense. However, it was unclear whether pyrin ordinarily acts as an inhibitor of IL-1 that is lacking in FMF, or as an inducer of IL-1 that is activated in FMF.
To address this question, Dr. Kastner and his colleagues turned to animal models, breeding mice with targeted genetic changes. Initially, they hypothesized that the MEFV mutation knocked out the production of the normal pyrin protein and that the loss of functional pyrin caused FMF. To test that idea, Kastner and his collaborators created what researchers refer to as a knock-out mouse model of FMF in which they disrupted MEFV's production of pyrin, expecting that the loss of functional pyrin protein would produce symptoms of the disorder in the mice. Surprisingly, the knock-out mice showed no signs of the disease.
Dr. Kastner's team then switched its strategy. Instead of knocking out the normal MEFV gene, they created targeted changes in the MEFV gene to produce an altered form of the pyrin protein. Previous studies in humans had shown that most FMF patients have mutations in MEFV involving only one letter in the genetic code. A single-letter change can produce a substitution of a single amino acid in the corresponding pyrin protein that can change the way the protein works.
Because most of the FMF-associated mutations appeared in a particular area of the gene, researchers selectively inserted those changes in the same area of the mouse genome associated with the human disease. The result is called a knock-in mouse. In the current study in Immunity, Dr. Kastner, senior author of the study, and his collaborators showed that FMF knock-in mice have severe spontaneous inflammatory symptoms comparable to FMF patients.
Studies in these knock-in mice demonstrated that the FMF-associated pyrin mutations cause inflammation by activating IL-1. The mechanism of IL-1 activation in these mice appears to be independent of other known mechanisms of IL-1 activation and may play a role in many other types of inflammation. Of potential importance to FMF patients, strategies that block IL-1 completely blocked inflammation in the knock-in mice.
"Our finding using this mouse model clarifies the pathogenesis of FMF and supports the initiation of trials of IL-1 inhibitors in FMF patients who do not respond to colchicine or cannot tolerate the medication," said lead author Jae Jin Chae, Ph.D., staff scientist in the Inflammatory Disease Section of NHGRI's Medical Genetics Branch. Dr. Chae also observed that bone marrow transplantation was successful in treating the knock-in mice. The risks of bone marrow transplantation currently preclude its use in the human disease.
In future studies using the animal model, the researchers hope to understand why such high percentages of populations from the Mediterranean Basin carry a gene mutation for FMF. They theorize that the mutation may have conferred an advantage at some point in history when a pathogen — perhaps an epidemic — challenged the immune system of the region's population. The researchers hope to make a connection between this possible evolutionarily selective advantage and a type of known pathogen.
Study coauthors included investigators from NHGRI's Medical Genetics Branch and Genetic and Molecular Biology Branch and the National Cancer Institute's Dermatology Branch and Laboratory Animal Sciences Program.
For more on Familial Mediterranean fever, please go to: Learning About Familial Mediterranean Fever.
Posted: May 31, 2011