Background
Familial Mediterranean fever (FMF) is a hereditary autoinflammatory disease in which the body's own immune system overreacts to minimal or inappropriate stimuli, producing recurrent attacks of fever and painful inflammation. The disease is caused by mutations in MEFV, the gene that encodes a protein called pyrin. Inside our cells, pyrin sits at the center of a large protein complex known as an inflammasome, a built-in alarm system that detects bacteria and other dangers and triggers inflammation to protect the body.
Normally, pyrin is held in check by a built-in "brake". The brake is released when cells sense bacterial toxins or certain drugs, or when a person inherits a mutation in pyrin itself or in its regulators. Once the brake is off, pyrin gathers into large clumps that release powerful inflammatory signals, producing the painful flares of FMF. But ever since MEFV was identified in 1997, a key question has remained unanswered: after the brake is released, what partner protein does pyrin engage with to assemble its inflammatory complex?
Many different mutations in MEFV are linked to FMF. Some cause severe attacks; others lead to milder symptoms. The international Infevers database now lists more than 400 of these mutations, but the majority are classified as "variants of uncertain significance", meaning doctors do not yet know whether each one actually drives disease.
Key findings
The two new studies focus on the same molecular pair: pyrin and a small cellular GTPase called CDC42, best known as a regulator of cell shape and movement. The first study, conducted by Mariko Aoki, Alberto Iannuzzo and Philippe Mertz under the supervision of Takahiro Yasumi and Jérôme Delon, began in the clinic as part of an international collaborative effort. The team identified six patients from three unrelated families with severe inflammatory symptoms who all carried the same previously unreported mutation, called T43I, in the CDC42 gene. Normally, CDC42 meets pyrin only briefly, just long enough to turn on the inflammatory switch when needed, and then lets it go. Using AI-based structural modeling and biochemical experiments, the team showed that the T43I mutation reshapes a small hairpin loop on the surface of CDC42, making the hairpin cling to the tail end of pyrin (a region called the B30.2 domain) far more tightly than normal. Once mutant CDC42 has pyrin in that firm grip, the cellular alarm becomes abnormally prone to activation: cells from these patients form abnormal protein clumps, release high levels of inflammatory signals (IL-1β and IL-18), and ultimately rupture. These findings establish CDC42 as the long-sought direct partner of pyrin: the "key" that, when turned in the right lock, ignites inflammation. But when the key is warped, the lock spins too easily—and pyrin spirals out of control.
The companion study, conducted by Naoya Iwata and colleagues under the supervision of Takahiro Yasumi and Yoshitaka Honda, started from the opposite end. The team adopted a "genotype-first" approach, introducing 265 MEFV variants into human cells one by one and using a cell-based assay to read out what these changes actually do. The result is a functional atlas of the MEFV gene that sorts the variants into clear classes (gain-of-function, loss-of-function and neutral) and pinpoints several previously uncharacterized disease-causing mutations. The team then mapped these mutations onto a 3D model of pyrin. The classical FMF mutations (M680I, M694V and M694I) all clustered on a single small patch of the B30.2 domain. This was the very "keyhole" that the first study had shown to contact CDC42. Follow-up experiments confirmed that these mutations make pyrin grip CDC42 more tightly, speeding up the formation of pyrin clumps inside the cell and triggering inflammation. When the researchers removed CDC42 from the cells, this hyperactivity stopped. But they also found that a separate group of non-FMF mutations worked differently: they activated pyrin largely independently of CDC42. In other words, more than one mechanism can drive pyrin activation.
Together, the two studies complete a picture that has eluded the field for nearly three decades. Mutations on the "key" side of the system (CDC42) make the lock open too easily; mutations on the "keyhole" side (the pyrin B30.2 domain) make the lock turn too easily. Either way, the outcome is the same: CDC42 interacts with pyrin and gathers it into clumps inside the cell. From these clumps, inflammatory signals are sent out and pyrin-driven inflammatory attacks are triggered. This is the molecular step the field has been missing, and it explains how FMF and related diseases driven by faulty pyrin regulation get switched on.
Looking ahead
Beyond explaining the disease, the discovery has immediate practical value. The functional atlas of MEFV variants converts hundreds of previously uninterpretable genetic results into much clearer, more actionable answers, opening the way to faster and more accurate diagnosis for patients and their families who had been left in limbo. The identification of CDC42 as a direct regulator of pyrin also points to a new axis of inflammasome control—one that could be targeted to prevent the runaway activation seen in both CDC42-associated autoinflammation and the most severe forms of FMF. It also opens a path to precision medicine: because some MEFV mutations drive inflammation through CDC42 while others do not, future treatments could be tailored to each patient's molecular subtype, rather than treating all pyrin-associated diseases as one. The work also offers a proof of concept for a "genotype-first" strategy, in which complex disease mechanisms are derived from a comprehensive map of genetic variants. The same approach could be applied to many other genetic diseases.
Glossary
Familial Mediterranean fever (FMF): A genetic autoinflammatory disorder caused by mutations in the MEFV gene, characterized by recurrent fevers and painful inflammation of the abdomen, chest, and joints. In Japan, FMF is officially designated as an intractable disease, with around 500 patients reported in a 2009 nationwide survey.
Inflammasome: A large protein complex inside cells that acts as a built-in "fire alarm" of the immune system. When activated, it triggers the release of pro-inflammatory signals such as IL-1β and IL-18 and a form of inflammatory cell death called pyroptosis.
Pyrin (MEFV): A sensor protein at the heart of one type of inflammasome. Mutations in the MEFV gene that encodes pyrin cause familial Mediterranean fever and a broader family of pyrin-associated autoinflammatory diseases.
Paper Information
Aoki, M., Iannuzzo, A., Mertz, P., Feng, S., Iwata, N., Perugini, C., Tsuchida, N., El Masri, R., Kuchitsu, Y., Tacine, B., Coppola, S., Shibata, H., Cescato, M., Nishitani-Isa, M., Terré, A., Kawasaki, Y., Dalmon, S., Nishimura, K.,Magnotti, F., ... Delon, J. (2026). An N-terminal CDC42 T43I variant reveals the mechanism of pyrin inflammasome activation. Science Immunology. DOI: 10.1126/sciimmunol.aea0515
Iwata, N., Kuchitsu, Y., Hijikata, A., Shibata, H., Nishitani-Isa, M., Aoki, M., Izawa, K., Yoshitomi, H., Takita, J., Ueno, H., Taguchi, T., Yasumi, T., & Honda, Y. (2026). A genotype-first approach reveals the molecular basis of pyrin inflammasome activation. Science Immunology. DOI: 10.1126/sciimmunol.aea0705
