A molecular handshake between two immune proteins may explain why familial Mediterranean fever (FMF) erupts into painful inflammation—and could help doctors interpret hundreds of genetic variants that have long remained medical mysteries. Two studies published in Science Immunology identify the small signaling protein CDC42 as a direct regulator of pyrin, the immune sensor at the center of FMF and related autoinflammatory disorders. The findings reveal how mutations in either protein can push the immune system toward dangerous overactivation.
FMF is an inherited disease caused by mutations in MEFV, the gene that produces pyrin. Patients experience recurring attacks of fever and inflammation, often affecting the abdomen, chest, joints, or other tissues. Unlike autoimmune diseases, in which the immune system attacks specific body targets, autoinflammatory diseases arise from excessive activation of the body’s innate immune defenses. In FMF, pyrin forms part of an inflammasome—a multiprotein alarm complex that detects cellular danger and activates powerful inflammatory pathways.
Under normal conditions, pyrin is restrained by molecular “brakes.” Bacterial toxins, certain drugs, or other danger signals can release those restraints, allowing pyrin to assemble into clusters inside the cell. These structures organize the pyrin inflammasome, which activates inflammatory enzymes and promotes the release of interleukin-1 beta and interleukin-18. The process can also trigger pyroptosis, an inflammatory form of programmed cell death in which the cell ruptures and releases alarm signals. Until now, however, scientists did not know which partner pyrin directly engaged after its inhibitory brake was removed.
The first study began with six patients from three unrelated families who developed severe inflammatory symptoms and carried the same previously unreported CDC42 mutation, known as T43I. CDC42 is a small GTPase best known for controlling the internal scaffolding of cells, including their shape, movement, and organization. Researchers led by Mariko Aoki, Alberto Iannuzzo, Philippe Mertz, Takahiro Yasumi, and Jérôme Delon combined clinical observations, biochemical experiments, and artificial-intelligence-based structural modeling to investigate how the mutation altered immune signaling.
Their analysis indicated that the T43I substitution changes a small hairpin-shaped loop on the surface of CDC42. The altered loop binds much more tightly to the B30.2 domain at the end of pyrin than the normal protein does. Under healthy conditions, CDC42 and pyrin interact only briefly, allowing the immune system to respond to genuine danger without remaining permanently switched on. The mutant CDC42, by contrast, appears to hold pyrin in a state that is unusually ready to assemble into inflammatory clusters.
Cells carrying the CDC42 T43I variant formed abnormal pyrin-containing structures, released excessive amounts of interleukin-1 beta and interleukin-18, and eventually underwent pyroptosis. These results establish CDC42 as the long-sought direct molecular partner of pyrin. The discovery also explains how a mutation outside the MEFV gene can produce a pyrin-driven autoinflammatory syndrome: CDC42 does not need to be the immune sensor itself to cause disease. By gripping pyrin too strongly, it can make the sensor respond to signals that would normally be harmless or insufficient.
The companion study approached the problem from the opposite direction. Naoya Iwata and colleagues tested 265 MEFV variants individually in human cells, using a functional assay to determine whether each mutation increased, reduced, or failed to alter pyrin activity. This genotype-first strategy produced a functional map of the gene, helping distinguish disease-causing gain-of-function variants from neutral changes and mutations that impair pyrin activity. Such information could be particularly valuable because the Infevers database contains more than 400 reported MEFV variants, many of which are still classified as having uncertain clinical significance.
When the researchers mapped the variants onto a three-dimensional model of pyrin, several classic FMF mutations—including M680I, M694V, and M694I—clustered within the B30.2 domain. This region was the same molecular surface identified in the CDC42 study as the site of interaction between the two proteins. Follow-up experiments showed that these FMF-associated pyrin variants also strengthened the pyrin–CDC42 interaction, accelerating the formation of pyrin clusters and triggering inflammation. Removing CDC42 from the cells prevented this excessive activity, confirming that CDC42 is essential for this particular activation route.
Together, the studies suggest that pyrin activation can be viewed as a molecular lock-and-key system. Mutations in CDC42 can make the key engage too forcefully, while mutations in pyrin’s B30.2 domain can make the lock turn too easily. In both cases, the result is premature clustering of pyrin and activation of the inflammasome, sometimes after only minor stimulation. The researchers also identified non-FMF MEFV mutations that activate pyrin largely independently of CDC42, showing that multiple molecular routes can lead to the same inflammatory outcome. This distinction may eventually support more precise treatments, with therapies tailored to the mechanism driving an individual patient’s disease.
The findings have immediate diagnostic significance and may influence the development of future inflammasome-targeting medicines. A functional atlas of MEFV variants could help physicians interpret genetic test results more confidently, reducing uncertainty for patients and families. Meanwhile, the CDC42–pyrin axis offers a new target for controlling diseases caused by excessive pyrin signaling, including CDC42-associated autoinflammation and severe forms of FMF. By revealing the molecular step that connects genetic mutations to runaway inflammation, the research provides both a long-missing explanation for FMF and a possible blueprint for precision medicine across a broader range of inherited immune disorders.
Subject of Research: Molecular mechanisms of pyrin inflammasome activation, familial Mediterranean fever, CDC42-associated autoinflammation, and functional analysis of MEFV variants.
Article Title: An N-terminal CDC42 T43I variant reveals the mechanism of pyrin inflammasome activation.
Web References: https://doi.org/10.1126/sciimmunol.aea0515; https://doi.org/10.1126/sciimmunol.aea0705
References: Aoki, M., et al. (2026). “An N-terminal CDC42 T43I variant reveals the mechanism of pyrin inflammasome activation.” Science Immunology. DOI: 10.1126/sciimmunol.aea0515. Iwata, N., et al. (2026). “A genotype-first approach reveals the molecular basis of pyrin inflammasome activation.” Science Immunology. DOI: 10.1126/sciimmunol.aea0705.
Image Credits: Illustrator: Ivana Duic.
Keywords: Familial Mediterranean fever, pyrin, CDC42, inflammasome, autoinflammatory disease, MEFV, IL-1β, IL-18, pyroptosis, precision medicine.
Tags: autoinflammatory disease mechanismsCDC42 immune signalingfamilial Mediterranean fever pathogenesisgenetic mutations in MEFVgenetic variants in autoinflammatory disordersimmune proteins interactioninflammasome activation triggersinflammatory pathways in FMFinnate immune system dysregulationmolecular basis of inflammationpyrin and CDC42 regulatory pathwayspyrin inflammasome regulation
