A Mechanistic Framework for Somatic Mosaicism in Autoinflammatory Diseases Beyond Mendelian Inheritance

Hereditary autoinflammatory diseases have traditionally been defined as disorders caused by germline mutations in innate immune pathway genes transmitted through Mendelian inheritance, an assumption challenged by the discovery that postzygotic somatic mosaicism can reproduce similar autoinflammatory phenotypes. Aim of the review is to synthesize the mechanistic evidence for somatic and gonadal mosaicism across the major monogenic autoinflammatory diseases and to reassess the adequacy of "hereditary" as a categorical descriptor for this disease class. A structured narrative review of PubMed/MEDLINE was conducted (2002-2026), screening 241 records for evidence of somatic mosaicism in NLRP3, MEFV, TNFRSF1A, NOD2, NLRC4, UBA1, and MVK, of which 35 met inclusion criteria for qualitative synthesis; rather than proceeding disease-by-disease, evidence was organized around the underlying mechanisms by which a minority mutant-cell population produces disease. Four mechanistic categories were identified: (1) dominant effect without clonal expansion, most extensively documented in NLRP3-mediated cryopyrin-associated periodic disease; (2) clonal growth advantage, which introduces lineage-restricted diagnostic blind spots and, when gonadal tissue is involved, has implications for genetic counseling risk; (3) "passenger" mechanism converging with age-related clonal hematopoiesis in late-onset disease; and (4) no germline counterpart at all for UBA1-driven VEXAS syndrome. The detection of variants depends heavily on sequencing depth and tissue/lineage sampling, but variant allele fraction does not consistently predict clinical severity. Mevalonate kinase deficiency, a recessively inherited autoinflammatory disorder, has no reported mosaic cases, a pattern attributed to its biallelic genetic architecture rather than under-ascertainment. "Hereditary", which conflates transmission and cellular distribution, is no longer an adequate categorical descriptor for autoinflammatory diseases as a class, though it remains accurate for individual patients and specific variants.