BACKGROUND: Poikiloderma, hereditary fibrosing, with tendon contractures, myopathy, and pulmonary fibrosis (POIKTMP) is a rare genetic multisystemic fibrosing disorder caused by FAM111B gene mutations. Given its rarity, the molecular underpinnings of POIKTMP remain elusive. FAM111B, a trypsin-like serine protease, initially studied in cancer, exhibits germline variants not consistently linked to tumours, suggesting broader functions beyond cell proliferation.

METHODS: In this study, we compiled and compared the clinical features of 41 POIKTMP patients, which included the description of 4 newly identified cases. Functional studies involved the exploration of patient-derived cells carrying FAM111B missense variants using omics technologies.

FINDINGS: Our results show that the phenotypic spectrum of POIKTMP encompassed renal failure, dental anomalies, hypoparathyroidism, and potentially neuropathy. Notably, variants clustering within the D-box domain of FAM111B protein tend to present a more severe phenotype. Most importantly, loss of FAM111B expression perturbed ubiquitin-proteasome system (UPS) function, leading to increased content of ubiquitin-protein conjugates and a sterile type I interferon signature.

INTERPRETATION: These findings highlight a dysfunctional UPS as a potential central driver of POIKTMP's molecular pathogenesis, presenting promising therapeutic avenues.

FUNDING: Association Française contre les Myopathies (AFM - 20760), Fondation Génavie (657298), Fondation Thellie, I-SITE NExT Junior Talent, Biogenouest, Infrastructures en Biologie Santé et Agronomie (IBiSA) and Conseil Régional de Bretagne.