A de novo dominant-negative PSMB8 mutation causes severe CANDLE/PRAAS due to arrested proteasome biogenesis

OBJECTIVES: Proteasome-associated autoinflammatory syndromes (PRAAS) include a group of autoinflammatory interferonopathies caused by 20S proteasome dysfunction. We characterised pathomechanisms and treatment responses of patients with a de novo, dominant-negative (DN)-proteasome subunit beta type-8 (PSMB8) variant. METHODS: Patients with the DN-PSMB8 p.G209R variant encoding a mutant β5i subunit of the 20S immunoproteasome were evaluated. Interferon biomarkers, proteasome activity, structural modelling, and proteotoxic stress responses were assessed. Patients' T cells underwent integrated transcriptomic and proteomic profiling to characterise immune dysregulation, proteotoxic stress responses, mitochondrial function, and type I interferon (IFN-I) associated stress signalling. RESULTS: Patients with DN-PRAAS presented with early-onset systemic inflammation, panniculitis, cytopenias, infections, and porto-sinusoidal vascular liver disease (PSVD), indicating broader immune dysfunction that partially responds to Janus kinase inhibition and/or interferon-α/β receptor blockade (anifrolumab). Mechanistically, the PSMB8 p.G209R variant caused steric hindrance that impaired β5i propeptide processing and final 20S proteasome formation, resulting in intracellular protein aggregation, impaired mitochondrial metabolism, and altered neutral lipid processing. The IFN-I signature of patients' T cells was reduced by blockade of 2 integrated stress response (ISR)-regulating kinases, protein kinase R (PKR) and general control nonderepressible 2 (GCN2), and by Janus kinase signalling. CONCLUSIONS: The DN-PSMB8 p.G209R variant broadens the clinical and mechanistic PRAAS spectrum by causing 20S proteasome maturation arrest and uncovering a convergence between mitochondrial dysfunction and the ISR. Our findings implicate cytopenia in a pattern of vascular pathology, including PSVD of the liver. We further identify PKR and GCN2 as key mediators of maladaptive IFN-I responses and potential therapeutic targets.