Identification of rare missense variants reducing cathepsin O secretion in families with intracranial aneurysm

Using whole-exome sequencing plus identity-by-descent mapping in two large families with multiple intracranial aneurysm (IA) cases, this study identified two rare missense CTSO variants (p.Val316Ile and p.Ala43Val) that segregated with IA. CTSO expression was detected in murine circle-of-Willis vessels and human IA domes, and CTSO protein was found in VSMC lysates and conditioned media, consistent with secretion. Functional assays showed that cyclic stretch increased CTSO secretion without increasing Ctso mRNA, linking haemodynamic-type strain to extracellular CTSO availability. CTSO depletion reduced VSMC transmigration, accelerated adhesion to fibronectin (FN), increased FAK phosphorylation, and increased FN accumulation without corresponding Fn1 mRNA changes; SPR demonstrated high-affinity CTSO–FN binding. AFM nanoindentation revealed that CTSO depletion increased VSMC stiffness, and exogenous CTSO reversed the stiffening. Importantly, cells expressing the IA-associated CTSO variants showed markedly reduced CTSO secretion and recapitulated the increased FN phenotype, supporting a mechanism in which impaired CTSO secretion promotes FN deposition, enhanced VSMC–FN adhesion, and VSMC stiffening—processes implicated in maladaptive arterial remodeling that may predispose to IA.