Aims: Intracranial aneurysm (IA) is a common cerebrovascular abnormality characterized by localized dilation and wall thinning in cerebral arteries, which can rupture and lead to fatal subarachnoid hemorrhage. Although genetic factors can contribute to IA, the genetic predisposition of IA is largely unknown. This study aims to identify rare functional variants associated with IA in families with multiple affected subjects and explore their impact on IA pathophysiology.

Methods and results: By combining whole exome sequencing and identity-by-descent analyses, we have identified two rare missense variants in the CTSO gene associated to IA in two large families with multiple affected subjects. We found that the cysteine-type papain-like cathepsin O (CTSO) encoded by CTSO is expressed in the circle of Willis of mice and in the wall of human IA domes. Stretching of vascular smooth muscle cells (VSMC) induced CTSO secretion. CTSO controls VSMC migration and adhesion to the extracellular matrix, and directly interacts with fibronectin (FN). CTSO depletion, or expression of the two CTSO variants, which are poorly secreted, increased the amount of FN. Moreover, CTSO depletion augmented VSMC stiffness, which was reduced by the addition of exogenous CTSO.

Conclusion: Collectively, our findings identify CTSO as a potential new player in arterial remodeling, regulating FN deposition and VSMC function, supporting the causal role of rare coding CTSO variants in familial forms of IA.