Immunohistochemical Analysis of Sox17 Associated Pathway in Brain Arteriovenous Malformations

Background

Sox17 has emerged as an important factor in vascular remodeling because of the potential linkage with Wnt/β-catenin, Notch, and the inflammatory pathway. Brain arteriovenous malformation (BAVM), as an angiogenic and inflammatory disorder, might possess an aberrant regulation of the Sox17 associated pathway. We sought to investigate the expression of the Sox17 associated pathway in BAVMs.

Methods

Using immunohistochemical methods, 16 paraffin specimens of BAVM nidus were analyzed. Specimens were obtained from patients during surgical procedures.

Results

Expression of Sox17, Hey1, and β-catenin was observed in all specimens. Large veins possessed a distinct pattern of expression; thick-walled veins had a stronger intensity, whereas thin-walled veins had a weaker intensity, of Sox17, Hey1, and β-catenin (P < 0.001). Thick-walled veins also had a higher expression of Sox17, Hey1, and β-catenin compared with large arteries (P < 0.05). Hey1 and β-catenin expression was also higher in thick-walled veins compared with brain microvessels (P < 0.01). In addition, the difference in expression of the Sox17 associated pathway (Hey1 and β-catenin) was observed in medium and small arteries compared with large arteries in BAVM nidus and brain microvessels (P < 0.01).

Conclusions

The Sox17 associated pathway was activated in the BAVM nidus. Our results indicate that arterial identity is gained in thick-walled veins; this might reflect the process of arterialization of the veins as a result of hemodynamic stress. In addition, high expression of the Sox17 associated pathway in medium and small arteries indicates that BAVM vessels are intrinsically active.

Introduction

Brain arteriovenous malformations (BAVMs) are composed of tangles of abnormally developed vasculature without intervening capillaries.¹ As a consequence, abnormal shunting of arteries and veins occurs and results in high-pressure vascular channels, which are at risk of rupture, often with catastrophic outcomes, particularly in younger patients.² BAVMs are generally considered a congenital disorder; however, clinical evidence regarding involution, regrowth, and de novo formation argue against the static condition of BAVMs.3, 4, 5

Knowledge of the pathobiology of BAVMs has increased; we and other groups have reported that abnormal angiogenesis, inflammation, and certain genetic factors are involved in the mechanism of BAVMs.6, 7, 8, 9, 10, 11, 12, 13 However, the pathobiology remains elusive and contradictory. Shoemaker et al.¹⁴ reported that expression of lymphatic markers increases in BAVMs. In contrast, Zhuge et al.12, 13 described Notch activation in BAVMs, as generally conceived that Notch activation in endothelial cells (ECs) represses vein and lymphatic markers, which promotes the arterial identity. Furthermore, despite a higher incidence of BAVMs in patients with hereditary hemorrhagic telangiectasia, endoglin expression was found to be increased in sporadic BAVMs, whereas the opposite occurred in patients with hereditary hemorrhagic telangiectasia.⁹ Genetic studies also failed to demonstrate an association between endoglin and the incidence of sporadic BAVMs.¹⁰ Hence, it is suggested that multiple pathways or a heterogeneous pathway, rather than a single pathway, is involved in BAVM pathobiology.

It was previously reported that the Sox17 locus is one of the most susceptible loci associated with intracranial aneurysms, suggesting its importance for blood vessel maintenance.15, 16 Sox17 is a transcription factor belonging to the SoxF family, which contains high-mobility-group box.¹⁷ Although Sox17 has been considered an important regulator in biologic processes, its significance in vascular biology is only beginning to be recognized. It was previously reported that Sox17 is a pathway that links Wnt/β-catenin and Notch in angiogenesis.¹⁸ The interplay between Wnt and Notch signaling is well noted in various biologic processes. In vascular development, Wnt and Notch interact with each other to determine proper EC differentiation, vascular remodeling, and arteriovenous specification. Aberrant regulation of Wnt and Notch has been reported to alter normal vascular development.19, 20, 21, 22 Wnt signaling increases Sox17 transcription, subsequently activating Notch and promoting arterial identity.¹⁸ The relationship between Sox17 and Notch seems to be complex, as Notch was also reported to provide negative feedback for Sox17 to restrict angiogenesis.²³ The interaction between Wnt, Notch, and Sox17 is essential for proper vascular development. In pathologic conditions, it was previously reported that Sox17 is involved in angiogenesis in tumor microvessels.²⁴ The presence of Sox17 improves tumor vascularization and inflammatory cell recruitment in tumor microvessels. It was recently reported that Sox17^+/− mice specifically exhibit intracranial aneurysm in hypertension,²⁵ further emphasizing the potential of Sox17 in intracranial vessel remodeling.

Sox17 involvement in BAVMs is intriguing; furthermore, the potential of Sox17 interaction with several key angiogenic and inflammatory pathways could provide a novel perspective of the complexity of BAVMs. In this study we sought to investigate activation of Sox17 associated pathways in BAVMs.