Cyclo-oxygenase 2, a putative mediator of vessel remodeling, is expressed in the brain AVM vessels and associates with inflammation

Defining the Role of Oral Pathway Inhibitors as Targeted Therapeutics in Arteriovenous Malformation Care

Arteriovenous malformations (AVMs) are vascular malformations that are prone to rupturing and can cause significant morbidity and mortality in relatively young patients. Conventional treatment options such as surgery and endovascular therapy often are insufficient for cure. There is a growing body of knowledge on the genetic and molecular underpinnings of AVM development and maintenance, making the future of precision medicine a real possibility for AVM management. Here, we review the pathophysiology of AVM development across various cell types, with a focus on current and potential druggable targets and their therapeutic potentials in both sporadic and familial AVM populations.

Cellular loci involved in the development of brain arteriovenous malformations

Brain arteriovenous malformations (bAVMs) are abnormal vessels that are prone to rupture, causing life-threatening intracranial bleeding. The mechanism of bAVM formation is poorly understood. Nevertheless, animal studies revealed that gene mutation in endothelial cells (ECs) and angiogenic stimulation are necessary for bAVM initiation. Evidence collected through analyzing bAVM specimens of human and mouse models indicate that cells other than ECs also are involved in bAVM pathogenesis. Both human and mouse bAVMs vessels showed lower mural cell-coverage, suggesting a role of pericytes and vascular smooth muscle cells (vSMCs) in bAVM pathogenesis. Perivascular astrocytes also are important in maintaining cerebral vascular function and take part in bAVM development. Furthermore, higher inflammatory cytokines in bAVM tissue and blood demonstrate the contribution of inflammatory cells in bAVM progression, and rupture. The goal of this paper is to provide our current understanding of the roles of different cellular loci in bAVM pathogenesis.

Ischemia-induced inflammation in arteriovenous malformations

OBJECTIVE

The pathophysiology of development, growth, and rupture of arteriovenous malformations (AVMs) is only partially understood. However, inflammation is known to play an essential role in many vascular diseases. This feasibility study was conducted to investigate the expression of enzymes (cyclooxygenase 2 [COX-2] and NLRP3 [NOD-, LRR-, and pyrin domain–containing protein 3]) in the AVM nidus that are essential in their inflammatory pathways and to explore how these influence the pathophysiology of AVMs.

METHODS

The study group comprised 21 patients with partially thrombosed AVMs. The cohort included 8 ruptured and 13 unruptured AVMs, which had all been treated microsurgically. The formaldehyde-fixed and paraffin-embedded samples were immunohistochemically stained with a monoclonal antibody against COX-2 and NLRP3 (COX-2 clone: CX-294; NLRP3: ab214185). The authors correlated MRI and clinical data with immunohistochemistry, using the Trainable Weka Segmentation algorithm for analysis.

RESULTS

The median AVM volume was 2240 mm3. The proportion of NLRP3-positive cells was significantly higher (26.23%–83.95%), compared to COX-2 positive cells (0.25%–14.94%, p < 0.0001). Ruptured AVMs had no higher expression of NLRP3 (p = 0.39) or COX-2 (p = 0.44), compared to nonruptured AVMs. Moreover, no patient characteristics could be reported that showed significant correlations to the enzyme expression.

CONCLUSIONS

NLRP3 consistently showed an approximately 10-fold higher expression level than COX-2, making the inflammatory process in AVMs appear to be mainly associated with ischemic (NLRP3)–driven rather than with mechanical (COX-2)–driven inflammatory pathways. No direct associations between NLRP3 and COX-2 expression and radiological, standard histopathological, or patient characteristics were found in this cohort.

Involvement of Neutrophil Extracellular Traps in Cerebral Arteriovenous Malformations

BACKGROUND

Cerebral arteriovenous malformations (cAVMs) represent tangles of abnormal vasculature without intervening capillaries. High-pressure vascular channels due to abnormal arterial and venous shunts can lead to rupture. Multiple pathways are involved in the pathobiology of cAVMs including inflammation and genetic factors such as KRAS mutations. Neutrophil release of nuclear chromatin, known as neutrophil extracellular traps (NETs), plays a multifunctional role in infection, inflammation, thrombosis, intracranial aneurysms, and tumor progression. However, the relationship between NETs and the pathobiology of cAVMs remains unknown. We tested whether NETs play a role in the pathobiology of cAVMs.

METHODS

We analyzed samples from patients who had undergone surgery for cAVM and immunohistochemically investigated expression of citrullinated histone H3 (CitH3) as a marker of NETs. CitH3 expression was compared among samples from cAVM patients, epilepsy patients, and normal human brain tissue. Expressions of thrombotic and inflammatory markers were also examined immunohistochemically in samples from cAVM patients.

RESULTS

Expression of CitH3 derived from neutrophils was observed intravascularly in all cAVM samples but not other samples. Nidi of AVMs showed migration of many Iba-I-positive cells adjacent to the endothelium and endothelial COX2 expression, accompanied by expression of IL-6 and IL-8 in the endothelium and intravascular neutrophils. Unexpectedly, expression of CitH3 was not necessarily localized to the vascular wall and thrombus.

CONCLUSIONS

Our results offer the first evidence of intravascular expression of NETs, which might be associated with vascular inflammation in cAVMs.