Key biomarkers in cerebral arteriovenous malformations: Updated review

The formation of vascular networks consisting of arteries, capillaries, and veins is vital in embryogenesis. It is also crucial in adulthood for the formation of a functional vasculature. Cerebral arteriovenous malformations (CAVMs) are linked with a remarkable risk of intracerebral hemorrhage because arterial blood is directly shunted into the veins before the arterial blood pressure is dissipated. The underlying mechanisms responsible for arteriovenous malformation (AVM) growth, progression, and rupture are not fully known, yet the critical role of inflammation in AVM pathogenesis has been noted. The proinflammatory cytokines are upregulated in CAVM, which stimulates overexpression of cell adhesion molecules in endothelial cells (ECs), leading to improved leukocyte recruitment. It is well-known that metalloproteinase-9 secretion by leukocytes disrupts CAVM walls resulting in rupture. Moreover, inflammation alters the angioarchitecture of CAVMs by upregulating angiogenic factors impacting the apoptosis, migration, and proliferation of ECs. A better understanding of the molecular signature of CAVM might allow us to identify biomarkers predicting this complication, acting as a goal for further investigations that may be potentially targeted in gene therapy. The present review is focused on the numerous studies conducted on the molecular signature of CAVM and the associated hemorrhage. The association of numerous molecular signatures with a higher risk of CAVM rupture is shown through inducing proinflammatory mediators, as well as growth factors signaling, Ras-mitogen-activated protein kinase-extracellular signal-regulated kinase, and NOTCH pathways, which are accompanied by cellular level inflammation and endothelial alterations resulting in vascular wall instability. According to the studies, it is assumed that matrix metalloproteinase, interleukin-6, and vascular endothelial growth factor are the biomarkers most associated with CAVM and the rate of hemorrhage, as well as diagnostic methods, with respect to enhancing the patient-specific risk estimation and improving treatment choices.

The Role and Therapeutic Implications of Inflammation in the Pathogenesis of Brain Arteriovenous Malformations

Brain arteriovenous malformations (bAVMs) are focal vascular lesions composed of abnormal vascular channels without an intervening capillary network. As a result, high-pressure arterial blood shunts directly into the venous outflow system. These high-flow, low-resistance shunts are composed of dilated, tortuous, and fragile vessels, which are prone to rupture. BAVMs are a leading cause of hemorrhagic stroke in children and young adults. Current treatments for bAVMs are limited to surgery, embolization, and radiosurgery, although even these options are not viable for ~20% of AVM patients due to excessive risk. Critically, inflammation has been suggested to contribute to lesion progression. Here we summarize the current literature discussing the role of the immune system in bAVM pathogenesis and lesion progression, as well as the potential for targeting inflammation to prevent bAVM rupture and intracranial hemorrhage. We conclude by proposing that a dysfunctional endothelium, which harbors the somatic mutations that have been shown to give rise to sporadic bAVMs, may drive disease development and progression by altering the immune status of the brain.

RNA sequencing analysis between ruptured and un-ruptured brain AVM

BACKGROUND

A brain arteriovenous malformation (BAVM) is a tangle of abnormal blood vessels connecting the arteries and veins in the brain and is associated with a higher risk for intracerebral hemorrhage (ICH). RNA sequencing technology has been recently used to investigate the mechanism of diseases owing to its ability to identify the gene changes on a transcriptome-wide level. This study aims to gain insights into the potential mechanism involved in BAVM rupture.

METHODS

Sixty-five BAVM nidus samples were collected, among which 28 were ruptured and 37 were un-ruptured. Then, next-generation RNA sequencing was performed on all of them to obtain differential expressed genes (DEGs) between the two groups. In addition, bioinformatics analysis was performed to evaluate the involved biological processes and pathways by GO and KEGG analysis. Finally, we performed a univariate Cox regression analysis to obtain the early rupture-prone DEGs.

RESULTS

A total of 951 genes were differentially expressed between the ruptured and un-ruptured BAVM groups, of which 740 genes were upregulated and 211 genes were downregulated in ruptured BAVMs. Then, bioinformatics analysis showed the biological processes and pathways related to the inflammatory processes and extracellular matrix organization were significantly enriched. Meanwhile, some downregulated genes are involved in cell adhesion and genes participating in response to muscle activity and the terms of nervous system development. Finally, one hundred twenty-five genes, many were involved in inflammation, were correlated with the early rupture of BAVMs.

CONCLUSIONS

The upregulated genes in the ruptured BAVM group were involved in inflammatory processes and extracellular matrix organization. Some of the downregulated genes participated in cell adhesion and myofibril assembly, indicating the role of enhanced inflammation and reduced inflammation vessel strength in BAVMs rupture.

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.

KRAS/BRAF mutations in brain arteriovenous malformations: A systematic review and meta-analysis

INTRODUCTION

Somatic KRAS mutations have been identified in the majority of brain arteriovenous malformations (AVM) specimens. The aim of our study was to evaluate the prevalence of Kirsten rat sarcoma (KRAS)/murine sarcoma viral oncogene homolog B1 (BRAF) mutations in brain AVM.

METHODS

A systematic literature review was performed in November 2019. We reviewed MEDLINE/PubMed, Cochrane Library, and ClinicalTrials.gov for citation or ongoing trials from January 2010 to March 2020.

RESULTS

6 studies were identified as meeting the inclusion criteria of this review. The total frequency of KRAS mutations in 1726 patients with AVM was 55%. The prevalence of BRAF mutation was 7.5%. The prevalence of AVMs with grade 2 was the most (39%). Frontal and parietal lobes were the commonest sites of AVMs (21%). the most prevalent presentation of patients with AVM was hemorrhage (62%).

CONCLUSION

Our findings support a high prevalence of somatic activating mutations in KRAS and less commonly, BRAF in the overwhelming majority of brain AVMs. Practically and importantly, this pathway homogeneity in CNS arteriovenous malformations also supports the development of targeted therapies with RAS/RAF pathway inhibitors. However, more studies are needed to confirm this hypothesis.

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

BACKGROUND

Brain arteriovenous malformations (bAVM) may rupture causing disability or death. BAVM vessels are characterized by abnormally high flow that in general triggers expansive vessel remodeling mediated by cyclo-oxygenase-2 (COX2), the target of non-steroidal anti-inflammatory drugs. We investigated whether COX2 is expressed in bAVMs and whether it associates with inflammation and haemorrhage in these lesions.

METHODS

Tissue was obtained from surgery of 139 bAVMs and 21 normal Circle of Willis samples. The samples were studied with immunohistochemistry and real-time quantitative polymerase chain reaction (RT-PCR). Clinical data was collected from patient records.

RESULTS

COX2 expression was found in 78% (109/139) of the bAVMs and localized to the vessels' lumen or medial layer in 70% (95/135) of the bAVMs. Receptors for prostaglandin E2, a COX2-derived mediator of vascular remodeling, were found in the endothelial and smooth muscle cells and perivascular inflammatory cells of bAVMs. COX2 was expressed by infiltrating inflammatory cells and correlated with the extent of inflammation (r = .231, p = .007, Spearman rank correlation). COX2 expression did not associate with haemorrhage.

CONCLUSION

COX2 is induced in bAVMs, and possibly participates in the regulation of vessel wall remodelling and ongoing inflammation. Role of COX2 signalling in the pathobiology and clinical course of bAVMs merits further studies.

Neurovascular inflammation in the pathogenesis of brain arteriovenous malformations

Brain arteriovenous malformations (bAVM) arise as congenital or sporadic focal lesions with a significant risk for intracerebral hemorrhage (ICH). A wide range of interindividual differences is present in the onset, progression, and severity of bAVM. A growing body of gene expression and polymorphism-based research studies support the involvement of localized inflammation in bAVM disease progression and rupture. In this review article, we analyze the altered responses of neural, vascular, and immune cell types that contribute to the inflammatory process, which exacerbates the pathophysiological progression of vascular dysmorphogenesis in bAVM lesions. The cumulative effect of inflammation in bAVM development is orchestrated by various genetic moderators and inflammatory mediators. We also discuss the potential therapies for the treatment of brain AVM by targeting the inflammatory processes and mediators. Elucidating the precise role of inflammation in the bAVM growth and hemorrhage would open novel avenues for noninvasive and effectual causal therapy that may complement the current therapeutic strategies.

Basic research and surgical techniques for brain arteriovenous malformations

Arteriovenous malformations (AVMs) are hemorrhagic vascular diseases in which arteries and veins are directly connected with no capillary bed between the two. We herein introduce the results of basic research of this disease and surgical techniques based on our data and experiences. The results obtained from our research show that cell death- and inflammation-related molecules changed or became activated compared with control specimens. These findings indicate that chronic inflammation occurs in and around the nidus of AVMs. Various molecules are involved in the mechanisms of cell death and angiogenesis during this process. Confirmation of blood flow in the nidus is very important to avoid hemorrhagic complications during surgical removal of the nidus. The risk of hemorrhage increases when the blood flow in the nidus is not reduced. We reported the advantages of serial indocyanine green videoangiography, which is used to assess the blood flow during AVM nidus removal. Since publication of the ARUBA trial and Scottish Audit, treatments with high morbidity have not been allowed. It is especially important for neurosurgeons to treat low Spetzler-Martin grade AVMs with low morbidity. J. Med. Invest. 67 : 222-228, August, 2020.

KRAS G12D or G12V Mutation in Human Brain Arteriovenous Malformations

BACKGROUND

Brain arteriovenous malformations (BAVMs) are vascular malformations composed of tangles of abnormally developed vasculature without capillaries. Abnormal shunting of arteries and veins is formed, resulting in high-pressure vascular channels, which potentially lead to rupture. BAVMs are generally considered a congenital disorder. But clinical evidence regarding involution, regrowth, and de novo formation argue against the static condition of this disease. Recently, the presence of the somatic activating KRAS mutations in more than half of BAVM cases was reported, suggesting the role of KRAS function in the pathogenesis.

METHODS

KRAS mutation in codon35 (G→A, G12D; G→T, G12V) was examined by a digital polymerase chain reaction analysis using genome purified from paraffin-embedded slides of human BAVMs. We also examined protein expression of KRAS G12D in lesions to corroborate results from digital polymerase chain reaction analysis.

RESULTS

We detected codon35 G→A mutation in 15 (39.5%) among 38 samples and codon35 G→T mutation in 10 (27.0%) among 37 samples we could assess mutations. There were no samples positive for both codon35 G→A and G→T mutation. The ratio of codon35 G→A mutation ranged from 0.60% to 12.28% and that of G→T was from 1.20% to 8.99%. We next examined protein expression of KRAS G12D in BAVM lesions in immunohistochemistry. A KRAS G12D mutant was detected mainly in endothelial cells of dilated vessels in lesions.

CONCLUSIONS

KRAS mutations in codon35 were detected in about two thirds of specimens examined. KRAS function may actively contribute to the pathobiology of BAVM and can become a therapeutic target.

Histopathological Features of Brain Arteriovenous Malformations in Japanese Patients

Clinical features of high risk brain arteriovenous malformations (BAVMs) are well characterized. However, pathological evidences about the differences that are possessed by high risk patients are still lacking. We reviewed archived routine hematoxylin-eosin specimens from a total of 54 surgical treated BAVMs. The histopathological features in nidus were semi-quantitatively analyzed. We obtained the pathological differences of BAVMs nidus between several clinical features. Among the analyzed pathological features, the significant differences were observed in degree of venous enlargement and intimal hyperplasia. Juvenile, female, diffuse nidus, high Spetzler-Martin grade, and low flow patients had a lesser degree of those parameters compared to adult, male, compact nidus, low Spetzler-Martin grade and high flow patients. High risk profiles of BAVMs patients were well-reflected in the nidus pathology. Therefore, juvenile, female, diffuse nidus, and low flow in Japanese BAVMs patients might have different vascular remodeling process that predispose to higher tendency of hemorrhage.

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.

Brain arteriovenous malformation modeling, pathogenesis, and novel therapeutic targets

Patients harboring brain arteriovenous malformation (bAVM) are at life-threatening risk of rupture and intracranial hemorrhage (ICH). The pathogenesis of bAVM has not been completely understood. Current treatment options are invasive, and ≈ 20 % of patients are not offered interventional therapy because of excessive treatment risk. There are no specific medical therapies to treat bAVMs. The lack of validated animal models has been an obstacle for testing hypotheses of bAVM pathogenesis and testing new therapies. In this review, we summarize bAVM model development and bAVM pathogenesis and potential therapeutic targets that have been identified during model development.

Distinctive distribution of lymphocytes in unruptured and previously untreated brain arteriovenous malformation

AIM

To test the hypothesis that lymphocyte infiltration in brain arteriovenous malformation (bAVM) is not associated with iron deposition (indicator of microhemorrhage).

METHODS

Sections of unruptured, previously untreated bAVM specimens (n=19) were stained immunohistochemically for T-lymphocytes (CD3⁺), B-lymphocytes (CD20⁺), plasma cells (CD138⁺) and macrophages (CD68⁺). Iron deposition was assessed by hematoxylin and eosin and Prussian blue stains. Superficial temporal arteries (STA) were used as control.

RESULTS

Both T lymphocytes and macrophages were present in unruptured, previously untreated bAVM specimens, whereas few B cells and plasma cells were detected. Iron deposition was detected in 8 specimens (42%; 95% confidence interval =20-67%). The samples with iron deposition tended to have more macrophages than those without (666±313 vs 478±174 cells/mm²; P=0.11). T-cells were clustered on the luminal side of the endothelial surface, on the vessel-wall, and in the perivascular regions. There was no correlation between T lymphocyte load and iron deposition (P=0.88). No macrophages and lymphocytes were detected in STA controls.

CONCLUSIONS

T-lymphocytes were present in bAVM specimens. Unlike macrophages, the load and location of T-lymphocytes were not associated with iron deposition, suggesting the possibility of an independent cell-mediated immunological mechanism in bAVM pathogenesis.

Ferumoxytol-enhanced MRI to Image Inflammation within Human Brain Arteriovenous Malformations: A Pilot Investigation

Inflammation cell infiltration and cytokine expression are seen in the vascular walls and intervening stroma of resected brain arteriovenous malformation (bAVM) specimens, even in unruptured and previously untreated lesions. Macrophages may play a critical role in bAVM progression to rupture, and could serve as a marker for rupture risk. We assessed feasibility of imaging macrophages within the bAVM nidus using ferumoxytol-enhanced MRI in four patients with already diagnosed bAVMs using iron-sensitive imaging (ISI; T2*-GE-MRI sequence). Patients were imaged at baseline and at either 1 day (n=2) or 5 days (n=2) after infusion of 5mg/kg of ferumoxytol. Residual intravascular ferumoxytol obscured evaluation for uptake in bAVM vascular walls and stroma at the 1-day time point. The two cases imaged at 5 days showed less intravascular tracer but had signal loss in the nidal region consistent with ferumoxytol localization. One case underwent surgical resection; there was prominent vascular wall CD68 staining. Ferumoxytol-enhanced-MRI for assessing bAVM inflammatory cell burden appears feasible and has the potential to be developed as a biomarker to study lesional inflammatory events.

Differential gene expression in relation to the clinical characteristics of human brain arteriovenous malformations

Arteriovenous malformations (AVMs) of the central nervous system are considered as congenital disorders. They are composed of abnormally developed dilated arteries and veins and are characterized microscopically by the absence of a capillary network. We previously reported DNA fragmentation and increased expression of apoptosis-related factors in AVM lesions. In this article, we used microarray analysis to examine differential gene expression in relation to clinical manifestations in 11 AVM samples from Japanese patients. We categorized the genes with altered expression into four groups: death-related, neuron-related, inflammation-related, and other. The death-related differentially expressed genes were MMP9, LIF, SOD2, BCL2A1, MMP12, and HSPA6. The neuron-related genes were NPY, S100A9, NeuroD2, S100Abeta, CAMK2A, SYNPR, CHRM2, and CAMKV. The inflammation-related genes were PTX3, IL8, IL6, CXCL10, GBP1, CHRM3, CXCL1, IL1R2, CCL18, and CCL13. In addition, we compared gene expression in those with or without clinical characteristics including deep drainer, embolization, and high-flow nidus. We identified a small number of genes. Using these microarray data we are able to generate and test new hypotheses to explore AVM pathophysiology. Microarray analysis is a useful technique to study clinical specimens from patients with brain vascular malformations.

Molecular imaging of cerebrovascular lesions

Inflammation is a key component in the pathogenesis of cerebrovascular lesions. Two agents have emerged as promising possibilities for imaging cerebrovascular lesions. These agents are ferumoxytol and myeloperoxidase (MPO)-specific paramagnetic magnetic resonance (MR) contrast agent. Ferumoxytol is an iron oxide nanoparticle coated by a carbohydrate shell that is used in MRI studies as an inflammatory marker as it is cleared by macrophages. Ferumoxytol-enhanced MRI allows noninvasive assessment of the inflammatory status of cerebral aneurysms and arteriovenous malformations and, possibly, may differentiate "unstable" lesions that require early intervention from "stable" lesions that can be safely observed. Several pilot studies have also suggested that MPO-specific paramagnetic MR contrast agent, di-5-hydroxytryptamide of gadopentetate dimeglumine, may allow imaging of inflammation in the wall of saccular aneurysms in animal models. However, studies in human subjects have yet to be performed. In this paper, we review current data regarding ferumoxytol-enhanced MRI and MPO-specific paramagnetic MR contrast agent and discuss current and future applications.

The emerging role of ferumoxytol-enhanced MRI in the management of cerebrovascular lesions

Inflammation is increasingly being understood to be a key component to the pathophysiology of cerebrovascular lesions. Ferumoxytol, an iron oxide nanoparticle coated by a carbohydrate shell, has been used in MRI studies as an inflammatory marker because it is cleared by macrophages. Ferumoxytol-enhanced MRI has emerged as an important tool for noninvasive assessment of the inflammatory status of cerebrovascular lesions, namely aneurysms and arteriovenous malformations. Moreover, preliminary evidence suggests that ferumoxytol-enhanced MRI could be applied as a non-invasive tool to differentiate “unstable” lesions that require early intervention from “stable” lesions in which observation may be safe. Assessment of the effects of anti-inflammatory pharmacological interventions on cerebrovascular lesions is also a potentially crucial application of the technique. Future improvements in technique and MRI signal quantification will certainly pave the way for widespread and efficient use of ferumoxytol-enhanced MRI in clinical practice. In this paper, we review current data regarding ferumoxytol-enhanced MRI and discuss its current/potential applications and future perspectives.