First described by Luschka and Virchow in the mid 19th century,1 arteriovenous malformations (AVMs) are congenital vascular lesions consisting of anomalous tangles of vessels that have direct connections between arteries and veins without intervening capillaries.2 These lesions can cause death and disability.1,2 Microsurgery, endovascular embolization, and stereotactic radiosurgery (SRS) have the potential to improve on the natural history of these lesions, and in select cases may offer a cure when applied thoughtfully.1,2 A growing body of literature is beginning to reveal new ideas on how these lesions may be modulated biologically. AVM-derived brain endothelial cells (AVM-BECs) are thought to be highly active cells demonstrating abnormal functions and overexpressing some pro-angiogenic growth factors.3 This group of cells is thought to undergo rapid proliferation and migration, and tend to produce aberrant tubule structures.3 Moreover, these cells have dysregulation of angiogenesis; for instance, thrombospondin-1 (TSP-1), which antagonizes Vascular Endothelial Growth Factor A (VEGF-A), a key proangiogenic molecule, is found to be at low levels in AVM-BECs. Inhibition of “inhibitor of DNA-binding protein A” (Id-1) (a TSP-1 transcriptional repressor) will allow normalization of function and behavior of this group of cells.4,5 A group of non-coding RNA that inhibits gene expression, microRNA-18a (miR-18a) would be a promising option to increase TSP-1 in AVM-BECs by inhibiting its inhibitor Id-1.
To investigate the possibility that microRNA-18a can be potentially used as a therapeutic agent for AVMs by enhancing their brain endothelial cell function, Ferreira et al6 conducted an experiment on AVM-BECs obtained from surgical specimens of 6 patients after AVM resection. These specimens were compared to control BECs isolated from normal cortex tissues from 4 patients who underwent surgery for epilepsy. The authors treated the cells for 24 hours with lipofectamine-delivered (2 μg/mL) miR-18a, naked miR-18a mimic, scrambled miRNA sequence, and small interfering RNA for green fluorescent protein (siGFP). An orbital shaker produced the shear flow and cells were analyzed for TSP-1, VEGF-A, VEGF-B, VEGF-C, and VEGF-D using enzyme-linked immunosorbent assay reagents. The cells were observed for proliferation and tubule formation and the gene expression was confirmed by quantitative polymerase chain reaction.
The results showed that miR-18a was significantly less expressed and TSP-1 less secreted in AVM-BECs than in control BECs. TSP-1 secretion by AVM-BECs significantly increased by Lipofectamine-delivered miR-18a and naked miR-18a mimic but did not increase in control BECs. When the cells were subjected to different shear flow magnitudes (static, venous flow and arterial flow), AVM-BECs secreted significantly more TSP-1 and had reduced Id-1mRNA only with naked miR-18a (and not by Lipofectamine-delivered miR-18a) under static and arterial flow (results from static vs venous flow were not statistically different). This showed that naked miR-18a increased the TSP-1 by inhibiting its inhibitor (Id-1) irrespective of the flow conditions. Under arterial flow only, naked miR-18a significantly decreased the VEGF-A and VEGF-D release in AVM-BECs and not in normal BECs, demonstrating that these cytokines are major targets of inhibition in angiogenesis. Furthermore, naked miR-18a decreased proliferation and increased tubule formation and branching of AVM-BEC compared to control BEC under arterial flow, demonstrating that miR-18a enhance AVM-BEC structure and function.
This experiment highlights the therapeutic potential of miR-18a for AVMs. The study also underlines its feasible clinical application by showing that miR-18a can be administered without a transfection reagent. This could theoretically allow an intravenous infusion of the miR-18a to correct the functionally anomalous AVM-BECs because these cells are in direct contact with the blood stream. Alternatively, such an agent could be delivered through endovascular catheters. This study suggests that a new era of biological treatments for brain arteriovenous malformations may be on the horizon.
1. Bendok BR, El Tecle NE, El Ahmadieh TY, et al.. Advances and innovations in brain arteriovenous malformation surgery. Neurosurgery. 2014;74(suppl 1):S60–S73.
2. Yashar P, Amar AP, Giannotta SL, et al.. Cerebral arteriovenous malformations: issues of the interplay between stereotactic radiosurgery and endovascular surgical therapy. World Neurosurg. 2011;75:638–647.
3. Jabbour MN, Elder JB, Samuelson CG, et al.. Aberrant angiogenic characteristics of human brain arteriovenous malformation endothelial cells. Neurosurgery. 2009;64(1):139–146; discussion 46-48.
4. Stapleton CJ, Armstrong DL, Zidovetzki R, Liu CY, Giannotta SL, Hofman FM. Thrombospondin-1 modulates the angiogenic phenotype of human cerebral arteriovenous malformation endothelial cells. Neurosurgery. 2011;68:1342–1353; discussion 53.
5. Lawler PR, Lawler J. Molecular basis for the regulation of angiogenesis by thrombospondin-1 and -2. Cold Spring Harbor Perspect Med. 2012;2(5):a006627.
6. Ferreira R, Santos T, Amar A, et al.. MicroRNA-18a improves human cerebral arteriovenous malformation endothelial cell function. Stroke. 2014;45(1):293–297.