PIK3CA and CCM mutations fuel cavernomas through a cancer-like mechanism.

Vascular malformations are thought to be monogenic disorders that result in dysregulated growth of blood vessels. In the brain, cerebral cavernous malformations (CCMs) arise owing to inactivation of the endothelial CCM protein complex, which is required to dampen the activity of the kinase MEKK31-4. Environmental factors can explain differences in the natural history of CCMs between individuals5, but why single CCMs often exhibit sudden, rapid growth, culminating in strokes or seizures, is unknown. Here we show that growth of CCMs requires increased signalling through the phosphatidylinositol-3-kinase (PI3K)-mTOR pathway as well as loss of function of the CCM complex. We identify somatic gain-of-function mutations in PIK3CA and loss-of-function mutations in the CCM complex in the same cells in a majority of human CCMs. Using mouse models, we show that growth of CCMs requires both PI3K gain of function and CCM loss of function in endothelial cells, and that both CCM loss of function and increased expression of the transcription factor KLF4 (a downstream effector of MEKK3) augment mTOR signalling in endothelial cells. Consistent with these findings, the mTORC1 inhibitor rapamycin effectively blocks the formation of CCMs in mouse models. We establish a three-hit mechanism analogous to cancer, in which aggressive vascular malformations arise through the loss of vascular 'suppressor genes' that constrain vessel growth and gain of a vascular 'oncogene' that stimulates excess vessel growth. These findings suggest that aggressive CCMs could be treated using clinically approved mTORC1 inhibitors.

Cerebral cavernous malformation proteins at a glance.

Loss-of-function mutations in genes encoding KRIT1 (also known as CCM1), CCM2 (also known as OSM and malcavernin) or PDCD10 (also known as CCM3) cause cerebral cavernous malformations (CCMs). These abnormalities are characterized by dilated leaky blood vessels, especially in the neurovasculature, that result in increased risk of stroke, focal neurological defects and seizures. The three CCM proteins can exist in a trimeric complex, and each of these essential multi-domain adaptor proteins also interacts with a range of signaling, cytoskeletal and adaptor proteins, presumably accounting for their roles in a range of basic cellular processes including cell adhesion, migration, polarity and apoptosis. In this Cell Science at a Glance article and the accompanying poster, we provide an overview of current models of CCM protein function focusing on how known protein-protein interactions might contribute to cellular phenotypes and highlighting gaps in our current understanding.

Distinct functions for Rap1 signaling in vascular morphogenesis and dysfunction.

Rap1 signaling is important for both major processes of vessel formation: vasculogenesis, or de novo vessel formation, and angiogenesis, sprouting of new vessels from pre-existing ones. We provide an overview of genetic studies in mice and zebrafish and discuss some of the proposed underlying mechanisms derived from cellular models, with particular emphasis on Rap1's role in angiogenesis, maintenance of endothelial barrier and connection with cerebral cavernous malformation (CCM), a neurological deficit that leads to seizures and lethal stroke. Lastly, we provide a brief summary of studies in cardiac and smooth muscle cells, where the Epac-Rap1 signaling axis is emerging as an important regulator of contractility.

Loss of CCM3 impairs DLL4-Notch signalling: implication in endothelial angiogenesis and in inherited cerebral cavernous malformations.

CCM3, a product of the cerebral cavernous malformation 3 or programmed cell death 10 gene (CCM3/PDCD10), is broadly expressed throughout development in both vertebrates and invertebrates. Increasing evidence indicates a crucial role of CCM3 in vascular development and in regulation of angiogenesis and apoptosis. Furthermore, loss of CCM3 causes inherited (familial) cerebral cavernous malformation (CCM), a common brain vascular anomaly involving aberrant angiogenesis. This study focused on signalling pathways underlying the angiogenic functions of CCM3. Silencing CCM3 by siRNA stimulated endothelial proliferation, migration and sprouting accompanied by significant downregulation of the core components of Notch signalling including DLL4, Notch4, HEY2 and HES1 and by activation of VEGF and Erk pathways. Treatment with recombinant DLL4 (rhDLL4) restored DLL4 expression and reversed CCM3-silence-mediated impairment of Notch signalling and reduced the ratio of VEGF-R2 to VEGF-R1 expression. Importantly, restoration of DLL4-Notch signalling entirely rescued the hyper-angiogenic phenotype induced by CCM3 silence. A concomitant loss of CCM3 and the core components of DLL4-Notch signalling were also demonstrated in CCM3-deficient endothelial cells derived from human CCM lesions (CCMEC) and in a CCM3 germline mutation carrier. This study defined DLL4 as a key downstream target of CCM3 in endothelial cells. CCM3/DLL4-Notch pathway serves as an important signalling for endothelial angiogenesis and is potentially implicated in the pathomechanism of human CCMs.

An analysis of flow dynamics in cerebral cavernous malformation and orbital cavernous angioma using indocyanine green videoangiography.

BACKGROUND: Cerebral cavernous malformations (CCMs) are known to be vascular anomalies with low perfusion because of being angiographically occult. We attempted direct visualization of blood flow within CCMs and orbital cavernous angiomas (CAs), and analyzed flow dynamics using indocyanine green videoangiography (ICG-VAG). METHODS: This series included seven CCMs and two orbital CAs. ICG-VAG was performed to visualize blood flow of the lesions before resection. Time to peak of staining was evaluated by reviewing recorded ICG-VAG. RESULTS: In five of seven CCMs, stain was identified. CCMs were seen as avascular areas in both arterial and venous phases, and were stained gradually. Stain was maximized late after venous phase. The orbital CAs were also stained lately, but more intensely than CCMs. CONCLUSIONS: The present study directly demonstrated slow and low perfusion within CCM and orbital CA using ICG-VAG. On the basis of characteristic flow dynamics of CCMs, intraoperative ICG-VAG provides useful information in microsurgical resection.

Evidence for anti-angiogenic and pro-survival functions of the cerebral cavernous malformation protein 3.

Mutations in CCM1, CCM2, or CCM3 lead to cerebral cavernous malformations, one of the most common hereditary vascular diseases of the brain. Endothelial cells within these lesions are the main disease compartments. Here, we show that adenoviral CCM3 expression inhibits endothelial cell migration, proliferation, and tube formation while downregulation of endogenous CCM3 results in increased formation of tube-like structures. Adenoviral CCM3 expression does not induce apoptosis under normal endothelial cell culture conditions but protects endothelial cells from staurosporine-induced cell death. Tyrosine kinase activity profiling suggests that CCM3 supports PDPK-1/Akt-mediated endothelial cell quiescence and survival.

Retrosigmoid suprafloccular transhorizontal fissure approach for resection of brainstem cavernous malformation.

OBJECTIVE: This study examined the usefulness of a surgical approach (retrosigmoid suprafloccular transhorizontal fissure approach) for resection of brainstem cavernous malformations (CMs). METHODS: An anatomic study concerning the retrosigmoid suprafloccular transhorizontal fissure approach was performed with 3 cadaveric heads. Clinical course was retrospectively reviewed for 10 patients who underwent microsurgical resection of brainstem CMs with this approach. Medical, surgical, and neuroimaging records of these patients were evaluated. RESULTS: In the anatomic study, after standard suboccipital retrosigmoid craniotomy, the horizontal fissure on the petrosal surface of the cerebellum was dissected between the superior semilunar lobule and flocculus. With this approach, the root entry zone of the trigeminal nerve and the middle cerebellar peduncle could be exposed by superior retraction of the superior semilunar lobule. The lateral surface of the pons was then easily visible around the root entry zone. When this approach was used for 10 brainstem CMs, complete resection was achieved in 9 patients (90%). No mortality was encountered in this study. New neurological deficits occurred in the early postoperative period for 4 patients but were transient in 3 patients. Neurological status at final follow-up was improved in 4 patients (40%), unchanged in 5 patients (50%), and worse in 1 patient (10%) compared with preoperative conditions. CONCLUSION: The retrosigmoid suprafloccular transhorizontal fissure approach is useful for the resection of lateral pontine CMs.

Angiographic balloon test occlusion and therapeutic sacrifice of major arteries to the brain.

OBJECTIVE: Treatment of certain cerebral aneurysms, caroticocavernous fistulae, and tumors of the neck or cranial base may involve therapeutic arterial sacrifice, which requires preoperative knowledge of the feasibility of permanent occlusion of the internal carotid artery (ICA) or vertebral artery or arteries. METHODS: Retrospective study of transcranial Doppler ultrasonography-monitored angiographic balloon test occlusion and therapeutic sacrifice of the ICA or vertebral artery. RESULTS: We performed transcranial Doppler-guided balloon test occlusion in 136 patients at a procedural risk equivalent to that of conventional neuroangiography, and with correct prediction of the hemodynamic result of therapeutic arterial sacrifice in all instances. Patients with an immediate drop in ipsilateral middle cerebral artery (MCA) velocity to 65% or more of baseline values upon ICA balloon occlusion tolerated ICA sacrifice well, whereas hemodynamic infarction is likely in those with a corresponding drop in MCA velocity to 54% or less. When ICA balloon occlusion caused a drop in MCA velocity to between 55 and 64% of baseline, the pulsatility of the MCA signal had to be analyzed. Patients who tolerated bilateral vertebral artery closure had reversal of flow and an increase in velocity in the P1 section of the posterior cerebral artery. In 212 patient-years of observation after therapeutic arterial sacrifice, no de novo aneurysms formed. CONCLUSION: Angiographic balloon test occlusion with transcranial Doppler monitoring can be performed ultra-swiftly at a risk equal to conventional neuroangiography and with correct prediction of the hemodynamic outcome of arterial sacrifice. Elective therapeutic arterial occlusion is a safe and efficient treatment of large cerebral aneurysms and caroticocavernous fistulae.