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No tight junctions at endothelial cell interfaces were found; however, several examined endothelial cell interfaces demonstrated apparent gaps between endothelial cell processes where basal lamina was exposed directly to the lumen of the sinusoids. Heavy hemosiderin deposits were found underlying the vascular channels within microns of the basal lamina without evidence of disrupted vessels. No astrocytic foot processes were seen within lesions. Glial fibrillary acidic protein immunocytochemistry confirmed that astrocyte processes stopped at the border of the lesions. Conclusions-The absence of blood-brain barrier components may lead to leakage of red blood cells into these lesions and the surrounding brain in the absence of major haemorrhage, thus accounting for the propensity of cavernous malformations to cause seizures. These data also raise the possibility that KRIT1 plays a part in the formation of endothelial cell junctions and expression of a mature vascular phenotype. (J Neurol Neurosurg Psychiatry 2001;71:188-192) Keywords: cavernous malformation; electron microscopy; glial fibrillary acidic protein; blood-brain barrier Cavernous malformations in the CNS consist of endothelial lined vascular sinusoids embedded in a connective tissue matrix. There is no normal nervous tissue within the lesion itself. The normal brain or spinal cord tissue at the periphery of the lesion is haemosiderin stained and gliotic. Blood flow within the channels is stagnant as evidenced by the presence of thrombus in various stages of organisation. These histological qualities lead to the classic mulberry appearance on gross examination and the diagnostic appearance of a reticulated core of mixed signal intensity surrounded by a hypointense rim on T2 weighted MRI sequences. 1 Despite their prominence on MRI, cavernous malformations are seen poorly, if at all, on conventional digital subtraction angiography. This is confirmation of the minimal blood flow through these vascular lesions.Despite the fact that numerous studies have examined cerebral cavernous malformations using histology and immunocytochemistry, 2-9 to our knowledge, only one ultrastructural study of these lesions has been published. 10 There are reports of electron microscopic studies of cavernous haemangiomas in the retina 11 and orbit 12 ; however, these lesions are distinct from cavernous malformations of the CNS. The previous ultrastructural analysis of cavernous malformations identified structural abnormalities of tight junctions between endothelial cells potentially related their pathophysiology. 10 The blood-brain barrier consists of the interplay of three major microvascular components. The tight junctions between endothelial cell constitute the major permeability barrier, and the overall biology of the barrier is shaped by the interactions of the endothelium with the pericytes/smooth muscle cells and the astrocyte foot processes that cover most of the abluminal surface of the microvasculature. 13 The molecular constituents of the tight junction include the...

Cerebral cavernous malformation (CCM) is a common autosomal dominant disorder characterized by venous sinusoids that predispose to intracranial hemorrhage. CCM is genetically heterogeneous, with loci at 7q, 7p and 3q. Mutations in KRIT1 account for all cases linked to 7q (CCM1), but the pathogenesis of CCM is not understood. Krev Interaction Trapped 1 (krit1) was originally identified through its interaction with the Ras-family GTPase krev1/rap1a in a two-hybrid screen, inferring a role in GTPase signaling cascades. We demonstrated additional 5'-coding exons for krit1, extending the N-terminus by 207 amino acids compared to the previously reported protein. Remarkably, by two-hybrid analysis and co-immunoprecipitation, full-length krit1 fails to interact with krev1/rap1a but shows strong interaction with integrin cytoplasmic domain-associated protein-1 (icap1). Icap1 binds to a NPXY motif in the cytoplasmic domain of beta1 integrin and participates in beta1-mediated cell adhesion and migration. The novel N-terminus of krit1 contains a NPXY motif that it is required for icap1 interaction. Like beta1 integrin, krit1 interacts with the 200 amino acid isoform of icap1 (icap1alpha), but not a 150 amino acid form that results from alternative splicing (icap1beta). In a competition assay, induced expression of krit1 diminishes the interaction between icap1alpha and beta1 integrin. Taken together, these data suggest that beta1 integrin and krit1 compete for the same site on icap1alpha, perhaps constituting a regulatory mechanism. Loss-of-function KRIT1 mutations, as observed in CCM1, would shift the balance with predicted consequences for endothelial cell performance during integrin beta1-dependent angiogenesis.