Transgenic mice expressing mutant Notch3 develop vascular alterations characteristic of cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy.

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is an increasingly recognized adult-onset autosomal dominant vascular dementia, caused by highly stereotyped mutations in the Notch3 receptor. CADASIL is a widespread angiopathy characterized by a degeneration of vascular smooth muscle cells (VSMCs) and the abnormal accumulation of electron-dense granular material called GOM and Notch3 protein, because of an impaired clearance. Evidence that VSMCs are the primary target of the pathogenic process is supported by the restricted expression of Notch3 in these cells but mechanisms of their degeneration remain essentially unknown. We generated transgenic mice in which the SM22alpha promoter drove, in VSMCs, the expression of a full-length human Notch3 carrying the Arg90Cys mutation, a CADASIL archetypal mutation. Transgenic mice showed no evidence of prominent brain parenchyma damage but demonstrated the two hallmarks of the CADASIL angiopathy, GOM deposits and Notch3 accumulation, within both the cerebral and peripheral arteries. Of interest, arteries of the tail were more severely affected with prominent signs of VSMC degeneration. Time-course analysis of vessel changes revealed that disruption of normal VSMC anchorage to adjacent extracellular matrix and cells, VSMC cytoskeleton changes as well as starting signs of VSMC degeneration, which were detected around 10 months of age, preceded Notch3 and GOM accumulation appearance, which were observed only by 14 to 16 months of age. In conclusion, we have generated transgenic mice that recapitulate the characteristic vascular lesions observed in CADASIL. Our results indicate that Notch3 or GOM accumulation are unlikely to be the prerequisites for the induction of VSMC degeneration and suggest that degeneration of VSMCs may rather be triggered by the disruption of their normal anchorage, based on the important role of adhesion for cell survival.

Lessons from CADASIL.

Vascular dementia (VaD) includes several different vascular mechanisms and changes in the brain. Among VaD, CADASIL is an inherited angiopathy caused by mutations in the Notch3 gene. The pathological hallmark of CADASIL is a granular osmiophilic material deposit (GOM) that is not only found in the brain, but also in the peripheral vascular tree. Consequently, a window into the brain was opened from a strictly neurological disease with tremendous consequences thanks to a skin biopsy. The latter was and continues to be used as a diagnostic tool for CADASIL, despite an immunohistochemical test that is now available. The skin biopsy first used as a diagnostic tool revealed the existence of numerous other VaDs presenting systemic vascular changes. Later, skin biopsy became a research tool, and a morphological skin vessel change classification was proposed on 300 patients. Interestingly, similar skin vessel lesions appear to be related to the same biological modifications. In addition, an early destruction of the medial muscle cells was noticed in 74% of cases. Because vascular smooth muscle cells secrete a powerful endothelial permeability factor (VEGF), their destruction could lead to a decrease in vascular permeability. Cocultures of endothelial cells with vascular muscle cells showed that their presence doubled vascular permeability. Thus, alteration or the loss of vascular muscle cells likely results in hypopermeability, in addition to vessel wall hypotonia and a watershed hypoperfusion. The wealth of information brought forth by knowledge of CADASIL provided new tools for research and clues for understanding the consequences of vascular impairments in dementia.

Morphometric analysis of ultrastructural vascular changes in CADASIL: analysis of 50 skin biopsy specimens and pathogenic implications.

Cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) is a systemic vascular disease caused by Notch 3 gene mutations. On electron microscopy a specific granular osmiophilic material (GOM) is found surrounding the vascular smooth muscle cells. In 1993, we first proposed the use of skin biopsy to diagnose patients and to identify relatives of patients with CADASIL. We analyze here our experience with skin biopsies from 50 patients with CADASIL and compare the findings with those of 20 normal skin biopsy specimens. A morphometric analysis of skin vessel morphology on electron microscopy was performed by systematic measurements of several blood vessel diameters, as well as of areas of lumen, endothelial cell and smooth muscle cell cross-sectional areas, vessel wall area, arterial media and extracellular matrix areas. We found relative absence of stenosis but marked destruction of smooth muscle cells, resulting in decrease of vessel wall thickness and loss of extracellular matrix area, producing vessel wall weakness. Similar changes were also observed in brain arterioles from 5 patients with CADASIL. Our results suggest that hypotonicity of the arteriolar tree may constitute an important pathogenetic mechanism in CADASIL. Other than hypotonicity, the early and severe destruction of smooth muscle cells may potentially result in decreased secretion of vascular endothelial growth factor, loss of vascular permeability and damaging hemodynamic consequences. Blood vessel morphology of skin vessels correlated well with changes in brain arterioles. Vascular morphology in skin biopsy samples contributes to our understanding of the pathogenesis of CADASIL. It could be important to perform skin biopsies in future therapeutic trials of CADASIL as a direct measure of therapeutic effectiveness.