Cellular and Molecular Neuropathology of Cerebral Small Vessel Disease and Dementia

Abstract

Cerebrovascular disorders are inherently heterogenous. They entail a variety of clinical, pathological and cognitive features. In recent years, cerebral small vessel disease (SVD) has been at the forefront mainly because it is attributed to a common cause of strokes and responsible for long-term sequalae including disability. Advances in neuroimaging, particularly magnetic resonance imaging (MRI) have expounded on the radiological definition of SVD involving white matter hyperintensities and parenchymal changes whereas it is difficult to appreciate covert pathology in intracranial arteries and arterioles. SVD pathology incorporates small cortical and subcortical infarcts, microinfarcts, microbleeds, perivascular spacing and white matter attenuation. Cerebral vessels undergo loss of smooth muscle cells and disruption of the extracellular matrix within basement membranes with consequences on interstitial fluid drainage. The distribution and quantity of SVD pathology involving both parenchymal lesions and arteriopathy vary with age, gender, vascular risk factors and genetically determined disorders. However, both types of lesions invariably correlate with progression of impairment or worsen cognitive function. SVD is part and parcel of almost all types of dementias. The incorporation of SVD as a biomarker is much warranted in the biological definition of dementia. Therapeutic interventions to reduce SVD pathology via risk control will have a major impact on the burden of dementia.

 INTRODUCTION

Cerebral small vessel disease (SVD) is increasingly described on magnetic resonance imaging (MRI) to denote parenchymal changes, particularly white matter hyperintensities (WMH).^1–3) The most common consequences of SVD are (i) lacunar (small deep) infarcts; (ii) primary non-traumatic intracerebral brain haemorrhage (ICH) and (iii) subcortical ischaemic vascular dementia, which may be caused by multiple lacunar infarcts or by diffuse white matter ischaemic damage. In addition to age, hypertension and diabetes mellitus appear the main risk factors for SVD. These risk factors are almost equally important for non-lacunar infarcts⁴⁾ and are a cause of cognitive impairment and dementia (Table 1). The pathogenesis of lacunar infarcts has been the topic of intensive research⁵⁾ and imaging studies have suggested that the majority (>90%) of lacunes are found at the edge or have proximal predilection to WMH.⁶⁾ However, the pathological definition of SVD implies much more and emphasizes distinct covert changes within walls of cerebral vessels. Small cerebral vessels bearing smooth muscle cells or myocytes have diameters from 40 up to 900 μm and they emerge from the leptomeningeal arteries, enter the brain parenchyma from the surface of the brain to extend a variable depth into the parenchyma comprising both basal and pial penetrators. The major structural pathologies described in penetrating small arteries in SVD are arteriolosclerosis, fibrinoid necrosis and micro-aneurysms. Small cerebral arteries and arterioles change with age and are actually affected in many disorders including hereditary angiopathies, inflammatory and infective vasculitides and toxic disorders.

Table 1 Spectrum of disorders with SVD pathology associated with cognitive impairment or dementia

Primary or Secondary Vascular Disorder(s)*	Common conditions	Vascular Distribution	Predominant Tissue changes	Form(s) of VCI- mild or major VCI (VaD) †
Arteriolosclerosis	Sporadic small vessel disease	Perforating and penetrating arteries, lenticulostriate arteries	Cortical infarcts, lacunar infarcts/lacunes, microinfarcts, WML	Small vessel dementia; subcortical ischaemic vascular dementia; strategic infarct dementia
	Hypertensive vasculopathy			Hypertensive encephalopathy with impairment; strategic infarct dementia
Non-atherosclerotic non-inflammatory vasculopathies	Arterial dissections (carotid, vertebral and intracranial), fibromuscular dysplasia, dolichoectatic basilar artery, large artery kinking and coiling, radiation induced angiopathy, moyamoya disease	Vertebral, basilar, Branches of MCA, mural haematoma perforating artery; SVD	No pattern of brain infarctions: haemodynamic, thromboembolic, or due to occlusion of a perforating artery. Subarachnoid haemorrhage; lacunar infarcts, PVS	Mild VCI
Vasculitides	Vasculitis (infectious and non-infectious); rheumatoid arthritis	Various cerebral and systemic vessels	White matter changes, SVD like lesions	Mild VCI
Amyloid angiopathies	Sporadic and Familial CAAs (Amyloid β, prion protein, cystatin C, transthyretin, gelsolin)	Leptomeninges, intracerebral arteries	Cortical microinfarcts, lacunar infarcts, WML	Mild and Major VCI (Independently affect cognition)
Monogenic stroke disorders	CADASIL, CARASIL, retinal vasculopathy with cerebral leukodystrophies (RVCLs), Moyamoya disease, Hereditary angiopathy, nephropathy, aneurysm and muscle cramps (HANAC)	Leptomeningeal arteries, intracerebral subcortical arteries	Lacunar infarcts/lacunes, microinfarcts, WML	Mild and Major VCI
Monogenic disorders involving stroke	Fabry disease, familial hemiplegic migraine, hereditary haemorrhagic telangiectasia, Vascular Ehlers-Danlos syndrome, Marfan syndrome, Psuedoxanthoma elasticum, Arterial tortuosity syndrome, Loeys-Dietz syndrome, polycystic kidney disease; Neurofibromatosis type 1 (von Ricklinghausen disease), Carney syndrome (Facial lentiginosis and myxoma)	Branching arteries	Cortical and subcortical infarcts, haemorrhagic infarcts	Mild and Major VCI
Metabolic disorders	Mitochondrial disorders (MELAS, MERRF, Leigh’s disease, Myoclonic epilepsy with ragged red fibres), Menkes disease, Homocystinuria, Tangier’s disease	Intracerebral small arteries, territorial arteries	Cortical and subcortical stroke-like lesions, microcystic cavitation, cortical petechial haemorrhages, gliosis, WML	Mild VCI

Data in Table modified from Kalaria et al,¹⁰⁹⁾ and several original references.⁸⁾ Several disorders may also occur with other co-morbidities such as coronary artery disease, congestive heart failure, hypertension, diabetes, hyperlipidaemia, hypercoagulability, renal disease, atrial fibrillation and valvular heart disease. *Other miscellaneous causes of stroke including mechanical, invention induced or rare genetic syndromes such as trauma, iatrogenic, decompression sickness, air or fat embolism, transplantation and Werner’s syndrome can lead to cognitive impairment. † VCI determined when two or more cognitive domains are affected per minimal harmonisation guidelines or minor VCI.¹¹⁰⁾ Abbreviations: CAA, cerebral amyloid angiopathy; CADASIL, cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy; CARASIL; cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy; ICH, intracerebral haemorrhage; MCA, middle cerebral artery; MELAS, Mitochondrial Myopathy, Encephalopathy, Lactic Acidosis and Stroke-like Episodes; MERRF, PCA, posterior cerebral artery; PVS, perivascular spaces; SLE, systemic lupus erythematosus; SVD, small vessel disease; VaD, vascular dementia; VCD, vascular cognitive disorder; WML, white matter lesion.

In recent times, C Miller Fisher should probably receive the credit for meticulous analysis of cerebral SVD or arteriolar pathology. Fisher suggested segmental arterial disorganization and small vessel atherosclerosis are the two most common causes of lacunar infarcts leading lacunar syndromes, whereas fibrinoid necrosis was associated with large lacunes and ICH.⁷⁾ Intracranial atherosclerosis occurs in very old age, particularly in the branches of the main cerebral arteries.⁸⁾ There are few qualitative differences in atherosclerotic plaques and the repertoire of the reactive cells associated with atherosclerosis in carotid versus intracranial arteries. In this review article, we focus on recent updates in the understanding of arteriolar pathology in the context of sporadic and familial SVD type of disorders (Table 1).

Figure 1

Schematic of a coronal brain section showing different types of cerebral SVD pathology. Modified from Kalaria and Sepulveda-Falla.³⁾ Numbers in boxes [1-7] correspond 1) perivascular and deep WM attenuation, 2) lacunar infarcts (<1.5cm), 3) WM infarcts (1-2 cm), 4) microinfarcts (<0.5 cm), 5) lobar or deep microbleeds or haemosiderin, 6) CAA or CAA related ICH, 7) superficial siderosis and 8) perivascular spaces. Increased perivascular spacing occurs because of reduction in arterial vascular tone and failure of IPAD¹¹³⁾. The WM changes ensues due to a chronic hypoxic state and decline in oligodendrocytes^114,115). White arrows in panels show location of key lesion(s). B, Microscopic images of cerebral arterioles showing gradual changes a, some loss of SMC; b and c, degrees of fibrinoid necrosis and d, hyalinosis or glassy like appearance of vessel wall. Abbreviations: ICH, intracerebral haemorrhage; SVD, small vessel disease; WM, white matter. Magnification bar: 100 μm.

 Features of Vascular Pathology in SVD

 Arteriolosclerosis, Fibrinoid Necrosis and Micro-Aneurysms

Arteriolosclerosis describes non-fibrinoid hyaline thickening in arteriolar vessels of 40–300 μm-diameter.^9,10) Normal intracerebral arteries are relatively thin-walled and have a wide lumen in relation to the wall thickness with their sclerotic index normally being <0.3. Arteriolosclerosis tends to be more associated with ischaemic white matter disease and vascular dementia or vascular cognitive impairment rather than lacunar infarcts.¹¹⁾ The thickened walls narrow the lumen and increase the sclerotic index (SI = 1 – [internal diameter/ external diameter]), a measure devised to indicate severity of degenerative fibrous thickening of the tunica media.^12,13) Previous studies indicate thickened fibrotic arteries seldom rupture.

Fibrinoid necrosis appears not to be necessarily very different¹⁴⁾ from Miller Fisher’s description of lipohyalinosis.¹⁵⁾ In the early stages, the walls are thickened by eosinophilic fibrinoid material, composed mainly of plasma proteins, with abundant fibrin, formed by leakage of the blood-brain barrier (BBB) together with remnants of smooth muscle cells. At this fibrinoid-necrosis stage of SVD, the BBB is disrupted and the affected vessels are prone to rupture. In time, the fibrinoid material is replaced by collagen produced by fibroblasts and the arteriolar walls become ‘glassy’ in appearance. Lipids are usually only a minor component in these lesions, and hyalinosis refers to acellular fibrosis.⁹⁾ It has been recommended that the term ‘lipohyalinosis’ should therefore be abandoned and replaced by descriptions based on appropriate stains: fibrinoid change if histological or immunocytochemical stains verify the presence of fibrin, and fibrosis if collagen is the main constituent of thickened arterial walls. The homogeneous eosinophilia in haematoxylin- and eosin (H&E)-stained sections may result from either fibrinoid change or collagenous fibrosis. These two are probably consecutive changes and can be readily distinguished by use of special stains but appear deceptively similar to H&E.¹⁴⁾ A description probably better describing these vessels would be fibro-hyalinosis, a term implying a rather uniform SVD change extending also to non-hypertensive regressive conditions.

According to the traditional view, Charcot–Bouchard or miliary micro-aneurysms arise in the context of hypertension, at weakened sites in vessel walls.¹⁴⁾ They resemble small sacs, 0.3 to 2 mm across, arising from parent arteries/arterioles 100–300 μm in diameter. The walls of the aneurysms consist of hyaline connective tissue, damaged smooth muscle cells and elastica interna. Rupture of microaneurysms typically produces globular haemorrhages; if ‘healed’ by thrombosis and fibrosis, these are transformed into fibrocollagenous balls.

Alkaline phosphatase histochemistry and high-resolution micro-radiography showed the great majority of ‘micro-aneurysms’ to be complex tortuosities. These are most common at the interface between the grey and white matter and their numbers increase with age, but hypertension has no effect on their prevalence. Micro-aneurysms are also not found in relation to lacunar infarcts. However, definitive identification of microaneurysms in routine diagnostic analysis is very rare.

 Causes of Arteriopathies

The main causes of arteriolosclerosis are ageing, hypertension, cerebral amyloid angiopathy (CAA) and among common familial disorders such as cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy (CADASIL) (Table 1). These can be distinguished by location and type of staining to demonstrate protein accumulation or loss of arterial vascular smooth muscle cells. Hypertension or ageing-related arteriolosclerosis is usually pronounced in subcortical arteries and arterioles. Longstanding hypertension likely increases risk of rupture of small arteries with consequent ICH. Arteriolosclerosis in CAA is largely leptomeningeal and cortical, and in CADASIL predominantly affects leptomeningeal and subcortical vessels, often sparing those in the cortex. Vessels affected by arteriolosclerosis show tinctorial staining for collagen and are intensely immunopositive for collagen I or collagen IV, for example. Arterioles with CAA are Congo Red and Thioflavin S positive, and remarkably often amyloid β (Aβ)-immunopositive. In CADASIL and other monogenic SVD disorders, arteriolar walls show granular basophilia in sections stained with H&E or Periodic Acid Schiff (PAS) but specificity can be shown by immunopositivity of fragments of mutated proteins such as the ectodomain of NOTCH3 in CADASIL¹⁶⁾ and others (Table 2).

Table 2 Molecular genetics and pathology of cerebral arteriopathies

Disorder	Onset Age (yrs.) ‡	Gene (mutations)	Protein	Protein type/cellular abnormalities	Proposed dysfunction(s)
CADASIL (most common); Autosomal dominant)	20-60	NOTCH3 (dominant >280)	NOTCH3	Transmembrane cell signalling receptor; SMC loss; ECM changes	Aberrant cell-cell signalling, activates unfolded protein response and impaired gene transcription (NICD)
CARASIL Autosomal récessive	20-30	HTRA1 (>50 in symptomatic carriers or typical CARASIL)	HTRA1	Serine protease; ECM dysregulation	Promotes serine-protease-mediated cell death, suppresses TGFβ expression
COL4A1 and COL42- related disorders / PADMAL*	14-50	COL4A1; COL4A2 (dominant in coding regions; 3’ UTR in COL4A1)	COL4A1 and A2	Collagen IV, α1 and α2 chains; ECM disruption	Weakening of vascular BM; ER stress responses.
RVCL disorders: HERNS (Chinese descent); CRV (cerebroretinal vasculopathy); HVR (hereditary vascular retinopathy)	30-50	TREX1 (dominant)	TREX1	3’ → 5’-prime exonuclease DNase III	Disruption of cell death mechanisms, impaired DNA degradation and repair
CARASAL	20-40	CTSA (dominant)	Cathepsin-A	Lysosomal peptidase complexes with β -galactosidase, neuraminidase; predicted ECM dysregulation	ER stress response; disruption of multi- enzyme complexes in lysosomes
Hereditary small vessel disease of the brain (SVDB) †	36-52	Not known	-	-	Unknown functions

* COL4A1 and A2 gene disorders consist of 5 major phenotypes:¹¹¹⁾ (1) perinatal haemorrhage with porencephaly, (2) hereditary infantile hemiparesis, retinal arteriolar tortuosity and leukoencephalopathy (HIHRATL)¹¹²⁾, (3) SVD with Axenfeld-Rieger anomaly (anterior segment dysgenesis of the eye), (4) hereditary angiopathy with nephropathy, aneurysms and muscle cramps (HANAC) and (5) PADMAL,⁹³⁾ which also incorporates subcortical angiopathic leukoencephalopathy and hereditary multi-infarct dementia of the Swedish type.^91,92) † Several other disorders prominently characterised by leukoencephalopathy and cognitive impairment are described in isolated families. ‡ Age of onset signifies when first cerebrovascular event or gait disturbance due to spasticity was recorded. Abbreviations: BM, basement membrane; CADASIL, cerebral autosomal dominant arteriopathy with subcortical infarcts and leukoencephalopathy; CARASAL, cathepsin A related arteriopathy with strokes and leukoencephalopathy; CARASIL, cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy; ECM, extracellular matrix; NICD, Notch intracellular domain; PADMAL, pontine autosomal dominant microangiopathy and leukoencephalopathy; RVCL, autosomal dominant retinal vasculopathy with cerebral leukodystrophy.

 Chronic Hypertensive Disease

Hypertension is not itself a disease of blood vessels, but its deleterious effects are mediated by structural changes in blood vessels, particularly in small arteries and arterioles. Chronic hypertension shifts the autoregulatory limits to the right, towards higher pressure values. This is a protective response allowing maintenance of a constant cerebral blood flow (CBF) even at increased arterial pressure and prevents the ill effects of the excessive systemic pressure on delicate capillaries involving specific molecular mechanisms.^17,18) To maintain the upper and lower limits of autoregulation resistance vessels or arteries must undergo gradual change within the vessel wall.⁹⁾ Symptoms of cerebral hypoperfusion develop when the mean arterial blood pressure falls to about 40 per cent of baseline levels. In hypertensive patients, such a reduction is reached at a correspondingly higher level of arterial pressure than in normotensive people. Thus, one-third of asymptomatic hypertensive patients were found to have focal or diffuse cerebral hypoperfusion.¹⁹⁾ This may be exacerbated by excessive antihypertensive medication, to ischaemic levels severe enough to cause tissue damage in both grey and white matter, especially along the arterial border zones.²⁰⁾ Similarly, in a hypertensive patient with stroke, decreases in blood pressure to levels tolerated by normotensives may worsen the ischaemia as a result of arterial changes.

 Sporadic cerebral amyloid angiopathy

Sporadic CAA is predominantly a silent disease without overt clinical symptoms. The most common type of CAA is associated with deposition of Aβ (Aβ-CAA),^21,22) the cleavage product of Aβ precursor protein (APP). Deposition of Aβ in the walls of cerebral blood vessels occurs in sporadic CAA, Alzheimer disease, and Down syndrome but may also occur in other disorders such as dementia pugilistica,²³⁾ and cerebral and spinal vascular malformations.²⁴⁾ Aβ-CAA is also common in iatrogenic Creutzfeldt-Jakob disease (iCJD), observed in ~90% shared in both growth hormone and dura mater iCJD cases.²⁵⁾ Sporadic Aβ-CAA rarely affects people <60 years of age. In those >60, the prevalence is a little over 30 per cent²⁶⁾ and increases with age.²⁷⁾

Unlike detection of parenchymal Aβ deposition with tracers such as Pittsburgh compound B (PiB) there are no established methods to detect Aβ-CAA in the living brain.²⁸⁾ Aβ-CAA is an independent risk for progressive dementia, associated with ischaemic damage to the white matter, and petechial cortical haemorrhages or infarcts. CAA comprises a group of protein misfolding disorders characterized by the extracellular deposition of fibrillar proteins with amyloid properties in the walls of blood vessels of the brain and meninges.²⁹⁾ It is a relatively common cause of brain haemorrhage in the elderly.³⁰⁾ Over 25 unrelated proteins are known to be involved in amyloidosis³¹⁾ and of these seven are associated with CAA (Table 1). Brains with CAA are often characteristic because of the distribution of the vascular disease, particularly SVD. The predominant form of Aβ that accumulates in arterioles and arteries is Aβ40, whereas that in capillaries is mainly Aβ42.³²⁾ On the basis of different distributions, several ways of grading Aβ-CAA patterns have been devised over the years.^22,33) Each of the up to four patterns³³⁾ is claimed to be distinct and are useful for quantification of research material but for routine purposes the system proposed by Thal et al³⁴⁾ seems to have been most often applied.³⁵⁾ Type 1 CAA corresponds to Aβ in cortical capillaries and other vessels whereas Type 2 involves leptomeningeal and cortical vessels, with the exception of cortical capillaries. The type 1 of Thal³⁴⁾ corresponds to Type 3 described by Mann et al.³³⁾ The differing patterns in CAA within subjects could reflect variations in the efficiency of Intramural Periarterial Drainage (IPAD), as its failure leads to CAA. Alternatively, it may relate to the relative amounts of Aβ, with higher levels of Aβ40 promoting a more ‘aggressive’ form of CAA.³³⁾

Electron microscopy reveals extracellular deposits of randomly orientated, straight, unbranched filaments of indefinite length with a diameter of approximately 6–9 nm. CAA tends to be associated with accentuated perivascular neurofibrillary pathology, particularly if the CAA is severe. Systematic morphometric analysis of sections of frontal, temporal and parietal cortex from 51 AD brains revealed that phospho-tau labelling of neurites around Aβ-laden arteries and arterioles significantly exceeded that around non-Aβ-laden blood vessels, which was, in turn, greater than cortical immunolabelling away from blood vessels.³⁶⁾

With respect to the pathogenesis of CAA three main sources of Aβ accumulation in vessel walls may be considered. (i) Systemic : Aβ is derived from cells throughout the body and is carried in plasma and transported bidirectionally to and from the brain parenchyma by specific receptors in the vessel walls.³⁷⁾ (ii) Vascular: Aβ is produced locally by vascular smooth muscle cells, endothelium and pericytes,³⁸⁾ all of which express APP.⁴²⁴ (iii) Drainage : Aβ accumulates because of failure of IPAD from the CNS. Aβ formed by neurons within the CNS is cleared from the interstitial fluid by several processes, including enzymatic degradation within the brain parenchyma and the walls of blood vessels,^39,40) transcytosis across the BBB, with endothelial cell uptake mediated by specific receptors including lipoprotein receptor-related protein 1,⁴¹⁾ and IPAD, together with other constituents of the interstitial fluid, along the perivascular extracellular matrix to meningeal arteries and probably cervical lymph nodes. This drainage may be impaired in older people, as vascular disease reduces arterial pulsations (thought to supply the motive force for perivascular drainage), with resulting accumulation of Aβ in the arterial wall, which further impedes vascular pulsatility.⁴²⁾ Precipitation within the perivascular extracellular matrix of amyloidogenic solutes such as Aβ in the course of their removal from the brain is probably the cause of most types of CAA.⁴³⁾ Reduced Aβ-degrading enzyme activity within the vessel wall may be a contributory factor: Miners et al.⁴⁴⁾ showed that neprilysin activity was lowest in meningeal blood vessels from patients with most severe CAA, even after adjusting for smooth muscle content, and that raising or lowering neprilysin activity respectively decreased or increased the death of human cerebrovascular smooth muscle cells on exposure to Aβ. Exclusively neuronal production of Aβ is sufficient to cause CAA.⁴⁵⁾

Affected blood vessels in Aβ-CAA may show segmental dilation or fibrinoid necrosis. Serial sectioning and computer-assisted three-dimensional image analysis suggest that the following sequential steps lead to blood vessel rupture and haemorrhage:⁴⁶⁾ (i) accumulation of amyloid in the arterial wall, (ii) destruction of smooth muscle cells, (iii) consequent dilation (formation of micro-aneurysms) of the artery and (iv) breakdown of the BBB, (v) deposition of plasma proteins in the vessel wall (fibrinoid necrosis), and finally (vi) rupture and haemorrhage. Fibrinoid change was more marked than deposition of amyloid at sites of dilation and rupture. This is consistent with the proposal that the deposition of Aβ in walls of cortical vessels may not directly cause microhemorrhages.⁴⁷⁾ Fibrinoid change was also significantly associated with possession of APOE ε2,⁴⁸⁾ which carries an increased risk of haemorrhage in Aβ-CAA. Loss of vascular smooth muscle cells in CAA seems to depend at least partly on their uptake of Aβ and this, in turn, is influenced by APOE genotype. Degradation of extracellular matrix proteins (e.g. MMP-9) weaken vessel walls and is pivotal in the rupture of Aβ-laden arteries.⁴⁹⁾ Other changes in the vessel wall may aggravate the angiopathy. For example, iron accumulation and calcification of vessel walls has been reported in hereditary cerebral haemorrhage with amyloid angiopathy of the Dutch type (HCHWA-D) that correlate with the striped cortex observed on in vivo 7T MR scans.⁵⁰⁾

 GENETICALLY DETERMINED SVDS

 Hereditary amyloid β-peptide cerebral amyloid angiopathy (CAA)

Primarily Aβ-CAA occurs in Dutch (HCHWA-D) and Flemish (HCHWA-F) types and in the different familial forms of Alzheimer disease (FAD).³⁾ Mutations in APP that cause haemorrhages are mostly located within the Aβ domain and involve substitutions of amino acids 21–23 of Aβ, whereas those responsible for FAD are most often located in APP next to but outside of Aβ. Enhanced Aβ accumulation within vessel walls causes cell loss and increasing propensity to rupture and obstruction with resultant haemorrhages and infarcts. In the Italian and Iowa families, CAAH is a common feature, along with dementia. In those FADs associated with presenilin-1 mutations, CAA has been reported to be more common if the presenilin-1 mutation is located beyond codon 200.⁵¹⁾

 Other hereditary CAAs

Hereditary cerebral haemorrhage with amyloid angiopathy of the Icelandic type (HCHWA-I, hereditary cystatin C amyloid angiopathy) is a rare autosomal dominant CAA associated with fatal brain haemorrhages in young and middle-aged normotensive adults.⁵²⁾ In addition to the cerebral and meningeal arteries, ACys-amyloid is deposited in extracerebral tissues, including the skin.

Worster-Drought et al ⁵³⁾ originally described an autosomal dominant CAA with non-neuritic plaques and neurofibrillary tangle formation, clinically characterized by dementia, spastic tetraparesis and cerebellar ataxia with onset around the sixth decade. The disease was subsequently called familial British dementia (FBD) caused by the defective gene BRI2.⁵⁴⁾ A mutation at another site on the same BRI2 gene causes heredopathia ophthalmo-oto-encephalica.²⁹⁾ This disorder, renamed familial Danish dementia (FDD), is characterized by cataracts and ocular haemorrhages in the third decade, followed by hearing problems and, in the fourth to fifth decades, cerebellar ataxia and dementia.

Although CAA in both FBD (with deposition of ABri) and FDD (with deposition of ADan) is severe, with concentric splitting and occlusion of affected vessels, ICH is rare. CAA also affects small arteries and arterioles in white matter as well as in systemic organs. In FDD, Aβ is sometimes co-deposited with ADan in both vessels and parenchyma. In both disorders, amyloid plaques, hyperphosphorylated tau-positive neurofibrillary tangles and neurites are also present in the brain parenchyma.

In gelsolin-related familial amyloidosis of the Finnish type,⁵⁵⁾ the amyloid AGel is deposited systemically, particularly in the skin, peripheral nerves and cornea. Two mutations in the GEL gene on chromosome 9 cause AGel amyloidosis. Remarkably, CNS deposition of AGel is widespread in spinal, cerebral, and meningeal blood vessels, and extensive extravascular deposits are present in the dura, spinal nerve roots and sensory ganglia.⁵⁶⁾

 Transthyretin-related and other CAAs

TTR CAA is caused by mutations in the transthyretin (TTR) gene, located on chromosome 18. Patients may suffer from dementia, cerebellar ataxia, motor dysfunction, and decreased vision and hearing. Currently >100 different mutations have been identified in TTR. Mutated TTR is prone to form amyloid fibrils. In addition to systemic ATTR deposition, common consequences of which include peripheral neuropathy and cardiomyopathy, nine mutations are associated with oculoleptomeningeal amyloidosis with prominent CAA of meningeal vessels.⁵⁷⁾ Patients with the p.Val30Met mutation in TTR 4.4% exhibit retinal amyloidotic angiopathy^58), and there is early CNS involvement showing a topographic spread from leptomeninges to cortical vessels and finally involving deep vessels, although with early brainstem and spinal cord involvement.⁵⁹⁾

CAA in patients with prion disease usually results from vascular deposition of Aβ. CAA due to prion protein (PrP) deposition is rare and associated with stop mutations in PRNP. PrP amyloid (APrP) may be extensively deposited in parenchymal and leptomeningeal vessels and in the surrounding neuropil.⁶⁰⁾

 Hereditary Non-Amyloid Angiopathies

Molecular genetic studies have identified several monogenic conditions characterised by SVD and predispose to ischaemic and haemorrhagic strokes and diffuse white matter disease⁶¹⁾ (Table 2). CADASIL is the most common hereditary SVD. Other hereditary SVDs include cerebral autosomal recessive arteriopathy with subcortical infarcts and leukoencephalopathy (CARASIL), the collagen type IV (COL4A1/A2)-related disorders, retinal vasculopathy with cerebral leukodystrophies (RVCLs), and cathepsin A related arteriopathy with strokes and leukoencephalopathy (CARASAL).⁶¹⁾ Although hereditary SVDs vary in phenotype, they demonstrate convergent effects of microangiopathy on cerebral grey and white matter, leading to cognitive impairment. Collectively, all these hereditary SVDs involve mutations in single genes, whose products are responsible for cell signalling or extracellular matrix development and maturation.⁶²⁾(Table 2).

 CADASIL as a Model SVD

Heterogeneity within the CADASIL phenotype is now increasingly recognised. Multiple small infarcts, detectable on T1-weighted MRI cause cognitive decline between 40 and 70 years of age, primarily in executive frontal lobe functions, followed by impairment of memory and other cognitive functions, leading to the development of a subcortical dementia in ~80% after 65 years of age. The characteristic neuropathological feature is non-atherosclerotic and non-amyloid arteriopathy,⁶³⁾ affecting penetrating small and medium-sized arteries of the white matter but also leptomeningeal blood vessels. Large intracerebral haemorrhages are rare but cerebral microbleeds are often described to occur in subcortical structures.^64,65) The extracellular matrix (ECM) within the basal lamina is disrupted¹⁶⁾ and may accumulate other proteins (see below) with impact on IPAD. In accord with other SVDs, alterations in retinal microvessel are also apparent in CADASIL. Arteriolar narrowing is the most consistent change and can be revealed by fundoscopy and fluorescence angiography.⁶⁴⁾ Autopsy shows thickened arterial walls with fibrosis, accumulation of eosinophilic material, pericyte degeneration and loss of smooth muscle cells in the central retinal artery and branches.⁶⁶⁾ Other pathological features of note include severe astrocytopathy associated with white matter disease.⁶⁷⁾

One of the key pathological characteristics of CADASIL is the deposition of electron dense extracellular granular osmiophilic material (GOM), which contains extracellular domains of NOTCH3 (N3ECD).^16,63,68) Presence of GOM within dermal biopsies offers a means of intra vitam diagnosis and was reported to be completely congruent with genetic screening.⁶⁹⁾ Immunohistochemical demonstration of N3ECD in skin arteries is also very useful in establishing diagnosis⁷⁰⁾ provided clinical symptoms are consistent with the disease. Up to 280 distinct mutations within the 34 epidermal growth factor receptor (EGFr) domains of NOTCH3 have been associated with CADASIL. The vast majority of these (~70%) are missense point mutations with most frequently occurring in exon 4 of the gene.⁷¹⁾ Almost all of the mutations result in either a substitution of a wild-type cysteine by another amino acid or vice versa in one of the 34 epidermal growth factor-like (EGF) repeats in N3ECD. Cysteine-sparing mutations, such as p.Asp80Gly, appear to cause CADASIL with a phenotype indistinguishable from cysteine mutations including aggregation properties of N3ECD.⁷²⁾ Upon recent analysis of large exome databases involving global populations, the frequency of the archetypal cysteine altering NOTCH3 (NOTCH3cys) mutations in the EGFr domains were found to be 100-fold higher than expected based on estimates of CADASIL prevalence⁷¹⁾. Recent advances suggest individuals with a mutation located in EGFr domains 1-6 are predisposed to more severe classical phenotype including characteristic clinical and pathological features compared with those with a mutation in 7-34 domains^71,73).

The pathogenesis of CADASIL remains elusive. The uneven number of cysteine residues affects the formation of disulphide bridges and therefore changes the three-dimensional structure of the NOTCH3 receptor and consequently its functions.⁷⁴⁾ The formation of abnormal disulphide bridges could affect receptor trafficking, processing, specificity for ligand binding and/or signal transduction. Most mutations in NOTCH3 do not appreciably impair signal transduction activity, NOTCH3 processing, or signalling to CBF1/RBP-Jκ activation. It is more likely that the misfolded, non-degradable N3ECDs that accumulate within GOM⁷⁵⁾ over the years affect the function of vascular smooth muscle cells and block the drainage routes.⁷⁶⁾

Proteomic studies of CSF, isolated cerebral vessels and cultivated vascular smooth muscle cells from CADASIL patients have provided various clues to the pathogenesis. Proteins involved in inflammatory responses (Factor B, serum amyloid P component, periostin),^77,78) protein degradation and folding, endoplasmic reticulum stress with activation of Rho kinase and unfolded protein response,^79,80) vascular functions (clusterin and endostatin),⁸¹⁾ extracellular matrix proteins (tissue inhibitor of metalloproteinases 3 and vitronectin),⁸²⁾ and serine proteases (high-temperature requirement protein A1, HTRA1)⁸³⁾ are evident that may or may not complex with N3ECD. Majority of the studies favour that mutant NOTCH3 induces gain of toxic involving misfolding or aggregation of proteins. Prolonged retention of mutant NOTCH3 aggregates in the endoplasmic reticulum decreases cell growth and increases sensitivity to other stresses.⁸⁴⁾

 CARASIL

CARASIL was first described as Maeda syndrome.^85,86) CARASIL is characterised by severe non-amyloid arteriopathy, leukoencephalopathy and lacunar infarcts together with spinal anomalies and alopecia. As in CADASIL, the recurrent strokes lead to insidious deterioration with most subjects becoming cognitively impaired in older age. Typical CARASIL is caused by mutations in the HTRA1 gene, which encodes the serine protease. Both nonsense and missense as well as frameshift plus splicing site mutations have been reported in the HTRA1 gene that causes typical CARASIL or HTRA-1 related SVD.⁸⁶⁾ Heterozygous HTRA1-related SVD has a milder clinical presentation of CARASIL. Differing locations of mutations found in symptomatic carriers with mild disease and classical CARASIL suggests that distinct molecular mechanisms influence the development of SVD. Mutations result in haploinsufficiency or reduced HTRA1 protease activity (21%-50%) or loss of the protein.⁸⁵⁾

The arteriopathy is described by extensive loss of medial smooth muscle cells, intimal proliferation, and splitting of the internal elastic lamina with severe disruption of the ECM in the pial arteries, perforating arteries, and arterioles. CARASIL extends the spectrum of diseases associated with the dysregulation of transforming growth factor (TGF) to read transforming growth factor (TGF)-β signalling. It may also influence the metabolism of APP, which includes several HTRA1 cleavage sites. However, defective TGF-β signalling due to mutations in the TGF-β receptors leads to hereditary haemorrhagic telangiectasia, whereas activation of TGF-β signalling contributes to Marfan’s syndrome and related disorders.⁸⁷⁾ Dysregulation of the inhibition of signalling by TGF-β growth factors has also been linked to alopecia and spondylosis.

 Collagen type IV (COL4) Disorders

Recently several conditions akin to SVD features have been found to be associated with mutations in collagen IV (COL4 ) A1 and A2 genes.^88–90) Two previous conditions described in large German and Swedish families as subcortical angiopathic encephalopathy (SAE)⁹¹⁾ and hereditary multi-infarct dementia of the Swedish type⁹²⁾ were originally thought to be CADASIL- like but are now classified as COL4 type disorders. Both these disorders are consistent with the clinical features of what is now coined as PADMAL for pontine autosomal dominant microangiopathy and leukoencephalopathy.⁹³⁾ Neurological manifestations of COL4A1 and COL4A2 mutations may vary even within families.^94,95) Depending on the age of onset, affected individuals present with infantile hemiparesis, seizures, visual loss, dystonia, strokes, migraine, mental retardation, cognitive impairment and dementia. Single or recurrent ICH may occur in non–hypertensive young adults (foetus > adult) in the deep brain regions: spontaneously, subsequent to trauma or as a result of anticoagulant use (Table 2).

Autosomal dominant COL4A1 -related disease with 100% penetrance has been described in European Caucasian families, with 100% penetrance. More than 50 mutations have been described in the coding region of COL4A1/COLA2 that mostly involve glycine substitutions, some of which also cause ICH. Most glycine (gly) substitutions occur in the conserved Gly-X-Y motifs within the triple-helical collagenous domain. They also cause COL4-related ICH with an estimated ~20% penetrance.^96,97) In addition, 10 different mutations in the 3’ UTR of COL4A1 have been described^98,99) that all cause PADMAL-like syndrome and some develop ICH although the frequency is not known.

Arteriolar pathology is characterised by moderate to severe segmental loss of arteriolar myocytes, fibro-hyalinosis, intimal proliferation and microvascular degeneration often also involving the endothelium. COL4A1 mutations are also associated with variable degrees of retinal arteriolar tortuosity, and abnormalities of endothelial basement membranes in the skin. Mutations in COL4A1 and A2 actually reduce expression perhaps weakening the vessel walls and giving way to bleeds. The mutation in 3’ UTR of COL4A1 disrupts miR-29 binding and increases COL4A1 protein expression.¹³⁾ Mutant COL4A1/COL4A2 chains accumulate in vascular smooth muscle cells and cause endoplasmic reticulum (ER) stress responses, as in CADASIL,⁸⁴⁾ possibly leading to cytotoxicity.^100,101) Mutant COL4A1 accumulates may also disrupt TGF-β signalling¹⁰²⁾ and cause abnormal angiogenesis underlying ICH development.

 Retinal vasculopathies with cerebral leukodystrophy (RVCL)

Hereditary endotheliopathy with retinopathy, nephropathy and stroke (HERNS), cerebroretinal vasculopathy (CRV) and hereditary vascular retinopathy (HVR) were reported independently but linkage analysis demonstrated that they are allelic disorders or different phenotypes of same disease spectrum.¹⁰³⁾ (Table 2). They cause progressive central visual impairment. Lesions occur in the pons, cerebellum and basal ganglia in addition to the frontal and parietal lobes, and consist of foci of coagulative necrosis with negligible inflammation. Whilst heterozygous mutations in TREX1 cause RVCL, homozygous mutations in the same gene are linked to the typical autosomal recessive form of Aicardi-Goutières syndrome,¹⁰⁴⁾ which manifests as a progressive encephalopathy of early onset, brain atrophy, demyelination, basal ganglia calcifications and chronic lymphocytic proliferation. It is unclear how the carboxyl truncating mutations in TREX1 lead to the phenotype or pathogenesis of RVCL involving arteriolosclerosis but it seems obviously due to disruption of the predicted transmembrane domain with subsequent dissemination of TREX1 throughout the cell.¹⁰⁵⁾

 Cathepsin A related arteriopathy with strokes and leukoencephalopathy (CARASAL)

CARASAL is a rare hereditary SVD affecting small cerebral arteries in adults due to mutations in the cathepsin- A (CTSA) gene found on chromosome 20q13.12. CTSA encodes for the serine carboxypeptidase cathepsin-A, a member of the peptidase S10 family, with various functions. Patients with CARASAL exhibit adult-onset leukoencephalopathy with early onset clinical manifestations involving the brainstem. CARASAL is delineated from CADASIL and CARASIL but also from other SVDs with secondary leukoencephalopathies, such as granulomatous encephalitis, lymphoma, vasculitis, mitochondrial disorders and WMHs due to diabetes, hypertension, or smoking and also from primary leukoencephalopathies, such as GM1 gangliosidosis, Krabbe disease, or Tay-Sachs-disease.¹⁰⁶⁾

It is predicted that mutations in cathepsin-A cause loss of several function(s) including dysregulation of the ECM. Cathepsin-A forms a complex with neuraminidase-1 and elastin binding protein, forming the elastin binding protein receptor. This receptor complex plays a role in the formation of elastic fibres, which are also a component of arterioles. CARASAL also belongs to a group of leukodystrophies with abnormalities in components of the neurovascular unit affecting tight junctions and astrocytic end-feet juxtaposed to endothelial cells.¹⁰⁷⁾

 Rarer vasulopathies and angiopathies

Other families with hereditary SVDs have been described that are not explained by any of the known gene defects. These include hereditary systemic angiopathy (HSA) in which the retinal microvessels undergo progressive occlusion leading to ischaemic retinopathy with subsequent optic disc atrophy and formation of capillary aneurysms. Pathological changes include foci of coagulative necrosis in the white matter with prominent perivascular inflammation, oedema, astrocytic gliosis. There is evidence of proliferation of microvessels, many with hyperplastic endothelium and severely thickened walls, and some showing fibrinoid necrosis or thrombosis.¹⁰⁸⁾

 Conclusion

In recent years, there have been several advances in the understanding of the pathophysiology of sporadic and familial SVDs. Parenchymal pathology attributed to SVD indicates that this is largely caused by arteriolar wall modifications due to age-related loss of cellular elements within vascular smooth muscle cells and disruption of the extracellular matrix within the basal lamina. Among the genetic forms of cerebral SVD, CADASIL remains the most aggressive type of arteriopathy. However, an overall theme in all these SVD type of disorders including monogenic causes indicates severe loss of vascular smooth muscle cells and dysregulation of the extracellular matrix or basal lamina with variable consequences on IPAD. Reducing SVD via vascular risk factors is a viable strategy to tackling the burden of dementia.

 ACKNOWLEDGEMENTS

Funding: RK’s research is supported by previous grants from the Alzheimer’s Research UK (ARUK PG2013-22) and Medical Research Council, UK (MRC, G0500247). ROC’s research was supported by a grant from Alzheimer’s Research UK NCG-2019B-001.

Author Contribution: All authors were involved in drafting and editing the manuscript.

Disclosures: The authors declare that they have no competing interests.

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