Invited Review - Understanding cause and effect in Alzheimer's pathophysiology: Implications for clinical trials.

Alzheimer's disease (AD) pathology is multi-faceted, including extracellular accumulation of amyloid-ß (Aß), accumulation of tau within neurons, glial activation and loss of neurons and synapses. From a neuropathological perspective, usually at a single time-point and often at the end-stage of the disease, it is challenging to understand the cause and effect relationships between these components. There are at least four ways of trying to unravel these relationships. First, genetic studies demonstrate mutations that influence Aß production, but not tau, can initiate AD; whereas genetic variants influencing AD risk are related to innate immunity and lipid metabolism. Second, studies at early time points show that pathology begins decades before the onset of dementia and indicate different anatomical locations for initiation of Aß and tau accumulation. Third, cause and effect can be studied in experimental models, but most animal models do not fully replicate AD pathology. However, induced pluripotent stem cells (iPSCs) to study live human neurons has introduced a new perspective. Fourth, clinical trials may alter AD pathology giving insights into cause and effect relationships. Therefore, a sequence of (i) neocortical Aß accumulation followed by (ii) a microglial inflammatory reaction to Aß, causing neuritic dystrophy which promotes (iii) spread of tau from the limbic system to the neocortex with (iv) progressive tau accumulation and spread resulting in (v) neurodegeneration, explains the evidence. It is proposed that different therapeutic targets are required for different stages of the disease process: Aß for primary prevention, microglia for secondary prevention, and tau for established disease.

A novel method to visualise the three-dimensional organisation of the human cerebral cortical vasculature.

Current tissue-clearing protocols for imaging in three dimensions (3D) are typically applied to optimally fixed, small-volume rodent brain tissue - which is not representative of the tissue found in diagnostic neuropathology laboratories. We present a method to visualise the cerebral cortical vasculature in 3D in human post-mortem brain tissue which had been preserved in formalin for many years. Tissue blocks of cerebral cortex from two control cases, two Alzheimer's brains and two cases from Alzheimer's patients immunised against Aß42 were stained with fluorescent Lycopersicon esculentum agglutinin (Tomato lectin), dehydrated and cleared using an adapted three-dimensional imaging of solvent cleared organs (3DISCO) protocol to visualise the vascular endothelium. Tissue was imaged using light sheet and confocal microscopy and reconstructed in 3D using amira software. The method permits visualisation of the arrangement of the parallel penetrating cortical vasculature in the human brain. The presence of four vascular features including anastomosis, U-shaped vessels, spiralling and loops were revealed. In summary, we present a low cost and simple method to visualise the human cerebral vasculature in 3D compatible with prolonged fixation times (years), allowing study of vascular involvement in a range of normative and pathological states.

Systems proteomic analysis reveals that clusterin and tissue inhibitor of metalloproteinases 3 increase in leptomeningeal arteries affected by cerebral amyloid angiopathy.

AIMS: Amyloid beta (Aß) accumulation in the walls of leptomeningeal arteries as cerebral amyloid angiopathy (CAA) is a major feature of Alzheimer's disease. In this study, we used global quantitative proteomic analysis to examine the hypothesis that the leptomeningeal arteries derived from patients with CAA have a distinct endophenotypic profile compared to those from young and elderly controls. METHODS: Freshly dissected leptomeningeal arteries from the Newcastle Brain Tissue Resource and Edinburgh Sudden Death Brain Bank from seven elderly (82.9 ± 7.5 years) females with severe capillary and arterial CAA, as well as seven elderly (88.3 ± 8.6 years) and five young (45.4 ± 3.9 years) females without CAA were used in this study. Arteries from four patients with CAA, two young and two elderly controls were individually analysed using quantitative proteomics. Key proteomic findings were then validated using immunohistochemistry. RESULTS: Bioinformatics interpretation of the results showed a significant enrichment of the immune response/classical complement and extracellular matrix remodelling pathways (P < 0.05) in arteries affected by CAA vs. those from young and elderly controls. Clusterin (apolipoprotein J) and tissue inhibitor of metalloproteinases-3 (TIMP3), validated using immunohistochemistry, were shown to co-localize with Aß and to be up-regulated in leptomeningeal arteries from CAA patients compared to young and elderly controls. CONCLUSIONS: Global proteomic profiling of brain leptomeningeal arteries revealed that clusterin and TIMP3 increase in leptomeningeal arteries affected by CAA. We propose that clusterin and TIMP3 could facilitate perivascular clearance and may serve as novel candidate therapeutic targets for CAA.

Haemoglobin scavenging after subarachnoid haemorrhage.

Rapid and effective clearance of cell-free haemoglobin after subarachnoid haemorrhage (SAH) is important to prevent vasospasm and neurotoxicity and improve long-term outcome. Haemoglobin is avidly bound by haptoglobin, and the complex is cleared by CD163 expressed on the membrane surface of macrophages. We studied the kinetics of haemoglobin and haptoglobin in cerebrospinal fluid after SAH. We show that haemoglobin levels rise gradually after SAH. Haptoglobin levels rise acutely with aneurysmal rupture as a result of injection of blood into the subarachnoid space. Although levels decline as haemoglobin scavenging occurs, complete depletion of haptoglobin does not occur and levels start rising again, indicating saturation of CD163 sites available for haptoglobin-haemoglobin clearance. In a preliminary neuropathological study we demonstrate that meningeal CD163 expression is upregulated after SAH, in keeping with a proinflammatory state. However, loss of CD163 occurs in meningeal areas with overlying blood compared with areas without overlying blood. Becauses ADAM17 is the enzyme responsible for shedding membrane-bound CD163, its inhibition may be a potential therapeutic strategy after SAH.

Rat astrocytic tumour cells are associated with an anti-inflammatory microglial phenotype in an organotypic model.

AIM: Microglia form a high proportion of cells in glial tumours but their role in supporting or inhibiting tumour growth is unclear. Here we describe the establishment of an in vitro model to investigate their role in astrocytomas. METHODS: Rat hippocampal slices were prepared and, after 7 days to allow microglia to become quiescent, rat C6 astrocytic tumour cells were added. Over the following 7 days, infiltration and cell death were studied using fluorescent C6 tumour cells and confocal microscopy; immunophenotyping of microglia was performed using CD68 (phagocytosis), MHCII (antigen-presentation) and Iba1 (microglial marker regardless of functional state). Cell proliferation was assessed using Ki67 and qPCR to detect cytokine expression. Sham and control groups were included. RESULTS: Microscopy showed proliferation of C6 tumour cells with both infiltration of tumour cells into the hippocampal tissue and of microglia among the tumour cells. Confocal experiments confirmed increasing tumour cell infiltration into the hippocampal slice with time (P<0.001), associated with cell death (σ=0.313, P=0.022). Ki67 showed increased proliferation (P<0.001), of both tumour cells and Iba1+ microglia and increased microglial phagocytosis (CD68: P<0.001). Expression of pro-inflammatory cytokines IL1, IL6 and TNFα were downregulated with expression of the anti-inflammatory cytokine TGFß1 maintained. CONCLUSION: This model allows study of the proliferation and infiltration of astrocytic tumour cells in central nervous system tissue and their interaction with microglia. Our data suggest that microglial function is altered in the presence of tumour cells, putatively facilitating tumour progression. Manipulation of the microglial functional state may have therapeutic value for astrocytic tumours.

The neuroinflammatory response in humans after traumatic brain injury.

AIMS: Traumatic brain injury is a significant cause of morbidity and mortality worldwide. An epidemiological association between head injury and long-term cognitive decline has been described for many years and recent clinical studies have highlighted functional impairment within 12 months of a mild head injury. In addition chronic traumatic encephalopathy is a recently described condition in cases of repetitive head injury. There are shared mechanisms between traumatic brain injury and Alzheimer's disease, and it has been hypothesized that neuroinflammation, in the form of microglial activation, may be a mechanism underlying chronic neurodegenerative processes after traumatic brain injury. METHODS: This study assessed the microglial reaction after head injury in a range of ages and survival periods, from <24-h survival through to 47-year survival. Immunohistochemistry for reactive microglia (CD68 and CR3/43) was performed on human autopsy brain tissue and assessed 'blind' by quantitative image analysis. Head injury cases were compared with age matched controls, and within the traumatic brain injury group cases with diffuse traumatic axonal injury were compared with cases without diffuse traumatic axonal injury. RESULTS: A major finding was a neuroinflammatory response that develops within the first week and persists for several months after traumatic brain injury, but has returned to control levels after several years. In cases with diffuse traumatic axonal injury the microglial reaction is particularly pronounced in the white matter. CONCLUSIONS: These results demonstrate that prolonged microglial activation is a feature of traumatic brain injury, but that the neuroinflammatory response returns to control levels after several years.

Review: activation patterns of microglia and their identification in the human brain.

Microglia in the central nervous system are usually maintained in a quiescent state. When activated, they can perform many diverse functions which may be either beneficial or harmful depending on the situation. Although microglial activation may be accompanied by changes in morphology, morphological changes cannot accurately predict the function being undertaken by a microglial cell. Studies of peripheral macrophages and in vitro and animal studies of microglia have resulted in the definition of specific activation states: M1 (classical activation) and M2 (sometimes subdivided into alternative activation and acquired deactivation). Some authors have suggested that these might be an overlapping continuum of functions rather than discrete categories. In this review, we consider translational aspects of our knowledge of microglia: specifically, we discuss the question as to what extent different activation states of microglia exist in the human central nervous system, which tools can be used to identify them and emerging evidence for such changes in ageing and in Alzheimer's disease.

Association between APOE genotype, neuropathology and dementia in the older population of England and Wales.

AIMS: Apolipoprotein E (APOE) genotype is the major genetic risk factor for sporadic Alzheimer's disease (AD) but it is unclear how this is mediated. Most studies of APOE genotype have used case-control design to compare groups differing by two variables: i.e. dementia and AD pathology, so it is unclear to which of these variables APOE genotype is more strongly related. The prospective Medical Research Council Cognitive Function and Ageing Study neuropathology cohort is population-based sample in which donations are unbiased by dementia status. METHODS: We investigated the association between APOE genotypes and neuropathological and cognitive data in this cohort (n = 310). RESULTS: APOEε4 was associated with an increased risk of diffuse plaques, neuritic plaques, tangles and cerebral amyloid angiopathy. APOEε4 was not associated with infarcts, lacunes, haemorrhages or small vessel disease. APOEε2 appeared to have a protective effect on AD pathology and also on the risk of cortical atrophy. APOE genotype had a non-significant effect on the presence of dementia after adjusting for AD pathology. CONCLUSIONS: APOE genotype is associated with each of the key features of AD pathology but not with cerebrovascular disease other than cerebral amyloid angiopathy. The excess risk of dementia in those with an APOEε4 allele is explained by the pathological features of AD. However, it remains unclear to what extent cognitive dysfunction is caused by these specific pathological features or more directly by closely related APOE-associated mechanisms.

Microglial alterations in human Alzheimer's disease following Aß42 immunization.

AIMS: In Alzheimer's disease (AD), microglial activation prompted by the presence of amyloid has been proposed as an important contributor to the neurodegenerative process. Conversely following Aß immunization, phagocytic microglia have been implicated in plaque removal, potentially a beneficial effect. We have investigated the effects of Aß42 immunization on microglial activation and the relationship with Aß42 load in human AD. METHODS: Immunostaining against Aß42 and microglia (CD68 and HLA-DR) was performed in nine immunized AD cases (iAD - AN1792, Elan Pharmaceuticals) and eight unimmunized AD (cAD) cases. RESULTS: Although the Aß42 load (% area stained of total area examined) was lower in the iAD than the cAD cases (P=0.036), the CD68 load was higher (P=0.046). In addition, in the iAD group, the CD68 level correlated with the Aß42 load, consistent with the immunization upregulating microglial phagocytosis when plaques are present. However, in two long-surviving iAD patients in whom plaques had been extensively cleared, the CD68 load was less than in controls. HLA-DR quantification did not show significant difference implying that the microglial activation may have related specifically to their phagocytic function. CD68 and HLA-DR loads in the pons were similar in both groups, suggesting that the differences in microglial activation in the cortex were due to the presence of AD pathology. CONCLUSION: Our findings suggest that Aß42 immunization modifies the function of microglia by increasing their phagocytic activity and when plaques have been cleared, the level of phagocytosis is decreased below that seen in unimmunized AD.

Decompressive craniectomy for acute disseminated encephalomyelitis.

BACKGROUND: Acute disseminated encephalomyelitis (ADEM) is a rare, acute demyelinating condition. Although it usually presents in an acute or subacute manner over days, its clinical course may be rapid with symptoms and signs of severe intracerebral mass effect secondary to cerebral oedema. METHODS: Case report and literature review. RESULTS: We report a case of a patient presenting with a hyperacute course manifested by rapid loss of consciousness and focal neurological signs. Management with emergency hemicraniectomy and steroids resulted in rapid neurological improvement and minimal long-term deficit. CONCLUSIONS: We believe that only surgical decompression is likely to be life saving in similar cases of hyperacute cerebral oedema due to ADEM. The wide decompression performed was concordant with that indicated for traumatic brain swelling. Such aggressive management is vindicated by the rapid recovery shown by our patient within days of surgery and the finding of minimal neurological sequelae at 3 months.

Population variation in glial fibrillary acidic protein levels in brain ageing: relationship to Alzheimer-type pathology and dementia.

BACKGROUND: The cellular pathology of astrocytes in brain ageing and their role in modulating the brain's response to neurodegenerative pathology remain incompletely understood. METHODS: Using quantitative ELISA, we have investigated glial fibrillary acidic protein (GFAP) expression in the population-based neuropathology cohort of the Medical Research Council Cognitive Function and Ageing Study to determine: (1) the population variation in the astroglial hypertrophic response, (2) its relationship to the presence of Alzheimer-type pathology, and (3) its association with cognition. RESULTS: Increasing GFAP was found with increasing Braak stage, levels increasing even at early stages. Within Braak stages, GFAP did not differ between demented and non-demented individuals, but there was greater variance in GFAP in the demented. Possession of ApoE epsilon4 was associated with slightly increased GFAP levels (not significant) for given amyloid beta protein loads. CONCLUSION: In a population-based sample, increasing gliosis precedes development of Alzheimer lesions. Population variation in GFAP with varying Alzheimer lesion burdens suggests that they are not the only driver for astrogliosis. GFAP was not independently predictive of dementia, but the variation in astrocytic responses may be a factor modulating brain responses to neurodegenerative pathology.

Consequence of Abeta immunization on the vasculature of human Alzheimer's disease brain.

A major feature of Alzheimer's disease is the accumulation of amyloid-beta peptide (Abeta) in the brain both in the form of plaques in the cerebral cortex and in blood vessel as cerebral amyloid angiopathy (CAA). Experimental models and human clinical trials have shown that accumulation of Abeta plaques can be reversed by immunotherapy. In this study, we hypothesized that Abeta in plaques is solubilized by antibodies generated by immunization and drains via the perivascular pathway, detectable as an increase in cerebrovascular Abeta. We have performed a follow up study of Alzheimer's disease patients immunized against Abeta42. Neuropathological examination was performed on nine patients who died between four months and five years after their first immunization. Immunostaining for Abeta40 and Abeta42 was quantified and compared with that in unimmunized Alzheimer's disease controls (n = 11). Overall, compared with these controls, the group of immunized patients had approximately 14 times as many blood vessels containing Abeta42 in the cerebral cortex (P<0.001) and seven times more in the leptomeninges (P = 0.013); among the affected blood vessels in the immunized cases, most of them had full thickness and full circumference involvement of the vessel wall in the cortex (P = 0.001), and in the leptomeninges (P = 0.015). There was also a significantly higher level of cerebrovascular Abeta40 in the immunized cases than in the unimmunized cases (cortex: P = 0.009 and leptomeninges: P = 0.002). In addition, the immunized patients showed a higher density of cortical microhaemorrhages and microvascular lesions than the unimmunized controls, though none had major CAA-related intracerebral haemorrhages. The changes in cerebral vascular Abeta load did not appear to substantially influence the structural proteins of the blood vessels. Unlike most of the immunized patients, two of the longest survivors, four to five years after first immunization, had virtually complete absence of both plaques and CAA, raising the possibility that, given time, Abeta is eventually cleared from the cerebral vasculature. The findings are consistent with the hypothesis that Abeta immunization results in solubilization of plaque Abeta42 which, at least in part, exits the brain via the perivascular pathway, causing a transient increase in the severity of CAA. The extent to which these vascular alterations following Abeta immunization in Alzheimer's disease are reflected in changes in cognitive function remains to be determined.

Solutes, but not cells, drain from the brain parenchyma along basement membranes of capillaries and arteries: significance for cerebral amyloid angiopathy and neuroimmunology.

UNLABELLED: Elimination of interstitial fluid and solutes plays a role in homeostasis in the brain, but the pathways are unclear. Previous work suggests that interstitial fluid drains along the walls of arteries. AIMS: to define the pathways within the walls of capillaries and arteries for drainage of fluid and solutes out of the brain. METHODS: Fluorescent soluble tracers, dextran (3 kDa) and ovalbumin (40 kDa), and particulate fluospheres (0.02 microm and 1.0 microm in diameter) were injected into the corpus striatum of mice. Brains were examined from 5 min to 7 days by immunocytochemistry and confocal microscopy. RESULTS: soluble tracers initially spread diffusely through brain parenchyma and then drain out of the brain along basement membranes of capillaries and arteries. Some tracer is takenf up by vascular smooth muscle cells and by perivascular macrophages. No perivascular drainage was observed when dextran was injected into mouse brains following cardiac arrest. Fluospheres expand perivascular spaces between vessel walls and surrounding brain, are ingested by perivascular macrophages but do not appear to leave the brain even following an inflammatory challenge with lipopolysaccharide or kainate. CONCLUSIONS: capillary and artery basement membranes act as 'lymphatics of the brain' for drainage of fluid and solutes; such drainage appears to require continued cardiac output as it ceases following cardiac arrest. This drainage pathway does not permit migration of cells from brain parenchyma to the periphery. Amyloid-beta is deposited in basement membrane drainage pathways in cerebral amyloid angiopathy, and may impede elimination of amyloid-beta and interstitial fluid from the brain in Alzheimer's disease. Soluble antigens, but not cells, drain from the brain by perivascular pathways. This atypical pattern of drainage may contribute to partial immune privilege of the brain and play a role in neuroimmunological diseases such as multiple sclerosis.

Genetic variation in the 5-HT2A receptor and altered neocortical [3H] ketanserin binding in Alzheimer's disease.

A common intronic single nucleotide polymorphism (T102C) in the 5-HT2A receptor gene is associated with the development of different neuropsychiatric symptoms, including hallucinations and depressive symptoms in Alzheimer's disease (AD). Differential 5-HT2A receptor binding has also been associated with the development of these symptoms in AD. However, the relationship between 5-HT2A (T102C) genotype and 5-HT2A receptor binding in AD and control human brains has not been examined. We examined the association between different 5-HT2A (T102C) genotypes and [(3)H] ketanserin binding in the temporal and frontal cortex of 20 AD and 14 control human brains. In homozygotes, but not heterozygotes, there was a significant reduction in B(max) values for [(3)H] ketanserin binding in both areas of cortex in AD compared with control subjects. This study suggests a mechanism for the generation of different neuropsychiatric symptoms in AD from a single nucleotide polymorphism with reduced receptor binding in T102C 5-HT2A receptor gene homozygotes correlating with susceptibility to depressive symptoms, whereas the relative preservation of receptor binding in heterozygotes with AD correlating with susceptibility to hallucinations.

No evidence for the presence of apolipoprotein epsilon4, interleukin-lalpha allele 2 and interleukin-1beta allele 2 cause an increase in programmed cell death following traumatic brain injury in humans.

BACKGROUND: Brain injury after trauma is an important cause of mortality and morbidity in society. There is evidence in both man and laboratory animals that in addition to necrosis, cell loss may occur as a result of programmed cell death (PCD). The cellular and molecular responses after head injury are partly influenced by genetic polymorphisms of apolipoprotein E and the pro-inflammatory cytokine IL-I. AIM: The principal aim of this study was to determine whether the presence of the ApoE epsilon4, IL- 1 alpha2 or IL- 1beta2 allele types influenced the amounts of PCD after head injury compared with controls. METHODS: Paraffin sections from the hippocampus of 38 patients (32 M : 6 F, aged 15 - 75, mean 38 years, survival 7- 576 hours; mean 36 hours) who died after a head injury were stained by Tunel histochemistry and quantified, and genotyping was undertaken by PCR "blind" to clinical detail. RESULTS: There were more Tunel+ cells (neurons and glia) after head injury than in controls with statistically increased numbers in all sectors of the hippocampus including the dentate fascia. However, there was no correlation between ApoEepsilon4, IL- 1 alpha allele 2 and IL- 1beta allele 2 and the amount of Tunel positivity. CONCLUSION: Given that both the ApoE and IL-1 influence outcome after various forms of acute brain injury, further work will be required to determine the mechanism underlying this relationship.

Association of APOE e4 and cerebrovascular pathology in traumatic brain injury.

BACKGROUND: Previous studies have found the e4 allele of the apolipoprotein E gene (APOE e4) is associated with an unfavourable outcome after head injury, but this has not been related to specific pathological features. OBJECTIVES: This study tested the postulate that head injured patients with APOE e4, amounting to approximately a third of the population, are selectively predisposed to one or more of the different pathological features that constitute the response to traumatic brain injury (TBI), and that this underlies the association of APOE e4 with poor clinical outcome. METHODS: Included in the study were 239 fatal cases of TBI (1987-1999) for which APOE genotypes were determined from archival tissue. For each case, specific pathological features of trauma were recorded by researchers blinded to the APOE e4 status. Of the 239 cases examined, 83 (35%) were APOE e4 carriers and 156 (65%) were non-carriers. RESULTS: Possession of APOE e4 was associated with a greater incidence of moderate or severe contusions (42% v 30% for carriers versus e4 non-carriers; p = 0.05) and there was a trend towards a greater incidence of severe ischaemic brain damage (54% v 42%; p = 0.08). Significant differences were not noted between the other pathological features examined. CONCLUSIONS: Possession of APOE e4 is associated with a greater incidence of moderate/severe contusional injury and severe ischaemic brain damage in fatal cases of TBI. This may be relevant to the relatively poor outcome from traumatic brain injury in patients with APOE e4 identified in clinical studies.

The association between APOE epsilon4, age and outcome after head injury: a prospective cohort study.

Previous preliminary studies have suggested that possession of the APOE epsilon4 allele is associated with a poor outcome after head injury. This study was designed to confirm and extend those observations in a larger study with examination of additional variables. We prospectively identified admissions to a Neurosurgical Unit for head injury, collected demographic and clinical data, determined APOE genotypes and obtained follow-up information at 6 months. A total of 1094 subjects were enrolled (age range: 0-93 years, mean 37 years). Outcome was assessed using the Glasgow Outcome Scale. There was no overall association between APOE genotype and outcome, with 36% of APOE epsilon4 carriers having an unfavourable outcome compared with 33% of non-carriers of APOE epsilon4. However, there was evidence of an interaction between age and APOE genotype on outcome (P = 0.007) such that possession of APOE epsilon4 reduced the prospect of a favourable outcome in children and young adults. The influence of APOE genotype in younger patients after head injury can be expressed as, at age <15 years, carriage of APOE epsilon4 being equivalent to ageing by 25 years. This finding is consistent with experimental data suggesting that the effect of APOE genotype on outcome after head injury may be expressed through the processes of repair and recovery.

Cerebral amyloid angiopathy in traumatic brain injury: association with apolipoprotein E genotype.

OBJECTIVE: In view of the association of the apolipoprotein E (APOE) epsilon 4 allele with poor outcome after traumatic brain injury we determined the frequency of cerebral amyloid angiopathy (CAA) and the extent of haemorrhagic pathology in relation to APOE genotype in an autopsy series of 88 head injured cases. METHODS: Tissue sections from the frontal and temporal lobes were immunostained for amyloid-beta peptide (A beta) and stained for Congo red to identify vascular amyloid pathology. A semiquantitative assessment of contusions, the total contusion index, was used to estimate the severity of the haemorrhagic pathology. APOE genotypes were determined by polymerase chain reaction of genomic DNA extracted from paraffin embedded tissue sections. RESULTS: CAA was present in 7/40 (18%) epsilon 4 carriers compared with 1/48 (2%) non-epsilon 4 carriers (p = 0.021, 95% confidence interval (CI) for difference in proportions with CAA 3% to 29%) with 6/40 (4 with CAA) epsilon 4 carriers being homozygotes. Thus the risk of having CAA for epsilon 4 carriers was 8.4 times that for the non-epsilon 4 carriers. However, there was no clear tendency for patients with CAA to have more severe or more numerous contusions (median contusion index 19 (CAA) v 14.5, p = 0.23, 95% CI for difference in medians -5 to 14). CONCLUSIONS: Presence of CAA in head injured cases was significantly associated with possession of an APOE epsilon 4 allele but not with the severity of contusions.