Association Between Interleukin-1A Polymorphism and Cerebral Amyloid Angiopathy–Related Hemorrhage
Background and Purpose— It has been suggested that the interleukin-1A (IL-1A) allele 2 is a risk factor for Alzheimer’s disease (AD). Because cerebral amyloid angiopathy–related hemorrhage (CAAH) often coexists with AD, we examined the IL-1A polymorphism in CAAH.
Methods— In a case-control study, patients with pathologically verified CAAH, AD patients without intracerebral hemorrhage, and neuropathologically normal control subjects were studied. DNA was extracted from brain tissue, and IL-1A was genotyped. Logistic regression was used to examine the IL-1A polymorphism in CAAH patients with and without AD compared with AD and non-AD control subjects.
Results— There were 42 patients with CAAH, 232 AD patients, and 167 non-AD control subjects. In age-adjusted analyses, there was no association between possession of IL-1A allele 2 and risk of CAAH compared with AD control subjects (odds ratio [OR], 0.94; 95% confidence interval [CI], 0.45 to 1.97; P=0.87) or non-AD control subjects (OR, 0.94; 95% CI, 0.47 to 1.87; P=0.86). Stratifying for the presence of apolipoprotein E ε2 or ε4 demonstrated the known increased risk of CAAH from these lipoprotein E alleles. Subgroup analyses demonstrated a nonsignificant excess of the IL-1A 2,2 genotype in patients with CAAH and AD compared with those CAAH patients who did not have histological evidence indicating AD (OR, 2.17; 95% CI, 0.15 to 122.3; P=0.64). Comparisons between CAAH patients with AD and AD control subjects and between CAAH patients without AD and non-AD control subjects did not demonstrate an association between CAAH and possession of either the IL-1A allele 2 or the 2,2 genotype.
Conclusions— The IL-1A allele 2 or 2,2 genotype does not appear to be a major risk factor for CAAH.
Pathological1 and population2 studies suggest that inflammation plays a role in Alzheimer’s disease (AD). Interleukin-1 (IL-1), a proinflammatory cytokine mainly of microglial origin, has been recently implicated in AD; homozygosity for IL-1A allele 2 is more frequent in AD subjects than in control subjects.3 However, an excess of IL-1A allele 2 or the 2,2 genotype has not been a consistent finding in all studies,4 and the role of the IL-1A polymorphism in AD is not yet clear. A closely related condition in which amyloid β-protein (Aβ) is deposited in the walls of leptomeningeal and cortical blood vessels, cerebral amyloid angiopathy (CAA), frequently occurs in AD but usually remains clinically silent. The mononuclear-phagocyte system has been implicated in the deposition of vascular Aβ.5 In the minority of patients with CAA who present with lobar hemorrhage (CAAH) with or without coexisting evidence of AD, the role of inflammatory cytokines is unclear.
Apolipoprotein E (APOE for gene; apoE for protein) polymorphism has highlighted some of the complexities in CAAH and AD6; the ε4 allele is a risk factor for both CAA and AD,7 whereas the ε2 allele protects against AD but is associated with an increased risk of hemorrhage in patients with CAA.8 This has provoked interest in CAA9 and CAAH. Susceptibility genes implicated in AD are worthy of careful examination in CAAH.
Because we have previously found increased microglial activation in CAAH patients compared with CAA without hemorrhage,10 we examined the association between the IL-1A polymorphism in patients with pathologically verified CAAH compared with control subjects with and without significant AD pathology.
Forty-two patients with pathologically verified (autopsy, n=36; biopsy, n=6) CAAH were studied. Five were US citizens and 37 were UK citizens; their mean±SD age was 71.6±9.4 years (range, 46 to 97 years). Neuropathological diagnosis of AD was determined with the Consortium to Establish a Register for Alzheimer’s Disease (CERAD) criteria. AD was considered present only in cases with an age-related plaque frequency C score indicating a diagnosis of AD. The allele frequency and genotypes were compared with those in 232 patients (27 US citizens, 205 UK citizens) with clinical histories of dementia who satisfied CERAD criteria for postmortem neuropathological diagnosis of AD (AD controls) (mean age, 81.5±7.8 years; range, 52 to 99 years) and in 167 nondemented patients (36 US citizens, 133 UK citizens) without significant AD pathology or intracerebral hemorrhage (non-AD controls) (mean age, 74.5±9.6 years; range 50, to 99 years). All patients were white.
DNA was extracted from formalin-fixed, paraffin-embedded tissue for the CAAH patients and from fresh brain samples from the control groups. IL-1A (-889) genotyping was performed as previously described.3
Allele frequencies and genotypes in the CAAH group were compared with those in the AD and non-AD controls. Logistic regression was used to investigate the association between CAAH and IL-1A (-889) allele 2 possession. All analyses were adjusted for age. The following case versus control comparisons were modeled: (1) all CAAH patients versus AD controls; (2) all CAAH patients versus non-AD controls; (3) CAAH patients with AD versus AD controls; and (4) CAAH patients without definite AD versus non-AD controls. To examine whether possession of the APOE ε2 or ε4 allele modified the association with IL-1A allele 2, the analyses were repeated, stratified on possession of the ε2 or ε4 allele. A fifth model—CAAH patients with definite CERAD neuropathological evidence of AD versus CAAH patients without definite neuropathological evidence of AD—was also examined, but because of small numbers, Fisher’s exact test was used and stratification by APOE ε2 or ε4 allele possession was not possible. All analyses were performed with Stata, version 7.0 (Stata Corp).
The distributions of the IL-1A (-889) and APOE genotypes and alleles in the CAAH and control individuals are shown in Table 1. CAAH patients had an excess of neither the IL-1A allele 2 nor the 2,2 genotype compared with AD and non-AD control subjects. Among the CAAH patients, 35 had sufficient tissue for assessment of AD pathology according to CERAD criteria (Table 2). Three of 21 patients (14.3%) with CAAH and CERAD histological evidence indicating AD had the 2,2 genotype compared with 1 of 14 (7.7%) with CAAH without significant AD pathology, but this was not statistically significant (odds ratio [OR], 2.17; 95% confidence interval [CI], 0.15 to 122.3; P=0.64). Numbers were too small to allow stable estimation of whether APOE genotype altered the association.
In age-adjusted analyses (Table 3), there was no evidence for an association between possession of IL-1A allele 2 and risk of all CAAH compared with AD or non-AD control subjects. However, there was evidence that CAAH risk was lower among those who did not have either the ε2 or ε4 allele—ie, ε3/ε3 genotype—whereas risk appeared to be moderately elevated in patients possessing either the ε2 or ε4 APOE alleles. Separate analyses between CAAH patients with AD (CAAH with AD) and the AD control subjects and between CAAH patients without AD (CAAH without AD) and the non-AD control subjects yielded findings similar to the analyses between all CAAH patients and non-CAAH patients, with no association observed between possession of IL-1A allele 2 and risk of hemorrhage with or without AD. Restriction of the analyses to IL-1A 2,2 genotypes only did not alter any of these findings (data not presented).
Our data do not support the hypothesis that a polymorphism of IL-1A (allele 2) plays a major role in the development of CAAH. This proinflammatory cytokine and the inflammatory system in general may be more important in the pathogenesis of AD than of CAA. Although not statistically significant, the subgroup of CAAH patients with AD had a higher frequency of the IL-1A 2,2 genotype than CAAH patients without AD. The importance of APOE ε2 in the development of CAAH has already been demonstrated in this group of patients,8 and a recent population-based study confirmed that possession of an ε2 or ε4 allele is a significant independent risk factor for lobar hemorrhage.11
Deposition of Aβ in cortical and leptomeningeal blood vessels is influenced by age and APOE ε4 allele status.7 Activation of the mononuclear phagocyte system has been demonstrated in familial Dutch CAA and, less consistently, in and around amyloid-laden blood vessel walls in AD.5,12 The involvement of an inflammatory component in the initiation of sporadic CAA is, controversial, however, because it has also been reported that the mononuclear phagocyte system is not activated in CAA.13
In rare cases of sporadic CAA, an associated vasculitis has been described.14,15 There is increasing in vitro evidence of the interactions between microglia and Aβ or apoE. However, it remains to be determined in patients with coexisting CAA and vasculitis whether one condition provokes the other or if coexistence of CAA and vasculitis is a chance occurrence. We were therefore interested to observe an excess of perivascular-activated microglia in patients with CAAH, which was not associated with possession of the APOE ε2 allele.10 The pathogenetic significance of this is unclear. Microglial activation may lead to the development of vasculopathic complications such as fibrinoid necrosis or microaneurysm formation in blood vessels already laden with Aβ and cause CAAH. Alternatively, the presence of activated microglia in CAAH may merely reflect a “detritus-clearing” response to cortical hemorrhage.
One of the major strengths of the present study is the use of pathological specimens, ensuring accurate phenotypes; any potential confounding effect from coexistence of these pathologies was addressed. However, a number of limitations merit attention in this type of genetic association study. Although the study represents the largest group of patients with pathologically verified CAAH to be reported, the sample size is small, and a small effect of the IL-1A polymorphism cannot be excluded, particularly in the subgroup analyses.
Analysis of APOE has shed much light on the complex nature of CAAH, following the identification of the ε4 allele as a dose-dependent risk factor for sporadic and late-onset familial AD. However, it is clear from pathological studies that although AD and CAAH frequently overlap, major differences exist9 to the extent that either condition can exist without the other. The APOE findings in CAAH and the present negative study for IL-1A polymorphism in CAAH provide further evidence that AD and CAAH are end results of distinct Aβ pathological pathways.
Dr Vinters has had grant support from PHS AG 16570 and P01 AG 12435. We wish to thank Professor Sue Griffin, Professor Gordon Wilcock, Professor Margaret Esiri, Dr Deborah Dewar, Dr Tim Moss, Dr Sian MacGowan, Dr Robert Mrak, and Dr Lilian Murray for their help with the control subjects for this study.
- Received December 17, 2002.
- Revision received May 13, 2003.
- Accepted June 11, 2003.
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