Objectives Cerebral amyloid angiopathy (CAA) is associated with lobar intracerebral haemorrhage (ICH). While only the ε4 allele of the apolipoprotein E (APOE) gene is associated with the presence of CAA, both APOE-ε4 and ε2 are associated with lobar ICH. The generally accepted explanation is that APOE-ε4 promotes vascular amyloid deposition, while APOE-ε2 promotes progression to severe CAA with associated vasculopathic changes that cause vessel rupture and ICH. We assessed the evidence for these allele-specific effects.

Methods We systematically identified published studies with data on APOE genotype and histopathological assessment of postmortem brains for CAA severity. We obtained unpublished data from these for meta-analyses of the effects of ε4-containing (ε4+) and ε2-containing (ε2+) genotypes on progression to severe CAA.

Results Of six eligible studies (543 eligible participants), data were available from 5 (497 participants, 353 with CAA). Meta-analyses showed a possible association of ε4+ genotypes with severe CAA (ε4+ vs ε4−: severe vs mild/moderate CAA, OR 2.5, 95% CI 1.4 to 4.5, p=0.002; severe vs moderate CAA, OR 1.7, 95% CI 0.9 to 3.1, p=0.11). For ε2+ versus ε2− genotypes, there was no significant association, but the very small number of participants with ε2+ genotypes (22) precluded reliable estimates.

Conclusions We found a possible association of severe CAA with APOE-ε4 but not APOE-ε2. However, our findings do not exclude a biologically meaningful association between APOE-ε2 and severe CAA. Further work is needed to elucidate fully the allele-specific associations of APOE with CAA and their mechanisms.