Herpes Simplex Virus Type 1 in the Brain, Apolipoprotein E Genotype and Alzheimer’s Disease

The ε4 allele of apolipoprotein E (apoE) is an important risk factor for Alzheimer's disease (AD), however, it is not required nor sufficient to cause the disease on its own. Herpes viruses cause acute and chronic diseases of the central nervous system and have been implicated in AD. Using a sensitive polymerase chain reaction method, latent herpes simplex virus type 1 (HSV-1) has been detected from five different brain regions (hippocampus, frontal cortex, occipital cortex, cerebellum and striatum) of neuropathologically confirmed AD and control tissue. HSV-1 positivity was then correlated with AD, presence of the virus in specific brain regions, and apoE genotype. The results confirm that the ε4 allele of apoE is a risk factor for AD, while HSV-1 alone is not. This held true for all five brain regions examined. Furthermore, no synergy between the two factors could be found when any one of the brain regions was examined individually or when the data were pooled. These findings emphasize that the ε4 allele of apoE is a risk factor for AD and that HSV-1, either alone or in combination with apoE, does not represent an increased risk for AD. Furthermore, no particular brain region seems to be more infected with HSV-1 than another, even in those regions most affected in AD.