The herpes simplex type 1 virus (HSV-1) that causes cold sores may increase susceptibility to Alzheimer’s disease (AD) if a particular variant of a specific gene is also present.

The variant, known as ApoE4, has been linked to AD for some time. In fact, according to some scientists, evidence points to ApoE4 as a contributing, perhaps causative, factor in AD. The possibility that it works with the herpes virus is new.

Researchers at the University of Rochester in New York found that HSV-1 tends to remain latent with other versions of ApoE. However, ApoE4 carriers were more likely to suffer active infections and the characteristic cold sores or fever blisters.

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The researchers said the results support a model in which ApoE4 in the presence of HSV-1 renders brain cells more vulnerable to lytic infection and possibly apoptosis. They suspect that the gene-virus combination “synergistically promotes neuronal death such as occurs in AD.” The study appeared ahead of print in the online edition of Neurobiology of Aging.

William Netzer, PhD, of The Rockefeller University’s Fisher Center for Alzheimer’s Disease Research in New York City, said, “One of the most important challenges in AD research is to understand how susceptibility genes, such as ApoE4, interact with other genes or environmental triggers to cause AD.”

While the study does not determine the role of HSV-1 in AD, Dr. Netzer said, the topic is important to pursue. “If HSV-1 and ApoE4 do conspire in AD, an HSV-1 vaccine—if effective”—could be possible.