Two nearly ubiquitous herpes viruses are abundant in the brains of people with Alzheimer’s disease and appear to integrate themselves into the patient’s own genome, where the viruses play havoc with genes involved in Alzheimer’s pathogenesis, among other things, a new study reports.
The genomic analysis of hundreds of Alzheimer’s disease (AD) brain samples found human herpesvirus 6a and 7 (HHV-6a, HHV-7) in the entorhinal cortex and the hippocampus, the initial sites of beta-amyloid overexpression in the disease, first authors Benjamin Readhead, MBBS , Jean-Vianney Haure-Mirande, PhD , and colleagues reported June 21 in Neuron.
Dr. Sam Gandy
The viruses appear to interact with genes implicated in the risk for AD and for regulation and processing of the amyloid precursor protein, including presenilin-1 (PSEN1), BACE1, BIN1, PICALM, and several others. Their presence was directly related to the donors’ Clinical Dementia Rating score, and a mouse model suggests a potential pathway linking HHV infection and brain amyloidosis through a microRNA that’s been previously linked to AD.
It’s impossible to say whether HHV-6a and HHV-7 infections, which occur in nearly 100% of small children, trigger late-life amyloid pathology or whether the viruses reactivate and cross into the brain after amyloid-related damage has already begun, said Keith Fargo, PhD , director of scientific programs and outreach at the Alzheimer’s Association. But the data in this paper are strong enough to give real credence, for the first time, to the idea that Alzheimer’s disease may have an infective component.
“This paper, which is quite dense, presents an idea we have seen before, but which has been mostly dismissed,” Dr. Fargo said in an interview. “For the first time, a world-class group of researchers have completed a landmark paper packed with evidence. It’s not definitive evidence yet, but it will certainly bring that hypothesis into the mainstream in a way it has not been before. The Alzheimer’s research world will sit up and take notice.”
The viruses were present in about 20% of AD brain samples taken from four separate brain banks, but not in control samples or in samples from patients with other neurodegenerative diseases. The commonality suggests that the association is real, and something unique to Alzheimer’s, said Sam Gandy, MD, PhD , one of the paper’s senior authors, and a professor of neurology at Mount Sinai Medical Center, New York.
Joel T. Dudley, PhD , director of the Mount Sinai Institute for Next Generation Healthcare, was the other senior author.
“It seems obvious to us that the AD brains around this country are accumulating the genomes of this particular pair of viruses,” Dr. Gandy said in an interview. “For whatever reason, these people were accumulating the genomes of an infectious agent which crossed the blood-brain barrier, went into the brain, and was present there when they were dying of AD. It was not a remote relationship, such as we would see with serology. This was there when they were dying, and it’s hard to imagine it was doing anything good.”
HHV6