Alzheimer's Disease: The Virus Hypothesis
Compelling evidence is challenging the longstanding amyloid hypothesis.
For decades, the "amyloid hypothesis" has dominated research on Alzheimer's disease (AD). According to this hypothesis, Aβ—a sticky, microscopic brain protein fragment—accumulates in the brain, strangling communication between brain cells and eventually killing them.
That hypothesis is increasingly being questioned, however, largely on the basis that more than 100 clinical trials targeting amyloid plaques have failed to actually slow disease progression in AD.1 Although it's still widely recognized that an imbalance between production and clearance of Aβ is an early feature of AD, researchers are increasingly looking to other hypotheses regarding the disease's etiology.
One minority view receiving attention is the viral hypothesis. It's now well established that there's a link between viruses—in particular, herpes viruses—and AD. But the nature of the link is unknown: Are viruses simply opportunistic tagalongs in a disease process of a different origin? Do they accelerate already established AD? Or do they truly play a causal role?
Evidence for a Viral Role
As far back as the 1990s, several studies demonstrated that HSV1 DNA is present in latent form in the brains of a high percentage of older adults (about 60%), including those with AD and those without.6-8 It's not commonly found in the brains of younger individuals, however.9,10 Thus, it's been hypothesized that the virus tends to enter the brain in older age as a result of age-related weakening of the immune system.
Although the virus commonly is present even in the brains of older adults who are cognitively normal, there are intriguing links between HSV1 and AD. First, in herpes simplex encephalitis, a rare but serious disease typically caused by HSV1, the virus infiltrates the hippocampus and frontal lobe in particular, causing memory loss and cognitive deficits similar to those seen in AD.11 Even when it does not cause encephalitis, HSV1 is closely linked with AD pathology: Studies of cultured cells, including human neural cells, have shown that infection with HSV1 causes the accumulation of both Aβ and P-tau, which are the main components of the amyloid plaques and neurofibrillary tangles characteristic of AD.12-16 Furthermore, in mouse models, infection with HSV1 leads to formation of Aβ deposits in the brain.12
Another intriguing connection between HSV1 and AD is that in human AD brains where HSV1 is present, the virus's DNA is preferentially located within amyloid plaques. In one autopsy study of AD patients, 90% of amyloid plaques contained HSV1 DNA, and 72% of HSV1 DNA was found within amyloid plaques. By comparison, in normal control brains, much less of HSV1 DNA was found in amyloid plaques (only 24%).10 Thus, there's a strong link between HSV1 DNA and amyloid plaques in AD patients, but a far weaker association in individuals who are cognitively normal.
If viruses, specifically HSV1, play a causal role in AD, an obvious question is why so many who are infected with the virus never go on to develop AD. However, a key feature of many viruses and bacteria is that it is possible to be infected without being obviously impacted. Tuberculosis is a relevant example: Millions are infected with the bacterium that causes tuberculosis, but only a small percentage of those infected actually develops tuberculosis.
In the case of herpes viruses, genetic factors (specifically, the APOE genotype) appear to influence susceptibility to herpes—and to AD. Carriers of the APOE-E4 allele are more likely to become infected by and more likely to experience severe herpes labialis (caused by HSV1),8 as well as several other infectious disorders.17,18 In mice, the APOE-E4 allele has been shown to influence the invasiveness of HSV1 in the brain, with those carrying one or both alleles having higher viral loads.
Thus, APOE appears to play an important role in the virus's ability to spread in the brain.19 Further, in humans, the combination of the APOE-E4 allele with the presence of HSV1 in the brain significantly increases risk for AD, accounting for 60% of all AD cases.8 Thus, the APOE genotype may account in large part for why some infected with HSV1 develop AD and others do not.
Are Recurrent Viral Reactivations at the Root of Alzheimer's?
Growing evidence demonstrates that HSV1 does indeed reactivate in the brain, potentially recurrently.9 There's strong evidence from postmortem human brains that such reactivations occur under conditions of immunosuppression in particular.20 If recurrent HSV1 infections in the brain were severe, leading to overt herpes simplex encephalitis, they would most likely be fatal without rapid diagnosis and treatment. The fact that recurrent activation of HSV1 in the brain typically is not obvious suggests that it most likely recurs in a localized or mild manner, but still with the potential to cause significant cell death over time.21
Important evidence suggesting that recurrent HSV1 infection is linked with AD comes from recent research on anti-HSV antibodies, namely IgM and IgG. The presence of IgM is associated with recent reactivation of HSV1 virus; by contrast, the presence of IgG demonstrates only that a person was infected with HSV1 at some point in the past.
Two different studies, one of which was a large, well-designed prospective investigation, have found that IgM-positive individuals are far more likely to develop dementia in subsequent years than are those who are negative.22,23 Furthermore, other research has shown that individuals who are IgM-positive are significantly more likely than those who are only IgG-positive to have low plasma levels of Aβ1-40 and 1-42, which are considered biomarkers of AD because a decrease in their plasma levels is thought to signal accumulation of Aβ in the brain.24
What About Other Pathogens?
One study in particular involved a large, multiscale analysis of viral activity in multiple datasets comprising postmortem samples from individuals with AD as well as controls who were cognitively normal.25 The researchers examined more than 500 viral species and found that several herpes viruses were more abundant in the brains of individuals with AD than in cognitively normal controls. Although HSV1 was associated with AD to some degree, the viruses that were most strongly associated were actually human herpes virus 6A (HHV-6A) and 7 (HHV-7), both of which are extremely common in North America.
According to Benjamin Readhead, MBBS, an assistant research professor in the Neurodegenerative Disease Research Center at Arizona State University in Tempe, Arizona, and lead author of the study, the difference was not that the viruses were present in AD brains and not in cognitively normal brains; rather, the viruses were common in both, but the viral load was noticeably greater in AD. Indeed, both HHV-6A and HHV-7 proved more abundant in AD brains across multiple brain regions. The findings were replicated in two additional independent and geographically dispersed cohorts.
Not only were herpes viruses more prevalent in AD but the same study also found that HHV-6A and HHV-7 viruses were interacting with human genes—including genes that are known regulators of amyloid processing and other aspects of Alzheimer's biology. "We were able to observe that these viruses are regulating the expression of these genes that we know are right in the middle of Alzheimer's pathophysiology," Readhead says.
While the study adds credence to the hypothesis that viruses could play a causal role in AD, Readhead stresses that it fell short of actually demonstrating a causal connection. "There is a significantly increased abundance of certain common pathogens—in particular human herpes virus HHV-6A, HHV-7, and HSV1—and we are able to show this in several different cohorts," Readhead says. "What that doesn't really address though is whether these are opportunistic bystanders that emerge in the presence of neuropathology that's being driven by other factors or whether they are actually a contributor to the disease. There is also the middle possibility that in the presence of established disease, it could be that these infections or these pathogens might accelerate things."
Aβ as Antiviral Defense Mechanism
But several studies have challenged that assumption, including one new study from Moir's own research group. This research suggests that Aβ, rather than accumulating without cause, accumulates as a defense mechanism—that it traps viruses in its fibrils and forms plaques as a way of sequestering those viruses. "Aβ is not a piece of metabolic junk, and its ability to form fibrils is actually part of innate immune response to infection," Moir says.
As far back as 2010, for example, Moir and his colleagues demonstrated that Aβ functions as an antimicrobial peptide. Specifically, they compared Aβ with LL-37, an archetypal human antimicrobial peptide, and found that Aβ exerts an antimicrobial influence against eight common pathogens, with a level of potency equal to or greater than LL-37. They also demonstrated that tissue taken from AD brains showed significantly higher antimicrobial activity than did tissue from age-matched controls, and that antimicrobial activity correlated significantly with tissue Aβ concentration.26
Several years later, another study showed that Aβ protects against both fungal and bacterial infections in mice, worm, and cell culture models of AD. The evidence was consistent with a classic antimicrobial peptide model of entrapment of invading microbes by amyloid fibrils.27
Importantly, Aβ reacts with antimicrobial activity regardless of which specific pathogen enters the brain, Moir says. This fact may help to explain why not all AD brains reflect the presence of the same pathogen. (As Moir notes, a high percentage of AD patients has herpes viruses in their brains, but the presence of herpes virus still falls far short of explaining all AD cases.)
According to Moir, it's possible there's not just one single pathogen or infection behind AD; rather, it could be a collection of pathogens. Alternatively, he adds, the cutting edge of research on viruses and AD suggests that AD might result from a disruption in the virome rather than from a classic infection. In the latter case, he suggests, AD might be analogous to inflammatory bowel disease, which is not caused by a single pathogen but rather by an imbalance or disruption in the microbiome.
Microbes in the brain "don't just sit there," Moir says. "Sometimes they cooperate, sometimes they compete. They are a real microbiome in that they interact with one another and they are in a kind of balance. If that balance gets out of whack, it may be that the immune system rises to try to do battle with it, as it does with inflammatory bowel disease, but what you end up with is chronic inflammation as the immune system tries to wrest back control. Part of that mechanism for wresting back control is Aβ and amyloid production."
Could Antivirals Inhibit AD Progression?
In one study, researchers first infected Vero cell cultures with HSV1, then administered acyclovir, penciclovir, and foscarnet, three antiviral agents that inhibit replication. All three reduced Aβ and P-tau accumulation—P-tau to near zero, and Aβ to near normal levels (though foscarnet was the least effective of the three).28
In another study by the same authors, BAY 57-1293, a different antiviral that targets a different stage of HSV1 replication, was found to be even more efficient at reducing Aβ and P-tau formation.29
Other research has examined the impact of intravenous immunoglobulin, which acts at the stage of initial infection, before the virus has begun replicating. This antiviral, too, has been shown effective at reducing accumulation of Aβ and P-tau; moreover, intravenous immunoglobulin can be used in combination with acyclovir.30
A recent study from Taiwanese researchers provides the first population-level evidence that antivirals may inhibit the development of AD.31 The nationwide cohort study enrolled 33,000 subjects, approximately one-quarter of whom were newly diagnosed with HSV infection and the rest of whom showed no evidence of infection. Subjects were then followed for 15 years. Those with HSV infection proved 2.5 times more likely to develop dementia than were those without infection—a significant finding in itself. Even more significant, however, was the impact of antiviral therapy: Those with HSV who were treated with antivirals had a 10-fold reduction in their risk of developing dementia compared with those who did not receive therapy.
According to Readhead, the fact that antivirals had a dramatic effect on dementia risk in the Taiwanese cohort is suggestive. "That [finding] is one of the strongest pieces of evidence pointing to a potential causal role for these viruses," he says.
The Way Forward
"We know a great deal about the disease's neuropathology but very little about what causes Alzheimer's," says Devanand, who suggests researchers are still only exploring associations.
Although it's clear that there's a link with viruses, there's still ambiguity about the exact nature of the link. A crucial next step, Devanand says, is prospective clinical trials of antiviral drugs to evaluate what impact they have on AD. (Devanand's own research group has one such trial underway.) If successful, these trials would provide the clearest evidence of a causal role for viruses in AD; more important, they could provide a desperately needed pharmacological treatment for AD patients.
"Whether any of this will work, we don't know, but we need to try, because everything else we've tried has failed," Devanand says.
— Jamie Santa Cruz is a health and medical journalist based in Parker, Colorado.
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