Introduction

On Thursday, 24 February 2011, Ruth Itzhaki, University of Manchester, U.K.; Elisa Porcellini, University of Bologna, Italy; Luc Letenneur, INSERM, Bordeaux, France, and Richard Smeyne, St. Jude Children’s Research Hospital, Memphis, Tennessee, shared some of their latest research and field audience questions. Paul Klapper, herpes virologist at Manchester Royal Infirmary, U.K., and Terrence Town, Cedars-Sinai Medical Center, Los Angeles, were on hand for discussion afterward.

Herpes simplex virus 1 (HSV1) is attracting attention from a growing number of research groups as a possible trigger for Alzheimer's disease. Recent work has tied the virus to AD biomarkers, and offered some epidemiological and genetic support for the long-proposed connection between HSV reactivation and AD risk.
  

To one group of investigators, the data are strong enough to warrant a treatment trial with antiviral drugs—which are readily available and inexpensive—and they applied for funding to the British government this month. What do you think? Share your comments.

Ruth Itzhaki's Presentation

Elisa Porcellini's Presentation

Luc Letenneur's Presentation

Richard Smeyne's Presentation

Background

Background Text
By Esther Landhuis

Could the same bugs that give us cold sores set off Alzheimer’s disease in some people? Bizarre as it may sound at first blush, the notion that microbes such as herpes simplex virus may contribute to AD has buzzed around for years, perhaps drawing as much skepticism as intrigue. The Alzforum hosted a Live Discussion on this topic in 2004, and now, a spate of recent literature and budding developments on the human trials front gives us reason to take another look. The popular press on both sides of the Atlantic is starting to pay attention, too (see The Times; The Huffington Post).

Viruses are blamed primarily for fever blisters and other short-lived afflictions, so it’s no wonder they receive scant consideration as potential triggers for fatal neurodegenerative diseases that develop over decades. However, this possibility may not be so far-fetched—after all, many bugs lurk within the body for years in a resting state. “This process of dormancy followed by activation makes infectious agents prime candidates as factors in chronic disease,” Ruth Itzhaki and Matthew Wozniak of the University of Manchester, United Kingdom, suggest in a recent review (Wozniak and Itzhaki, 2010). Their commentary mentions prior long-fought battles to prove correspondingly “heretical” ideas—such as Helicobacter pylori as a cause of stomach ulcers, and human papillomaviruses as a cause of cervical cancer.

The 2004 Live Discussion The Pathogen Hypothesis featured heated discussion of Itzhaki’s work on herpes simplex virus type 1 (HSV1) and Brian Balin’s data showing induction of amyloid plaques in the brains of wild-type mice infected with Chlamydia pneumoniae (Little et al., 2004). More recently, Balin, Philadelphia College of Osteopathic Medicine, Pennsylvania, and colleagues have detected Chlamydia antigens alongside amyloid deposits and tau tangles in postmortem AD brain tissue (Hammond et al., 2010).

Itzhaki and Wozniak, and lately other groups as well, are making similar inroads with HSV1. The UK researchers have detected HSV1 DNA in elderly brains by PCR (Jamieson et al., 1991), and via antibodies to HSV proteins in the cerebrospinal fluid (Wozniak et al., 2005). They showed it localizes to amyloid plaques (Wozniak et al., 2009), and linked the virus to other AD biomarkers. In their cell culture system, HSV1 infection drives up AD-specific tau phosphorylation (Wozniak et al., 2009), increases levels of intracellular Aβ, BACE1, and the γ-secretase component nicastrin, and triggers plaque formation in the brains of wild-type mice (Wozniak et al., 2007). Other labs working on HSV1 have also reported links to tau and Aβ. Carola Otth and colleagues at University Austral de Chile, Valdivia, found that HSV1 induces tau cleavage by caspase-3 in primary neuron and astrocyte cultures (Lerchundi et al., 2010). Just last month, researchers led by Jesus Aldudo at CIBERNED in Madrid, Spain, reported that HSV1 increases intracellular Aβ production in autophagosomes and inhibits their breakdown within those compartments (Santana et al., 2011). And Elaine Bearer and colleagues at the University of New Mexico Health Sciences Center, Albuquerque, have evidence that intracellular HSV1 interacts with amyloid precursor protein (APP), and that this interplay enhances viral transport and disrupts APP transport and distribution. These data are in press at PLoS ONE. Taken together, these findings, some say, rise to a level of evidence where other laboratories should attempt to reproduce these data and test in their own experiments whether HSV1 could play a causal role in AD.

Recent epidemiological studies by independent groups appear to support the argument. Federico Licastro, first author Elisa Porcellini, and colleagues at the University of Bologna, Italy, analyzed data from a highly regarded genomewide association study of several thousand European AD patients and controls (see Lambert et al., 2009). They found a set of eight AD-linked gene variants that may increase the brain’s susceptibility to viral infections (Porcellini et al., 2010). The genes identified were nectin-2 (NC2), also known as poliovirus receptor-related protein 2 (PVRL2); apolipoprotein E (ApoE); glycoprotein carcinoembryonic antigen-related cell adhesion molecule-16 (CEACAM-16); B cell lymphoma-3 (BCL3); translocase of outer mitochondrial membrane 40 homolog (TOMM40); complement receptor-1 (CRl); ApoJ or clusterin (CLU); and C-type lectin domain A family-16 member (CLEC16A). These variants form a genetic signature that may determine individual brain susceptibility to pathogen infection, or susceptibility to pathogen damage, particularly by HSV and related viruses. This “may be one complex genetic trait influencing the risk of neurodegeneration leading to clinical AD in old age,” the authors write.

In another study, first author Luc Letenneur at INSERM in Bordeaux, France, and colleagues reported a connection between HSV reactivation and AD risk in 512 French seniors who were cognitively normal when they enrolled in the large, prospective PAQUID study that followed them for 14 years (Letenneur et al., 2008). Previous analyses had measured serum anti-HSV antibodies in small cohorts of AD patients and controls (e.g., Renvoize et al., 1987; Ounanian et al., 1990). The studies produced inconsistent results, possibly because they only examined immunoglobulin G (IgG) antibodies, which characterize past infections or reactivations. In the recent study, Letenneur and colleagues monitored both IgG and IgM anti-HSV antibodies in serum and found that only the latter, which reveals recent HSV reactivation, was associated with elevated AD risk.

How might this work? On the mechanistic front, Claudio Grassi of Catholic University Medical School, Rome, Italy, and colleagues last July reported that HSV1 infection disrupts calcium homeostasis, leading to APP phosphorylation and Aβ42 accumulation, in rat primary cortical neurons (Piacentini et al., 2010). A month earlier, Isamu Mori of Shubun University in Aichi, Japan, proposed that HSV1 may use viral accessory genes to promote its spread within the nervous system (Mori, 2010). Walter Lukiw and James Hill of Louisiana State University Health Sciences Center, New Orleans, have data for a different means by which the herpes virus may evade destruction. They found that HSV1 infection upregulates miRNA-146a, a brain-enriched microRNA associated with pro-inflammatory signaling in stressed neurons and AD (Hill et al., 2009). Last October, they followed with a report that they were able to dampen this miRNA-146a boost in human primary brain cells with aciclovir, an antiviral drug used to treat HSV infections (Lukiw et al., 2010; see also Itzhaki comment on Piacentini et al., 2010; Lukiw et al., 2010; and Porcellini et al., 2010).

Itzhaki and Wozniak themselves have unpublished data on antiviral drug effects on signature AD pathologies as well. They found that antivirals greatly reduce the amounts of Aβ and phospho-tau induced by HSV1 (manuscript in preparation). Moreover, Itzhaki and seven other investigators in three British cities on 9 February submitted a proposal for an antiviral drug trial in AD to the British Medical Research Council. The placebo-controlled study would test an aciclovir prodrug in people with mild to moderate AD who are seropositive for HSV1, measuring cognition and daily function. The team comprises PI Nitin Purandare and co-applicants Alistair Burns, Ruth Itzhaki, Graham Dunn, Julie Morris (all of the University of Manchester, U.K.), Paul Klapper (Manchester Royal Infirmary, Manchester), Clive Holmes (University of Southampton), and Naji Tabet (Institute of Post Graduate Medicine, Brighton & Sussex Medical School, U.K.). In a recent review (Holmes and Cotterell, 2009), Clive Holmes has argued for early intervention against infection as an AD prevention strategy. So do Nicolaas Verhoeff, Kunin-Lunenfeld Applied Research Unit in Toronto, Canada, and colleagues in an independent commentary (Honjo et al., 2009).

It is hard to estimate the proportion of AD patients who may benefit from antiviral treatment, Itzhaki told ARF, but there are some epidemiological data suggesting that AD risk among people with HSV1 DNA in the brain is higher in ApoE4 carriers than in non-carriers (see Itzhaki et al., 1997; Itzhaki et al., 2001). The ApoE link is controversial, however, as one other epidemiological study confirmed the association (Itabashi et al., 1997), whereas another merely found a trend (Beffert et al., 1998). Incidentally, Itzhaki and colleagues found that ApoE4 is a risk factor for HSV-induced cold sores (Itzhaki et al., 1997), and for infection by HSV2, the genital herpes virus (Jayasuriya et al., 2008). Another study confirmed the HSV2 finding (Koelle et al., 2010).

Recent animal work seems to support the idea that the ApoE isoform does influence HSV infection in the brain. Researchers led by Howard Federoff, who moved from the University of Rochester, New York, to Georgetown University in Washington, DC, analyzed HSV1-infected transgenic mice expressing the ApoE2, 3, or 4 allele exclusively. They reported that the ApoE4 mice expressed higher levels of the viral early genes and less of the latency gene, conditions that would promote more frequent reawakening of the virus (Miller and Federoff, 2008). Others have also infected ApoE-transgenic mice with HSV1, and found greater viral load in the brains of ApoE4 animals (Burgos et al., 2007; Bhattacharjee et al., 2008). And in a review published last year, Gerald Rimbach of Christian-Albrechts University, Kiel, Germany, analyzed available data from human trials and basic science research to propose possible mechanisms for how ApoE4 may drive up AD risk in people with active HSV infection (Kuhlmann et al., 2010).

So, esteemed scientists all across the field, here you have it. In your mind, does this new data since 2004 shift your prior stance? In preparation for our live hour, Alzforum asked around among AD researchers. As is to be expected for a controversial topic, some replied on the record, others off. David Holtzman of Washington University, St. Louis, wrote: “I think this topic is of interest. In order for there to be more interest, there need to be experiments which demonstrate (or not) in animals and in humans that there is or isn’t a cause/effect relationship. For example, in animal models, do particular types of herpes virus actually accelerate AD-type pathology and its associated neurodegeneration? If so, what type? In humans, is there evidence in living humans that those developing AD pathology and neurodegeneration (pre- and post-symptomatically) have differences (or not) in prior or current CNS infections with different herpes or other viruses?”

Others said more epidemiology is needed—in particular, to show that active HSV1 is seen more commonly, or that its effects are more damaging, before and during the onset of typical AD symptoms than in age-matched elderly who did not develop AD or its precursor, mild cognitive impairment (MCI). This could be done by demonstrating links between HSV1 titers and AD biomarkers (e.g., low cerebrospinal fluid Aβ42/40 ratio, high PIB signal in brain amyloid imaging). Some people reinforced the need for more human data showing a true relationship by pointing out that the high prevalence of HSV in human brain combined with the great stickiness of plaques makes it unsurprising that the two should sometimes be found together. Yet others pointed out that for a theory in this large and noisy field to gain traction, it is usually necessary that at least three independent labs show similar data.

How about it? Are these studies possible? Does existing evidence make a strong enough case for other labs to attempt independent confirmation? In what ways is the proof of causation by Aβ stronger than for HSV? What would be reasonable to do next?

imageThe following questions were raised during the Webinar. Some were not addressed.

Q: The virus is safe as long as it is latent, and indeed cytoprotective. What is waking it up and producing the IgM+ effects? Other pathogens?

Paul Klapper: I'm not sure I would agree with the concept of a virus being "safe." In latency, herpes simplex virus is believed to only exist as episomal circularized DNA. In this form, it is dormant but can be reactivated. It therefore always carries the potential for cell destruction. In its circularized form, it is essentially dormant, and thus to suggest that latency is “cytoprotective” seems inappropriate.

The production of IgM antibody in peripheral blood could indicate reactivation at any peripheral site. As Letteneur explained, it is not possible to routinely sample CSF, which might give a better indication of intrathecal reactivation. Itzhaki does, however, have data showing intrathecal synthesis of HSV antibody in Alzheimer’s patients. In a known HSV infection of the brain—herpes encephalitis—we know there is a profound and vigorous immune response to the virus. This response is prolonged in survivors of the disease. The finding of intrathecal synthesis of HSV antibody in the CNS indicates the virus has been there and has replicated, i.e., has at least produced HSV proteins. In latency, no proteins are produced.

Ruth Itzhaki: We have suggested that HSV1 is reactivated in the CNS, in the same way as it is known to be in the PNS, by events such as stress, immunosuppression, and peripheral infections. It seems unlikely that in the latent state, the virus is “safe,” as there is evidence that inflammation continues then. Anyway, it is difficult to distinguish between a very low level of persistent infection and true latency.

Q: Are there any fluoridated antivirals that could be used in PET scans to detect the virus in vivo?

Paul Klapper: The short answer is no. Richard Price in New York did produce work that we also followed in Manchester during the 1980s. We were trying to come up with a non-invasive method of diagnosing herpes encephalitis. In those days, we used radio-iodinated antivirals. We used iodo-vinyl deoxyuridine (a derivative of bromo-vinyl deoxyuridine, which has the same mode of action as aciclovir) labeled with Iodine -131. We were able to visualize intracranial accumulation of the drug (following intravenous administration) using a gamma camera. We had work in progress to produce fluoro-vinyl deoxyuridine with the intention of using this initially in NMR CT scanning (it is possible to detect accumulation of fluoro compounds using NMR spectroscopy) and then PET (at the time PET was not practicable because there was only one operational PET scanner in the whole of the U.K.). Work on this was suspended when we switched to using PCR on CSF as a means of detecting the virus in herpes encephalitis. Labeling does, however, remain a possible way of detecting and studying active virus replication within the brain. However, within the present context, the mechanism that Itzhaki proposes is not that the virus is continuously replicating in brain. It may reactivate infrequently, and there may be only limited replication during reactivation. Thus, to use PET with a targeting antiviral to demonstrate this reactivation would be problematic. It would essentially require continuous scanning to be able to detect reactivation at the time that it occurred. This would not be practicable.

Q: PET imaging of humans using positron-labeled HSV probes is a great idea.

Paul Klapper: True, we had the idea more than 20 years ago. For the reasons outlined above, it is not going to get us much further forward in studies of Alzheimer's.

Q: Chimerix has an antiviral that gets into the brain....

Paul Klapper: There are numerous antivirals that enter the brain and many anti-HSV compounds. None of them achieves free passage to the brain. Usually we can predict their penetrative ability using the octanol:water partition coefficient of the drug. This essentially measures how well the drug will traverse lipid membranes. In relation to the planned clinical trial, the decision was made to use aciclovir (administered as the pro-drug valaciclovir in order to allow oral dosing) because of the extensive experience and pharmacokinetic data available on the use of aciclovir in treatment of herpes encephalitis.

Q: Does aciclovir readily enter the brain? What level of dosing do you think would be required?

Paul Klapper: When administered intravenously, the drug does not readily enter the brain. It has to traverse the blood-brain barrier. This means that, in order to achieve adequate therapeutic concentrations within the CNS, the intravenous level has to be adequately high. The trials of treatment of herpes encephalitis during the 1980s showed that 10 mg/kg dosage every eight hours was required to achieve adequate intra-CNS concentration of the drug. Huge amounts are not required because the beauty of aciclovir is that the drug will selectively concentrate within virus-infected cells as a result of phosphorylation of the drug by the virus-specific enzyme thymidine kinase. Intravenous treatment is not practicable in elderly demented patients; hence, the pro-drug valaciclovir will be used. Some 55 percent of the orally delivered drug will appear in systemic circulation, meaning that adequate systemic levels can be achieved to ensure therapeutic concentrations of the drug within the CNS.

Q: We treated our pediatric patient with cerebral HSV2 with ganciclovir and suppressed the HSV2 replication; hence, it must cross the blood-brain barrier.

Paul Klapper: Ganciclovir has good activity against HSV and penetrates the blood-brain barrier with similar efficiency to aciclovir. Given a choice, however, I would always opt for aciclovir. Aciclovir has a much better safety profile than ganciclovir. Even a single dose of ganciclovir carries a risk of causing infertility in a patient and can commonly cause neutropenia. While it is appropriate for treatment of infections with a virus not effectively treated by aciclovir, for example, cytomegalovirus, first-line treatment of HSV will always be aciclovir. In neonates, as in the elderly, the blood-brain barrier is more permeable than in adults; hence, monitoring for excessive drug concentrations (which can cause reversible neurotoxicity) through inadequate renal function becomes important.

Ruth Itzhaki: The Chimerix compound (CMX001) is one of several antivirals that can enter the brain. I wrote to Chimerix about investigating it, but they never replied. Incidentally, another bonus of using antivirals to treat AD is that, presumably, they would restore Aβ to its normal very low level rather than eliminating it. (Whether Aβ is elicited by infection as an innate immune response of the cell or to aid virus replication is an issue we’d like to investigate, but the damage it causes is likely due to its subsequent overproduction.)

Q: For antivirals to be effective, there needs to be ongoing viral replication. Any thoughts about this with respect to AD patients?

Paul Klapper: Absolutely right. So late-stage Alzheimer’s is not going to be treatable with antivirals. What needs to happen is treatment of early-stage HSV reactivation that, as shown in Itzhaki's model, is a trigger for the development of plaques. As it is not possible to predict when reactivation will occur, long-term suppressive treatment is the only hope. This works in the peripheral nervous system where we use continuous aciclovir/valaciclovir treatment to suppress reactivation in patients who have repeated and frequent reactivation of oro-pharyngeal or genital herpes—hence, the rationale for a trial of suppressive treatment in mild to moderate Alzheimer’s. If we suppress reactivation, do we slow or arrest the process of disease development?

Q: On the question of selective vulnerability of the entorhinal cortex and hippocampus, is this a trigeminal-olfactory connection with HSV1 infection? We think Chlamydia goes through the olfactory system to damage the EC and hippocampus.

Paul Klapper: There seems to be some confusion about herpes simplex and its interaction with the peripheral nervous system. Neuroanatomists will tell you that there is no clear route for HSV to track from the trigeminal ganglion back to the limbic areas of the brain—the site of herpes encephalitis. However, does the virus actually need to reactivate from this site to reach the brain? Not necessarily. It is possible that during primary infection, herpes simplex virus is delivered to sensory nerves other than the trigeminal nerve. It is entirely credible that the virus enters olfactory nerves at this time and establishes latency within the CNS as well as the PNS. The triggers that cause the virus to reactivate in the PNS must not be the same as within the CNS; otherwise, there would be more evidence of CNS latency. However, as Ruth and others have shown, virus can be found in normal brain, supporting the proposition that virus is in the brain and obviating the more tortuous proposal that virus reactivated within the trigeminal nerve ganglion then undergoes anterograde transport through the spinal cord to reach the brain.

Q: I would suggest that the olfactory pathway is a key to the entry into the vulnerable areas of the brain having earliest involvement in AD.

Paul Klapper: The olfactory nerve route for delivery of virus to the CNS is, as outlined above, a credible route for delivery of virus to the CNS during primary HSV infection.

Q: We proposed the idea that HSV would travel into the trigeminal nucleus in the brain stem (Bearer, 2004).

Paul Klapper: There is an extremely large literature on HSV in nervous tissue because of studies over more than 50 years. Studies on peripheral nerve transit, and reactivation, have continued during this time with even more vigor. Many of these studies provide a very strong experimental basis for the proposal that HSV infection is a potential trigger for Alzheimer's.

Q: If 85 percent of elderly humans are infected with HSV, why do only a few (10 percent) get AD?

Paul Klapper: We ask a similar question of herpes encephalitis. Why, when 85 to 95 percent of the population carry one or both types of HSV, is herpes encephalitis so rare? This is one of the most complex of human viruses. It has very complex interactions with both the innate and adaptive immune system. We are unlikely to properly explain either herpes encephalitis or Alzheimer’s in the near future. The more we uncover, the more we realize how little we know. Herpes encephalitis does, however, exist and must be treated. The accumulated evidence amassed by Itzhaki and other researchers suggest HSV may be a trigger for Alzheimer’s. Given this, we should be conducting a trial of treatment even if we cannot completely understand the process.

Ruth Itzhaki: Our studies on human brains indicate that it is HSV1 in brain, plus ApoE4 genotype status, that confers a high risk of AD. Therefore, although HSV1 reaches the brains of ApoE2 and 3 carriers as well as ApoE4 carriers, i.e., all are probably equally susceptible to infection, we propose that, when the virus reactivates, the ApoE4 carriers suffer much greater damage, and after repeated reactivations, eventually develop AD.

Q: There is a lot of work on the swine alpha herpes virus, pseudorabies virus, that is used in animal models. How well this replicates human disease is a question.

Paul Klapper: There is a very wide variety of work on the use of 160+ herpes viruses in a huge range of animal models of PNS and CNS infection over the years. The reality is that none of the models can properly mimic human disease. We have tried in herpes encephalitis, and while we can model selected facets of the human disease, they are all incomplete. I suspect the same is true in trying to model HSV in Alzheimer’s. The best model is the human disease itself.

Q: Have you discussed HSV vaccines?

Paul Klapper: Vaccines for HSV have been in development since the 1930s. We still do not have one, and there is a very real difficulty in delivering the vaccine before we become infected. Most of us acquire HSV1 soon after we lose passively acquired maternal immunity. There is then a very short time in which to deliver protective vaccination, and it would have to be a very effective vaccine to ensure lifelong immunity. I do not believe vaccination is a practicable proposition for this virus.

Q: HSV does not infect mice the same way as in humans.

Paul Klapper: Agreed. There is no exact mimic of human disease.

Q: Are there mouse herpes viruses that can be used in these models? Does HSV infect mouse cells, such as N2a cells?

Paul Klapper: I’m not sure about these particular cells, but while HSV is exclusively a human virus, under experimental conditions in the laboratory, it can infect a wide variety of cell types (including mouse cells)—unlike some other viruses, such as varicella-zoster or cytomegalovirus, which even under experimental conditions can only infect a very select series of cells and cell types.

Ruth Itzhaki: It was suggested that only immunocyto-histochemical methods for Aβ, which are not specific and might detect APP rather than Aβ, have been used with HSV1-infected cells or mouse brains. Several studies detected Aβ using ELISA and/or Western blot, as well ICC with several different Aβ antibodies (see Wozniak et al., 2007; Piacentini et al., 2010; Santana et al., 2011). Our study (Wozniak et al. 2007) also used an APP antibody and showed that in infected SH-SY5Y cells, the APP level decreased while Aβ increased. Piacentini et al. showed a similar effect in rat cortical neurons. On the idea that neuronal damage might reactivate HSV1, why invoke an unknown damaging agent when we know that HSV1 is present in many elderly brains and that it does reactivate there—and that HSV1 damages cells? Also, infected ApoE-transfected mice would be a better model for sporadic AD than HSV1-infected transgenic mice overproducing Aβ.

Q: Why are there tens of thousands of lymphocytes in everyone's trigeminal ganglia, even from birth, through very old age? What is their function?

Paul Klapper: Immune defense.

Q: Do we have access to a group of elderly adults who, geographically, have not been exposed to HSV1?

Paul Klapper: HSV1 has a worldwide distribution. Following primary infection, a lifelong latent infection is established. Periodically, the virus reactivates, causing continued immune activation. Presence or absence of HSV-specific antibody therefore provides a good indicator of prior infection. The prevalence of antibody is linked to socioeconomic status: In poor, overcrowded populations, the virus spreads readily and the prevalence of antibody approaches 100 percent. In populations of high socioeconomic status, virus spreads less readily and antibody prevalence is lower. Your target population for sero-negative elderly adults is, thus, wealthy elderly adults (with the proviso that they are not nouveau riche!).

Ruth Itzhaki: If there are any, they’ve not yet been identified. However, as I said in my talk, we know nothing about risk factors for AD in the 40 percent of people who do not have the joint risk factors of brain HSV1 and ApoE4 carriage.

Q: Richard Smeyne's presentation reminded me of a recent study (Braak and del Tredici 2011) showing AD-like tau changes in the locus ceruleus at quite a young age. Is this compatible with your findings?

Paul Klapper: Smeyne mentioned Von Economo's encephalitis (also known as encephalitis lethargica) that occurred in epidemic form during the 1920s, and by 1940 had all but died out. The reports of the Matheson Commission make really interesting reading in relation to the neuropathology of this condition. The epidemic followed the pandemic of influenza A after World War I. While a specific definition of tau changes was not possible in 1920, the descriptions are consistent. Though influenza was not demonstrated in the brains of patients dying with this encephalitis, herpes simplex was found in some cases, and this was long before herpes encephalitis was first properly described in 1947. Smeyne’s results are interesting, but HSV rather than influenza provides a more understandable trigger for Alzheimer’s. Perhaps what Smeyne’s results show us is that influenza, and possibly other viruses, are actually the trigger for reactivation of HSV latent within the CNS.

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  1. To fuel the debate, there is a database relevant to herpes simplex and Alzheimer's disease. This annotates various Alzheimer's disease susceptibility genes in relation to the viral lifecycle, and a database of herpes simplex host/viral interactions is also provided.

    KEGG pathway analysis of the viral lifecycle also shows that the virus interferes with many processes relevant to Alzheimer's disease.

    Conversely, the KEGG pathways etched out by the many Alzheimer's disease susceptibility gene candidates can be related to viral lifecycles and pathogen defense: The immune network is also heavily represented. The relationships between genes and infection suggest that each may condition the risk-promoting effects of each other, perhaps accounting for some of the heterogeneity in association studies.

  2. I have been intrigued by these ideas since Itzhaki and others presented some of the data in Salzburg at the AD/PD meeting in 2007. Here are some issues that come to mind:

    • Most cases of AD are sporadic, representing a complex disease with genetic and environmental contributors. Why would anybody not want to test the hypothesis of HSV as a potential environmental contributor with CNS tropism to AD pathogenesis in suitable animal models? We must not forget about environmental triggers (or co-factors) in complex diseases (e.g., role of EBV in multiple sclerosis), and we don’t really have an abundance of such candidates in AD.
    • Is there any study in the neuropathology literature that has examined brains of survivors of a monophasic HSV encephalitis to determine whether Aβ and tau are dysregulated versus survivors of other encephalitides?
    • Do patients who are on chronic antiviral treatment with Zovirax (e.g., for prevention of recurrence of genital herpes) have a lesser risk of AD development or AD progression?
    • Do patients with classical herpes simplex type 1 infections carry tau-positive tangles in their ganglion Gasseri of cranial nerve 5 at postmortem examination?
  3. Further to my Limbic-Predilection "Hypothesis" paper (Ball, 1982) suggesting HSV1 as a cause of AD, the key question that cell biologists must now answer is
    why neurofibrillary tangles always appear first in the aging and the demented person's brain in the hippocampus and the adjacent entorhinal cortex of the temporal lobe. The answer to this puzzle (Ball, 2005) will also fit very beautifully with the theory that repeated reactivation of <em>latent</em> HSV-1 in the trigeminal ganglia (just a few millimeters from those same parts of temporal lobe) allows the virus to travel, not centrifugally down to lips/face/eye terminations of the trigeminal nerve, but rather centripetally along other branches of same nerves, into those neurons situated in the hippocampus/entorhinal cortex. I await your February 24 Webinar discussions with great anticipation!

    References:

    . "Limbic predilection in Alzheimer dementia: is reactivated herpesvirus involved?". Can J Neurol Sci. 1982 Aug;9(3):303-6. PubMed.

    . The essential lesion of Alzheimer disease: a surprise in retrospect. J Alzheimers Dis. 2006;9(3 Suppl):29-33. PubMed.

  4. Perhaps one of the important pieces of evidence is the high enrichment of herpes simplex binding proteins in plaques and tangles in the Alzheimer's disease brain, with p values rivaling those found in GWAS! (See <a href="http://www.polygenicpathways.co.uk/platang.htm" target="_new">Carter, 2010</a>.) These structures also contain many immune-related proteins, and one interpretation is that they represent tombstones for the warriors in a pathogen/immune battle. As viral proteins are rarely, if ever, detected in AD brains, this battle may have been won, but at the cost of neuronal destruction, mediated via inflammation and complement-related lysis (e.g., the complement receptor 1 and clusterin, which prevent formation of the <a href="http://en.wikipedia.org/wiki/Membrane_attack_complex" target="_new">membrane attack complex</a>).

    <p>This complex is found in AD neurons. It has often been thought that unraveling the causes of plaques and tangles would offer some insight into pathology, and this places the herpes virus firmly at center stage.

    </p><p>β amyloid is a major plaque constituent, and is toxic, but then so are apoptosis, glutamate, calcium, and free radicals, all of which are involved in AD pathology. Ruth Itzhaki and others have shown that viral infection increases β amyloid deposition, as does chlamydial infection (Ala et al., 2004; Little et al., 2004; Hoffman et al., 2009).

    </p><p>It has also been shown that <em>Helicobacter pylori</em> eradication prolongs the lifespan of Alzheimer's disease patients. Two case studies have shown that <em>Cryptococcus neoformans</em> eradication can evoke almost total remission in patients initially diagnosed with Alzheimer's disease/dementia (Kountouras et al., 2010).

    </p><p>Clinical studies have unfortunately shown that there is little one can do once the disease is established, but perhaps if the aging population were regularly screened and treated for herpes and other pathogens, there would be no plaques, no β amyloid, and less Alzheimer's disease.

    References:

    . Chlamydia pneumoniae induces Alzheimer-like amyloid plaques in brains of BALB/c mice. Neurobiol Aging. 2004 Apr;25(4):419-29. PubMed.

    . Five-year survival after Helicobacter pylori eradication in Alzheimer disease patients. Cogn Behav Neurol. 2010 Sep;23(3):199-204. PubMed.

    . Reversible dementia: a case of cryptococcal meningitis masquerading as Alzheimer's disease. J Alzheimers Dis. 2004 Oct;6(5):503-8. PubMed.

    . Cryptococcal meningitis misdiagnosed as Alzheimer's disease: complete neurological and cognitive recovery with treatment. J Alzheimers Dis. 2009;16(3):517-20. PubMed.

    . Alzheimer's disease plaques and tangles: cemeteries of a pyrrhic victory of the immune defence network against herpes simplex infection at the expense of complement and inflammation-mediated neuronal destruction. Neurochem Int. 2011 Feb;58(3):301-20. PubMed.

  5. I have been skeptical that abnormal β amyloid or tau were the primary cause of Alzheimer's disease for some time. As a person who led the development of the main drug used to treat the cognitive symptoms of the disease, it was my experience that discovering new treatments is often based on serendipity and a keen ability to integrate disparate information rather than a detailed examination of pathology after a disease is well established.

    If fact, for many CNS diseases, the "cause" is often identified only after an effective treatment is found. First, an effective treatment is identified largely by chance, and its mechanism of action is identified later. I believe this is because the brain is so complex it is largely resistant to mechanistic analysis.

    We have known for a long time that a number of chronic degenerative diseases of the nervous system are late sequelae of earlier infectious diseases. Examples include Lyme disease, syphilis, and the late consequences of the Spanish flu epidemic. There is certainly no reason to dismiss the possibility of an infectious cause. Thus, a search for an infectious cause is clearly indicated. Perhaps this could be done via an association study of blood antibody levels using statistical methodology similar to that used for gene association studies.

    My plea would be that we accept the idea that Alzheimer's will be a tough disease to cure and that therefore we expand our efforts to include strategies that are more "off the beaten track."

    Aricept certainly was well "off the beaten track" when I started the clinical program.

  6. Our work has focused on intracellular/axonal transport. In the process of investigating the signatures on cargo that recruit microtubule-based transport machinery, we used herpes simplex virus and discovered an interaction between HSV particles and cellular APP. Our next paper reporting live imaging inside cells of this interaction between virus and cellular APP was just accepted for publication in PLoS ONE.

    We would be delighted to comment on this at the Webinar later this week.

    References:

    . Herpes simplex virus dances with amyloid precursor protein while exiting the cell. PLoS One. 2011;6(3):e17966. PubMed.

  7. HSV1 has been identified as a cause of deficits in memory and executive functioning following replication within the CNS in diverse neurological pathologies, such as chronic psychiatric disease as schizophrenia and bipolar disorders (Schretlen et al., 2010; Shirts et al., 2008; Dickerson et al., 2008; Dickerson et al., 2004; Dickerson et al., 2003).

    Additionally, there is a case report about a 55-year-old female who developed HSE during brain irradiation and antioedematous dexamethasone treatment for leptomeningeal metastasized breast tumor with epileptic seizures (Koudriavtseva et al., 2010). This report suggests that cancer treatments could be contributory factors in the HSV1 reactivation, particularly in immuno-compromised hosts, where recurrent subclinical neuronal damage could happen. Early HSV diagnosis before and during cancer treatment may be considered.

    These antecedents reinforce the necessity of more studies about the neuronal-HSV relationship. For example, it would be interesting to evaluate the link between different pharmacologic treatments, dietary supplements, or basal neurological conditions and HSV reactivation.

    References:

    . Neuroanatomic and cognitive abnormalities related to herpes simplex virus type 1 in schizophrenia. Schizophr Res. 2010 May;118(1-3):224-31. PubMed.

    . Antibodies to cytomegalovirus and Herpes Simplex Virus 1 associated with cognitive function in schizophrenia. Schizophr Res. 2008 Dec;106(2-3):268-74. PubMed.

    . Association between cognitive functioning, exposure to Herpes Simplex Virus type 1, and the COMT Val158Met genetic polymorphism in adults without a psychiatric disorder. Brain Behav Immun. 2008 Oct;22(7):1103-7. PubMed.

    . Infection with herpes simplex virus type 1 is associated with cognitive deficits in bipolar disorder. Biol Psychiatry. 2004 Mar 15;55(6):588-93. PubMed.

    . Association of serum antibodies to herpes simplex virus 1 with cognitive deficits in individuals with schizophrenia. Arch Gen Psychiatry. 2003 May;60(5):466-72. PubMed.

    . Fatal herpetic encephalitis during brain radiotherapy in a cerebral metastasized breast cancer patient. J Neurooncol. 2010 Oct;100(1):137-40. PubMed.

  8. In 2000, Frank LaFerla and I published a paper in Biochemistry (Cribbs et al., 2000) describing fibril formation and neurotoxicity by a herpes simplex virus glycoprotein B fragment with homology to the Alzheimer's Aβ peptide. Because the majority of sporadic AD cases cannot be attributed to genetic factors alone, investigators have focused attention on environmental factors that may modulate the onset and/or progression of the disease. Head injury and infectious agents are environmental factors that have been periodically implicated, but no plausible mechanisms have been clearly identified. Frank found the sequence homology and was familiar with speculation of a link between viral infection and AD by prominent virologists centered on the neurotropic herpes viruses, with herpes simplex virus 1 (HSV1) considered a likely candidate. We generated synthetic peptides of the gB fragment that were homologous to Aβ and the biophysical and biological properties of these peptides compared to those of Aβ. We showed that the gB fragments form β-pleated sheets, self-assemble into fibrils, are thioflavin-positive, and ultrastructurally indistinguishable from Aβ. They also accelerate the formation of Aβ fibrils in vitro, and are toxic to primary cortical neurons at doses comparable to those of Aβ. At the time, we suggested a possible role for HSV1 in the pathophysiology of sporadic cases of AD.

    References:

    . Fibril formation and neurotoxicity by a herpes simplex virus glycoprotein B fragment with homology to the Alzheimer's A beta peptide. Biochemistry. 2000 May 23;39(20):5988-94. PubMed.

  9. Following Piacentini’s (1) paper showing that HSV1 produces marked changes in neuronal excitability and intracellular Ca2+ signaling that cause APP phosphorylation and Aβ accumulation in rat cortical neurons, we further investigated the effects of this virus on APP processing in neuronal cells (2). We found that HSV1 infection in SH-SY5Y cells and rat cortical neurons causes multiple cleavages of APP, which result in intra- and/or extracellular accumulation of different APP fragments with established neurotoxicity. These include: 1) APP fragments (APP-Fs) of 35 and 45 kDa (APP-F35 and APP-F45) that comprise portions of Aβ; 2) N-terminal APP-Fs that are secreted; 3) intracellular C-terminal APP-Fs (CTFs); and 4) Aβ1-40 and Aβ1-42, in monomeric and oligomeric forms.

    <p>Some of these cleavages are produced by cellular enzymes involved in the amyloidogenic APP pathway (β- and γ-secretases and caspase-3-like enzymes). It is reasonable to speculate that intra- and extracellular accumulation of these species in the CNS resulting from repeated HSV1 reactivations could play a co-factorial role in AD pathogenesis.

    </p><p>During the Webinar, we would be pleased to comment on data reported in our recent papers.

    References:

    . HSV-1 promotes Ca2+ -mediated APP phosphorylation and Aβ accumulation in rat cortical neurons. Neurobiol Aging. 2011 Dec;32(12):2323.e13-26. Epub 2010 Jul 31 PubMed.

    . APP processing induced by herpes simplex virus type 1 (HSV-1) yields several APP fragments in human and rat neuronal cells. PLoS One. 2010;5(11):e13989. PubMed.

  10. I’d like to echo Paul and Ruth’s comments about the interpretation of latent HSV1 as “safe.” An analogy that may be helpful is that of a loaded gun; I don’t think many would argue that the bullet is safe in a loaded pistol. Latent HSV represents the potential for damaging infection once the trigger is pulled—whether by cellular stress associated with aging, neuroinflammation, or other. I also don’t think there is evidence that HSV is neuroprotective in any state.

    Regarding the increased abundance of HSV IgM antibodies in AD patients versus healthy elderly, I found Luc Letenneur’s data both interesting and thought-provoking. Is it possible that AD patients and controls actually have similar degrees of HSV reactivation, but that the immune response to the virus is different? In other words, maybe it’s not that AD patients have more virus reactivation than controls, but rather that they are unable to make a switch from IgM to IgG antiviral antibodies. There is some support for the latter possibility. In the late 1990s, we and others found that the CD40 receptor-ligand pathway was dysregulated in mouse models of AD and in AD patients. This pathway is chiefly responsible for IgM-to-IgG isotype switching in plasma cells, and might explain the Letenneur data.

    Other questions raised were whether mouse herpes viruses can be used in these models, and whether HSV infects mouse cells, such as N2a cells. Regarding the former, this does occur in principle. Regarding N2a cells, we, including collaborator Homayon Ghiasi, actually tried to infect both SweAPP (a gift from Gopal Thinakaran) and wild-type N2a cells with a range of HSV titers, but were unable to detect cytopathic effects or altered Aβ1-40 or Aβ1-42 secretion from these cells. So, at least at this stage, in our hands, HSV does not seem to infect N2a cells well.

  11. Almost every human disease has been associated with a virus or other pathogen, usually several. Borna, herpes, and hepatitis for bipolar disorder; toxoplasmosis, herpes, and borrelia for schizophrenia; enterovirus for diabetes; congenital rubella and measles (not the vaccine!) for autism; flu and coronavirus for Parkinson's; Helicobacter pylori for ulcers and gastric cancer; Epstein-Barr virus for multiple sclerosis and cancer; adenovirus for childhood obesity; herpes, borrelia, and Helicobacter for Alzheimer's; XMRV retrovirus for chronic fatigue and prostate cancer; many viral infections precede multiple sclerosis relapses, and so on. I doubt whether anyone can find a disease that has not been associated with pathogens. These likely act together with genes. But you can do something about some pathogens, whereas you can't change the genes. It has been estimated that gastric cancer could be cut by 40 percent if a vaccine to Helicobacter pylori was developed, but no government has adopted public health measures for this, and funding agencies and drug companies show little interest. The priorities in public health and funding are all wrong, and the current obsession with genes is partly to blame.

  12. In answer to Terence’s question as to whether AD patients and controls have similar degrees of HSV reactivation, there is evidence suggesting that in brain,this is indeed the case. We sought IgG antibodies to HSV1 in CSF, the rationale being the known persistence of intrathecal immune activation and continued intrathecal synthesis of IgG for several years after herpes simplex encephalitis. (This could be due to a very low-level persistent infection of brain or to a failure to suppress the initial intrathecal immune response after recovery from infection.) We detected IgG in both elderly controls and AD patients (see Wozniak at al., 2005), suggesting that past or chronic HSV infection had occurred in the brain, possibly recurrently, and that the whole functional viral genome must be present. Also, occurrence of reactivation appeared to be independent of ApoE genotype. Hence, we proposed that it is not the occurrence or the frequency of reactivation that depends on ApoE, but instead, the extent and severity of damage.

    <p>As to murine cells, HSV1 certainly can infect them, both in vivo and in vitro (though humans are the only natural host for HSV1, as Paul said). We have worked with N2a cells, too, and found it difficult to detect cytopathic effects in them (probably reflecting their tendency to clump even in the absence of HSV1). However, immunofluorescence clearly showed that HSV1 does infect the cells. Also, we found that intracellular accumulation of Aβ1-42 and AD-like tau occurs in these cells.

    References:

    . Productive herpes simplex virus in brain of elderly normal subjects and Alzheimer's disease patients. J Med Virol. 2005 Feb;75(2):300-6. PubMed.

  13. I’m a caregiver not a medical professional. However, has anyone considered the herpes roster virus?

    It is a common illness, but the reoccurrence as shingles is not universal. It lies latent for decades and has slow growth—as does AD. It affects nerve cells specifically—as does AD. It uses the CD46 molecule as a receptor site—neuronal cells! It is triggered in immuno-compromised and/or persons undergoing psychological stress—common to AD patients. Shingles can cause mysterious pains for years after an outbreak, also common to AD patients

    The obvious symptoms are treated (rash pain), but is it possible the infection continues to attack the brain slowly?

    Sorry if this is all nonsense to professionals, but a quick study of the history of AD patients and incidence of shingles may be revealing.

  14. One of the questions raised in the Webinar related to factors able to reactivate the herpes virus.

    <p>These factors include heat, 17-β estradiol, interleukin 6, and NGF deprivation. Vitamin A supplementation in rats increases cerebral levels of NGF. Transient cerebral ischemia lowers brain NGF levels. Hypoxia is also able to increase the replication of herpes simplex (1). Many of these factors are relevant to Alzheimer's disease, which is characterized by low cerebral NGF levels (2), vitamin A deficiency (3), and high circulating levels of IL6 (4); cerebral hypoperfusion (ischemia/hypoxia/hypoglycemia) related to carotid atherosclerosis is also a feature of AD (5).

    </p><p>Licastro et al. have recently elaborated on their work on the susceptibility genes in relation to the virus, pointing out that many GWAS genes comprise a network related to viral defense (6).

    </p><p>This may also extend to other pathogens implicated in Alzheimer's disease and atherosclerosis, such as <em>C. pneumoniae</em>, <em>H. pylori</em>, <em>C. neoformans</em>, <em>B. burgdorferri</em>, and oral pathogens such as <em>P. gingivalis</em> (7): It would also appear that β amyloid (reportedly an antimicrobial and antiviral peptide) (8,9) and γ-secretase, which cleaves multiple viral and microbial receptors, are heavily involved in pathogen defense (7).

    </p><p>Miklossy has recently shown that spirochetes and treponemes are also implicated in Alzheimer's disease: They are present in the AD brain and are able to promote β amyloid deposition (10).

    </p><p>Just as Alzheimer's disease is polygenic, it is also probably polymicrobial. This may be very good news, as many microbes can be killed. It remains to be seen whether such treatments are effective in prevention or in the early stages of Alzheimer's disease.

    References:

    . The Fox and the Rabbits—Environmental Variables and Population Genetics (1) Replication Problems in Association Studies and the Untapped Power of GWAS (2) Vitamin A Deficiency, Herpes Simplex Reactivation and Other Causes of Alzheimer's Disease. ISRN Neurology. 2011 Apr;

    . NGF-cholinergic dependency in brain aging, MCI and Alzheimer's disease. Curr Alzheimer Res. 2007 Sep;4(4):351-8. PubMed.

    . Serum levels of beta-carotene, alpha-carotene and vitamin A in patients with Alzheimer's disease. Eur J Neurol. 1999 Jul;6(4):495-7. PubMed.

    . Inflammatory markers in matched plasma and cerebrospinal fluid from patients with Alzheimer's disease. Dement Geriatr Cogn Disord. 2003;16(3):136-44. PubMed.

    . Vascular risk factor detection and control may prevent Alzheimer's disease. Ageing Res Rev. 2010 Jul;9(3):218-25. PubMed.

    . Gene signature in Alzheimer's disease and environmental factors: the virus chronicle. J Alzheimers Dis. 2011;27(4):809-17. PubMed.

    . Alzheimer's Disease: APP, Gamma Secretase, APOE, CLU, CR1, PICALM, ABCA7, BIN1, CD2AP, CD33, EPHA1, and MS4A2, and Their Relationships with Herpes Simplex, C. Pneumoniae, Other Suspect Pathogens, and the Immune System. Int J Alzheimers Dis. 2011;2011:501862. Epub 2011 Dec 29 PubMed.

    . The Alzheimer's disease-associated amyloid beta-protein is an antimicrobial peptide. PLoS One. 2010;5(3):e9505. PubMed.

    . Acyclovir or Aβ42 peptides attenuate HSV-1-induced miRNA-146a levels in human primary brain cells. Neuroreport. 2010 Oct 6;21(14):922-7. PubMed.

    . Alzheimer's disease - a neurospirochetosis. Analysis of the evidence following Koch's and Hill's criteria. J Neuroinflammation. 2011;8:90. PubMed.

References

Webinar Citations

  1. The Pathogen Hypothesis

Paper Citations

  1. . Antiviral agents in Alzheimer's disease: hope for the future?. Ther Adv Neurol Disord. 2010 May;3(3):141-52. PubMed.
  2. . Chlamydia pneumoniae induces Alzheimer-like amyloid plaques in brains of BALB/c mice. Neurobiol Aging. 2004 Apr;25(4):419-29. PubMed.
  3. . Immunohistological detection of Chlamydia pneumoniae in the Alzheimer's disease brain. BMC Neurosci. 2010;11:121. PubMed.
  4. . Detection of herpes simplex virus type 1 DNA sequences in normal and Alzheimer's disease brain using polymerase chain reaction. Biochem Soc Trans. 1991 Apr;19(2):122S. PubMed.
  5. . Productive herpes simplex virus in brain of elderly normal subjects and Alzheimer's disease patients. J Med Virol. 2005 Feb;75(2):300-6. PubMed.
  6. . Herpes simplex virus type 1 DNA is located within Alzheimer's disease amyloid plaques. J Pathol. 2009 Jan;217(1):131-8. PubMed.
  7. . Alzheimer's disease-specific tau phosphorylation is induced by herpes simplex virus type 1. J Alzheimers Dis. 2009;16(2):341-50. PubMed.
  8. . Herpes simplex virus infection causes cellular beta-amyloid accumulation and secretase upregulation. Neurosci Lett. 2007 Dec 18;429(2-3):95-100. PubMed.
  9. . Tau cleavage at D421 by caspase-3 is induced in neurons and astrocytes infected with herpes simplex virus type 1. J Alzheimers Dis. 2011;23(3):513-20. PubMed.
  10. . Herpes simplex virus type I induces the accumulation of intracellular β-amyloid in autophagic compartments and the inhibition of the non-amyloidogenic pathway in human neuroblastoma cells. Neurobiol Aging. 2012 Feb;33(2):430.e19-33. Epub 2011 Jan 26 PubMed.
  11. . Genome-wide association study identifies variants at CLU and CR1 associated with Alzheimer's disease. Nat Genet. 2009 Oct;41(10):1094-9. PubMed.
  12. . Alzheimer's disease gene signature says: beware of brain viral infections. Immun Ageing. 2010;7:16. PubMed.
  13. . Seropositivity to herpes simplex virus antibodies and risk of Alzheimer's disease: a population-based cohort study. PLoS One. 2008;3(11):e3637. PubMed.
  14. . A sero-epidemiological study of conventional infectious agents in Alzheimer's disease. Age Ageing. 1987 Sep;16(5):311-4. PubMed.
  15. . Antibodies to viral antigens, xenoantigens, and autoantigens in Alzheimer's disease. J Clin Lab Anal. 1990;4(5):367-75. PubMed.
  16. . HSV-1 promotes Ca2+ -mediated APP phosphorylation and Aβ accumulation in rat cortical neurons. Neurobiol Aging. 2011 Dec;32(12):2323.e13-26. Epub 2010 Jul 31 PubMed.
  17. . Herpes simplex virus type 1 persists in the aged brain through hypothetical expression of accessory genes. J Neurovirol. 2010 Jun;16(3):203-7. PubMed.
  18. . HSV-1 infection of human brain cells induces miRNA-146a and Alzheimer-type inflammatory signaling. Neuroreport. 2009 Oct 28;20(16):1500-5. PubMed.
  19. . Acyclovir or Aβ42 peptides attenuate HSV-1-induced miRNA-146a levels in human primary brain cells. Neuroreport. 2010 Oct 6;21(14):922-7. PubMed.
  20. . Role of infection in the pathogenesis of Alzheimer's disease: implications for treatment. CNS Drugs. 2009 Dec;23(12):993-1002. PubMed.
  21. . Alzheimer's disease and infection: do infectious agents contribute to progression of Alzheimer's disease?. Alzheimers Dement. 2009 Jul;5(4):348-60. PubMed.
  22. . Herpes simplex virus type 1 in brain and risk of Alzheimer's disease. Lancet. 1997 Jan 25;349(9047):241-4. PubMed.
  23. . Association of HSV1 and apolipoprotein E-varepsilon4 in Alzheimer's disease. J Neurovirol. 2001 Dec;7(6):570-1. PubMed.
  24. . Herpes simplex virus and risk of Alzheimer's disease. Lancet. 1997 Apr 12;349(9058):1102. PubMed.
  25. . HSV-1 in brain and risk of Alzheimer's disease. Lancet. 1998 May 2;351(9112):1330-1. PubMed.
  26. . Apolipoprotein E-epsilon 4 and recurrent genital herpes in individuals co-infected with herpes simplex virus type 2 and HIV. Sex Transm Infect. 2008 Dec;84(7):516-7. PubMed.
  27. . APOE genotype is associated with oral herpetic lesions but not genital or oral herpes simplex virus shedding. Sex Transm Infect. 2010 Jun;86(3):202-6. PubMed.
  28. . Isoform-specific effects of ApoE on HSV immediate early gene expression and establishment of latency. Neurobiol Aging. 2008 Jan;29(1):71-7. PubMed.
  29. . Apolipoprotein E genotype influences vertical transmission of herpes simplex virus type 1 in a gender specific manner. Aging Cell. 2007 Dec;6(6):841-2. PubMed.
  30. . Effect of human apolipoprotein E genotype on the pathogenesis of experimental ocular HSV-1. Exp Eye Res. 2008 Aug;87(2):122-30. PubMed.
  31. . Apolipoprotein E genotype and hepatitis C, HIV and herpes simplex disease risk: a literature review. Lipids Health Dis. 2010;9:8. PubMed.
  32. . Perspectives on herpes-APP interactions. Aging Cell. 2004 Apr;3(2):81-4. PubMed.
  33. . The pathological process underlying Alzheimer's disease in individuals under thirty. Acta Neuropathol. 2011 Feb;121(2):171-81. PubMed.

Other Citations

  1. See responses to some questions raised during the Webinar.

External Citations

  1. The Times
  2. The Huffington Post
  3. PVRL2
  4. ApoE
  5. BCL3
  6. TOMM40
  7. CRl
  8. CLU
  9. aciclovir

Further Reading

Papers

  1. . Impaired cued and contextual memory in NPAS2-deficient mice. Science. 2000 Jun 23;288(5474):2226-30. PubMed.
  2. . Alzheimer's disease gene signature says: beware of brain viral infections. Immun Ageing. 2010;7:16. PubMed.
  3. . Herpes simplex virus dances with amyloid precursor protein while exiting the cell. PLoS One. 2011;6(3):e17966. PubMed.
  4. . Gene signature in Alzheimer's disease and environmental factors: the virus chronicle. J Alzheimers Dis. 2011;27(4):809-17. PubMed.