Is AD caused by viral infection?

Is AD caused by viral infection? 2 ARC Publishing HSV1 infection of a wide variety of cells in culture, including human neuronal-type, causes accumulation of beta amyloid (Aβ) and of abnormally phosphorylated tau (P-tau)—the main components respectively of the characteristic amyloid plaques and neurofibrillary tangles seen in AD brains. The increase in Aβ probably occurs via activation of PKR and de-repression of BACE1 expression. Implicating HSV1 further in AD is the finding that HSV1 DNA is very specifically localised in amyloid plaques in AD brains. This association of viral DNA with plaques does not prove causality, but considered together with the HSV1-induced formation of Aβ. It suggests that HSV1 is a major cause of Aβ formation in brain and of its toxic oligomers. Several genome-wide association studies (GWAS) have examined genetic links between HSV1 and its host cells. They show that a limited number of genes, when combined, are strongly associated with AD, even though the effect of any single gene or SNP is very weak. Possibly these genes code for proteins that interact in various processes, leading to a synergistic effect on AD pathogenesis. Also, HSV1 might bind to many cell proteins, thereby modulating their expression, including many encoded by susceptibility genes for various neurological diseases, including AD. Epidemiological studies have examined serum IgG and IgM, also IgG avidity index, and serological data relating to several different microbes; all show association between HSV1 reactivation and AD development, and between infectious burden and cognitive decline, with HSV1 particularly implicated. Cytomegalovirus (CMV) has been implicated also, possibly influencing immune response to other pathogens, thereby triggering the immune dysregulation involved in some age-related diseases, and suggesting specifically reactivation of HSV1 with CMV action and age. There is evidence that Aβ has anti-bacterial action, as part of the innate immune system, and recently it has been shown to have specifically anti-HSV1 action; however, it is likely that if eventually over-produced, it becomes toxic. Other relevant, harmful effects of infection include dynamic interactions between HSV1 and amyloid precursor protein (APP), the precursor of Aβ, which would facilitate viral transport and interfere with normal APP transport and distribution; induction of toll-like receptors in HSV1-infected astrocyte cultures, which has been linked to the likely effects of reactivated HSV1 in brain; infection-induced acute or chronic inflammation in triple-transgenic mice, which would exacerbate tau pathological features, further supporting the triggering of inflammation by infectious agents in brain, leading to cognitive impairment via effects on tau. Only 2 papers, 12 and 14 years ago, have challenged the data—specifically on viral presence in brain. No other opposing studies have since been published. One can conclude that there is overwhelming supportive evidence that HSV1 in brain of APOE-e4 carriers confers a major risk of AD.