Herpes simplex virus type 1 in brain and risk of Alzheimer's disease

Summary

Background

The apolipoprotein E ε4 (APOE-ε4) allele is a risk factor for Alzheimer's disease (AD), but it is neither essential nor sufficient for development of the disease. Other factors—genetic or environmental—must therefore have a role. By means of a PCR we have detected herpes simplex virus type 1 (HSV1) in latent form in brains of elderly people with and without AD. We have postulated that limited reactivation of the virus causes more damage in AD patients than in elderly people without AD because of a difference in the hosts. We now report the APOE genotypes of AD patients and non-AD sufferers with and without HSV1 in brain.

Methods

DNA was extracted from 84 samples of brain from 46 AD patients (39 temporal lobe, 39 frontal lobe, three hippocampus) and from 75 samples of brain from 44 non-AD elderly people (33 temporal lobe, 36 frontal lobe, six hippocampus). PCR amplification was used to detect HSV1 thymidine kinase gene and the host APOE gene.

Findings

By multiple logistic regression, the APOE-ε4 allele frequency was significantly higher in the patients positive for HSV1 in brain than in the HSV1-negative AD group, the HSV1-positive non-AD group, or the HSV1-negative non-AD group (52·8% vs 10·0%, 3·6%, and 6·3%, respectively). The odds ratio for APOE-ε4 in the HSV1-positive AD group compared with HSV1-negative non-AD group was 16·8 (95% CI 3·61-77·8) and in the HSV1-negative AD group, 1·67 (0·21-13·4). We also compared APOE genotypes of 40 people who had recurrent cold sores and 33 non-sufferers; the APOE-ε4 allele frequencies were 36% and 9%, respectively (p<0·0001).

Interpretation

These findings suggest that the combination of HSV1 in brain and carriage of an APOE-ε4 allele is a strong risk factor for AD, whereas either of these features alone does not increase the risk of AD. The findings in people with cold sores support our hypothesis that APOE-ε4 and HSV1 together are damaging in the nervous system.

Introduction

Most human beings are infected early in life with herpesvirus, which persists latently thereafter in the peripheral nervous system, usually in the trigeminal ganglia. The virus periodically reactivates, in response to such factors as stress and exposure to ultraviolet light, with production of active virus. However, the host response varies in that some people develop herpes labialis (cold sores) whereas others remain free of symptoms.

Herpes simplex virus type 1 (HSV1) has been proposed as a possible factor in Alzheimer's disease, based on the following: several neurological disorders are caused by common viruses; HSV1 is ubiquitous; it has a predilection for latent residence in the peripheral nervous system; and it targets, in acute HSV1 encephalitis, the same regions of the central nervous system (temporal and frontal cortex, and hippocampus) as those most affected in AD.¹ However, whether HSV1 resides latently also in the human central nervous system has been uncertain. Attempts to detect HSV1 DNA in the central nervous system in patients with or without AD have been inconclusive,² mainly because of the insensitivity of the hybridisation techniques used. Similarly, studies seeking viral proteins were inconclusive3, 4 though usually negative—not surprisingly, since such proteins would presumably be cleared by the immune system.

We have used a PCR, with stringent precautions against possible artefacts, to search for HSV1 in human brain. We have detected viral DNA, by use of primers for viral thymidine kinase and ICP0 sequences, in brains of elderly people with and without AD,5, 6, 7 in the regions most affected in AD (temporal and frontal cortex, hippocampus), but not in the occipital cortex, a region far less affected. We did not detect the virus in brains of younger people, and we therefore suggested that in old age, viral entry into the CNS is facilitated by the decline in immune function with age. We found no evidence of another neurotropic herpesvirus, varicella zoster,⁸ in brains of 17 AD patients (24 samples) and 12 non-AD elderly people (20 samples). This finding is interesting, because varicella zoster also tends to become latent in neuronal cells, and is consistent with the hypothesis that HSV1 has a specific role in AD.

Reverse PCR showed the presence of LAT transcript but not TK transcript in brains positive for HSV1, which shows that the virus was latent.⁶ We postulated⁷ that in the central nervous system, periodic mild reactivation of HSV1 (in response to such factors as stress and immunosuppression) results in damage that is more severe in those people destined to develop AD, because of a difference in viral or host characteristics. Reactivation of latent HSV1 in brain occurs in patients receiving immunosuppressive treatment⁹ and in mice latently infected with HSV1 in brain, from which the virus was recovered by cocultivation.¹⁰ Furthermore, there have been reports of recurrent herpes simplex encephalitis¹¹ and of mild cases of this disorder, after which patients recovered completely apart from a slight loss of memory.¹²

Possession of one or both type 4 alleles of the apolipoprotein E gene (APOE-ε4) has been implicated as a risk factor in sporadic and late-onset AD.¹³ We decided to identify the APOE genotypes of our patients¹⁴ and to look for an association with HSV1.