PCR search for the herpes simplex virus type 1 genome in brain sections of patients with familial Alzheimer's disease.

Abstract

An association between Chlamydia pneumoniae and Alzheimer's disease (AD) remains questionable (3, 5). Herpes simplex virus type 1 (HSV-1), the most ubiquitous neurotropic virus in humans, has been proposed as a risk factor for AD, although this issue is still under debate. Itzhaki et al. raised the possibility that the presence of HSV-1 in the brain is a risk factor for AD in elderly people carrying the apolipoprotein E ɛ4 allele (4). Strandberg noticed a significant association between serologically measured viral burden and cognitive impairment in elderly people (6). On the other hand, Beffert et al. argued that HSV-1 in the brain does not confer an increased risk for AD when combined with the apolipoprotein E ɛ4 allele and that ɛ4 carriers are not more susceptible to HSV-1 infection than are non-ɛ4 carriers (1). Familial AD, the least common type of AD, is characterized by the early onset of the disease (at 35 to 60 years of age) with early neuropathological alterations, including β-amyloid deposition. Mutations on the amyloid precursor protein, presenilin-1, and presenilin-2 genes have been identified in familial AD patients. However, other possible mechanisms underlying the pathology of familial AD, including the involvement of HSV-1 in the deterioration phase of the disease, have not been addressed. By using nested PCR, we examined formalin-fixed, paraffin-embedded, postmortem brain tissue from three individuals with familial AD, six without AD, and two with sporadic AD for the presence of HSV-1 glycoprotein D (Table ​(Table1).1). A mutation on the presenilin-1 gene had been identified in patient 1. HSV-1 DNA was detected in the brains of all three familial-AD individuals, preferentially in the frontal and temporal lobes, whereas it was not found in any parts of the brains of non-AD group members except patient 9, whose frontal lobe was HSV-1 positive. In situ hybridization (using a DAKO GenPoint tyramide signal amplification system and an ENZO HSV DNA probe) detected HSV DNA in the frontal and temporal cortex neurons of patients 1 and 2, with the virus-specific signal being detected predominantly in the cytoplasm in a dotlike staining pattern. Furthermore, high-sensitivity immunohistochemistry (DAKO ENVISION+) detected HSV-1 antigens in the frontal- and temporal-cortex neurons of patients 1 and 2 in a cytoplasmic staining fashion. A moderate to high level of β-amyloid deposition was detected in the brains of all three familial- and both sporadic-AD patients, whereas it was not evident in the non-AD brains except that of patient 9, whose brain exhibited moderate β-amyloid deposition. TABLE 1. Detection of the HSV-1 genome in brain sections of patients with familial AD These results (the presence of viral DNA and antigens in neuronal cytoplasm) are suggestive of limited reactivation of HSV-1, coupled with β-amyloid deposition, in the brains of individuals with familial AD. In a latent state, virus genomes are harbored in the nucleus but not in the cytoplasm of neurons (7). Significantly, β-amyloid fibrils have been demonstrated to stimulate infection with enveloped viruses, including HSV, in vitro (8). Additionally, glycoprotein B of HSV-1, the internal sequence of which has homology to the carboxyl-terminal region of the β-amyloid peptide, has been shown to promote fibril formation in vitro (2). Thus, there appears to exist a close association between β-amyloid deposition and HSV-1 reactivation in the human brain. Since repeated reactivation of HSV-1 in neurons likely promotes neurodegeneration in AD brains, preventive and therapeutic measures against HSV-1 may be applied not only to sporadic but also to familial AD.