Investigators led by Steve Jacobson, PhD lab at the National Institute of Neurological Diseases and Stroke (NINDS) in Bethesda Maryland analyzed RNA sequence datasets using the Broad Institute PathSeq tool to look for 118 human viruses. Out of 901 Alzheimer’s (AD) cases and controls examined, they found HHV-6A and HHV-6B RNA in 8/657 (1.2%) of AD patients and 1/244 (0.4%) of controls. None were positive for HSV1 RNA. The group utilized RNA-seq datasets from two different repositories, the Mount Sinai Brain Bank (MSBB) and the Religious Orders Study/ Memory and Aging Project (ROSMAP).
The investigators also utilized DNA samples from ROSMAP and the John Hopkins Brain Resource Center to search for HHV-6 DNA using digital droplet PCR (ddPCR). Excluding ciHHV-6+ samples, they found 20/ 510 (3.9%) of AD brains positive compared to 6/166 (3.6%) non-AD controls. The NINDS group’s finding of a low prevalence of HHV-6A/B DNA is in contrast to previous studies that have found a fairly high prevalence of both HHV-6 and HSV1 in various brain tissues, using PCR, qPCR, nested PCR:
- Lin 2002 – 40%
- Cermelli 2003–26.8%
- Chan 2001 – 24.3%
- Chapenko 2016 – 68%
- Hemling 2003-- 87.5%
The authors did not proved a comparison of their ddPCR assay to conventional qPCR assays or the new WHO international standard, so it is difficult to assess relative sensitivity for brain tissue samples.
Total RNA detection for any virus in the Mount Sinai Brain bank was 12.6% in AD brains compared to 8.1% in control brains. The ROSMAP prevalence was lower with viral RNA detectable in 2.9% and 5.1% of AD and control brains respectively.
Although the authors did find 1% of samples positive for HHV-6A/B RNA, they did not determine whether those samples came from ciHHV-6+ individuals. Even samples from healthy ciHHV-6 individuals are known to produce significant levels of mRNA (Boutolleau, 2006).
Other viruses detected were CMV, EBV, JCV and HIV, all at low levels, with the highest prevalence being EBV.
The PathSeq method has been used to detect JC virus and dengue virus during active infections, with PathSeq scores (which correlate with viral load) above 5000 (Reoma 2019, Johnson 2019). The PathSeq scores for HHV-6 in the present study were mostly between 1 and 10.
The authors note that while their results do not support the role of HHV-6 in AD pathogenesis, they also do not rule it out. They also acknowledge that the virus may be present at copy number below the limit of their laboratory detection. Finally, they note that the viruses may play a role as a “triggering event” and then become less evident at later stages.
Read the full-text: Allnutt 2020