Translational challenges of human herpesvirus 6 chromosomal integration

Inherited chromosomally integrated human herpesvirus 6 and autoimmune connective tissue diseases

BACKGROUND

Entire genome of human herpesvirus 6 (HHV-6) that integrates into human chromosomes is called chromosomally integrated HHV-6 (ciHHV-6). Several viral infections have been suggested to be involved in autoimmune connective tissue diseases (CTDs). Reactivated HHV-6 from the integrated viral genome can induce immune responses against the virus. Thus, it is plausible that ciHHV-6 is associated with autoimmune CTDs.

OBJECTIVES

We sought to determine whether the prevalence of ciHHV-6 was significantly higher in patients with autoimmune CTDs than in a healthy population.

STUDY DESIGN

A total of 846 peripheral blood samples collected from autoimmune CTD patients were analyzed. Since there was a large number of samples, they were pooled into 24 samples per group. Copy numbers of HHV-6 DNA were measured by real-time PCR. The threshold level for distinguishing between ciHHV-6 and active viral infection and the reliability of pooled DNA analysis were examined as initial validation experiments.

RESULTS

The threshold level was 1.6 × 10^6 copy/mL in whole blood. The reliability of pooled DNA analysis to identify one ciHHV-6 sample among 23 HHV-6 DNA-negative samples was high. No HHV-6 DNA was detected in any of the pooled DNA samples collected from the patients. The probability of the present study including the 846 autoimmune CTD patient's samples was statistically not different with a healthy Japanese population which was 0.2 % or 0.6 %.

CONCLUSIONS

There was no significant difference in the prevalence of ciHHV-6 between a healthy population and patients with autoimmune CTDs.

Identification of chromosomally integrated human herpesvirus 6 by droplet digital PCR

BACKGROUND

Human herpesvirus 6 (HHV-6) latently infects a majority of adults. In about 1% of the population HHV-6 exists in a chromosomally integrated form (ciHHV-6) that resides in every somatic and germ cell and can be transmitted through the germ line. Patients with ciHHV-6 have been misdiagnosed and unnecessarily treated for active HHV-6 infection, sometimes with important side effects, based on results from quantitative molecular HHV-6 tests.

METHODS

A droplet digital PCR (ddPCR) assay was developed to identify ciHHV-6 in cellular patient samples by precisely determining the ratio of HHV-6 to cellular DNA. We validated the assay on confirmed ciHHV-6 patient samples and a cell line derived from a ciHHV-6 patient, and we analyzed hematopoietic stem cell transplant patients suspected of having ciHHV-6. We additionally evaluated whether the assay could be applied to stored plasma samples from a study of clinical correlates of HHV-6.

RESULTS

The ddPCR assay accurately identified ciHHV-6 in cellular samples (buffy coat, peripheral blood mononuclear cells), giving a ratio very close to 1 HHV-6/cell [mean (SD), 1.02 (0.03)] in fluorescence in situ hybridization-confirmed sample). In stored plasma samples, the assay performance was set by design to have 100% sensitivity, which resulted in 82% specificity for ciHHV-6.

CONCLUSIONS

The possibility of ciHHV-6 is often overlooked in patients with detectable HHV-6 viral loads by quantitative PCR. Our ddPCR test provides rapid and accurate laboratory identification of ciHHV-6 from easily obtained cellular samples. In addition, the assay provides excellent sensitivity and specificity using stored plasma samples, facilitating retrospective analysis of the clinical significance of ciHHV-6.