Investigators led by Bhupesh Prusty, PhD, at the University of Würzburg in Germany have identified two cases of possible germline inherited chromosomally integrated HHV-7. High viral loads were found in the hair follicles of the patients and one of the parents, but not in the blood, suggesting to the authors that inherited ciHHV-7 integration is less stable than that of ciHHV-6. Integration was possibly missed previously, say the authors, because clinical strains of the virus differ significantly from the standard laboratory strains.
Since HHV-7 shares similarities with the HHV-6 genome, including the direct repeat elements that enable the viral genome to integrate in host cell telomeres via homologous recombination, there has been suspicion that HHV-7 germ line integration might also occur. However, no evidence has been found to date and no HHV-7 was found in over 5,000 cord blood samples tested by University of Rochester investigators (Hall 2004).
The German group noticed that some primers that worked well in laboratory strains inexplicably failed to detect HHV-7 in clinical samples. Therefore, they identified a primer pair based on HHV-7 U90 ORF that was able to detect all cases of HHV-7 germline integration.
The investigators established a latent HHV-7 cell line carrying approximately 1 genome of HHV-7 per cell, and they note that integrated HHV-7 can be activated using chemical stimuli such as Trichostatin A and hydrocortisone.
HHV-7 is the cause of febrile seizures in infants (Ward 2005, Epstein 2012) and reactivates occasionally in transplant patients, especially those with skin conditions (Drago 2016, Dzieciatkowski 2011). It has also been associated with several skin diseases such as lichen planus (Nahidi 2017) and pityriasis rosea (Drago 2015).