The Microbe Blog (at http://www.smallthingsconsidered.us)

Elio Schaechter and Merry Youle

http://schaechter.asmblog.org/schaechter/ 2010/08/when-the-end-is-the-story.html

by Welkin Johnson

Sometimes, discovery in biology is about discerning rules and sometimes it is about pursuing exceptions. In this spirit, Human Herpesvirus six (HHV-6), the etiologic agent of the common childhood illness roseola infantum, is shaping up to be an intriguing exception. Members of the Herpesviridae maintain their large double-stranded DNA genomes (typically 100-250 kb) as autonomous, covalently closed circles (episomes) during latent infection of host tissues. Nevertheless, there is now convincing evidence that the HHV-6 genome can, at least on occasion, become integrated into host-cell chromosomes.

The first hints that this could happen came from a handful of clinical reports of individuals with exceptionally high levels of HHV-6 DNA in peripheral blood. Further, a smattering of case reports described families in which HHV-6 DNA appears to be inherited vertically. Fluorescence in situ hybridization (FISH) studies using HHV-6 sequences as probes revealed a close physical association between HHV-6 DNA and human chromosomes in the cells of the afflicted individuals. Combined, this gives rise to the rather fantastic notion that HHV-6 can, from time to time, find its way into an individual's germline DNA and be passed on to the next generation.

A notable peculiarity of the HHV-6 genome is the presence at both the 5' and 3' ends of hexanucleotide repeats composed of the sequence TTAGGG-the sequence of the mammalian telomeric repeat, strings of which are found at the ends of every cellular chromosome. Telomeric repeats are added to the 3' ends of chromosomes by the cellular DNA polymerase known as telomerase, and are essential for the cell to distinguish the ends of linear chromosomes from 3' ends generated by DNA breaks. A functional role for the TTAGGG repeats in the viral replication cycle has not yet been established, although they most certainly play a role. Arbuckle and colleagues undertook a detailed molecular study of several families with inherited HHV-6. Using FISH they confirm the presence of HHV- 6 DNA associated with chromosomes in multiple individuals from each of four families. Although the particular chromosome involved differed from one family to the next, HHV-6 DNA was invariably found close to the end of one chromosome, the same chromosome within each family. Using PCR amplification they confirmed that the HHV-6 DNA was indeed covalently integrated into the host genome. The maintenance of an integrated copy of a herpesviral genome through at least two host generations is nothing short of amazing.

Over 90% of humans are seropositive for HHV-6, yet reports of families with inherited, chromosomally integrated HHV-6 are few. This suggests that integration into human germline DNA is not a typical outcome of HHV-6 infection. Nevertheless, the mere existence of integrated HHV-6 DNA in these families raises a somewhat heretical idea: could this particular herpesvirus have evolved to use integration as an essential step in its infectious cycle? The study by Arbuckle and colleagues provides some clues. Using peripheral blood lymphocytes from families with integrated HHV-6 DNA, the investigators were able to induce lytic viral replication in culture, thus demonstrating that integrated HHV-6 DNA is functionally capable of expressing progeny virus. In a separate experiment, standard cell lines (JJHAN and HEK293T) were experimentally infected with a laboratory strain of HHV-6. After allowing the virus to replicate and spread in these cells, the investigators were able to detect newly integrated HHV-6 DNA, proving that integration can also occur as a consequence of viral replication. Does this mean that integration is the rule for HHV-6? Herpesviruses replicate their DNA in the nucleus, so integration by homologous recombination may happen as a matter of chance, without having any bearing on the biology of the virus. In the case of HHV-6, homology provided by the telomeric repeats may simply increase the probability. However, the same investigators were unable to detect episomal forms of HHV-6 DNA in experimentally infected cells. Although a negative result, this is consistent with the possibility that HHV-6's strategy for replication and latency is distinct from that of other herpesviruses.

The precise molecular events that give rise to inherited HHV-6 remain to be deciphered. The list of questions raised by these observations is long indeed, and should prove intriguing to virologists. Such questions also mean that the Roseoloviruses are primed for a share of the limelight heretofore accorded to their more famous HHV cousins.

Arbuckle, J. H., M. M. Medveczky, J. Luka, et al. 2010. The latent human herpesvirus-6A genome specifically integrates in telomeres of human chromosomes in vivo and in vitro. Proc. Natl. Acad. Sci. USA 107:5563-5568.