Nucleic Acids Res 40:7347C7357. PCR (qPCR), droplet digital PCR, and fluorescent hybridization, we could demonstrate that HHV-6A/B integrated in most human cell lines tested, including telomerase-positive (HeLa, MCF-7, HCT-116, and HEK293T) and telomerase-negative cell lines (U2OS and GM847). Our results also indicate that inhibition of DNA replication, using phosphonoacetic acid, did not affect HHV-6A/B integration. Certain clones harboring ciHHV-6A/B spontaneously express viral genes and proteins. Treatment of cells with phorbol ester or histone deacetylase inhibitors triggered the expression Goserelin Acetate of many viral genes, including = 20,000), the prevalence of iciHHV-6A/B in the province of Quebec (Canada) was found to be 0.6%, 60% of which were iciHHV-6B (6). Comparable results were obtained in different parts of the world, with iciHHV-6A/B prevalence estimates ranging between 0.5% and 2% (reviewed in reference 3). The consequences of harboring an integrated copy of HHV-6A/B in all somatic cells remains poorly understood. Gravel and colleagues recently demonstrated that patients with iciHHV-6A/B are at greater risk of developing angina pectoris than are age-matched controls and independently of other known associated cardiovascular risk factors (6). Additional large-scale studies are required to determine whether iciHHV-6A/B represents an inherited risk factor for the development of other diseases. Whether HHV-6A/B integration represents a mechanism of viral latency remains a hot research topic. Several studies provided evidence that integrated virus can be excised from chromosomes, resulting in the generation of progeny of infectious virions (7,C9). Arbuckle et al. were the first to show that HHV-6A can integrate into cell lines (7). Although HHV-6A/B integration can occur in several distinct chromosomes, the integration sites are generally near the internal end of the host telomeres (reviewed in references 2 and 3). So far, the factors involved in HHV-6A/B integration remain unknown. Intriguingly, the viral genome harbors telomeric repeats that are identical to the human telomere sequences, suggesting that homologous recombination (HR) events between host and viral telomere sequences could facilitate integration. In support of this, Marek’s disease virus (MDV) telomeric repeats are reported to play a role in MDV integration into host chromosomes (10, 11). A recent study also confirmed the importance of viral telomeric sequences for efficient HHV-6A integration (12). Beyond that, it is unclear if these processes require cellular and/or viral proteins. Trempe and colleagues demonstrated that the HHV-6A/B U94 protein possesses some of the biological properties needed for homologous recombination and likely also viral integration (13). However, U94 was recently reported to be dispensable for HHV-6A integration (14). A prerequisite for the analysis of HHV-6A/B integration mechanisms is a reliable and efficient experimental system for viral integration. In this study, we describe the development of an HHV-6A/B integration system in several human cell lines. The system can be used to estimate integration frequency as well as to study the spontaneous and chemically induced HHV-6A/B gene expression and production of infectious virions from an integrated state. RESULTS HHV-6 chromosomal integration assay using single-cell cloning. To establish a reliable and efficient integration system, we tested several human cell lines for their susceptibility to HHV-6A/B chromosomal integration (Table 1). Following infection, cells were seeded at 1 cell/well, and approximately 1 month later, HHV-6A/B DNA was isolated from individual clones and analyzed by quantitative PCR (qPCR) and/or droplet digital PCR (ddPCR). We could detect HHV-6A/B DNA in clones of most human cell lines tested, albeit at various frequencies. The frequency of clones that harbor the virus genome varied between 1% and 22% depending on the cell line and the viral stocks used. The difference between the cell lines could be due to some degree to their susceptibility to HHV-6A/B infection. For Goserelin Acetate U2OS, HeLa, and MCF-7, HHV-6A and HHV-6B were equally efficient at integration. HEK293T cells preferentially supported HHV-6B integration, but only one experiment was performed. Lastly, out of 478.However, the low integration efficiency in these cells does not allow for a quantitative assessment of integration. the expression of many viral genes, including = 20,000), the prevalence of iciHHV-6A/B in the province of Quebec (Canada) was found to be 0.6%, 60% of Goserelin Acetate which were iciHHV-6B (6). Comparable results were obtained in different parts of the world, with iciHHV-6A/B prevalence estimates ranging between 0.5% and 2% (reviewed in reference 3). The consequences of harboring an integrated copy of HHV-6A/B in all somatic cells remains poorly understood. Gravel and colleagues recently demonstrated that patients with iciHHV-6A/B are at greater risk of developing angina pectoris than are age-matched controls and independently of other known associated cardiovascular risk factors (6). Additional large-scale studies are required to determine whether iciHHV-6A/B represents an inherited risk factor for the development of other diseases. Whether HHV-6A/B integration represents a mechanism of viral latency remains a hot research topic. Several studies provided evidence that integrated virus can be excised from chromosomes, resulting in the generation of progeny of infectious virions (7,C9). Arbuckle et al. were the first to show that HHV-6A can integrate into cell lines (7). Although HHV-6A/B integration can occur in several distinct chromosomes, the integration sites are generally near the internal end of the host telomeres (reviewed in references 2 and 3). So far, the factors involved in HHV-6A/B integration remain unknown. Intriguingly, the viral genome harbors telomeric repeats that are identical to the human telomere sequences, suggesting that homologous recombination (HR) events between host and viral telomere sequences could facilitate integration. In support of this, Marek’s disease virus (MDV) telomeric repeats are reported to play a role in MDV integration into host chromosomes (10, 11). A Rabbit polyclonal to FDXR recent study also confirmed the importance of viral telomeric sequences for efficient HHV-6A integration (12). Beyond that, it is unclear if these processes require cellular and/or viral proteins. Trempe and colleagues demonstrated that the HHV-6A/B U94 protein possesses some of the biological properties needed for homologous recombination and likely also viral integration (13). However, U94 was recently reported to be dispensable for HHV-6A integration (14). A prerequisite for the analysis of HHV-6A/B integration mechanisms is a reliable and efficient experimental system for viral integration. In this study, we describe the development of an HHV-6A/B integration system in several human cell lines. The system can be used to estimate integration frequency as well as to study the spontaneous and chemically induced HHV-6A/B gene expression and production of infectious virions from an integrated state. RESULTS HHV-6 chromosomal integration assay using single-cell cloning. To establish a reliable and efficient integration system, we tested several human cell lines for their susceptibility to HHV-6A/B chromosomal integration (Table 1). Following infection, cells were seeded at 1 cell/well, and approximately 1 month later, HHV-6A/B DNA was isolated from individual clones and analyzed by quantitative PCR (qPCR) and/or droplet digital PCR (ddPCR). We could detect HHV-6A/B DNA in clones of most human cell lines tested, albeit at various frequencies. The frequency of clones that harbor the virus genome varied between 1% and 22% depending on the cell line and the viral stocks used. The difference between the cell lines could be due to some degree to their susceptibility to HHV-6A/B infection. For U2OS, HeLa, and MCF-7, HHV-6A and HHV-6B were equally efficient at integration. HEK293T cells preferentially supported HHV-6B integration, but only one experiment was performed. Lastly, out of 478 NIH 3T3 (murine fibroblasts) clones tested, none were positive for HHV-6A or HHV-6B, despite intracellular detection of HHV-6 DNA measured 48 h post-HHV-6 exposure (threshold cycle [for GAPDH, 28.6 3.8). TABLE 1 HHV-6 integration frequency in various cell lines hybridization (FISH) on several clonal cell lines. FISH analyses confirmed that the virus genome is indeed localized at the ends of metaphase chromosomes. A representative result of HHV-6 integrated in the telomeric region of cellular chromosomes is presented in Fig. 1D. Open in a separate window FIG 1 Characterization of clones with integrated HHV-6. (A and.