Chromosomal transmission of human herpesvirus 6 DNA in acute lymphoblastic leukaemia

The Eruptive Fevers at Sixes and Sevens

Sixth Disease (roseola infantum) and its primary causative agent, HHV-6, share names that numerically concur. This article examines and answers the question of whether that correspondence is by design or coincidental by briefly reviewing the history and nomenclature of the HHV viruses and the classic febrile rashes of childhood while highlighting some clinical and microbiologic features of HHV-6 infection.

Inherited Chromosomally Integrated Human Herpesvirus 6: Laboratory and Clinical Features

Inherited chromosomally integrated human herpesvirus 6 (iciHHV-6) is a condition in which the complete HHV-6 genome is integrated into the chromosomes of the host germ cell and is vertically transmitted. The aims of this study were to identify iciHHV-6 prevalence in hospitalized patients and clinical features in individuals carrying this integration. HHV-6 PCR on hair follicles was used to confirm iciHHV-6 status when the blood viral load was more than 5 Log10 copies/mL. From January 2012 to June 2022, HHV-6 DNAemia was investigated in 2019 patients. In particular, 49 had a viral load higher than 6 Log10 copies/mL and HHV-6 DNA in hair follicles was positive. A viral load between 5.0 and 5.9 Log10 copies/mL was observed in 10 patients: 6 infants with acute HHV-6 infection and 4 patients with leukopenia and HHV-6 integration. Therefore, the iciHHV-6 prevalence in our population was 2.6% (53/2019). Adult patients with integration presented hematological (24%), autoimmune (11%), autoimmune neurological (19%), not-autoimmune neurological (22%), and other diseases (19%), whereas 5% had no clinically relevant disease. Although in our study population a high percentage of iciHHV-6 adult hospitalized patients presented a specific pathology, it is still unknown whether the integration is responsible for, or contributes to, the disease development.

Investigation of Inherited Chromosomally Integrated Human Herpesvirus-6A+ and -6B+ in a Patient with Ulipristal Acetate-Induced Fulminant Hepatic Failure

Inherited chromosomally integrated (ici) human herpes virus 6 (HHV-6) is estimated to occur in 0.6–2.7% of people worldwide. HHV-6 comprises two distinct species: HHV-6A and HHV-6B. Both HHV-6A and HHV-6B integration have been reported. Several drugs are capable of activating iciHHV-6 in tissues, the consequences of which are poorly understood. We report herein a case of a woman with iciHHV-6A+ and iciHHV-6B+, who developed ulipristal acetate (a selective progesterone receptor modulator)-induced fulminant hepatic failure that required liver transplantation. We confirmed the presence of ~one copy per cell of both HHV-6A and HHV-6B DNA in her hair follicles using multiplex HHV-6A/B real-time PCR and demonstrated the Mendelian inheritance of both iciHHV-6A and iciHHV-6B in her family members over three generations. Because of the rarity of this presentation, we discuss herein the possible links between reactivated HHV-6 from iciHHV-6A and/or iciHHV-6B and adverse drug reactions, suggesting that iciHHV-6 could be screened before the introduction of any hepatotoxic drugs to exclude HHV-6 active disease or combined idiosyncratic drug-induced liver injury in these patients.

The Presence of Human Herpesvirus 6 in the Brain in Health and Disease

The human herpesvirus 6 (HHV-6) -A and -B are two dsDNA beta-herpesviruses infectingalmost the entire worldwide population. These viruses have been implicated in multipleneurological conditions in individuals of various ages and immunological status, includingencephalitis, epilepsy, and febrile seizures. HHV-6s have also been suggested as playing a role inthe etiology of neurodegenerative diseases such as multiple sclerosis and Alzheimer's disease. Theapparent robustness of these suggested associations is contingent on the accuracy of HHV-6detection in the nervous system. The effort of more than three decades of researching HHV-6 in thebrain has yielded numerous observations, albeit using variable technical approaches in terms oftissue preservation, detection techniques, sample sizes, brain regions, and comorbidities. In thisreview, we aimed to summarize current knowledge about the entry routes and direct presence ofHHV-6 in the brain parenchyma at the level of DNA, RNA, proteins, and specific cell types, inhealthy subjects and in those with neurological conditions. We also discuss recent findings relatedto the presence of HHV-6 in the brains of patients with Alzheimer's disease in light of availableevidence.

Endogenization and excision of human herpesvirus 6 in human genomes

Sequences homologous to human herpesvirus 6 (HHV-6) are integrated within the nuclear genome of about 1% of humans, but it is not clear how this came about. It is also uncertain whether integrated HHV-6 can reactivate into an infectious virus. HHV-6 integrates into telomeres, and this has recently been associated with polymorphisms affecting MOV10L1. MOV10L1 is located on the subtelomere of chromosome 22q (chr22q) and is required to make PIWI-interacting RNAs (piRNAs). As piRNAs block germline integration of transposons, piRNA-mediated repression of HHV-6 integration has been proposed to explain this association. In vitro, recombination of the HHV-6 genome along its terminal direct repeats (DRs) leads to excision from the telomere and viral reactivation, but the expected "solo-DR scar" has not been described in vivo. Here we screened for integrated HHV-6 in 7,485 Japanese subjects using whole-genome sequencing (WGS). Integrated HHV-6 was associated with polymorphisms on chr22q. However, in contrast to prior work, we find that the reported MOV10L1 polymorphism is physically linked to an ancient endogenous HHV-6A variant integrated into the telomere of chr22q in East Asians. Unexpectedly, an HHV-6B variant has also endogenized in chr22q; two endogenous HHV-6 variants at this locus thus account for 72% of all integrated HHV-6 in Japan. We also report human genomes carrying only one portion of the HHV-6B genome, a solo-DR, supporting in vivo excision and possible viral reactivation. Together these results explain the recently-reported association between integrated HHV-6 and MOV10L1/piRNAs, suggest potential exaptation of HHV-6 in its coevolution with human chr22q, and clarify the evolution and risk of reactivation of the only intact (non-retro)viral genome known to be present in human germlines.

Inherited chromosomally integrated HHV-6 possibly modulates human gene expression

Approximately, 1% of human population possesses a copy of human herpesvirus 6A and 6B (HHV-6A/B) in the genome. This viral element is referred to as inherited chromosomally integrated HHV-6A/B (iciHHV-6A/B) and is encoded in all of their cells. A recent study revealed that iciHHV-6A/B potentially increases the immune responses against HHV-6. However, it remains unclear whether iciHHV-6A/B affects human gene expression. Here, we perform global transcriptome analysis using the datasets obtained from various human tissues. We detected two and four individuals positive for iciHHV-6A and iciHHV-6B, respectively, and revealed that the transcriptional expression of iciHHV-6A/B was sporadic in the human body. Transcriptome analysis identified the human genes differentially expressed between iciHHV-6A/B-positive and -negative individuals. Particularly, the expression of some genes encoding immunoglobulins decreased in sigmoid colon of iciHHV-6A/B-positive individuals. Our findings suggest that iciHHV-6A/B may be associated with human health maintenance.

Chromosomal Integration by Human Herpesviruses 6A and 6B

Upon infection and depending on the infected cell type, human herpesvirus 6A (HHV-6A) and 6B (HHV-6B) can replicate or enter a state of latency. HHV-6A and HHV-6B can integrate their genomes into host chromosomes as one way to establish latency. Viral integration takes place near the subtelomeric/telomeric junction of chromosomes. When HHV-6 infection and integration occur in gametes, the virus can be genetically transmitted. Inherited chromosomally integrated HHV-6 (iciHHV-6)-positive individuals carry one integrated HHV-6 copy per somatic cell. The prevalence of iciHHV-6⁺ individuals varies between 0.6% and 2%, depending on the geographical region sampled. In this chapter, the mechanisms leading to viral integration and reactivation from latency, as well as some of the biological and medical consequences associated with iciHHV-6, were discussed.

The human vascular endothelial cell line HUV-EC-C harbors the integrated HHV-6B genome which remains stable in long term culture

Human herpes virus 6 (HHV-6) is a common human pathogen that is most often detected in hematopoietic cells. Although human cells harboring chromosomally integrated HHV-6 can be generated in vitro, the availability of such cell lines originating from in vivo tissues is limited. In this study, chromosomally integrated HHV-6B has been identified in a human vascular endothelial cell line, HUV-EC-C (IFO50271), derived from normal umbilical cord tissue. Sequence analysis revealed that the viral genome was similar to the HHV-6B HST strain. FISH analysis using a HHV-6 DNA probe showed one signal in each cell, detected at the distal end of the long arm of chromosome 9. This was consistent with a digital PCR assay, validating one copy of the viral DNA. Because exposure of HUV-EC-C to chemicals did not cause viral reactivation, long term cell culture of HUV-EC-C was carried out to assess the stability of viral integration. The growth rate was altered depending on passage numbers, and morphology also changed during culture. SNP microarray profiles showed some differences between low and high passages, implying that the HUV-EC-C genome had changed during culture. However, no detectable change was observed in chromosome 9, where HHV-6B integration and the viral copy number remained unchanged. Our results suggest that integrated HHV-6B is stable in HUV-EC-C despite genome instability.

Latency, Integration, and Reactivation of Human Herpesvirus-6

Human herpesvirus-6A (HHV-6A) and human herpesvirus-6B (HHV-6B) are two closely related viruses that infect T-cells. Both HHV-6A and HHV-6B possess telomere-like repeats at the terminal regions of their genomes that facilitate latency by integration into the host telomeres, rather than by episome formation. In about 1% of the human population, human herpes virus-6 (HHV-6) integration into germline cells allows the viral genome to be passed down from one generation to the other; this condition is called inherited chromosomally integrated HHV-6 (iciHHV-6). This review will cover the history of HHV-6 and recent works that define the biological differences between HHV-6A and HHV-6B. Additionally, HHV-6 integration and inheritance, the capacity for reactivation and superinfection of iciHHV-6 individuals with a second strain of HHV-6, and the role of hypomethylation of human chromosomes during integration are discussed. Overall, the data suggest that integration of HHV-6 in telomeres represent a unique mechanism of viral latency and offers a novel tool to study not only HHV-6 pathogenesis, but also telomere biology. Paradoxically, the integrated viral genome is often defective especially as seen in iciHHV-6 harboring individuals. Finally, gaps in the field of HHV-6 research are presented and future studies are proposed.

Cell Culture Systems To Study Human Herpesvirus 6A/B Chromosomal Integration

Human herpesviruses 6A/B (HHV-6A/B) can integrate their viral genomes in the telomeres of human chromosomes. The viral and cellular factors contributing to HHV-6A/B integration remain largely unknown, mostly due to the lack of efficient and reproducible cell culture models to study HHV-6A/B integration. In this study, we characterized the HHV-6A/B integration efficiencies in several human cell lines using two different approaches. First, after a short-term infection (5 h), cells were processed for single-cell cloning and analyzed for chromosomally integrated HHV-6A/B (ciHHV-6A/B). Second, cells were infected with HHV-6A/B and allowed to grow in bulk for 4 weeks or longer and then analyzed for the presence of ciHHV-6. Using quantitative PCR (qPCR), droplet digital PCR, and fluorescent in situ 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 of many viral genes, including U39, U90, and U100, without the production of infectious virus, suggesting that the tested stimuli were not sufficient to trigger full reactivation. In summary, both integration models yielded comparable results and should enable the identification of viral and cellular factors contributing to HHV-6A/B integration and the screening of drugs influencing viral gene expression, as well as the release of infectious HHV-6A/B from the integrated state.IMPORTANCE The analysis and understanding of HHV-6A/B genome integration into host DNA is currently limited due to the lack of reproducible and efficient viral integration systems. In the present study, we describe two quantitative cell culture viral integration systems. These systems can be used to define cellular and viral factors that play a role in HHV-6A/B integration. Furthermore, these systems will allow us to decipher the conditions resulting in virus gene expression and excision of the integrated viral genome resulting in reactivation.

Herpesvirus Latency: On the Importance of Positioning Oneself

The nucleus is composed of multiple compartments and domains, which directly or indirectly influence many cellular processes including gene expression, RNA splicing and maturation, protein post-translational modifications, and chromosome segregation. Nuclear-replicating viruses, especially herpesviruses, have co-evolved with the cell, adopting strategies to counteract and eventually hijack this hostile environment for their own benefit. This allows them to persist in the host for the entire life of an individual and to ensure their maintenance in the target species. Herpesviruses establish latency in dividing or postmitotic cells from which they can efficiently reactivate after sometimes years of a seemingly dormant state. Therefore, herpesviruses circumvent the threat of permanent silencing by reactivating their dormant genomes just enough to escape extinction, but not too much to avoid life-threatening damage to the host. In addition, herpesviruses that establish latency in dividing cells must adopt strategies to maintain their genomes in the daughter cells to avoid extinction by dilution of their genomes following multiple cell divisions. From a biochemical point of view, reactivation and maintenance of viral genomes in dividing cells occur successfully because the viral genomes interact with the nuclear architecture in a way that allows the genomes to be transmitted faithfully and to benefit from the nuclear micro-environments that allow reactivation following specific stimuli. Therefore, spatial positioning of the viral genomes within the nucleus is likely to be essential for the success of the latent infection and, beyond that, for the maintenance of herpesviruses in their respective hosts.

Virome characterisation from Guthrie cards in children who later developed acute lymphoblastic leukaemia

Background:

Some childhood acute lymphoblastic leukaemias (ALL) can be traced back to a prenatal origin, where a virus infection could be involved in the first pre-leukaemic clone development. The DNA virome of 95 children who later developed ALL was characterised from neonatal blood spots (NBS) using unbiased next-generation sequencing (NGS) and compared with the virome of 95 non-ALL controls.

Methods:

DNA was individually extracted from the ALL-patients and controls, pooled, randomly amplified and sequenced using the Illumina MiSeq Sequencing System.

Results:

Virus-like sequences identified in both groups mapped to human endogenous retroviruses and propionibacterium phage, considered a part of the normal microbial flora. Potential pathogens human herpesvirus type 6 (HHV-6) and parvovirus B19 were also identified, but only few samples in both ALL and controls tested positive by PCR follow-up.

Conclusions:

Unbiased NGS was employed to search for DNA from potential infectious agents in neonatal samples of children who later developed ALL. Although several viral candidates were identified in the NBS samples, further investigation by PCR suggested that these viruses did not have a major role in ALL development.

The putative U94 integrase is dispensable for human herpesvirus 6 (HHV-6) chromosomal integration

Human herpesvirus 6 (HHV-6) can integrate its genome into the telomeres of host chromosomes and is present in the germline of about 1 % of the human population. HHV-6 encodes a putative integrase U94 that possesses all molecular functions required for recombination including DNA-binding, ATPase, helicase and nuclease activity, and was hypothesized by many researchers to facilitate integration ever since the discovery of HHV-6 integration. However, analysis of U94 in the virus context has been hampered by the lack of reverse-genetic systems and efficient integration assays. Here, we addressed the role of U94 and the cellular recombinase Rad51 in HHV-6 integration. Surprisingly, we could demonstrate that HHV-6 efficiently integrated in the absence of U94 using a new quantitative integration assay. Additional inhibition of the cellular recombinase Rad51 had only a minor impact on virus integration. Our results shed light on this complex integration mechanism that includes factors beyond U94 and Rad51.

Inherited chromosomally integrated human herpesvirus 6 as a predisposing risk factor for the development of angina pectoris

Inherited chromosomally integrated human herpesvirus-6 (iciHHV-6) results in the germ-line transmission of the HHV-6 genome. Every somatic cell of iciHHV-6+ individuals contains the HHV-6 genome integrated in the telomere of chromosomes. Whether having iciHHV-6 predisposes humans to diseases remains undefined. DNA from 19,597 participants between 40 and 69 years of age were analyzed by quantitative PCR (qPCR) for the presence of iciHHV-6. Telomere lengths were determined by qPCR. Medical records, hematological, biochemical, and anthropometric measurements and telomere lengths were compared between iciHHV-6+ and iciHHV-6- subjects. The prevalence of iciHHV-6 was 0.58%. Two-way ANOVA with a Holm-Bonferroni correction was used to determine the effects of iciHHV6, sex, and their interaction on continuous outcomes. Two-way logistic regression with a Holm-Bonferroni correction was used to determine the effects of iciHHV6, sex, and their interaction on disease prevalence. Of 50 diseases monitored, a single one, angina pectoris, is significantly elevated (3.3×) in iciHHV-6+ individuals relative to iciHHV-6- subjects (P = 0.017; 95% CI, 1.73-6.35). When adjusted for potential confounding factors (age, body mass index, percent body fat, and systolic blood pressure), the prevalence of angina remained three times greater in iciHHV-6+ subjects (P = 0.015; 95%CI, 1.23-7.15). Analyses of telomere lengths between iciHHV-6- without angina, iciHHV-6- with angina, and iciHHV-6+ with angina indicate that iciHHV-6+ with angina have shorter telomeres than age-matched iciHHV-6- subjects (P = 0.006). Our study represents, to our knowledge, the first large-scale analysis of disease association with iciHHV-6. Our results are consistent with iciHHV-6 representing a risk factor for the development of angina.

Herpesvirus Genome Integration into Telomeric Repeats of Host Cell Chromosomes

It is well known that numerous viruses integrate their genetic material into host cell chromosomes. Human herpesvirus 6 (HHV-6) and oncogenic Marek's disease virus (MDV) have been shown to integrate their genomes into host telomeres of latently infected cells. This is unusual for herpesviruses as most maintain their genomes as circular episomes during the quiescent stage of infection. The genomic DNA of HHV-6, MDV, and several other herpesviruses harbors telomeric repeats (TMRs) that are identical to host telomere sequences (TTAGGG). At least in the case of MDV, viral TMRs facilitate integration into host telomeres. Integration of HHV-6 occurs not only in lymphocytes but also in the germline of some individuals, allowing vertical virus transmission. Although the molecular mechanism of telomere integration is poorly understood, the presence of TMRs in a number of herpesviruses suggests it is their default program for genome maintenance during latency and also allows efficient reactivation.

Presence of HHV-6 genome in spermatozoa in a context of couples with low fertility: what type of infection?

Human herpesvirus-6 (HHV-6) is a betaherpesvirus whose genome may integrate into human chromosomes. Chromosomally integrated HHV-6 (ciHHV-6) may be transmitted vertically from parents to children. HHV-6 DNA has been detected in semen, but its integrated or extrachromosomal status has not yet been characterised. The aim of this study was to determine the prevalence of HHV-6 DNA and to search for ciHHV-6 forms in spermatozoa purified from semen obtained from subjects explored for low fertility. A total of 184 sperm samples were purified using PureSperm(®) . HHV-6 viral load and species identification were performed by real-time polymerase chain reaction. Of 179 sperm specimens analysed, three were positive for HHV-6 (1.7%). Two samples (1.1%) had viral loads of 680 232 and 2 834 075 copies per million spermatozoa, compatible with loads expected for a ciHHV-6 form. The viral load of the third positive sample (73 684 copies per million spermatozoa) was lower than would be expected for ciHHV-6 infection, implying that the HHV-6 DNA detected in spermatozoa corresponds mainly to ciHHV-6. However, viral DNA may also be detected at a low level that is not in favour of the presence of ciHHV-6. Further studies are necessary to determine the origin of detected viral genomes.

Associations between human herpesvirus-6, human papillomavirus and cervical cancer

Cervical cancer (CxCa) is the second most common cancer among women globally. Human papillomavirus (HPV) infection is thought to be a necessary, but not sufficient, causal factor in CxCa development. Why some women are able to clear HPV infection with no adverse effects, whereas others develop cancer, remains unclear. HHV-6 has demonstrated transformative abilities and has been shown to be present in the genital tract. However, based on the current evidence, we cannot conclude that HHV-6 is a co-factor in HPV-associated carcinogenesis. Nonetheless, future research is warranted because of several crucial gaps in the literature.

Quantitative analysis of human herpesvirus-6 genome in blood and bone marrow samples from Tunisian patients with acute leukemia: a follow-up study

BACKGROUND

Infectious etiology in lymphoproliferative diseases has always been suspected. The pathogenic roles of human herpesvirus-6 (HHV-6) in acute leukemia have been of great interest. Discordant results to establish a link between HHV-6 activation and the genesis of acute leukemia have been observed. The objective of this study was to evaluate a possible association between HHV-6 infection and acute leukemia in children and adults, with a longitudinal follow-up at diagnosis, aplasia, remission and relapse.

METHODS

HHV-6 load was quantified by a quantitative real-time PCR in the blood and bone marrow samples from 37 children and 36 adults with acute leukemia: 33 B acute lymphoblastic leukemia (B-ALL), 6 T acute lymphoblastic leukemia (T-ALL), 34 acute myeloid leukemia (AML).

RESULTS

HHV-6 was detected in 15%, 8%, 30% and 28% of the blood samples at diagnosis, aplasia, remission and relapse, respectively. The median viral loads were 138, 244, 112 and 78 copies/million cells at diagnosis, aplasia, remission and relapse, respectively. In the bone marrow samples, HHV-6 was detected in 5%, 20% and 23% of the samples at diagnosis, remission and relapse, respectively. The median viral loads were 34, 109 and 32 copies/million cells at diagnosis, remission and relapse, respectively. According to the type of leukemia at diagnosis, HHV-6 was detected in 19% of the blood samples and in 7% of the bone marrow samples (with median viral loads at 206 and 79 copies/million cells, respectively) from patients with B-ALL. For patients with AML, HHV-6 was present in 8% of the blood samples and in 4% of the bone marrow samples (with median viral loads at 68 and 12 copies/million cells, respectively). HHV-6 was more prevalent in the blood samples from children than from adults (25% and 9%, respectively) and for the bone marrow (11% and 0%, respectively). All typable HHV-6 were HHV-6B species. No link was shown between neither the clinical symptoms nor the abnormal karyotype and HHV-6 activation. A case of HHV-6 chromosomal integration was shown in one patient with AML.

CONCLUSION

This study confirms the absence of role of HHV-6 in the genesis of acute leukemia but the virus was reactivated after chemotherapy treatment.

Chromosomally-integrated human herpesvirus 6 in familial glioma etiology

Human herpesvirus 6 (HHV-6) is a highly neurotropic beta-herpesvirus with demonstrated transformative properties. HHV-6 infection has been implicated in the etiologies of cancers, including lymphoma and leukemia; conditions with brain involvement, including epilepsy and encephalitis; and other disorders. HHV-6 is also the only human herpesvirus that has been proven to integrate into the chromosomes of a proportion (1-12%) of infected individuals. Because several traditional genetic association studies have failed to identify a variant that can account for the established relationship between family history and glioma risk, the possibility that chromosomally-integrated HHV-6 (CI-HHV-6), as a heritable factor, may explain a proportion of familial glioma cases warrants evaluation. To test this hypothesis, the prevalence of CI-HHV-6 in familial glioma cases and related and unrelated cancer-free control groups should be compared. Among glioma-affected families, the inheritance pattern of CI-HHV-6 could be evaluated by constructing pedigrees. If CI-HHV-6 is found to be associated with familial glioma risk, this knowledge could potentially lead to the future development of novel therapeutic and preventive approaches, including vaccines and immunotherapies targeted at the HHV-6 sequences.

Viral detection in hydrops fetalis, spontaneous abortion, and unexplained fetal death in utero

This study was undertaken to investigate the occurrence of viral infection in fetal death by examining tissues for the presence of DNA of several viral agents. Tissue specimens including heart, kidney, liver, lung, and placenta of 73 cases of fetal death were examined with 27 cases of elective termination of pregnancy as a control group. DNA extracted from these samples was tested for the presence of HSV, CMV, EBV, VZV, HHV-6, HHV-7, and PVB19. Viral DNA was found in one or more tissue samples from 25/73 cases (34%): CMV in 20, HSV in 5, parvovirus B19 in 5, HHV-7 in 3, and HHV-6 in 2. The presence of HHV-6 in fetal tissue has been reported rarely. No study so far has reported the detection of HHV-7 in fetal tissues with normal or adverse outcomes. Viral DNA was not found in any of the termination of pregnancy samples. Among the positive cases, eight had dual infection. One further case was positive for three viruses: HSV, CMV, and HHV-7. HHV-6 was the sole infectious agent in two cases, HHV-7 in one case, PVB19 in three, and CMV in ten cases. The finding of multiple viral DNA in 12% of the cases suggests the involvement of complex risk factors in cases of fetal loss. Although the cause of fetal death often includes other factors (e.g., chromosomal abnormalities) these data suggest the incidence of viral infective etiology may be higher than considered previously. However, larger studies are required to establish this link.

The molecular biology of human herpesvirus-6 latency and telomere integration

The genomes of herpesviruses establish latency as a circular episome. However, Human herpesvirus-6 (HHV-6) has been shown to specifically integrate into the telomeres of chromosomes during latency and vertically transmit through the germ-line. This review will focus on the telomere integration of HHV-6, the potential viral and cellular genes that mediate integration, and the clinical impact on the host.

A high circulating copy number of HHV-6 due to chromosomal integration in a child with acute lymphoblastic leukemia

We report a case of a 3.5-year-old female with a very high copy number of human herpesvirus 6 (HHV-6) detected by PCR in blood during acute lymphoblastic leukemia induction therapy. The patient was unsuccessfully treated with antiviral drugs. HHV-6 genome was shown to be constitutively integrated into chromosome 22q-tel, likely to be inherited from the mother who was found to carry high HHV-6 copy number. This case highlights the importance of excluding HHV-6 chromosomal integration before diagnosing HHV-6 infection or reactivation in immunocompromised patients.

Herpesviruses and chromosomal integration

Herpesviruses are members of a diverse family of viruses that colonize all vertebrates from fish to mammals. Although more than one hundred herpesviruses exist, all are nearly identical architecturally, with a genome consisting of a linear double-stranded DNA molecule (100 to 225 kbp) protected by an icosahedral capsid made up of 162 hollow-centered capsomeres, a tegument surrounding the nucleocapsid, and a viral envelope derived from host membranes. Upon infection, the linear viral DNA is delivered to the nucleus, where it circularizes to form the viral episome. Depending on several factors, the viral cycle can proceed either to a productive infection or to a state of latency. In either case, the viral genetic information is maintained as extrachromosomal circular DNA. Interestingly, however, certain oncogenic herpesviruses such as Marek's disease virus and Epstein-Barr virus can be found integrated at low frequencies in the host's chromosomes. These findings have mostly been viewed as anecdotal and considered exceptions rather than properties of herpesviruses. In recent years, the consistent and rather frequent detection (in approximately 1% of the human population) of human herpesvirus 6 (HHV-6) viral DNA integrated into human chromosomes has spurred renewed interest in our understanding of how these viruses infect, replicate, and propagate themselves. In this review, we provide a historical perspective on chromosomal integration by herpesviruses and present the current state of knowledge on integration by HHV-6 with the possible clinical implications associated with viral integration.

Human herpesvirus 6 in hematological malignancies

Pathogenetic roles of human herpesvirus (HHV)-6 in lymphoproliferative diseases have been of continued interest. Many molecular studies have tried to establish a pathogenic role for HHV-6 in lymphoid malignancies. However, whether HHV-6 plays a role in these pathologies remains unclear, as positive polymerase chain reaction results for HHV-6 in those studies may reflect latent infection or reactivation rather than presence of HHV-6 in neoplastic cells. A small number of studies have investigated HHV-6 antigen expression in pathologic specimens. As a result, the lack of HHV-6 antigen expression on neoplastic cells argues against any major pathogenic role of HHV-6. The role of HHV-6 in childhood acute lymphoblastic leukemia (ALL) has also been of interest but remains controversial, with 2 studies documenting higher levels of HHV-6 antibody in ALL patients, and another 2 large-scale studies finding no significant differences in HHV-6 seroprevalences between ALL patients and controls. Alternatively, HHV-6 is increasingly recognized as an important opportunistic pathogen. HHV-6 reactivation is common among recipients of allogeneic stem cell transplantation (SCT), and is linked to various clinical manifestations. In particular, HHV-6 encephalitis appears to be significant, life-threatening complication. Most HHV-6 encephalitis develops in patients receiving transplant from an unrelated donor, particularly cord blood, typically around the time of engraftment. Symptoms are characterized by short-term memory loss and seizures. Magnetic resonance imaging typically shows limbic encephalitis. Prognosis for HHV-6 encephalitis is poor, but appropriate prophylactic measures have not been established. Establishment of preventive strategies against HHV-6 encephalitis represents an important challenge for physicians involved with SCT.

Unique route for human herpesvirus transmission: HHV-6 and chromosomal integration

Evaluation of: Hall CB, Caserta MT, Schnabel K et al.: Chromosomal integration of human herpesvirus 6 is the major mode of congenital human herpesvirus 6 infection. Pediatrics 122(3), 513–520 (2005). Human herpesvirus (HHV) -6 is the cause of exanthem subitum, a common childhood illness. In common with the other human HHVs, it establishes lifelong latency with occasional reactivation, but unlike the other members of its family, it has an alternative form of persistence, specifically, integration of viral sequences into host chromosomes characterized by very high HHV-6 DNA loads in blood. This phenomenon was first discovered in the early 1990s but, until recently, was considered extremely rare. However, accumulating evidence suggests that chromosomal integration is uncommon rather than rare and that its usual mode of transmission is in the germline. The present article confirms this possibility; it is the culmination of an ongoing prospective survey of congenital HHV-6 infection that will no doubt shed further light on the, as of yet, unknown clinical consequences of the fascinating phenomenon of chromosomally integrated HHV-6.

Human herpesvirus 6 integrates within telomeric regions as evidenced by five different chromosomal sites

Fluorescent in situ hybridization (FISH) was used to investigate the chromosomal integration sites of human herpesvirus 6 (HHV-6) in phytohemagglutinin-stimulated leukocytes and B lymphocytes from Epstein-Barr virus transformed lymphoblastoid cell lines (LCLs). Five different chromosomal integration sites were found in nine individuals. Only one site was identified in each individual, each site was in the vicinity of the telomeric region and was on either the p or q arm of only one of the two chromosome homologues. The sites were 9q34.3, 10q26.3, 11p15.5, 17p13.3, and 19q 13.4, of which three have not been previously identified. For 9q34.3 the site of integration was further mapped using a locus-specific probe for 9q34.3 together with a pan-telomeric probe and both co-localized with the HHV-6 signal. Similarly an arm-specific telomeric probe for 19q co-localized with the HHV-6 signal. It was therefore concluded that the site of integration is actually within the telomere. The number of viral DNA copies/cell was calculated in blood, LCL cells and hair follicles and was one or more in every case for each of the nine individuals. This result was confirmed by FISH where 100% of cells gave an HHV-6 signal. These findings add to previous reports suggesting that integrated HHV-6 DNA is found in every cell in the body and transmitted vertically. Finally, including our data, worldwide seven different chromosomal sites of HHV-6 integration have now been identified. Large epidemiological studies of chromosomal integration are required to identify further telomeric sites, geographical or racial variation and possible clinical consequences.

Chromosomal integration of human herpesvirus 6 is the major mode of congenital human herpesvirus 6 infection

OBJECTIVE

We examined the frequency and characteristics of chromosomally integrated human herpesvirus 6 among congenitally infected children.

METHODS

Infants with and without congenital human herpesvirus 6 infection were prospectively monitored. Cord blood mononuclear cell, peripheral blood mononuclear cell, saliva, urine, and hair follicle samples were examined for human herpesvirus 6 DNA. Human herpesvirus 6 RNA, serum antibody, and chromosomally integrated human herpesvirus 6 levels were also assessed.

RESULTS

Among 85 infants, 43 had congenital infections and 42 had postnatal infections. Most congenital infections (86%) resulted from chromosomally integrated human herpesvirus 6; 6 infants (14%) had transplacental infections. Children with chromosomally integrated human herpesvirus 6 had high viral loads in all sites (mean: 5-6 log(10) genomic copies per mug of cellular DNA); among children with transplacental infection or postnatal infection, human herpesvirus 6 DNA was absent in hair samples and inconsistent in other samples, and viral loads were significantly lower. One parent of each child with chromosomally integrated human herpesvirus 6 who had parental hair samples tested had hair containing human herpesvirus 6 DNA. Variant A caused 32% of chromosomally integrated human herpesvirus 6 infections, compared with 2% of postnatal infections. Replicating human herpesvirus 6 was detected only among chromosomally integrated human herpesvirus 6 samples (8% of cord blood mononuclear cells and peripheral blood mononuclear cells). Cord blood human herpesvirus 6 antibody levels were similar among children with chromosomally integrated human herpesvirus 6, transplacental infection, and postnatal infection and between children with maternal and paternal chromosomally integrated human herpesvirus 6 transmission.

CONCLUSIONS

Human herpesvirus 6 congenital infection results primarily from chromosomally integrated virus which is passed through the germ-line. Infants with chromosomally integrated human herpesvirus 6 had high viral loads in all specimens, produced human herpesvirus 6 antibody, and mRNA. The clinical relevance needs study as 1 of 116 newborns may have chromosomally integrated human herpesvirus 6 blood specimens.

[HHV-6 infection and acute lymphoblastic leukemia in a child]

We report the case of a child who was infected by HHV-6 and who started an acute lymphoblastic leukemia two months later. This case reminds that an etiologic role have been suggested for many viral infections in some leukemias in childhood, particularly the human herpesvirus 6 (HHV-6).

Human herpesvirus 6 DNA levels in cerebrospinal fluid due to primary infection differ from those due to chromosomal viral integration and have implications for diagnosis of encephalitis

The prevalence and concentration of human herpesvirus 6 (HHV-6) DNA in the cerebrospinal fluid (CSF) of the immunocompetent in primary infection was compared with that in viral chromosomal integration. Samples from 510 individuals with suspected encephalitis, 200 young children and 310 older children and/or adults, and 12 other patients were tested. HHV-6 DNA concentration (log(10) copies/ml) was measured in CSF, serum, and whole blood using PCR. Serum HHV-6 immunoglobulin G antibody was measured by indirect immunofluorescence. Primary infection was defined by antibody seroconversion and/or a low concentration of HHV-6 DNA (<3.0 log(10) copies/ml) in a seronegative serum. Chromosomal integration was defined by a high concentration of viral DNA in serum (>/=3.5 log(10) copies/ml) or whole blood (>/=6.0 log(10) copies/ml). The prevalences of CSF HHV-6 DNA in primary infection and chromosomal integration were 2.5% and 2.0%, respectively, in the young children (<2 years) and 0% and 1.3%, respectively, in the older children and/or adults. The mean concentration of CSF HHV-6 DNA in 9 children with primary infection (2.4 log(10) copies/ml) was significantly lower than that of 21 patients with viral chromosomal integration (4.0 log(10) copies/ml). Only HHV-6B DNA was found in primary infection, whereas in viral integration, 4 patients had HHV-6A and 17 patients HHV-6B. Apart from primary infection, chromosomal integration is the most likely cause of HHV-6 DNA in the CSF of the immunocompetent. Our results show that any diagnosis of HHV-6 encephalitis or other type of active central nervous system infection should not be made without first excluding chromosomal HHV-6 integration by measuring DNA load in CSF, serum, and/or whole blood.

The prevalence of chromosomally integrated human herpesvirus 6 genomes in the blood of UK blood donors

A lesser-recognized form of human herpesvirus 6 (HHV-6) persistence is integration of the viral genome in a host chromosome and high viral copy numbers in blood or sera are characteristic of this phenomenon. A cross-sectional study was performed to determine the frequency of high HHV-6 viral loads in whole blood (>6 log(10) copies/ml) in a population of blood donors in London, UK. Blood samples from 500 anonymized blood donors were collected from one donation center, DNA extracted, and quantitative realtime PCR used to measure viral load. Four samples (0.8%) were found to have high viral copy numbers of HHV-6 (median 6.7 log(10) copies/ml; range 6.5- 6.9 log(10) copies/ml). Cellular DNA was also quantitated using qRT-PCR for beta-globin. By comparing these two results, we calculated that there were between two and five copies of HHV-6 present per cell in these four donors. The median viral load detected in plasma from the four individuals was 3.8 log(10) copies/ml (range 3.5-4.0 log(10) copies/ml). All samples were HHV-6 variant B. In addition, a retrospective analysis of all diagnostic blood samples performed for HHV-6 in our center showed a prevalence of 2.9% of high viral loads characteristic of integration. In conclusion, high viral copy numbers of HHV-6, representing a population of viral integration, is detected in 0.8% of UK blood donors. The presence of high HHV-6 viral loads in healthy normal individuals reiterates the need to consider the confounding effect of HHV-6 viral integration in any laboratory diagnosis of HHV-6 infection.

Human herpesvirus 6 chromosomal integration in immunocompetent patients results in high levels of viral DNA in blood, sera, and hair follicles

Six immunocompetent patients with human herpesvirus 6 (HHV-6) chromosomal integration had HHV-6 and beta-globin DNA quantified in various samples by PCR. The mean HHV-6 DNA concentration (log(10) copies/milliliter) in blood was 7.0 (>/=1 HHV-6 DNA copies/leukocyte), and in serum it was 5.3 (>/=1 HHV-6 DNA copies/lysed cell). The mean HHV-6 DNA load (log(10) copies)/hair follicle was 4.2 (>/=1 copies/hair follicle cell), demonstrating that viral integration is not confined to blood cells. The characteristically high HHV-6 DNA levels in chromosomal integration may confound laboratory diagnosis of HHV-6 infection and should be given due consideration.

The natural history and laboratory diagnosis of human herpesviruses-6 and -7 infections in the immunocompetent

BACKGROUND

Human herpesviruses-6 and -7 (HHV-6/7) are widespread in all populations. In some individuals HHV-6 is found integrated into human chromosomes, which results in a high viral load in blood. HHV-6 variant B (HHV-6B) and HHV-7 primary infections, although usually silent, not infrequently cause the childhood exanthem roseola infantum and are sometimes accompanied by neurological illness. HHV-6 variant A (HHV-6A) is not associated with any disease.

OBJECTIVES

The present review focuses on the immunocompetent individual and considers the epidemiology of the two viruses and their role as human pathogens. It discusses the importance of satisfactory diagnostic tests to distinguish them, compares those currently available, and recommends how best to differentiate primary from persistent infection in each case.

RESULTS

It is explained that at the present time antibody avidity immunofluorescence tests are the most reliable discriminators of the two types of infection. In primary infection these tests can be supplemented by PCR for viral DNA in blood but careful interpretation is required for HHV-6 in view of the high persistent viral DNA load seen with chromosomal integration. Since the contribution of primary HHV-6 and -7 infections to the burden of severe neurological illness in young children is only now emerging as significant, the need to test for these viruses in such cases is stressed.

CONCLUSIONS

1. Primary HHV-6/7 infections must be distinguished from persistent infections. 2. Chromosomal integration of HHV-6 requires urgent study. 3. HHV-6A/B must be distinguished in clinical situations. 4. Where serious neurological disease/encephalitis is temporally related to immunisation it is particularly important to test for HHV-6/7 primary infection since otherwise the condition might wrongly be diagnosed as a vaccine reaction. 5. Because less is currently known about HHV-7 and HHV-6A than HHV-6B, future studies should concentrate on the former two. 6. Improvements in diagnostic tests are required for each virus.

Human herpesviruses-6 and -7 infections

PURPOSE OF REVIEW

To summarize the biology and clinical consequences of infection with the closely related human herpesviruses-6 and -7 (HHV-6/7) in children.

RECENT FINDINGS

Over the last year there has been a paucity of paediatric publications on HHV-6 and only two studies focused on HHV-7. Steady progress has been made regarding the biology and clinical consequences of HHV-6 infection whereas the effect of HHV-7 infection remains a neglected topic. However, both viruses have been shown to contribute significantly and equally to the burden of disease in young children with suspected encephalitis or severe convulsions with fever. There continues to be uncertainty as to the effects of HHV-6 infection after stem cell transplant, although there is general agreement that it contributes to encephalitis. In contrast, HHV-7 seems to have little clinical impact after stem cell transplant, although central nervous system infection and disease have recently been reported in children. Understanding the contribution of chromosomal integration and inheritance of both HHV-6 variants A and B (HHV-6A/B) and their effect on diagnosis is emerging.

SUMMARY

There is an urgent need for more research on HHV-6 and -7 in children, particularly in relation to chromosomal integration of HHV-6A and B, and clinical consequences of HHV-7 infection.

Unexpected occasional persistence of high levels of HHV-6 DNA in sera: Detection of variants A and B

Previously it was thought that in the immunocompetent human herpesvirus-6 [HHV-6] DNA was present transiently in serum during early primary infection but not thereafter. In this study, HHV-6 serum IgG avidity was detected by immunofluorescence and HHV-6 variants A/B [HHV-6A/B] serum DNA by semi-quantitative PCR [titre-log(10) copies/ml] in: (a) young children <3 years old from an encephalitis Survey, and a control Anonymised Serum Bank and (b) children/adults referred for diagnosis. The results showed that 11 out of 15 children [all <2 years] with primary infection proven by seroconversion had transient low levels of serum HHV-6B DNA [mean titre 2.6]. However, 3.3% (6/184) of Survey Children had significantly higher levels [mean titre 5.3; 2 HHV-6A; 4 HHV-6B; P < 0.001]. Similarly high level serum DNA [mean titre 4.0; 4 HHV-6A; 6 HHV-6B] was found in 1.5% (10/653) of the Serum Bank Children. Moreover, seven young children <3 years old [four Survey Children and three referred for diagnosis] had high titre serum HHV-6 DNA [mean 4.8] persisting i.e., in all available samples [median 186 days]. Three older children >3 years old and 4 adults [3 of whom were the mothers of 3 of the young children with persisting HHV-6] also had persisting high titre viral DNA [mean 4.2; median 108 days]. Thus in contrast to acute primary infection, where only HHV-6B DNA is found transiently, both HHV-6A and B DNA persist in serum at high titre in occasional individuals of all ages. The significance of this newly described phenomenon in relation to diagnosis, clinical consequences and congenital infection are discussed.

Inheritance of Chromosomally Integrated Human Herpesvirus 6 DNA

Human herpesvirus 6 (HHV-6) genome has been detected in several human lymphoproliferative disorders with no signs of active viral infection, and found to be integrated into chromosomes in some cases. We previously reported a woman with HHV-6–infected Burkitt’s lymphoma. Fluorescence in situ hybridization showed that the viral genome was integrated into the long arm of chromosome 22 (22q13). The patient’s asymptomatic husband also carried HHV-6 DNA integrated at chromosome locus 1q44. To assess the possibility of chromosomal transmission of HHV-6 DNA, we looked for HHV-6 DNA in the peripheral blood of their daughter. She had HHV-6 DNA on both chromosomes 22q13 and 1q44, identical to the site of viral integration of her mother and father, respectively. The findings suggested that her viral genomes were inherited chromosomally from both parents. The 3 family members were all seropositive for HHV-6, but showed no serological signs of active infection. To confirm the presence of HHV-6 DNA sequences, we performed polymerase chain reaction (PCR) with 7 distinct primer pairs that target different regions of HHV-6. The viral sequences were consistently detected by single-step PCR in all 3 family members. We propose a novel latent form for HHV-6, in which integrated viral genome can be chromosomally transmitted. The possible role of the chromosomally integrated HHV-6 in the pathogenesis of lymphoproliferative diseases remains to be explained.

Inheritance of Chromosomally Integrated Human Herpesvirus 6 DNA

Human herpesvirus 6 (HHV-6) genome has been detected in several human lymphoproliferative disorders with no signs of active viral infection, and found to be integrated into chromosomes in some cases. We previously reported a woman with HHV-6–infected Burkitt’s lymphoma. Fluorescence in situ hybridization showed that the viral genome was integrated into the long arm of chromosome 22 (22q13). The patient’s asymptomatic husband also carried HHV-6 DNA integrated at chromosome locus 1q44. To assess the possibility of chromosomal transmission of HHV-6 DNA, we looked for HHV-6 DNA in the peripheral blood of their daughter. She had HHV-6 DNA on both chromosomes 22q13 and 1q44, identical to the site of viral integration of her mother and father, respectively. The findings suggested that her viral genomes were inherited chromosomally from both parents. The 3 family members were all seropositive for HHV-6, but showed no serological signs of active infection. To confirm the presence of HHV-6 DNA sequences, we performed polymerase chain reaction (PCR) with 7 distinct primer pairs that target different regions of HHV-6. The viral sequences were consistently detected by single-step PCR in all 3 family members. We propose a novel latent form for HHV-6, in which integrated viral genome can be chromosomally transmitted. The possible role of the chromosomally integrated HHV-6 in the pathogenesis of lymphoproliferative diseases remains to be explained.

Fine mapping of an apparently targeted latent human herpesvirus type 6 integration site in chromosome band 17p13.3

An unusually high level of latent HHV-6 infection has been documented in the peripheral blood and/or bone marrow cells of a small group of patients with predominantly malignant lymphoid disorders, and in at least one healthy individual. We have shown previously in peripheral blood mononuclear cells (PBMCs) of three patients, two with a history of lymphoma and one with multiple sclerosis, a specific target site for latent integration of the full-length HHV-6 viral genome on the distal short arm of chromosome 17, in band p13.3. Fluorescence in situ hybridization (FISH) procedures were used to map more precisely the location of the viral integration site in one of those patients, relative to two known oncogenes mapped previously, namely CRK, and the more telomeric ABR oncogene. It is shown that the HHV-6 integration site is located at least 1,000 kb telomeric of ABR, and is very likely to map close to or within the telomeric sequences of 17p. This finding is significant given that human telomeric-like repeats flank the terminal ends of the HHV-6 genome. Cytogenetic studies showed evidence of karyotype instability in the peripheral blood cells infected latently.