HHV6 exhibits transforming capacities

The 3.9 kbp Sall-L fragment located within the direct repeat region of HHV-6 variant A was shown to transform the murine NIH3T3 cell line, human epidermal keratinocytes RHEK-1, and both primary and established rodent cells (Razzaque et al., 1993; Thompson et al., 1994). This transforming activity was localized to the DR7 gene and cells expressing DR7 protein-induced tumours when injected into immunodeficient nude mice, while cells expressing truncated DR7 protein did not (Kashanchi et al.,1997). Moreover, DR7 protein from HHV-6A strain U1102 binds to the human tumour-suppressor protein p53, which is a major control point of the cell cycle, involving the inhibition of p53-activated transcription (Collot et al., 2004).

Because DR7 from variants A and B share only 42.2% homology, it seemed interesting to us to look for the same oncogenic properties for DR7B. Consequently, using a two-hybrid system, we demonstrated that DR7B, like DR7A, was able to bind to human p53, indicating that the p53-binding site on the DR7 protein could be on the common portion of both viral proteins (unpublished data).

Recently, we looked for the expression of DR7 protein in tissue samples from HD previously detected as positive by HHV-6 structural MAbs. DR7 was strongly positive and it was principally found in RS cells, and to a lesser extent in other lymphoid cells (Fig. 1), signalling the expression of this oncogenic protein in transformed cells.

As a result of these transforming capacities, a latently HHV-6-infected cell line, named "Katata" cell line, was established from pathological tissues derived from

Fig. 1 Expression of DR7 protein in tissue samples from patients suffering from HD, previously detected positive by HHV-6 structural monoclonal antibodies. DR7 was strongly positive and was principally found in RS cells, and to a lesser extent in other lymphoid cells. (for colour version: see colour section on page 355).

Fig. 1 Expression of DR7 protein in tissue samples from patients suffering from HD, previously detected positive by HHV-6 structural monoclonal antibodies. DR7 was strongly positive and was principally found in RS cells, and to a lesser extent in other lymphoid cells. (for colour version: see colour section on page 355).

an HHV-6-positive and EBV-negative Burkitt's lymphoma (Bandobashi et al., 1997). HHV-6 was also present in two leukaemia cell lines.

HHV-6 retains p53 within the cytoplasm

Recently, it was shown that p53 increases in the cytoplasm of HHV-6-infected cells, this stability of p53 being dependent on deubiquitination. The infected cells revealed resistance to apoptosis (Takemoto et al., 2004).

HHV-6 can transactivate genes Transactivation of human genes

An interesting feature associated with HD is the abundant constitutive activation of the p50/p65 nuclear factor kappa B (NFkB) complex in cultured RS cells, this factor being normally observed only for limited time intervals after stimulation with diverse inducers. Moreover, constitutive NFkB activation has been shown to be required for survival and proliferation of RS cells, preventing them from undergoing apoptosis under stress conditions (Bargou et al.,1997). These findings tended to identify NFkB as an important component for understanding the HD pathogenesis. We found enhanced expression of p50 and p65 mRNAs and NFkB activation, as demonstrated by the use of reporter genes, in HHV-6B-infected cells compared to mock-infected cells. The increase in the NFkB levels in infected cells could be considered to be the result of transcriptional upregulation of the p50 and p65 genes (unpublished data).

Transactivation of viral genes

EBV transactivation. The EBV, considered as an oncogenic virus, is found in 50-60% of HHV-6-positive LD tissues. In this case, the intensity of signals obtained for EBV is weaker than for HHV-6, suggesting that EBV sequences are markedly less represented (Torelli et al.,1992). Techniques used do not indicate if both viruses were present in the same cells.

More recently, by superinfection of EBV latently infected cells with HHV-6A, an HHV-6 transactivation effect on EBV replication cycle has been demonstrated. This activation, mediated via a cyclic AMP-responsive element located within the EBV Zebra promoter requires infectious virus. This promoter controls the EBV gene product ZEBRA, which is responsible for disrupting EBV latency and initiating the lytic replication cascade (Flamand et al.,1996).

Activation of LMP1 and EBNA2 expression has also been reported, after HHV-6A infection of EBV-positive Burkitt's lymphoma cell lines. The authors have demonstrated the presence of positive and negative regulatory elements responsive to HHV-6A infection in LMP1 regulatory sequences (Cuomo et al., 1998).

HHV-8 transactivation. HHV-6, which was reported to be rarely present in HHV-8-related LD (Asou et al., 2000), is able to induce HHV-8 replication as demonstrated by coculture of HHV-6-infected T cells with HHV-8. HHV-8 replication results from activation of the first promoter activated during HHV-8 replication (0RF-50 promoter) (Lu et al., 2005).

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