Retroviruses in foreign species and the problem of provirus silencing q J. Svoboda a, * , J. Hejnar a , J. Geryk a , D. Elleder a , Z. Vernerova  b a Institute of Molecular Genetics, Academy of Sciences of the Czech Republic, Flemingovo n. 2, 166 37 Prague 6, Czech Republic b Department of Pathology, First Faculty of Medicine, Charles University, Prague, Czech Republic Received 29 May 2000; received in revised form 18 August 2000; accepted 5 October 2000 Received by G. Bernardi Abstract Retroviruses are known to integrate in the host cell genome as proviruses, and therefore they are prone to cell-mediated control at the transcriptional and posttranscriptional levels. This plays an important role especially after retrovirus heterotransmission to foreign species, but also to differentiated cells. In addition to host cell-mediated blocks in provirus expression, also so far unde®ned host speci®cities, deciding upon the pathogenic manifestation of retrovirus heterotransmission, are in play. In this respect, we discuss especially the occurrence of wasting disease and immunode®ciency syndrome, which we established also in avian species using avian leukosis virus subgroup C (ALV-C) inoculated in mid-embryogenesis in duck or chicken embryos. The problem of provirus downregulation in foreign species or in differentiated cells has been in the recent years approached experimentally. From a series of observations it became apparent that provirus downregulation is mediated by its methylation, especially in the region of proviral enhancer-promoter located in long terminal repeats (LTR). Several strategies have been devised in order to protect the provirus from methylation using LTR modi®cation and/or introducing in the LTR sequence motifs acting as antimethylation tags. In such a way the expression of retroviruses and vectors in foreign species, as well as in differentiated cells, has been signi®cantly improved. The complexity of the mechanisms involved in provirus downregulation and further possibilities to modulate it are discussed. q 2000 Elsevier Science B.V. All rights reserved. Keywords: Retrovirus heterotransmission; Retroviral vector; Methylation; CpG; Long terminal repeat(s); Histone deacetylase 1. Introduction The topic of this review is focused on selected aspects of retrovirus heterotransmission, but also touches the problem of retroviral genome silencing in differentiated and other types of cells. In case of heterotransmission, many host cell factors are in play, which decide whether or to what degree the retroviral genome will be expressed and what pathogenic consequences may be triggered. As is known generally, the retroviral genome becomes integrated in the cell genome as a provirus and, therefore, it is not surprising that it is highly in¯uenced by the host-cell gene-regulation machinery. In discussing such downregulation of the provirus we are in fact dealing with post-integration blocks in provirus expression, the nature of which is epigenetic and mediated by new host cell factors. The unusual cell milieu, in concert with which the virus has not been evolving, can be lacking some factors like those enabling viral RNA export from the nucleus, or provide unusual factors like those changing the viral RNA splicing. However, of main importance is cell transcriptional regulation, which in many cases leads to provirus silencing. It is, therefore, not surpris- ing that in phylogenetically distant host cells the provirus can integrate, but in many cases does not produce an infec- tious progeny. We call such host cells non-permissive, in contrast to permissive cells where formation of infectious virions takes place. Non-permissivenes to retroviral infection has been for a Gene 261 (2000) 181±188 0378-1119/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0378-1119(00)00481-9 www.elsevier.com/locate/gene Abbreviations: ALV, avian leukosis virus(es); ALV-B, ALV-C, ALV-D, avian leukosis virus subgroup B, C, and D, respectively; aprt, adenosine- phosphoribosyltransferase gene; b-geo, fused b-galactosidase and neomy- cin resistance gene; BLV, bovine leukemia virus; CAT, chloramphenicol acetyltransferase; cHS4, chicken hypersensitive site 4; EC, embryonic carcinoma; GFP, green ¯uorescence protein; HDAC, histone deacetylase; HIV, human immunode®ciency virus; IFN-SAR, interferon scaffold attach- ment region; LCR, locus control region(s); LTR, long terminal repeat(s); MeCP, methyl-CpG-binding protein; MEL, murine erythroleukemia; MLV, murine leukemia virus(es); MSV, murine sarcoma virus; NCR, negative control region; neo, neomycin resistance gene; PR RSV, Prague strain of RSV; RSV, Rous sarcoma virus; SIV, simian immunode®ciency virus; SIVcpz, chimpanzee SIV; SIVsm, sooty mangabey SIV; TSA, trichostatin; X-MLV, xenotropic MLV q Extended version of the lecture ªModi®cation of retrovirus pathogeni- city by transspecies transferº presented at the workshop ªMicrobial Varia- tion and Evolutionº at Ischia, Italy (organizers G. Bernardi and B. Fantini). * Corresponding author. Tel.: 142-2-243-10-238; fax: 142-2-243-10- 955. E-mail address: svoboda@img.cas.cz (J. Svoboda).