- Email: [email protected]
Our retroviral heritage
REVIEWS
RetrovirusesInfect
Ourretroviral heritage a wide range
ofvertebrate
species, from fish to humans. During their replication, retroviruses copy their RNA genome into DNA utilizing a virally encoded enzyme, RNA-dependent DNA poiymerase (reverse transcriptase, RTL Retrovimses belong to a broader group of genetic elements replicating via reverse transcription, including hepatitis B virus, cauliflower mosaic virus, and retroefements of prokatyotes Lbwiaconfda0rbavefifbntwearede~ceaded and eukaryotes, such as human LINE elements. Some fivmuirasesaswellasjivm~apezW%iktbereisciear infectiously transmitted retroviruses are not associated evidescetbatviraidiscase~sacbaspolloarwlrabks, with dii at all, whereas others are deadly pathogens, affkded ancknt chrflfratiorc,vtnrcrWen?lKU&Wd such as human immunodekiency virus. tbeealqyearS+iSccnt*ry,s~apcribemirsewmy Retrovirus particles contain diploid RNA genomc: mtnwbalgnwmrcsuan~e ?f-*sm that form a haploid DNA provirus in the infected cell. between i@ctkw aadgeneiic modes OjtranSmkti Similar to lysogenic phage, the newly synthesized pro-dpmpostemas btfow tbe discomy of reverse virus then becomes inserted into the host’s chromosomai traascriptioaial9M Tbarenfnswbnbadcarlkr DNA with the help of another viral enzyme, integrase @ovi&dtaemWaa~fwgrmr-#ne#raasmissioaof The site of integration in the host genome is essentially rel&ses were subject toj?iead& ridicak Today, the random, although insertion into active euchromatin i\ sbwniiag of genetic elenmts betwten chromosomes and more usual. If integration occurs within a host gene i, RN& and tbegewratioa ofpmcesseddp can disrupt it and knock out its function, whereas ii commonplace. It is iimely, bwuwn; lo revisit tbe topic of integration occurs adjacent to a gene, the promoter or buman miogenoas retrovfnrscs Abe su&ecl @is arlick? enhancer sequences in the retrovims tong terminal repeat (LTR) can upregulate gene expression. Indeed, insertional activation of cel!u!3r oncogenes is a common HERV gcnomcs am t.-.. --smittcd as pan of the host’s normal genetic material, so the selective pressure to mechanism of retroviral oncogenesis. On many occasions during vertebrateevolution, pro- maintain functional ORFs for viral replication no long3 viruses have become integrated into the DNA of the applies. This has led to the vast majority of HERVs acquiring rdndom mutations or deletions disrupting one host’s germ lie and, thereby, become transmitted to the or more of the main ORFs. The possession of a particunext generation as a mend&an trait. These genomes are known as endogenous retroviruses (ERVs)t. Clearly, the lar ORF might however, confer selective advantage to the host. Some HERVs have maintained biological activities more pathogenic the ERV the less likely it is to be mainsimilar to those of infectious viruses, including trantamed in the host genome. Yet, many viruses have been scription, ORF translation and, in some cases, particle passed in this way for millions of years and for hundreds production. Although HERVs were originally thought of of thousands of host generations. Mendeiian proviruses acquired in recent evolutionary history, however, ofin as merely ‘junk DNA’, a number of studies reflect the increasing interest in the biological significance of these retain their capacity to encode infectious virions although the host can control and suppress endogenous vii’~s genetic element&~. Here, we illustrate the diversity of HERVs, the roles that HERV elements play in the normal production by several different mechanisms. On tare function of the host genome and their pathogenic occasions, retroviruses package host RNA transcripts, and potential. Examples from non-human hosts will be integrate them upon reverse transcription, into newly included where appropriate. infected cells, for example, oncogenes. But we do not yet know of any examples of par-asexual exchange of host HRRV chtssifWion and diversity genes by retroviruses or cross-species transfer into a new germ line, other than the viral genomes themselves. Although retroviruses were initially classified on the basis of particle structure and pathogenicity, they are now divided by nudeotide sequence information. In adHumallendogenousretfoviruaes dition, those retroelements regulating gene expression It has been known for over 20 years that the human are often referred to as ‘complex’, while elements iackgenome contains multiple DNA sequences closely related ing such control are termed ‘simple’. Endogenous retroto infectious retroviruses. Most human endogenous retrovirus (HER9 genomes have been acquired at least viral genomes probably all belong to the simple genera. ten million and possibly up to a hundred million years HERVs are often cktssihed further on the basis of the identity of the tRNA that acts as a primer for reverse ago* and might constitute up to 0.1% of our germ-line DNA. As shown in Fig. 1, in comparison with a distantly transcriptions. A primer-binding site (PBS) complementary to tRNA sequences is krcated imrnediitety downrelated long interspersed element (LINE) retroposon, full srream from the 5’ LTR. For clarity, we use the PBSlength HERVs show the same basic geriome o:gat?izrelated nomenclature, citing aitematives. ation as exogenous infectious retroviruses, possessing The human genome contains several families of HERV regions with sequence similarity to the LTRs and major elements, ranging in copy number from one to lC!OO open reading frames (ORFs) of retrovimses, namely gug (reviewed in Ref. 4). Although most HERVs are trans(encoding viral core proteins), pnI (encoding viral enzymes) and env (encoding envelope proteins). LINES, lationally defective, those that express an ORF for g@ can produce virus-like particles that package mukipfe unlike HERV, are not bounded by LTRs, but do possess retroviral transcripts, including defective ones of related ORFs with pcdhomology and associated RT activity.
REVIEWS
genomesj.6. As the human genome becomes fully documented and expressed sequences are characterized, it will be interesting to see to what extent ‘fossil’retrovimses affect gene organization
and expression. e.R. HEW
HBRveXpre5sion Active HERV tmnscription has been reported in normal peripheral blood mononuclear cells’, mitogenstimulated T-lymphocyte&s, leukocytes from
patients
with
u3 R u5
u3 R u5
chronic
myeloid leukaemia and preleukaemic syndrome*s.t*, the piacenta’)t?, and testicular tumourst3. A wide variety of factors influence HERV RNA expression, including the precise site of integration, regulatory se-
Matrix
(MA) Protease (PR) Majar capsid (CA) Nualeccapsid (NC)
Reverse transcriptass (RT) lntegrasa (IN)
Envelope Transmambrana Protein(TM) surface protein (SU)
??
??
quences within the 5’ LTR (Ref. 14), and steroidssts. While the presence of HERV transcripts has been repotted in many cell types, a direct role for the transcripts remains unclear. Because most of the transcripts are tanslationally defective they might merely reflect a response to metabolic pmcesses occurring in the host cell. Although the vast majority of HERV loci are translationally defective due to random mutations, it is now clear that complete ORFs exist in several HERV families. Expression of such loci could potentially lead to the expression of HERV protein products or even virus particles (discussed below). The maintenance of a HERV ORF over a signi% cant evolutionary time period sug-
Fffiutts 1. (a) Comparisonof full-length hunlan endogenousretmvirus(HERV) and LINE-1 genotnes.and main ORF (open reading frame) products.Smallarnws representshon repeatscf cellular DNA. I’denotesthe LINE-1 internal promoter.(I>) Genenlizd relrovirusparticle structwe.
immunosuppressive cytokine helping the placenta to be retained as an allograftt~J~?r. The conservation of ERV-3 throughout primate evolution, its tight transcriptional regulation and high level of expression point to the host adopting this viral protein for a useful function.
PresenceofHERVpartWsinhosttisues HERV particles can occasionally be detected in host tissues. Particle formation might result from expression of either a single complete retroviral lorus or from complementation between distinct but individually defective loci. To date, no infectious HERV has been demonstrdted, although many ERV genomes of other host species yield infectious panicles. The most detailed studies describing the presence of HERV panicles have characterized virions in nomral placenta and in temtocarcinoma cell lines (reviewed in Ref. 4). In buth examples, the panicles appear to contain rciwvird .X~UC~ICCJ ILlYLL the only family of HERV known to encode proteins fmm all major ORFs. Biological activity for several cloned protein producz~ of these ORFs has Been demonstratcul including gag and proteasez3, integras@, and that of a recently discovered smaller ORF (Ref. 25). Although
ID Il?Ef.I,33, ’---...---^--‘+d :z 11CDII” ‘.L,,.-~..” \..C.S __,,
TIG MARCH 1997 VOL. 13 No. 3
117
REVIEWS
hotmone@Jj.
The
presence
Of antigens
n&ted
to M4TV
in human mammary carcktomas has been repot&g. HERV-K possibly encodes these antigens, although a recent study found little immunol~cal cross-reactivity between MhlTV and HERV-K, at least within the env regio@. Further research will be necessary before identiC.z^.. -I _^^. as ^^ LL ^ lluIlyuL I...-^^ rq”tU”Phl” ^^.. :..,.t,r “, ..F ‘%“&m, .yu,g ^1 rn?n.r Iu.l\” c,e,.le.a .U..1 . . Recently, increased levels of specific antibodies to HERV-K gag protein have been reported in patients with seminomat3. While the presence of gag protein in the ht-
mour cells was demonstrated as in teratocarcinomas. the role of HERV-K In testicular turnours remains obscure.
Particle assembly
Recipient target cell
/
DNA
I \
Ftaunx 2. The principle of retroviral gene therapy. Packaging cells assemble recombinant virions carrying thenpeutic vector RNA. Following tra~uh~ctionof rargetcells, vectorRNA is reverse trxwribed. integratedinto the host cell genomeand stably expressed.i%te the possessionof a suiktble packagingsignnal (+) by the therapeuticvectorRNA, but not by tho.w ending virion proteins (4. prevents packaging of unwanted IWA transcripts
into virions. Abbreviation: L’l’R,long terminal repeat.
biological activity for the RT of a cloned poI gene has yet to be described, the particles carry readily detectable active RT enxymei~6. The production of HERV particles carrying retroviral genomes and active RT from the T47D human mammary carcinoma cell line has recently been reportedj.26. Interestingly, as with mouse mammary tumor virus (MMTV), the levels of HERV-KlO transcripts and RT activity produced by the cell line are both responsive to steroid
There are a variety of well-documented mechanisms by which the presence of a HERV element affects host genome structure and function. HERVs can affect the structural organization of the host genome, as illustrated by the loci encoding the C2 and C4 components of the human complement system. A retroposon derived from HERV-K C responsible for the restriction fragment length polymorphisms seen in the C2 component locust, while for C4. the Integration of a HERV-K element, before the separation of primate and human species, is responsible for the length polymorphiims seen with this geneso-3’. HERV-K loci have been identified in close proximity to several other host genes, including the breast and ovarian cancer susceptibility gene ERG41 on chromosome 17 (Ref. 33) and the glucoseGphosphate dehydrogenase locus on the X chromosomes. Such Endings might merely be a consequence of the sheer number of elements present in the normal human genome as an interaction between the HERV and the proximal gene remains to be shown. HERVs. and related retrotnmsposons, can affect normal gene expression by acting as insertional mutagens. In fact, analysis of single gene disorders has revealed relatively few examples of integrating retroid elements producing host gene knockouts. In somatic tissue a pS3 knockout in a murine Sarcoma has been observed3s. The inbred Ipr-Ipr mouse is one example in which an ERV disrupts apoptosis in the thymus leading to the development of an autoimmtne condition resembling human systemic lupus erythematosus 6LE)s. To date, in the absence of such elegant genetics, the potential association and significance of HERVs in the development of human autoimmune diseases, such as Sjogrens syndrome 6s). rheumatoid arthritis (RA) and multiple sclerosis (MS), has been the subject of much speculation but little rewards’-7-40. Ancient HERV integtations have affected the expression of adjacent host genes. Although retrovirus integration sites are widespread”, they are not entirely random-“-4‘t. Because HERVs possess LTR regions that can regulate gene transcription and are often found integrated in association with transcriptionally active sites, the potential for affecting host gene transcription becomes apparent. Such an effect by several retroelement families has been home nrtr for host genes, such as amylase, calbindin and phospholipase A2 (Refs 45-48). Many examples are known of exogenous and activated endogenous retroviruses re-integrating next to protooncoRenes and activating them*. However. few cases are found in humans, compared with gene activation
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spread of infection from activated ERV. Interestingly, both the suppressor genes are, themselves, derived from ERV. The Fu-I gene, which is more closely related to HERVs than the Fu4 gene, suppresses murine leukemia virus replication by inhibiting proviral integration and has recently heen cloned%. The Fy-4 locus, like a similar dominant inhibitor in chickens, suppresses viral replication by specifically blocking cell surface recep tars with expressed endogenous envelope product.557. Mice have a further locus, Rmcf;which acts in a similar manner to Fu4, and protects against infection by exogenous recombinant mink cell focus-forming viruses%. Mutation of key binding sites on cellular receptors might also have resulted from selection against vimemiaw. Animals
that
are
phylogenetically
distant
from
the
species bearing the ERV might not have evolved such resistance mechanisms. Thus, a class of murine retroViNSeS that cannot replicate in mouse cells on account of receptor incompatibility can propagate to high titres in human cells in culture. Such viruses are described as xenotropic? Chicken ERVs rarely replicate in their own species but readily infect cultured cells of quail, pheasant and turlley. Likewise, a cat ERV replicates in human cells, as does one from baboons, although neither replicafe in their own host soecies57 The xenotropic hoit range of some animal ERVs remained a curiositv until the advent of xenozraf% We have long known that the human tumour xe&grafts in immunodeficiem mice frequently become infected by xenotropic mouse ERV (Ref. 61), and that the feline ERV, RD114 virus, was first identified as RT activity in a human mmour cell line that had been passaged as a xenogmfr. If xenografts cm become contaminated with ERVs of their new hosts, the possibility also exists that an ERV could spread in the opposite direction, from xenograft to host. Currently, xenogmfted tissues and organs from baboons and pigs are being discussed for human use. We already know that the baboon ERV, BaEV, readily infects human cells in culture$ recently we have found that a oorcine ERV can also infect human cell&. Although these viruses do not appear to be pathogenic in the hosts in which thev are naturally endogenous, the situation could be quite humans, in whom the titres, and might spread to their co%acts.-
Gxlchlsiins
ERV genomes with ORFs and funcTional LTRs can produce complete infectious viruses when expressed. ERV activation can happen spontaneously or it can be induced experimentally with chemicals, such as 5azacytidine or j-bromo-2-deoxyuridine. Once virus is produced by an activated cell, it can spread infectiously in the tissues of its host, provided the host cells are susceptible to re-infection. The resulting vinemia is often pathogenic owing to insertion4 mutagenesis or oncogenesis. Consequently, animals harbouring infectious ERVs have evolved mechanisms to prevent re-infection with their own potentially infectious ERV genomes. Mice, for example, have dominantly acting alleles at hvo distinct loci (FL?-I and Fo-4) that suppress the
The internation and mendelian transmission of endogenous r&oviru.ses represents the most intimate of host-parasite relations. No’otonly has the virus gained a free ride through eoms of host evolution, but, on ocwsion. the host has adapted to its own use a protein encoded by the invading vimI genome. The adoption of vital genes that have phylogenetically distinct, originally infectious origins exemplifies this phenomenon. Endogenous mtroViNSeS can protect the host from more pathogenic, exogenous infection and can provide additions to the gene pool, A cost, however, can be the occasional pathogenic consequences of virus activation and expression. Modem molecular genencs has permined the delineal_iOlt of thousands of HERV benomes. The degree to which HERVs play a role in human disease and to which they can compromise the safety of retmviml-mediated gene thempy requires funher study. The risk of acquiring
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REVIEWS
animal endogenous retrovinl infections through xenogratis also requires attention. Investigations of HERVs are still at a relatively early stage. htit promise to yield results on the complex interaction further interesting between retrovinl elements and their human host.
Sttcleic
etrd. etal. et
Acknowledgement D.J. Griffths
\Ve thank
for his critical appmisa). of
the manuscripi.
References I Varmus. 1-I.E. Hughes.
SE. :wd Coffin. J. teds) (19977) RrtnxirUses Cold spring Harbor Ld?untory Press 2 Shill, A.. Coutrvas. E.E. and Rush, M.G. (1991) I’rio/“g) 182. i9i-502 3 aeon. E.. Kate. N. and Cohen. hl (1989) Cwr Top .tJictvbiol. Ittttttttttol.l-ix. 115-132 4 Wilkinson. D.A.. hl:lger. D.L. and Leon& J-AC (1974) in The R&wirirlcrL.tVol. 3) (Levy. J.A.. rd.). pp. 46i-i3i. l’le”um lJre*\ 5 Patience. C. c’lrrl. (1996)./. lid 70, 26%-2657 6 Simpwn, G.R. et cl/. (1996) ~nulog~222, -lil-456 7 tir,drr~.w”. XLL. rt n/. (19%) AIDS Rev Htrst R~ltor~itmw 12. R33-R-i0 8 Krieg. A.M. (1992) A/DS Rev. Httsr. R~/tw’t,rts~.~8. 1991-1998 9 Kellehrr. CA e,rr/ (1996) J Gm. l’trnl 77. IlOl-1110 IO Boyd. M.T.. Xlaclra”. N and Oxier. DC. (19X9) Lnttcet I. 811-817 22 Brodsky. 1.. Foley. B. and G~llespw. D. (1993) Lwk Lptph 11. 119-123 12 Boyd. XLT. cl crl. (1993) litv/og~~ 196. 90%9O9 13 Sauter. M. cl 01. J. Vtd 69. +l++Ll 24 Sj0ttmm. E.. Anden%en. S. and Johmxn. T. 1996) J Vttnl. 70. 188-198 15 Ono. bl.. Kawakami. M. and lishlkuho. H. (19X7)] I’& 61. 2059-2062 16 O’Connril. C.D. d N/ (19%) Iiro/o~r 13X. 22i-235 17 Cohen. M. d nl. (19%) tTrn&v 147. +*9-*%-I 18 Nilsxm. B.O. et cl/. (19%) Iitw Getw6. 221-227 19 Andewo”. AC. e/n/ 1996)J /~wsI Detwct/o/ 106. 125-12X 20 Vemhlrs, PJ.\V. et nl. 199i) Vt,u/og~ 111. i89-i9Z 22 Hamguchi. S.. Good. R A and Day. N K. (1‘99j) /nrwtwm/ To&y 16. 59%603 22 Boiler. K. et a/. (1‘9933) ~iro/og~ 196. 3+9-353 23 Miiller-Lantxh. N. el rr/. 1993) AIDS Itcs. Htrttr Reltw~Dxs~~9. 3-43-350 24 Kiramum. Y c’, a/ (1996) J I’id. 70. 3302-3306 25 Lower. R. ef n/. Cl99j)J I’nnl 69. 141-l-t9 26 Seifarth. \V. d ol (199% J. I’ttul 69. G,0~-6+16 27 Ohno. T. el (I/. (1979) Proc Sat/ Acd Scr L’S A 76. 2460-2464 28 Vogetseaeder. U’. el a/ 1995) AIDS Rev Httttt Rettvtvtxws 11,869-872
ettrl.
(1995)
( (
(
(
(
29 Zhu. Z.B. et cl/. (1992) J. hp. .lk:/. l-i. 17%-17X7 30 Dangel. A.W. d nl. (199.1) Jttttttttno~~ttelic~40. -12%136 31 Taxdxhji. .\I. el nl. (1994) Acids Rcs. 22. i211-5217 32 Chu, X., Rittner, C and Schneider. P.M. (1995) C/h /nf,nurmxwel. 12. 7-1-81 33 Jones, K.A. et nl. (19?Ii)-r)Htmt. ,~Jo/. Gewt. 3, 1927-1934 34 Srd!acek, 2. (1993) Httttt iJo/. Gettd. 2. 1865-1869 35 hlitreiter. K. et nl. (1994) ti1u/ogv2t% 837-8-11 36 \Vu. J. (1993) J. J3f.1 dJd 178. .i61-+6X 37 Pen”“. H. rti. (1991) I*fti&357. S62-863 38 Krieg. A.~L, Gourley. U.F. and I’erl. A. (1992) FASmj. 6. 2i37-2% 39 Perl. A. and Ranki. K. (1993) Ttwtr/s.lJtcrobio/. 1. li3-Ii6
etn/.
41 Mee.\e, E. (1996) Q~“gerw/. Cell Gcwl. 72. -IO--Q 42 Shih. C-C.. Stoye, J.P. and Coffin, J.M. (19X8) Cc//H. 531-537 43 Tanwio. D. and hkmuelidix L. (1991) 101. l-t&156 44 Cm@, R. (1‘992) Trwrtls Gettet 8. 1X7-190 45 Samuelbon, L C. el N/ (19%) .IJo/. Cd. Bid 10. Lil3-2520 46 Lm. A.Y. and Abmham. B.A. (1991) Cn~cerRes. il. +lO7-‘Ill0 47 Tmg. C.N. &N/. t 19922) Germs Det: 6. l_ti7-1465 48 Feuchter-hlunhy, A.E.. Freeman. J.D. and .\lager. D.L. 1993) Strclerc Act& Km. 21. 135-I+3 49 Mane. 8. 333. X7-90 50 \Vuo. S.L.C. (199-1) Twttds GctteJ. 10. Ill-112 51 Scadden. D.T.. Fuller. B. and Cunningham. J.U. (1990) J 1iml &&.-124-427 52 Hatzoglou. hl. et N/ (199O) ~~!III~I.Gew Ther. I. 38%397 53 Purcell. D.F (1996),/. I’/rv/. 70. xX7-897 54 Cowt, FL. (1995) J litd. 69.7+30-7-136 55 Goodchild. Freemm. J.D. and hlagrr. D.L. (1995) itulo~~ 206. 164-173 56 Be% S. (1996) Srrfttw382. X26-X.?9 57 VI&s. R.A. (1993) in ?be R~t~ot~tritlrte(\~~~l. 2) (I.e\y. J.A.. cd.). pp I-103. I’lmum I’re\s 58 Bullcr. R S.. Ahmrd. A :md Porti\. J L (19X7) J. I’nol 61. 29-3-l 59 \‘&s. R A. :md Tailor, C S. (1995) (lell81, i31-533 6O Levy. J.A (1973) Scwtw 182. llil-1153 61 \Vew R.A. (1978) ,\itt/ Cttttcw Jttst .lJortoRr.-18. 183-1X9 62 P.mence. C.. Takeuch~. \I’ and \Veiw. R.A. .IJd (in press)
Chwttosottto
(
I
etrd.(1988xX) Acttttw
eta/. etN/ N.L da/
,Vht
C Palfemx. D.A.WiIkinson andR.A. Weiss LUPin the Chester Bmtty Luborutoti~5, 77x Ittsfifttfe ofCmcet Rewmch. 237 F~lhnn~ Rod, Lottht, 1iK SW3 GjB.
Genetwork is a rezular column of news and information ahout Internet (pp. 121-122). resources for researchersin genetics and development Genetwork is compiled and edited with thwhelp of Steven E. Brenner (Stmctunl Biology Cent% National Institute of BioScience and Human Technology, Higashi l-l Tsukuba. Ihanki 305, Japan; [email protected]); and Fran Lewitter (Scientific Computing, Whitehead Institute for Biomedical Research, Nine Cambridge Center. Cambridge MA 02142-1479, USA;
TIC MARCH 1997 VOL. 1: Nu. 3
120
RetrovirusesInfect
Ourretroviral heritage a wide range
ofvertebrate
species, from fish to humans. During their replication, retroviruses copy their RNA genome into DNA utilizing a virally encoded enzyme, RNA-dependent DNA poiymerase (reverse transcriptase, RTL Retrovimses belong to a broader group of genetic elements replicating via reverse transcription, including hepatitis B virus, cauliflower mosaic virus, and retroefements of prokatyotes Lbwiaconfda0rbavefifbntwearede~ceaded and eukaryotes, such as human LINE elements. Some fivmuirasesaswellasjivm~apezW%iktbereisciear infectiously transmitted retroviruses are not associated evidescetbatviraidiscase~sacbaspolloarwlrabks, with dii at all, whereas others are deadly pathogens, affkded ancknt chrflfratiorc,vtnrcrWen?lKU&Wd such as human immunodekiency virus. tbeealqyearS+iSccnt*ry,s~apcribemirsewmy Retrovirus particles contain diploid RNA genomc: mtnwbalgnwmrcsuan~e ?f-*sm that form a haploid DNA provirus in the infected cell. between i@ctkw aadgeneiic modes OjtranSmkti Similar to lysogenic phage, the newly synthesized pro-dpmpostemas btfow tbe discomy of reverse virus then becomes inserted into the host’s chromosomai traascriptioaial9M Tbarenfnswbnbadcarlkr DNA with the help of another viral enzyme, integrase @ovi&dtaemWaa~fwgrmr-#ne#raasmissioaof The site of integration in the host genome is essentially rel&ses were subject toj?iead& ridicak Today, the random, although insertion into active euchromatin i\ sbwniiag of genetic elenmts betwten chromosomes and more usual. If integration occurs within a host gene i, RN& and tbegewratioa ofpmcesseddp can disrupt it and knock out its function, whereas ii commonplace. It is iimely, bwuwn; lo revisit tbe topic of integration occurs adjacent to a gene, the promoter or buman miogenoas retrovfnrscs Abe su&ecl @is arlick? enhancer sequences in the retrovims tong terminal repeat (LTR) can upregulate gene expression. Indeed, insertional activation of cel!u!3r oncogenes is a common HERV gcnomcs am t.-.. --smittcd as pan of the host’s normal genetic material, so the selective pressure to mechanism of retroviral oncogenesis. On many occasions during vertebrateevolution, pro- maintain functional ORFs for viral replication no long3 viruses have become integrated into the DNA of the applies. This has led to the vast majority of HERVs acquiring rdndom mutations or deletions disrupting one host’s germ lie and, thereby, become transmitted to the or more of the main ORFs. The possession of a particunext generation as a mend&an trait. These genomes are known as endogenous retroviruses (ERVs)t. Clearly, the lar ORF might however, confer selective advantage to the host. Some HERVs have maintained biological activities more pathogenic the ERV the less likely it is to be mainsimilar to those of infectious viruses, including trantamed in the host genome. Yet, many viruses have been scription, ORF translation and, in some cases, particle passed in this way for millions of years and for hundreds production. Although HERVs were originally thought of of thousands of host generations. Mendeiian proviruses acquired in recent evolutionary history, however, ofin as merely ‘junk DNA’, a number of studies reflect the increasing interest in the biological significance of these retain their capacity to encode infectious virions although the host can control and suppress endogenous vii’~s genetic element&~. Here, we illustrate the diversity of HERVs, the roles that HERV elements play in the normal production by several different mechanisms. On tare function of the host genome and their pathogenic occasions, retroviruses package host RNA transcripts, and potential. Examples from non-human hosts will be integrate them upon reverse transcription, into newly included where appropriate. infected cells, for example, oncogenes. But we do not yet know of any examples of par-asexual exchange of host HRRV chtssifWion and diversity genes by retroviruses or cross-species transfer into a new germ line, other than the viral genomes themselves. Although retroviruses were initially classified on the basis of particle structure and pathogenicity, they are now divided by nudeotide sequence information. In adHumallendogenousretfoviruaes dition, those retroelements regulating gene expression It has been known for over 20 years that the human are often referred to as ‘complex’, while elements iackgenome contains multiple DNA sequences closely related ing such control are termed ‘simple’. Endogenous retroto infectious retroviruses. Most human endogenous retrovirus (HER9 genomes have been acquired at least viral genomes probably all belong to the simple genera. ten million and possibly up to a hundred million years HERVs are often cktssihed further on the basis of the identity of the tRNA that acts as a primer for reverse ago* and might constitute up to 0.1% of our germ-line DNA. As shown in Fig. 1, in comparison with a distantly transcriptions. A primer-binding site (PBS) complementary to tRNA sequences is krcated imrnediitety downrelated long interspersed element (LINE) retroposon, full srream from the 5’ LTR. For clarity, we use the PBSlength HERVs show the same basic geriome o:gat?izrelated nomenclature, citing aitematives. ation as exogenous infectious retroviruses, possessing The human genome contains several families of HERV regions with sequence similarity to the LTRs and major elements, ranging in copy number from one to lC!OO open reading frames (ORFs) of retrovimses, namely gug (reviewed in Ref. 4). Although most HERVs are trans(encoding viral core proteins), pnI (encoding viral enzymes) and env (encoding envelope proteins). LINES, lationally defective, those that express an ORF for g@ can produce virus-like particles that package mukipfe unlike HERV, are not bounded by LTRs, but do possess retroviral transcripts, including defective ones of related ORFs with pcdhomology and associated RT activity.
REVIEWS
genomesj.6. As the human genome becomes fully documented and expressed sequences are characterized, it will be interesting to see to what extent ‘fossil’retrovimses affect gene organization
and expression. e.R. HEW
HBRveXpre5sion Active HERV tmnscription has been reported in normal peripheral blood mononuclear cells’, mitogenstimulated T-lymphocyte&s, leukocytes from
patients
with
u3 R u5
u3 R u5
chronic
myeloid leukaemia and preleukaemic syndrome*s.t*, the piacenta’)t?, and testicular tumourst3. A wide variety of factors influence HERV RNA expression, including the precise site of integration, regulatory se-
Matrix
(MA) Protease (PR) Majar capsid (CA) Nualeccapsid (NC)
Reverse transcriptass (RT) lntegrasa (IN)
Envelope Transmambrana Protein(TM) surface protein (SU)
??
??
quences within the 5’ LTR (Ref. 14), and steroidssts. While the presence of HERV transcripts has been repotted in many cell types, a direct role for the transcripts remains unclear. Because most of the transcripts are tanslationally defective they might merely reflect a response to metabolic pmcesses occurring in the host cell. Although the vast majority of HERV loci are translationally defective due to random mutations, it is now clear that complete ORFs exist in several HERV families. Expression of such loci could potentially lead to the expression of HERV protein products or even virus particles (discussed below). The maintenance of a HERV ORF over a signi% cant evolutionary time period sug-
Fffiutts 1. (a) Comparisonof full-length hunlan endogenousretmvirus(HERV) and LINE-1 genotnes.and main ORF (open reading frame) products.Smallarnws representshon repeatscf cellular DNA. I’denotesthe LINE-1 internal promoter.(I>) Genenlizd relrovirusparticle structwe.
immunosuppressive cytokine helping the placenta to be retained as an allograftt~J~?r. The conservation of ERV-3 throughout primate evolution, its tight transcriptional regulation and high level of expression point to the host adopting this viral protein for a useful function.
PresenceofHERVpartWsinhosttisues HERV particles can occasionally be detected in host tissues. Particle formation might result from expression of either a single complete retroviral lorus or from complementation between distinct but individually defective loci. To date, no infectious HERV has been demonstrdted, although many ERV genomes of other host species yield infectious panicles. The most detailed studies describing the presence of HERV panicles have characterized virions in nomral placenta and in temtocarcinoma cell lines (reviewed in Ref. 4). In buth examples, the panicles appear to contain rciwvird .X~UC~ICCJ ILlYLL the only family of HERV known to encode proteins fmm all major ORFs. Biological activity for several cloned protein producz~ of these ORFs has Been demonstratcul including gag and proteasez3, integras@, and that of a recently discovered smaller ORF (Ref. 25). Although
ID Il?Ef.I,33, ’---...---^--‘+d :z 11CDII” ‘.L,,.-~..” \..C.S __,,
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hotmone@Jj.
The
presence
Of antigens
n&ted
to M4TV
in human mammary carcktomas has been repot&g. HERV-K possibly encodes these antigens, although a recent study found little immunol~cal cross-reactivity between MhlTV and HERV-K, at least within the env regio@. Further research will be necessary before identiC.z^.. -I _^^. as ^^ LL ^ lluIlyuL I...-^^ rq”tU”Phl” ^^.. :..,.t,r “, ..F ‘%“&m, .yu,g ^1 rn?n.r Iu.l\” c,e,.le.a .U..1 . . Recently, increased levels of specific antibodies to HERV-K gag protein have been reported in patients with seminomat3. While the presence of gag protein in the ht-
mour cells was demonstrated as in teratocarcinomas. the role of HERV-K In testicular turnours remains obscure.
Particle assembly
Recipient target cell
/
DNA
I \
Ftaunx 2. The principle of retroviral gene therapy. Packaging cells assemble recombinant virions carrying thenpeutic vector RNA. Following tra~uh~ctionof rargetcells, vectorRNA is reverse trxwribed. integratedinto the host cell genomeand stably expressed.i%te the possessionof a suiktble packagingsignnal (+) by the therapeuticvectorRNA, but not by tho.w ending virion proteins (4. prevents packaging of unwanted IWA transcripts
into virions. Abbreviation: L’l’R,long terminal repeat.
biological activity for the RT of a cloned poI gene has yet to be described, the particles carry readily detectable active RT enxymei~6. The production of HERV particles carrying retroviral genomes and active RT from the T47D human mammary carcinoma cell line has recently been reportedj.26. Interestingly, as with mouse mammary tumor virus (MMTV), the levels of HERV-KlO transcripts and RT activity produced by the cell line are both responsive to steroid
There are a variety of well-documented mechanisms by which the presence of a HERV element affects host genome structure and function. HERVs can affect the structural organization of the host genome, as illustrated by the loci encoding the C2 and C4 components of the human complement system. A retroposon derived from HERV-K C responsible for the restriction fragment length polymorphisms seen in the C2 component locust, while for C4. the Integration of a HERV-K element, before the separation of primate and human species, is responsible for the length polymorphiims seen with this geneso-3’. HERV-K loci have been identified in close proximity to several other host genes, including the breast and ovarian cancer susceptibility gene ERG41 on chromosome 17 (Ref. 33) and the glucoseGphosphate dehydrogenase locus on the X chromosomes. Such Endings might merely be a consequence of the sheer number of elements present in the normal human genome as an interaction between the HERV and the proximal gene remains to be shown. HERVs. and related retrotnmsposons, can affect normal gene expression by acting as insertional mutagens. In fact, analysis of single gene disorders has revealed relatively few examples of integrating retroid elements producing host gene knockouts. In somatic tissue a pS3 knockout in a murine Sarcoma has been observed3s. The inbred Ipr-Ipr mouse is one example in which an ERV disrupts apoptosis in the thymus leading to the development of an autoimmtne condition resembling human systemic lupus erythematosus 6LE)s. To date, in the absence of such elegant genetics, the potential association and significance of HERVs in the development of human autoimmune diseases, such as Sjogrens syndrome 6s). rheumatoid arthritis (RA) and multiple sclerosis (MS), has been the subject of much speculation but little rewards’-7-40. Ancient HERV integtations have affected the expression of adjacent host genes. Although retrovirus integration sites are widespread”, they are not entirely random-“-4‘t. Because HERVs possess LTR regions that can regulate gene transcription and are often found integrated in association with transcriptionally active sites, the potential for affecting host gene transcription becomes apparent. Such an effect by several retroelement families has been home nrtr for host genes, such as amylase, calbindin and phospholipase A2 (Refs 45-48). Many examples are known of exogenous and activated endogenous retroviruses re-integrating next to protooncoRenes and activating them*. However. few cases are found in humans, compared with gene activation
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spread of infection from activated ERV. Interestingly, both the suppressor genes are, themselves, derived from ERV. The Fu-I gene, which is more closely related to HERVs than the Fu4 gene, suppresses murine leukemia virus replication by inhibiting proviral integration and has recently heen cloned%. The Fy-4 locus, like a similar dominant inhibitor in chickens, suppresses viral replication by specifically blocking cell surface recep tars with expressed endogenous envelope product.557. Mice have a further locus, Rmcf;which acts in a similar manner to Fu4, and protects against infection by exogenous recombinant mink cell focus-forming viruses%. Mutation of key binding sites on cellular receptors might also have resulted from selection against vimemiaw. Animals
that
are
phylogenetically
distant
from
the
species bearing the ERV might not have evolved such resistance mechanisms. Thus, a class of murine retroViNSeS that cannot replicate in mouse cells on account of receptor incompatibility can propagate to high titres in human cells in culture. Such viruses are described as xenotropic? Chicken ERVs rarely replicate in their own species but readily infect cultured cells of quail, pheasant and turlley. Likewise, a cat ERV replicates in human cells, as does one from baboons, although neither replicafe in their own host soecies57 The xenotropic hoit range of some animal ERVs remained a curiositv until the advent of xenozraf% We have long known that the human tumour xe&grafts in immunodeficiem mice frequently become infected by xenotropic mouse ERV (Ref. 61), and that the feline ERV, RD114 virus, was first identified as RT activity in a human mmour cell line that had been passaged as a xenogmfr. If xenografts cm become contaminated with ERVs of their new hosts, the possibility also exists that an ERV could spread in the opposite direction, from xenograft to host. Currently, xenogmfted tissues and organs from baboons and pigs are being discussed for human use. We already know that the baboon ERV, BaEV, readily infects human cells in culture$ recently we have found that a oorcine ERV can also infect human cell&. Although these viruses do not appear to be pathogenic in the hosts in which thev are naturally endogenous, the situation could be quite humans, in whom the titres, and might spread to their co%acts.-
Gxlchlsiins
ERV genomes with ORFs and funcTional LTRs can produce complete infectious viruses when expressed. ERV activation can happen spontaneously or it can be induced experimentally with chemicals, such as 5azacytidine or j-bromo-2-deoxyuridine. Once virus is produced by an activated cell, it can spread infectiously in the tissues of its host, provided the host cells are susceptible to re-infection. The resulting vinemia is often pathogenic owing to insertion4 mutagenesis or oncogenesis. Consequently, animals harbouring infectious ERVs have evolved mechanisms to prevent re-infection with their own potentially infectious ERV genomes. Mice, for example, have dominantly acting alleles at hvo distinct loci (FL?-I and Fo-4) that suppress the
The internation and mendelian transmission of endogenous r&oviru.ses represents the most intimate of host-parasite relations. No’otonly has the virus gained a free ride through eoms of host evolution, but, on ocwsion. the host has adapted to its own use a protein encoded by the invading vimI genome. The adoption of vital genes that have phylogenetically distinct, originally infectious origins exemplifies this phenomenon. Endogenous mtroViNSeS can protect the host from more pathogenic, exogenous infection and can provide additions to the gene pool, A cost, however, can be the occasional pathogenic consequences of virus activation and expression. Modem molecular genencs has permined the delineal_iOlt of thousands of HERV benomes. The degree to which HERVs play a role in human disease and to which they can compromise the safety of retmviml-mediated gene thempy requires funher study. The risk of acquiring
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animal endogenous retrovinl infections through xenogratis also requires attention. Investigations of HERVs are still at a relatively early stage. htit promise to yield results on the complex interaction further interesting between retrovinl elements and their human host.
Sttcleic
etrd. etal. et
Acknowledgement D.J. Griffths
\Ve thank
for his critical appmisa). of
the manuscripi.
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Chwttosottto
(
I
etrd.(1988xX) Acttttw
eta/. etN/ N.L da/
,Vht
C Palfemx. D.A.WiIkinson andR.A. Weiss LUPin the Chester Bmtty Luborutoti~5, 77x Ittsfifttfe ofCmcet Rewmch. 237 F~lhnn~ Rod, Lottht, 1iK SW3 GjB.
Genetwork is a rezular column of news and information ahout Internet (pp. 121-122). resources for researchersin genetics and development Genetwork is compiled and edited with thwhelp of Steven E. Brenner (Stmctunl Biology Cent% National Institute of BioScience and Human Technology, Higashi l-l Tsukuba. Ihanki 305, Japan; [email protected]); and Fran Lewitter (Scientific Computing, Whitehead Institute for Biomedical Research, Nine Cambridge Center. Cambridge MA 02142-1479, USA;
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