REVIEW New Insight on the Role of Extrachromosomal Retroviral DNA a Cara1 and MS Reitz Jr2
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Leukemia (1997) 11, 1395–1399 1997 Stockton Press All rights reserved 0887-6924/97 $12.00 REVIEW New insight on the role of extrachromosomal retroviral DNA A Cara1 and MS Reitz Jr2 1Basic Research Laboratory, National Cancer Institute, National Institutes of Health, Bethesda, MD; and 2Institute of Human Virology, University of Maryland Biotechnology Institute and School of Medicine, Baltimore, MD 21201, USA During infection with different retroviruses, high levels of unin- reverse-transcription process; circular with a single LTR tegrated extrachromosomal DNA accumulate in infected cells. (c1LTR), present in the nucleus of the infected cell and prob- While extrachromosomal linear DNA is the immediate precur- sor of the integrated provirus, the function, if any, of extra- ably derived from homologous recombination between the chromosomal circular DNA has been unclear. Several groups two viral LTR ends of linear E-DNA; and circular with two have attempted to address the possible function, activity, and tandem LTRs (c2LTR), present in the nucleus of the infected importance of this unintegrated DNA during the life cycle of cell derived either from the head to tail joining of the two viral retroviruses and the course of retroviral-associated diseases. LTR ends of linear E-DNA or from intramolecular autointegrat- This review summarizes recent work in this field and tries to ive events. Lately, a number of reports have suggested that the analyze some aspects of extrachromosomal forms of retroviral DNA and their possible application as a molecular biological extrachromosomal forms of viral DNA and, more specifically, tool. circular E-DNA may have a role in the replication cycle of Keywords: HIV-1; extrachromosomal DNA; integrase; transcrip- retroviruses. tion vaccinia Extrachromosomal DNA as a marker of viral replication Introduction Reverse transcription takes place in the cytoplasm of the All retroviruses enter target cells via interaction between the infected cell. In quiescent cells, the final product of reverse envelope glycoprotein present on the retroviral envelope and transcription, linear E-DNA is found only in the cytoplasm of the specific receptor(s) exposed on the outermost membrane the cell, which is bound to several proteins of the preinte- of the target cell. This interaction determines the fusion gration complex while it is absent inside the nuclei (Figure between the viral envelope and cellular membranes, thus 1).6–9 After the cell has been activated, the fate of linear inducing the release of the viral core into the cytoplasm of E-DNA differs between lentiretroviruses and oncoretroviruses. the infected cell. After the viral core has been internalized In lentiretroviruses, such as the human immunodeficiency in the target cell, reverse transcriptase mediates cytoplasmic virus type 1 (HIV-1), linear E-DNA is translocated upon cellu- reverse transcription of the viral genomic RNA into double- lar activation, both in dividing and nondividing cells, into the stranded linear complementary DNA by using the free nucleo- nucleus of the host cell. This translocation is an ATP-depen- tides and ions present in the cytoplasm of the cell and the dent active process involving the presence of several kary- tRNA primer bound to the primer binding site of the retroviral ophilic elements present in at least two proteins of the preinte- RNA. Once completed, double stranded linear complemen- gration complex (p17gag and vpr) (Figure 1a).3,10–12 In tary DNA enters into the nucleus as part of a preintegration oncoretroviruses, such as the Moloney murine leukemia virus complex which, at the very least, contains the viral enzyme (MMLV), linear E-DNA remains in the cytoplasm until nuclear integrase (IN). The linear viral DNA is integrated into host division during the M phase of the cell cycle breaks down the cellular DNA after both ends of linear DNA have undergone nuclear envelope and permits nuclear import of the preinte- loss of two bases at each end.1 Such integrated proviral DNA gration complex together with unintegrated linear E-DNA by is then competent for transcription of viral mRNAs that, after a passive mechanism (Figure 1b).9,13 It has been suggested that export into the cytoplasm, are translated into viral proteins to this difference may be attributable to a difference in the size produce new infectious viral particles. of the preintegration complexes of each retrovirus,9 since Typically, when reverse-transcribed linear extrachromo- translocation of preintegration complexes through the nuclear somal viral DNA (linear E-DNA) reaches the nucleus of the pore seems to be size dependent. However, the karyophilic infected cell, it can either integrate in the proviral form or elements encoded by HIV-1, which determine nuclear translo- circularize and remain as extrachromosomal DNA (circular E- cation of the viral preintegration complex, appear to be absent DNA).1–3 While it is known that linear E-DNA is the substrate in the MMLV preintegration complex. Thus, different retro- for integration,4 circular E-DNA is thought to be simply a side viruses may have developed different capabilities to infect tar- product of the reverse-transcription process.1,5 get cells in different phases of the cell cycle. Therefore, retro- E-DNA is found in infected cells in at least three different viral replication will either require cellular activation in the forms: linear, present in the cytoplasm and in the nucleus of absence of cell replication, as in the case of lentiretroviruses the infected cell and constituting the final product of the (HIV-1),3,8,12 or cell replication, as in the case of oncoretro- viruses (MMLV).9,13 In either case, however, circular E-DNA forms are present only in the nucleus of the infected cells. For this reason, detection of circular E-DNA in the nuclei of the Correspondence: A Cara, Mount Sinai Medical Center, Division of Infectious Diseases, One Gustave Levy Place, Box 1090, Atran 6-620, infected cells is considered to be a marker for the nuclear New York, NY 10029, USA translocation process. In addition, circular E-DNA is always Received 4 December 1996; accepted 23 May 1997 present during the acute phase of infection in vitro and in Review A Cara and MS Reitz 1396 Figure 1 Schematic representation of the early events of replication in (a) lentiretroviruses and (b) oncoretroviruses. vivo, and, for this reason, it is also considered a marker of an additional step to the simple amplification of DNA. Ampli- active viral replication.14–16 fication of circular E-DNA could easily be substituted for intra- Several studies of E-DNA have been performed in patients cellular RNA analysis as a measure of viral replication, thus infected with HIV-117–19 or human T cell leukemia virus type saving time and eliminating reverse transcription of RNA into I20,21 as well as in animals infected with simian immunodefi- cDNA. For these reasons, measurement of the level of circular ciency virus (SIV),22 feline leukemia virus23, MMLV,24 equine E-DNA during the course of infection, since it is a marker of infectious anemia virus,25 spleen necrosis virus,26 avian viral replication, appears to be a simple tool to follow the leukosis virus,27 avian reticuloendotheliosis virus,28 and bov- course of retroviral infections in vivo. However, due to the ine leukemia virus.29 In all cases, recovery of E-DNA was con- lower levels of circular E-DNA, with respect to integrated current with viral replication. Moreover, a more active viral DNA and viral RNA during the course of the infection and replication corresponded with an increased amount of E-DNA the consequent difficulties in detecting such smaller amounts and particularly circular E-DNA, suggesting a connection of viral DNA, we would argue against such an approach. between the stage of any associated diseases and the amount of E-DNA. With regard to HIV-1 infection, levels of extrachromosomal Extrachromosomal DNA is transcriptionally active viral DNA have been correlated with HIV-1 expression and CD4+ cell decline in vivo.18,19 Moreover, a decrease in HIV-1 During the past few years, several reports have suggested that unintegrated DNA recovery has been associated with antire- circular E-DNA is not simply a dead end product of the troviral therapy.30–32 It has been suggested that the level of reverse transcription process but rather participates actively in viral RNA is a better marker of HIV-1 replication and stage of the course of infection. Indeed, circular E-DNA has been the disease than is the decrease in CD4+ cell counts or the shown to be transcriptionally active, although at low levels level of total viral DNA.33 However, although amplification and only for a short period of time.34–40 In itself, this is not of extracellular RNA is necessary to quantitate the levels of surprising since both linear and circular E-DNA possess the serum-associated virus, testing for viral replication by analyz- promoter and termination signals essential for synthesis of ing the levels of intracellular RNA using RT-PCR introduces viral mRNA and consequently viral proteins. The fact that the Review A Cara and MS Reitz 1397 transcriptional potential of E-DNA appears to be quite low in the IN protein. With respect to the first possibility, it is very suggests that the structural conformation of this DNA does not unlikely that IN protein binds circular E-DNA. Indeed, while make it an efficient template for viral expression. several proteins of the preintegration complex are bound to Indeed, several lines of evidence suggest that the transcrip- linear E-DNA, only histones seem to bind circular E-DNA.41,42 tional activity of unintegrated circular c1LTR and c2LTR E- Possibly, when linear E-DNA circularizes to form either c1LTR DNA and linear E-DNA is much lower than that of its inte- or c2LTR E-DNA, the proteins of the preintegration complex grated counterpart.40 First, transcriptional activity of E-DNA are displaced from the LTR termini, and the circular forms are appears to be impeded by several proteins of the preinte- not recognized as suitable substrates for integration.