HI virus (graphic)
Retroviruses ( Retroviridae ) are a large family of enveloped viruses with single (+) - strängigem- RNA - genome , whose genetic information (ss (+) - RNA) accordingly, in the form of ribonucleic acid is present. In contrast to “normal” RNA viruses, however, the RNA of retroviruses must first be transcribed into a DNA molecule by means of reverse transcription before it can be incorporated as such into the genome of the host cell and become active there.
Retroviruses can be broadly divided into simple and complex retroviruses. Furthermore, a distinction is made between infectious exogenous retroviruses (sometimes abbreviated as XRV ) and endogenous retroviruses (ERV), which are inherited vertically via the germline and thus become part of the host cell genome.
Retroviruses primarily infect animal cells and are ubiquitous in vertebrates : They infect mammals , birds , amphibians , reptiles and fish , but are usually very host-specific . They include the causative agents of some widespread infectious diseases that occur pandemically or epidemically in both humans and animals . HIV and HTLV-1 , among others, are known to cause disease in humans .
Historically, the retroviruses were initially after its electron microscopic retroviruses D phenotype in type A, B, C or divided . A classification that also took into account biochemical properties and cell tropism followed later . The classification differentiated oncornaviruses , spumaviruses and the lentiviruses .
The current, currently binding taxonomy by the International Committee on Taxonomy of Viruses (ICTV) divides the retroviruses into two subfamilies and eleven genera as follows , primarily due to their genetic relationships :
Family : Retroviruses ( Retroviridae )
- Subfamily : Orthoretroviruses ( Orthoretrovirinae )
- Subfamily : Foamy or Spumaretroviruses ( Spumaretrovirinae )
Five retroviruses are known to date in humans: human T-lymphotropic virus 1 (HTLV-1) and human T-lymphotropic virus 2 (HTLV-2), both delta retroviruses; Human Immunodeficiency Virus -1 (HIV-1), Human Immunodeficiency Virus Type II (HIV-2), both lentiviruses; and Xenotropic murine leukemia virus-related virus (XMRV), a gammaretrovirus. The human retroviruses are so closely related to those of other primates that both groups are often combined under the name primate retroviruses . It is assumed that the corresponding human retroviruses arose from the transmission of monkey retroviruses to humans. In the case of HTLV-1 and HTLV-2, this transmission probably took place thousands of years ago, for HIV-1 and HIV-2 it was probably in the 20th century. The foamy viruses have been split up into different genera and now form a subfamily of the retro viruses (ICTV status November 2018).
Simple and complex retroviruses
The subdivision into simple and complex retroviruses takes place according to the genome organization or the translation of accessory proteins. The former include alpha, gamma, epsilon and most beta retro viruses, the latter the delta retro viruses as well as lenti and foamy viruses.
External systematics and pararetroviruses
The Retroviridae include with other virus families ( Belpaoviridae , Metaviridae and Pseudoviridae ) to the reverse transcribed RNA viruses of the virus order Ortervirales which retain the Caulimoviridae as reverse-transcribed DNA include viruses. The Caulimoviridae are sometimes classified as pararetroviruses ( Baltimore group 7) with the not closely related, but also reverse transcribing DNA viruses of the Hepadnaviridae .
History of retrovirology
Diseases such as bovine leukosis or Jaagsiekte in sheep had been known in domestic animals since the 19th century, but their cause remained unclear. In 1904 the first disease caused by retroviruses, infectious equine anemia , was shown by two French veterinarians, Vallée and Carré, to be transmitted to other horses with a filtrate. Oncogenic (tumor-causing) retroviruses have been studied since 1908, when the Danish pathologists Vilhelm Ellermann and Oluf Bang showed that cell-free filtrates could transmit chicken leukemia to other chickens and thus described the first contagious cancer. This virus is now known as avian leukemia virus (ALV), but initially the leukemia was not considered true leukemia and leukemias were not considered true tumors. These early studies went largely unnoticed within the scientific community, and it was only much later that their relevance in relation to retroviruses could be recognized. In 1911, Peyton Rous found that filtered extracts from chicken sarcoma could infect healthy chickens, which then also developed tumors . The virus was later named Rous sarcoma virus (RSV) after him and Rous received the Nobel Prize in 1966, 54 years after it was first described . It was not found until 1961 that Rous sarcoma viruses contain ribonucleic acid (RNA), which is why (until 1974) they were referred to as RNA tumor viruses .
The discovery that viruses can cause tumors was also confirmed in mammals in 1936: John J. Bittner described the mouse mammary tumor virus (MMTV). In 1951 the murine leukemia virus (MLV) was isolated and the vertical transmission from parents to offspring was described for the first time . In 1964, Howard M. Temin proposed the provirus hypothesis because it was observed that cells that were "transformed" by RSV (acquired tumor properties) retained the transformed properties even in the absence of the virus. For this reason, Temin postulated, based on temperate bacteriophages , of which it was already known that they could integrate into the genome of their host, that the RNA tumor viruses do the same. As early as 1960, André Lwoff suggested that DNA tumor viruses ( polyomaviruses ) could integrate into the genome of their host. In 1968 it was shown that this assumption is correct. The fact that RNA tumor viruses can also be inherited via the germline was still considered bizarre.
Endogenous retroviruses were discovered in the late 1960s. The assumption that entire viral genomes are passed on by their hosts according to Mendel's rules was a completely new idea, and Temin's provirus hypothesis was still not generally accepted, in some cases even considered impossible. The reverse transcriptase , is transcribed by RNA in DNA was detected in 1970, and the family of RNA tumor viruses renamed as a result of 1974 in retroviruses. After the discovery of reverse transcriptase, Temin's provirus hypothesis finally proved to be correct.
The first viral proteins were also described at the beginning of the 1970s and, in the course of the following years, the replication cycle of the retroviruses was gradually elucidated. In 1978 the LTR regions ( Long Terminal Repeats ) were discovered in the genome of the retroviruses, two years later a jumping scheme was proposed for the complex process of reverse transcription. The technique of DNA sequencing , which emerged in the early 1980s, led to the first publication of the complete genomic sequence of a retrovirus, the Moloney murine leukemia virus , in 1981 .
In 1980, the human T-cell leukemia virus type 1 (HTLV-1), the first retrovirus to infect humans, was first described after many years of unsuccessful searches for retroviruses in all possible human tumor tissues. A short time later, Luc Montagnier and Françoise Barré-Sinoussi (Nobel Prize for Medicine 2008) discovered HIV-1, HIV-2 followed in 1986. Since 1988 at the latest when it became clear that HIV is the cause of the immunodeficiency disease AIDS , retrovirology developed from a rather short one exotic basic research on the most intensively researched area in virology with great importance for health sciences .
Infectious retrovirus particles have a diameter of about 100 nm. They have a capsid that is surrounded by a virus envelope that has been pinched off from the cytoplasmic membrane of the host cell and is interspersed with viral glycoproteins, as well as a "core" within the capsid made of other proteins and a ribonucleic acid complex.
The single-stranded RNA genome of the retroviruses is linear and 7-12 kilobase pairs (kb) in size. Retroviruses are the only RNA viruses that are diploid ; H. each retrovirus has an extra copy of its genome . They are from the host's own transcription - enzymes translated and synthesized and require a specific cellular tRNA . The proviral genome of a simple retrovirus usually contains three genes and two long terminal repeats (LTRs), which are located at the beginning and at the end, and which contain information to control the expression of the viral genes. The three genes are gag ( group-specific antigen ), pol ( polymerase ) and env (envelope). gag encodes the matrix , capsid and nucleocapsid proteins. pol codes for the viral enzymes protease , reverse transcriptase (with RNase H) and integrase . With beta and delta retroviruses, the protease has its own reading frame ( pro ) and with alpha retroviruses, the information for the protease is in the gag gene. env encodes the proteins of the envelope. Regarding regulatory sequences, there is a sequence designated by e (psi) in the 5 'area , which is a signal for the packaging of the RNA into the virus particles, a primer binding site (PBS) to which the respective tRNA can attach and a promoter. In the 3 'area there are one or more polypurin tracts , which are essential for reverse transcription.
Complex retroviruses such as B. the HI virus belonging to the lentiviruses, the HTLV belonging to the deltaretroviruses or the foamy viruses contain other regulatory genes, which are referred to as accessory genes . For HIV-1 these are tat , rev , vif , nef , vpu and vpr , for HTLV-1 rex , rof , tax , tof and for foamy viruses tas and bet .
The life cycle of a retrovirus consists of several steps: infection of the cell, reverse transcription, overcoming the nuclear envelope, integration into the host genome, expression of the viral proteins and the RNA genome and the formation of new virus particles.
Entry into the host cell
After the glycoprotein of the virus envelope has bound to its cellular receptor (s) , the viral membrane fuses with the membrane of the cell and releases the capsid into the interior of the cell, the cytoplasm. What happens to the capsid in the cytoplasm has not yet been clarified in detail; it probably breaks down into its individual building blocks and releases the proteins contained inside, such as reverse transcriptase and the RNA genomes, into the cytoplasm of the host cell.
Main article: Reverse transcriptase
Retroviruses are the only single-stranded plus-strand-oriented RNA viruses in which the genome cannot be used immediately as a template ( mRNA ) for infection : When the virus has introduced this RNA into the infected cell , the RNA has to be transcribed into double-stranded DNA . This process is called reverse transcription . The virus also brings reverse transcriptase with it in its virus particles. This first transcribes the single-stranded RNA of the virus into single-stranded DNA and then into double-stranded DNA. With reverse transcription, the two LTR sequences are generated, which are essential for the further course of the infection.
Normally, transcription occurs on the DNA as a template , with a complementary strand of RNA being synthesized; The retroviruses and the retroelements (also called class I transposons ) are an exception . Since the process is relatively imprecise due to the lack of proofreading capability of the reverse transcriptase, frequent mutations of the virus occur. These enable the virus to adapt quickly to antiviral drugs and thus develop resistance .
Overcoming the nuclear envelope
Some types of retroviruses, for example gamma retroviruses, can not actively penetrate the nuclear envelope . They therefore only infect cells that are dividing and use the moment of cell division for integration when the genome is not protected by the nuclear envelope. Other genera such as the alpharetroviruses and above all the lentiviruses can actively overcome the nuclear envelope and thus also infect dormant cells . Entry into the cell nucleus occurs after the pre-integration complex (PIC) has formed in the cytoplasm. Since the nuclear pores are smaller than the PIC, which is about the size of a ribosome , it must be an active transport process. Both cellular and viral proteins are involved in this process, which is not yet fully understood. A widely accepted model describes entry into the cell nucleus through the nuclear pores with the help of karyophilic signals from the proteins contained in the PIC.
Integration into the host genome
Main article: Integrase
The integration of the viral genome into the host genome is an essential step in the replication cycle of the virus. It is catalyzed by an enzyme called integrase , which is found in all retroviruses and retrotransposons . The integrase binds to the viral and the host DNA and forms a complex with these, which is referred to as a pre-integration complex (PIC, from English preintegration complex ). Theoretically, the integration can take place at any location in the host genome, but depending on the type of retrovirus, certain preferred chromosomal regions appear. What exactly influences the place of integration has not yet been fully clarified, but it is certain that the amino acid sequence of the integrase plays a role, which results in specific possibilities for interaction between integrase and factors in chromatin . The figure on the right shows the sequence of DNA strand breaks and subsequent reassembly during the integration of the viral genome. The integrase monomers are shown in gray, the viral DNA in red and the host DNA in black, the red dots the 5 'ends of the viral DNA:
- The viral cDNA is bound to the integrase and is part of the pre-integration complex.
- The integrase removes two nucleotides from the 3 'ends of the viral DNA, creating 5' overhangs.
- The integrase cuts the host DNA at a random location so that 5 'overhangs of 4-6 nucleotides are formed, and connects the truncated 3' ends of the viral DNA with the host DNA.
- The base pairing of the host DNA is broken in the affected area.
- DNA repair enzymes supplement the 4–6 bases of the other strand of the host DNA and ligases finally connect their two free ends with those of the viral DNA.
- During the process, the virus genome loses the two terminal nucleotides, while the 4–6 bases of the host DNA are duplicated and then flank the provirus.
Expression and Particle Formation
After integration, cellular transcription factors and the cell's RNA polymerase II are recruited to transcribe the proviral DNA. The promoter and enhancer structures required for this are located in the LTR of the provirus. In complex retroviruses, some viral proteins (e.g. tat ) also act as transcription enhancers. Different mRNAs arise through alternative splicing.
The resulting mRNAs are transported into the cytoplasm. This is where the various viral proteins are translated . The Env proteins are synthesized on the membrane of the endoplasmic reticulum, so that the Env proteins are anchored directly in the cell membrane, where they assemble to form trimers. All other virus proteins are synthesized on free ribosomes . The Gag and Gag / Pol precursor proteins are myristylated at the amino terminal end and attach to the cell membrane. Particle formation then takes place on the cell membrane: Gag and Gag / Pol precursor proteins accumulate and interact with each other and with glycoproteins in the cell. Only the unspliced mRNAs from which Gag and Pol were translated have the packaging signal Psi and the leader sequence. With the help of the Psi signal, the mRNAs bind to the zinc finger motifs of the nucleocapsid proteins - this ensures that only unspliced and thus complete genomes are packed into the virus particle. When it comes into contact with the mRNAs, the membrane on the cell surface collapses and cuts off an immature virus particle. Only within this particle does the viral protease begin to assemble into dimers, to cut itself out of the precursor proteins in an autocatalytic cut and then to break the Gag and Gag / Pol precursor proteins into the individual components (matrix, capsid, nucleocapsid, reverse transcriptase and Integrase). Inside the particle, the capsid proteins assemble to form a conical capsid. Only at the end of this ripening process is the particle infectious.
Endogenous retroviruses and evolutionary development
The exact time of formation of the first retrovirus-like particles is unclear, the oldest sequences suggest an age of at least 250 million years, presumably they are much older. Retroviruses likely evolved from retrotransposons . This means that they represent an infectious, further developed form of the retro elements . According to this, reverse transcription would be one of the oldest mechanisms in retrovirus development, which may already have emerged in the RNA world . In any case, there is great similarity between retroviruses and the transposons from different living organisms, for example the Ty elements from baker's yeast and the Copia and Ulysses elements from Drosophila melanogaster . These retrotransposons encode a reverse transcriptase and have a structure similar to that of the retroviruses.
Evidence that the organization of DNA in bacteria differs fundamentally from that in archaea and complex cells ( eukaryotes ) give rise to the thesis that their cellular ancestors (LUGA or LUCA) still belonged to the RNA world . The storage of genetic information in DNA is then seen as a skill that was initially 'invented' by retroviruses and which cellular organisms have acquired several times through transmission of them. This results in the bacteria on the one hand, and the archaea and eukaryotes on the other. (The basic structure of the ribosomes as protein factories and the genetic code, on the other hand, agree so well in all cellular organisms that the LUCA must already have it.)
Integration into their host's genome is one of the most unusual and notable features of retroviruses. The large number of similar sequences in vertebrates and retroviruses shows that retroviruses have very often infected the cells of the germline of their hosts in the past . Retroviruses inherited in this way to the offspring are referred to as endogenous retroviruses (ERV) in order to distinguish them from the horizontally transmitted, exogenous retroviruses . With the increasing number of sequenced organisms , more and more endogenous retroviruses were discovered. The amount of retroviral DNA in vertebrates varies between 5 and 10%, the human genome consists of about 8% of retroviral sequences. These data provide insight into the long host-virus coevolution. With the endogenous retroviruses, the infectious viruses that emerged from transposons became parts of the genomes again.
So far, 31 different ERV "families" have been described in the human genome, which probably go back to 31 different cases of germline infections by retroviruses (in English referred to as genome invasion event ). This baseline event was followed by an increase in the ERV copy number, either by reinfection of the germ line cells or by retrotransposition within the cell. Over the generations, the activity of the ERVs continues to decrease, as mutations accumulate and entire sections of the ERVs can be lost, until finally the activity of the viruses ceases completely. The age of the individual ERV families or ERV lines can be estimated from the size and shape of the phylogenetic pedigrees of the lines. Most of the human ERV lines (HERV) therefore originated before the evolutionary development of Old World and New World monkeys about 25 to 30 million years ago.
Diseases caused by retroviruses
Retroviruses infect many different living beings. The species affected range from mussels to humans, with most being found among vertebrates . Retroviruses cause a large number of different diseases in their hosts, including tumors (lymphomas, sarcomas ), neurological diseases and immune deficiencies . Some of these diseases cause great damage to agriculture by affecting farm animals or causing human pandemics ( AIDS ). Other infections remain asymptomatic, which is why these retroviruses are considered non-pathogenic.
Some diseases in rodents induced by retroviruses are model systems with which the infection mechanisms of the retroviruses and the development of the tumors caused by some retroviruses can be examined in detail. Modern tumor biology is partly based on data that could be generated on the basis of these models.
Very many oncogenes known today were first discovered in experiments with animal retroviruses that had incorporated these oncogenes into their genome. Examples are:
- SRC - Rous Sarcoma Virus (RSV), en. Rous sarcoma virus
- ABL - Abelson Murine Leukemia Virus (Ab-MLV), en. Abelson murine leukemia virus
- KRAS - Kirsten Murines Sarcoma Virus (Ki-MSV), also called Kirsten Murines Leukemia Virus (Ki -MLV), en. Kirsten murine sarcoma virus (other species than (Ab-) MLV)
- HRAS - Harvey Murines Sarcoma Virus (Ha-MuSV), en. Harvey murine sarcoma virus
- KIT - HZ4-feline sarcoma virus (HZ4-FeSV)
- ERBB - Erythroblastosis Avian Leukosevirus (Erythroblastosis-ALV), ALV en. Avian leucosis virus
- MYC - MC29 Myelocytoma Avian Leukosis Virus (MC29-ALV), en. Avian myelocytomatosis virus 29
- CRK - CT10 Avian Sarcoma Virus (CT10-ASV), en. Avian sarcoma virus CT10
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