Retrovirus: Definition, Structure, and Diseases

A retrovirus is a kind of virus that utilizes RNA as its hereditary component. If a retrovirus does infect a cell, then it inserts an imitation of its RNA genome. Once it gets inside the cytoplasm of the host cell, the virus utilizes its reverse transcriptase enzyme for producing DNA. After this, the novice DNA is incorporated into the genome of the host cell through an integrase enzyme. At this end, the retroviral DNA is called a provirus. 

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Various retroviruses are capable of causing human diseases, like AIDS and some kinds of cancer. To know what is a retrovirus, people must know that retroviruses have got different subfamilies; they contain three fundamental groups:

• Oncoretroviruses (oncogenic retroviruses)

• Spumaviruses (foamy viruses)

• Lentiviruses (slow retroviruses)

The oncogenic retroviruses can cause some diseases like cancer. The slow retroviruses can cause serious immunodeficiency and death too in humans as well as other animals. The spumaviruses or foamy viruses tend to be benign, and they aren’t connected to any disease in animals and humans.

Retrovirus is a virus that belongs to the family of Retroviridae. It characteristically carries the genetic blueprint in the form of ribonucleic acid (RNA). Retrovirus is named after an enzyme known as reverse transcriptase. The reverse transcriptase transcribes the RNA into DNA (deoxyribonucleic acid). It is a process that constitutes a reversal of the actual direction of cellular transcription from DNA into RNA. The reverse action makes the genetic material from a retrovirus to permanently incorporate into the DNA genome of the infected cell. The enzyme is popularly used in biological sciences to synthesize genes. The best example of a retrovirus is Human Immunodeficiency Virus (HIV).

Retrovirus Structure

Retrovirus virions consist of the outer lipid envelope of glycoprotein. The virions contain two identical single-stranded RNA molecules that are present as a dimer. Although the virions do not have the same biology or morphology, their components are very similar.

Some of the main virion components are:

1. Envelope

It is composed of lipids and glycoprotein encoded by the env gene. The retroviral envelope conducts three distinct functions.

• Protection from the lipid bilayer

• Allowing the retrovirus to get in and out of the host cells through endosomal membrane trafficking.

• Directly enabling cells by fusing their membranes.

2. RNA

It has a dimer RNA with a cap of 5’ end and a poly tail at 3’ end. The RNA genome has terminal noncoding regions vital in replication and internal areas that encode virion protein for gene expression.

3. Proteins

It contains gag proteins, protease (PR), pol protein, and env proteins.

• The group-specific antigen (gag) is a significant component of the viral capsid. It possesses two nucleic acid binding domains, including matrix (MA) and nucleocapsid (NC).

• Protease is differently expressed in different viruses. During maturation of virion, it functions in proteolytic cleavages to produce mature gag and pol proteins.

• Env proteins are responsible for the entry of virions into the host cell. Due to the functional copy of env gene retroviruses are distinct from retroelements.

Retrovirus Life Cycle

Retroviruses have a single-stranded RNA genome that transforms into a unique form of replication. After it has entered the host cell, a reverse transcriptase enzyme synthesizes a double-stranded DNA from the RNA genome of retroviral. The copy of the DNA genome gets into the host genome inside the nucleus via an enzyme called integrase. As a result, the retroviral genome is transcribed into RNA whenever the host genome transcribes, allowing retrovirus to replicate. 

Retrovirus Replication

Step by step replication of a retrovirus

1. Retrovirus infects normal cells

2. Viral RNA is introduced in the host cell

3. Reverse transcription takes place

4. Viral DNA produces reverse transcriptase

5. Genetic material enters the host cells nucleus

6. Viral DNA integrates into the host genome

7. Viral genes are transcribed and translated

8. Virus particles gathers and come out of host cell

9. A new virus can infect other cells.

Retroviral Vectors

A retroviral vector consists of proviral sequences that accommodate the gene of interest for incorporating both into target cells. The vector contains cellular and viral gene promoters like CMV promoters to increase the expression of a gene of interest in the target cell. The use of packaging cells has been the most crucial advancement in vector technology.

Usually, the packaging cells are fibroblast derivatives that contain sequences of coding DNA independently, known as DNA plasmids, expressing viral gene products like gag and pol. The virions containing the vector genome have produced that bud off into the culture medium when the retroviral vector, along with the gene of interest is introduced into the packaging cells by techniques of non-viral transfections. They can infect and stably integrate with the genome of dividing target cells. The retroviral vector cannot replicate further as it does not encode the viral structural proteins that are provided by the packaging cell.

Difference Between Virus and Retrovirus

1. Genetic Material       

• Retrovirus – they have RNA as genetic material

• Virus – they have RNA as well as DNA as genetic material

2. Host Cells

• Retrovirus – retrovirus do not destroy the host cell

• Virus – virus can destroy host cell once they start replicating

3. Reverse Transcription

• Retrovirus – retrovirus undergoes reverse transcription to convert the RNA into DNA.

• Virus – viruses do not undergo reverse transcription processes.

Retrovirus Examples

Some retroviral diseases:

A retroviral disease is a highly common thing, and commonly, three retroviruses affect humans. They are:

Numerous retroviruses do cause some serious diseases in humans, birds, and other mammals. Among many retrovirus examples, human retroviruses are one. They comprise HIV-I, HIV-2, and AIDS. Again, HTLV (human T-lymphotropic virus) causes serious diseases in humans. The MLVs or murine leukaemia viruses are the real causes of cancer in mouse hosts. In molecular biology, retroviruses turn into worthwhile research tools, and they are used in some gene delivery systems efficiently.

• HIV – This disease is transmitted via the sharing of needles and bodily fluids. Again, children too can transmit the virus from their mothers through breastfeeding or childbirth. As HIV attacks as well as destroys the CD 4 T cells, and they are vital for aiding the body to fight infections, the immune system becomes weaker and weaker. 

When an HIV infection is not managed via medication, then a person can form AIDS or acquire immunodeficiency syndrome. AIDS is caused by retrovirus, and it is considered the last phase of an HIV infection and it can result in the growth of tumours and opportunistic infections. The worst thing is they always turn out to be life-threatening.

Human T- cell lymphotropic virus (HTLV) type 1 and (HTLV) type 2

• HTLV (Human T-cell lymphotropic virus) types one and two – Both HTLV 1 and HTLV 2 are closely-connected retroviruses. Commonly, HTLV1 is found in countries like some parts of America, the Caribbean, and Japan. HTLV1 is related to the growth of T cell leukaemia, and it is also connected with a kind of neurological disorder that affects people’s spinal cord, named HTLV1-connected tropical spastic paraparesis.

• HTLV2 is commonly found in South, Central, and North America, and it gets transmitted as HTLV1. Again, it is connected to some neurodegenerative diseases as well as the growth of some blood cancers.

Retrovirus causes

Retroviruses have undergone an explosion in the last four decades. The term “retrovirus' 'means it behaves a little backward from the actual way, and so, people think about genetics. Hence, retroviruses have got an RNA genome. When they get inside cells that RNA was reverse-transcribed into DNA, then it goes backwards. After this, the DNA gets inside the cell’s genome, and when the cell splits, it does copy this, and then it starts expressing RNA. 

Various types of retroviruses are linked with diseases, and they include AIDs, leukaemia, and cancer. Retroviruses are tamed for many years for use in gene therapy. Hence, it becomes possible to extract all the genes that permit the retroviruses to imitate themselves. And so, when you utilize the integrating capability of retroviruses, then you can take something that can commonly turn harmful to people and also utilize it in the form of a therapeutic vehicle for making them better.

What is retrovirus genetic material?

The majority of the viruses have either DNA or RNA as their genetic component, and then the nucleic acid might be double- or single-stranded. The whole infectious virus particle, known as a virion, comprises the nucleic acid besides an exterior shell of protein.