The Retrovirus

By Mritika Senthil


“Not all of an individual’s DNA is obtained from their evolutionary ancestors.”


The aforementioned statement largely contradicts one of the most significant societal perceptions. Denunciation of one’s ancestors has become a common reaction to possession of genetic disorders or traits. However, can the human genome become influenced by a factor outside of inheritance? This is where the “retrovirus” plays a role. Defined as a virus composed of RNA rather than the more prevalent DNA, at least eight types of retroviruses are determined to constitute an individual’s DNA, comprising a portion of their genetic makeup. These viruses are grouped together based on the facets of their common structures and replications. There are multiple aspects that play a role in the process of retroviruses controlling alien biological functions.


Image Credit: Flickr @ Pascal

Reverse Transcription

Retroviruses begin with an enzyme, reverse transcriptase. This substance is able to generate complementary DNA (cDNA) by transcribing its RNA into DNA after entering the cytoplasm, or the thick solution that fills a cell and is enclosed by its membrane, of a host cell that it invades. This process is known as reverse transcription. As a result, the retrovirus is now reconcilable with the host cell’s genetic material. Afterwards, the process of genome integration occurs. The newly established DNA approaches the nucleus of this cell. Here, an “integrase” enzyme inserts the viral DNA into that of the host cell. Now, it has become a part of the host cell genome and its DNA is referred to as a “provirus.” The virus implements the facilities of the host, meaning that it is now also able to transcribe and translate, producing products such as viral DNA and viral RNA. These result in the creation of new virus copies.


Prior Implications

The idea of a retrovirus appears abstract until real-world examples are provided; the infectious retroviruses HTLV-1 and HIV respectively promote diseases such as adult T-cell leukemia (ATL) and AIDS. The retrovirus’s insinuation is most commonly exhibited through the spreading of HIV, which can holistically infect one’s blood, wounds, etc. Control of such ailments occurs through usage of antiretroviral drugs. Through a system known as “highly active antiretroviral therapy” (HAART), multiple drugs target different viral targets. This is the most prevalent for HIV.


A notable aspect is the idea that retroviruses can be present in species aside from humans. For example, the simian immunodeficiency virus (SIV), closely related to HIV, is a retrovirus found in gorillas and chimpanzees.


However, not all implications of this virus are negative. Human endogenous retroviruses (HERVs) have been thought to spread through human DNA during cycles of evolution. Therefore, associated genes currently constitute 1-5% of the human genome. More specifically, the idea of placental morphology in response to placental development has long been thought to be associated with this factor.


Further implications of these viruses are associated with principal technological developments that have been modified to mediate solid genetic modification of treated cells. Such uses include those in clinical gene therapy, where long-term genetic defects are aimed to be corrected, most commonly in stem and progenitor cells, and for research purposes.


Current Implications

A significant noted symptom of COVID-19 was respiratory inadequacy. This has recently been studied extensively in lab settings with the use of retroviruses, specifically the aforementioned SIV and the murine leukemia virus, due to many concurrent protein compounds. The aggregate data from these results has led to greater developments in not just an understanding of COVID-19, but of coronaviruses in general.


Based on the above information, it is clear that retroviruses are unique in a series of microorganisms. Their replications, insinuations, and applications have allowed them to prevail in mammal genomes, altering a range of areas from industries such as healthcare management to daily bodily functions.


What Did You Learn?


Q: What is the function of the integrase enzyme in reverse transcription?

A: At this point, the retroviral DNA would be reaching a place of influence for the host cell’s nucleus. The integrase enzyme inserts the viral DNA into that of the host cell it invades. As a result, it can become a part of the host cell genome and is referred to as a provirus.


Q: What are human endogenous retroviruses (HERVs)?

A: Human endogenous retroviruses (HERVs) are those which have been distributed through human DNA during the process of evolution. Their associated genes currently constitute 1-5% of the human genome. It is important to note that they are distinct from infectious retroviruses (such as those associated with acquiring HIV) and can have positive implications as well as negative ones.


References:

https://jvi.asm.org/content/78/19/10628

https://www.britannica.com/science/retrovirus

https://www.health.state.mn.us/diseases/retrovirus/retrovirus.html

https://www.healthline.com/health/what-is-a-retrovirus

https://www.theguardian.com/science/2013/jan/27/20-human-body-facts-science

https://www.medicinenet.com/script/main/art.asp?articlekey=5344

https://www.sciencedirect.com/topics/immunology-and-microbiology/retrovirus

https://www.cancer.gov/publications/dictionaries/cancer-terms/def/retrovirus

https://www.khanacademy.org/science/ap-biology/gene-expression-and-regulation/translation/v/retroviruses


Image Credits:

Public Domain, https://www.flickr.com/photos/pasukaru76/16404681476/in/photolist-qZChgN-7Nd4eP-2546K9J-7Nh32C-aJwgKX-jUUiox-251yzYJ-a5aV3L-7Nh38Q-25d3DHW-7Nh35w-dfZuzh-jDwYG-2hJf5bW-28kdquK-5D5KgZ-5Da9G5-2j2WJLs-7Nh3gy-Q7KYoQ-2iFNKia-Pz4cGn-SF6Bxh-2jqurn3-2hGrWVg-NorStk-LVEJoc-2j31Vqe-9MCeT-CLDYxA-CTsQVA-u8ZEmQ-uqMdkz-8agQ8K-uqMjVP-uqMB38-tSWFH9-uofVBS-2j31W79-ttJDiP-2h9idM5-ttyzt5-u8ZQ9A-uofACu-ttJSAR-u98AV4-25DZzxV-2juW3cZ-uofMHf-uqaSFq


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