Found the reason why 10% of the human genome is made up of retrovirus genes
Last reviewed: 16.10.2021
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Virologists have discovered a mechanism that allowed retroviruses to reproduce effectively over millions of years as an integral part of mammalian DNA.
The main task of viruses is the transfer of its own genes to subsequent generations. In an effort to multiply at any cost - all viruses do not stand out from other living organisms, including humans.
The difference consists in the exceptional simplicity of the device of the virus, which is reduced to one molecule - the carrier of the genome, viral RNA, DNA or in some variants of their combination. Another difference between the virus is the dual state of the virus - the "dead" extracellular state of the virus particles, or the virions that show little or no independent activity, and "alive" when the virus enters the host cell and begins to multiply, integrating into the cellular mechanism of DNA transcription.
But, it turned out relatively recently, with the discovery of endogenous retroviruses, in some viruses the stage of the virion may be absent.
These viruses transmit their own genes to subsequent generations, without leaving the cell (hence their name - endogenous, in other words, intragenic), and their genetic program, embedded with cellular DNA, is considered an integral part of the host genome.
It has now been established that about 8-10% of the human genome consists of nucleotide sequences of retroviruses that infect our evolutionary ancestors tens of millions of years ago.
In other words, one-tenth of human DNA is a retroviral genes several millions of years old ("retro" means that these viruses use the reverse transcription mechanism to replicate their own genome: first, the viral enzyme reverse transcriptase begins to synthesize one strand of DNA on viral RNA, after this already synthesizes the second on this thread, and then this viral DNA, having penetrated through the shell of the cell nucleus, is built into the host cell and works as a matrix for the synthesis of viral RNAs already by the forces of the host th cell).
Researchers from the Oxford Institute, together with the Center for AIDS Research of Aarano Diamond (New York, USA) and the Rega Institute (Belgium) decided to learn the mechanism by which ancient retroviruses could register in our genes in large numbers.
For this purpose, they studied the genomes of 38 mammals. From these genomes, they isolated sites that had sequences of retroviral nucleotides, and then compared them in silico ("in silicon," in other words using specialized computer-mathematical methods) for the similarity, difference and part they occupy in the viral portion of DNA.
As the analysis demonstrated, in a certain category of endogenous retroviruses at some episode of their evolution, the env gene responsible for the protein, which helps the virus to penetrate the cell, was lost.
Losing the ability to infect other cells did not mean losing the opportunity for self-replication, only now the entire lifetime of the virus began to pass inside the host cell parallel to its own life span using viral mobile DNA sites - retrotransposons.
Although the most interesting is that with the loss of certain retroviruses of the infectious function, these viruses quickly increased their representation within the genome, which resulted in the predominance of the genetic material of viruses in mammalian DNA.
Comparing different genomes, the researchers summarized the versatility of this phenomenon: the loss of the infecting possibility gave a 30-fold increase in the amount of viral material.
Are endogenous retroviruses dangerous for health?
With the task of spreading their own genes, viruses are coping superbly, having registered themselves in human DNA as passengers without damaging transport. For owners, they are, in most cases, not pathogenic, not infectious, do not form virion particles, in other words, they do not infect anyone, and are present under regulatory control of cellular DNA transcription.
If you look from a different angle, some endogenous retroviruses (like some exogenous ones that cause, for example, Raus sarcoma, lymphoma and myelopathy) have oncogenic potential and have all the chances to stimulate the development of cancer, although it triggers processes that force the body to pay for such risks, it is unclear. The study of "fossil" viruses in our genome is just beginning, so the most cognitive discoveries that make us look at our body from a completely different angle are still ahead