Implications to the cell physiology are diverse, and as claimed by some authors, it highlighted the significance of genome integration in the evolution of vertebrate species and other living organisms, despite the studies analyzing the role of integrated invading genes in the etiopathogenesis of autoimmune diseases and cancer (Mustafin and Khusnutdinova, 2019). Usually, a small genome is inserted into a larger genome, commonly reported among viruses and their hosts (Xavier Ruiz and Arnold, 2020). The genome integration process into another genome relies on a multi-functional enzyme complex that plays a key role, catalyzing scissions in the hosting DNA molecule and joining the inserting invading genome or parts of it, and/or, prior to integration, converting the genomic RNA into a DNA sequence. Viral genome integration, in its host chromosome, is not a common process for all viruses, neither the mechanism could be ruled out for other viruses, known for missing enzymes favoring the integration process (Gonzalez and Spouge, 2013 Hirai et al., 2019). Therefore, at the same time, but independently with Temin and Mizutani, he unraveled the mechanism of retrovirus genome integration into the host chromosome, announcing the existence and function of an enzymatic complex, the reverse transcriptase (RT), which produces and integrates a proviral DNA molecule utilizing the genomic retroviral RNA as template (Pederson, 2020).

Basically, virus genome coding sequences orchestrate the cell metabolic pathway leading to virus replication and production of progeny virus, available for continuous infection of surrounding tissue and other hosts, by gaining access to the extracellular microenvironment or by intimate cell to cell membrane extensive contacts.īaltimore wisely classified virus genomes into 7 classes, based on the mechanisms of viral replication and transcription (Koonin et al., 2021), as also confirmed Temin’s suspicion of the DNA nature of retrovirus genome during its infectious cycle in the host cell. Virus synthesizes and assembles its molecules exclusively utilizing the host cells’ biochemical machinery. So, would the integration of unconventional reverse transcribed viral RNA of SARS-CoV-2, and of other RNA viruses, as the Bornaviruses, lead to the production of transcripts and proteins inducing antigenemia and stimulating constant immune response, or else would result in the excessive activation of neighboring cellular genes with pathogenic role to the host cell? Therefore, viral capsids or solely viral gene segments, actively and selectively transported to the cell nucleus, could be found taking into account the exuberant virome reaching the cell nucleus to perform their replicative cycle. Rather than re-infection, SARS-CoV-2 gene activation, from host chromosome integrated material, would explain the detection of virus gene segments as in viremia. Rarely, but potentially occurring, the genome of non-oncogenic RNA viruses could gain access to the cell nucleus, and eventually integrate their gene segments or genome in the host chromosome as it has been postulated for the current agent of the Corona Virus Disease 2019 (COVID-19), the Severe Acute Respiratory Syndrome Coronavirus type 2 (SARS-CoV-2). Some of these viruses cause lytic infection alternated with latent and persistent infection, leading to chronic inflammation, ultimately resulting in autoimmune diseases and cancer. Quite commonly, oncogenic viruses integrate their genome in the host cell genome or interact in their episomal form.

But it could also be interpreted that the cellular genome is the source of autonomous gene segments that escaped from the cells, at some conditions, as a threat to the cell's survival. Nevertheless, during billion years long evolutionary processes, the cell's genome revealed a mosaic of viral genomes or gene segments, giving rise to speculations that the genome of any cell was constructed and shaped by the invasion of virus genomes. Abstract: Genetic information exchange between virus and host cells apparently seems to be detrimental, as pluricellular organisms could develop diseases.

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