When most people hear the word “virus,” a chill goes down their spine. Viruses are dreaded for their ability to knock us off our feet, and feared for their ability to kill us in our prime. Viruses are blamed for everything from a runny nose and a missed day of work to a pandemic that kills tens of thousands. Clearly, we should associate a negative connotation with this dastardly word, whose etymology links “virus” to snake venom and steaming poison, right? The answer may surprise you.
Indeed, while many viruses can make us sick, keep us home from work, and send us to the hospital -- or even the morgue if we are quite unfortunate -- there are many beneficial viruses in the world. Life as we know it would be very different, if not impossible, without viruses.
Movers and shakers: Viruses and Evolution
On the most basic level, viruses are simply small discrete packages of genetic material, whether RNA or DNA. Because these nucleic acids are the heritable material that enables evolution to occur in living things, you might expect viruses to have some role in evolution. And, indeed, this is the case.
Retroviruses, the kind that integrate their genetic material into that of their hosts, are famous for leaving parts of themselves behind in the host genome. As much as 4.9 percent of the human genome can be traced to retroviral origins, occurring as what are called endogenous retroviruses. The nucleic acid sequences of endogenous retroviruses are quite repetitious, making it possible for genes to “jump” around in the genome during DNA replication and cell division. Such jumps often occur imprecisely, resulting in the gene duplications, deletions, and translocations that are the molecular basis for evolution. It is also possible for viruses to transfer genes between hosts, when their enzymes inadvertently copy host gene sequences along with their own genomes. Such “lateral gene transfer” events are therefore a major source of genetic novelty. Without viruses and the genetic novelty they introduce, evolution simply would not be as commonplace for complicated organisms such as mammals and, in particular, humans.
The enemy of my enemy is my friend: Viruses and immunity
Viruses have a bad reputation for causing disease, but they also have a large -- and largely unappreciated -- role in preventing it. As recently discussed in Science, there are viruses that can protect us from bacterial infections. These viruses, called bacteriophages, often reside in mucus and help the body to keep mucosal surfaces free of pathogenic bacteria. These viruses appear to have co-evolved with the hosts of their hosts, possessing molecules that help anchor them to mucus and keep them near the bacteria they infect and kill.
Bacteriophages have also been used to treat existing bacterial infections, in place of pharmacological interventions. In some cases, such so-called “phage therapy” offers greater specificity, efficacy, and fewer side effects than a pharmacological intervention might. However, such therapies have not been widely used, nor approved for use by the FDA.
In addition to their use in treating and preventing infections, viruses also have a beneficial role to play when no other infectious agent is present. Indeed, viruses have recently been shown to help guard against autoimmune disorders in mice. Mice that were infected with a virus were protected from inflammatory bowel syndrome when compared to mice that were not infected with viruses or bacteria. In light of this new data, scientists have proposed the existence of a commensal or symbiotic “virome,” in addition to the long-understood commensal “microbiome.” This paradigm shift, thinking of some viruses as neutral or beneficial to their hosts, is an exciting development in the field of virology.
The war on drugs: Viruses as therapeutics
Viruses, by virtue of being well adapted to replicate in their human hosts, expertly produce the gene products that they need from within host cells. Viruses’ ability to convert host cells into factories for their own nucleic acids and proteins enables them to perform beneficial functions for the host as well, when the viruses are modified for this purpose.
So-called “gene therapy” relies on getting foreign DNA expressed in host cells, and viruses are the consummate professionals at this very task of gene expression. However, for the purpose of gene therapy, viruses are modified to express functional copies of host genes that are mutated or otherwise nonfunctional. Such treatment could potentially correct inherited genetic disorders or repair the oncogenes driving a particular cancer’s malignancy. Scientists can also make viruses that produce antiviral or antibacterial genetic sequences, coding for such proteins as antibodies; these viruses can be used to treat infectious diseases and cancers as well. Unlike many cancer treatments, viral therapy has the virtue as well of being highly targeted – because any given virus will only infect the cells that express the cell surface receptor particular to that virus, it is relatively straightforward to tune where in the body a given virus will end up. Viruses are also very good at getting therapeutic genes into difficult-to-access areas of the body, such as the brain, protecting their nucleic acid payload with a tough protein shell before delivering it to the intended cell.
Like any treatment, however, viral gene therapy does come with some caveats, complications, and drawbacks, however. In some cases, introduction of foreign nucleic acids can disrupt the homeostasis of the target cell, occasionally transforming it into a cancerous cell. If the patient has immunity to a virus related to that used in the therapy, the patient may experience a robust immune response that can be life threatening, or the patient may simply reject the therapy. Fortunately, the adenoviruses, adeno-associated viruses, and retroviruses often used in gene therapy are generally well tolerated.
