Viruses - A Vector to Remember

Invisible killers. An unsanitary butcher in Indonesia shakes hands with a tourist who sneezes in the airport and contaminates a traveler to New York who exposes a taxi driver who dooms the city to mortality en masse. Is this your view of viruses? It certainly is the view of the average person on the street (who won't want to shake your hand after you broach the topic of viruses). Perhaps your first thought was of a computer virus and a blue screen error. Viruses are bad, right?

WRONG! Viruses can be extremely bio-awesome, if you give them a chance. They are a hot tool in biotech, and one that is under-appreciated.

Molecular FedEx

Think of a virus as a little delivery van. The virus carries packets of information from one place and delivers them to a cell. Scientists use viruses to change the genetic instructions inside a cellular factory. There are many different types of viruses. Some infect bacteria (these are usually referred to as "bacteriophage" or just "phage"). Some infect yeast cells. Some infect bird cells. Some infect mammalian cells.

Viral Life-Cycle

Here's the general life-cycle of a virus:

• Bump into a cell
• If it's a compatible cell-type, latch on to the cell membrane
• Inject genetic information into cell
• Genetic information uses the cell's machinery to produce more viruses
• Viruses build up inside cell until the cell is lysed (fancy word for "burst open")
• Viruses hang out until they bump into another cell and the cycle repeats

Take just a moment to dredge up what you know about the central dogma and appreciate how a virus fits into that framework.

• RNA is translated into proteins.
• Proteins are the workhorses of the cell. They provide structure and chemical reactions. 

All a virus does is highjack the translation machinery of a cell (viruses can't make their own proteins) by inserting their own genetic information. That information usually contains instructions to make new little viruses. It's that simple.

Viruses Are Tools

You may have noticed that in general, the cell doesn't fair so well if a virus manages to inject genetic information inside of it. So viruses really are evil, right? WRONG. They're just delivery vans. It's the type of information the van delivers that makes it good or bad for the cell.  In nature, viruses are finely tuned to self-replicate, and that's usually bad news for the cell (and for us if we catch it). However, in the lab, scientists can put whatever information they want inside the virus. Viruses are really good at putting DNA into cells. They're useful tools.

We mentioned in other posts that plasmids can be used to put new genetic information into a cell. Viruses are another way. We call "ways to put DNA into cells" "vectors". Plasmids are vectors. Viruses are vectors. Got it?

Plasmids are all well and good for simple bacterial projects. But they're not much use on living, multicellular organisms like mice or people. Viruses are not only capable of infecting living cells-they can be finely tuned to deliver DNA to specific tissues. It's possible to deliver DNA only to the heart tissue, or the liver. And it's possible to do this without harming the host tissue. A common virus for this sort of application is the Adeno-Associated Virus, which comes in most any flavor you need.

Theoretical Application

The following if fictional, but illustrates the potential of viral therapy (and why researchers are so excited about it).
 Cystic Fibrosis is caused by a genetic mutation. The mutation causes a chloride ion channel to be leaky. All this means is that cells aren't able to control how much salt is inside or outside. That causes a build-up of mucous in the lungs from the imbalanced electrolytes. The mucous invites bacterial infections, which can be fatal over time.
What if you could replace the malfunctioning gene with a functional one? What if you could fix the chloride ion channel and restore normal function to the lungs? That is exactly what viruses are good for. Just pop a functional copy of the gene in a viral vector, infect the patient, and a couple of days later, they're good as new.

Envision the Vector