Friday, November 20, 2009

XMRV the next craze?

So for my first official post on this blog I am going to draw on inspiration from Abbie’s erv blog from Science Blogs. Xenotropic murine leukemia virus-related virus (XMRV) is a new retrovirus that has been found in some patients that have chronic fatigue syndrome (CFS) and in some prostate cancers. Remember: correlation does not mean causation (read below). Although we understand some of this virus’ cousins, it has not been fully characterized. Due to its detection in some high-profile diseases, this virus has been kicked around in the media, bounced around in blogs, and has now become the catchall for conspiracy theorists and general nutjobs. There are a few things that need some clarification:

1. Attributing the virus to many unrelated diseases. There is no evidence that this virus causes any disease, yet. There is an association of this virus being in people with a defective antiviral gene (RNAaseL), but this could mean that they are more susceptible to infection. Unlike the most well-known retrovirus HIV, this virus cannot infect cells that are not rapidly dividing. People with CFS have markers of chronic immune activation, rapidly dividing immune cells that are good targets for this virus. Prostate cancers are groups of rapidly dividing cells. It will take a lot of work to determine what relation XMRV has to both of these stories, but there are three possibilities. A.) The virus is a causative agent. B.) The virus is marker of an underlying condition, meaning the virus happens to preferentially infect people who have certain conditions. A good example is how people with AIDS display high prevalence of specific opportunistic infections. C.) The virus infects many different people and the infection has nothing to do with the disease. It is best not to jump to any conclusions until some large-scale studies have been done. Study of this virus is very much in its infancy.

2. The anti-vaccine movement. If you give some of these people any new observation they will latch on like a leech. Some argue this virus came from contaminated cells used to produce vaccines. These are some of the facts. There are very few full-length sequences of this virus, but they are remarkably similar. Normally one associates retroviruses with lots of sequence diversity. Some people see this as proof that these viruses had a common origin. There are other retroviruses that do have more conserved sequences. In the patients where they detect this virus there are generally few copies of the virus floating around, unlike HIV, where there could be millions of copies per ml of blood. A lower error rate combined with low replication rate can cause low diversity. In addition, these viruses are rather streamlined when compared to HIV, so they do not have the same sequence-wiggle room in accessory genes. Similar viruses XMLVs were common contaminants in labs. Some may argue that that this provides an avenue to get into production lines of vaccines. However, all this shows us is that these viruses are very good at infecting rapidly dividing human cells in the absence of an immune system. Contamination in a vaccine would kill a lot of the cells, drastically lower production, and would likely be noticed at some point. Furthermore, there are very low levels of infected people (if it were in a common vaccine the rate should be much higher). Immune-compromised people have received vaccines, and this virus has not popped up and killed them.

3. No the government did not make it. I have no evidence to say someone said it but I am heading this one off.

Hopefully this clears the air on some issues.

Kevin



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