Monday, April 03, 2006

Dog bites man: Germs cause prostate cancer, mental disease   posted by agnostic @ 4/03/2006 10:45:00 AM

Carl Zimmer describes a new paper from PLoS Pathogens which demonstrates a causal link between a virus and prostate cancer (full article here). Nothing will surprise readers of Paul Ewald's Plague Time, the popularized version of a journal article he co-authored with Gregory Cochran (google "infectious_causation_of_disease.pdf"). The basic logic is simple: diseases which have been around for awhile, are common, and impose fitness costs should be weeded out by natural selection if they were genetic in origin; so, such diseases are likely to be infectious. This simple point is left out of a lot of the articles on newly discovered infectious origins of common diseases (the prostate cancer article, for example), which is a shame since this theoretical guide would lead to better allocation of resources for curing diseases -- namely away from genetic studies and toward microbial studies. Indeed, a putative genetic link for prostate cancer failed to be replicated, and the PLoS article shows why: if a variant of a gene is only involved in susceptibility to infection, then a link between the defective variant and the cancer will show up only if that individual has already been infected. This point generalizes.

Also in the current issue of PLoS Pathogens is this article showing that infection of neurons with the Borna virus impairs neuronal function -- not the most basic functions, but for example long term potentiation, which is a fancy neuroscience term for the long-term strengthening of connections between neurons based on external stimuli, which is thought to underlie higher functions such as learning and memory. I'll confess that the molecular information is over my head, but for those who understand it, the authors show what molecular mechanisms are involved in the neuronal impairment caused by the Borna virus. Plague Time mentioned a link between the Borna virus and schizophrenia, mood disorders, and chronic fatigue syndrome, and only six years later we have a decent understanding of how it can cause mental disease. In fact, the PLoS authors suggest that the effects of a Borna-infected brain are similar to those observed in Alzheimer's and autism. Now, whether or not the Borna virus turns out to be the culprit for these diseases is an open question, but the implication is that this is where time and energy should be devoted if we want to cure common mental diseases.