Drug Resistance in Malaria

P. falciparum gametocytes on a blood smear

The department seminar last week was given by Dr. Timothy Anderson, a scientist working right here in San Antonio at The Texas Biomedical Research Institute.  His lab is looking at genes responsible for drug resistance in the parasite that causes malaria and tracking these mutations as they spread through endemic regions.   Drug resistance is an increasing problem for endemic areas where the drugs that are currently used to treat malaria are becoming less effective.

Before diving into the details, some background: Malaria is caused by parasites of the genus Plasmodium.  These organisms have a complex life cycle and are transmitted by mosquitoes to humans where they infect and kill red blood cells.   Malaria is endemic to tropical regions around the world.  (To learn more about the distribution of malaria, the CDC has a malaria map application here).  The species of plasmodium that causes the most severe infection is P. falciparum and Dr. Anderson's lab is looking at the types and distribution of resistance mutations in this species.

One gene in particular, pfmdr-1, has been well studied and shown to play a role in drug resistance.  Copy number variations, as well as single nucleotide polymorphisms (SNPs) in this gene can lead to drug resistance.  Dr. Anderson's lab genotyped 160 infections from Malawi to determine the rate of CNVs and SNPs associated with resistance*.  All the parasites had a single copy of pfmdr-1.  Although no CNVs were seen in this study, this finding is important for setting a baseline of CNV prevalence for future surveillance studies to reference.  It was also found that 34% of the parasites had variations at 4 of the 5 SNP sites studied.  After determining the prevalence of these mutations in the population, they looked at susceptibility to various anti-malarial drugs and found that several of these genotypes were associated with increased resistance to the drugs tested.

In order to develop effective strategies for drug development and delivery it's important to understand the underlying mechanisms of resistance and how resistance moves through a population.  This study and studies like it are critical for understanding how to minimize the spread of drug resistance and for the implementation of smart drug policies that save lives.

*Nkhoma et al. Parasites bearing a single copy of the multi-drug resistance gene (pfmdr-1) with wild-type SNPs predominate amongst Plasmodium falciparum isolates from Malawi. Acta Trop (2009) vol. 111 (1) pp. 78-81

First seminar of the semester!

(this post is from September 1, 2011 but I didn't get it published until today... ah procrastination)

So this is the final year of my graduate studies and I'd like to write at least something about each seminar I attend.  The first seminar for the Microbiology and Immunology department was today. Woot! It is presented by Duncan Wilson from Albert Einstein College of Medicine.  The title was "Studying Herpes Simplex Virus Microtubule-traffic using an in vitro System."

Dr. Wilson is known for determining where the virus gets its final membrane, which is not as straight forward a question as it might seem.  That was more than 20 years ago and today he is working on answering questions regarding how the virus gets around inside a cell and from one cell to another within the nervous system.


Herpes simplex virus (HSV) is in a family of viruses that are enveloped and use linear double-stranded DNA, which is encased in a capsid.  The capsid is surrounded by a tegument of unknown function and the whole thing is wrapped in a membrane derived (as Dr. Wilson's lab showed) from the trans-golgi network (image above).

HSV infects neurons and in order to move to the nuclear pore for replication, as well as to get from one neuron to the next, they use motor proteins that the cells normally use for trafficking cellular molecules along microtubules (left image).  It isn't currently clear whether the whole virus travels along microtubules or if just the capsid with the DNA traverses the cells.  This has lead to two models being hypothesized, the separate model (capsid without envelope) and the married model (complete virus). This is an important question not only for the sake of understanding HSV biology but has implications for drug development strategies.

In order to address this question many researchers are looking at the proteins involved in the process to get an idea of what is going on.  Dr. Wilson's lab has identified a large tegument protein that is involved in anterograde trafficking.  Because of it's location in the tegument and not on the envelope of the virus, this seems to support the hypothesis that the capsid is responsible for trafficking of the virus.  However, some of the electron microscopy data show a capsid that is partially enveloped, suggesting that perhaps a model that is a bit of a hybrid of the separate and married models is actually represent what is going on in the cells. 

Prions!

I study infectious diseases because I find them so incredibly interesting and varied.  Even though my work focuses on viruses, I have mad love for all infectious agents and prions are some of the most interesting.  I mean, infectious proteins? How interesting is that?  These disease are caused by an altered form of a protein that can cause the native form to become altered.  In mammals, a protein named PrP (prion protein) is responsible for these diseases.  The altered form is called PrP^Sc (for scrapie, the name of the prion disease found in sheep) and the native form PrP^C (cellular).  The altered form of the protein is resistant to degradation and form aggregates resulting in plaques in the brain. These diseases fall into three categories based on mode of transmission:  sporadic (caused by a random mutation in the PrP gene), familial (inheritance of mutant PrP) or transmissible (infectious).   


You likely know about the widely publicized infectious prion diseases like mad-cow disease or Creutzfeldt–Jakob disease but my favorite prion disease is actually not infectious.  It is a hereditary form called fatal familial insomnia (FFS).  That's right, insomnia.  If you have the misfortune of inheriting this disease, you will die of insomnia. It sounds so so terrible.  Luckily, it is exceedingly rare.  

HIV in the gut

Yesterday I attended a seminar by Dr. Ronald Veazy, a scientist in the Pathology Department at Tulane National Primate Research Center.  His research focuses on the effects of HIV infection on the gut.  His lab works with the animal model of HIV, which is SIV infection of Asian macaques.

For a little background on this model, HIV originated from an SIV (simian immunodeficiency virus) that naturally infects chimpanzees (though now there is some debate on how "natural" this infection is. more on this later...).  SIV doesn't result in the same disease course in chimps because they have had some time to adapt to the virus.  So while they are not able to get rid of the infection, they are able to control the virus to some extent and death from SIV in chimps seems to be pretty rare.

However, Asian primates are not naturally infected with SIV and when infected display a disease course much like AIDS in humans.  This was discovered when a captive rhesus macaque was accidentally infected with an SIV from a sooty mangabee (SIVsm), which gave rise to the virus that is currently used in the animal model, which we now call SIVmac.

Now back to Dr. Veazey's work.  Following the discovery of HIV/AIDS, it was thought that the pathogenesis of the virus was predominantly a result of depletion of immune cells called CD4 positive T cells or helper T cells in the blood (from now on CD4+ cells).  These cells play a central role in the immune response to invaders and are the target cells of HIV/SIV.  The virus uses the CD4 molecule on the surface of these cells (along with a co-receptor) to enter the cells and turn them into little virus factories.  These cells can be isolated from the blood and counted to determine the state of the immune system during HIV infection.  However, these cells are also found in large numbers in the gut, in what we call gut-associated lymphoid tissue or GALT.  The depletion of these cells in the gut was not appreciated as a source of pathogenesis until around 1998 when a group of scientists, including Dr. Veazey, discovered that in macaques these cells are quickly depleted following infection and, unlike the cells in the circulation, these populations of CD4+ cells do not recover after this initial depletion. Tracking these populations in humans was difficult (gut biopsies had to be taken, which is something to avoid in people with impaired immune systems).  So there was no evidence of this in humans and it was thought to be a peculiarity of the macaque model until 2004 when it was shown to occur in humans with HIV as well.  In the mean time, the co-receptor for HIV was discovered (a molecule called CCR5) and was shown to have high expression in CD4+ cells in the gut.  So we now know that T cell depletion in the gut plays a central role in HIV pathogenesis, though the exact mechanism is still under some debate.  It was initially assumed that the decrease in CD4+ T cells itself directly leads to immunodeficiency.  However, it now seems that the depletion of CD4+ cells in the gut leads to gut "leakiness" that in turn results in systemic inflammation at levels too high to sustain, thus exhausting the immune system and leading to immunodeficiency.  Regardless of whether this depletion is the key to disease progression or is just one of many factors that leads to AIDS, Dr. Veazey's research has been incredibly important in the discovery of an important factor in HIV/AIDS pathogenesis.

Flu Map

As a simple way to start things off, I'll just share an interesting tool I found on weatherunderground.com.  It's a map of the States that shows recent flu activity by state.

http://www.wunderground.com/maps/us/2xFlu.html