Wednesday, 24 August 2011

On The Origins Of HIV (part 2 of 3)

So the question is this: what discovery was made in 1999 that painted the picture of the evolutionary origins of HIV? It all stated with a frozen sample of SIV which was found in the species Pan troglodytes troglodytes, commonly referred to as the chimpanzee. This sample of SIVcpz (with the cpz standing for chimpanzee) was discovered and analysed by Gao et al. who published their findings in the journal Nature. In their paper they listed 5 criteria which provide strong evidence of a link between SIV and HIV. These were…

1) Similarities in viral genome organisation
2) Phylogenetic relatedness (a way of looking at how closely related two things are in an evolutionary sense)
3) Prevalence in natural host
4) Geographical coincidence
5) Plausible routes of transmission

SIVcpz satisfied all of these criteria. In particular, this strain of SIVcpz showed very close genetic similarity to HIV-1 (the pandemic strain of the virus) indicating that HIV may have stemmed from SIV (it is thought to be that way round, as SIV has been known of for much longer and is now believed to be around 32,000 years old, much older than the human counterpart).

Once strong evidence of a link between HIV and SIV had been found, the next mystery to solve was why SIV mutated to produce HIV. Many theories have been proposed, the most plausible of these being known as the “hunter theory”. This theory centres on the premise that SIV was able to get into humans through the preparation of bush meat in Africa. The killing and eating of chimpanzee meat would have allowed for the blood of the chimps to come into contact with any wounds or cuts the hunters may have suffered. This mixing of blood gave the chance for SIV to enter a new human host. Once inside humans, the SIV would have come under attack from our immune system. as with any virus. This in turn would have applied selection pressure to the SIV, forcing it to evolve rapidly in order to ensure its survival. This rapid evolution eventually made the SIV perfectly adapted to survive in its new host and less well adapted to survive in its former host – thus completing the transition from SIV to HIV. And so it began…

From this humble beginning the infection of a few hunter-gatherers in Africa, HIV was able to spread worldwide and infect some 33.3 million people. However, the question still remained when did SIV jump from chimps to man and subsequently evolve to HIV, a process known as zoonosis. The earliest samples of HIV date back to just over 50 years ago. A plasma sample of HIV was found in an adult male in what is now the Democratic Republic of Congo, dating to 1959 (Zhu et al. 1998) and later, a lymph node sample was found in an adult female from the same area dating to 1960 (Worborey et al. 2008). Using these early samples Worobey et al. came up with an estimate for the zoonosis to have occurred sometime between 1884 and 1924, a date estimated by using the mutation rate between the two early samples of HIV and then comparing rate of change with the change made from the SIV genome. Think of this in terms of a car journey – if you know the distance between points A and B during a particular journey is 30 miles and you know this took an hour, you can easily work out the mph speed. If you also knew where you started your journey and how far you’ve travelled, let’s say 120 miles, you can work out that you started the journey 4 hours ago (assuming a constant speed). Using this metaphor, the genomes of the two different HIV samples can be thought of as points A and B and SIV can be thought of as the starting point.

So that concludes what is generally accepted as the origins of HIV. In my next post we will have a look at some of the conspiracy theories surrounding HIV and why they deserve the title “conspiracy” in order to round off this 3 part blog post.  

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