Urbanization in Africa at dawn of 20th century marked outbreak of HIV
UA-led research indicates the HIV/AIDS pandemic began around 1900 in sub-Saharan
The research, led by Michael Worobey, an assistant professor of ecology and evolutionary biology at The University of Arizona in
Worobey and his collaborators screened a number of tissue samples and uncovered the world's second-oldest genetic sequence of HIV-1 group M, which dates from 1960. They then used it, along with dozens of other previously known HIV-1 genetic sequences, to construct a range of plausible family trees for this viral strain. The lengths of the tree branches represent the periods of time when the virus genetically diverged from its ancestors. The timing and number of these genetic mutations enabled the scientists to calibrate the probable range of rates at which the trees have grown. That is, the probable rates of evolution of HIV-1 group M. Based on this range of rates, the scientists projected back in time to the period when the trees most likely took root: around the turn of the 20th century. This marks the probable time of origin of HIV-1 group M, according to Worobey and the others.
Using newly developed techniques, the scientists recovered the 48-year-old HIV gene fragments from a wax-embedded lymph-node tissue biopsy from a woman in
"From that point on, the next oldest sequences that anyone has recovered are from the late 1970s and 1980s, the era when we knew about AIDS. Now for the first time we have been able to compare two relatively ancient HIV strains. That helped us to calibrate how quickly the virus evolved and make some really robust inferences about when it crossed into humans, how quickly the epidemic grew from that time and what factors allowed the virus to enter and become a successful human pathogen." Research shows that HIV spread from chimps to humans in southeastern
By 1960 a large number of people in this region were infected with HIV, reflected by the considerable amount of genetic diversity of the virus. From there events seeded the epidemic in different parts of the world. By 1981, people started realizing that something was happening and the rest is history.
Worobey said laying the technical groundwork for analyzing samples of HIV's ancient history was extraordinarily painstaking.
"The DNA and RNA in these samples is in a really sorry state. It's highly fragmented, so instead of a nice, pearl-strand of DNA or RNA, you have a jumbled mass that's all jammed together. It's been gratifying, but a ridiculous amount of work." Worobey said his research in the near term will be on recovering more samples and assembling the fragmented DNA and RNA sequences to form a clearer picture of HIV's history. He said the Nature paper "does a lot to snap everything into sharp focus and allows us to understand the timing of these events and the growth of the epidemic."
"There's still a lot of interesting work we can do with these techniques. We have lots more samples to analyze and hopefully recover nucleic acids from and it's pretty exciting to be in that position," Worobey said.
"I think the picture that has emerged here, where changes the human population experienced may have opened the door to the spread of HIV, is a good reminder that we can make changes now that could help reverse the epidemic. If HIV has one weak spot, it is that it is a relatively poorly transmitted virus. From better testing and prevention, to wider use of antiretroviral drug therapy, there are a number of ways to reduce transmission and force this virus back into extinction. Our results suggest that there are reasons for such optimism."
Worobey's colleagues on the paper include Marlea Gemmel, Dirk E. Teuwen, Tamara Haselkorn, Kevin Kunstman, Michael Bunce, Jean-Jacques Muyembe, Jean-Marie M. Kabongo, Raphael M. Kalengayi, Eric Van Marck, M. Thomas P. Gilbert and Steven M. Wolinsky.
Contact Michael Worobey at 520-626-3456 or by e-mail at worobey@email.arizona.edu.