Ancestor of HIV-1 Pandemic Strains First Occurred About 1930, Los Alamos Researchers Say

JUN-08-00 12:35 From:LANL PA-1

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T-516 P 03/03 Job-T34

HIV-1 ANCESTOR
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The researchers then worked backward to estimate the most likely model to describe the
evolution of the virus. The rate of evolution determined using this tree - as well as a biologically
motivated model for considering error in both time and genetic distance - allowed the researchers to
determine the time of the ancestral sequence to be within the first half of the 2o' century.
The phylogenetic tree for HIV was calculated using maximum likelihood methods. These are
considered among the best methods for answering statistical questions of this kind, but require large
amounts of computational resources. Because the researchers used Nirvana, the fastest unclassified
supercomputer in the world, they were able to process vast numbers of candidate phylogenies
relatively quickly and with considerable accuracy, and to statistically analyze their evolutionary
model.
To calculate the time of origin, the researchers used strategies that permitted them either to
assume a constant rate of evolution or to factor in a changing rate. Agreement between the two
calculations enhanced confidence in the results.
The researchers also tested their methodology by using it to estimate the timing of two
historically documented points in the HIV family tree the sampling time of the oldest available
HIV sequence (1959) and the origin of the epidemic in Thailand (near 1986). The Los Alamos
method estimated these events accurately.
The research was funded through Los Alamnos' Laboratory-Directed Research and
Development progrum, thc National Institutcs of Realth and the Pediauric AIDS Foundation.
Los Alamos National Laboratory is operated by the University of California for the
Department of Energy.

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