DNA evidence has been used to link suspects to crime scenes and even to people they might have infected with HIV. Now, using tools from evolutionary biology, researchers have shown that they can establish the direction of transmission of HIV in criminal cases involving men who intentionally infected women partners during unprotected sex.

A new paper, published online November 15 in Proceedings of the National Academy of Sciences, explains how a phylogenetic approach can yield the history of how the viruses evolved in different individuals and thus provide "evolutionary forensics" evidence for who transmitted the virus to whom.

"This is the first case study to establish the direction of transmission," Mike Metzker, of Baylor College of Medicine and coauthor of the new study, said in a prepared statement.

"Phylogenetic analysis allows us to reconstruct the history of the infection events," David Hillis, of University of Texas, Austin, and a coauthor of the new study, said in a prepared statement. "We can identify the source in a cluster of infections because some isolates of HIV from the source will be related to HIV isolates in each of the recipients."

The HIV analysis is not as simple as DNA screening used to match a blood or hair sample with an individual. In those who are infected, "there is not just one strain but a population of strains because HIV mutates all the time when it makes new virions," Metzker said. To resolve this, researchers traced their way back to a likely "common ancestor" viral strain in each individual because at the point of transmission, "there is a genetic bottleneck in which only one or two viruses get transmitted to the recipient," Metzker said.

For two U.S. criminal cases (State of Texas v Philippe Padieu and State of Washington v Anthony Eugene Whitfield), Metzker and his colleagues isolated and sequenced two gene regions (env and pol) on the virus found in blood samples taken from the accused, the victims and other HIV positive individuals living nearby.

After performing a phylogenetic analysis on the blinded samples (that is, the researchers had no information on whether samples came from the accused, the victims or others), Metzger, Hillis and their colleagues were able to determine which individual was most likely to have infected others in the group. The analysis, along with other evidence, contributed to the 2009 conviction of Padieu (for six counts of aggravated assault with a deadly weapon—HIV) and the 2004 conviction of Whitfield (for 17 counts of first-degree assault).

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