CSI: Evolutionary forensics edition

U. TEXAS-AUSTIN (US) — Using evolutionary forensics, scientists were able to determine the source of HIV infection in two separate criminal cases in which men were convicted of intentionally infecting their female sexual partners.

The research, known as phylogenetic analysis, shows it’s possible to identify the source of a cluster of diseases by analyzing the evolution of a virus within its host and between individuals.

In the cases, State of Washington vs. Anthony Eugene Whitfield and State of Texas vs. Philippe Padieu, it was shown that Whitfield and Padieu knowingly spread HIV to multiple female partners through unprotected sex.

Researchers were able to pinpoint these two origins of the women’s infections from blind (identities unknown to them) blood samples.

“Our research demonstrates that the source of a disease transmission cluster can be identified through phylogenetic analysis,” says David Hillis, professor of integrative biology at the University of Texas at Austin. the

“This kind of analysis can and has been used both to exonerate the falsely accused as well as evidence in convictions.”

The scientists published their results this week in the early edition of Proceedings of the National Academy of Sciences.

HIV evolves rapidly within infected individuals over the course of a few months, so the viruses cannot be compared between a source and a newly infected individual for an exact match. It’s not as easy as directly comparing two individuals’ DNA to confirm paternity.

Even two samples of HIV from the same person, taken several months apart, will be different because the virus is evolving so quickly.

“Within a given person, there is not just one strain but a population of strains because HIV mutates all the time when it makes new virions (viral particles),” says Mike Metzker, associate professor of molecular and human genetics at Baylor College of Medicine.

“During transmission, however, there is a genetic bottleneck in which only one or two viruses get transmitted to the recipient.”

In order to understand the origins of the infections of the women victims, therefore, the scientists needed to reconstruct the evolutionary history of the viruses.

To do so, Metzker’s lab first isolated and sequenced the HIV genes from blind blood samples taken from the accused, from the victims, and from other HIV positive individuals who lived in the same area as the victims and the accused.

Metzker sent the gene sequences (still blind to identity) to the Hillis lab where they constructed evolutionary histories for the viruses by performing the phylogenetic analysis.

In each of the cases, they identified the sample from the unknown individual that shared the subsets of HIV genes that were related to the other individuals in the case.

“This individual was the only person who could have infected the others,” Hillis says.

In each case, the source individual was revealed at the trial to be the defendant when the blinded code numbers on the blood sample were revealed to the court.

“We didn’t know that information until the trial,” Hillis says. “We just knew that we had identified the only person whose HIV population was consistent as the source of the multiple infections.”

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