HIV ancestry traced in family tree.

HIV ancestry traced in family tree

The AIDS virus and its family tree are providing a new view of the virus' disease-causing ability.

Japanese researchers reported last week that an apparently harmless virus found in African monkeys is but a distant relative of the virus that causes AIDS in humans and probably is not to blame for the AIDS epidemic. Their findings, appearing in the June 2 NATURE, contrast with earlier assertions that the AIDS-causing human immunodeficiency virus (HIV) is the result of a recent evolutionary "jump" from the monkey virus. That assumption was based in part on evidence from contaminated laboratory specimens, scientists reported earlier this year (SN: 2/27/88, p.133).

Masanori Hayami of the University of Tokyo and his co-workers analyzed the entire DNA sequence of the simian immunodeficiency virus, SIV.sub.AGM., that commonly infects African green monkeys. The virus stimulates production of anti-bodies in green monkeys but causes no overt symptoms. By comparing its genetic sequence with those of related immunodeficiency viruses, the researchers found that SIV.sub.AGM is equally and distantly related to the two human immunodeficiency viruses, HIV-1 and HIV-2. This indicates the human AIDS viruses evolved independently for "a long time," the researchers say.

Indeed, says Carel Mulder of the University of Massachusetts Medical School in Worcester. "The fact that the SIV.sub.AGM is so remarkably different from the human AIDS viruses indicates that the human viruses cannot have originated from African green monkeys in recent times, as had been predicted by many people."

In the June 9 NATURE, other scientists agree that a monkey origin is unlikely . But these researchers, led by Temple E. Smith at Dana-Farber Cancer Institute in Boston, dispute several of the Japanese conclusions. On the basis of his group's own sequencing experiments, plus their analysis this week of the virus used by the Japanese, Smith said in an interview that SIV.sub.AGM is much more closely related to HIV-2 than to HIV-1. He also says all three virus groups appeared not later than 40 years ago, and probably are not more than a century old. Smith and his co-workers looked at thousands of data points in their tree construction.

However, some of the differences found by the Japanese may help reveal the mechanism behind HIV's exreme pathogenicity. For example, a small, supplementary "message" is encoded in the DNA of SIV.sub.AGM in the region that codes for production of a protein component of the viral envelope. The presence and exact location of this "in-frame stop codon" may change significantly the structure of an envelope protein and may affect virulence, the researchers note.

Perhaps more intriguing, the Japanese researchers found that SIV.sub.AGM lacks a gene -- the so-called "R" gene -- found in both HIV's and in a related simian immunodeficiency virus, SIV.sub.MAC., which causes an AIDS-like disease in macaque monkeys. Scientists still don't know the function of the "R" gene, but it may prove critical to an understanding of what makes SIV.sub.AGM nonpathogenic or what makes the African green monkey resistant to the virus.