November 9, 1999 - Buried within the genetic blueprint of every human is a snippet of DNA that resembles a gene sequence from the human immunodeficiency virus (HIV). Humans have been carrying this unwanted genetic baggage around for more than 30 million years, according to researchers from the Howard Hughes Medical Institute (HHMI) at Duke University.
"We're all walking around with a little bit of an HIV-like sequence in our genes," said Bryan Cullen, an HHMI investigator at Duke University.
According to Cullen and his colleagues, an ancient family of viruses, known as HERV-K (for human endogenous retrovirus K), took up permanent residence in the genetic material of Old World monkeys shortly after they diverged from New World monkeys. The viruses then traveled with their simian and pre-human hosts as these species moved along the evolutionary path that led to Homo sapiens. Cullen's group published its findings in the November 9, 1999, issue of the Proceedings of the National Academy of Sciences.
During infection, some viruses insert their DNA into the host's genome and direct the host's cellular machinery to make the proteins needed to assemble more viruses. If this gene insertion takes place in a cell that will become an egg or a sperm, the host's offspring will have a copy of the virus in every single cell. "Once it's in there, it doesn't get out," Cullen said.
Because of these viral gene insertion events, genetic material from inactive viruses accounts for roughly 3 percent of the human genome. Cullen says that 30-50 copies of HERV-K exist in the human genome, and that some of the copies appear to be active at a low level in normal testicular and placental tissue. The HERV-K genes show even more activity in certain cancers, especially those involving the testes, "but there doesn't seem to be a harmful effect from the activity of these genes," Cullen said.