A Planet of Viruses - Carl Zimmer [20]
Retroviruses are a major threat to human health when they’re free-living, but even after they become endogenous they remain dangerous. Mutations can give them back the ability to make full-blown viruses that can escape and cause new infections and even cause cancer. Endogenous retroviruses that can only insert new copies of their DNA into their host genome are dangerous as well, because they can cause genes that are shut down to switch on at the wrong times. The threat from endogenous retroviruses is so great, in fact, that our ancestors evolved weapons that exist only to keep these viruses from spreading.
Paul Bieniasz, a virologist at Rockefeller University, discovered two of these weapons in 2007 by reviving an endogenous retrovirus, as Hiedmann’s team had revived Phoenix the year before. Bieniasz dubbed his resurrected virus HERV-K[con]. When he infected human cells with it, he found that the cells could fight the virus using two proteins called APOBEC3. Bieniasz’s experiments suggest that APOBEC3 homes in on endogenous retroviruses as they are making new copies of themselves destined to be inserted back into the host’s genome. The protein upsets the gene-copying process so that the new copies of the viruses pick up extra mutations. The extra mutations act like a hail of bullets. Some of them don’t cause any harm, but if one of them hits a vital spot in the virus’s DNA, it can cripple the virus so that it can no longer reproduce.
Proteins like APOBEC3 disable endogenous retroviruses, but they don’t eliminate them. Over millions of years, our genomes have picked up a vast amount of DNA from dead viruses. Each of us carries almost a hundred thousand fragments of endogenous retrovirus DNA in our genome, making up about 8 percent of our DNA. To put that figure in perspective, consider that the twenty thousand protein-coding genes in the human genome make up only 1.2 percent of our DNA. Scientists have also observed millions of smaller pieces of “jumping DNA” in the human genome. It’s possible that many of those pieces evolved from endogenous retrovirus, having been stripped down to the bare essentials required for copying DNA.
Endogenous retroviruses may be dangerous parasites, but scientists have discovered a few that we have commandeered for our own benefit. When a fertilized egg develops into a fetus, for example, some of its cells develop into the placenta, an organ that draws in nutrients from the mother’s tissues. The cells in the outer layer of the placenta fuse together, sharing their DNA and other molecules. Heidmann and other researchers have found that a human endogenous retrovirus gene plays a crucial role in that fusion. The cells in the outer placenta use the gene to produce a protein on their surface, which latches them to neighboring cells. In our most intimate moment, as new human life emerges from old, viruses are essential to our survival. There is no us and them—just a gradually blending and shifting mix of DNA.
THE VIRAL FUTURE
The Young Scourge
Human Immunodeficiency Virus
Every week, the Centers for Disease Control and Prevention publish a thin newsletter called Morbidity and Mortality Weekly Report. The issue that appeared on July 4, 1981, was a typical assortment of the ordinary and the mysterious. Among the mysteries that week was a report from Los Angeles, where doctors had noticed an odd coincidence. Between October 1980 and May 1981, five men were admitted to hospitals around the city with the same rare disease, known as pneumocystis pneumonia.
Pneumocystis pneumonia is caused by a common fungus called Pneumocystis jiroveci. The spores of P. jiroveci are so abundant that most people inhale it at some point during their childhood. Their immune system quickly kills off the fungus and produces antibodies that ward off any future