Fairy Tales and Building Blocks

The latest issue of the New Yorker has two stories that are well worth reading. The first, The Book of Exodus, is an amazing, near-unbelievable article about a Muslim scholar in World War II Sarajevo who risked him life to protect a Jewish treasure. His act set off a cascade of selflessness that could serve as a fable about human kindness and its unexpected rewards. Unfortunately, the article isn’t available online (here’s a link to the abstract), but everyone should try to track it down.

The other story may not be as accessible, but it’s just as fascinating. Michael Specter reveals in Darwin’s Surprise that viruses have repeatedly embedded themselves in our DNA over the course of evolution, changing our genetic code and even enabling some of the developments that determine what it means to be human.

As Specter states, “It takes less than two per cent of our genome to create all the proteins necessary for us to live. Eight per cent, however, is composed of broken and disabled retroviruses, which, millions of years ago, managed to embed themselves in the DNA of our ancestors.”

Retroviruses are distinct from other viruses in that their genetic information is stored on single-stranded RNA instead of the more standard double-stranded DNA. In most organisms, including our own, DNA acts as a sort of library for our genetic information. When a cell needs this information to carry out a process, it’s transcribed to RNA, which acts as a set of disposable instructions to guide the cellular machinery through the necessary steps to be taken.

In retroviruses, such as HIV, the process occurs in reverse. The virus injects its RNA into the cell, where it hijacks the cellular machinery to reverse the typical process. In the end, the virus’ RNA has been permanently written into the cellular DNA. If a sperm or egg cell is infected in such a manner, and the virus doesn’t kill the host or its descendent, the virus can permanently be written into the DNA of our species.

What does that mean? For one thing, scientists can reconstruct garbled bits of virus DNA from our genome—those that have been scrambled over time—to reconstitute ancient retroviruses. In carefully controlled conditions, these historical artifacts can teach us more about how retroviruses work, helping us build better defenses against them. For instance, the article reveals that scientists at the University of California at San Francisco and the University of Toronto have recently discovered that cells infected with HIV see an uptick in the activity of these long-embedded viruses. Specifically, the viruses produce proteins that can be detected on the surface on the infected cells; a vaccine could potentially target these proteins.

Endogenous retroviruses, as viruses that have embedded themselves into the genetic code are called, can also provide a map of genetic lineages throughout the past. As Specter says, “Darwin’s theory makes sense…only if humans share most of those viral fragments with relatives like chimpanzees and monkeys. And we do, in thousands of places throughout our genome. If that were a coincidence, humans and chimpanzees would have had to endure an incalculable number of identical viral infections in the course of millions of years, and then, somehow, those infections would have had to end up in exactly the same place within each genome.”

Even as these endogenous retroviruses have been inserted into our genome, they’ve changed it as well. Indeed, Specter suggests that the defining characteristic of mammals—the ability to bear young internally—may have been stimulated by the viruses. “The protein syncytin, which causes placental cells to fuse together, employs the exact mechanism that enables retroviruses to latch on to the cells they infect.”

Thus, we adapt and evolve.