A similar switch in DNA base pairs boosts the immune system in two very different ways
When we think of genetic mutations, we often think of the harm they can cause. But some mutations offer hope for health instead. Reuben Harris, associate professor of biochemistry, molecular biology and biophysics, studies mutations of a beneficial sort: those that cells use to protect against invaders. Some of these mutations, he is learning, offer promise for fighting such deadly opponents as HIV and cancer.
As a post-doctoral scholar in the late-1990s, Harris was fascinated by how B cells—immune system cells that make antibodies—are able to strengthen the affinity of the antibodies they make against a particular invader after they encounter the invader.
“At the time, we knew that mutations in antibody genes were responsible for stronger immune responses, but we didn’t know how these mutations formed,” he says. Another laboratory demonstrated that a protein called activation-induced deaminase (AID) was involved, but they didn’t know how it worked. Harris and colleagues discovered that the secret to the protein’s success was its ability to lop an amine group off the DNA base cytosine, transforming it into the RNA base uracil. Because of the way DNA bases pair, this can lead to a permanent change when the DNA is copied.
“It took many replica experiments and the use of other unique conditions to definitively show that AID was mutating DNA,” Harris recalls. “One experiment in particular provided the ‘eureka moment’—a supernatural high, if I were to try and put the feeling into words. After that it was mostly grinding out supporting data and controls.”
The discovery of this amazing collaboration between mutation and a healthy immune response paved the way for yet another one. It turns out that AID is just one member of a larger family of proteins that turn DNA cytosines to uracils. What tasks might other members be taking on?
As a new faculty member, Harris turned his attention to two other members of the family, APOBEC3F and APOBEC3G, which are known to be valuable allies in cells under onslaught from retroviruses like HIV-1. He discovered that these proteins help the cell resist retroviruses by performing the nucleotide quick-change on the genetic material the viruses insert into a cell’s DNA. That change, it turns out, mucks up the virus enough to prevent replication.
Harris is now working to understand the mechanism behind the method. His laboratory has shown that another human protein can block HIV-1 replication, and has made considerable progress toward understanding how these proteins are regulated and how they access and interact with (and only with) their targets. Just a few months ago, Harris, Hiroshi Matsuo, and colleagues took a big step forward when they published a high-resolution structure of a key part of human APOBEC3G in the journal Nature.
A better understanding of exactly how deaminases pull off the nucleotide switch, Harris says, will provide valuable insights for developing strategies to disable retroviruses, boost antibody-mediated immune defenses, and even combat cancers caused by lapses in the mechanisms that control these powerful proteins.
“We need to know much more,” Harris says.
Based on his track record so far, we likely soon will. — Mary Hoff
