Hepatitis B virus (HBV) infects about 296 million people and causes significant mortality, leading to cirrhosis or cancer as it targets the liver with few symptoms initially. Recent research has unveiled new mechanisms of HBV’s polymerase protein that could lead to innovative treatments, potentially moving beyond the limitations of current therapies that manage but do not eliminate the infection.
The hepatitis B
An unusual organism
The HBV genome is a masterpiece of economy—and as a consequence is unusually conservative. More than half of it contains overlapping reading frames, regions where nucleotides encode more than one protein. Because a single mutation within one of these frames can lead to a change in the other, the virus must maintain strict control over these intricately connected regions to prevent potentially catastrophic effects.
And yet HBV is clearly flexible enough to adapt to new environments and hosts. “It’s a very successful virus in humans, and its close relatives infect a variety of birds and mammals,” Schneider says.
How HBV manages this delicate balance between genetic rigidity and flexibility has been poorly understood, because these overlapping frames are difficult to tease apart. Their collective action obscures the mechanisms of individual proteins.
Long on researchers’ radar has been the pol protein, which plays essential roles in HBV replication. A multipurpose molecule, its importance is evinced by its size; it’s far larger than other viral proteins and wraps around two-thirds of HBV’s circular genome, sharing one of the overlapping reading frames with three other proteins.
To better understand its dynamic components, Rice’s team employed a new approach they developed last year that delivers
“Imagine two sheets of transparent paper with different text stacked on each other. If you can remove one of the sheets, it will be easier to read,” Schneider says. “That is what this RNA delivery system allows us to do.”
Next, they used deep mutational scanning—a high-throughput method that can reveal the mechanics and behaviors of tens of thousands of protein variants. It enabled them to test nearly every possible variant in the pol protein and see how it responded to each change.
Stalled ribosomes
One of their first unexpected discoveries was the strict requirement for