Tuberculosis (TB) has taken over a billion lives in the past 140 years. Although the disease is both preventable and treatable, resistance to traditional therapies is growing. The WHO calls multidrug-resistant TB “a public health crisis and a health security threat,” citing a 10 percent increase in cases from 2018 to 2019. In this Harvard Horizons presentation, Harvard University PhD student Harry Won describes a new approach to treating the disease: by targeting and destroying critical parts of TB’s cellular “machine.”
This transcript has been edited for clarity and correctness.
Mycobacterium tuberculosis causes tuberculosis, a devastating disease which presents a pressing global health threat. Every year, there are 10 million new cases of TB, and it claims over a million and a half lives, making it one of the world's leading infectious killers. In the past 200 years, TB has caused over a billion deaths, vastly more than the infectious specters of the past many are more familiar with, such as smallpox or the bubonic plague.
The advent of antibiotics in the early 20th century was an incredible advance that has saved an astonishing number of lives. But decades of using the same antibiotics, and having few new treatments, has resulted in the ever-increasing spread of multi-drug resistant tuberculosis, MDR-TB. Now there are over a half million new cases of MDR-TB every year, and it has only a 56% cure rate. And that is if patients have access to the full course of treatment, and this involves taking four or more antibiotics over the course of two years.
Our lab focuses on creating new knowledge that we hope ultimately addresses this problem and crushes the scourge of tuberculosis.
The old ways of making antibiotics really rely on gumming up proteins or molecular machines, preventing the cell from completing some critical activity such as multiplying or making energy. Now, if we were to liken a bacterial cell to an electric pencil sharpener, this would be like jamming silly putty in the blades, preventing them from working.
Because the old ways of making antibiotics had stopped giving us powerful, new drugs, we looked to our colleagues developing cancer drugs to see if we could maybe find some new strategies. What if we could get the pencil sharpener to chew up its own wiring? What if we could somehow turn bacteria against themselves?
This approach is called "targeted protein degradation," and it employs a two-headed molecule called the "degrader." These molecules are bispecific. They're sticky on both ends for different proteins, and it brings them close together. This results in the flagging of a target protein with a garbage label that then directs it for destruction by the proteasome, which is just a molecular garbage disposal that cells have to recycle old proteins. The goal here is to use a normal way of clearing out old proteins, and redirecting it to destroy ones that are causing disease. Yet we haven't seen this approach tried before in an infectious disease.
In our development of a TB degrader, one side would bind to a protease—this is just a different type of garbage disposal that bacteria have—the other side to some essential TB protein, a target protein that is critically important for bacteria to live. This degrader would bridge both proteins, resulting in the target protein being cut up. In this way, we would be forcing the bacteria to destroy the very things they need to survive.
My first step was to create a proof of concept system where we could test whether we could, in fact, deliver bacterial proteins for self-degradation. As I was in the early stages of my dissertation, and we were really pioneering this new approach in bacteria, we wanted to figure out how we might get the system to work in a model of TB before diving right in to making a molecule.
To do so, I fused the target protein and the protease with the partner A and B proteins. Now, these proteins get forced to stick together in the presence of a molecular glue I'll call "inducer." The idea here is that adding inducer will deliver the target proteins right to the protease, resulting in its destruction. I created bacterial strains that have the target protein tagged with the partner A and glowing GFP proteins. When we add inducer, the cells on the left would not deliver the target protein to the protease, but the cells on the right would.
With GFP, we can measure levels of the target protein in the cell using a microscope. And so behind me is a microscopy video of bacterial cells multiplying. And where the cells are glowing bright green, there's plenty of that protein around in the cell. We can see that over the course of 12 hours, there is destruction of the target protein and the GFP tag only when it is delivered to the protease, resulting in loss of GFP signal on the right. This was fantastic news, as it showed us that in fact, we could target bacterial proteins for self-degradation.
The next question was, could we kill bacteria by targeting the right proteins for self-degradation? We've since found that targeted self-degradation of an essential cell membrane protein is able to restrict bacterial growth. We can see on the right that when we are targeting this membrane protein for destruction, there is profoundly less bacterial growth on Petri dishes compared to when we don't. This was exactly what we wanted to see.
We've also found that targeted self-degradation of an important RNA production protein is able to make bacteria more sensitive to the antibiotic to which TB strains are most often resistant. We can see on the right in blue that when we're targeting this RNA production protein for destruction, we require around three times less of a specific antibiotic to arrest growth compared to when we don't.
Altogether, these data suggest that we could make a degrader that might kill TB outright, or could combo with old antibiotics, perhaps against drug-resistant TB strains.
My next steps are to search for molecules from which we can create our degrader, and then take it through the lengthy drug development process. My dream is that we will be able to create a targeted degrader antibiotic that is powerfully effective even against multi-drug resistant tuberculosis. My dream is that we will save millions of lives and make our world a healthier place.
