Scientists have discovered an in-built suicide system in the tuberculosis-causing bacterium, Mycobacterium tuberculosis, that when activated in mice, boosted the effectiveness of frontline TB treatment.
The findings, published in Molecular Cell, could provide the basis for a new way to combat the disease in humans, which remains one of the world’s biggest killers.
“The potential to hijack this suicide system to kill TB bacteria is really exciting, and could give us insights into how to treat other infectious diseases too,” said Luiz Pedro Carvalho, Crick Group Leader and co-author of the paper.
Previous genetic studies of Mycobacterium tuberculosis showed that the bacterium has up to 80 toxin-antitoxin (TA) systems: sets of closely linked genes that encode both a toxic protein and an antitoxin that neutralises it.
When the bacteria are growing normally, toxin activity is blocked by the antitoxin’s presence. But under lack of nutrients and other stressful conditions, dedicated enzymes rapidly degrade the antitoxin molecules. Once uninhibited, the toxin proteins slow growth, allowing the bacteria to survive a stressful environment.
One particular TA system has a more drastic effect: in the absence of its antitoxin, the toxin kills the bacteria, making it an attractive candidate as a new TB treatment.
In this study, the team started by solving the high-resolution structure of the TA system, to get a better understanding of how the toxin and antitoxin fit together. Luiz’s lab then performed detailed analysis of the chemical reaction sped up by the toxin– making use of the MRC Biomedical NMR Centre housed at the Crick – to reveal how it works.
“We found that when the toxin separates from its antidote, it becomes activated and starts to degrade essential cellular metabolites called NAD+ molecules, which leads to death of bacterial cells,” said Luiz. “As far as we know, this toxin is the first reported enzyme with this NAD+ phosphorylase activity.”
Experiments in TB-infected mice, carried out by the group of Olivier Neyrolles at the IPBS in France, revealed a synergistic effect of the toxin and the frontline anti-TB drug isoniazid. Genetically-induced toxin production in the TB bacteria led to a five-fold reduction in bacterial load and treatment with isoniazid led to a 10-fold reduction. But the combination of the two led to a 100-fold reduction in TB bacteria in lungs and increased survival of the animals.
“Why the bacteria have this kind of in-built suicide system isn’t clear, but we hope that we can exploit it as a drug target,” said Luiz.
“If we find molecules that can disrupt the TA system – and thus trigger cell death – in TB patients, that would be the perfect drug,” says Annabel Parret, EMBL staff scientist in the Wilmanns group, who led the project.
The team will now screen thousands of small molecules to see if they have this capability.