Studying tuberculosis with a zebrafish model

As one of the oldest known human infectious diseases, tuberculosis (TB) is still an enormous global problem. It caused an estimated 1.3 million deaths and 10 million new infection cases in 2017, according to WHO 2018 figures. Despite the fact that we have found the effective antibiotics for it since the 1960s. And the regimen that was developed in 1960s is still used today to treat TB. Now there’s an emergence of drug resistant TB. It is a very serious problem because drug resistant TB, especially extensively drug resistant TB, makes TB once again the lethal disease than it was in the pre-antibiotic era. What’s worse, the TB vaccine (BCG) that was discovered in the 1920s offers only limited protection against TB. TB recently surpassed HIV as one of the leading 10 causes of death worldwide and it only results from a single agent, the bacterium called Mycobacterium tuberculosis (Mtb).

So what should we do in the face of falling drugs and an ineffective TB vaccine? One way to tackle it is to go back and look at the basic pathogenesis of the disease and see what we can target. My supervisor, Professor Lalita Ramakrishnan, previously helped to develop the genetically tractable and optically transparent zebrafish-Mycobacterium marinum (Mm) model to study these various phases during TB pathogenesis.

In a disease like TB, you’ve got two players: the bacterium and the host. The goal is to develop a much deeper understanding of the tricks that the bacterium uses to establish itself in the host and to produce disease. Mycobacterium marinum causes TB in fish. It is a very close bacterial relative of the human TB bacterium and is much easier and safer to work with. When you look at the pathology of the fish disease, it looks very similar to what human TB bacteria produces in humans.

We use the zebrafish as a host model organism and people call it a “living laboratory”. Zebrafish have a zebra pattern and are very tiny. The zebrafish baby could fit into just half the length of the Queen's crown on a one pence coin. Moreover, the adult fish is only about 4cm in length. However, with the help of powerful microscopy, the zebrafish is now a very important model used to study diseases in vivo. For the first two to three weeks it is optically transparent so we can follow the fate of individual host immune cells and individual bacterium to see where they are going and what they are doing, without hurting the fish. It sounds simple, but its power has been extraordinary. From a bacterial point of view, we have found completely new ways that bacteria develop to manipulate the host's immune system. And from the host aspect, we’ve found genetic mutations that confer susceptibility, or even resistance, to bacterial infection.

For example, by doing forward genetic screening, our lab found a genetic allele, LTA4H, that regulates the severity of fish tuberculosis, and this regulation is proved to be the same in clinical TB infection of patients with LTA4H single nucleotide polymorphism (SNP). Too little or too much LTA4H both cause severe TB, whereas a middling amount causes the least severe TB. This finding also predicts the effectiveness of a widely used clinical TB treatment. People have used dexamethasone as an anti-inflammatory adjuvant to antibiotic TB treatment for many years. It has improved the survival rate of LTA4H high people, but LTA4H low people have been harmed by the treatment. This has just been verified in a repeat study and now there is a trial going on to show that dexamethasone is harmful for those with low inflammatory genotypes. So the zebrafish model may also contribute to a personalised medicine approach.

I have also found a zebrafish mutant that is resistant to mycobacterial infection. By studying its molecular mechanism, I hope to provide new ideas, from the host point of view, for the treatment of TB, including multidrug-resistant TB.

Last but not the least, it’s also critically important that our fish are treated wonderfully. We couldn’t have made the discoveries that we’ve made without these fish!

*Jingwen Fan [2017] is doing a PhD in Medicine. She presented her research at this year's Gates Cambridge Day of Research, for which she won Best Microtalk. Photo of bacteria fighting with immune cells supplied by Jingwen Fan.