Aug. 16, 2008 Synthetic Biology is bearing fruit: the tuberculosis pathogen can be fooled by a widely used food additive. This is shown in a paper by researchers at the Department of Biosystems Science and Engineering (D-BSSE) published recently in PNAS.
Not so long ago, even in Switzerland, people were falling ill with tuberculosis (TB). Only in the last few decades has the disease largely lost its terror, at least in this country; and the high-altitude sanatoria that specialised in it are concentrating on other fields. However, the illness is now gaining ground again throughout the world. The WHO records around nine million new cases of the disease each year, and about 50 million people are infected with a strain of Mycobacterium tuberculosis that is resistant to the antibiotics Isoniazid and Rifampicin. The only effective remedy is Ethionamid, structurally related to Isoniazid. The disadvantage is that this drug is fatal in high doses. However, high dosages are often needed to fight the pathogen.
Blocking with a trick
This is because Mycobacterium has a defence against even Ethionamid. It produces a protein, EthR, that blocks the production of the enzyme EthA, thus preventing EthA converting Ethionamid from an inactive precursor into a substance that kills the TB pathogen. In other words, Mycobacterium has the weaponry to either arm or disarm the antibiotic by suppressing its conversion.
This mechanism – recognised as an Achilles heel by ETH Zurich researchers led by Martin Fussenegger – was discovered only recently. Using an approach from the field of Synthetic Biology, the Professor of Biosystems Science and Engineering at the D-BSSE and his research group have now succeeded in taking the blocker EthR out of circulation. This makes the antibiotic Ethionamid considerably more effective, and Fussenegger hopes it could be used in smaller doses so it would no longer be toxic to patients.
A lucky strike but no accident
The researchers grafted the Mycobacterium’s entire gene network needed for this signal pathway into a mammalian cell and used it to test various substances that might block EthR. During their search, the biologists hit on a substance that is outstandingly suitable as an EthR blocker: 2-phenylethyl-butyrate.
This substance with a complicated name is an everyday food additive and is permitted in many countries including the USA. Fussenegger says that it is widespread and is therefore available cheaply. He says that it was a stroke of luck that they hit on precisely this substance so quickly. It was not an accident, however, because trumping EthR needs substances with particular properties that can be narrowed down very quickly using Synthetic Biology.
Looking for money for tests on mice
Martin Fussenegger is now looking for money to enable the combination to be tested in mice. The mouse tests would last about 10 months and would cost between 800,000 and 900,000 Swiss francs. He says that this project cannot be completed within his group, but probably only another small step is needed to enable the antibiotic and 2-phenyl-butyrate combination therapy to be tested in humans as well, precisely because the latter substance is a permitted food additive. As he points out, “It’s a small sum of money to help millions of people.”
Fussenegger is convinced that the planned tests on mice will have a positive outcome and that this combination will also be tolerated well by humans. He stresses that, “Every newly synthesised artificial agent entails a risk that is many, many times greater than this well-known substance.” He says that we must also reconsider whether we want or are able to put another antibiotic in place of each one that becomes useless. Every antibiotic has become ineffective relatively quickly after being put on the market, whereas the combination therapy makes it more difficult for bacteria to develop resistance against it. The ETH Zurich Professor says, “The substance that has been discovered interferes with a metabolic pathway that is important for Mycobacterium. This is why it will be much more difficult for the bacterium to adapt to it.”
From playing games to serious applications
There are still only a few examples in which the real benefit of Synthetic Biology has been so clearly demonstrable as in this research work by Martin Fussenegger and his colleagues. For example, the American Jay Keasling used a comparable approach in 2005 in his work to prepare Artemisin artificially in yeasts as an anti-malarial agent.
Synthetic Biology is a relatively new subject area within Biology. The procedure is reminiscent of the construction of electronic equipments, in which circuits that can be controlled well are built from well-characterized components like capacitors and transistors. Synthetic Biologists build artificial biological systems in an analogous way, using individual components such as single genes and enzymes for which the reactivity and the products formed are accurately known.
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- Weber W et al. A synthetic mammalian gene circuit reveals anti-tuberculosis compounds. PNAS, online publication July 9 2008 DOI: 10.1073/pnas.0800663105
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