Prof Liang Zhao Xun and his team.
Cancer is one of the most common diseases faced by mankind today, and it has been a constant race for doctors and scientists to overcome this disease. Over the years, various treatments have been developed to combat it. Amongst the many treatments that are currently available – such as chemotherapy, radiation therapy and hormone therapy – is targeted therapy.
Targeted therapy, like the name suggests, is a treatment that targets the DNA of cancer cells in a bid to control and interrupt how these cells grow, divide and spread. The treatment involves the use of drugs such as antitumour antibiotics, which aim to stimulate cancer apoptosis (cell death).
One such family of compounds that is being studied for its potent anticancer activity are the naturally occurring anthraquinone-fused enediynes (AQEs). AQEs, such as dynemicin A, are known by a distinctive molecular architecture: a 10-membered bicyclic enediyne “warhead,” along with a DNA-binding anthraquinone moiety. This 10-membered bicyclic enediyne “warhead,” once exposed to chemical triggers, will undergo a rearrangement to form a reactive diradical (an atom or molecule with two unpaired electrons, and are highly chemically reactive). The name “warhead” is most apt to describe what happens next.
When the enediyne “warhead” of the AQEs is inserted into chromosomal DNA, the diradical extracts hydrogen atoms from the DNA backbone. Hydrogen atoms are crucial in holding the DNA strand together, and their removal by the enediyne “warhead” causes the DNA to break apart. If used in cancer cells, the enediyne “warhead” destroys the cells’ DNA, preventing the cell from continued division and spreading. Thus, AQEs can be seen as potent anticancer weapons.
Targeted cancer therapies rely on two types of drugs: small molecule drugs and large molecule drugs. Large molecule drugs are unable to enter the cell and work instead by attacking proteins or enzymes on the surface of the cell. AQEs are being studied as small molecule drugs, where the drugs are small enough to enter a cancer cell and work from within the cell. However, despite the potential anticancer properties of AQEs, how the enediyne “warhead,” the anthraquinone and the overall structure of AQEs are biosynthesised is still not well understood at this point.
In order to decipher how the structures of AQEs are formed, a team of scientists, led by NTU School of Biological Sciences Prof Liang Zhao Xun, demonstrated how the study on a family of recently discovered microbial metabolite, sungeidines, yielded unprecedented insights into the biosynthetic steps leading to the formation of the AQEs’ structure. Both the sungeidines’ and dynemicin’s biosynthetic gene clusters (BGCs) share the essential characteristics of a warhead cassette and a dozen other ancillary genes. They are therefore believed to be evolutionarily related.
The currently known four AQE BGCs, dynemicin, tiancimycin, uncialamycin and yangpumicin, exclusively share nine genes that are likely to code enzymes based on protein sequence homology and predicted protein folds. Out of these nine genes, the newly discovered sungeidines only possess four homologous genes (genes inherited in different species by a common ancestor). This led the team to hypothesize that the sungeidine pathway is a “degenerative” one, a pathway that evolved from an AQE precursor through gene loss.
The sungeidine-producing Micromonospora sp. MD118 growing on solid agar plate.
Based on that hypothesis, Prof Liang’s team decided to work backwards and “resurrect” the sungeidine pathway and try to restore its ability to synthesize the anthraquinone-containing AQE scaffold. This “resurrection” was done by retrofitting the sungeidine pathway with downstream enzymes from the dynemicin pathway. The resurrected sungeidine pathway was indeed able to synthesize the anthraquinone-containing AQE scaffold. Through this process, the team was able to deduce the biosynthetic steps of the AQE: from how enzymes first converted the δ-thiolactone anthracene section into anthraquinone, then through the removal of a hydroxymethyl group (–CH2–OH) and installation of an epoxidate group (a three-membered ring structure containing an oxygen atom with two carbon atoms), the formation of the dynemicin AQE scaffold was brought about.
Through this retrofitting strategy of studying AQEs and how they are biosynthesised, the team hopes that it will be a framework for future studies into the functions and mechanisms of AQE-synthesizing enzymes. It is expected that the detailed knowledge on the biosynthesis of AQEs will enable scientists to engineer the biosynthetic pathway to generate more potent and less toxic AQE derivatives for targeted cancer therapies.
Read the publication here.