A group at UCSD recently published in Nature that they have programmed bacteria to lyse itself at intervals, releasing drugs that kill tumour cells. Yet another application of synthetic biology, this advancement enables better control of bacteria levels, reducing the risk of adverse immunological responses in patients.
Synthetic biology is based on the design of biological circuits (very much like electrical engineering) within organisms, programming them to perform the specified function. My simplified version of the group’s circuit design is as follows:
There are basically four components in their circuit:
- The drug, haemolysin E, a pore-forming anti-tumour toxin.
- GFP or green fluorescent protein, which makes bacteria glow green as they are about to lyse. They used a super-folder version which ensures GFP is robustly folded even when fused to poorly folded peptides. Previous versions of GFP tend to follow the folding ability of its fused protein which can impact its fluorescence .
- LuxI – an enzyme that catalyses the production of AHL (acylhomoserine lactone), a small molecule that controls the expression of all promoters. AHL freely diffuses among cells, ensuring every bacteria is synchronized. AHL binds to its receptor LuxR to carry out its functions.
- Lysis factor – The authors call this protein E, a lysis protein derived from bacteriophage lysis gene (φX174 E) that triggers lysis of the bacteria.
When the bacteria population is low, AHL is synthesized at a basal level but tends to be at a low concentration as it diffuses out into the extracellular environment. Only when the bacteria population reaches a specific level (see quorum sensing), does the level of AHL build up to sufficient amounts. Binding of AHL to LuxR activates the promoters, driving the expression of all four components – triggering the bacteria to glow green, then explode due to production of the lysis factor, releasing the drug that bathes the surrounding cancer cells. A few outliers survive, seeding the next round of bacterial growth. And the process repeats. There’s a cool video of the whole process, but you probably need a Nature subscription to watch it.
The group has demonstrated efficacy in vitro as well as in mice, where bacteria were made to carry and release different types of anti-tumour reagents – namely the toxin, an immune response stimulator or an apoptosis inducer. A combination of all three factors caused the greatest tumour regression. The bacteria were also effective when given orally, and when used together with chemotherapeutic reagent, 5-fluorouracil, increased the mean survival time of animals harbouring incurable colorectal metastases by 50%.
The use of bacteria has been gaining popularity in cancer treatments, especially with the increasing ease of genetic engineering. Several clinical trials are being run and there is already an FDA-approved bacterial therapy for bladder cancer which uses Bacillus Calmette-Guérin (BCG) (yup, the same injection we get in school to immunize us against tuberculosis) to stimulate an immune response against cancer cells. There are several advantages that come with using bacteria for cancer treatment:
- They are easily genetically modified – providing limitless possibilities for modulating their ability to sense environmental cues and carry out desired actions.
- They can be grown in vast amounts very rapidly.
- They are self-propelled enabling better tumour penetrance. Bacteria can even be made to sense chemicals produced by the tumour and propel themselves towards it.
- Anaerobic bacteria can thrive in low-oxygen conditions, often found in tumours. In contrast, chemotherapy which normally targets rapidly dividing cells, often fail to target quiescent cells in the depths of the tumour where glucose/oxygen are lacking.
- They are externally detectable by MRI/PET/bioluminescence/fluorescence, enabling easy monitoring of treatment efficacy and tumour state.
Of course, there are various challenges as well. The most worrying of which is probably ensuring these bacteria do not mutate into monster bacteria that go on a rampage in our bodies. The high resistance that bacteria have already developed against numerous antibiotics goes to show making them genetically stable may be an uphill task. Bacteria are also nasty triggers of the immune response, and having fevers and chills in addition to the already damaging side effects of chemotherapy would not be something to look forward to. Getting bacteria to target cancer efficiently in different subgroups of patients and establishing effective combination therapy regimes with existing treatments would also take a lot of time, money and effort to get right.
All of this has never stopped Man before though. George Church’s group has already come up with several safety mechanisms to generate safer bacteria that require a synthetic amino acid that can only be made in the lab to survive. And now by controlling the population of bacteria by synchronous lysing, immunostimulatory effects can also be tightly controlled. The possibilities it seems, are endless.
Din, M. O., Danino, T., Prindle, A., Skalak, M., Selimkhanov, J., Allen, K., … Hasty, J. (2016). Synchronized cycles of bacterial lysis for in vivo delivery. Nature, advance online publication. Retrieved from http://dx.doi.org/10.1038/nature18930
Zhou, S. (2016). Synthetic biology: Bacteria synchronized for drug delivery. Nature, advance online publication. Retrieved from http://dx.doi.org/10.1038/nature18915