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Our goal for this years competition is to create a genetic circuit which enables E. coli cells to detect and attack cancer cells. To do this, the cells should produce a tumor inhibitor or toxin effective at killing cancer cells, and respond to environmental cues indicating the presence of cancer cells by undergoing [http://en.wikipedia.org/wiki/Lysis lysis], releasing the anti-cancer agent into the surrounding environment. In principle, bacteria could be used in "search and destroy" missions against cancer inside the human body. We wish to develop our genetic circuit as a proof-of-concept of one such strategy.
A sketch of our current planned design for the circuit is shown below.
Ideally, the toxin should only be released if cancer cells are actually encountered, and never otherwise. To achieve this, the lysis-inducing part of the circuit should be regulated such that it is highly unlikely to activated in other situations. One possible way is to use a signal molecule that is solely associated with cancer cells to activate the lysis device. Another possibility is to use a combination of environmental cues that together indicate a high likelyhood of cancer presence. We have chosen the latter strategy, and the environmental cues we aim to use are low O2 and high lactate concentrations.
Each of the two cues activates a separate gene, which encode different proteins. Together, these two proteins should activate the unit responsible for lysis. Therefore, if only one of the cues are present, the cell does not lyse, and toxin is not released in high levels. (Some leakage of toxin must be expected even under normal conditions with no activation of the circuit). Selective activation in the presence of cancer cells would be crucial for the concept to be effective in a medical situation.
As shown in the sketch, we plan to express the toxin-producing gene with a constitutive promoter. This means that the toxin production always be on. Ideally, the toxin should be maximally effective against cancer cells and minimally toxic to the toxin-producing cell. We have evaluated several candidate molecules. At the moment, we are leaning towards using the molecule Colicin E1.
The goal of making bacterial cells detect and attack cancer cells present several challenges. First, how can cancer cells be reliably and effectively detected? Early in the process after deciding on the goal, we considered using several human signalling molecules such as HGF and VEGF as cancer indicators, as these are known to be over-produced by tumors. However, we were unable to determine quickly if these would be able to enter and affect E. coli cells, due to their large size. In contrast, O2 and lactate are small molecules which are easily taken up by the cells, and are known to directly regulate the expression of various genes.
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