THE PROBLEM

Thallium is the element between lead and mercury on the periodic table. It is a byproduct of glassmaking, electronics factories, drug production, and the extraction of metals from ore (EPA). Like its periodic table neighbors, thallium is a heavy metal that is highly toxic to humans--in fact, it is more potent than lead. While the EPA’s maximum contaminant level (MCL) for lead is 0.015 milligrams per liter, the MCL for thallium is only 0.002 milligrams per liter. Thallium is also known as the “poisoner’s poison” because it takes several days to kill, is odorless, and is tasteless. Thallium owes much of its toxicity to its size, which is similar to that of potassium, so thallium can easily enter cells via potassium pathways (RSC). In low doses, thallium causes hair loss and problems with the kidney, liver, and intestines, which can lead to vomiting and diarrhea (ATSDR, EPA).

Although thallium poisoning from drinking water does not pose a huge problem in most areas of the world, thallium’s high toxicity merits a powerful detection system. Current monitoring methods require extensive preparation and lab equipment, and they are not accessible to the general population (NEMI). Thus, we aim to develop an inexpensive, simple system that people can use to check for thallium in their drinking water.

THE SOLUTION

In 2014, Pardee et al. developed a paper-based detection system that was cheap, easy to use, and efficient, producing results in less than an hour. Their system relied on RNA-based sensors to sense small molecules. These sensors were freeze-dried onto paper with cell extract, which contains the translational and transcriptional machinery a cell uses to express proteins. We are adapting their system to sense metals by using a DNAzyme-based sensor. Read more about our solution in our Project Overview.

References

Environmental Protection Agency (EPA). Table of Regulated Drinking Water Contaminants. 2016, July 15.

Royal Society of Chemistry (RSC). Chemistry in Its Element - Thallium.

National Environmental Methods Index (NEMI).

Project Description

We are developing a cell-free sensor that conditionally expresses a chromoprotein only in the presence of thallium. The metal ion detection is based on a DNAzyme that is cleaved in the presence of thallium. The released DNA strand activates a toehold switch regulated by the T7 promoter system, which will promote transcription of T3 and T7 RNA polymerases. The transcription of additional T7 RNA polymerase will amplify the signal from thallium. The T3 RNA polymerase will activate the expression of a chromoprotein. Our circuit will also incorporate a differential amplifier to remove noise from other heavy metals such as mercury. For the differential amplifier, a second DNAzyme with a greater affinity for other metals will be cleaved and produce a DNA strand complementary to that from the thallium DNAzyme. This will remove noise from other heavy metals that may also cleave the thallium DNAzyme.

Early in the brainstorming process, our team wanted to develop a biosensor, since previous projects that hijacked glucometers and pregnancy tests sparked our interest. In deciding what to detect, we looked for analytes that were less widely studied and eventually decided upon thallium. Thallium, like lead and mercury, its neighbors on the periodic table, is toxic to humans. Because it has a similar atomic radius as potassium, thallium follows potassium pathways in the body. Thallium poisoning in low dosages results in hair loss and damage to the peripheral nervous system. In high doses, thallium is lethal. Thus, detection of thallium is important, especially in areas with industries that use thallium.

We are also looking for collaborators. If you are interested, please contact us at pitt.igem.2016@gmail.com. We look forward to an exciting summer!