THE PROBLEM

In many countries, water is a necessity. It's used for drinking, showering, cooking, washing dishes, and doing laundry. Yet as the recent crisis in Flint, Michigan shows, its purity and safety is not guaranteed. In Flint, city officials trying to save money on water temporarily supplied the city with water from the Flint River. The water contained bacteria, so residents were advised to boil the water. However, the chlorine used to treat the bacteria reacted with other compounds in the water to produce total trihalomethanes (TTHM), which could be carcinogenic. And, as news headlines declared, the water contained dangerously high levels of lead because the water corroded the pipelines and allowed lead to seep in. City officials did not proactively treat the water to prevent corrosion. And although residents complained about the water quality, city officials maintained for several months that the water was safe to drink (NPR).

Lead's toxicity is a result of its similarity to minerals our bodies need such as zinc, iron, and calcium. It can enter the body through inhalation, ingestion, or absorption through the skin. Through the pathways of these minerals, lead is distributed throughout the body. Lead is especially dangerous for young children and fetuses, in whom it interferes with development, resulting in symptoms ranging from speech and language problems to decreased bone and muscle growth. Other common sources of lead poisoning in children are lead-based paint, used in old houses, and contaminated soil (KidsHealth). In adults, lead poisoning can lead to high blood pressure and kidney problems (EPA). In children and adults, even a trace amount of lead can cause problems, but the maximum contaminant level (MCL) in drinking water set by the EPA is 15 parts per billion (ppb). In 2015, a year after Flint changed its water source to the Flint River, the a resident's water was found to contain over 13,000 ppb of lead. In the same time period, an increased percentage of children in Flint had elevated blood lead levels (NPR).

The Flint water crisis has sparked national concern about lead contamination in water. Cities are scrambling to test their water lest they become the "next Flint." In the past few months, the city of Pittsburgh has found lead levels that are dangerously close to the EPA's MCL or higher. This awareness is beneficial for the residents, but the power still lies within city officials. Residents need a device to test for lead in their own homes. Current lead sensors for home use exist, but they cost about twenty to thirty US dollars (USD)--inaccessible for over 40 percent of Flint's population and over 14 percent of the US population, which lives under the poverty line (NPR; US Census Bureau). Thus, we are developing Hot Metal Switch, a sensor that could cost mere cents, for residents to test their own water whenever they wish.

THE SOLUTION

Hot Metal Switch is based on a paper-based sensor developed by the Collins group. 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 as part of the detection circuit. In the presence of lead, the circuit will express LacZ, producing a visible color change. Read more about our solution in our Project Overview.
The design of our circuit makes it easily adaptable to other metals. To demostrate this, we used a thallium DNAzyme to sense thallium. 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. The EPA’s MCL for for thallium is only 2 ppb. 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.

References

National Public Radio (NPR). Lead-Laced Water In Flint: A Step-By-Step Look At The Makings Of A Crisis. 2016, April 20.

KidsHealth from Neymour. Lead Poisoning. 2016.

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

United States Census Bureau. Income and Poverty in the United States: 2014. 2015, September.

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

Agency for Toxic Substances & Disease Registry (ATSDR). Toxicological Profile for Thallium.2015, January 21.

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!