The new technology, described in the Sept. 2 issue of Science, offers the possibility of designing cell-death-inducing programs specific to any type of cancer, which could effectively kill tumors while leaving healthy tissues unharmed.
To create their tumor-killing program, the researchers designed a logic circuit — a system that makes a decision based on multiple inputs. In this case, the circuit is made of genes that detect molecules specific to a type of cervical cancer cell. If the right molecules are present, the genes initiate production of a protein that stimulates apoptosis, or programmed cell death. If not, nothing happens.
Because the genes used to create the circuits can be easily swapped in and out, this approach could also yield new treatments or diagnostics for many other diseases, according to Ron Weiss, an MIT associate professor of biological engineering and one of the leaders of the research team. “This is a general technology for disease-state detection,” he says.
Yaakov Benenson of ETH Zurich (the Swiss Federal Institute of Technology in Zurich) co-led the research team, which also included first author and MIT postdoc Zhen Xie, MIT postdoc Liliana Wroblewska and Laura Prochazka of ETH Zurich.
The researchers chose a type of genetic material called microRNA as their target. These snippets of RNA, discovered only 10 years ago, help regulate gene expression by selectively destroying messenger RNA, which relays DNA’s instructions to the rest of the cell.
There are about 1,000 different microRNA sequences found in humans. Cancer cells often have too much of some microRNAs and too little of others; each distinct type of cancer has its own microRNA profile.
In this paper, the researchers wanted to distinguish a particular type of cervical cancer cell, known as HeLa, from other cancer cells. To do that, they identified six microRNAs — some found in large numbers in HeLa cells, others in low numbers — that, taken together, differentiate HeLa from any other cancer cell.
Then, the researchers created a synthetic gene for a protein, called hBax, that promotes cell death. They designed the gene with two separate safeguards against the killing of healthy, non-HeLa cells: It can be turned off by high levels of microRNAs that are ordinarily low in HeLa, and can also be deactivated by low levels of microRNAs that are normally plentiful in HeLa. A single discrepancy from the target microRNA profile is enough to shut off production of the cell-death protein.
If all microRNA levels match up with the HeLa profile, the protein is produced and the cell dies. In any other cell, the protein never gets made, and the synthetic genes eventually break down.
“It’s a simple scheme, but they’re able to do a very impressive calculation with a very large number of inputs,” says Wendell Lim, a professor of biochemistry and biophysics at the University of California at San Francisco. Lim, who was not involved in this project, says similar circuits could also find applications in regenerative medicine to guide the growth of new tissues.
A complex state
This system is the first that can detect six different biological markers inside a cancer cell; most others look for only one or two, which Weiss says is not enough. “Cancer is a complex cellular state,” he says. “In order to be able to make an informed, accurate and precise decision about whether an individual cell is cancerous or not, you really have to look at the system as a whole and look at multiple inputs.”
The researchers are now working on optimizing the circuit to eliminate any false positives and developing new circuits that can identify additional cell types. They also hope to test the approach in living animals and eventually humans.
They are also investigating possible methods to package and deliver the DNA that comprises the circuit, in collaboration with MIT chemical engineers Robert Langer and Daniel Anderson.