Directed Evolution
The Stanford approach is grounded on the fact that all biological processes are driven by the interaction of proteins, the molecules that fit together in lock-and-key fashion to perform all the tasks required for living things to function.
In nature proteins evolve over millions of years. But bioengineers have developed ways to accelerate the process of improving these tiny parts using technology called directed evolution. This particular application was the subject of the doctoral thesis of Mihalis Kariolis, a bioengineering graduate student in Cochran's lab.
Using genetic manipulation, the Stanford team created millions of slightly different DNA sequences. Each DNA sequence coded for a different variant of Axl.
The researchers then used high-throughput screening to evaluate over 10 million Axl variants. Their goal was to find the variant that bound most tightly to Gas6.
Kariolis made other tweaks to enable the bioengineered decoy to remain in the bloodstream longer and also to tighten its grip on Gas6, rendering the decoy interaction virtually irreversible.
Yu Rebecca Miao, a postdoctoral scholar in Giaccia's lab, designed the testing in animals and worked with Kariolis to administer the decoy Axl to the lab mice. They also did comparison tests to show that sopping up Gas6 resulted in far fewer secondary cancer nodules.
Irimpan Mathews, a protein crystallography expert at the SLAC National Accelerator Laboratory, joined the research effort to help the team better understand the binding mechanism between the Axl decoy and Gas6.
Protein crystallography captures the interaction of two proteins in a solid form, allowing researchers to take X-ray-like images of how the atoms in each protein bind together. These images showed molecular changes that allowed the bioengineered Axl decoy to bind Gas6 far more tightly than the natural Axl protein.
Next steps
Years of work lie ahead to determine whether this protein therapy can be approved to treat cancer in humans. Bioprocess engineers must first scale up production of the Axl decoy to generate pure material for clinical tests. Clinical researchers must then perform additional animal tests in order to win approval for and to conduct human trials. These are expensive and time-consuming steps.
But these early, hopeful results suggest that the Stanford approach could become a non-toxic way to fight metastatic cancer.
Glenn Dranoff, a professor of medicine at Harvard Medical School and a leading researcher at the Dana-Farber Cancer Institute, reviewed an advance copy of the Stanford paper but was otherwise unconnected with the research. "It is a beautiful piece of biochemistry and has some nuances that make it particularly exciting," Dranoff said, noting that tumors often have more than one way to ensure their survival and propagation.
Axl has two protein cousins, Mer and Tyro3, that can also promote metastasis. Mer and Tyro3 are also activated by Gas6.
"So one therapeutic decoy might potentially affect all three related proteins that are critical in cancer development and progression," Dranoff said.
Early but promising tests in lab mice suggest that a bioengineered 'decoy' protein, administered intravenously, can halt the spread of cancer from the original tumor site. Years of subsequent tests lie ahead. But this approach might one day provide an alternative, or supplement, to chemotherapy.
(Photo Credit: Stanford Engineering)
This image shows Jennifer Cochran, associate professor of bioengineering, and Amato Giaccia, professor of radiation oncology at Stanford.
(Photo Credit: Stanford Engineering)
Source: Stanford School of Engineering