Finding a cancer ‘off switch’ for mutant Ras

Scientists from AstraZeneca/MedImmune’s biotech units have advanced our understanding of Ras biology in another high impact publication, this time in Nature Communications.

Building from our recent high impact publication in Science Translational Medicine to develop an antisense oligonucleotide against KRAS as an anti-cancer therapy, our scientists have uncovered another approach in which the mutant Ras protein can be neutralised.

In a study published today in Nature Communications our scientists at MedImmune, the global biologics research and development arm of AstraZeneca, and AstraZeneca’s IMED Biotech Unit—alongside researchers at Oxford University—have successfully inhibited a mutant form of Ras linked to cancer. This advancement raises hope that highly specific small biologics can be developed to ‘turn off’ a cancer-causing mutation — a concept and approach that had been previously considered unfeasible by the scientific community.

Understanding Mutant Ras

Ras is a protein that resides inside cells and plays a critical a role in cellular proliferation. Guanosine triphosphate (GTP) binds to normal Ras inside the cell, creating an active form—or ‘on’ form—of Ras that allows other proteins to bind to Ras and carry the signal onward. Ras then turns GTP into guanosine diphosphate (GDP), causing Ras to change into an off-position shape to which downstream signaling proteins cannot bind. Eventually, GDP is removed from Ras allowing GTP to bind again, reactivating Ras by changing it back to the active shape.

Mutant Ras causes cells to multiply uncontrollably because it is ‘switched on’, suggesting this causes cancer cells to reproduce uncontrollably resulting in aggressive disease resistant to most therapies. Because of this, mutant Ras has been a prime target for designing anti-cancer therapies. To target it with a biologic, we needed to get a ‘large molecule’ into the cell.

Collaboration to Challenge Convention

Researchers at MedImmune and IMED Biotech Units took on two approaches to target mutant Ras.

“First, we had the hypothesis that the active form of mutant Ras is not permanently ‘switched on’ but is changed into the ‘off’ state before being reactivated. If true, it might be possible to permanently capture this inactive form using a biologic tool.” commented Ralph Minter, Director, Fellow, Antibody Discovery and Protein Engineering at MedImmune. Ralph went on to say, “To do this we created a specific designed ankyrin repeat protein, or DARPin for short, which is an antibody-like protein yet one-tenth of the size of a full-length antibody.”

DARPins are very stable inside cells and easily engineered to perform specific functions, in this case to bind to the inactive mutant Ras-GDP and keep it in the off-state and prevent it from being switched on. IMED Biotech scientists added their expertise in protein crystallography, and together the teams confirmed the mechanism of action.

“This was a great opportunity to work with the MedImmune team as we were able to use our extensive assay and structural biology platforms built around Ras to quickly demonstrate the mechanism of the DARPins.” said Chris Phillips, Associate Director of Structural Biology, IMED Biotech Unit.

The next step was to demonstrate that the specific DARPin, K27, inhibits Ras signaling in living cells. Oxford University scientists provided expertise to show that cells that contain mutant Ras, artificially induced to express K27, exhibited inhibition of all known Ras signaling pathways. K27 also stopped cell growth and proliferation, compared with cells not expressing K27, providing the final evidence supporting this potential approach in cancer therapy.

This work, while preliminary, shows a biologic tool could be designed to bind to mutant Ras and keep it in the off-state, laying the groundwork for new strategies for designing anti-cancer therapeutics.