MRI sees gene expression in living animals

In a first, Carnegie Mellon University scientists have "programmed" cells to make their own contrast agents, enabling unprecedented high-resolution, deep-tissue imaging of gene expression. The results, appearing in the April issue of Nature Medicine, hold considerable promise for conducting preclinical studies in the emerging field of molecular therapeutics and for monitoring the delivery of therapeutic genes in patients.
"For 20 years it has been the chemist's job to develop agents that can be used to enhance MRI contrast," said Eric Ahrens, assistant professor of biological sciences in the Mellon College of Science at Carnegie Mellon. "Now, with our approach, we have put this job into the hands of the molecular biologist. Using off-the-shelf molecular biology tools we can now enable living cells to change their MRI contrast via genetic instructions."
"The new imaging method is a platform technology that can be adapted for many tissue types and for a range of preclinical uses in conjunction with emerging molecular therapeutic strategies," Ahrens said.
Ahrens' new approach uses magnetic resonance imaging (MRI) to monitor gene expression in real-time. Because MRI images deep tissues non-invasively and at high resolution, investigators don't need to sacrifice animals and perform laborious and costly analysis.
To trigger living cells into producing their own contrast agent, Ahrens gave them a gene that produces a form of ferritin, a protein that normally stores iron in a non-toxic form. This metalloprotein acts like a nano-magnet and a potent MRI "reporter."
A typical MRI scan detects and analyzes signals given off by hydrogen protons in water molecules after they are exposed to a magnetic field and radiofrequency pulses. These signals are then converted into an image. Ahrens' new MRI reporter alters the magnetic field in its proximity, causing nearby protons to give off a distinctly different signal. The resulting image reveals dark areas that indicate the presence of the MRI reporter.