Better Genetic Fix
Precision tools for therapy
Context: For a genetic engineer, putting an entire gene into a cell is much easier than correcting a few misspellings in DNA. The relatively blunt tools of recombinant DNA are ill-suited for fine tasks like editing specific DNA sequences in living cells. Now, researchers at Sangamo BioSciences in Richmond, CA, have invented the most precise tools to date for altering DNA sequences in living cells.
Methods and Results: Michael Holmes and colleagues assembled collections of proteins known as zinc fingers. Different combinations of zinc fingers can recognize DNA sequences of up to 30 letters, which is enough to pinpoint specific spots in the genome. To change the DNA at the intended site, the researchers add an enzyme that cuts DNA and triggers a natural process that repairs breaks in DNA by copying sequences from matching strands. The last part of the DNA-changing machinery is the DNA sequence to be copied. The researchers demonstrated their technique by inserting the DNA-binding machines into human white blood cells that had mutations that cause severe combined immune deficiency. The machines fixed the mutations in nearly a fifth of the cells, a phenomenal success rate.
Why It Matters: Experimental therapies for cancer, transplant rejection, and immune disorders remove blood cells or stem cells, alter their DNA, and infuse them back into the patient. In the past, such techniques have caused fatal side effects because researchers could not uniformly control where those genetic alterations took place. The Sangamo technique avoids this risk by pinpointing exactly where a new DNA sequence will go. It could also effectively erase a gene. Sangamo hopes to turn off a gene for a protein on white blood cells that HIV uses to enter cells. But the benefits go beyond possible therapies: the Sangamo technique promises to become a standard precision tool for biotechnology.
Source: Urnov, F. D., et al. 2005. Highly efficient endogenous human gene correction using designed zinc-finger nucleases. Nature (in press).
How to image brain plaques
Context: The sticky plaques characteristic of Alzheimer’s disease build up in the brains of patients well before cognitive symptoms appear. But the most reliable noninvasive method for detecting these plaques – positron emission tomography (PET) – is prohibitively expensive and unwieldy, requiring hard-to-handle, short-lived radioactive materials. Now, two groups of researchers have demonstrated more-practical methods for monitoring the plaque development characteristic of Alzheimer’s disease. One of them promises higher-resolution images than PET.
Methods and Results: Conventional imaging chemicals do not work well with amyloid plaques because the brain is separated from potentially toxic chemicals in the blood by the blood-brain barrier; nor can the typically water-loving chemicals readily access the fatty plaques. Martin Hintersteiner of the Novartis Institutes of Biomedical Research in Basel, Switzerland, found a dye that crosses the blood-brain barrier in mice and binds to plaques. In a procedure called near-infrared imaging, the dye yields a quantitatively stronger signal as the number of plaques in the brain increases. In a separate study, Makoto Higuchi and colleagues at Riken Brain Science Institute and Dojin Laboratories in Japan found another dye that works in conjunction with magnetic-resonance imaging, a technique common in both research and the treatment of patients. The resulting images correlated well with images of the same mice obtained after staining slices of their brains.
Why It Matters: Usually, researchers studying Alzheimer’s must dissect animal brains to see the effects of treatments. But monitoring a living brain over time would yield much more useful information and might even help in early diagnosis. Because Hintersteiner’s and Higuchi’s imaging techniques cost as little as a fiftieth as much as PET, and because the chemicals are easier to work with, live experiments once considered out of reach can now be performed on animals – and, with the MRI technique, potentially even people.
Sources: Higuchi, M., et al. 2005. 19F and 1H MRI detection of amyloid ß plaques in vivo. Nature Neuroscience 8:527-33.
Hintersteiner, M., et al. 2005. In vivo detection of amyloid-ß deposits by near-infrared imaging using an oxazine-derivative probe. Nature Biotechnology 23:577-83.