How to prevent deafness in Beethoven
A gene defect in humans causes progressive hearing loss in humans, resulting in deafness by their mid-20s. The same genetic mutation causes deafness in the Beethoven-mouse model.
A more precise CRISPR-Cas9 gene-editing system
“We developed a better, as more precise version for CRISPR-Cas9 gene-editing to prevent the hearing loss. It allows us to selectively shut down the defective copy of the geneTmc1, which is the disease-causing mutation for the hearing loss in Beethoven mice. The healthy copy of the gene remains unaffected. This is really remarkable, because we are talking about a single incorrect DNA letter in the defectiveTmc1copy, in a score of three billion letters in the mouse genome,” says Bence György. He conducted the research during his post-doc at Harvard Medical School, with co-workers from Boston Children’s hospital. Bence is now Head of the Translational Group in theClinical Center of the Institute of Molecular and Clinical Ophthalmology Basel, Switzerland (IOB).
High precision gene silencing
The classic CRISPR-Cas9 gene editing works with gRNA as guiding molecule. gRNA recognizes the mutation in the target DNA and guides Cas9 to snip it. “Unfortunately this guide RNA is not entirely precise. To prevent cutting the wrong DNA we worked with a modified Cas9 enzyme from Streptococcus pyogenes instead of the ‘standard’ from Staphylococcus aureus. In addition, our optimized system integrates a second level of recognition. Next to gRNA we used a modified Cas9 enzyme which recognizes the single letter change in the Beethoven mice DNA. In experiments in Beethoven cells containing one defective and one normal copy of the gene, at least 99 percent DNA-snips occurred exclusively in the defective copy of the gene,” Bence explains.
The road is long, but promising
Targeting single-point DNA mutations means hope for patients with many diseases. Scanning the global ClinVar database, which contains all known genetic mutations linked to human diseases, the new tool could potentially selectively inactivate 3,759 mutant gene variants. Collectively those are responsible for one-fifth of dominant human genetic mutations, including several disease genes that cause eye disease.
A therapy for any of these diseases is far in the future, but the proof of principle for selectively silencing mutant genes will help to accelerate their development.