Only one duplicate of a transformed Tmc1 quality causes dynamic hearing loss prompting significant deafness, both in people and in mice. It also addresses a major problem facing the field of genome editing: how to deliver the protein and RNA needed for the CRISPR-Cas9 technique into the cells of a living animal. Because the study was performed in mice, the implications for treating humans are still unclear. The mutation causes progressive hearing loss in young individuals, who eventually become deaf. That is on account of, up to this point, researchers didn't have the innovation to specifically treat the fundamental issue: the hereditary changes that damage hearing.
This was particularly important for the Beethoven mice, because the animals carry one mutated copy and one normal copy of the Tmc1 gene. The work represents the first time that a genome-editing protein has been ferried directly into the relevant cells to halt progression of genetic hearing loss.
The virus was injected into the mice's cochleas - a part of the inner ear which picks up sounds.
Nearly half of all deafness cases are caused by genetic factors, but treating inherited hearing loss is tricky. When it comes to using CRISPR-Cas9 for gene editing, researchers typically insert the DNA encoding the Cas9 complex into a cell, and let the cell use its own machinery to produce the gene-editing arsenal.
CRISPR-Cas9 technology is an enzyme that acts as an injectable "molecular scissor" and can be used by scientists to edit DNA strands.
Up to now, the biggest barrier to altering it has been the presence of a second, "good" copy of... In contrast, delivery of DNA encoding the Cas9 and guide RNA results in more modest DNA specificity and greater potential side effects.
Liu's group utilized a strategy that they had detailed in 2015. They packaged Cas9 and the guide RNA into a greasy bundle that slips inside cells - and doesn't stick around.
That let Cas9 hit the bad quality duplicate, and blur away before it could hurt the great one, says Liu, the Richard Merkin Professor and Vice-Chair of the Faculty at the Broad Institute, and Professor of Chemistry and Chemical Biology at Harvard University.
Liu's team began this work years before his team invented a more recent genome editing tool known as base editors.
Researchers use CRISPR-Cas9 to target a mutation in the Tmc1 gene that causes the loss of hair cells in the inner ear.
Following two months, hair cells in treated ears took after those in solid creatures - thickly pressed and tufted with hairlike groups.
The researchers used rodent models of human genetic disease since a rodent cochlea-the organ in the inner ear that sends sounds to the brain-is strikingly similar to that of humans, Gao says. On average, after four weeks, treated ears could hear sounds about 15 decibels lower than untreated ears.
"We set out to develop a genome-editing strategy to try to address this genetic hearing loss by disrupting the underlying genetic variant", said co-senior author David Liu, the Richard Merkin Professor, director of the Merkin Institute of Transformative Technologies in Healthcare, and core institute member at the Broad Institute, professor of chemistry and chemical biology at Harvard University, and HHMI investigator.
Other CRISPR "knockout" programs are approaching human studies, such as the lead program of Intellia Therapeutics (NASDAQ: NTLA), which is working with Regeneron Pharmaceuticals (NASDAQ: REGN) to disable the gene responsible for the misfolded protein that leads to the rare disease transthyretin amyloidosis.
That might be the key to future hearing loss treatments of all kinds, says Holt. Novartis is running a phase 1/2 trial testing a gene therapy to deliver the gene for a protein (human atonal transcription factor) and spur the growth of sound-sensing hair cells in the inner ear. "The conventional thinking in the field is that once you've lost your hair cells, it's hard to get them back".