Professor David Liu of the Broad Institute, a leading scholar in gene editing, and Professor Jeffrey Holt of Harvard Medical School, who specializes in otolaryngology, recently published a new study in the journal Science that offers a potential treatment strategy for many people with hereditary deafness.
Using novel monobase editing techniques, they successfully corrected a genetic error that causes deafness in the mice’s inner ear, allowing the mice to regain some of their hearing. The researchers believe that this method, which has been further refined, will also help improve hearing in deaf patients.
It is worth noting that this is the first successful example of the use of genome editing techniques to repair recessive pathogenic mutations.
The study was recommended by Science’s official website (Photo: Science Screenshot)
Although hearing loss is not fatal, the impact on life and society is enormous. Technical equipment such as cochlear implants can help, but for patients, the fundamental problem of deafness is not solved. About half of people with deaf ears are caused by genetic mutations. In recent years, the advent of gene editing tools has given these genetic ally deaf people hope for their roots.
Mutations in genes that encode transmembrane channel-like protein 1 (TMC1) are a common cause of deafness. Two years ago, Professor Liu’s team made a breakthrough by using a CRISPR-Cas9 gene-editing method to repair a dominant disease mutation of Tmc1 in mice, slowing hearing loss in mice.
A so-called dominant disease mutation, in which only one of the two copies of a gene goes wrong and causes a genetic disease. In this case, “erasing” the copy of the gene in question and letting the good copy work may have therapeutic effects.
Professor Liu Ruqian, gene editor (Photo: Harvard University)
However, “most genetic diseases are not caused by dominant mutations, but by recessive mutations, including most inherited hearing loss.” Professor Liu Ruqian said. Invisible disease-causing mutations mean that both copies of genes from both parents have problems. Therefore, it is not just about “erasing” problematic genes, but “fixing” at least one copy of the gene to restore their normal function.
In the new study, scientists hope to repair Tmc1’s recessive disease-causing mutations. The mutation differs from the normal version of only one amino acid, but is sufficient to cause the hair cells in the inner ear to deteriorate rapidly, unable to effectively convert sound signals into nerve cells sent to the brain, causing hearing loss.
For the purpose of repairing genes, Dr. Wei-His Yeh, co-author of the study, and colleagues first developed and optimized a single base editor. Traditional CRISPR is cut on the genome, while the single base editor is different, allowing a single base conversion of DNA without cutting off double-stranded DNA.
Because the single base editor is large and not suitable for commonly used adeno-related virus (AAV) vectors, the researchers designed a clever two-carrier delivery method: the single base editor is divided into two halves and packaged in two AAVs. When they enter the same cell, the two halves are combined again, heading to the DNA chain to find the editing target.
This delivery may sound complicated, but it actually proves effective and, crucially, rarely causes unnecessary deletion or insertion of DNA. “We hardly see any evidence of off-target editing, ” says Professor Liu. “
In the mouse cochlea, the green cells were transferred to the restored Tmc1 gene (Photo: Resources 2; Credit: Olga Shubina-Oleinik, Boston Children’s Hospital)
The researchers first verified in a petri dish that this single-base editing system based on a dual AAV vector can effectively correct Tmc1 recessive pathogenic mutations. They then tested the therapeutic potential of this method in mice with mutations in the Tmc1 gene. One day after the mice were born, they injected a gene-editing system into the mice’s inner ear.
The treatment saved hair cells in the inner ear of mice. The researchers were pleased to observe that genetically edited animals had well-shaped hair cells in their inner ears and were able to detect the signaling of those hair cells.
The single base editor delivered by the double AAV gives the inner ear of Tmc1 mutant mice a normal form of hair cells (Photo: Resources 1)
So can these hair cells help mice regain their hearing? Dr Yeh said she did an informal test: a clapped, mouse that had completely lost hearing jumped up and turned to look at her – they heard it! Of course, the researchers also conducted formal tests, with brain waves showing that genetically deaf mice were able to respond to sounds as low as 60 decibels after four weeks of treatment, compared with those without treatment who did not respond to noise of up to 110 decibels.
The researchers say they will continue to improve the gene therapy so that the treated mice can hear lighter sounds like normal mice, and allow the hearing to improve longer.
The results “support the further development of gene editing techniques to correct point mutations that cause genetic disorders, including hereditary deafness,” the researchers concluded. We look forward to the efforts of scientists to bring good news to more patients.