The best things come in small packages: new ‘mini gene’ therapy for Usher F1

New mini gene therapy could successfully restore hearing and sight to those who suffer from Usher 1F.

A recent paper from researchers at Harvard Medical School (MA, USA) has proposed a new ‘mini gene’ therapy to fix the DNA mutation that leads to both sight and hearing loss in people suffering from the genetic disease Usher syndrome type 1F.

There are currently no treatments for Usher 1F, a genetic disorder characterized by the birth of children who are typically born profoundly deaf, have poor balance and progressively lose their sight until they become totally blind by the time they reach adulthood. The coupled progressive deterioration of sight and hearing is thought to be due to a mutation of the gene that codes for the protein protocadherin-15, which has a slightly different role in each sense but is vital for both to function correctly.

The research team had previously established that protocadherin-15 combines with another protein called ‘cadherin 23’ in the hair cells of the inner ear to create filamentous structures that mechanically open ion channels when sound waves cause vibrations in the inner ear. The opening of these ion channels allows the electrical impulses to be sent to the brain and be detected as sound. When protocadherin 15 is not produced due to genetic mutations, these signals cannot enter the hair cells, preventing sound waves from being converted to electrical signals and ultimately rendering the patient deaf.

To fix this issue, co-senior author David Corey proposed a gene therapy approach to deliver the gene for protocadherin 15 to the hair cells. However, the genetic code for the protein is too big to fit in the viral capsules typically used for gene therapy. Therefore, the researchers had to find a way of reducing the DNA code to a suitable size while still being able to code correctly for the protein.

First, the team enlisted the help of co-senior author Marcos Sotomayor (Ohio State University, OH, USA) to map all 25,000 atoms of protocadherin 15 protein using x-ray crystallography and a cryo-electron microscope. Sotomayor revealed that the protein is structured to form 11 links in a chain with a binding region at one end that connects to cadherin 23.

With the protein structure understood, Sotomayor created a panel of eight truncated versions of the protein where between 3-5 of the 11 links were removed. These protein variants were then reverse-engineered to acquire the genetic code required to produce each of them. These “mini genes” were then tested in inner ear cells in vitro to validate that they still bound to cadherin 23 and retained their function. Of the eight mini genes, three were small enough to fit inside a viral capsid and so were selected for the next stage of the experiment.

Using mice that had been genetically modified to not produce protocadherin 15 each of the three mini genes was tested to evaluate their performance. There only one mini gene Was able to successfully produce a protein in vivo that was able to bind with cadherin 23 and open the ion channels, allowing electrical signals to be transported to the brain. Further auditory testing showed that the previously deaf mice could now hear.

These are very promising results; however, the ultimate goal for this research team isn’t to fix deafness in Usher syndrome sufferers but rather to prevent blindness, “the whole project was designed to study the ear with the idea that something that works in the ear can later be applied to the eye, as an article of faith…While the best test system is the mouse inner ear, the immediate goal is a treatment for blindness,” noted Coney.

The reasons for this are twofold. Usher 1F patients are born totally deaf and may lack hair cells, likely preventing this gene therapy from yielding much success, while the blindness is a slow onset, providing a bigger window of opportunity to correct the condition and prevent patients losing their sight. Furthermore, cochlear implants can partially resolve the deafness, while there is no treatment to protect their sight.

The team initially focused on the use of this gene therapy to restore hearing for logistical reasons. The immediate hearing loss, when compared to the slow onset blindness allowed the mini genes to be tested in much more rapidly established mouse models with a more immediate readout. This allowed the team to establish that the mini gene could successfully be used to code for a functional protein in vivo before starting on the issue of fixing sight.

The team is now testing the mini genes in the eyes of zebrafish, which experience more rapid vision loss when protocadherin 15 is knocked out in the retina providing results faster than a mouse model. The research group hopes that if these trials with zebrafish are successful, they can start using it to treat blindness in primates and eventually humans.