USH1F is caused by mutations in the protein Protocadherin-15 (PCDH15). Because PCDH15 is a very large protein, the genetic sequence that cells use to make PCDH15 protein is too long to fit into the viruses that are used to deliver gene therapy treatments.
In a previous study, these researchers tried to overcome this “size” problem by examining the PCDH15 protein and designing smaller versions of it (called mini-PCDH15). They hoped these mini-PCDH15s would accomplish the same function as the regular large version of PCDH15. When the researchers used a virus to deliver their different mini-PCDH15 proteins to mice with mutant PCDH15 and hearing loss, they improved hearing function. However, they found that some of their mini-PCDH15 proteins worked better than others, and they were unsure why.
In this study, they wanted to identify why some mini-PCDH15 proteins work better than others at restoring hearing in mice with mutant PCDH15. They selected three of the mini-PCDH15 proteins they had tested in mice previously, two of which restored hearing function in mice and one which did not, and they did experiments to try to identify what was unique about the mini-PCDH15 proteins that worked well. They found that one of the biggest differences between the mini-PCDH15 proteins that worked and those that did not was their stability at higher temperatures, and their flexibility. The best mini-PCDH15 protein was flexible and stable at higher temperatures, whereas the worst mini-PCDH15 was rigid and less stable at high temperatures.
What this means for Usher syndrome: By understanding what characteristics are important to maintain as a protein is shortened, researchers will be able to design better versions of mini-PCDH15 for gene therapies in the future. In addition to PCDH15, many other proteins involved in Usher syndrome are also very large. The viruses used for gene therapies cannot deliver their full-length genetic sequences. So, this same “mini-protein” strategy could be used to overcome this problem.
