Gene-editing: the next frontier in retinal disease treatment

In July this year, Associate Professor Alex Hewitt, head of clinical genetics at the Centre for Eye Research Australia (CERA) in Melbourne and a principal researcher at the Menzies Institute for Medical Research at the University of Tasmania, received a Research Excellence Award as the top-ranked NHMRC Practitioner Fellowship applicant.The award recognised Assoc Prof Hewitt and his team’s investigations into gene-based therapies for eye diseases such as glaucoma and age-related macular degeneration (AMD), as well as less common retinal dystrophies for which there are currently no effective treatments.While the CERA team had been involved in gene discovery research for some time, it was only within the last two years that it began to place a concerted focus on gene editing.{{quote-A:R-W:450-I:2-Q: Gene editing could revolutionise the treatment of inherited eye disease, -WHO:Associate Professor Alex Hewitt, head of clinical genetics at the Centre for Eye Research Australia (CERA) in Melbourne}}“The gene-editing breakthroughs in the last few years have been amazing, and we are now looking to adapt the technology that other people have developed for eye disease applications,” Assoc Prof Hewitt explained.He noted that in the UK, inherited retinal diseases had become the leading cause of blindness in working-aged individuals. “These are important diseases that often haven’t had the limelight that other common complex diseases have.”Although there is support available for patients with inherited retinal disease, such visual aids, Assoc Prof Hewitt noted that “there is no magic script we can fill out that will halt their disease”.“Knowing that gene-editing technology could potentially be adapted to cure inherited retinal diseases is the major impetus for my research,” he said.HARNESSING CRISPRAccording to Assoc Prof Hewitt, a key development in the field of gene editing was the discovery that bacteria possessed an adaptive immune syst similar to that observed in humans.Bacteria use this syst, known as CRISPR (clustered regularly interspaced short palindromic repeats), to fend off viruses, which spread by hijacking the bacteria’s own DNA to reproduce their genetic material. CRISPR comprises a series of proteins that are able ‘cut out’ a specific region or sequence of DNA, such as foreign DNA inserted by an invading virus.“Research groups have adapted CRISPR to work in mammalian cells, which allows us to very accurately go in and edit the genome,” Assoc Prof Hewitt said. “It is still a fair way away from being ready for clinical use – there are many further technological advances that have to occur and safety and regulatory licensing considerations – but essentially there are potentially some eye diseases whereby you could bypass the disease-causing variant by editing out the genetic change.”{{image3-a:l-w:400}}Assoc Prof Hewitt said that currently, the technology was effective for deleting areas of DNA but that it was not very good at correcting disease-causing variants, especially in vivo – in the retina.In layman’s terms, he suggested the genome could be thought of as a cookbook. “If there’s a typo in one part of a recipe, you might be lucky enough to be able to skip that whole step and still get a nice cake. That’s where you could just delete that whole sentence or whole paragraph and not actually disrupt the recipe,” he explained. “However, for some recipes it will be crucial that you actually correct the typo to ensure you have a nice cake at the end.“In the dish (in vitro), using st cells and other cells taken in the laboratory, we can correct the mutation, but at the moment, the efficiency of the technology is low and is not good enough to be used in vivo in retinal cells.”The technology has been successfully used on liver cells, but Assoc Prof Hewitt noted that these were significantly different to retinal cells. “If you can only correct one in 1,000 cells in the liver, then you can just wait for the liver to regenerate with those healthy cells – but that won’t happen in the retina. Correcting the ‘spelling mistake’ in the retina is still definitely a challenge and requires ongoing work, but we have recently shown in studies on mice that you can delete or disrupt a gene in a retina fairly easily.”‘Off-target effects’ are another challenge in gene editing. Instead of cutting out the target DNA, it is possible that CRISPR will identify and edit a similarly coded section of DNA in an area that affects a different gene.“If, for example, CRISPR goes to a gene that suppresses cancer then it would be a concern if we turned on or activated a cancer-causing gene using this technology,” Assoc Prof Hewitt said.SOCIAL ACCEPTANCEOutside of the lab, Assoc Prof Hewitt said an important consideration in gene-editing research was social acceptance: “If people are worried about genetically-modified food, how are they going to feel about genetically-modified humans?”To answer this question, the CERA team conducted a social media survey last year involving 12,000 people across 185 countries to ascertain the public’s perceptions to bryonic editing as an arm of in vitro fertilisation and the editing of cells for the treatment of disease.{{quote-A:R-W:450-Q:59% of respondents agreed with the use of gene editing to cure life-threatening or debilitating disease, with only about 10% disagreeing with this application.}}The results showed that 59% of respondents agreed with the use of gene editing to cure life-threatening or debilitating disease, with only about 10% disagreeing with this application.“Most people supported the medical applications of this technology, be it in the bryo or in somatic cells in the tissue, but there wasn’t good support at all for enhancent editing – in other words, the editing of non-medical traits such as eye colour,” Assoc Prof Hewitt commented.“All the work that we’re focusing on is in the applications of the eye. We’re not actually doing any bryo editing insofar as trying to alter subsequent generations. We are just focusing on the correction of genetic variance in vivo and using induced pluripotent st cells to study and model disease.”ST CELL MODELLINGSince purchasing an automated st cell-generating robot in 2014, CERA is said to have become the ‘go-to place’ for researchers seeking st cells for eye research. A team led by Assoc Prof Alice Pébay has built an impressive repository of patient-specific st cell lines for a variety of diseases, including inherited retinal diseases as well as AMD and glaucoma.Assoc Prof Hewitt explained that st cell technology erged approximately 10 years ago. A Japanese orthopaedic surgeon developed a process by which an adult cell such as a skin cell could be used to generate undifferentiated st cells. These so-called induced pluripotent st cells could then be differentiated into other tissue types – for example, retinal cells.“This means we can study retinal cells without actually taking a sample from the back of the eye, which is terribly exciting,” Assoc Prof Hewitt said.“By studying the cells from the eyes of people who have become blind due to different diseases, and by comparing the protein and gene expression profile of those cells compared to those from people with healthy eyes, we can hopefully delineate the important pathways in these diseases and open up new therapeutic targets that could lead to the development of drugs to correct those sorts of alterations.”FUNDING HURDLESUsing st cells as a disease-modelling tool has made it easier for the CERA to progress its gene-editing research, and the field in general is moving very quickly, according to Assoc Prof Hewitt. However, a major barrier to the technology’s development is funding.“Medical research funding has never been tighter in Australia, and it would be great to see if we could ulate America’s philanthropic level of funding,” Assoc Prof Hewitt said. “That would allow our research team to pursue high-risk scientific questions that might lead to breakthroughs faster.”He added that given the significant potential for gene editing to address retinal diseases, it was important to ensure there would be equity and access to the erging technology once available.“Gene editing could revolutionise the treatment of inherited eye disease,” Assoc Prof Hewitt stated. “We have to be sure that this technology is not rushed into care because that will just delay the field further, but I’m certainly very optimistic about the improvents and advances and breakthroughs that are currently occurring.”