GENE THERAPY FOR OCULAR DISORDERS

Precision modification of the genome will soon become a reality in the clinic, Luigi Naldini MD, PhD, director, HSR-TIGET, San Raffaele Telethon Institute for Gene Therapy told a session of the EURETINA Winter meeting.
Prof Naldini presented an overview of some recent advances and applications of gene therapy for the treatment of a broad range of genetic diseases, including ocular disorders. 'The concept of gene therapy, exploiting genes as therapeutics and transferring those genes into cells to express, or to replace a malfunctioning gene or to instruct a novel function,' was a familiar concept. However, Prof Naldini explained that efficient gene transfer in the clinic is largely dependent on engineered viruses to mediate efficient insertion of foreign genes into target sites. The main vectors used for gene therapy are derived from adeno-associated virus, lentiviruses and adenoviruses.

Targetting the eye
He outlined several challenges to be met by gene therapy, including regulation of delivered transgenes and insertional mutagenesis. Efficient vectors will be required to target many organs. The eye may be a suitable target as there are relatively few cell types to reach. Long-lived tissue cells are somewhat more amenable to stable gene transfer and they don't necessarily need integration of the vector. Contrastingly, stem cells require stable insertion of the gene to allow for maintenance of the delivered gene throughout the progeny of the cell.

There are significantly improved vectors available to researchers today, including lentiviral vectors, originally designed to improve efficiency of conventional gamma retroviral vectors. One example of the use of lentiviral vectors is a trial currently being performed at Prof Naldini's institute demonstrating the efficiency of these gene therapy vectors. The trial uses hematopoietic stem cells taken from a patient and then modified to correct the genetic fault before re-transplantation back into the patient to repopulate the whole hematopoietic lineage.

Prof Naldini's team, using such technique, has developed a treatment for a lysosomal storage disorder which affects most tissues in the body, including the CNS and sometimes the retina, in which significant tissue damage can occur. 'The rationale of using HSC therapy is because the progeny of the cells will migrate into the tissue to become macrophages which are scavenger cells and so if the cells are now genetically modified so that they are able to scavenge and destroy the storage material, you can then ameliorate the effect'.

The disease being targeted is a severe storage disorder of early childhood for which there is no current treatment, a disorder known as metachromatic leukodystrophy. Transplantation of genetically modified autologous stem cells to metachromatic leukodystrophy patients, more than one year after transplantation, show that a majority of the blood cells of such patients successfully carry the corrected transgene.

The genetically modified cells express the required enzyme to a high level and, according to Prof Naldini, 'where before there was no [gene] expression, now there is up to ten-fold normal expression and this has allowed the child, for which we expected a dramatic decrease in motor performance and mental capacity, to prevent this disease at one-year follow-up and this is now being applied to other patients today'.

He proposed to delegates that such 'gene therapy may become a real option or choice potentially applicable to any patient in comparison to donor-derived allogenic transplant which has limited application'.
Another challenge for gene therapy is to develop tight controls that limit the expression of the transgene to the cell type in which it is required. 'Whether to prevent transgene toxicity or to prevent an immune response we need to be able to stringently target the transgene expression into the desired cell type. To do this we use promoters derived from tissue specific genes so your transgene will mostly express in the tissue that you want'.

In addition, Prof Naldini stated that 'while we don't invent much, we do learn from nature and nature has actually developed a system for the post-transcriptional regulation of a gene based on micro RNA'.
Micro RNAs are non-coding RNAs which act to regulate other genes by repressing translation.