Researchers, led by investigators at the Schepens Eye Research Institute of Massachusetts Eye and Ear, and the Department of Ophthalmology, Harvard Medical School, Boston, have shown that recently developed gene editing technology is capable of depleting VEGF2 expression demonstrating a potential therapeutic approach to retinal angiogenesis. The technology, commonly referred to as CRISPR / Cas 9, was delivered to cell models using viral vectors where it achieved complete blockage of VEGF-induced activation of Akt (an intrinsic marker of angiogenesis), and the halting of the proliferation, migration and tube formation of primary human retinal microvascular endothelial cells. As angiogenesis is a feature of many disorders including neo-vascular age-related macular degeneration (ARMD), proliferative diabetic retinopathy, and tumor growth and metastasis, the technology may hold significant potential as a future therapeutic approach.
CRISPR / Cas-9 technology, formally referred to as “clustered regularly interspaced short palindromic repeats”, originally emerged from research into prokaryotic immune defence systems. Repetitive sequences isolated from a number of prokaryotic and archaebacteria, first identified in 1987 by Yoshizumi Ishino and Atsuo Nakata, found an unusual set of 29 nucleotide repeats interspersed with five intervening 32 nucleotide sequences, seemingly without any discernible function. Over a period of 10+ years, as increasing numbers of DNA sequences were deposited in the public databases, similarly structured repeat sequences were reported from several different bacterial and archael strains. Independent research showed that the spacer sequences separating the direct repeats appeared to have a phage associated origin and that viruses were unable to infect cells that carried spacer sequences corresponding to their own genomes. In essence, the system as a whole appeared to represent an un-expected and sophisticated immune system for prokaryotes, essentially a new mechanism that provided an immune memory of previous phage infections and facilitated rapid clearing of subsequent phage invasions that had previously infected the cell. Researchers were quick to adapt such primordial sequences by modifying transcripts to essentially turn off selected genes using short guide sequences to the gene of interest. To date, significant venture capital has been invested in the technology now being employed for multiple industrial applications.
Returning to angiogenesis, VEGF production, and in particular the VEGF2 receptor, is known to mediate the majority of the blood vessel growth associated with proliferative diabetic retinopathy and neo-vascular ARMD. A modified CRISPR / Cas 9 system using an endothelial specific gene promoter (ICAM2) with VEGF2 guide sequences was cloned into an AAV delivery system and used to infect primary human retinal microvascular endothelial cells. Expression of the CRISPR / Cas 9 system almost completely blocked Akt activation and halted tube formation, both of which represent key steps in physiological angiogenesis in vivo. As highlighted by the researchers, while current “anti-VEGF agents can reduce angiogenesis and vascular leakage, therapeutic challenges remain, including the need for chronic treatment and the fact that a significant number of patients do not respond. Gene therapy targeting genomic VEGFR2 in vascular ECs, using AAV-CRIPSR/Cas9, may provide a potential novel alternative approach.”