An investigation into the feasibility of generating cell models for macular corneal dystrophy

Background and Aim: The CRISPR/Cas۹ genome editing technology has revolutionized manipulation of genomes. This tool consists a guide RNA and an RNA-guided endonuclease which enable sequence specific double strand breaks (DSB) in DNA regions of interest. DSBs are repaired by either non-homologous end joining or homology-directed repair which, respectively, allow introduction of a disease causing mutation or correction of an existing mutation. Corneal dystrophies are usually inherited disorders characterized by accumulation of insoluble deposits in the cornea. Macular corneal dystrophy, MCD, is an autosomal recessive stromal corneal dystrophy caused by mutations in CHST۶ which encodes a sulfotransferase. The aim of this proof-of-principle study is to investigate the feasibility of generating cell models for macular corneal dystrophy using CRISPR/Cas۹ gene editing strategy.Methods: CHST۶ was screened for mutations in ۲۱ unrelated MCD patients. The mutations identified and all previously reported MCD causing CHST۶ mutations were mapped unto the gene's sequence in order to identify the most appropriate target region for CRISPR manipulation. Once identified, two candidate sgRNAs were designed and cloned into appropriate CRISPR construct vectors. HEK۲۹۳FT cells were transfected with the vectors, and the T۷E۱ mismatch cleavage assay was used to identify the better guide RNA. Subsequently, HEK cells were co-transfected with the selected construct and an ssODN. The ssODN was a single-stranded DNA oligonucleotide that contained a point mutation of interest and acts as template for introduction of the mutation into HEK cell genomes by HDR after gRNA/CRISPR directed cleavage. Single GFP positive cells that represent transfected cells were separated using a fluorescence-activated cell sorter. The cells were cultured to obtain clones. DNA from the clones was isolated and sequenced by Sanger sequencing to verify introduction of the mutation.Results: Sequencing of DNA of forty single cell clones evidenced presence of one or two mutated alleles in fifteen clones.Conclusion: The results are encouraging vis a vis potential of creating cell models for MCD research. The protocol needs to be repeated in the more relevant corneal keratocytes or stromal stem cells. Preliminary studies indicate that the transfection of these cells, as compared to HEK cells, is more challenging.

Keywords:

CRISPR/Cas۹ , gene editing , macular corneal dystrophy , mutation