The goal of autologous cell replacement for retinal degenerative disease depends on the ability to correct a patient’s pathogenic mutation before transplantation. Here we have generated patient-specific iPSCs from an XLRP patient and showed that transfection of CRISPR gRNA/Cas9 alongside a donor homology template corrects a point mutation within the RPGR gene ORF15 region, a challenging DNA sequence to manipulate. We believe this is the first report of successful gene correction in human iPSCs associated with inherited retinal degeneration. The present study is significant because it demonstrates that gene editing can be engineered to precisely target a repeat-rich region with high GC content. RPGR spans a 59,000-bp region8 in a human genome containing over 3 billion bases, yet 13% of sequencing reads had correction of a single nucleotide mutation within the repetitive, low-complexity ORF15 region, demonstrating that despite presumed technical hurdles based on the genomic sequence, the RPGR gene is open to CRISPR repair. This rate of repair indicates that CRISPR gene editing may reasonably be part of an autologous transplantation treatment for other photoreceptor degenerations in the future.

The alternative and currently the more developed, therapeutic modality for genetic retinal disease is gene supplementation therapy. Clinical trials for RPE-associated Leber congenital amaurosis reported in 2008 that gene therapy9 is safe and beneficial for patients. However, recently published three-year follow-up data found that initial gains in retinal sensitivity waned over time and were not accompanied by meaningful improvements in objective measures of visual function10. Moreover, gene therapy in animal models may produce much greater and more sustained rescue in humans at comparable vector doses, suggesting that interspecies genetic differences could limit translation to humans. These and other limits of gene therapy—such as its relative inability to treat dominantly inherited or late-stage disease—might be better addressed through a regenerative gene-corrected iPSC transplantation approach.

The 13% correction rate that we achieved is comparable to other studies of homology-directed CRISPR editing11 and exceeds previous iPSC gene editing studies reporting correction rates closer to 1%12. Such a rate can provide sufficient therapeutic cells, but gene-editing technology is also rapidly improving. The piggyBac transposon is a mobile genetic element that can insert and excise exogenous constructs in a “footprint free” manner for CRISPR in iPSCs11. Minimizing error-prone non-homologous end-joining at DNA cleavage sites through inhibition of DNA ligase IV can also increase the yield of repaired cells13. Definitive analysis of off-targeting rates will be required before eventual clinical use. With repaired patient-specific iPSCs in hand, the next step for iPSCs transplantation therapy is differentiating clonally corrected iPSCs into photoreceptors capable of integrating within the retina. Although a number of challenges remain, significant progress has been made in preclinical transplantation experiments using post-mitotic photoreceptor precursor cells or iPSC-derived en bloc retinal tissue14,15.