Genetic correction and analysis of induced pluripotent stem cells from a patient with gyrate atrophy.

Gene-corrected patient-specific induced pluripotent stem (iPS) cells offer a unique approach to gene therapy. Here, we begin to assess whether the mutational load acquired during gene correction of iPS cells is compatible with use in the treatment of genetic causes of retinal degenerative disease. We isolated iPS cells free of transgene sequences from a patient with gyrate atrophy caused by a point mutation in the gene encoding ornithine-δ-aminotransferase (OAT) and used homologous recombination to correct the genetic defect. Cytogenetic analysis, array comparative genomic hybridization (aCGH), and exome sequencing were performed to assess the genomic integrity of an iPS cell line after three sequential clonal events: initial reprogramming, gene targeting, and subsequent removal of a selection cassette. No abnormalities were detected after standard G-band metaphase analysis. However, aCGH and exome sequencing identified two deletions, one amplification, and nine mutations in protein coding regions in the initial iPS cell clone. Except for the targeted correction of the single nucleotide in the OAT locus and a single synonymous base-pair change, no additional mutations or copy number variation were identified in iPS cells after the two subsequent clonal events. These findings confirm that iPS cells themselves may carry a significant mutational load at initial isolation, but that the clonal events and prolonged cultured required for correction of a genetic defect can be accomplished without a substantial increase in mutational burden.

DNA methylation assay for X-chromosome inactivation in female human iPS cells.

Remarkable interest in the epigenetic status of human induced pluripotent stem (iPS) cells inspired numerous studies of their X-inactivation patterns. However, both the presence and the absence of X-inactivation have been described to date in undifferentiated iPS cells. The reasons for the discordant results between different studies are unclear, and further X-inactivation testing is warranted for all female human iPS cell lines. Some of the inconsistency in the current data most likely results from the use of different X-inactivation assays by different authors. We provide a detailed protocol for a simple, reliable and affordable X-inactivation assay based on promoter methylation and CAG-repeat polymorphism in the human androgen receptor (AR) gene at Xq11.2. This assay is commonly used in clinical genetic laboratories and we propose that it could be ideal for routine assessment and monitoring of the X-inactivation status in female human iPS cell lines.