Induced Pluripotent Mesenchymal Stromal Cell Clones Retain Donor-Derived Differences in DNA Methylation Profiles

Reprogramming of somatic cells into induced pluripotent stem cells (iPSC) is an epigenetic phenomenon. It has been suggested that iPSC retain some tissue-specific memory whereas little is known about inter-individual epigenetic variation of iPSC clones. In this study we have reprogrammed mesenchymal stromal cells (MSC) from human bone marrow by retrovirus-mediated overexpression of OCT-3/4, SOX2, c-MYC, and KLF4. Global DNA-methylation profiles of the initial MSC, MSC-derived iPSC (iP-MSC) and embryonic stem cells (ESC) were then compared using a high density DNA-methylation array covering more than 450,000 CpG sites. Overall, DNA-methylation patterns of iP-MSC and ESC were similar whereas some CpG sites revealed highly significant differences, which were not related to parental MSC. Furthermore, hypermethylation in iP-MSC versus ESC was particularly enriched in developmental genes as well as shore regions next to CpG islands indicating that these differences are not due to tissue-specific memory or random de novo methylation. Subsequently, we searched for CpG sites with donor-specific variation in MSC preparations. These “epigenetic fingerprints” were highly enriched in non-promoter regions and outside of CpG islands – and they were maintained upon reprogramming into iP-MSC. In conclusion, DNA methylation profiles of iP-MSC clones from the same donor were closely related despite heterogeneity of MSC. On the other hand, iP-MSC maintain donor-derived epigenetic differences. In the absence of isogenic controls for disease modeling applications, it would therefore be more appropriate to compare iPSC from different donors rather than a high number of different clones from the same patient. 16 samples were hybridised to the Illumina Infinium 450k Human Methylation Beadchip