Some people are convinced that they’ve lived a past life, recalling snippets of someone else’s memories from a different time and place. Now, without the benefit of hypnosis, researchers have discovered that induced pluripotent stem (iPS) cells also retain transcriptional memories of past lives as somatic cells, which can be at least partially explained by incomplete DNA methylation during cell reprogramming.
Although iPS cells look and behave a lot like embryonic stem (ES) cells, it’s becoming clear that they show important differences in things like DNA methylation and miRNA expression. So a team of talented researchers led by Miguel Ramalho-Santos at the University of California San Francisco systematically compared the transcriptional profiles and genome-wide methylation patterns of human ES cells and iPS cells generated from three different somatic cell types (hepatocytes, skin fibroblasts, and melanocytes). Some of their interesting findings:
- All iPS cells kept transcriptional memories of their past lives as somatic cells. For example, several differentially expressed genes between ES cells and iPS cells were also differentially expressed between ES cells and the iPS originals.
- Somatic genes expressed in iPS cells were hypomethylated compared with the same genes in ES cells.
- These incompletely silenced genes tended to be genomically isolated from other genes repressed during cell reprogramming, suggesting that the silencing machinery is inefficiently recruited to these remote locales.
In these experiments, the team analyzed low-passage (<P20) iPS cells. The researchers speculate that some of the transcriptional memories may fade with time (i.e., extensive passaging), but may never be completely abolished.
Even if you don’t believe in reincarnation, you can reinvent yourself as an iPS guru with the help of this paper at Nature Cell Biology, April, 2011.