Monday, May 17, 2010

Journal Club: GeneSifter Aids Stem Cell Research

Last week’s Nature featured an article entitled “Aberrant silencing of imprinted genes on chromosome 12qF1 in mouse induced pluripotent stem cells [1]” in which GeneSifter Analysis Edition (GSAE) was used to compare gene expression between genetically identical mouse embryonic stem (ES) cells and induced pluripotent stem cells (iPSCs).

Stem Cells 

Stems cells are undifferentiated, pluripotent, cells that later develop into the specialized cells of tissues and organs. Pluripotent cells can divide essentially without limit, become any kind of cell, and have been found to naturally repair certain tissues. They are the focus of research because of their potential for treating diseases that damage tissues. Initially stem cells were isolated from embryonic tissues. However, with human cells, this approach is controversial. In 2006 researchers developed ways to “reprogram” somatic cells to become pluripotent cells [2]. In addition to being less controversial, iPSCs have other advantages, but there are open questions as to their therapeutic safety due to potential artifacts introduced during the reprogramming process. 

Reprogramming cells to become iPSCs involves the overexpression of a select set of transcription factors by viral transfection, DNA transformation, and other methods. To better understand what happens during reprogramming, researchers have examined gene expression and DNA methylation patterns between ES cells and iPSCs and have noted major differences in mRNA and microRNA expression as well as DNA methylation patterns. As noted in the paper, a problem with previous studies is that they compared cells with different genetic backgrounds. That is, the iPSCs harbor viral transgenes that are not present in the ES cells, and the observed differences could likely be due to factors unrelated to reprogramming. Thus, a goal of this paper's research was to compare genetically identical cells to pinpoint the exact mechanisms of reprogramming. 

GeneSifter in Action 

Comparing genetically similar cells requires that both ES cells and iPSCs have the same transgenes. To accomplish this goal, Stadtfeld and coworkers devised a clever strategy whereby they created a novel inducible transgene cassette and introduced it into mouse ES cells. The modified ES cells were then used to generate cloned mice containing the inducible gene cassette in all of their cells. Somatic cells could be converted to iPSCs by adding the appropriate inducing agents to the tissue culture media.
Even though ES cells and iPSCs were genetically identical, ES cells were able to generate live mice whereas iPSCs could not. To understand why, the team looked at gene expression using microarrays. The mRNA profiles for six iPSC and four ES cell replicates were analyzed in GeneSifter. Unsupervised clustering showed that global gene expression was similar for all cells. When the iPSC and ES cell data were compared using correlation analysis, the scatter plot identified two differentially expressed transcripts corresponding to a non-coding RNA (Gtl2) and small nucleolar RNA (Rian). The transcripts’ genes map to the imprinted Dlk1-Dio3 gene cluster on mouse chromosome 12qF1. While these genes were strongly repressed in iPSC clones, the expression of housekeeping and pluripotentency cells was unaffected as demonstrated using GeneSifter’s expression heat maps.

Subsequent experiments that looked at gene expression from over 60 iPSC lines produced from different types of cells and chimeric mice that were produced from mixtures of iPSCs and stem cells showed that the gene silenced iPSCs had limited development potential. Because the Dlk3-Dio cluster imprinting is regulated by methylation, methylation patterns revealed that the Gtl2 allele had acquired an aberrant silent state in the iPSC clones. Finally, by knowing that Dlk3-Dio cluster imprinting is also regulated by histone acetylation, the authors were able to treat their iPSCs with a histone deacetylase inhibitor and produce live animals from the iPSCs. Producing live animals from iPSCs in a significant milestone for the field.

While histone deacetylase inhibitors have multiple effects, and more work will need to be done, the authors have completed a tour de force of work in this exciting field, and we are thrilled that our software could assist in this important study. 

Further Reading

1. Stadtfeld M., Apostolou E., Akutsu H., Fukuda A., Follett P., Natesan S., Kono T., Shioda T., Hochedlinger K., 2010. "Aberrant silencing of imprinted genes on chromosome 12qF1 in mouse induced pluripotent stem cells." Nature 465, 175-181.

2. Takahashi K., Yamanaka S., 2006. "Induction of pluripotent stem cells from mouse embryonic and adult fibroblast cultures by defined factors." Cell 126, 663-676.


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