By adding the right concoction of ingredients, scientists can reprogram your everyday somatic cell into an induced pluripotent stem cell (IPSC) – that is, a cultured cell that has the ability to differentiate into almost any other cell type in response to specific environmental factors, similar to an embryonic stem cell.1 This innovative technology allows the study of the molecular mechanisms of early development and disease, without the ethical restrictions associated with embryonic stem cells.
Not surprisingly, the possibility of utilizing induced pluripotent stem cells in the field of regenerative medicine is of important focus to many scientists. In a recent post, we touched on the potential ability of vitamins A and C to enhance the erasure of epigenetic memory required for cell reprogramming. Because these special types of cells can propagate indefinitely and form any other cell type in the body – such as neurons, liver, and heart cells – we may be able to replace lost organs, repair tissue, and even generate type O red blood cells, which can be used in transfusions for people with any blood type.
Great…so what’s the problem?
Unfortunately, there are drawbacks to this technology, namely the efficiency of reprogramming. Many IPSC’s do not actually gain complete pluripotency.2 Epigenetic modifications are heavily implicated during the reprogramming process whereby the epigenetic makeup of the cell is completely overhauled to first encourage the expression of pluripotent genes and then remodelled to encourage the expression of genes associated with the final cell type that the IPSC will become.3 As the epigenome plays a crucial role in reprogramming, inconsistencies of pluripotency across IPSC lines may be due to epigenetic barriers.
TRIM28: a novel epigenetic barrier
A team of scientists headed by Dr. Miles from The Netherlands Cancer institute recently uncovered a novel epigenetic barrier to the efficient induction of pluripotent stem cell reprogramming.4 Published in a recent issue of STEM CELLS, the paper highlights the use of a shRNA screen targeting over 670 epigenetic modifiers, revealing the involvement of TRIM28 in the resistance of cells transitioning from somatic to pluripotent state.
TRIM28, or Tripartite motif-containing 28, is involved in mediating transcriptional control by interacting with a certain domain in numerous transcription factors. Previous research shows that it plays a role in cellular differentiation and proliferation, DNA damage repair response, transcriptional regulation, and apoptosis.
By blocking the expression TRIM28 during reprogramming, the group demonstrated increases in the number of cells reaching pluripotency, as well as increased expression of a selection of 143 genes.
“Analysis of the list of genes… revealed the most statistically significant gene ontology term was ‘unclassified’. This result indicates TRIM28 does not regulate a specific pathway during reprogramming,” states the authors.
It is known that TRIM28 gene encodes for a protein known to be involved in transcriptional regulation via the recruitment and formation of protein complexes that maintain repressive chromatin.5 Given this, researchers proposed the gene expression alterations, hence reprogramming differences, were likely to be associated with chromatin modification.
Subsequent tests supported this notion by establishing a proportion of the 143 genes to be located near H3K9me3 – a repressive histone H3 modification which has shown to influence the transcription of genes that impedes the IPSC reprogramming process. When TRIM28 expression was blocked, the closer genes are to the H3K9me3 the greater the increase in expression. This suggests the role of TRIM28 in repressing the expression of genes involved in reprogramming via the maintenance of H3K9me3 heterochromatin site.
Why is this important?
Due to the potential to produce almost any other type of cell, the technology of IPSC has sparked excitement in the clinical sciences. The implementation of IPSC to repair damaged or diseased tissue 6 or to test/develop personalised medicines 7 is on the horizon. By establishing barriers preventing the efficient transition of differentiated cells to pluripotent cells scientist can refine IPSC generation to make the future clinical use of IPSC’s both safe and efficient.
Source: Miles, D. C., de Vries, N. A., Gisler, S., Lieftink, C., Akhtar, W., Gogola, E., … & Beijersbergen, R. L. (2017). TRIM28 is an Epigenetic Barrier to Induced Pluripotent Stem Cell Reprogramming. STEM CELLS, 35(1), 147-157.
