Polycomb Proteins Pave the Way for Proper Stem Cell Differentiation

Embryonic stem cells have the ability to differentiate into one of the more than 200 cell types. Differentiation requires a strictly regulated program of gene expression that turns certain genes on at specific times and shuts other genes off. Loss of this regulatory circuit prevents stem cells from properly differentiating into adult cell types, and an inability to differentiate has also been linked to the onset of cancer.

Researchers at the BRIC, University of Copenhagen have identified a crucial role of the molecule Fbx110 in embryonic stem cell differentiation. Kristian Helin from the BRIC said, “Our new results show that this molecule is required for he function of one of the most important molecular switches that constantly regulated the activity of our genes. If Fbx110 is not present in embryonic stem cells, the cells cannot differentiate properly and this can lead to developmental defects.”

What is the function of Fbx110? Fbx110 recruits members of the “Polycomb” gene family to DNA. Polycomb proteins, in particular PRC1 & 2, are known to modulate the structure of chromatin, even though they do not bind DNA. Fbx110,, however binds DNA, but it also binds PRC1 . Therefore, Fbx110 seems to serve as an adapter that recruits Polycomb proteins to DNA.

Polycomb proteins bound to nucleosomes
Polycomb proteins bound to nucleosomes

Postdoctoral fellow Xudong Wu, who led the experimental part of this investigation, said, “Our results show that Fbx110 is essential for recruiting PRC1 to genes that are to be silenced in embryonic stem cells. Fbx110 binds directly to DNA and to PRC1, and this way it serves to bring PRC1 to specific genes. When PRC1 is bound to DNA it can modify the DNA associated proteins, which lead to silencing of the gene to which it binds.”

Timing of gene activity is crucial during development and must be maintained throughout the lifespan of any cell. Particular genes are active at a certain times and inactive at other times, and PRC1 seems to be part of the reason for this coordination of gene activity. PRC1 is dynamically recruited to and dissociates from genes according to the needs of the organism.

When cancer arises, this tight regulation of gene activity is often lost and the cells are locked in an inchoate state. This loss of terminal differentiation causes increases cell proliferation and the accumulation of other mutations that allow the cancer cells to undergo continuous self-renewal through endless cell divisions. Such an ability is denied to mature cells because of their tightly controlled programs of gene expression.

Wu added, “Given the emerging relationship between cancer and stem cells, our findings may implicate that an aberrant activity of Fbx110 can disturb PRC function and promote a lack of differentiation in our cells. This makes it worth studying whether blocking the function of Fbx110 could be a strategy for tumor therapy.”

In collaboration with a biotech company called EpiTherapeutics, the BRIC researchers want to develop Fbx110 inhibitors as potential novel therapies for cancer.

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Professor of Biochemistry at Spring Arbor University (SAU) in Spring Arbor, MI. Have been at SAU since 1999. Author of The Stem Cell Epistles. Before that I was a postdoctoral research fellow at the University of Pennsylvania in Philadelphia, PA (1997-1999), and Sussex University, Falmer, UK (1994-1997). I studied Cell and Developmental Biology at UC Irvine (PhD 1994), and Microbiology at UC Davis (MA 1986, BS 1984).