Lifting the cover on stem cells and cancer

Why do generic stem cells – such as those found in the early embryo or adult organs – specialize into the hundreds of types that make up the body? And why does this process sometimes break down, leading to cancer? Researchers have identified a number of genes that trigger differentiation in stem cells, but the picture is far from complete. Now Frank Rosenbauer's lab at the MDC, collaborating with Sten Eirik Jacobsen’s lab at the Lund Stem Cell Center in Sweden and partners from England, the United States, and Italy, have discovered a new mechanism that controls whether stem cells stay generic or differentiate in specific ways. A chemical code called methylation, which cells use to control large numbers of genes, also determines the fate of some stem cells. The work, published in the October 4, 2009, online issue of Nature Genetics, should help researchers understand why blood cells sometimes deviate from normal developmental routes. It may also lead to new approaches in the treatment of leukemia and other types of cancer.

ABOVE: Normal healthy bone marrow reveals a wide range of developing blood cells and the stem cells from which they arise. BELOW: Removing a key methylating enzyme, however, causes most of these cells to disappear.  

Most of a cell's genes are kept "silent" until they are needed to prompt a particular developmental step or another important process. Activating them often requires removing protein "brakes" docked onto DNA. But the cell also has a general way to silence entire regions of the genome, by blanketing it with clusters of methyl groups. Proteins called methylating enzymes roam the cell nucleus and attach these small hydrocarbons to DNA.

This needs to happen at the right level and right time. In some types of cancer, important genes remain silent because DNA is overmethylated. One therapeutic strategy has been to block the enzymes that carry out the process.

Ann-Marie Bröske and Lena Vockentanz from Rosenbauer's lab wondered whether changes in methylation patterns affect stem cell development. To find out, they developed strains of mice that lacked a key methylating enzyme called DNMT1. Since embryos cannot survive without this molecule, the lab used a method called a conditional knockout, removing both copies of the mouse DNMT1 gene only from developing blood cells.

Blood is one of the best-studied systems for understanding stem cells. Red, white, and other blood cells have a short lifespan and must be constantly made from hemopoietic stem cells (HSCs). These cells have to reproduce frequently and hold onto their generic properties to maintain a large pool from which specialized cells can be made at any time. Defects in this process lead to leukemia and other forms of cancer.

Bröske, Vockentanz and their colleagues observed that animals without any DNMT1 at all were unable to replenish their blood or lymph cells and soon died. An investigation of their bone marrow, where HSCs are stored and blood cells are born, revealed an almost complete absence of generic or developing cells. The HSC pool was drained, with none left to reproduce or differentiate.

This demonstrated that DNA methylation is essential to maintain HSCs – and made it impossible to study the effects of changes in methylation on individual stem cell programs – so the scientists had to take another approach. Instead of removing DNMT1 completely, they used strains of mice in which one copy of the molecule was completely inactive, while the other could only methylate DNA at low levels.

The mice survived, and Bröske, Vockentanz and their colleagues could now examine specific HSC functions. They found that the stem cells had a defect in self-renewal, a major program that keeps stem cell numbers at a steady level. This suggested that methylation plays an essential role in maintaining them. Most surprisingly however, the HSCs went on to develop into red blood cells and some other types. But they were unable to produce white blood cells or lymph, which are vital to the immune system.

Rosenbauer says this reveals a major, previously unrecognized mechanism by which the fates of stem cells are determined. The amount of methylation plays a critical role at an early stage, in deciding what intermediary type of stem cell an HSC will become. With low amounts, genes become active and trigger specialization along one route. At the same time this overpowers the silencing mechanism, needed for differentiation into lymph.

Another experiment revealed the importance of methylation in cancer. One cause of tumors is that stem cells keep dividing, rather than committing themselves to a particular fate. High levels of methylation – a hallmark of many dangerous cancers – may be necessary for the tumor cells to keep reproducing.

The researchers investigated the effects of methylation in a mouse model of leukemia. When DNMT1 was lowered in cancer cells, animals developed tumors much more slowly and had much longer lifespans. Reducing methylation slowed the rate at which cancer stem cells reproduced by about 12 times. A similar effect was obtained by blocking DNMT1 with drugs.

"In cancer, heavy methylation silences genes that would otherwise tell cells to stop dividing and specialize," Rosenbauer says. "The drugs might allow differentiation signals to become active. This could establish a new, important link between the mechanisms that guide stem cell development and cancer."

- Russ Hodge

Highlight Reference:

DNA methylation protects hematopoietic stem cell multipotency from myeloerythroid restriction. Ann-Marie Bröske, Lena Vockentanz, Shabnam Kharazi, Matthew Huska, Elena Mancini, Marina Scheller, Christiana Kuhl, Andreas Enns, Marco Prinz, Rudolf Jaenisch, Claus Nerlov, Achim Leutz, Miguel Andrade-Navarro, Sten Eirik Jacobsen, and Frank Rosenbauer. Nature Genetics 41 (11): 1207-1215. November 2009.

Link to the original paper
Wikipedia article on methylation
Wikipedia article on epigenetics